Citation
The effects of common surface pretreatments on the shear strength of bonded concrete overlays

Material Information

Title:
The effects of common surface pretreatments on the shear strength of bonded concrete overlays
Creator:
Pultorak, Andrew Stephen ( author )
Place of Publication:
Denver, Colo.
Publisher:
University of Colorado Denver
Publication Date:
Language:
English
Physical Description:
1 electronic file (134 pages) : ;

Thesis/Dissertation Information

Degree:
Master's ( Master of science)
Degree Grantor:
University of Colorado Denver
Degree Divisions:
Department of Civil Engineering, CU Denver
Degree Disciplines:
Civil engineering

Subjects

Subjects / Keywords:
Concrete -- Testing ( lcsh )
Shear (Mechanics) ( lcsh )
Concrete construction -- Maintenance and repair ( lcsh )
Concrete construction -- Maintenance and repair ( fast )
Concrete -- Testing ( fast )
Shear (Mechanics) ( fast )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Review:
The durability of a concrete repair is highly dependent on the shear strength of the interface between new and old concrete. Therefore, the engineer designing the repair makes every effort to maximize this strength. To that end, pretreatments, such as prewetting the substrate and/or applying bonding agents, are commonly specified. The efficacy of these pretreatments is often debated, and previous studies have produced contradictory results. This research was undertaken to determine the effects of prewetting the substrate and applying a bonding agent, both in combination and individually. The bond strength in tension and the shear strength of the bond were measured using a variety of methods, including in-place testing and testing of extracted specimens. The results indicate that both prewetting and the use of a bonding agent can be beneficial to the shear strength of bonded overlays.
Bibliography:
Includes bibliographical references.
System Details:
System requirements: Adobe Reader.
Statement of Responsibility:
by Andrew Stephen Pultorak.

Record Information

Source Institution:
University of Colorado Denver Collections
Holding Location:
Auraria Library
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
987247335 ( OCLC )
ocn987247335
Classification:
LD1193.E53 2016m P95 ( lcc )

Downloads

This item has the following downloads:


Full Text
THE EFFECTS OF COMMON SURFACE PRETREATMENTS ON THE
SHEAR STRENGTH OF BONDED CONCRETE OVERLAYS
by
ANDREW STEPHEN PULTORAK B.S., University of Colorado Denver, 2006
A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements of the degree of Master of Science Civil Engineering
2016


This thesis for the Master of Science degree by Andrew Stephen Pultorak has been approved for the Civil Engineering Program by
Prof. Kevin L. Rens, Chair Frederick R. Rutz Chengyu Li
December 17, 2016
11


Pultorak, Andrew Stephen (M.S., Civil Engineering)
The Effects of Common Surface Pretreatments on the Shear Strength of Bonded Concrete Overlays Thesis directed by Associate Professor Frederick R. Rutz
ABSTRACT
The durability of a concrete repair is highly dependent on the shear strength of the interface between new and old concrete. Therefore, the engineer designing the repair makes every effort to maximize this strength. To that end, pretreatments, such as prewetting the substrate and/or applying bonding agents, are commonly specified. The efficacy of these pretreatments is often debated, and previous studies have produced contradictory results. This research was undertaken to determine the effects of prewetting the substrate and applying a bonding agent, both in combination and individually. The bond strength in tension and the shear strength of the bond were measured using a variety of methods, including in-place testing and testing of extracted specimens. The results indicate that both prewetting and the use of a bonding agent can be beneficial to the shear strength of bonded overlays.
The form and content of this abstract are approved. I recommend its publication.
Approved: Frederick R. Rutz
m


ACKNOWLEDGEMENTS
I wish to express my deepest gratitude to Dr. Fred Rutz, whose advice, mentorship, wisdom, and enthusiastic participation made this project possible. My sincere appreciation goes to Dr. Kevin Rens and Dr. Chengyu Li of the Civil Engineering Department at UCD for their service as members of my examination committee. I also wish to thank CTL-Thompson, Inc., including Bud Wemer, Zack Ballard, and Dan Barrett, for donating their time, expertise, and for generously allowing me to use their testing equipment. A tremendous thank-you to Tom Thuis and Peter Sillstrop of the Electronic Calibration and Repair Lab at UC-Denver for fabricating parts and aiding me in setting up the compression testing equipment. My sincerest appreciation goes to my fellow students: Christian Rosen, Anne Swan, and Chris Cardillo, who helped me with the unpleasant task of placing and then removing 2'A tons of concrete. I also wish to acknowledge the contributions of others who have assisted me, including, but not limited to: Randon Grimes, Jed Williamson, and Chris Sheehan.
Finally thank you to my wife, Jessica, and my sons, Ethan and Ben, for their love, understanding, and support throughout this project.
IV


TABLE OF CONTENTS
CHAPTER
I OVERVIEW......................................................................1
1.1 Introduction..............................................................1
1.2 Research Goal and Significance............................................2
1.3 Outline...................................................................2
II BACKGROUND....................................................................3
2.1 Introduction..............................................................3
2.2 Mechanism of Concrete to Concrete Bond....................................4
2.3 Methods of Testing Bond Strength..........................................4
2.3.1 Pull off test (ASTMC1583)..............................................5
2.3.2 Direct shear (guillotine) test.............................7
2.3.3 Slant shear test.......................................................8
2.3.4 Jacking test...........................................................9
2.4 Pretreatments............................................................10
2.4.1 Prewetting............................................................10
2.4.2 Bonding agents........................................................11
2.5 Literature Review........................................................11
2.5.1 Early studies.........................................................11
2.5.2 Modem studies.........................................................12
2.5.3 Code references.......................................................14
2.5.4 Previous research at the University of Colorado Denver..............15
v


2.5.5 Summary of literature review............................................15
III RESEARCH PROGRAM.................................................................17
3.1 Construction of Substrate and Overlay Slabs................................17
3.2 Testing....................................................................24
3.2.1 Slump test..............................................................24
3.2.2 Slab concrete compression test..........................................25
3.2.3 Pull-off test...........................................................26
3.2.4 Direct shear (guillotine) test..........................................28
3.2.5 Slant shear test........................................................30
3.2.6 Jacking test............................................................34
IV RESULTS..........................................................................37
4.1 Slump Cone Results.........................................................37
4.2 Sack Concrete Sieve Analysis...............................................37
4.3 Compression Testing Results................................................37
4.4 Specimen Identifiers.......................................................39
4.5 Pull-off Test Results......................................................40
4.6 Direct Shear Test Results..................................................42
4.7 Slant Shear Test Results...................................................44
4.8 Jacking Test Results.......................................................46
V DISCUSSION.......................................................................48
5.1 Variation of Compressive Strength..........................................48
vi


5.2 Effects of Strength Gain on Test Results................................50
5.3 Bond Strength in Tension................................................51
5.4 Shear Strength at the Bonded Interface..................................53
5.4.1 Representative shear strengths from previous research.................53
5.4.2 Direct shear test.....................................................54
5.4.3 Slant shear tests.....................................................56
5.4.4 Jacking test..........................................................56
5.4.5 Comparison of shear test results......................................58
5.4.6 Comparison of tension and shear test results..........................58
5.4.7 Variations in data....................................................60
5.4.8 Statistical significance..............................................61
VI CONCLUSIONS..................................................................63
6.1 Summary.................................................................63
6.2 Conclusions.............................................................63
6.3 Suggestions for Future Research.........................................64
REFERENCES..............................................................................66
APPENDIX
A Pull Off test data...........................................................69
B Direct Shear (guillotine) test data..........................................76
C Slant Shear test data.......................................................100
D Jacking test data...........................................................Ill
E Slant Shear discussion......................................................118
vii


..4
..5
16
19
24
37
38
39
39
40
40
41
41
42
42
43
43
44
44
45
45
45
46
46
LIST OF TABLES
Common examples of surface profiling and pretreatments..
Summary of common tests for bond strength...............
Summary of results regarding pretreatments..............
Pretreatment summary....................................
Testing Summary.........................................
Results of slump cone test..............................
Compression force at failure............................
Compression stress at failure...........................
Average substrate compressive strength..................
Pull-off test samples...................................
Pull-off test results (SI units)........................
Pull-off test results (U.S. customary units)............
Pull-off test bond strength adjustment (SI units).......
Pull-off test bond strength adjustment (U.S. customary units)
Direct shear test samples...............................
Direct shear test results (SI units)....................
Direct shear test results (U.S. customary units)........
Direct shear test strength adjustment (SI units)........
Direct shear test strength adjustment (U.S. customary units)..
Slant shear test samples................................
Slant shear test results (SI units).....................
Slant shear test results (U.S. customary units).........
Jacking test samples....................................
Jacking test results (SI units).........................
vm


4.8- 3 Jacking test results (U.S. customary units)...........................................47
4.8- 4 Jacking test strength adjustment (SI units)...........................................47
4.8- 5 Jacking test strength adjustment (U.S. customary units)...............................47
5.4-1 Results of statistical analysis.......................................................62
E-l Maximum shear at slant interface (SI Units)..........................................121
E-2 Maximum shear at slant interface (U.S. customary units)..............................121
E-3 Slant shear strength adjustment (U.S. customary units)...............................122
ix


LIST OF FIGURES
FIGURE
2-1 Example pull-off test (ASTM C15 83) apparatus........................................6
2-2 Direct shear test in-progress........................................................7
2-3 Preparing to test slant shear sample on compression machine..........................8
2-4 Diagram showing (1) the slant shear specimen under uniaxial compression stress and (2)
the resulting stresses on the bond surface..........................................9
2- 5 Block specimen undergoing jacking test..............................................10
3- 1 Typical substrate form, ready to receive concrete...................................17
3-2 Surface profding the substrate......................................................18
3-3 Finished profde compared with ICRI CSP 6 example....................................19
3-4 Example of saturated, surface dry appearance immediately before placement...........20
3-5 Application of cement slurry bonding agent..........................................20
3-6 Plan view of slabs and pretreatments................................................22
3-7 Isometric rendering of a typical substrate slab with two overlay slabs..............22
3-8 As constructed view showing pull-off test sampling locations........................23
3-9 As constructed view showing pull-off, direct shear, slant shear, and jacking test sampling locations................................................................................23
3-10 Example slump cone test on overlay concrete (note integral color)...................25
3-11 Compression Testing Equipment.......................................................26
3-12 Cleaning the surface of the pull-off test specimen..................................27
3-13 NDT 007 James Bond Tester...........................................................27
3-14 MTS testing equipment with direct shear apparatus...................................29
3-15 Direct Shear (Guillotine) Apparatus.................................................30
3-16 Diagram showing (1) the slant shear specimen with slant angle a subjected to uniaxial compression, (2) the stresses on the bond surface, and (3) the forces on the bond surface .........................................................................................31
3-17 Checking the slant angle with an inclinometer prior to coring for a slant shear specimen
...................................................................................31
x


3-18 Mohrs circle under 1 ksi (0.145 MPa) uniaxial compression at slant angles of 45 and 60
degrees...............................................................................33
3-19 Representative slant shear cylinder after testing under uniaxial compression..........34
3-20 Same specimen as Figure 3-19, opened to reveal surface of splitting tension failure.34
3-21 Concrete saw and dust collection system...............................................35
3-22 Simplex RC306C hydraulic testing specimen on Slab 2A..................................36
5-1 28-day compressive strength by slab location..........................................49
5-2 Strength gain in concrete.............................................................51
5-3 Unadjusted average bond strengths in tension, grouped by pretreatment.................52
5-4 Adjusted average bond strengths in tension, grouped by pretreatment...................53
5-5 Unadjusted average direct shear strengths at the bonded interface, grouped by pretreatment
.....................................................................................54
5-6 Adjusted average direct shear strengths at the bonded interface, grouped by pretreatment ............................................................................................55
5-7 Unadjusted average jacking test strengths at the bonded interface, grouped by pretreatment
.....................................................................................56
5-8 Adjusted average jacking test strengths at the bonded interface, grouped by pretreatment...
57
5-9 Direct shear vs. pull-off test results (SI units)..................................59
5-10 Direct shear vs. pull-off test results (U.S. customary units)......................60
5-11 Coefficients of variation for each test type, grouped by pretreatment..............61
E-l Diagram showing (1) the slant shear specimen under uniaxial compression stress and (2)
the resulting stresses on the bond surface......................................119
E-2 Applied shear stress at bonded interface (cbond) as a ratio of applied compressive stress
(oo), varying surface roughness, after Austin, et al. (1999).................... 119
E-3 Compressive strength vs. measured slant shear strength............................123
xi


CHAPTER I
OVERVIEW
1.1 Introduction
When damage or deterioration to existing concrete is not so severe as to warrant complete removal, repairs frequently involve overlaying the existing concrete with a new concrete surface. Examples of this type of repair range from small patches on a wall or beam, to resurfacing existing concrete pavement or a bridge deck with a layer of new concrete. In the case of pavement, it is possible to design the new concrete to act independently from the existing concrete. However, it often much more economical to bond the new concrete, hereafter referred to as the overlay, to the existing concrete, hereafter referred to as the substrate. When sufficient bond exists between the overlay and substrate, the two slabs will act in unison under applied loading; this behavior is known as monolithic action (Bissonnette, et al., 2012). The durability of the repair, whether a patch in a structural member or an overlay of existing pavement, is highly dependent on the shear strength of the bonded interface between the overlay and substrate.
The desire to obtain good bond strength to prolong the life of a repair is apparent, and much time and effort has been spent to isolate the factors that produce consistently high bond strengths.
This research examines two such factors which are the cause of significant debate within the engineering community: 1) prewetting the substrate to increase its moisture content and 2) applying a bonding agent to the substrate surface. These practices are referred to in ACI 325 as pretreatments (2006). The research conducted as part of this report was performed using three substrate slabs on to which six overlay slabs were bonded. The substrate slabs were mechanically roughened and cleaned, after which the surfaces were prepared with a combination of pretreatments: with or without a bonding agent, and with or without prewetting. Pull-off, direct shear, push off, and slant shear tests were performed to evaluate the effects on bond strength.
1


1.2 Research Goal and Significance
This thesis seeks to quantify the effect of pretreatments on the shear strength of concrete-to-concrete bondings, including bonded overlays and other types of repairs. The results of the study may be applied to the construction of repairs and to the preparation of the interface between successive concrete placements (cold joints) to develop better strength at the bonded interface.
A variety of different tests were used to measure the effects of pretreatment on bonded strength, including in-place testing and testing performed on extracted samples. Each test measures the strength of the bond in a different way. The results illustrate the sensitivity of each particular test to each pretreatment type, as well as indicate the combination of pretreatments that will best enhance the bond strength over all types of tests.
1.3 Outline
There are six chapters in this thesis. The first chapter introduces the topic and goals of the research.
Chapter 2 is a literature review, summarizing past research into the shear strength of bonded overlays and the effects of pretreatments. The chapter briefly discusses the mechanisms of concrete to concrete bond, and examines the types of tests available to determine bond strength and shear strength.
In Chapter 3, the research program is discussed. This includes construction, preparation, and testing of the substrate and overlay slabs. The profiling and cleaning of the substrate surface and the application of the pretreatments is discussed.
In Chapter 4, the bond strength and shear strength data is tabulated.
Chapter 5 is a discussion of the experimental results from the four strength tests and the observed effects of the pretreatments.
Chapter 6 presents the conclusions of the research and summarizes future research needs.
The attached appendices include experimental data gathered during the execution of the research program.
2


CHAPTER II
BACKGROUND
2.1 Introduction
The ability to repair concrete by removing damaged or deteriorated areas and replacing or overlaying them with new concrete has long been understood. Frequently, the goal of repair, whether on a structural member or concrete pavement, is to place the new concrete in such a way that the old and new concrete are able to adequately transfer stresses between one another, such that the member as a whole behaves as if it were made of monolithic concrete. Depending on the loading to which the member is subjected, stresses could act either perpendicular to the interface, causing tension or compression, or parallel with the interface, causing shear. The mechanism resisting these stresses is referred to broadly as bond strength.
The author has compiled existing literature dating from 1919 (Rosengarten) to the present; these studies investigate the means of maximizing bond strength to produce more durable repairs. These studies generally agree that the best bond is obtained using a substrate surface free of laitance (the weak surface layer formed during finishing) and debris. Other recommendations and conclusions can be divided into two categories, as outlined in Table 2.1-1. The first is recommendations regarding surface profiling, which is the intentional roughening of the substrate surface to provide superior mechanical interlock. Much research has examined the effects of bruising, which is the introduction of microfractures into the surface of the substrate during profiling that may weaken the bond strength.
The second category concerns pretreatments, which are substances introduced into the substrate prior to concrete placement to enhance the strength of the bond. This study is primarily concerned with the second category.
3


Table 2.1-1 Common examples of surface profiling and pretreatments
Examples of Surface Profiling: Examples of Pretreatment:
Sand/shot blasting Prewetting/moistening
Impact hammering Cement bonding agent
Acid etching Latex bonding agent
Water jetting Epoxy bonding agent
2.2 Mechanism of Concrete to Concrete Bond
Bond strength is generally measured as either 1) the adhesion or tensile strength of the bond, or 2) the shear strength of the bonded interface. Assuming the bond strength is less than the tensile strength of the concrete, the bond strength will be governed by the failure mode of tensile cracking along the interfacial surface. The failure mode of the bond in shear is more complex, particularly for roughened surfaces. Subjected to shear, mechanical interlock between the two surfaces will contribute to the shear resistance; the failure mode will be a combination of shear and tensile cracking (Austin, S., etal., 1999).
2.3 Methods of Testing Bond Strength
Numerous tests are available to measure bond strength, both in tension and in shear. As Austin, et al. (1999) notes, any one test provides limited information that "taken in isolation, can result in a misunderstanding of the behaviour of bonded cementitious materials'. To avoid this bias, the selection of test should ideally involve a stress state to which the repaired member will be subjected to during service (Walls & Shrive, 1988). The test methods selected and discussed herein, as summarized on Table 2.3-1, subject the bonded interface to a variety of stress conditions, including tension, shear, and a combination of compression and shear.
4


Table 2.3-1 Summary of common tests for bond strength
Test: Test Location: Type:
Pull-off In-Place Direct Tension
Guillotine Laboratory Direct Shear
Slant Shear Laboratory Combination of Compression & Shear
Jacking In-Place Direct Shear
2.3.1 Pull offtest (ASTM C1583)
The pull-off test is a direct tension test suitable for testing in-place bond strength. The test has been in use in a variety of forms since before 1990 (Hindo, 1990). In 2004, the test was given the designation ASTM C1583 (2015), and it is currently (to the authors knowledge) the most commonly used testing procedure for bond strength. However, its accuracy has been repeatedly called into question, with one study for the U.S. Army Corps of Engineers noting that results obtained... can be described as variable or very variable (Vaysburd & McDonald, 1999).
The procedure begins by coring through the overlay and partially into the substrate. Vaysburd and McDonald (1999) found that the depth of the core into the substrate significantly affected the strength results. They recommended a minimum penetration into the substrate of 25 mm (1 in).
ASTM C1583, conversely, specifies a minimum depth into the substrate of 10 mm (0.5 in). After the core has been prepared and cleaned, a stainless steel puck is epoxied to the surface of the core and the epoxy is allowed to cure.
Once epoxy curing is complete, the apparatus is placed above the core (Figure 2-1). The specimen is loaded in tension by means of a threaded rod inserted into the top of the steel puck. The ASTM C1583 requires a constant loading rate of 35 15 kPa/s (5 2 psi/s); it has been found that higher rates of loading correspond to higher bond strength results (Vaysburd & McDonald, 1999).
The test must be performed with the apparatus set as near perpendicular to the bonding surface as is practical to minimize unintended eccentricity in the applied load.
5


Figure 2-1 Example pull-off test (ASTM Cl 583) apparatus
The test may result in one of several possible failure mechanisms. The partial core may fail entirely above or below the bonded surface, indicating that the strength of bond exceeds the tensile strength of the new or old concrete. Clearly, the observed result in such case represents the tensile strength of the failed concrete and does not represent a bond strength. Another possible mode consists of failure of the epoxy which secures the stainless steel puck to the overlay surface.
These failure modes indicate only that the bond strength exceeds the test result; they do not provide an actual value for bond strength. The pull off test will provide a representative bond strength only when failure occurs at or very near to the bond surface. For this reason, ASTM C1583 cautions that results may be averaged together only if they exhibit the same failure mode.
The pull-off test measures adhesion at the bonded surface, which is typically considerably lower than the shear strength of the bonded interface. A factor is required to convert the value determined with the pull-off apparatus to a value for shear strength. Rosen (2016), referencing a 2000 study by Delatte, et al., multiplied the measured pull-off strengths by 2.04 to estimate shear strength.
6


2.3.2 Direct shear (guillotine) test
Although there is no ASTM standard for the direct shear test, several are known to be in use in locations throughout the United States. This test is similar to an "Iowa-type shear test after Iowa Test Method 406-C (2000), which is referenced in ACI 325-06. It is also similar to a "Brookhaven National Laboratory guillotine shear test (Illinois Bureau of Materials and Physical Research, 2012). It differs from the aforementioned, single shear tests in that the shearing action is applied at two locations (double shear) on the sample: the first is at the bonded interface where failure will occur, and the second is about 76 mm (3 in) away from the interface and is present only to stabilize the specimen during testing. The apparatus is pictured in Figure 2-2 and its usage is described in detail, below:
Figure 2-2 Direct shear test in-progress
The test apparatus consists of a set of nested boxes, called a guillotine. Full depth core samples are taken perpendicular to the bond surface and transferred to the laboratory. After drying, the cores are placed in the guillotine with the bond plane centered between the edges of the nested
boxes. The apparatus is compressed, which induces shear on the bond plane until failure occurs. Test 406-C recommends a loading rate in the range of 45 to 60 kPa/s (400 to 500 psi/min).
7


2.3.3 Slant shear test
Slant shear tests are commonly used by manufacturers for testing the performance of proprietary bonding agents (Austin, S., et al., 1999); this test has been formally adopted as ASTM C882: "Standard Test Method for Bond Strength of Epoxy-Resin Systems used with Concrete in Sheaf. This test was first known as the Arizona Slant Shear test (Austin, S., et al., 1999) (Kriegh, 1976). In atypical laboratory testing scenario, concrete is placed in a cylinder mold with a plate installed that forms the interior face at a 60-degree angle. The plate is removed once the concrete has cured, and the bonding surface is prepared as desired before casting the 'overlay' concrete in the mold. The resulting specimen is then compressed to its ultimate strength (Figure 2-3).
Figure 2-3 Preparing to test slant shear sample on compression machine. Note difference in concrete color indicating the slanting bond surface
Multiple studies conclude that slant shear tests are among the most sensitive to the type and proportions of the materials used to create the bond surface Kriegh (1976). However, the test is also extremely sensitive to the roughness or profding of the substrate. Austin, et al. (1999) obtained bond failure solely with specimens prepared with relatively smooth substrates. In his research, he noted that several roughened specimens failed in compression instead of shear failure at the bonded surface. However, other researchers, such as Rosen (2016), have obtained good results for roughened surfaces.
8


The slant shear test exerts a combination of compression and shear on the bonded surface resulting from the angle of inclination of the surface. The compression force can be resolved into two components: a compression stress 'oN' normal to the bond surface, known as clamping force, and shearing stress 'tnt' parallel to the bond surface (Figure 2-4).
(1) (2)
Figure 2-4 Diagram showing (1) the slant shear specimen under uniaxial compression stress and
(2) the resulting stresses on the bond surface
Appendix E contains additional information related to the slant shear test, including a discussion of the effect of slant angle a' on the transformed stresses.
2.3.4 Jacking test
The jacking test is a direct shear test used for in place testing. The procedure involves sawcutting the overlay into smaller block specimens. A hydraulic jack is installed adjacent to the blocks and is secured to the substrate. Ideally, the jack should be oriented such that the shearing force is applied as closely as possible to the surface of the substrate, so as to minimize the overturning moment component of the force (Rosen, 2016). A steel plate can be inserted between the ram and the block to distribute the shearing force evenly across the face of the block. Shear is applied (Figure 2-5) until the sample fails, and the maximum applied force is recorded.
9


2.4 Pretreatments
Pretreatments, as the name implies, are basic additions to the smooth or profded substrate intended to increase the strength and durability of the concrete-to-concrete bond. The addition of water into the otherwise dry substrate is referred to as "prew etting". The cementitious material added to the surface of the substrate prior to concrete placement is referred to as a bonding agent. Both treatments have been in common use for over a century. Paragraphs, below, describe the intended benefit of pretreatments.
Figure 2-5 Block specimen undergoing jacking test
2.4.1 Pre wetting
The surface of a dry substrate, particularly one that has been made more porous by the removal of laitance during profiling, has a relatively high moisture demand. The effect of prewetting the substrate concrete fills the existing capillaries that would otherwise tend to draw water out of the new overlay concrete. This may result in a condition wherein not enough free water is present to fully hydrate the overlay cement; this may reduce concrete strength at the interface. The term Saturated Surface Dry (SSD) is often used to describe a condition where the continual wetting of the substrate in the period leading up to an overlay fills the pores of the old concrete. The surface is allowed to dry
10


prior to placement so as not to weaken the new concrete by increasing the water/cement ratio at the bond interface.
2.4.2 Bonding agents
The earliest bonding agents in common use consisted of a cement-water slurry (cement-neat) or a cement-sand-water slurry. These agents are still commonly specified as a means to achieve a more durable bond. Other bonding agent products, such as latex-modified grout or epoxy grout, have been introduced more recently. In any case, the agent is typically scrubbed into the surface of the substrate immediately before overlay concrete is placed.
The mechanism by which a bonding agent enhances concrete-to-concrete bond is not entirely clear. It may be that the action of scrubbing the agent into the substrate coats assimilates dust particles that were not removed by cleaning (Silfwerbrand & Paulsson, 1998).
2.5 Literature Review
Existing studies containing conclusions or recommendations regarding pretreatments were identified during the literature review phase. The execution and results of these studies are described in chronological order in the paragraphs below. Codified recommendations regarding pretreatments are also discussed. The summary section tabulates and compares nearly a century of research into pretreatments.
2.5.1 Early studies
In 1919, W. E. Rosengarten, a researcher with the Bureau of Public Roads (a forerunner of the Federal Highway Administration (FHWA)), published his findings on the strength of concrete-to-concrete bondings. Though concrete pavement had been used as early as 1896 in the United States (Pasko, 1997), it was not in common use until after 1910. When Rosengarten conducted his research, concrete pavement was entering a decade of service and much of it likely needed rehabilitation. Rosengarten prepared some of his specimens using a bonding agent, which he termed a cement butter layer (Rosengarten, 1919). He also wet the substrate of some of his samples prior to bonding the overlay. The impact on the strength of the bond was evaluated using both direct tension and direct
11


shear tests, as well as a flexure test. Rosengarten found that the bonding agent added 25 percent to the tensile strength of the bond, and also benefited the strength in shear. His results on prewetting, which he notes was common practice at the time, were inconclusive for tension. Rosengarten did see an increase in shear strength due to prewetting.
In 1956, Felt conducted his now widely cited research with the goal of identifying the factors which maximized the shear strength, and therefore the durability, of an overlay (Felt, 1956). Along with other variables, Felt investigated the effects of prewetting and the use of bonding agent. He began by evaluating small (240 x 240 x 84 mm) (9.5 x 9.5 x 7 in) bonded prisms to guide his selection of pretreatments for the remainder of the study. He followed this with much larger (400 x 1020 x 84 mm) (16 x 40 x 7 in) slabs, some of which were laboratory cast, and others in which the substrate was cut from pavement that had been in service for several decades. Felt tested his samples using a single shear, guillotine-type apparatus. He concluded that a dry substrate was preferable to damp and that a cement or cement-sand slurry bonding agent produced a superior bond. Noting the amount of scatter in his data, Felt wrote ... it became apparent that factors influencing bond of new and old concrete were not easily isolated and controlled. This sentiment has been echoed by many contemporary researchers. The difficulty in isolating the variables that have the principle effect on bond strength may explain the conflicting conclusions of the more modem literature, discussed below.
2.5.2 Modem studies
Wall and Shrive (1988) conducted a study that included finite element modeling of an interfacial bond, in conjunction with laboratory experimentation on 112 prismatic samples using a combined shear/compression test. Their FEM model indicated that consistency of the bonding agent was critical; a void in the bonding material was found to significantly increase the stress in the adjacent areas of the bond material. Based on laboratory experimentation, they found that superior bond strength can be achieved with or without a bonding agent; however, their results indicate less scatter in the data when a bonding agent was employed, an effect first noted by Felt. In disagreement
12


with Felts findings, Wall and Shrive observed an improvement in strength due to prewetting of the substrate.
In 1991, Saucier and Pigeon presented the results of a study based on the results of combined shear/compression tests on over 2,000 bonded concrete prisms. The prisms themselves were relatively small cubes with dimensions of75x75x75 mm (3 x 3 x 3 in) and were used in conjunction with a coarse aggregate size of 12 mm (0.5 in) (Saucier & Pigeon, 1991). The study results generally agreed with Felts conclusions: the use of a bonding agent will increase the strength of the bond, and prewetting the substrate does not improve bond strength. Saucier and Pigeon also experimented with allowing some drying of the bonding agent. Felt had recommended that the bonding agent be allowed to dry slightly on the substrate before placement of the overlay, stating: grout that has lost its water sheen is in proper condition for the concreting operation (Felt, 1956). Saucier and Pigeon allowed the bonding agent to dry for 45 minutes on some of their specimens prior to placing the overlay. They noted the dried agent caused a slight increase in bond strength when applied to an SSD substrate, but caused a decrease in bond strength when applied to a dry substrate.
In a large test involving over 150 substrate slabs with dimensions of 915x915x130 mm (36 x 36 x 5 in), Whitney, et al. (1992) found that the majority of debonding between substrate and overlay occurs during the early curing of the overlay. The study found that good bond strength could be achieved through the application of epoxy bonding agents, particularly in harsh environmental conditions. They noted that the application rate of the bonding agent did not seem to greatly affect the strength of the bond. The study also found that both high substrate surface temperatures before placement, and large changes in ambient temperature in the 24 hours following an overlay, adversely affected the measured bond strength.
In a 1998 article focusing on the rehabilitation of bridge decks in Sweden, Silfwerbrand and Paulsson advised against the use of bonding agents. They note that the use of a bonding agent creates two possible planes of weakness between the overlay and substrate (Silfwerbrand & Paulsson, 1998).
13


Wells and Stark found that substrate surfaces prepared by shotblasting produced consistently good bond strengths without the need for a bonding agent. However, they did find that bonding agents did have a positive effect on slabs prepared using other surface profiling methods (Wells & Stark, 1999).
Djazmati and Pincheria (2004) conducted a study on the effects of surface profiling and pretreatment at the interfacial bond between successive concrete placements (cold joints). In contrast to much of the literature on this topic, Djazmati and Pincheria examined bonding to recently placed substrate (as little as 24 hours old) to simulate cold joints formed during construction. They found that a joint that had been saturated with water was about half as strong as a dry joint. They recommended moist curing the joint for a minimum of 24 hours prior to placement, but cautioned that the joint surface should appear dry before commencing with the second placement. Djazmati and Pincheria also studied the effect of a resin emulsion bonding agent on a smooth joint. They found that the resin emulsion bonding agent, being substantially less stiff than concrete, caused the resulting joint to be much more flexible than monolithic concrete. Therefore, they recommend against the use of a resin emulsion bonding agent at cold joints.
In a report for the Bureau of Reclamation, Bissonnette, et al. (2012) concluded that moistening of the substrate is beneficial, and that optimum saturation in the substrate surface lies somewhere between 55 and 90 percent, though he states that fundamental issues remain unsolved with regard to moisture conditioning of the concrete substrate... Bissonnette recommended against the use of bonding agents for reasons similar to those expressed by Silfwerbrand and Paulsson (1990).
Julio, Branco, and Silva (2004) concluded that pre-wetting the substrate does not significantly influence the bond strength.
2.5.3 Codereferences
Both ACI 325 (2006) and ACI 345 (2011) express concern over the effectiveness of bonding agents and the potential for debonding if the bonding agent is improperly applied. The Portland Cement Association (1996) recommends a thin coat of bonding grout consisting of a cement-sand
14


slurry be scrubbed into the substrate surface prior to placement of bonded overlays. Meanwhile, the National Concrete Pavement Technology Center states that bonding agents are not required for concrete-to-concrete pavement overlays (Harrington, 2008).
2.5.4 Previous research at the University of Colorado Denver
This study builds on the findings of a previous program of research performed at the University of Colorado Denver by Christian Rosen (2016). Rosens work involved testing the effects of various substrate surface profiles, ranging from rough to smooth, on the shear strength of bonded overlays. He found that surface roughness had a significant effect on shear strength, with the roughest surface preparation producing the highest strengths. Rosen also examined the effects of the compaction of concrete on interfacial shear strength; he found that proper compaction of the overlay significantly increased shear strength.
2.5.5 Summary of literature review
It is apparent when reviewing the literature that the effectiveness of pretreatments remains the subject of debate. Table 2.5-1 summarizes the findings of twelve previously cited studies with regards to pretreatments. It should be acknowledged that this table greatly simplifies the results and conclusions of these studies for the purposes of comparison. Immediately obvious when reviewing this table is the lack of any clear trends that might tend to guide the practicing engineer. As Talbot, et al. (1994) remarks: the conclusions obtained by various investigators are unfortunately often influenced by the specific type of testing procedure used. Environmental factors, such as humidity, ambient temperature, and rate of evaporation may significantly affect the need for prewetting. The degree of preparation of the substrate, including profiling and cleaning, may determine the effectiveness of a bonding agent. These factors are not easily controlled and are often difficult to measure, even in a laboratory environment.
15


Table 2.5-1 Summary> of results regarding pretreatments
Year of Article/Report Author Results Regarding Pretreatments: increase in strength/durability, decrease in strength/durability, inconclusive)
Prewetting Bonding Agent
1919 Rosengarten +/- +
1956 Felt - +
1988 Wall and Shrive + +/-
1991 Saucier and Pigeon - +
1992 Whitney, et al. N/A +
1998 Silfwerbrand and Paulsson N/A -
1999 Wells and Stark N/A +/-
2004 Djazmati and Pincheria +/- -
2004 Julio, Branco, and Silva +/- N/A
2006 ACI 325 +/- +/-
2011 ACI 345 + +/-
2012 Bissonnette, et al. + -
16


CHAPTER III
RESEARCH PROGRAM
3.1 Construction of Substrate and Overlay Slabs
The research program evaluated the bond strength of concrete overlays using 6 different combinations of pretreatments. In total, three substrate (138 x 147 x 8.3 cm) (54 x 56 x 3% in) (Figure 3-1) and six overlay (91x61x8.9 cm) (36 x 24 x 3 Vi in) slabs were cast using a commercially available sack concrete mix with a design 28-day strength of 34.5 MPa (5,000 psi). The maximum coarse aggregate size for the sack concrete was 9.5 mm (3/8 in). The design slump was approximately 76 mm (3.0 in). Approximately 0.62 cubic meters (22 cubic feet) and 0.28 cubic meters (10 cubic feet) of concrete was placed for the substrate and overlay slabs, respectively. The concrete was mixed in small, 0.08 cubic meter (3 cubic foot) batches. Seven batches were necessary to construct the substrates; six were needed to construct the overlays.
Figure 3-1 Typical substrate form, ready to receive concrete
The substrate slabs were cast outdoors and covered with a tarp for the initial curing period. Cylinders, taken during concrete placement, were tested for compressive strength at 3, 7 and 28 days to establish the maturity curve for the mix. The average 28-day compressive strength for the substrate was 41.5 MPa (6,010 psi), with a relatively low standard deviation of 1.9 MPa (269 psi). The average 28-day compressive strength for the overlay was 38.3 MPa (5,558 psi), with a standard deviation of
17


9.10 MPa (1320 psi). All slabs were compacted with an immersion vibrator during placement; proper vibration has been shown to increase the shear strength at the bonded interface (Rosen, 2016). Both the substrate and the overlay slabs were unreinforced with the exception of #3 hairpin bars placed as anchor reinforcement for the bolts used in the jacking test. The hairpins were located entirely within the substrate slab with approximately 1 in (25 mm) clear coverage to the surface of the substrate.
It was decided that overlay placement would occur no sooner than 28 days following the placement of the substrate. This wait was imposed to allow the substrate 1) to gain strength such that the possibility of fracturing during surface profiling was minimized, and 2) to accomplish initial shrinkage prior to the placement of the overlay, to best simulate the differential shrinkage that typically occurs when an overlay is placed on existing concrete. Surface preparation commenced exactly 28 days following the substrate placement. Laitance was removed by means of a bush hammer attachment on a small, handheld demolition hammer (Figure 3-2). As shown in Figure 3-3, the prepared surface roughness was similar to Concrete Surface Profile 6 (CSP 6), medium scarification, as depicted in ICRI Guideline No. 310.2R (International Concrete Repair Institute, 2013). The surfaces were then thoroughly cleaned using compressed air followed by vacuuming.
Figure 3-2 Surface profiling the substrate
18


m
Figure 3-3 Finished profile compared with ICRICSP 6 example.
Following surface profiling and cleaning, the six bonding surfaces received different pretreatments, identified in Table 3.1-1.
Table 3.1-1 Pretreatment summary!
Overlay Slab No.: Moisture Condition of Substrate: Bonding Agent:
1A Dry None
IB Dry Wet Cement Slurry
2A SSD Wet Cement Slurry
2B SSD Dried Cement Slurry
3A Saturated with standing puddles Wet Cement Slurry
3B SSD None
Note: SSD = "Saturated Surface Dry
"Dry substrate slabs were not permitted to come into contact with water for several days prior to placement of the overlay. Saturated Surface Dry (SSD) slabs (Figure 3-4) were repeatedly moistened for a period of about one hour, then any standing or free water on the surface was allowed to evaporate prior to overlay placement. As the name implies, SSD refers to a condition wherein the pores of the existing concrete are filled with water, but excess moisture on the concrete surface has evaporated Slab 3A was initially prepared similarly to the SSD slabs, but puddles of standing water
19


were allowed to remain on the surface during overlay placement. All these surface conditions were intended to envelope possible field conditions.
Figure 3-5 Application of cement slurry bonding agent
20


The selected bonding agent was a cement and water slurry with a w/c ratio of 0.50. Previous research, including research referenced in ACI 325 (2006) and Saucier and Pigeon (1991), indicates that a bonding agent with a w/c ratio in excess of 0.60 could significantly weaken the bond. The bonding agent was scrubbed into the substrate with a stiff bristle'brush (Figure 3-5) as recommended by Wells and Stark (1999). In all but one case, the overlay was placed on the bonding agent immediately, prior to any drying or dulling of the slurry. In the case of Slab 2B, the slurry was allowed to sit for a period of several days, such that it was fully dry at the time of overlay placement. As a means of reducing the amount of formwork needed, two overlay slabs were placed on a common substrate slab. The bleed-over of moisture during pretreatment from the adjacent bonding surface was considered in the experiment layout. Pretreatments involving dry substrates were grouped together on Slab 1, while SSD or wet substrates were grouped together on Slab 2 and Slab 3.
Slabs 1A, IB, 2A, and 3A were placed when the substrate was 30 days old. A sudden rainstorm prevented the completion of the remaining overlays, which were placed five days later (substrate age of 35 days). The overlays slabs were formed from the same concrete mix as the substrate, but were treated with a commercially available colorant to help in the identification of the bonding surface. An overall plan showing the relative arrangement of the substrate and overlay pads is presented in Figure 3-6. An isometric view of atypical substrate slab and two overlay pads is presented in Figure 3-7. This figure shows the typical slab dimensions and the approximate locations where the various test samples were taken. The testing is discussed in detail in Chapter 3.2. Figure 3-8 and Figure 3-9 illustrate the as-constructed condition of the slabs.
21


Slab 1
Slab 1A:
Dry Substrate/No Bonding Agent
Slab IB:
Dry Substrate/Wet Bonding Agent
Slab 2
1
Slab 2A:
SSD Substrate/Wet Bonding Agent
Slab 2B:
SSD Substrate/
Dried Bonding Agent
Slab 3
o ooooo
3B

Slab 3A:
Wet Substrate/Wet Bonding Agent
Slab 3B:
SSD Substrate/No Bonding Agent
Figure 3-6-Plan view of slabs and pretreatments
Direct shear test (full depth)
Substrate
Overlay
Pull off test (partial depth)
Slant shear tests at 45
Hydraulic jack and
Figure 3-7- Isometric rendering of a typical substrate slab with two overlay slabs
22


Figure 3-8 As constructed view showing pull-off test sampling locations.
Figure 3-9 As constructed view showing pidl-off direct shear, slant shear, and jacking test sampling locations. Background to foreground: Slab 1, 2, and 3
23


3.2 Testing
All testing was performed in the Civil Engineering Laboratory at the University of Colorado Denver. Testing of the compressive strength of the substrate and overlay concrete was accomplished between 3 and 28 days after placement of the concrete. Testing of the bond strength was conducted in the order shown on Table 3.2-1. The pull off tests were conducted 40 days after the final overlay slab was constructed. Direct shear tests were conducted next, 65 days after the final overlay slab was constructed. Slant shear tests and jacking tests were conducted 80 and 90 days after the final overlay slab was constructed, respectively.
Table 3.2-1 -Testing Summary
Test: Test Location: Type: No. of Samples Tested: Standard (if applicable):
Compression Laboratory Compression 3 per batch (36 total) ASTM C39
Pull-off In-Place Direct Tension 3 per slab (18 total) ASTMC1583
Direct Shear (Guillotine) Laboratory Direct Shear 3 per slab (18 total) N/A
Slant Shear Laboratory Combination of Compression & Shear 4 per slab (24 total) ASTM C882
Jacking In-Place Direct Shear 4 per slab (24 total) N/A
Total 120 tests
3.2.1 Slump test
Slump was measured using a standard slump cone (Figure 3-10) test and was conducted for seven of the thirteen concrete batches. The design slump for the sack concrete product used in the testing program is 50 76 mm (2-3 in). Slump results are tabulated in Section 4.1.
24


Figure 3-10 Example slump cone test on overlay concrete (note integral color)
3.2.2 Slab concrete compression test
Samples were taken during placement of the substrate and overlay slabs in 102 (diameter) x 203 mm (4 4> x 8 in) plastic cylinder molds. Three samples were taken from twelve of the thirteen total batches for a total of 36 specimens. A single cylinder from each batch was tested in compression at 3-, 7-, and 28-days after placement. Testing was performed on a Fomey compression testing machine (Figure 3-11) equipped with an Admet data logger. The cylinders were capped prior to testing with reusable neoprene rubber pads surrounded by a steel extrusion controller. The Fomey machine was not equipped with a displacement sensor, therefore, only the compression load at failure was recorded. Load rate was controlled using a hand wheel and was adjusted so as to maintain the rate prescribed in ASTM C39 (2004) of 0.25 0.05 MPa/s (35 7psi/s). For a 4 in (102 mm) standard cylinder, the rate of load application is 440 88 lbs/s. Compression stress at failure was determined from the applied load at failure using Equation (3-1). Results from the compression strength tests can be found in Chapter 4.2.
25


(3-1)
Where:
o = compressive strength (MPa or psi)
P = compressive force at failure (kN or lbs) Ag = gross area of the sample (mm2 or in2)
Figure 3-11 Compression Testing Equipment 3.2.3 Pull-off test
Samples were prepared using a 66.7 mm (2 5/8 in) inner diameter coring bit with a 406 mm (16 in) coring depth capacity mounted on a wet core drill. A guide was placed on the bit such that the core would penetrate approximately 1.3 cm (0.5 in) into the substrate slab. The cores were taken as close to perpendicular to the bond plane as possible to minimize the eccentricity of the applied test load. Once coring was completed, the annular space was rinsed thoroughly with water to remove dust and debris. The surface of the overlay at the core location was treated with full-strength Muratic acid
26


to remove laitance, then thoroughly rinsed with water. After waiting for the surface to dry, any remaining dust was blown off with compressed air (Figure 3-12). 76.2 mm (3 in) diameter stainless steel pucks were epoxied onto the top of the partial cores and the epoxy was left to cure for a period of several days.
Figure 3-12 Cleaning the surface of the pull-off test specimen Tensile strength of the bond was tested using Non Destructive Testing Systems (NDT) 007 James Bond Tester furnished by CTL-Thompson (Figure 3-13). The location of the failure plane and the maximum tensile force was recorded for each test.
Figure 3-13 NDT 007 James Bond Tester
27


In accordance with the testing frequency recommendations of Part 8 (Sampling) of ASTM C1583, three tests were performed on each of the six overlay slabs. In 13 of the 18 tests, failure occurred at the bond line. The remaining tests failed on the surface of the substrate. Several tests were aborted due to failure of the epoxy bonding the stainless steel puck to the overlay. In these cases, the puck was rebonded to the specimen and the test was resumed at a later date.
The bond strength is calculated from the maximum tensile load using Equation (3-2); the relationship is similar to that used in the compression test. Results from the pull-off tests can be found in Chapter 4.5.
Where: o = bond strength (kPa or psi)
Pu = tensile force at failure (kN or lbs)
Ag = gross area of the specimen (mm2 or in2)
3.2.4 Direct shear (guillotine) test
A guillotine box apparatus was furnished by CTL-Thompson, Inc. Full depth core samples were taken using a 66.7 mm (2 5/8 in) inner diameter coring bit mounted to a wet core drill. The specimens were placed in the guillotine with the bond plane centered between the edges of the nested boxes (Figure 3-15) such that approximately 3.2 mm (1/8 in) of gap was observed between the inner and outer box walls. The apparatus was compressed, which induced shear on the bond plane until failure occurred. The testing was performed using an 89.0 kN (20,000 lbs) MTS compression testing machine with displacement control (Figure 3-14). The loading rate was set at 0.5 mm/min (0.02 in/min). The shear strength at the bonded interface is calculated from the applied force in Equation (3-3). Dividing the applied load by two is necessary because the shearing action is imposed equally on each leg of the box, though failure was found to occur only on the bonded interface.
28


Figure 3-14 MTS testing equipment with direct shear apparatus
It may be argued that the eccentricity in applied load resulting from the gap between the inner and outer walls of the guillotine box induces a bending moment on the specimen (and thus the bonded interface is not in pure shear). While the influence of moment is not easily avoidable, the construction of the apparatus with the narrow gap between boxes minimizes the effect.
Pu
T =
2 A
(3-3)
9
Where: r = shear strength at bonded interface (kPa or psi)
P = compressive load at failure (kN or lbs)
Ag = gross area of the specimen (mm2 or in2)
Although each leg of the box is profded so as to cradle the specimen uniformly, in practice it was observed that some localized crushing of concrete during the early stages of loading was necessary to "seat the sample. Once this crushing occurred, the stress-strain plot indicates a relatively linear relationship until failure occurs. The author understands that cast plaster caps around the sample and guillotine apparatus are sometimes used to fill the annular space, such that the loading is applied more uniformly. While the benefits of this approach are undeniable, it is difficult to
29


implement capping when testing a large number of samples due to the time involved. For these experiments, the samples were not capped.
Results from the direct shear tests can be found in Chapter 4.6.
Figure 3-15 Direct Shear (Guillotine) Apparatus
3.2.5 Slant shear test
As noted in Chapter 2.3.3, slant shear specimens are typically prepared using a cylindrical mold with a removable plate to form the slanted interface. Although this is a convenient method to produce many specimens, it was not the preferred method for this study. For this study, it was decided that slant shear specimens would be cored directly from the slabs, such that the surface profding and pretreatment would be identical across all four types of tests. This necessitated coring samples on an angle, and then sawing the ends perpendicular to the axis of the core. Figure 3-16 defines how the slant angle a was measured in this study.
Full depth core samples were taken at 45 degrees from normal (Figure 3-17) using a 66.7 mm (2 5/8 in) inner diameter coring bit mounted to a wet core drill. The slant angle was selected based on: 1) the maximum slant capability of the core drill stand used in the experimentation, and 2) the ability to obtain more samples than would have been possible had cores been attempted at a shallower angle (due to space limitations). The ends of the samples were sawed perpendicular, and then allowed to dry for a minimum of 5 days in accordance with ASTM C42 (2004). The samples were then
30


compressed to failure on a 1000 kN (220,000 lbs) MTS compression testing machine with displacement control. The loading rate was set at 0.10 mm/min (0.04 in/minute).
(1)
(2)
(3)
Figure 3-16 Diagram showing (1) the slant shear specimen with slant angle a' subjected to uniaxial compression, (2) the stresses on the bond surface, and (3) the forces on the bond surface
Figure 3-17 Checking the slant angle with an inclinometer prior to coring for a slant shear
specimen
31


The principle stresses at the bonded interface were determined using a 2-dimensional plane stress transformation. The normal or clamping stress is given by Equation (3-4), while the shearing stress at the interface is given by Equation (3-5). These variables are depicted in Figure 3-16. Pu/Ag
N
tnt
Where:
(l + cos(2a))
sin(2a))
(3-4)
(3-5)
oN = clamping force at failure (kPa or psi) tnt = shear stress at failure (kPa or psi)
Pu = compressive load at failure (kN or lbs)
Ag = gross area of the specimen (mm2 or in2) a = slant angle (degrees from horizontal)
This relationship can also be expressed in terms of forces acting on the bond surface Equation (3-6) gives the normal stress in terms of the clamping force, while Equation (3-7) gives the shearing stress in terms of the shear force.
N
n
tNt
Where:
Asurface V
(3-6)
(3-7)
Asurface
oN = clamping force at failure (kPa or psi)
Tnt = shear stress at failure (kPa or psi)
N = clamping force on bonded interface (kN or lbs)
V = shear force at bonded interface (kN or lbs)
Asurface = area of bonded interface (mm2 or in2)
This transformation can also be expressed graphically using Mohrs circle of stress, as shown in Figure 3-18. Under uniaxial stress, one quadrant of Mohrs circle passes through the origin, while the other quadrant is located at the maximum principle stress. The figure illustrates the computation of the normal and shear stresses on a 45 degree and 60 degree slant, under a hypothetical uniaxial
32


compressive stress of 1.0 ksi (0.145 MPa). At 45 degrees, the normal and shear stresses are equal. At 60 degrees, the normal stress is significantly reduced. Refer to Appendix E for a discussion of how the effect of friction significantly affects the observed shear resistance at the bonded interface.
Of the 24 slant shear specimens tested, none were found to have failed in shear at the bonded interface as anticipated. All samples failed in splitting tension in a manner similar to typical compressive cylinder tests. Figure 3-19 and Figure 3-20 illustrate the typical failure condition observed. It appears that the clamping force on the roughened bond plane was sufficient to resist the applied shear force on the 45 degree bond plane.
Results from the slant shear tests can be found in Chapter 4.7.
Figure 3-18 Mohrs circle under 1 ksi (0.145 MPa) uniaxial compression at slant angles of 45 and 60 degrees (Sign convention for o: + tension / compression) (Sign convention for x: + CW / CCW
rotation)
33


Figure 3-19 Representative slant shear cylinder after testing under uniaxial compression
Figure 3-20 Same specimen as Figure 3-19, opened to reveal surface of splitting tension failure. Note colored concrete is overlay; gray concrete is substrate
3.2.6 Jacking test
Sample blocks were prepared using a 350 mm (14 in) dry concrete saw with an adjustable shoe to set the depth of cut (Figure 3-21). The saw was connected to a wet/dry vacuum to minimize the dust generated by this operation. The blocks were cut to a preferred size of 152 x 152 mm (6x6
34


in) where possible; however, clearance between the test slabs and an adjacent wall necessitated adjusting the block dimensions for some of the 'A' slabs. The saw was adjusted such that the depth of the sawcut extended approximately 13 mm (0.5 in) into the substrate slab.
Figure 3-21 Concrete saw and dust collection system
A Simplex RC306C hydraulic jack with a 300 kN (30 ton) capacity was installed adjacent to the blocks. During casting of the substrate, a 25 mm (1.0 in) step had been formed into the surface to accommodate the jack body. This allowed the piston to exert load on the block as close to the bond surface as possible, thereby minimizing overturning moment resulting from eccentric application of the load. A steel plate with dimensions of 89 x 89 x 25 mm (3 Vi x 3 Vi x 1 in) was placed between the piston and the block to evenly distribute the test load.
35


Figure 3-22 Simplex RC306C hydraulic testing specimen on Slab 2A. On right, 1 in (25 mm) steel-
plate: on left, brace angle bolted to substrate
The samples were tested to failure and the maximum force in the jack was recorded. The
dimensions of the blocks were recorded by measuring the dimensions of the bonded interface after
failure, for increased accuracy. The shear stress at the bonded interface is given by Equation (3-8).
_ Pux-^c
_ LXW
Where: r = shear stress at failure (kN or lbs)
(3-8)
p = recorded pressure in jack at failure (kPa or psi)
Ac = area of the jack cylinder; 4,150 mm2 (6.44 in2) for the Simplex RC306C
L = length of the block specimen (mm or in) W= width of the block specimen (mm or in)
36


CHAPTER IV
RESULTS
4.1 Slump Cone Results
Slump was tested for 7 of the 13 total concrete batches used in the test program. Table 4.1-1 lists the measured slump (a blank entry indicates no measurement was taken).
Table 4.1-1 Results of slump cone test
Batch No. Slab ID Slump
mm in
Substrate Placement (April 7, 2016)
1 Substrate Slab 1 76 3.00
2 Substrate Slab 1 95 3.75
3 Substrate Slab 2 102 4.00
4 Substrate Slab 2
5 Substrate Slab 2 178 7.00
6 Substrate Slab 3
7 Substrate Slab 3
Overlay Placement 1 (May 7, 2016)
1 Overlay Slab 1A 76 3.00
2 Overlay Slab IB
3 Overlay Slab 2A 25 1.00
4 Overlay Slab 3A
Overlay Placement 2 (May 12, 2016)
1 Overlay Slab 2B 44 1.75
2 Overlay Slab 3B
4.2 Sack Concrete Sieve Analysis
A particle size distribution analysis was performed to determine the gradation of the aggregates within the proprietary sack concrete mix used in the research program. A representative sample was taken and tested on a laboratory sieve shaker. The analysis indicates that the proprietary concrete mix uses a coarse aggregate with a maximum particle size (Dioo) of 12.7 mm (3/8 in).
4.3 Compression Testing Results
102 x 203 mm (4x8 in) sample cylinders were taken and tested for 12 of the 13 concrete batches used in the test program. Table 4.3-1 lists the maximum compression force recorded by the
37


Admet data logger for each cylinder. Table 4.3-2 lists the corresponding uniaxial compression stress in the cylinder at failure. A blank entry indicates no compression testing was performed for that batch. The two tests taken for each substrate slab were averaged to produce the substrate compressive strength, as shown in Table 4.3-3.
Table 4.3-1 Compression force at failure
Batch No. Slab ID 3-Day 7-Day 28-Day
kN kips kN kips kN kips
Substrate Placement (April 7, 2016)
1 Substrate Slab 1 125 28.1 243 54.6 333 74.8
2 Substrate Slab 1 187 42.1 262 58.9 348 78.3
3 Substrate Slab 2 187 42.0 272 61.2 324 72.8
4 Substrate Slab 2
5 Substrate Slab 2 187 42.0 272 61.2 311 70.0
6 Substrate Slab 3 207 46.5 304 68.4 346 77.9
7 Substrate Slab 3 212 47.8 303 68.1 354 79.6
Overlay Placement 1 (May 7, 2016)
1 Overlay Slab 1A 171 38.4 220 49.5 333 74.8
2 Overlay Slab IB 212 47.6 203 45.7 385 86.4
3 Overlay Slab 2A 209 47.0 281 63.1 349 78.5
4 Overlay Slab 3A 227 51.0 237 53.3 368 82.7
Overlay Placement 2 (May 12, 2016)
1 Overlay Slab 2B 107 24.2 139 31.3 259 58.1
2 Overlay Slab 3B 168 37.8 172 38.7 172 38.6
38


Table 4.3-2 -Compression stress at failure
Batch No. Slab ID 3-Day 7-Day 28-Day
kPa psi kPa psi kPa psi
Substrate Placement (April 7, 2016)
1 Substrate Slab 1 15,396 2,233 29,941 4,343 41,046 5,953
2 Substrate Slab 1 23,099 3,350 32,295 4,684 42,961 6,231
3 Substrate Slab 2 23,028 3,340 33,600 4,873 39,959 5,796
4 Substrate Slab 2
5 Substrate Slab 2 23,028 3,340 33,589 4,872 38,396 5,569
6 Substrate Slab 3 25,524 3,702 37,545 5,445 42,725 6,197
7 Substrate Slab 3 26,204 3,801 37,348 5,417 43,679 6,335
Overlay Placement 1 (May 7, 2016)
1 Overlay Slab 1A 21,047 3,053 27,137 3,936 41,031 5,951
2 Overlay Slab IB 26,106 3,786 25,052 3,634 47,429 6,879
3 Overlay Slab 2A 25,771 3,738 34,643 5,025 43,051 6,244
4 Overlay Slab 3A 27,982 4,058 29,222 4,238 45,388 6,583
Overlay Placement 2 (May 12, 2016)
1 Overlay Slab 2B 13,256 1,923 17,146 2,487 31,888 4,625
2 Overlay Slab 3B 20,712 3,004 21,228 3,079 21,160 3,069
The results show that all substrate slabs, as well as the overlay slabs placed on May 7, achieved the design compressive strength of 34.5 MPa (5,000 psi). The compressive strengths of the overlay slabs placed on May 12 were lower than the design strength.
Table 4.3-3 Average substrate compressive strength
Slab ID Substrate (kPa) Substrate (psi)
Test 1 Test 2 Average Test 1 Test 2 Average
1 41,046 42,961 42,003 5,953 6,231 6,092
2 39,959 38,396 39,178 5,796 5,569 5,683
3 42,725 43,679 43,203 6,197 6,335 6,266
4.4 Specimen Identifiers
Each specimen tested as part of the pull-off, direct shear, slant shear, and jacking tests was assigned a sample I.D. The first two digits indicate the overlay slab where the specimen originated. The next number indicates the order in which the specimen was obtained, and the last letter indicates the type of test performed.
39


4.5 Pull-off Test Results
Three pull-off tests were performed on each of the six overlay slabs, for a total of 18 performed as part of the testing program. Table 4.5-1 lists the sample I.D. for each pull-off test; the letter T indicates a Tension test. Images of each sample have been included in the Appendix. All specimens were observed to fail on or near the bond surface. Thirteen of the specimens failed at the bond surface; the remaining five specimens failed on the surface of the substrate.
Table 4.5-1 Pull-off test samples
Slab ID Pretreatments Pull-off Test Sample I.D.
Test 1 Test 2 Test 3
1A Dry / No Agent 1A1T 1A3T 1A5T
IB Dry / Wet Agent 1B5T 1B7T 1B11T
2A SSD / Wet Agent 2A3T 2A5T 2A7T
2B SSD / Dried Agent 2B1T 2B3T 2B5T
3A Wet / Wet Agent 3A1T 3A3T 3A5T
3B SSD / No Agent 3B1T 3B3T 3B5T
The tensile strength at failure is listed on Table 4.5-2 (SI units) and Table 4.5-3 (U.S. customary units). A statistical analysis was performed on the dataset to compute the mean bond strength, standard deviation, and coefficient of variation.
Table 4.5-2 Pull-off test results (SI units)
Tensile Strength (kPa) Statistical Analysis
Slab ID Pretreatments Test 1 Test 2 Test 3 Average (kPa) Std. Dev. (kPa) cov
1A Dry / No Agent 800 317 1,213 111 366 47.2%
IB Dry / Wet Agent 1,069 1,296 1,048 1,138 112 9.9%
2A SSD / Wet Agent 1,262 1,096 1,055 1,138 89 7.9%
2B SSD / Dried Agent 379 372 124 292 119 40.7%
3A Wet / Wet Agent 1,007 1,296 1,531 1,278 214 16.8%
3B SSD / No Agent 814 1,096 1,758 1,223 396 32.4%
40


Table 4.5-3 Pull-off test results (U.S. customary units)
Slab ID Pretreatments Tensile Strength (psi) Statistical Analysis
Test 1 Test 2 Test 3 Average (psi) Std. Dev. (psi) COV
1A Dry / No Agent 116 46 176 113 53.1 47.2%
IB Dry / Wet Agent 155 188 152 165 16.3 9.9%
2A SSD / Wet Agent 183 159 153 165 13.0 7.9%
2B SSD / Dried Agent 55 54 18 42.3 17.2 40.7%
3A Wet / Wet Agent 146 188 222 185 31.1 16.8%
3B SSD / No Agent 118 159 255 111 57.4 32.4%
Past studies (Vaysburd & McDonald, 1999) indicate that the compressive strength of the concrete has a significant effect on the strength of the interfacial bond. Djazmati, et al. (2004) and Rosen (2016) compensate for unavoidable differences in compressive strength by dividing bond strength results by the square root of the compressive strength of the concrete (fc). Because the observed compressive strengths of the substrate and overlay differ in this study, the minimum compressive strength, fc(mm), was used for the adjustment. Table 4.5-4 (SI units) and Table 4.5-5 (U.S. customary units) list the factored bond strength results for the pull-off tests.
Table 4.5-4 Pull-off test bond strength adjustment (SI units)
Slab ID Pretreatments 28 Day Compressive Strength (kPa) Avg. Bond Strength in Tension (a) (kPa) Factored Bond Strength in Tension a / if C(min)
Substrate Overlay
1A Dry / No Agent 42,003 41,031 111 3.83
IB Dry / Wet Agent 42,003 47,429 1,138 5.55
2A SSD / Wet Agent 39,179 43,051 1,138 5.75
2B SSD / Dried Agent 39,179 31,888 292 1.63
3A Wet / Wet Agent 43,203 45,388 1,278 6.15
3B SSD / No Agent 43,203 21,160 1,223 8.41
41


Table 4.5-5 Pull-off test bond strength adjustment (U.S. customary units)
Slab ID Pretreatments 28 Day Compressive Strength (psi) Avg. Bond Strength in Tension (a) (psi) Factored Bond Strength in Tension a / if C(min)
Substrate Overlay
1A Dry / No Agent 6092 5951 113 1.46
IB Dry / Wet Agent 6092 6879 165 2.11
2A SSD / Wet Agent 5683 6244 165 2.19
2B SSD / Dried Agent 5683 4625 42.3 0.62
3A Wet / Wet Agent 6266 6583 185 2.34
3B SSD / No Agent 6266 3069 111 3.20
4.6 Direct Shear Test Results
Three direct shear tests were performed on each of the six overlay slabs, for a total of 18 performed as part of the testing program. All specimens were observed to fail on or near the bond surface. Sixteen of the specimens failed at the substrate surface; the remaining specimens (both on Slab 2B) failed on the surface of the overlay. Table 4.6-1 lists the sample I.D. for each direct shear test; the letter G indicates a Guillotine test. Images of each sample have been included in the Appendix.
Table 4.6-1 Direct shear test samples
Slab ID Pretreatments Direct Shear Test Sample I.D.
Test 1 Test 2 Test 3
1A Dry / No Agent 1A2G 1A4G 1A7G
IB Dry / Wet Agent 1B2G 1B8G 1B10G
2A SSD / Wet Agent 2A2G 2A4G 2A6G
2B SSD / Dried Agent 2B2G 2B4G 2B6G
3A Wet / Wet Agent 3A2G 3A4G 3A6G
3B SSD / No Agent 3B2G 3B4G 3B6G
The shear strength at failure is listed on Table 4.6-2 (SI units) and Table 4.6-3 (U.S. customary units). As with the pull-off results, a statistical analysis was performed on the dataset to compute the average shear strength, standard deviation, and coefficient of variation.
42


Table 4.6-2 Direct shear test results (SI units)
Shear Strength at Interface (kPa) Statistical Analysis
Slab ID Pretreatments Test 1 Test 2 Test 3 Average (kPa) Std. Dev. (kPa) cov
1A Dry / No Agent 2,296 2,544 2,062 2,301 197 8.6%
IB Dry / Wet Agent 3,316 3,027 2,916 3,087 169 5.5%
2A SSD / Wet Agent 3,309 3,075 2,365 2,916 402 13.8%
2B SSD / Dried Agent 655 1,496 1,420 1,191 380 31.9%
3A Wet / Wet Agent 3,027 2,330 2,606 2,654 286 10.8%
3B SSD / No Agent 2,675 2,730 3,192 2,866 232 8.1%
Table 4.6-3 Direct shear test results (U.S. customary units)
Shear Strength at Interface (psi) Statistical Analysis
Slab ID Pretreatments Test 1 Test 2 Test 3 Average (psi) Std. Dev. (psi) COV
1A Dry / No Agent 333 369 299 334 28.6 8.6%
IB Dry / Wet Agent 481 439 423 448 24.5 5.5%
2A SSD / Wet Agent 480 446 343 423 58.2 13.8%
2B SSD / Dried Agent 95 217 206 173 55.1 31.9%
3A Wet / Wet Agent 439 338 378 385 41.5 10.8%
3B SSD / No Agent 388 396 463 416 33.6 8.1%
Table 4.6-4 (SI units) and Table 4.6-5 (U.S. customary units) list the factored shear strength results for the direct shear tests. Reference the pull-off test results in Chapter 4.5 for further explanation of the adjustment factor.
43


Table 4.6-4 Direct shear test strength adjustment (SI units)
Slab ID Pretreatments 28 Day Compressive Strength (kPa) Avg. Interfacial Shear Strength (t) (kPa) Factored Interfacial Shear Strength T / V/C(min)
Substrate Overlay
1A Dry / No Agent 42,003 41,031 2,301 11.36
IB Dry / Wet Agent 42,003 47,429 3,087 15.06
2A SSD / Wet Agent 39,179 43,051 2,916 14.73
2B SSD / Dried Agent 39,179 31,888 1,191 6.67
3A Wet / Wet Agent 43,203 45,388 2,654 12.77
3B SSD / No Agent 43,203 21,160 2,866 19.70
Table 4.6-5 Direct shear test strength adjustment (U.S. customary units)
Slab ID Pretreatments 28 Day Compressive Strength (psi) Avg. Interfacial Shear Strength (t) (psi) Factored Interfacial Shear Strength T / V/C(min)
Substrate Overlay
1A Dry / No Agent 6,092 5,951 334 4.33
IB Dry / Wet Agent 6,092 6,879 448 5.74
2A SSD / Wet Agent 5,683 6,244 423 5.61
2B SSD / Dried Agent 5,683 4,625 173 2.54
3A Wet / Wet Agent 6,266 6,583 385 4.86
3B SSD / No Agent 6,266 3,069 416 7.50
4.7 Slant Shear Test Results
Four slant shear samples were taken from each of the six overlay slabs, for a total of 24. During extraction, four of these slant cores failed: three on Slab 2B, two on Slab IB, and one on Slab 3A. The remaining eighteen specimens were tested in compression. As described in Chapter 3.2.5, all specimens were observed to fail in splitting tension, with none failing in shear on the bond surface. Table 4.7-1 lists the sample I.D. for each direct shear test; the letter S indicates a Slant shear test. Images of each sample have been included in the Appendix.
44


Table 4.7-1 Slant shear test samples
Slab ID Pretreatments Slant Shear Test Sample I.D.
Test 1 Test 2 Test 3 Test 4
1A Dry / No Agent 1A1S 1A2S 1A3S 1A4S
IB Dry / Wet Agent IBIS 1B3S
2A SSD / Wet Agent 2A1S 2A2S 2A3S 2A4S
2B SSD / Dried Agent 2B1S
3A Wet / Wet Agent 3A2S 3A3S 3A4S
3B SSD / No Agent 3B1S 3B2S 3B3S 3B4S
The measured compression stress on the slant shear samples at failure is listed on Table 4.7-2 (SI units) and Table 4.7-3 (U.S. customary units).
Table 4.7-2 Slant shear test results (SI units)
Slab ID Pretreatments Compression Stress at Failure (kPa)
Test 1 Test 2 Test 3 Test 4
1A Dry / No Agent 37,411 25,662 33,219 36,860
IB Dry / Wet Agent 41,341 40,569
2A SSD / Wet Agent 44,747 37,687 38,501 38,763
2B SSD / Dried Agent 35,012
3A Wet / Wet Agent 29,979 27,593 36,391
3B SSD / No Agent 39,066 30,916 36,446 26,641
Table 4.7-3 Slant shear test results (U.S. customary units)
Slab ID Pretreatments Compression Stress at Failure (psi)
Test 1 Test 2 Test 3 Test 4
1A Dry / No Agent 5,426 3,722 4,818 5,346
IB Dry / Wet Agent 5,996 5,884
2A SSD / Wet Agent 6,490 5,466 5,584 5,622
2B SSD / Dried Agent 5,078
3A Wet / Wet Agent 4,348 4,002 5,278
3B SSD / No Agent 5,666 4,484 5,286 3,864
Based on the observed failure mode, it is apparent that the strength results were affected primarily by the compressive strength of the concrete, and not by the properties of the bond. Therefore, the slant shear test results have not been included in the results discussion or the
45


conclusions of this study. Refer to Appendix E for additional information regarding the slant shear test results.
4.8 Jacking Test Results
Four jacking tests were performed on each of the six overlay slabs, for a total of 24 performed as part of the testing program. All specimens were observed to fail on the bond surface. Table 4.8-1 lists the sample I.D. for each jacking test; the letter T indicates a Jacking test. Images of each sample have been included in the Appendix.
Table 4.8-1 Jacking test samples
Slab ID Pretreatments Jacking Test Sample I.D.
Test 1 Test 2 Test 3 Test 4
1A Dry / No Agent 1A1J 1A2J 1A3J 1A4J
IB Dry / Wet Agent 1B1J 1B2J 1B3J 1B4J
2A SSD / Wet Agent 2A1J 2A2J 2A3J 2A4J
2B SSD / Dried Agent 2B1J 2B2J 2B3J 2B4J
3A Wet / Wet Agent 3A1J 3A2J 3A3J 3A4J
3B SSD / No Agent 3B1J 3B2J 3B3J 3B4J
The shear strength at failure is listed on Table 4.8-2 (SI units) and Table 4.8-3 (U.S. customary units). As with the pull-off results, a statistical analysis was performed on the dataset to compute the average shear strength, standard deviation, and coefficient of variation.
Table 4.8-2 Jacking test results (SI units)
Shear Strength at Interface (kPa) Statistical Analysis
Slab ID Pretreatments Test 1 Test 2 Test 3 Test 4 Average (kPa) Std. Dev. (kPa) cov
1A Dry / No Agent 1,032 1,156 1,320 962 1,117 136 12.2%
IB Dry / Wet Agent 1,209 1,307 1,206 1,034 1,189 98 8.2%
2A SSD / Wet Agent 1,734 1,390 1,541 1,714 1,595 140 8.8%
2B SSD / Dried Agent 820 827 1,081 1,117 961 138 14.4%
3A Wet / Wet Agent 1,496 1,705 1,509 1,330 1,510 133 8.8%
3B SSD / No Agent 1,182 1,571 1,871 1,525 1,537 244 15.9%
46


Table 4.8-3 Jacking test results (U.S. customary units)
Shear Strength at Interface (psi) Statistical Analysis
Slab ID Pretreatments Test 1 Test 2 Test 3 Test 4 Average (psi) Std. Dev. (psi) COV
1A Dry / No Agent 150 168 192 140 162 19.7 12.2%
IB Dry / Wet Agent 175 190 175 150 173 14.2 8.2%
2A SSD / Wet Agent 252 202 224 249 231 20.3 8.8%
2B SSD / Dried Agent 119 120 157 162 139 20.1 14.4%
3A Wet / Wet Agent 217 247 219 193 219 19.3 8.8%
3B SSD / No Agent 172 228 271 221 223 35.4 15.9%
Table 4.8-4 (SI units) and Table 4.8-5 (U.S. customary units) list the factored shear strength results for the jacking tests. Reference the pull-off test results for further explanation of the adjustment factor.
Table 4.8-4 Jacking test strength adjustment (SI units)
Slab ID Pretreatments 28 Day Compressive Strength (kPa) Avg. Interfacial Shear Strength (t) (kPa) Factored Interfacial Shear Strength r / \ jTC(min)
Substrate Overlay
1A Dry / No Agent 42,003 41,031 1,117 5.52
IB Dry / Wet Agent 42,003 47,429 1,189 5.80
2A SSD / Wet Agent 39,179 43,051 1,595 8.06
2B SSD / Dried Agent 39,179 31,888 961 5.38
3A Wet / Wet Agent 43,203 45,388 1,510 7.26
3B SSD / No Agent 43,203 21,160 1,537 10.6
Table 4.8-5 Jacking test strength adjustment (U.S. customary units)
Slab ID Pretreatments 28 Day Compressive Strength (psi) Avg. Interfacial Shear Strength (t) (psi) Factored Interfacial Shear Strength r / \ jTC(min)
Substrate Overlay
1A Dry / No Agent 6,092 5,951 162 2.10
IB Dry / Wet Agent 6,092 6,879 172 2.21
2A SSD / Wet Agent 5,683 6,244 231 3.07
2B SSD / Dried Agent 5,683 4,625 139 2.05
3A Wet / Wet Agent 6,266 6,583 219 2.77
3B SSD / No Agent 6,266 3,069 223 4.02
47


CHAPTER V
DISCUSSION
5.1 Variation of Compressive Strength
In general, the sack concrete used in this study produced relatively consistent 28-day strengths, despite some variability in the measured slump prior to placement. The average 28-day strength for the substrate and first overlay placements was 6,174 psi (42,566 kPa) with a coefficient of variation of just 6.2%. In contrast, the average 28-day strength for the second overlay placement was just 3,847 psi (26,525 kPa) with a coefficient of variation of 28.6%. The reason for this significant difference is not entirely certain. The concrete sacks used in both overlay placements were taken from the same shipment, and were kept covered between placements. One possible explanation is that the bags for the second overlay placement experienced increased humidity due to rainfall, which may have partially hydrated the cement.
Figure 5-1 shows the measured 28 day compressive strength for each substrate and overlay slab. Had the variation of compressive strengths not exceeded the observed differences in the substrate and first overlay placements, they may have been reasonably ignored in the comparison of bond strengths. However, the deviations were deemed significant enough to warrant adjustment to the bond strength results. The adjustment was made by dividing the recorded bond strength in tension or shear strength at the bonded interface by the square root of the concrete strength. The minimum of the substrate and overlay compressive strengths was used in the adjustment factor; it was assumed that the weaker slab would fail first and control the strength result. Equation (5-1) gives the solution for the factored bond strength in tension; Equation (5-2) gives the solution for the factored shear strength at the bonded interface.
48


Compressive strength (kPa)
Adjusted Bond Strength in Tension =
(5-1)
/y/fc,min
Where: o = measured bond strength in tension (kPa or psi)
/ 6', nun minimum of overlay and substrate compressive strength (psi or kPa)
Adjusted Shear Strength at Interface = T
(5-2)
/ f
' J c,min
Where: r = measured shear strength at interface (kPa or psi)
f c,min minimum of overlay and substrate compressive strength (kPa or psi)
7,000
6,000
5.000 a
-B
60
4.000 g
Zl
(D
3.000 1
i-H
Oh
s
2.000 ^
1,000
0
Substrate Compressive Strength
Overlay Compressive Strength
Figure 5-1 28-day compressive strength by slab location
49


5.2 Effects of Strength Gain on Test Results
Common engineering practice is to assume that the majority of strength gain in concrete is complete after 28 days of maturity. Accordingly, 28 day compressive strengths were used as adjustment factors to account for the difference in compressive strength between slabs, a process discussed previously in Chapter 5.1. The possible effects of strength gain after 28 days are discussed in the following paragraphs.
Using the strength gain data from the 3, 7, and 28 day tests, a maturity curve (Figure 5-2) was established based on a logarithmic function. Although no samples were tested after 28 days, the strength gain after 28 days may be estimated using the logarithmic function. The curve predicts a strength of 5813 psi (40,076 kPa) at 28 days. At 56 days, the concrete is expected to gain an additional 773 psi (5332 kPa) compressive strength, about 13.3% of the 28 day strength. However, between 56 and 72 days, the period in which the majority of the bond strength testing was conducted, the concrete is only expected to gain an additional 280 psi (1933 kPa), or about 4.3% of the 56 day strength.
With the exception of the pull-off tests, the results from each test were obtained in a single day. Clearly, the strength of concrete did not change appreciably during the days testing; therefore, the results of any individual test type are unaffected by strength gain. It is only when the results of one type of test are compared with another that the effects of strength gain may be of concern. However, as was noted above, the estimated difference in compressive strength during the testing period is very minimal and may be reasonably ignored. Therefore, no adjustments were made to the bond strength results to compensate for the maturity of the concrete at the time of testing.
The logarithmic curve used to model strength gain was calculated using a least-squares best fit. This function is reproduced as Equation (5-3) in U.S. customary units.
50


/c' = 1115.6 ln(t) + 2095.1
(5-3)
Where: f c = compressive strength of concrete (est.) (psi)
t = maturity, in days
50.000
40.000
Ph
30.000
>
20,000
o
U
10,000
S 4
V
t/% 1:

fc- 1115.6 ln(t) +2095.1 R2 = 0.5987

7,000
h 6.000
- 3,000
- 2,000 1,000
0
10 20 30 40
Maturity Days
50
60
Figure 5-2 Strength gain in concrete
5.3 Bond Strength in Tension
Figure 5-3 plots the average unadjusted bond strength in tension for each of the six pretreatment categories. Error is displayed as one standard deviation about the mean (SD). The groupings shown on the horizontal axis represent the different types of pretreatments. The first term in the category title: Dry, "SSD. or Wet represents the moisture condition of the substrate prior to overlay placement. The second term indicates whether a cement slurry bonding agent was used.
The results of the pull-off tests were adjusted for variations in the compressive strength of concrete and the average adjusted strength was plotted on Figure 5-4. With the exception of Slab 2B (discussed further below), the comparison shows that similar bond strengths were achieved in all
51
Compressive Strength (psi)


samples prepared with an SSD substrate surface. The best performance was achieved in Slab 3B, which was prepared solely by prewetting the substrate. No detrimental impacts to strength were observed due to the overwet substrate surface treatment used in Slab 3A; these samples performed similarly to the SSD samples used in concert with a bonding agent.
The overlay on Slab 1A was applied directly to the profiled substrate with no prewetting and no bonding agent. This bond exhibited inferior performance relative to the other slabs. This is consistent with the conclusions of Bissonnette (2012) that a carefully controlled amount of moisture within the substrate can produce a better bond.
1A IB 2A 2B 3 A 3B
Figure 5-3 Unadjusted average bond strengths in tension (SD), grouped by pretreatment. N = 3
samples per pretreatment category.
The cement slurry on Slab 2B was allowed to dry for a period of several days prior to overlay placement. This bond was extremely poor relatively to the other samples tested. This substantiates a
52
Tensile strength at bonded interface (psi)


common concern with bonding agents, as expressed in ACI 325 and 345. The dried bonding agent, far from enhancing the bond strength, appears to act as a bond breaker. It is important that bonding agents remain wet until the moment the overlay concrete is placed.
The cement slurry bonding agent applied to a dry substrate (Slab IB) performed similarly to the slabs prepared by prewetting. It appears that a properly applied bonding agent may benefit strength to a similar magnitude as prewetting, but the effect of both pretreatments in combination (Slab 2A) is not additive.
1A
IB
2A
2B
3 A
3B
Figure 5-4 Adjusted average bond strengths in tension (SD), grouped by pretreatment. N=3
samples per pretreatment category.
5.4 Shear Strength at the Bonded Interface
5.4.1 Representative shear strengths from previous research
Published Typical' values for shear strengths at bonded interfaces vary considerably by
source. The AASHTO Bridge Design Specifications permit the designer to use a value of 1.93 MPa
53
Adjusted bond strength (US)


(280 psi) for "clean concrete girder surfaces free of laitence with surface roughened to an amplitude of 0.25 hf (2014). Felt considered a shear strength of 2.24 MPa (325 psi) (measured in direct shear using a guillotine type jig) to be average; he classified samples that exceeded 2.76 MPa (400 psi) as "superior (Felt, 1956). For adhesive bond strength, Silfwerbrand found an average value of about 1.0 MPa (145 psi) for surfaces prepared with a pnumatic hammer (Silfwerbrand, 1990). ACI 345 notes that 1.38 MPa (200 psi) is typically sufficent for durability (2006).
5.4.2 Direct shear test
Using typical shear strengths as a guide, Figure 5-5 indicates "superior bond strengths were achieved in all slabs prepared with pretreatments (with the exception of Slab 2B, discussed below). The factored average shear strength measurements, adjusted for compressive strength of the concrete, are shown on Figure 5-6.
1A
IB
2A
2B
3 A
3B
CD
M
C-IH
*-<
CD
^d
CD
^d
rt
o
n
c
p
a
c/3
Figure 5-5 Unadjusted average direct shear strengths (SD) at the bonded interface, grouped by
pretreatment. N=3 samples per pretreatment category.
54


The adjusted results show that the best performance was obtained from a substrate prepared solely by prewetting (Slab 3B). The overwet surface on Slab 3A appears to have had a detrimental effect on the measured strengths. The surface with no pretreatments (Slab 1A) performed poorly, as did the surface where the bonding agent was allowed to dry (Slab 2B).
The results indicate that a surface prepared solely with a bonding agent (Slab IB) will perform similar to a surface prepared with a combination of prewetting and a bonding agent. The good performance of Slab IB may be in part due to the wetting effect of the bonding agent. The bonding agent may form a barrier to prevent free water from the overlay concrete from being lost into the capillaries of the substrate. It may also encapsulate dust and other particles left behind after the cleaning that would otherwise prevent the overlay from bonding to the surface of the substrate (Silfwerbrand & Paulsson, 1998).
1A
IB
2A
2B
3 A
3B
Figure 5-6 Adjusted average direct shear strengths (SD) at the bonded interface, grouped by
pretreatment. N=3 samples per pre treatment categonK
55
Adjusted shear strength (US)


5.4.3 Slant shear tests
The observed failure mode of the slant shear specimens indicates that the results for the slant shear tests primarily reflect the average compressive strength of the overlay and substrate. These results do not appear to be influenced by the strength of the bond or by the pretreatments used.
5.4.4 Jacking test
Measured average jacking test results are shown on Figure 5-7, grouped by pretreatment type. The average results, adjusted for compressive strength of concrete, are shown on Figure 5-8. The highest adjusted strength results were obtained using an SSD slab with no bonding agent (Slab 3B). A SSD substrate surface used in conjuction with a bonding agent (Slab 2A) also performed well relative to the other surface preparations, as did the overwet substrate and bonding agent (Slab 3A), though neither performed as well as the prewet only substrate.
1A
IB
2A
2B
3 A
3B
CD
O
.03
C-IH
*-<
CD
^d
CD
^d
a
o
a
-a
GO
Figure 5-7 Unadjusted average jacking test strengths (SD) at the bonded interface, grouped by
pretreatment. N=4 samples per pre treatment categonK
56


The surface with no pretreatments (Slab 1A) obtained similar adjusted strengths as Slab 2B, where the bonding agent was allowed to dry. Other tests had obtained significantly lower strengths for the dried bonding agent relative to all other pretreatments. It is possible that the eccentricity caused by the location of the hydraulic piston during testing may be producing a clamping force at the opposite end of the block. This force may enhance the resistance to shearing due to mechanical interlock, such that even poorly pretreated surfaces exhibit shear resistance corresponding to their roughness.
Pretreating with a bonding agent alone (Slab IB) did not produce superior shear strength relative to the other preparation methods. This is in contrast to the direct shear test, where the bonding agent alone produced strengths similar to those observed with prewetting.
1A
IB
2A
2B
3 A
3B
GO
£
60
S
CD
a
CD
w
CD
+->
cn
p
<
Figure 5-8 Adjusted average jacking test strengths (SD) at the bonded interface, grouped by
pretreatment. N=4 samples per pre treatment category.
57


5.4.5 Comparison of shear test results
The direct shear and jacking tests both indicated that the best strength is achieved using prewetting only, with no bonding agent, and that the lowest strengths result from the improper application of a bonding agent (Slab 2B). Poor performance was also observed when no pretreatments were used (Slab 1A).
The shear tests produced conflicting results when comparing the effect of prewetting in conjunction with a bonding agent (Slabs IB and 2A): the jacking test indicated that prewetting had an beneficial effect on bond strength, while the direct shear results indicated the opposite. The results are inconclusive; it appears that a properly applied bonding agent may benefit in strength to a similar magnitude as prewetting for some testing conditions. However, it can be ascertained that the effect of both pretreatments used in combination is not additive.
5.4.6 Comparison of tension and shear test results
All three tests agree that the best bond strength in tension and shear is obtained when the substrate surface is SSD and no bonding agent is used. When a bonding agent is used in combination with a SSD surface, all tests indicate some reduction in strength in comparison with the prewet-only surface. The surface prepared with a dried bonding agent performed poorly in all types of tests. Additionally, the surface prepared with no pretreatments performed poorly relatively to the pretreated slabs for both tension and shear tests.
The tension and shear tests responded differently to the overwet substrate surface on Slab 3A. The shear tests both indicated that the overwet surface had a slightly detrimental effect on shear strength, which the pull-off test showed no distinct difference in strength between the overwet surface of Slab 3A and the SSD surface of Slab 2A.
The unadjusted direct shear results and pull-off test results are plotted on Figure 5-9 (SI units) and Figure 5-10 (US customary units). Clearly, it is not possible to obtain the strength of a single sample using more than one test, so the x,y coordinates of the datapoints are taken from the same test number (Test 1-3) for each of the two tests (pull-off and direct shear). The pull-off and direct shear
58


samples were cored on the same day in alternating order along the length of the test slab. Test sample one for the direct shear test was taken immediately adjacent to test sample 1 for the pull-off test, and so on for test samples two and three.
Figure 5-10 shows that, though there is considerable scatter in the data, there may be a linear relationship between shear strength at the bonded interface and bond strength in tension. If a best fit line is calculated using the least-squares method and this line is made to go through the origin, Equation (5-4) is obtained.
c3
Ph
-C
2000
1600
B 1200
60
S
<+H
<+H
O
p
Ph
800
400

y = 0.39 R2 = 0.4 llx 647

r''' *


500 1.000 1.500 2.000 2.500 3.000 3.500
Direct shear test strength (kPa)
Figure 5-9 Direct shear vs. pull-off test results (SI units)
59


400
* w
3>
£
60
S

-M
3
Ph
300
200
100
0

y = 0.391 lx R2 = 0.4647


< >"

0 100 200 300 400 500 600
Direct shear test strength (psi)
Figure 5-10 Direct shear vs. pull-off test results (U.S. customary units)
<7 = 0.3911 t (5-4)
Where: a = Tensile strength of bond (est.) (kPa or psi)
r = Shear strength at bonded interface (kPa or psi)
If the equation is inverted, the conversion factor from tensile strength to shear strength is calculated as 2.56. Considering the multitude of factors involved in this comparison, this correlates reasonably well with the 2.04 factor determined by Delatte, et al. (2000), and used by Rosen (2016).
5.4.7 Variations in data
Figure 5-11 plots the coefficient of variation for each pretreatment and test type. Relatively low variation was observed for most of the pretreatment/test combinations. Relatively high variation (CV > 30%) in the pull off test results was observed at Slabs 1A, 2B, and 3B. The variation at Slab 2B is easily explained: the adhesive strengths measured for this slab were so low that they were close to the lower limit of the measuring capacity of the device. At low levels of stress, small differences in measurement result in large coefficients of variation. The variation at Slabs 1A and 3B cannot be as
60


easily explained, as their average shear strengths were comparable to the neighboring slabs. Instead, it appears that the high variation in pull-off test results may be attributed to the lack of bonding agent on these slabs. The bonding agent, while not significantly increasing bond strength, decreases the variability of the bond, an effect first noted by Felt (1956) and corroborated by Saucier and Pigeon (1991). The same effect appears to occur in the jacking test results, though it is not nearly as pronounced. A similar effect was not observed in the direct shear test results.
O
c3
>
O
"H
"3
o
U
Pull-off Test Direct Shear Test A Jacking Test
50%
40%
30%
20%
10%
0%
Figure 5-11 Coefficients of variation for each test type, grouped by pretreatment



A A A
t I
Dry Dry/Agent SSD/Agent SSD/ Dried Agent Wet/Agent SSD
5.4.8 Statistical significance
A statistical analysis of the strength results was undertaken to further corroborate the observations regarding the effects of pretreatments. A similar analysis was performed by Wall and Shrive (1988). A Student's t-test was performed using datasets gathered from five combinations of pre-treatments to determine if the observed differences were statistically significant. A significant difference was deemed to occur at the 90 percent confidence interval. The results of the analysis are shown in Table 5.4-1.
61


Table 5.4-1 Results of statistical analysis
Pre-treatment Significant Difference (Y or IS 1) (90% Cl)
Moisture Bonding Agent Pull-off Direct Shear Jacking
Dry Substrate No Agent vs. Wet Agent N Y N
SSD Substrate Dry Agent vs. Wet Agent Y Y Y
Dry vs. SSD Substrate No Agent N Y Y
Dry vs. SSD Substrate Wet Agent N N Y
SSD vs. Wet Substrate Wet Agent N N N
Not surprisingly, all three tests indicate a significant difference between the strength obtained with a wet bonding agent versus one that has been left to dry. The direct shear test indicated a significant difference in strength when bonding agent was applied on a dry substrate, although the jacking test did not substantiate this result. The direct shear and jacking tests both identified that a significant difference in strength was achieved using an SSD substrate versus a dry substrate in the absence of a bonding agent. The jacking test also identified this difference when a bonding agent was used. None of the tests identified a significant statistical difference between a SSD and wet substrate.
62


CHAPTER VI
SUMMARY AND CONCLUSIONS
6.1 Summary
Prewetting the substrate and the use of a bonding agent are both common practices in the construction of bonded concrete overlays and other types of repairs, yet the efficacy of these pretreatments is often debated. The program of research described herein was undertaken to identify best practices with regards to pretreatment. To that end, three substrate and six overlay slabs were constructed, and each substrate surface was prepared with a different combination of pretreatments. Samples from each slab were subjected to a number of tests to evaluate the impact on bond strength in tension and shear strength at the bonded interface. The conclusions, below, are derived from the experimental data:
6.2 Conclusions
The results indicate that pretreatments can substantially improve the strength of concrete-to-concrete bond. Compared with an overlay constructed with no pretreatments, the overlay placed on the saturated surface dry substrate, using a properly applied bonding agent, achieved 46 percent greater bond strength in tension and 35 percent greater strength in shear. However, the inappropriate use of pretreatments can hinder the development of bond between substrate and overlay. When the bonding agent was improperly applied, the bond strength in tension decreased by 62 percent and the shear strength decreased by 31 percent, as compared to the slab constructed with no pretreatments.
Pre wetting of the substrate generally produced superior bond strengths, irrespective of the use of bonding agents. For some tests, overlays prepared with a combination of prewetting and a bonding agent performed somewhat better than overlays prepared solely by prewetting. Concern regarding overwetting the substrate surface appears to be largely unfounded; an overwet substrate surface (one containing small puddles of water) performed better in the tension test and slightly worse in the shear tests, but the overall effect was minor.
63


The effect on strength of the use of a bonding agent in the absence of prewetting was inconclusive. Two of the tests indicated superior bond strength, while the third saw no strength benefit. However, the application of a bonding agent did reduce the variability of the bond strength in the pull-off (tension) test. This effect was not observed in the shear tests. Allowing the bonding agent to dry on the substrate prior to overlay placement had an extreme negative effect on bond strength for all types of tests.
In general, reasonably good bond strengths were obtained using a saturated surface dry substrate alone. Prewetting the substrate substantially increased strength under both tensile and shear stresses, and the risk of overwetting the substrate appears to be minimal. The designer should consider specifying an SSD substrate whenever good concrete-to-concrete bond strength is desired.
The designer should weigh the increase to bond strength resulting from the application of a bonding agent with the potential drawbacks due to improper application. If the application is carefully controlled, the bonding agent may provide some benefit.
In some cases, the pull-off, direct shear, and jacking tests responded differently to combinations of pre-treatments. The designer should consider specifying the test which best mimics the loads to which the repair will be subjected.
6.3 Suggestions for Future Research
There are numerous ways in which this program of research may be extended. These can be generally grouped into two categories: 1) incorporating other methods of testing bond strength or 2) evaluating other methods of slab preparation and pretreatment.
In the first category, the literature teems with alternative methods of measuring bond strength in tension and shear strength at the bonded interface. Among these are a twist-off (torsion) type test described by Whitney, et al. (1992) and a tensile slant shear test described by Austin, et al. (1999). These and other tests could be incorporated into a new program of research to evaluate their sensitivities to the effects of pretreatments.
64


In the second category, the research discussed herein evaluated the effects of pretreatments on a single ICRI Concrete Surface Profde. It is reasonable to assume that the effects of bonding agents may become more pronounced as the amplitude of surface roughness decreases. A new program of research could be undertaken to determine the interaction between pretreatments and substrate surface roughness.
Silfwerbrand and Paulsson (1998) note that the scrubbing action during application of a bonding agent may assimilate dust particles that were not removed by cleaning, thereby improving the strength of the concrete-to-concrete bond. A program of research could be undertaken to determine if scrubbing water into the substrate surface would have a similar effect.
Certain authors, such as Whitney, et al. (1992) had previously observed that bonding agents can be very effective in hot weather when the temperature of the substrate is elevated. A program of research could be undertaken to assess the impact of ambient temperature on the effectiveness of pretreatments.
65


REFERENCES
AASHTO (American Association of State Highway and Transportation Officials). (2014). AASHTO LRFD Bridge Design Specifications, Customary U.S. Units, 7th Edition. AASHTO, Washington, DC.
ACI (American Concrete Institute). (2006). Concrete Overlays for Pavement Rehabilitation, Standard ACI 325.13R-06. ACI, Farmington Hills, MI.
ACI (American Concrete Institute). (2011). Guide to Highway Bridge Deck Construction, Standard ACI 345R-11. ACI, Farmington Hills, MI.
ASTM (American Society of Testing Materials). (2015). Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension (Pull-offMethod), Standard ASTM C1583. ASTM, Conshohocken, PA.
ASTM (American Society of Testing Materials). (2004). Standard Method of Test for Compressive Strength of Cylindrical Concrete Specimens, Standard ASTM C39. ASTM, Conshohocken, PA.
ASTM (American Society of Testing Materials). (2004). Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete, Standard ASTM C42. ASTM, Conshohocken, PA.
Austin, S., Robins, P., and Youguang, Pan. (1999). Shear Bond Testing of Concrete Repairs.
Cement and Concrete Research, 29(7), 1067-1076.
Bissonnette, B., Vaysburd, A. M., and von Fay, K. F. (2012). Best Pratices for Preparing Concrete Surface Overlays, Report Number MERE 12-17. U.S. Bureau of Reclamation, Dept, of the Interior, Denver, CO.
Delatte, N. J., Williamson, M. S., and Fowler, D. W. (2000). Bond Strength Development with
Maturity of High-early Strength Bonded Concrete Overlays." ACI Materials Journal. 97(2), 201-207.
Djazmati, B. and Pincheira, J. (2004). Shear Stiffness and Strength of Horizontal Construction Joints. ACI Structural Journal, 101(4), 484-493.
Felt, E. J. (1956). Resurfacing and Patching Concrete Pavements with Bonded Concrete.
Proceedings of the Thirty-Fifth Annual Meeting of the Highway Research Board. Vol 35. 444-469.
Harrington, D. (2008). Guide to Concrete Overlays: Sustainable Solutions for Resurfacing the
Rehabilitating Existing Pavements. National Concrete Pavement Technology Center, Ames, IA
Hindo, K. R. (1990). In-Place Bond Testing and Surface Preparation of Concrete. Concrete International, 12(4), 46-48.
66


Illinois Bureau of Materials and Physical Research. (2012). Standard Method of Test for Shear Strength of Bonded Polymer Concrete.
ICRI (International Concrete Repair Institute). (2013). Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair, Standard 310.2R-2013. ICRI, Rosemont, IL.
Julio, E. N., Branco, F. A., and Silva, V. D. (2004). Concrete-to-Concrete Bond Strength; Influence of the Roughness of the Substrate Surface. Construction and Building Materials, 2004(18), 675-681.
Kriegh, J. D. (1976). Arizona Slant Shear Test; a Method to Determine Epoxy Bond Strength. ACI Journal, July 1976, 372-373.
Pasko, T. J. (1997). Concrete Pavements Past, Present, and Future. Public Roads, 62(1).
Portland Cement Association (PCA). (1996). Resurfacing concrete floors, Standard IS144.07T. PCA, Skokie, IL.
Quikrete Companies. Quikrete 5000 Concrete Mix Specification, Standard Specification 03 31 00. Quikrete Companies, Atlanta, GA.
Rosen, C. J. (2016). Shear Strength at the Interface of Bonded Concrete Overlays. M.S. thesis, University of Colorado Denver, Denver, CO.
Rosengarten, W. E. (1919). Bonding New Cement Mortar and Concrete to Old Concrete. Concrete Products Magazine, 17(10), 19-20.
Saucier, F. and Pigeon, M. (1991). Durability of New-to-Old Concrete Bondings. Special Publication 128-43, American Concrete Institute, Farmington Hills, MI., 689-705.
Silfwerbrand, J. (1990). Improving Concrete Bond in Repaired Bridge Decks. Concrete International, 12(9), 61-66.
Silfwerbrand, J. and Paulsson, J. (1998). The Swedish Experience: Better Bonding of Bridge Deck Overlays. Concrete International, 20(10), 56-61.
Talbot, C., Pigeon, M., Beaupre, D., and Morgan, D. (1994). Influence of Surface Preparation on Long-term Bonding of Shotcrete. ACI Materials Journal, 91(6), 560-566.
Iowa Department of Transportation (IA DOT). (2000). Method of Test for Determining the Shearing Strength of Bonded Concrete, Standard Iowa 406-C. IA DOT, Ames, IA.
Vaysburd, A. M. and McDonald, J. E. (1999). An Evaulation of Equipment and Procedures for
Tensile Bond Testing of Concrete Repairs, Technical Report REMR-CS-61. U.S. Army Corps of Engineers, Washington, DC.
Wall, J.S. and Shrive, N.G. (1988). Factors Affecting Bond Between New and Old Concrete. ACI Materials Journal, 85(2), 117-125.
Wells, J.A., Stark, R.D., and Polyzois, D. (1999). Getting Better Bond in Concrete Overlays. Concrete International, 21(3), 49-52.
67


Whitney, D.P., Isis, P., McCullough, B.F., and Fowler, D.W. (1992). An Investigation of Various Factors Affecting Bond in Bonded Concrete Overlays, Report 920-5. Center for Transportation Research, University of Texas, Austin, TX.
68


APPENDIX A
PULL OFF TEST DATA
Appendix A contains recorded data from the ASTM C1583 tensile strength (pull-off) tests. Specimens are grouped by slab number. The tensile force at failure, measured using the pull-off apparatus, is given for each specimen. The tensile stress at failure is calculated based on the tensile force and gross bonded area.
69


ASTM C1583 PULL-OFF TEST DATA
Test: ASTM Cl583 (Pull-off test) Test Date: As Noted
Slab I.D.: Slab 1A
Moisture: Dry Tested by: ASP
Bonding Agent: None
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 1A1T (6/22/2016)
SPECIMEN 1A3T (6/21/2016)
SPECIMEN 1A5T (6/26/2016)
V SI Units Imperial Units
- \ 7. :'' '3L <> **h ft lA.' 8E2 Tensile Force at Failure Tensile Stress at Failure 4.23 kN 1213 kPa 950 lbs 176 psi
70


ASTM C1583 PULL-OFF TEST DATA
Test: ASTM Cl583 (Pull-off test) Test Date: As Noted
Slab I.D.: Slab IB
Moisture: Dry Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 1B5T (6/26/1016)
SI Units
Tensile Force at Failure
Tensile Stress at Failure
3.74 kN 1069 kPa
Imperial Units
840 lbs 155 psi
SPECIMEN 1B7T (6/26/2016)


l
SI Units
Tensile Force at Failure
Tensile Stress at Failure
4.54 kN 1296 kPa
Imperial Units
1020 lbs 188 psi
SPECIMEN 1B11T (8/4/2016)
f - SI Units Imperial Units
Tensile Force at 3.65 kN 820 lbs
S3M Failure Tensile Stress at 1048 kPa 152 psi
ft Mag i 1 Failure
m A *' A
Pi
\ plr X
jmL

aJW X
71


ASTM C1583 PULL-OFF TEST DATA
Test: ASTM Cl583 (Pull-off test) Test Date: As Noted
Slab I.D.: Slab 2A
Moisture: SSD Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 2A3T (6/26/1016)
SI Units Imperial Units
' -' Tensile Force at 4.41 kN 990 lbs
Failure
Tensile Stress at Failure 1262 kPa 183 psi
' r

SPECIMEN 2A5T (6/26/2016)
- SI Units Imperial Units
* V Tensile Force at Failure 3.83 kN 860 lbs
j X' Tensile Stress at Failure 1096 kPa 159 psi
K. .4
SPECIMEN 2A7T (8/4/2016)
SI Units
Tensile Force at Failure
Tensile Stress at Failure
3.69 kN 1055 kPa
Imperial Units
830 lbs 153 psi
72


ASTM C1583 PULL-OFF TEST DATA
Test: ASTM Cl583 (Pull-off test) Test Date: 6/22/2016
Slab I.D.: Slab 2B
Moisture: SSD Tested by: ASP
Bonding Agent: Cement Slurry (Dried)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 2B IT
SI Units
Tensile Force at Failure
Tensile Stress at Failure
1.34 kN 379 kPa
Imperial Units
300 lbs 55 psi
SPECIMEN 2B3T
SI Units
Tensile Force at Failure
Tensile Stress at Failure
1.29 kN 372 kPa
Imperial Units
290 lbs 54 psi
SPECIMEN 2B5T
SI Units Imperial Units
No Photo Available Tensile Force at Failure 0.45 kN 100 lbs
Tensile Stress at Failure 124 kPa 18 psi
73


ASTM C1583 PULL-OFF TEST DATA
Test: ASTM Cl583 (Pull-off test) Test Date: 6/26/2016
Slab I.D.: Slab 3A
Moisture: Overwet (small puddles) Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
SI Units Imperial Units
Tensile Force at Failure 3.52 kN 790 lbs
* $,*'k'. ly*1 m Tensile Stress at Failure 1007 kPa 146 psi

SPECIMEN 3A1T
SPECIMEN 3A3T
SI Units Imperial Units
^ A %'V-V fSSfcfc' -V r ,*vT -v V v Tensile Force at Failure Tensile Stress at Failure 4.54 kN 1296 kPa 1020 lbs 188 psi

SPECIMEN 3A5T

. A . -**-. 'V 5
' *'

SI Units
Tensile Force at Failure
Tensile Stress at Failure
5.34 kN 1531 kPa
Imperial Units
1200 lbs 222 psi
74


ASTM C1583 PULL-OFF TEST DATA
Test: ASTM Cl583 (Pull-off test) Test Date: 6/26/2016
Slab I.D.: Slab 3B
Moisture: SSD Tested by: ASP
Bonding Agent: None
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 3B1T
* SI Units Imperial Units
Tensile Force at Failure Tensile Stress at Failure 2.85 kN 814 kPa 640 lbs 118 psi
SPECIMEN 3B3T
g tg 1 X 4 5 6 7 mi 1 SI Units Imperial Units
Tensile Force at Failure Tensile Stress at Failure 3.83 kN 1096 kPa 860 lbs 159 psi
SPECIMEN 3B5T
. > SI Units Imperial Units
Tensile Force at Failure Tensile Stress at Failure 6.14 kN 1758 kPa 1380 lbs 255 psi
75


APPENDIX B
DIRECT SHEAR (GUILLOTINE) TEST DATA
Appendix B contains recorded data from the direct shear (guillotine) tests. Specimens are grouped by slab number. The maximum shearing force applied, measured using the MTS equipment, is given for each specimen. The shear stress at failure is calculated based on the recorded shearing force (divided by two) and gross bonded area. The stress-strain plot is based on the recorded load-displacement data, converted to stress and strain using the gross bonded area and diameter of specimen, respectively.
76


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab 1A
Moisture: Dry Tested by: ASP
Bonding Agent: None
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 1A2G
SI Units Imperial Units
Applied Load at Failure Shear Stress at Failure 16.0 kN 2296 kPa 3604 lbs 333 psi
SPECIMEN 1A4G
W 1 pil 2 3 4 5 SI Units Imperial Units
Applied Load at Failure Shear Stress at Failure 17.8 kN 2544 kPa 3995 lbs 369 psi
SPECIMEN 1A7G
| J. -' *+'/ ^ SI Units Imperial Units
Applied Load at Failure Shear Stress at Failure 14.4 kN 2062 kPa 3233 lbs 299 psi
77


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab 1A
Moisture: Dry Tested by: ASP
Bonding Agent: None
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 1A2G
Strain (in/in)
Maximum Load: 3604 lbs
Maximum Stress: 333 psi
Strain at Failure: 0.0080 in/in
78


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab 1A
Moisture: Dry Tested by: ASP
Bonding Agent: None
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 1A6G
Strain (in/in)
Maximum Load: 3995 lbs
Maximum Stress: 369 psi
Strain at Failure: 0.0086 in/in
79


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab IB
Moisture: Dry Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 1B2G
SI Units Imperial Units
Applied Load at 23.2 kN 5204 lbs
Failure
Shear Stress at 3316 kPa 481 psi
Failure
f

g s S l
SPECIMEN 1B8G
SI Units Imperial Units
Applied Load at Failure 21.2 kN 4757 lbs
Shear Stress at Failure 3027 kPa 439 psi
it ML
-71 234 567
SPECIMEN 1B10G

A
2 3 4 5 6 7
fB&i ...I"...
SI Units
Applied Load at 20.4 kN 4580 lbs
Failure
Shear Stress at 2916 kPa 423 psi
Failure
Imperial Units
80


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab IB
Moisture: Dry Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 1B2G
Strain (in/in)
Maximum Load: 5204 lbs
Maximum Stress: 481 psi
Strain at Failure: 0.0100 in/in
81


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab IB
Moisture: Dry Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
Maximum Load: 4757 lbs
Maximum Stress: 439 psi
Strain at Failure: 0.0113 in/in
82


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab IB
Moisture: Dry Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 1B10G
Strain (in/in)
Maximum Load: 4580 lbs
Maximum Stress: 423 psi
Strain at Failure: 0.0084 in/in
83


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab 2A
Moisture: SSD Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 2A2G
SI Units
Imperial Units
Applied Load at Failure
Shear Stress at Failure
23.1 kN 3309 kPa
5200 lbs 480 psi
SPECIMEN 2A4G
SPECIMEN 2A6G
SI Units
Imperial Units
Applied Load at Failure
Shear Stress at Failure
16.5 kN 2365 kPa
3711 lbs 343 psi
SI Units
Imperial Units
Applied Load at Failure
Shear Stress at Failure
3075 kPa
4828 lbs
446 psi
84


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab 2A
Moisture: SSD Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 2A2G
Strain (in/in)
Maximum Load: 5200 lbs
Maximum Stress: 480 psi
Strain at Failure: 0.0128 in/in
85


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab 2A
Moisture: SSD Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 2A4G
Strain (in/in)
Maximum Load: 4828 lbs
Maximum Stress: 446 psi
Strain at Failure: 0.0093 in/in
86


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab 2A
Moisture: SSD Tested by: ASP
Bonding Agent: Cement Slurry (Wet)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 2A6G
Strain (in/in)
Maximum Load: 3711 lbs
Maximum Stress: 343 psi
Strain at Failure: 0.0081 in/in
87


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab LD.: Slab 2B
Moisture: SSD Tested by: ASP
Bonding Agent: Cement Slurry (Dried)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 2B2G
i SI Units Imperial Units
Applied Load at Failure Shear Stress at Failure 4.6 kN 655 kPa 1023 lbs 95 psi
^Tn'ivmTiTi |'1W :Afl| 1 2 3 4 5 6 7 fl
SPECIMEN 2B4G
SI Units Imperial Units
Applied Load at 10.4 kN 2347 lbs
... -8Q^ Failure
jCe-- Shear Stress at 1496 kPa 217 psi
j Failure
. 1
JPI1* 2 3 4 5 6
SPECIMEN 2B6G


^WlWrm'I'I'l'tiTiTiT!1
!)1> 2 3 4 5
nTlTl.l.l. i..IiI.U|iIiViViTA SI Units
Applied Load at 9.9 kN 2234 lbs
Failure
Shear Stress at 1420 kPa 206 psi
Failure
Imperial Units
88


DIRECT SHEAR TEST DATA
Test: Direct Shear (Guillotine) Test Test Date: 7/11/2016
Slab I.D.: Slab 2B
Moisture: SSD Tested by: ASP
Bonding Agent: Cement Slurry (Dried)
Core Diameter: 66.7 nun (2 5/8 in)
SPECIMEN 2B2G
Strain (in/in)
Maximum Load: 1023 lbs
Maximum Stress: 95 psi
Strain at Failure: 0.0034 in/in
89


Full Text

PAGE 1

THE EFFECTS OF COMMON SURFACE P RETREAT MENTS ON THE SHEAR STRENGTH OF BO NDED CONCRETE OVERLA YS by ANDREW STEPHEN PULTO RAK B.S., University of Colorado Denver, 2006 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment o f the requirements of the degree of Master of Science Civil Engineering 2016

PAGE 2

ii This thesis for the Master of Science degree by Andrew Stephen Pultorak has been approved for the Civil Engineering Program by Prof. Kevin L. Rens, Chair Frederick R. Rutz Chengyu Li December 17 2016

PAGE 3

iii Pultorak, Andrew Stephen (M.S., Civil Engineering) The Effects of Common Surface Pretreatments on the Shear Strength of Bonded Concrete Overlays Thesis directed by Associate Professor Frederick R. Rutz ABSTRACT The durability of a concrete repair is highly dependent on the shear strength of the interface between new and old concrete. Therefore, the engi neer designing the repair makes every effort to maximize this strength. To that end, pretreatments, such as prewetting the substrate and/or applying bonding agents, are commonly specified. The efficacy of these pretreatments is often debated, and previous studies have produced contradictory results. Th is research was undertaken to determine the effects of prewetting the substrate and applying a bonding agent, both in combination and individually. T he bond strength in tension and the shear strength of the bo nd were measured using a variety of methods, including in place testing and testing of extracted specimens The results indicate that both prewetting and the use of a bonding agent can be beneficial to the shear strength of bonded overlays. The form and co ntent of this abstract are approved. I recommend its publication Approved: Frederick R. Rutz

PAGE 4

iv ACKNOWLEDGEMENTS I wish to express my deepest gratitude to Dr. Fred Rutz, whose advice, mentorship, wisdom, and enthusiastic participation made this proje ct pos sible. My sincere appreciation goes to Dr. Kevin Rens and Dr. Chengyu Li of the Civil Engineering Department at UCD for their service as members of my examination committee. I also wish to thank CTL Thompson, Inc., including Bud Werner, Zack Ballard, and D an Barrett, for donating their time, expertise, and for generously allowing me to use their testing equipment. A tremendous thank you to Tom Thuis and Peter Sillstrop of the Electronic Calibration and Repair Lab at UC Denver for fabricating parts and aidin g me in setting up the compression testing equipment. My sincerest appreciation goes to my fellow students: Christian Rosen, Anne Swan, and Chris Cardillo, who helped me with the unpleasant task of placing and then removing 2 tons of concrete. I also wish to acknowledge the contributions of others who have assisted me, including, but not limited to: Randon Grimes, Jed Williamson, and Chris Sheehan. Finally thank you to my wife, Jessica, and my sons, Ethan and Ben, for their love, understanding, and support throughout this project.

PAGE 5

v TABLE OF CONTENTS CHAPTER I OVERVIEW ................................ ................................ ................................ ......................... 1 1.1 Introduction ................................ ................................ ................................ ................ 1 1.2 Research Goal and Significance ................................ ................................ ................. 2 1.3 Outline ................................ ................................ ................................ ........................ 2 II BACKGROUND ................................ ................................ ................................ .................. 3 2.1 Introduction ................................ ................................ ................................ ................ 3 2.2 Mechanis m of Concrete to Concrete Bond ................................ ................................ 4 2.3 Methods of Testing Bond Strength ................................ ................................ ............. 4 2.3.1 Pull off test (ASTM C1583) ................................ ................................ .................. 5 2.3.2 Direct shear (guillotine) test ................................ ................................ .................. 7 2.3.3 Slant shear test ................................ ................................ ................................ ....... 8 2.3.4 Jacking test ................................ ................................ ................................ ............ 9 2.4 Pretreatments ................................ ................................ ................................ ............ 10 2.4.1 Prewetting ................................ ................................ ................................ ............ 10 2.4.2 Bondin g agents ................................ ................................ ................................ .... 11 2.5 Literature Review ................................ ................................ ................................ ..... 11 2.5.1 Early studies ................................ ................................ ................................ ........ 11 2.5.2 Modern studies ................................ ................................ ................................ .... 12 2.5.3 Co de references ................................ ................................ ................................ ... 14 2.5.4 Previous research at the University of Colorado Denver ................................ 15

PAGE 6

vi 2.5.5 Summary of literature review ................................ ................................ .............. 15 III RESEARCH PROGRAM ................................ ................................ ................................ .. 17 3.1 Co nstruction of Substrate and Overlay Slabs ................................ ........................... 17 3.2 Testing ................................ ................................ ................................ ...................... 24 3.2.1 Slump test ................................ ................................ ................................ ............ 24 3.2.2 Slab concrete compression test ................................ ................................ ............ 25 3.2.3 Pull off test ................................ ................................ ................................ .......... 26 3.2.4 Direct shear (guillotine) test ................................ ................................ ................ 28 3.2.5 Slant shear test ................................ ................................ ................................ ..... 30 3.2.6 Jacking test ................................ ................................ ................................ .......... 34 IV RESULT S ................................ ................................ ................................ ........................... 37 4.1 Slump Cone Results ................................ ................................ ................................ 37 4.2 Sack Concrete Sieve Analysis ................................ ................................ .................. 37 4.3 Compression Testing Results ................................ ................................ ................... 37 4.4 Specimen Identifiers ................................ ................................ ................................ 39 4. 5 Pull off Test Results ................................ ................................ ................................ 40 4.6 Direct Shear Test Results ................................ ................................ ......................... 42 4.7 Slant Shear Test Results ................................ ................................ ........................... 44 4.8 Jacking Test Results ................................ ................................ ................................ 46 V DISCUSSION ................................ ................................ ................................ .................... 48 5.1 Variation of Compressive Strength ................................ ................................ .......... 48

PAGE 7

vii 5.2 Effects of Strength Gain on Test Results ................................ ................................ .. 50 5.3 Bond Strength in Tension ................................ ................................ ......................... 51 5.4 Shear Strength at the Bonded Interface ................................ ................................ .... 53 5.4.1 Representative shear strengths from previous research ................................ ....... 53 5.4.2 Direct shear test ................................ ................................ ................................ ... 54 5.4.3 Slant shear tests ................................ ................................ ................................ ... 56 5.4.4 Jacking test ................................ ................................ ................................ .......... 56 5.4.5 Comparison of shear test results ................................ ................................ .......... 58 5.4.6 Comparison of tension and shear test results ................................ ....................... 58 5.4.7 Variations in data ................................ ................................ ................................ 60 5.4.8 Statistical significance ................................ ................................ ......................... 61 VI CONCLUSIONS ................................ ................................ ................................ ................ 63 6.1 Summary ................................ ................................ ................................ ................... 63 6.2 Conclusions ................................ ................................ ................................ .............. 63 6.3 Suggestions for Future Research ................................ ................................ .............. 64 REFERENCES ................................ ................................ ................................ ................................ ..... 66 APPENDIX A Pull Off test data ................................ ................................ ................................ ............... 69 B Direct Shear (guillotine) test data ................................ ................................ ..................... 76 C Slant Shear test data ................................ ................................ ................................ ........ 100 D Jacking test data ................................ ................................ ................................ .............. 111 E Slant Shear discussion ................................ ................................ ................................ .... 118

PAGE 8

viii LIST OF TABLES TABLE 2.1 1 Common examples of surface profiling and pretreatments ................................ .................. 4 2.3 1 Summary of common tests for bond strength ................................ ................................ ....... 5 2.5 1 Summary of results regarding pretreatments ................................ ................................ ...... 16 3.1 1 Pretreatment summary ................................ ................................ ................................ ........ 19 3.2 1 Testing Summary ................................ ................................ ................................ ................ 24 4.1 1 Results of slump cone test ................................ ................................ ................................ .. 37 4.3 1 Compression force at failure ................................ ................................ .............................. 38 4.3 2 Compression stress at failure ................................ ................................ .............................. 39 4.3 3 Average substrate compressive strength ................................ ................................ ............. 39 4.5 1 Pull off test samples ................................ ................................ ................................ ........... 40 4.5 2 Pull off test results (SI units) ................................ ................................ .............................. 40 4.5 3 Pull off test results (U.S. customary units) ................................ ................................ ......... 41 4.5 4 Pull off test bond strength adjustment (SI units) ................................ ................................ 41 4.5 5 Pull off test bond strength adjustment (U.S. customary units) ................................ ........... 42 4.6 1 Direct shear test sam ples ................................ ................................ ................................ .... 42 4.6 2 Direct shear test results (SI units) ................................ ................................ ....................... 43 4.6 3 Direct shear test results (U.S. customary units) ................................ ................................ .. 43 4.6 4 Direct shear test strength adjustment (SI units) ................................ ................................ .. 44 4.6 5 Direct shear test strength adjustme nt (U.S. customary units) ................................ ............. 44 4.7 1 Slant shear test samples ................................ ................................ ................................ ...... 45 4.7 2 Slant shear test results (SI units) ................................ ................................ ........................ 45 4.7 3 Slant shear test results (U.S. customary units) ................................ ................................ ... 45 4.8 1 Jacking test samples ................................ ................................ ................................ ........... 46 4.8 2 Jacking test results (SI units) ................................ ................................ .............................. 46

PAGE 9

ix 4.8 3 Jacking test results (U.S. customary units) ................................ ................................ ......... 47 4.8 4 Jacking test strength adjustment (SI units) ................................ ................................ ......... 47 4.8 5 Jacking test strength adjustment (U.S. customar y units) ................................ .................... 47 5.4 1 Results of statistical analysis ................................ ................................ .............................. 62 E 1 Maximum shear at slant interface (SI Units) ................................ ................................ .... 121 E 2 Maximum shear at slant interface (U.S. customary units) ................................ ................ 121 E 3 Slant shear strength adjustment (U.S. customary units) ................................ ................... 122

PAGE 10

x LIST OF FIGURES FIGURE 2 1 Example pull off test (ASTM C1583) apparatus ................................ ................................ 6 2 2 Direct shear test in progress ................................ ................................ ................................ 7 2 3 Preparing to test slant shear sample on c ompression machine ................................ ............. 8 2 4 Diagram showing (1) the slant shear specimen under uniaxial compression stress and (2) the resulting stresses on the bond surface ................................ ................................ ............. 9 2 5 Block specimen undergoing jacking test ................................ ................................ ............ 10 3 1 Typical substrate form, ready to receive concrete ................................ .............................. 17 3 2 Surface profiling the substrate ................................ ................................ ............................ 18 3 3 Finished profile compared w ith ICRI CSP 6 example. ................................ ...................... 19 3 4 Example of saturated, surface dry appearance immediately before placement .................. 20 3 5 Application of cement slurry bonding agent ................................ ................................ ...... 20 3 6 Plan view of slabs and pretreatments ................................ ................................ ................. 22 3 7 Isometric rendering of a ty pical substrate slab with two overlay slabs .............................. 22 3 8 As constructed view showing pull off test sampling locations. ................................ ......... 23 3 9 As constructed view showing pull off, direct shear, slant shear, and jacking tes t sampling locations ................................ ................................ ................................ .............................. 23 3 10 Example slump cone test on overlay concrete (note integral color) ................................ ... 25 3 11 Compression Testing Equipment ................................ ................................ ....................... 26 3 12 Cleaning the surface of the pull off test specimen ................................ ............................. 27 3 13 NDT 007 James Bond Tester ................................ ................................ ............................. 27 3 14 MTS testing equipment with direct shear apparatus ................................ .......................... 29 3 15 Direct Shear (Guillotine) Apparatus ................................ ................................ ................... 30 3 16 compression, (2) the stresses on the bond surface, and (3) the forces on the bond surface ................................ ................................ ................................ ................................ ............ 31 3 17 Checking the slant angle with an inclinometer prior to coring for a slant shear specimen ................................ ................................ ................................ ................................ ............ 3 1

PAGE 11

xi 3 18 t slant angles of 45 and 60 degrees ................................ ................................ ................................ ................................ 33 3 19 Representative slant shear cylinder after testing under uniaxial compression ................... 34 3 20 Same specimen as Figure 3 19, opened to reveal surface of splitting tension failu re ........ 34 3 21 Concrete saw and dust collection system ................................ ................................ ........... 35 3 22 Simplex RC306C hydraulic testing s pecimen on Slab 2A ................................ ................. 36 5 1 28 day compressive strength by slab location ................................ ................................ .... 49 5 2 Strength gain in concrete ................................ ................................ ................................ .... 51 5 3 Unadjusted average bond strengths in tension, grouped by pretreatment .......................... 52 5 4 Adjusted aver age bond strengths in tension, grouped by pretreatment .............................. 53 5 5 Unadjusted average direct shear strengths at the bonded interface, grouped by pretreatment ................................ ................................ ................................ ................................ ............ 54 5 6 Adjusted average direct shear strengths at the bonded interface, grouped by pretreatment ................................ ................................ ................................ ................................ ............ 55 5 7 Unadjusted average jacking test strengths at the bonded interface, grouped by pretreatment ................................ ................................ ................................ ................................ ............ 56 5 8 Adjusted average jacking test strengths at the bonded interface, grouped by pretreatment ... 57 5 9 Direct shear vs. pull off test results (SI units) ................................ ................................ .... 59 5 10 Direct shear vs. pull off test results (U.S. customary units) ................................ ............... 60 5 11 Coefficients of variation for each test type, grouped by pretreatment ................................ 61 E 1 Diagram showing (1) the slant shear specimen under uniaxial compression stress and (2) the resulting stresses on the bond surface ................................ ................................ ......... 119 E 2 Applied shear stress at bonded interface (c bond ) as a ratio of applied compressive stress ( 0 ), varying surface roughness after Austin, et al. (1999) ................................ .............. 119 E 3 Compressive strength vs. measured slant shear strength ................................ .................. 123

PAGE 12

1 CHAPTER I OVERVIEW 1.1 Introduction When damage or deterioration to existing concrete is no t so severe as to warrant complete removal, repairs frequently involve overlaying the existi ng concrete with a new concrete surface. Example s of this type of repair range from small patches o n a wall or beam to resurfacing existing concrete pavement or a b ridge deck with a layer of new concrete. In the case of pavement, it is possible to design the new concrete to act independently from the existing concrete. However, it often much more economical to bond the new concrete, hereafter referred to as the overl ay, to the existing substrate substrate, the two slabs will act in unison under applied loading ; this behavior is known as (Bissonnette, et al., 2012) The durability of the repair, whether a p atch in a structural member or an overlay of existing pavement is highly dependent on the shear strength of the bond ed interface between the overlay and substrate The desire t o obtain good bond strength to prolong the life of a repair is apparent, and much time and effort has been spent to isolate the factors that produce consistently hig h bond strength s. This research examines two such factors which are the cause of significant debate within the engineering community : 1) prewetting the substrate to increase its moisture content and 2) applying a bonding agent to the substrate surface These practices are referred to in ACI 325 (2006) Th e research conducted as part of this report was performed using three substrate slabs on to which six overlay slabs were bonded The substrate slabs were mechanically roughened and cleaned, after which the surfaces were prepared wit h a combination of pretreatments : with or without a bonding agent and with or without prewetting Pull off, direct shear, push off, and slant shear tests were performed to evaluate the effects on bond strength

PAGE 13

2 1.2 Research Goal and Significance T his thesis seeks to quantify the effect of pretreatments on the shear stre ngth of concrete to concrete bondings, including bonded overlays and other types of repairs. The results of the study may be applied to the construction of repairs and to the preparation of th e interface between successive concrete placements (cold joints) to develop better strength at the bonded interface A variety of different tests were used to measure the effects of pretreatment o n bonded strength, including in place testing and testing pe rformed on extracted samples. Each test measures the strength of the bond in a different way. The results illustrate the sensitivity of each particular test to each pretreatment type, as well as indicate the combination of pretreatments that will best enha nce the bond strength over all types of tests. 1.3 Outline There are six chapters in this thesis. The first chapter introduces the topic and goals of the research. Chapter 2 is a literature review, summarizing past research into the shear strength of bonded o verlays and the effects of pretreatments The chapter briefly discusses the mechanisms of concrete to concrete bond and examines the types of tests ava ilable to determine bond strength and shear strength In Chapter 3 the research program is discussed. This includes construction, p reparation and testing of the substrate and overlay slabs The profiling and cleaning of the substrate surface and the application of the pretreatments is discussed In Chapter 4 the bond stren gth and shear strength data is tabulated. Chapter 5 is a discussion of the experimental results from the four strength tests and the observed effects of the pretreatments Chapter 6 presents the conclusions of the research and summarizes future research ne eds. The attached appendices include experimental data gathered during the execution of the research program.

PAGE 14

3 CHAPTER II BACKGROUND 2.1 Introduction The ability to repair concrete by removing damaged or deteriorated areas and replacing or overlaying them with new concrete has long been understood. Frequently, t he goal of repair, whether on a structural member or concrete pavement, is to place the new concrete in such a way that the old and new concrete are able to adequately transfer stresses between one anothe r such that the membe r as a whole behaves as if it were made of monolithic concrete. Depending on the loading to whi ch the member is subjected, stresses could act either perpendicular to the interface, causing tension or compression, or parallel with the interface, causing shear The mechanism resisting these stresses is referred to broadly as bond strength The author has compiled existing literature dating from 1919 (Rosengarten) to the present; these studies investiga te the means of maximizing bond strength to produce more durable repairs. These studies generally agree that the best bond is obtained using a substrate surface free of laitance ( the weak surface layer formed during finishing) and debris. Other recommendat ions and conclusions can be divided into two categories, as outlined in Table 2.1 1 The first is recommendations regarding surface profiling, which is the intentional roughening of the substrate surface to provide superior mechanical interlock Much research has examined the effects of bruising, which is the introduction of microfractures into the surface of the substrate during profiling tha t may weake n the bond strength. The second category concerns pretreatments, which are substances introduced into the substrate prior to concrete placement to enhance the strength of the bond. This study is primarily concerned w ith the second category

PAGE 15

4 Table 2.1 1 Common examples of surface profiling and pretreatments Examples of Surface Profiling: Examples of Pretreatment: Sand/shot blasting Prewetting/moistening Impact hammering Cement bonding agent Acid etch ing Latex bonding agent Water jetting Epoxy bonding agent 2.2 Mechanism of Concrete to Concrete Bond Bond strength is generally measured as either 1) the adhesion or tensile strength of the bond, or 2) the shear strength of the bond ed interface Assuming the bond strength is less than the tensile strength of the concrete, the bond strength will be governed by the failure mode of tensile cracking along the interfacial surface. The failure mode of the bond in shear is more complex, particularly for roughened surfaces. Subjected to shear, mechanical interlock between the two surfaces will contribute to the shear resistance; the failure mode will be a combination of shear and tensile cracking (Austin, S., et al., 1999) 2.3 Methods of Testing Bond Strength Numerous tests are available to measure bond strength both in tension and in shear As Austin, et al. (1999) n t he selection of test should ideally involve a stress state to which the repaired member will be subjected to during service (Wal ls & Shrive, 1988) The test methods selected and discussed herein as summarized on Table 2.3 1 subject the bonded interface to a variety of str ess conditions, including tension, shear, and a combination of compression and shear.

PAGE 16

5 Table 2.3 1 Summary of common tests for bond strength Test: Test Location: Type: Pull off In Place Direct Tension Guillotine Laboratory Direct Shear Slant Shear Laboratory Combination of Compression & Shear Jacking In Place Direct Shear 2.3.1 Pull off test (ASTM C1583) The pull off test is a direct tensi on test suitable for testing in place bond strength The test has been in use in a variety of forms since before 1990 (Hindo, 1990) In 2004, the test was given the designation ASTM C1583 (2015) y used testing procedure for bond strength However, its accuracy has been repeatedly called into question, with one study for the U.S. (Vaysburd & McDonald, 1999) The procedure begins by coring through the ove rlay and partially into the substrate. Vaysburd and McDonald (1999) found that the depth of the core into the substrate significantly a ffected the strength results They recommended a minimum penetration into the substrate of 25 mm ( 1 in ). ASTM C1583, conversely specifies a minimum depth into the substrate of 10 mm ( 0.5 in ) After the core has been prepared and cleaned, a stainless steel puck is epoxied to the surface of the core and the epoxy is allowed to cure. Once epoxy curing is complete, the apparatus is placed above the core ( Figure 2 1 ) The specimen is loaded in tension by means of a threaded rod inserted into the top of the steel puck. The ASTM C1583 requires a constant loading rate of 35 15 kPa/s ( 5 2 psi/s ); it ha s been found that higher rates of loading correspond to higher bond strength results (Vaysburd & McDonald, 1999) The test must be performed with the apparatus set as near perpendicular to the bonding surface as is practical to minimize uni ntended eccentricity in the applied load.

PAGE 17

6 Figure 2 1 Example pull off test (ASTM C1583) apparatus The test may result in one of several possible failure mechanisms. The partial core may fail entirely above or below the bonded surface, indicating that the strength of bond exceeds the tensile strength of the new or old concrete. Clearly, the observed r esult in such case represents the tensi le strength of the failed concrete and does not represent a bond strength Another possible mode consists of failure of the epoxy which secures the stainless steel puck to the overlay surface. The se failure modes indicate only that the bond strength exceed s the test result; they do not provide an actual value for bond strength The pull off test will provide a representative bond strength only when failure occurs at or very near to the bond surfa ce. For this reason, ASTM C1583 cautions that results may be a veraged together only if they exhibit the same failure mode. The pull off test measures adhesion at the bonded surface, which is typically considerably lower than the shear strength of the bonded interface A factor is required to convert the value determi ned with the pull off apparatus to a value for shear strength. Rosen (2016) referencing a 2000 study by Delatte, et al., multiplied the measured pull off strengths by 2.04 to estimate shear strength

PAGE 18

7 2.3.2 Direct shear (guillotine) test Although there is no ASTM standard for the direct shear test, several are known to be in use in locations throughout the United States. This test is similar to an after Iowa Test Method 406 C (2000) which is referenced in ACI 325 06 Brookhaven (Illinois Bureau of Materials and Physical Research, 2012) It differs from the aforementi oned single shear tests in that the shearing action is applied at two locations (double shear) on the sample: the first is at the bond ed interface where failure will occur, and the second is about 76 mm ( 3 in ) away from the interface and is present only to stabilize the specimen during testing The apparatus is pictured in Figure 2 2 and its usage is described in detail, below: Figure 2 2 Direct shear test in progress The test apparatus consists of a set of nested boxes, calle d a guillotine. Full depth core samples are taken perpendicular to the bond surface and transferred to the laborato ry. After drying, the cores are placed in the guillotine with the bond plane centered between the edges of the nested boxes. The apparatus is compressed, which induces shear on the bond plane until failure occurs. Test 406 C recommends a loading rate in th e range of 45 to 60 kPa/s ( 400 to 500 psi/min ).

PAGE 19

8 2.3.3 Slant shear test Slant shear tests are commonly used by manufacturers for testing the performance of proprietary bonding agents (Austin, S., et al., 1999) ; this test has been formally adopted as ASTM Resin Systems used with Concrete in This test was first known as the Arizona Slant Shear test (Austin, S., et al., 1999) (Kriegh, 1976) In a typical laboratory testing scenario, concrete is placed in a cylinder mold with a plate installed that forms the interior face at a 60 degree angle. The plate is removed once the concrete has cured, and the bonding surface i the mold. The resulting specimen is then compressed to its ultimate strength ( Figure 2 3 ). Figure 2 3 Preparing to test slant shear sample on compression machine. Note difference in concrete color indicating the slanting bond surface Multiple studies conclude that slant shear tests are among the most sensitive to the type and proportions of the materials used to create the bond surface Kriegh (1976) However, the test is also extremely sensitive to t he roughness or profiling of the substrate. Austin et al (1999) obtained bond failure solely with specimens prepared with relatively smooth substrates. In his research, he noted that several roughened specimens f ailed in compression instead of shear failure at the bonded surface. However, other researchers, such as Rosen (2016) have obtained good results for roughened surfaces.

PAGE 20

9 The slant shear test exerts a combination of compression and shear on the bonded surface resulting from the angle of inclination of the surface. The compression force can be resolved into two components: a compression stress N normal to the bond surface, known as clamping force, and shearing stress NT parallel to the bond surface ( Figure 2 4 ) Figure 2 4 Diagram showing (1) the slant shear specimen under uniaxial compression stress and (2) the resulting stresses on the bond surface Appendix E contains additional information related to the slant shear test, including a discussion 2.3.4 Jacking t est The jacking test is a direct shear test used for in place testing. The procedure involves sawcutting the overlay into smaller block specimens. A hydraulic jack is installed adjacent to the blocks and is secured to the substrate. Ideally, the jack shoul d be oriented such that the shearing force is applied as closely as possible to the surface of the substrate, so as to minimize the overturning moment component of the force (Rosen, 2016) A steel plate can be inserted between t he ram and the block to distribute the shearing force evenly across the face of the block. Shear is applied ( Figure 2 5 ) until the sample fails and th e maximum applied force is recorded (1) (2)

PAGE 21

10 2.4 Pretreatments Pretreatments, as the name implies, are basic additions to the smooth or profiled substrate intended to increase the strength an d durability of the concrete to concrete bond. The addition of added to the surface of the substrate prior to concrete placement is referred to as a bonding agent. Both treatments have been in common use for over a century. Paragraphs, below, des cribe the intended benefit of pretreatments. Figure 2 5 Block specimen undergoing jacking test 2.4.1 Prewetting The surface of a dry substrate, particularly one that has been made more porous by the removal of laitance during profiling, has a relatively high moisture demand. The effect of prewetting the substrate concrete fills the existing capillaries that would ot herwise tend to draw water out of the new overlay concrete. This may result in a condition wherein not enough free water is present to fully hydrate the overlay cement; this may reduce concrete strength at the interface. The ter m Saturated Surface Dry (SSD ) is often used to describe a condition where the continual wetting of the substrate in the period leading up to an overlay fills the pores of the o ld concrete. The surface is allowed to dry

PAGE 22

11 prior to placement so as not to weaken the ne w concrete by increa sing the water/cement ratio at the bond interface 2.4.2 Bonding a gents The earliest bonding agents in common use consisted o f a cement water slurry (cement neat) or a cement sand water slurry. These agents are still commonly specified as a means to achieve a mo re durable bond. Other bonding agent products, such as latex modified grout or epoxy grout, have been introduced more recently In any case, the agent is typically scrubbed into the surface of the substrate immediately before overlay concrete is placed. Th e mechanism by which a bonding agent enhances concrete to concrete bond is not entirely clear. It may be that the action of scrubbing the agent into the substrate coats assimilates dust particles that were not removed by cleaning (Silfwerbrand & Paulsson, 1998) 2.5 Literature Review Existing s tudies containing conclusions or recommendations regarding pretreatments were identified during the literature review phase. The execution and results of these studies are described in chronological order in the paragraphs below. Codified recommendations regarding pretreatments are also discussed. The summary section tabulates and compares nearly a century of research into pretreatments. 2.5.1 Early s tudies In 1919, W. E. Rosengarten, a re searcher with the Bureau of Public Roads (a forerunner of the Federal Highway Administration (FHWA)) published his findings on the strength of concrete to concrete bondings. Though concrete pavement had been used as early as 1896 in the United States (Pasko, 1997) it was not in common use until after 1910. When Rosengarten conducted his research, concrete pavement was entering a decade of service and much of it likely needed rehabilitation. Rosengarten prepared some of his (Rosengarten, 1919) He also wet the substrate of some of his samples prior to bonding the overlay. The impact on the strength of the bond was evaluated u sing both direct tension and direct

PAGE 23

12 shear tests, as well as a flexure test. Rosengarten found that the bonding agent added 25 percent to the tensile strength of the bond and also benefited the strength in shear His results on prewetting which he notes w as common practice at the time, were inconclusive for tension. Rosengarten did see an increase in shear strength due to prewetting. In 1956, Felt conducted his now widely cited research with the goal of identifying the factors which ma ximized the shear st r ength, and therefore the durability, of an overlay (Felt, 1956) Along with other variables, Felt investigated the effects of prewetting and the use of bonding agent He began by evaluating small ( 240 x 240 x 84 mm ) (9.5 x 9.5 x 7 in) bonded prisms to guide his selection of pretreatments for the remainder of the study He followed this with much larger (400 x 1020 x 84 mm) (16 x 40 x 7 in) slabs, some of which were laboratory cast and others in which the substrate was cut f rom pavement that had been in service for several decades. Felt tested his samples using a single shear, guillotine type apparatus He concluded that a dry substrate was preferable to sand slurry bonding agent produced a supe rior bond. Noting the contemporary researchers. The difficulty in isolating the variable s that have the principle effect on bond strength may explain the conflicting conclusions of the more modern literature discussed below. 2.5.2 Modern s tudies Wall and Shrive (1988) conducte d a study that included finite element modeling of an interfacial bond in conjunction with laboratory experimentation on 112 prismatic samples using a combined shear/compression test. Their FEM model indicated that consistency of the bonding agent was cri tical ; a void in the bonding material was found to significantly increase the stress in the adjacent areas of the bond material Based on laboratory experimentation, they found that superior bond strength can be achieved with or without a bonding agent; ho wever, their results in dicate less scatter in the data when a bonding agent was employed an effect first noted by Felt In disagreement

PAGE 24

13 substrate. In 1991, Saucier and Pigeon presented the results of a study based on the results of combined shear/compression tests on over 2,000 bonded concrete prisms. The prisms themselves were relatively small cubes with dimensions of 75 x 75 x 75 mm (3 x 3 x 3 in) and were used in conjunction with a coarse aggregate size of 12 mm (0.5 in) (Saucier & Pigeon, 1991) The study results generally agreed and prewetting the substrate does not improve bond strength Saucier and Pigeon also experimented with allowing some drying of the bonding agent. Felt had recommended that the bonding agent be allowed to dry slightly on the substrate before placement of (Felt, 1956) Saucier and Pigeon allowed the bonding agent to dry for 45 minutes on some of their specimens prior t o placing the overlay. They noted the dried agent caused a slight increase in bond strength when applied to an SSD sub strate, but caused a decrease in bond strength when applied to a dry substrate. In a large test involving over 150 substrate slabs with d imensions of 915 x 915 x 130 mm ( 36 x 36 x 5 in ), Whitney et al. (1992) found that the majority of debonding between substrate and overlay occurs during the early curing of the overlay. The study found that good bond strength could be achieved through the application of epoxy bonding agents, particularly in harsh environmental conditions. They noted that the application rate of the bonding agent did not seem to greatly affect the strength of the bond. The study also fo und that both high substrate surface temperatures before placement and large changes in ambient temperature in the 24 hours following an overlay adversely affected the measured bond strength In a 1998 article focusing on the rehabilitation of bridge dec ks in Sweden, Silfwerbrand and Paulsson advise d against the use of bonding agents. They note that the use of a bonding agent creates two possible planes of weakness between the overlay and substrate (Silfwerbrand & Paulsson, 1998 )

PAGE 25

14 Wells and Stark found that substrate surfaces prepared by shotblasting produced consisten tly good bond strength s without the need for a bonding agent However, they did find that bonding a gents did have a positive effect on slabs prepared using oth er surface profiling methods (Wells & Stark, 1999) Djazmati and Pincheria (2004) conducted a study on the effects of surface profiling and pretreatment at the interfacial bond between successive concrete placements (cold joints). In contrast to much of the literature on this topic, Djazmati and Pincheria examined bonding to recently placed substrate (as little as 24 hours old) to simulate cold joints formed during construction. They found that a joint that had been saturated with water was about half as strong as a dry joint. They recommend ed moist curing the joint for a minimum of 24 hours prior to placement, but cau tion ed that the joint surface should appear dry before commencing with the second placement. Djazmati and Pincheria also studied the effect of a resin emulsion bonding agent on a smooth joint. They found that the resin emulsion bonding a gent, being substan tially less stiff than concrete, caused the resulting joint to be much more flexible than monolithic concrete Therefore, they recommend against the use of a resin emulsion bonding agent at cold joints. In a report for the Bureau of Reclamation, Bissonnet te et al. (2012) concluded that moistening of the substrate is beneficial, and that optimum saturation in the substrate surface l ies somewhere between 55 and 90 unsolved Bissonnette recommend ed against the use of bonding agents for reasons similar to those expressed by Silfwerbrand and Paulsson (1990) Julio, Branco, and Silva (2004) con clude d that pre wetting the substrate does not significantly influence the bond strength 2.5.3 Code r eferences Both ACI 325 (2006) and ACI 345 (2011) express concern over the effectiveness of bonding agents and the potential for debonding if the bonding agent is improperly applied The Portland Cement Association (1996) recom sand

PAGE 26

15 slurry be scrubbed into the substrate surface prior to placement of bonded overlays. Meanwhile, the National Concrete Pavement Technology Center states that bonding agents are not required fo r concrete to concrete pavement overlays (Harrington, 2008) 2.5.4 Previous r esearch at the University of Colorado Denver This study builds on the findings of a previous program of research performed at the University of Colorado Denver by Christian Rosen (2016) effects of various substrate surface profile s ranging from rou gh to smooth on the shear strength of bonded overlays He found that surface ro ughness had a significant effect on shear strength, with the roughest surface preparat ion producing the highest strengths Rosen also examined the effects of the compaction of concrete on interfacial shear strength; he found that proper compaction of the overlay significantly increased shear strength 2.5.5 Summary of literature r eview It is apparent when reviewing the literature that the effectiveness of pretreatments remains the subject of deba te. Table 2.5 1 summarize s the findings of twelve previously cited studies with regards to p retreatments. It should be acknowledged that this table greatly simplifies the results and conclusions of these studies for the purposes of comparison. Immediately obvious when reviewing this table is the lack of any clear trends that might tend to guide the practicing engineer. As Talbot et al. (1994) the conclusions obtained by various investigators are unfortunately often Environmental factors, such as humidity, ambient temperature, and rate of evaporation may s ignificantly affect the need for prewetting. The degree of preparation of the substrate, including profiling and cleaning, may determine the effectiveness of a bonding agent. These factors are not easily controlled and are often difficult to measure, even in a laboratory environment.

PAGE 27

16 Table 2.5 1 Summary of results regarding p retreatments Ye ar of Article/Report Author Results Regarding Pretreatments: : increase in strength/durability : decrease in strength/durability : inconclusive ) Prewetting Bonding Agent 1919 Rosengarten +/ + 1956 Felt + 1988 Wall and Shrive + + / 1991 Saucier and Pigeon + 1992 Whitney, et al. N/A + 1998 Silfwerbrand and Paulsson N/A 1999 Wells and Stark N/A +/ 2004 Djazmati and Pincheria +/ 2004 Julio, Branco, and Silva +/ N/A 2006 ACI 325 +/ +/ 2011 ACI 345 + +/ 2012 Bissonnette, et al. +

PAGE 28

17 CHAPTER III RESEARCH PROGRAM 3.1 Construction of Substrate and Overlay Slabs The research program evaluated the bond strength of concrete overlays using 6 different combinations of pretreatments. In total, three substrate ( 138 x 147 x 8.3 cm ) (54 x 56 x 3 in) ( Figure 3 1 ) and six overlay (91 x 61 x 8.9 cm) (36 x 24 x 3 in) slabs were cast using a commercially available sack concrete mix with a design 28 day strength of 34.5 MPa (5,000 psi ) The maximum coarse aggregate size for the sack concrete was 9.5 m m (3/8 in ). The design slump was approxim ately 76 m m (3.0 in) Approximately 0.62 cubic meters (22 cubic feet ) and 0.28 cubic meters (10 cubic feet) of concrete was placed for the substrate and overlay slabs, respectively. The concrete was mixed in small, 0.08 cubic meter ( 3 cubic foot) batches Seven batches were necessary to construct the substrates; six were needed to construct the overlay s Figure 3 1 Typical substrate form, ready to receive concrete The substrate slabs were cast outdoors and covered with a tarp for the initial curing period. Cylinders taken during concrete p lacement were tested for compressive strength at 3, 7 and 28 days to establish the maturity curve for the mix. The average 28 day compressive strength for the substrate was 41.5 MPa ( 6,010 psi ) with a relatively low standard deviation of 1.9 MPa ( 269 psi ). The average 28 day compre ssive strength for the overlay was 38.3 MPa ( 5,558 psi ) with a standard deviation of

PAGE 29

18 9.10 M Pa ( 1320 psi ). All slabs were compacted with an immersion vibrator during placement; proper vibration has been shown to increase the shear strength at the bonded in terface (Rosen, 2016) Both the substrate and the overlay slabs were unreinforced with the exception of #3 hairpin bars placed as anchor reinforcement for the bolts used in the jacking test. The hairpins were located entirely wi thin the substrate slab with approximately 1 in (25 mm) clear coverage to the surface of the substrate. It was decided that overlay placement would occur no sooner than 28 days following the placement of the substrate. This wait was imposed to allow the su bstrate 1) to gain strength such that the possibility of fracturing during surface profiling was minimized, and 2) to accomplish initial shrinkage prior to the placement of the overlay, to best simulate the differential shrinkage that typically occurs when an overlay is placed on existing concrete. Surface preparation commenced exactly 28 days following the substrate placement. Laitance was removed by means of a bush hammer attachment on a small, handheld demolition hammer ( Figure 3 2 ) As shown in Figure 3 3 t he prepared surface roughness was similar to Concrete Sur face Profile 6 (CSP 6), medium scarification as depicted in ICRI Guideline No. 310.2R (International Concrete Repair Institute, 2013) The surfaces were then thoroughly cleaned using compre ssed air followed by vacuuming. Figure 3 2 Surface profiling the substrate

PAGE 30

19 Figure 3 3 Finished profile compared with ICRI CSP 6 example Following surfa ce profiling and cleaning, the six bonding surfaces received different pretreatments, identified in Table 3.1 1 Table 3.1 1 P retreatmen t summary Overlay Slab No. : Moisture Condition of S ubstrate : Bonding Agent : 1A Dry None 1B Dry Wet Cement Slurry 2A SSD Wet Cement Slurry 2B SSD Dried Cement Slurry 3A Saturated with standing puddles Wet Cement Slurry 3B SSD None Note: SSD = were not permitted to come into contact with water for several days prior to placement of the overlay. Saturated Surface Dry (SSD) slabs ( Figure 3 4 ) were repeatedly moistened for a period of about one hour, then any standing or free water on the surface was allowed to evaporate prior to overlay placement. As the name implies, SSD refers to a condition wherein the pores of the existing concrete are filled with water, but excess moisture on the concrete surface has evaporated Slab 3A was initially prepared similar ly to the SSD slabs, but puddles of standing water

PAGE 31

20 were allowed to remain on the surface during overlay placement. All these surface conditions were intended to envelope possible field conditions. Figure 3 4 Example of saturated, surface d ry appearance immediately before placement Figure 3 5 Application of cement slurry bonding agent

PAGE 32

21 The selected bonding agent was a cement and water slurry with a w/c ratio of 0.50. Previous research including research referenced in ACI 325 (2006) and Saucier and Pigeon (1991) indicates that a bonding agent with a w/c ratio in excess of 0.60 could significantly weaken the bond. The b onding agent was scrubbed into the substrate with a stiff bristle brush ( Figure 3 5 ) as recommended by Wells and Stark (1999) In all but one case the overlay was placed on the bonding agent immediately, prior to any drying or dulling of the slurry. In the case of Slab 2B, the slurry was allowed to s it for a period of several days, such that it was fully dry at the time of overlay placement. As a means of reducing the amount of formwork needed, two overlay slabs were placed on a common sub strate slab. The bleed over of moisture during pretreatment from the adjacent bonding surface was considered in the experiment layout. Pretreatments involving dry substrates were grouped together on Slab 1, while SSD or wet substrates were grouped together on Slab 2 and Slab 3. Slabs 1A, 1B, 2A, and 3A were placed when the substrate was 30 days old. A sudden rainstorm prevented the completion of the remaining overlays, which were placed five days later (substrate age of 35 days). The overlays slabs were formed from the same concrete mix as the substrate but were treated with a commercially available colorant to help in the ident ification of the bonding surface An overall plan showing the relative arrangement of the substrate and overlay pads is presente d in Figure 3 6 An isometric view of a typical substrate slab and two overlay pads is presented in Figure 3 7 Th is figure shows the typical slab dimensions and the approximate locations where the various test samples were taken. The testing is discussed in detail in Chapter 3.2 Figure 3 8 and Figure 3 9 illustrate the as constructed condition of the slabs.

PAGE 33

22 Figure 3 6 Plan view of slabs and pretreatments Figure 3 7 Isometric rendering of a typical substrate slab with two overlay slabs 1A 1B 2A 2 B 3A 3 B Slab 1 Slab 2 S lab 3 Slab 1A: Dry Substrate/No Bonding Agent Slab 1B: Dry Substrate/Wet Bonding Agent S lab 2 A: SSD Substrate/Wet Bonding Agent Slab 2 B: SSD Substrate/ Dried Bonding Agent S lab 3 A: Wet Substrate/Wet Bonding Agent Slab 3 B: SSD Substrate/No Bonding Agent Slant shear tests at 45 Hydraulic jack and block samples Substrate Overlay Pull off test (partial depth) Direct shear test (full depth)

PAGE 34

23 Figure 3 8 As constructed view showing pull off test sampling locations. Figure 3 9 As constructed view showing pull off, direct shear, slant shear, and jacking test sampling locations. Background to foreground: Slab 1, 2, and 3

PAGE 35

24 3.2 Testing All testing was performed in the Civil Engineering Lab oratory at the U niversity of Colorado Denver Testing of the compressive strength of the substrate and overlay concrete was accomplished between 3 and 28 days after placement of the concrete. Testing of the bond strength was conducted in the order shown on Table 3.2 1 The pull off tests were conducted 40 days after the final overlay slab was constructed Direct shear tests were conducted next, 65 days after the final overlay slab was const ructed. Slant shear tests and jacking tests were conducted 80 and 90 days after the final overlay slab was constructed respectively. Table 3.2 1 Testing Summary Test: Test Location: Type: No. of Samples Test ed: Standard (if applicable): Compression Laboratory Compression 3 per batch ( 36 total) ASTM C39 Pull off In Place Direct Tension 3 per slab (18 total) ASTM C1583 Direct Shear (Guillotine) Laboratory Direct Shear 3 per slab (18 total) N/A Slant Shear Laboratory Combination of Compression & Shear 4 per slab (24 total) ASTM C882 Jacking In Place Direct Shear 4 per slab (24 total) N/A Total 120 tests 3.2.1 Slump t est Slump was measured using a standard slump cone ( Figure 3 10 ) test and was conducted for seven o f the thirteen concrete batches. The design slump for the sack concrete product used in the testing program is 50 76 mm ( 2 3 in ). Slump re sults are tabulated in Section 4.1

PAGE 36

25 Figure 3 10 Example slump cone test on overlay concrete (note integral color) 3.2.2 Slab c oncrete compression t est Samples were taken during placement of the substrate and overlay slabs in 102 (diameter) x 203 mm ( 4 x 8 in ) plastic cylinder molds. Three sa mples were t aken from twelve of the thirteen total batches for a total of 36 specimens. A single cylinder from each batch was tested in compression at 3 7 and 28 days after placement. Testing was performed on a Forney compression testing machine ( Figure 3 11 ) equ ipped with an Admet data logger. The cylinders were capped prior to testing with reusable neoprene rubber pads surrounded by a steel extrusion controll er. The Forney machine was not equipped with a displacement sensor, therefore, only the compression load at failure was recorded. Load rate was controlled using a hand wheel and was adjusted so as to maintain the rate prescribed in ASTM C39 (2004) of 0.25 0.05 MPa/s ( 35 7psi/s ). For a 4 in (102 mm) standard cylinder, the rate of load application is 440 88 lbs/s. Compression stress at failure was determined from the applied load at failure using E quation ( 3 1 ) Results from the compression strength tests can be found in Chapter 4.2

PAGE 37

26 ( 3 1 ) Where: = compressive strength ( MPa or psi) P u = compressive force at failure ( kN or lbs) A g = gross area of the sample ( mm 2 or in 2 ) Figure 3 11 Compression Testing Equipment 3.2.3 Pull off t est Samples were prepared using a 66.7 mm ( 2 5/8 in ) inner diameter coring bit with a 406 mm ( 16 in ) coring depth capacity mounted on a wet core drill A guide was placed on the bit such that the core would penetrate approximately 1.3 cm ( 0.5 in ) into the substrate slab. T he cores were taken as close to perpendicular to the bond plane as possible to minimize the eccentricity of the applied test load. Once coring was completed, the annular space was rin sed thoroughly with water to remove dust and debris. The surface of the overlay at the core locati on was treated with full strength Muratic acid

PAGE 38

27 to remove laitance, then thoroughly rinsed with water After waiting for the surface to dry, any remaining dust was blown off with compressed air ( Figure 3 12 ). 76.2 mm (3 in) diameter s tainless steel pucks were epoxied onto the top of the partial cores and the epoxy was left to cure for a period of several days. Figure 3 12 Cleaning the surface of the pull off test specimen Tensile strength o f the bond was tested using Non Destructive Testing Systems (NDT) 007 James Bond Tester furnished by CTL Thompson ( Figure 3 13 ) T he location of the failure plane and the maximum tensile force was recorded for each test. Figure 3 13 NDT 007 James Bond Tester

PAGE 39

28 In accordance with the testing frequency recommendations of Part 8 (Sampling) of ASTM C1583 three tests were performed on each of the six overlay slabs. In 13 of the 18 tests, failure occur red at the bond line. The remaining tests failed on the surface of the substrate. Several tests were aborted due to failure of the epoxy bonding the stainless steel puck to the overlay. In these cases, the puck was rebonded to the specimen and the test was resumed at a later date The bond strength is calculated from the maximum tensile load using Equation ( 3 2 ) ; the relationship is similar to that used in the compression test. Results from the pull off tests can be found in Chapter 4.5 ( 3 2 ) Where: = bond strength ( kPa or psi) P u = tensile force at failure ( k N or lbs) A g = gross area of the specimen ( mm 2 or in 2 ) 3.2.4 Direct s hear (g uillotine) t est A guillotine box apparatus was furnished by CTL Thompson, Inc. Full depth core samples were taken using a 66.7 mm ( 2 5/8 in ) inner diameter coring bit mounted to a wet core d rill. The specimens were placed in the guillotine with the bond plane centered between the edges of th e neste d bo xes ( Figure 3 15 ) such that approximately 3.2 mm ( 1/8 in ) of gap was observed between the inner and outer box walls. The apparatus was compressed, which induced shear on the bond plane until failure occurred The testing was performed using a n 89.0 kN ( 20,000 lbs ) MTS compression testing machine with displacement control ( Figure 3 14 ) The loading rate was set at 0.5 mm/min ( 0.02 in/min ). The shear strength at the bonded interface is calculated fr om the applied force in Equation ( 3 3 ) Dividing the applied load by two is necessary because the sh earing action is imposed equally on each leg of the box, though failure was found to occu r only on the bonded interface.

PAGE 40

29 Figure 3 14 MTS testing equipment with direct shear apparatus It may be argued that the eccentricity in applied load resulting from the gap between the inner and outer walls of the guillotine box induces a bending moment on the specimen (and thus the bonded interface is not in pure shear). While the influence of moment is not easily avoi dable, the construction of the apparatus with the narrow gap between boxes minimizes the effect. ( 3 3 ) Where: = shear streng th at bonded interface (kPa or psi ) P u = compressive load at failure ( kN or lbs) A g = gross area of the specimen ( mm 2 or in 2 ) Although each leg of the box is profiled so as to cradle the specimen uniformly, in practice it was observed that some localized crushing of concrete during the early stages of loading was ress strain plot indicates a relatively linear relationship until failure occurs. The author understands that cast plaster caps around the sample and guillotine apparatus are sometimes used to fill the annular space such that the loading is applied more uniformly. While the benefits of this approach are undeniable, it is difficult to

PAGE 41

30 implement capping when testing a large number of samples due to the time involved. For these experiments, the samples were not capped. Results from the direct shear tests c an be found in Chapter 4.6 Figure 3 15 Direct Shear (Guillotine) Apparatus 3.2.5 Slant shear t est As noted in Chapter 2.3.3 slant shear specimens are typically prepared using a cylindrical mold with a removable plate to form the slanted interface Although this is a convenient method to produce many specimens, it was not the preferred method fo r this study For this study, it was decided that slant shear specimens would be cored directly from the slabs, such that the surface profiling and pretreatment would be identical across all four types of tests. This necessitated coring samples on an angle and then sawing the ends perpendicular to the axis of the core. Figure 3 16 defines how the slant angle was measured in this study Full depth core samples were taken at 45 degrees from normal ( Figure 3 17 ) using a 66.7 mm ( 2 5/8 in ) inner diameter coring bit mounted to a wet core drill. The slant angle was selected bas ed on : 1) the maximum slant capability of the core drill stand used in the experimentation and 2) the ability to obtain more samples than would have been possible had cores been attempted at a shallower angle (due to space limitations) The ends of the sa mples were sawed perpendicular, and then allowed to dry for a minimum of 5 days in accordance with ASTM C42 (2004) The samples were then

PAGE 42

31 compressed to failure on a 1000 kN ( 220,000 lbs ) MTS compression testing machine w ith displacement control. The loading rate was set at 0.10 mm/min ( 0.04 in/minute ). Figure 3 16 uniaxial compression, (2) the stresses on the bond surface, and (3) the forces on the bond surface Figure 3 17 Checking the slant angle with an inclinometer prior to coring for a slant shear specimen (1) (2) (3)

PAGE 43

32 The principle stresses at the bonded interface were determined using a 2 dimensional plane stress transformation. The normal or clampin g stress is given by Equation ( 3 4 ) while the shearing stress at the interface is given by Equation ( 3 5 ) These variab les are depicted in Figure 3 16 ( 3 4 ) ( 3 5 ) Where: N = clamping force at failure ( kPa or psi) NT = shear stress at failure ( kPa or psi ) P u = compressive load at failure ( kN or lbs) A g = gross area of the specimen ( mm 2 or in 2 ) = slant angle (degrees from horizontal) This relationship can also be expressed in terms of forces acting on the bond surface A surface Equation ( 3 6 ) gives the normal stress in terms of the clamping force, while Equation ( 3 7 ) gives the shearing stress in terms of the shear force. ( 3 6 ) ( 3 7 ) Where: N = clamping force at failure (kPa or psi) NT = shear stress at failure (kPa or psi) N = clamping force on bonded interface (kN or lbs) V = shear force at bonded interface (kN or lbs) A surface = area of bonded interface (mm 2 or in 2 ) This transformation can also be expressed graphically as shown in Figure 3 18 Under uniaxial s the other quadrant is located at the maximum principle stress. The figure illustrates the computation of the normal and shear stresses on a 45 degree and 60 degree slant under a hypothe tical uniaxial

PAGE 44

33 compressive stress of 1 .0 ksi (0.145 MPa) At 45 degrees, the normal and shear stresses are equal. At 60 degrees, the normal stress is significantly reduced. Refer to Appendix E for a discussion of how the effect of friction significantly a f fects the observed shear resistance at the bond ed interface. Of the 24 slant shear specimens tested, none were fou nd to have failed in shear at the bond ed interface as anticipated. All samples failed in splitting tension in a manner similar to typical compressive cylinder tests. Figure 3 19 and Figure 3 20 illustrate the typical failure condition observed It appears that the clamping force on the roughened bond plane was sufficient to resist th e applied shear force on the 45 degree bond plane. Results from the slant shear tests can be found in Chapter 4.7 Figure 3 18 60 degrees (Sign convention for : + tension / compression) (Sign convention for : + CW / CCW rotation)

PAGE 45

34 Figure 3 19 Representative slant shear cylinder after testing under uniaxial compression Figure 3 20 Same specimen as Figure 3 19 opened to reveal surface of splitting tension failure Note colored concrete is overlay; gray concrete is substrate 3.2.6 Jacking t est Sample blocks were prepared using a 350 mm ( 14 in ) dry concrete saw with an adjustable shoe to set the depth of c ut ( Figure 3 21 ) The saw was connected to a wet/dry vacuum to minimize the dust generated by this operation. The b lock s were cut to a preferred size of 152 x 152 mm ( 6 x 6

PAGE 46

35 in ) where possible; however, clearance between the test slabs and a n adjacent wall necessitated The saw was adjusted such that the depth of the sawcut extended approximately 13 mm (0.5 in ) into th e substrate slab. Figure 3 21 Concrete saw and dust collection system A Simplex RC306C hydraulic jack with a 300 kN ( 30 ton ) capacity was installed adjacent to the blocks. During casting of the substrate, a 25 mm ( 1.0 in ) step had been formed into the surface to accommodate the jack body. This allowed the piston to exert load on the block as close to the bond surface as possible, thereby minimizing overturning moment resulting from eccentric application of the load A steel plate with dimensions of 89 x 89 x 25 mm ( 3 x 3 x 1 in ) was placed between the piston and the block to evenly distribute the test load.

PAGE 47

36 Figure 3 22 Simplex RC306C hydraulic test ing specimen on Slab 2A. On right 1 in (25 mm) steel plate; on left brace angle bolted to substrate The samples were tested to failure and the maximum force in the jack was recorded. The dimensions of the blocks were recorded by measuring the dimensions of the bonded interface after failure, for increased accuracy. The shear stress at the bonded interface is given by Equation ( 3 8 ) ( 3 8 ) Where: = shear stress at failure ( kN or lbs) p u = recorded press ure in jack at failure (kPa or psi ) A c = area of the jack cylinder ; 4,150 mm 2 ( 6.44 in 2 ) for the Simplex RC306C L = length of the block specimen ( mm or in) W = width of the block specimen ( mm or in)

PAGE 48

37 CHAPTER IV R ESULTS 4.1 Slump Cone Results Slum p was tested for 7 of the 13 total concrete batches used in the test program. Table 4.1 1 lists the measured slump (a blank entry indicates no measurement was taken). Table 4.1 1 Results of slump cone test Batch No. Slab ID Slump mm in Substrate Placement (April 7, 2016) 1 Substrate Slab 1 76 3.00 2 Substrate Slab 1 95 3.75 3 Substrate Slab 2 102 4.00 4 Substrate Slab 2 5 Substrate Slab 2 178 7.00 6 Substrate Slab 3 7 Substrate Slab 3 Overlay Placement 1 (May 7, 2016) 1 Overlay Slab 1A 76 3.00 2 Overlay Slab 1B 3 Overlay Slab 2A 25 1.00 4 Overlay Slab 3A Overlay Placement 2 (May 12, 2016) 1 Overlay Slab 2B 44 1.75 2 Overlay Slab 3B 4.2 Sack Concrete Sieve Analysis A particle size distribution analysis was performed to determine the gradation of the aggregates within the proprietary sack concrete mix used in the research program. A representative sample was taken and tested on a laboratory sieve shaker. The analysis indicates that the proprietary concret e mix uses a coarse aggregate with a maximum particle size (D 100 ) of 12.7 mm ( 3/8 in ). 4.3 Compression Testing Results 102 x 203 mm (4 x 8 in ) sample cylinders were taken and tested for 12 of the 13 concrete batches used in the test program. Table 4.3 1 lists the maximum compression force recorded by the

PAGE 49

38 Admet data logger for each c ylinder. Table 4.3 2 l ists the corresponding uniaxial compression stress in the cylinder at failure. A blank entry indicates no compression testin g was performed for that batch The two tests taken for each substrate slab were averaged to produce the substrate compressive stren gth, as shown in Table 4.3 3 Table 4.3 1 Compression force at failure Batch No. Slab ID 3 Day 7 Day 28 Day kN kips kN kips kN kips Substrate Placement (April 7, 2016) 1 Substrate Slab 1 125 28.1 243 54.6 333 74.8 2 Substrate Slab 1 187 42.1 262 58.9 348 78.3 3 Substrate Slab 2 187 42.0 272 61.2 324 72.8 4 Substrate Slab 2 5 Substrate Slab 2 187 42.0 272 61.2 311 70.0 6 Substrate Slab 3 207 46.5 304 68.4 346 77.9 7 Substrate Slab 3 212 47.8 303 68.1 354 79.6 Overlay Placement 1 (May 7, 2016) 1 Overlay Slab 1A 171 38.4 220 49.5 333 74.8 2 Overlay Slab 1B 212 47.6 203 45.7 385 86.4 3 Overlay Slab 2A 209 47.0 281 63.1 349 78.5 4 Overlay Slab 3A 227 51.0 237 53.3 368 82.7 Overlay Placement 2 (May 12, 2016) 1 Overlay Slab 2B 107 24.2 139 31.3 259 58.1 2 Overlay Slab 3B 168 37.8 172 38.7 172 38.6

PAGE 50

39 Table 4.3 2 Compression stress at failure Batch No. Slab ID 3 Day 7 Day 28 Day kPa psi kPa psi kPa psi Substra te Placement (April 7, 2016) 1 Substrate Slab 1 15,396 2,233 29,941 4,343 41,046 5,953 2 Substrate Slab 1 23,099 3,350 32,295 4,684 42,961 6,231 3 Substrate Slab 2 23,028 3,340 33,600 4,873 39,959 5,796 4 Substrate Slab 2 5 Substrate Slab 2 23,028 3,340 33,589 4,872 38,396 5,569 6 Substrate Slab 3 25,524 3,702 37,545 5,445 42,725 6,197 7 Substrate Slab 3 26,204 3,801 37,348 5,417 43,679 6,335 Overla y Placement 1 (May 7, 2016) 1 Overlay Slab 1A 21,047 3,053 27,137 3,936 41,031 5,951 2 Overlay Slab 1B 26,106 3,786 25,052 3,634 47,429 6,879 3 Overlay Slab 2A 25,771 3,738 34,643 5,025 43,051 6,244 4 Overlay Slab 3A 27,982 4,058 29,222 4,238 45,388 6,583 Overla y Placement 2 (May 12, 2016) 1 Overlay Slab 2B 13,256 1,923 17,146 2,487 31,888 4,625 2 Overlay Slab 3B 20,712 3,004 21,228 3,079 21,160 3,069 The r esults show that all substrate slabs, as well as the overlay slabs placed on May 7, achieved the design compressive strength of 34.5 MPa ( 5,000 psi ) The compressive strength s of the ov erlay slabs placed on May 12 were lower than the design strength. Table 4.3 3 Average substrate compressive strength Slab ID Substrate (kPa) Substrate (psi) Test 1 Test 2 Average Test 1 Test 2 Average 1 41,046 42,961 42,003 5,953 6,231 6,092 2 39,959 38,396 39,178 5,796 5,569 5,683 3 42,725 43,679 43,203 6,197 6,335 6,266 4.4 Specimen Identifiers Each specimen tested as part of the pull off, direct shear, slant shear, and jacking tests was assigned a sample I.D The first two digits indicate the overlay slab where the specimen originated. The next number indicates the order in which the specimen was obtained, and the last letter indicates the type of test performed.

PAGE 51

40 4.5 Pull off Test Results Three pull off tests were performed on each of the six overlay slabs, for a total of 18 performed as part of the testing program. Table 4.5 1 lists the sample I.D. for each pull off test ; t he ndicates a T ension test I mages of each sample have been included in the Appendix All specimens were observed to fail on or near the bond surface. Thirteen of the specimens failed at the bond surface; the remaining five specimens failed on the surface of the substrate. Table 4.5 1 Pull off test s ample s Slab ID Pretreatments Pull off Test Sample I.D. Test 1 Test 2 Test 3 1A Dry / No Agent 1A1T 1A3T 1A5T 1B Dry / Wet Agent 1B5T 1B7T 1B11T 2A SSD / Wet Agent 2A3T 2A5T 2A7T 2B SSD / Dried Agent 2B1T 2B3T 2B5T 3A Wet / Wet Agent 3A1T 3A3T 3A5T 3B SSD / No Agent 3B1T 3B3T 3B5T The tensile strength at failure is listed on Table 4.5 2 (SI un its) and Table 4.5 3 (U.S. customary units). A statistical analysis was performed on the dataset to compute the mean bond strength, standard deviation, and coefficient of variation. Table 4.5 2 Pull off test results (SI units) Slab ID Pretreatments Tensile Strength (kPa ) Statistical Analysis Test 1 Test 2 Test 3 Average (kPa ) Std. Dev. (kPa ) COV 1A Dry / N o Agent 800 317 1,213 777 366 47.2% 1B Dry / Wet Agent 1,069 1,296 1,048 1,138 112 9.9% 2A SSD / Wet Agent 1,262 1,096 1,055 1,138 89 7.9% 2B SSD / Dried Agent 379 372 124 292 119 40.7% 3A Wet / Wet Agent 1,007 1,296 1,531 1,278 214 16.8% 3B SSD / No Agent 814 1,096 1,758 1,223 396 32.4%

PAGE 52

41 Table 4.5 3 Pull off test results (U.S. customary units) Slab ID Pretreatments Tensile Strength (psi) Statistical Analysis Test 1 Test 2 Test 3 Average (psi) Std. Dev. (psi) COV 1A Dry / No Agent 116 46 176 113 53.1 47.2% 1B Dry / Wet Agent 155 188 152 165 16.3 9.9% 2A SSD / Wet Agent 183 159 153 165 13.0 7.9% 2B SSD / Dried Agent 55 54 18 42.3 17.2 40.7% 3A Wet / Wet Agent 146 188 222 185 31.1 16.8% 3B SSD / No Agent 118 159 255 177 57.4 32.4% Past studies (Vaysburd & McDonald, 1999) indicate that the compressive strength of the c oncrete has a significant effect on the s trength of the interfacial bond. Djazmati, et al. (2004) and Rosen (2016) compensate for unavoidable differences in compress ive strength by d ividing bond strength results by the square root of the compressive strength of the concrete ( c ) Because the observed compressive strengths of the substrate and overlay differ in this study, the minimum compressive strength c(min) w as used for the adjustment Table 4.5 4 (SI units) and Table 4.5 5 (U.S. customary units) list the factored bond strength results for the pull off tests. Table 4.5 4 Pull off test bond strength adjustment (SI units) 28 Day Compressive Strength (kPa ) Avg. Bond Strength in Tension ) Factored Bond Strength in Tension (m in ) Slab ID Pretreatments Substrate Overlay 1A Dry / No Agent 42,003 41,031 777 3.83 1B Dry / Wet Agent 42,003 47,429 1,138 5.55 2A SSD / Wet Agent 39,179 43,051 1,138 5.75 2B SSD / Dried Agent 39,179 31,888 292 1.63 3A Wet / Wet Agent 43,203 45,388 1,278 6.15 3B SSD / No Agent 43,203 21,160 1,223 8.41

PAGE 53

42 Table 4.5 5 Pull off test bond strength adjustment (U.S. customary units) 28 Day Compressive Strength (psi) Avg. Bond Strength in Tension Factored Bond Strength in Tension (m in ) Slab ID Pretreatments Substrate Overlay 1A Dry / No Agent 6092 5951 113 1.46 1B Dry / Wet Agent 6092 6879 165 2.11 2A SSD / Wet Agent 5683 6244 165 2.19 2B SSD / Dried Agent 5683 4625 42 .3 0.62 3A Wet / Wet Agent 6266 6583 185 2.34 3B SSD / No Agent 6266 3069 177 3.20 4.6 Direct Shear Test Results Three direct shear tests were performed on each of the six overlay slabs, for a total of 18 performed as part of the testing program. All specimens were observed to fail on or near the bond surface. Sixteen of the specimens failed at the substrate surface; the remaining specimens (both on Slab 2B) failed on the surface of the overlay. Table 4.6 1 lists the sample I.D. for each direct shear test; G uillotine test. I mages of each sample have been included in the Appendix. Table 4.6 1 Direct shear test sample s Slab ID Pretreatments Direct Shear Test Sample I.D. Test 1 Test 2 Test 3 1A Dry / No Agent 1A2G 1A4G 1A7G 1B Dry / Wet Agent 1B2G 1B8G 1B10G 2A SSD / Wet Agent 2A2G 2A4G 2A6G 2B SSD / Dried Agent 2B2G 2B4G 2B6G 3A Wet / Wet Agent 3A2G 3A4G 3A6G 3B SSD / No Agent 3B2G 3B4G 3B6G The shear s trength at failure is listed on Table 4.6 2 (SI units) and Table 4.6 3 (U.S. customary units) As with the pull off results, a statistical analysi s was performed on the dataset to compute the average shear strength, standard deviation, and coefficient of variation.

PAGE 54

43 Table 4.6 2 Direct shear test results (SI units) Slab ID Pretreatments Shear Strength at Interface (kPa ) Statistical Analysis Test 1 Test 2 Test 3 Average (kPa ) Std. Dev. (kPa ) COV 1A Dry / N o Agent 2,296 2,544 2,062 2,301 197 8.6% 1B Dry / Wet Agent 3,316 3,027 2,916 3,087 169 5.5% 2A SSD / Wet Agent 3,309 3,075 2,365 2,916 402 13.8% 2B SSD / Dried Agent 655 1,496 1,420 1,191 380 31.9% 3A Wet / Wet Agent 3,027 2,330 2,606 2,654 286 10.8% 3B SSD / No Agent 2,675 2,730 3,192 2,866 232 8.1% Table 4.6 3 Direct shear test results (U.S. customary units) Slab ID Pretreatments Shear Strength at Interface (psi) Statistical Analysis Test 1 Test 2 Test 3 Average (psi) Std. Dev. (psi) COV 1A Dry / N o Agent 333 369 299 334 28.6 8.6% 1B Dry / Wet Agent 481 439 423 448 24.5 5.5% 2A SSD / Wet Agent 480 446 343 423 58.2 13.8% 2B SSD / Dried Agent 95 217 206 173 55.1 31.9% 3A Wet / Wet Agent 439 338 378 385 41.5 10.8% 3B SSD / No Agent 388 396 463 416 33.6 8.1% Table 4.6 4 (SI units) and Table 4.6 5 (U.S. customary units) list the factored shear strength results for the dir ect shear tests Reference the pull off test results in Chapter 4.5 for further explanation of the adjustment factor.

PAGE 55

44 Table 4.6 4 Direct shear test strength adjustment (SI units) 28 Day Compressive Strength (kPa ) Avg. Interfacial Shear Strength ( ) (kPa ) Factored Interfacial Shear Strength (m in ) Slab ID Pretreatments Substrate Overlay 1A Dry / No Agent 42,003 41,031 2,301 11.36 1B Dry / Wet Agent 42,003 47,429 3,087 15.06 2A SSD / Wet Agent 39,179 43,051 2,916 14.73 2B SSD / Dried Agent 39,179 31,888 1,191 6.67 3A Wet / Wet Agent 43,203 45,388 2,654 12.77 3B SSD / No Agent 43,203 21,160 2,866 19.70 Table 4.6 5 Direct shear test strength adjustment (U.S. customary units) 28 Day Compressive Strength (psi) Avg. Interfacial Shear Strength ( ) (psi) Factored Interfacial Shear Strength (m in ) Slab ID Pretreatments Substrate Overlay 1A Dry / No Agent 6,092 5,951 334 4.33 1B Dry / Wet Agent 6,092 6,879 448 5.74 2A SSD / Wet Agent 5,683 6,244 423 5.61 2B SSD / Dried Agent 5,683 4,625 173 2.54 3A Wet / Wet Agent 6,266 6,583 385 4.86 3B SSD / No Agent 6,266 3,069 416 7.50 4.7 Slant Shear Test Results Four slant shear samples were taken from each of the six overlay slabs, for a total of 24. During extraction, four of these slant cores failed: three on Slab 2B two on Slab 1B, and one on Slab 3A. The remain ing eighteen specimens were tested in compressio n As described in Chapter 3.2.5 all specimens were observed to fail in splitting tension, with none failing in shear on the bond surface. Table 4.7 1 lists the sample I.D. S lant shear test. Images of each sample have been included in the Appendix.

PAGE 56

45 Table 4.7 1 Slant shear test sample s Slab ID Pretreatments Slant Shear Test Sample I.D. Test 1 Test 2 Test 3 Test 4 1A Dry / No Agent 1A1S 1A2S 1A3S 1A4S 1B Dry / Wet Agent 1B1S 1B3S 2A SSD / Wet Agent 2A1S 2A2S 2A3S 2A4S 2B SSD / Dried Agent 2B1S 3A Wet / Wet Agent 3A2S 3A3S 3A4S 3B SSD / No Agent 3B1S 3B2S 3B3S 3B4S The measured compression stress on the slant shear samples at failure is listed on Table 4.7 2 (SI units) and Table 4.7 3 (U.S. customary units) Table 4.7 2 Slant shear test results (SI units) Slab ID Pretreatments Compression Stress at Failure (kPa) Test 1 Test 2 Test 3 Test 4 1A Dry / No Agent 37,411 25,662 33,219 36,860 1B Dry / Wet Agent 41,341 40,569 2A SSD / Wet Agent 44,747 37,687 38,501 38,763 2B SSD / Dried Agent 35,012 3A Wet / Wet Agent 29,979 27,593 36,391 3B SSD / No Agent 39,066 30,916 36,446 26,641 Table 4.7 3 Slant shear test results (U.S. customary units) Slab ID Pretreatments Compression Stress at Failure (psi) Test 1 Test 2 Test 3 Test 4 1A Dry / No Agent 5,426 3,722 4,818 5,346 1B Dry / Wet Agent 5,996 5,884 2A SSD / Wet Agent 6,490 5,466 5,584 5,622 2B SSD / Dried Agent 5,078 3A Wet / Wet Agent 4,348 4,002 5,278 3B SSD / No Agent 5,666 4,484 5,286 3,864 Based on the observed failure mode, it is apparent that the strength results were affected primarily by the compressive strength of the concrete, and not by the properties of the bond. Therefore the slant shear test results have not been included in the results discussion or the

PAGE 57

46 conclusions of this study Refer to Appendix E for additional information regarding the slant shear test results. 4.8 Jacking Test Results Four jacking tests were performed on each of the six overlay slabs, for a total of 24 performed as part of the testing program. All spe cimens were observed to fail on the bond surface. Table 4.8 1 lists the sample I.D. for each jacking J acking test. Images of each sample have been included in the Appendix. Table 4.8 1 Jacking test sample s Slab ID Pretreatments Jacking Test Sample I.D. Test 1 Test 2 Test 3 Test 4 1A Dry / No Agent 1A1J 1A2J 1A3J 1A4J 1B Dry / Wet Agent 1B1J 1B2J 1B3J 1B4J 2A SSD / Wet Agent 2A1J 2A2J 2A3J 2A4J 2B SSD / Dried Agent 2B1J 2B2J 2B3J 2B4J 3A Wet / Wet Agent 3A1J 3A2J 3A3J 3A4J 3B SSD / No Agent 3B1J 3B2J 3B3J 3B4J The shear strength at failure is listed on Table 4.8 2 (SI units) and Table 4.8 3 (U.S. customary units) As with the pull off results, a statistical analysis was performed on the dataset to compute the average shear strength, standard deviation, and coefficient of variation. Table 4.8 2 Jacking test results (SI units) Slab ID Pretreatments Shear Strength at Interface (kPa) Statistical Analysis Test 1 Test 2 Test 3 Test 4 Average (kPa ) Std. Dev. (kPa ) COV 1A Dry / No Agent 1,032 1,156 1,320 962 1,117 136 12.2% 1B Dry / Wet Agent 1,209 1,307 1,206 1,034 1,189 98 8.2% 2A SSD / Wet Agent 1,734 1,390 1,541 1,714 1,595 140 8.8% 2B SSD / Dried Agent 820 827 1,081 1,117 961 138 14.4% 3A Wet / Wet Agent 1,496 1,705 1,509 1,330 1,510 133 8.8% 3B SSD / No Agent 1,182 1,571 1,871 1,525 1,537 244 15.9%

PAGE 58

47 Table 4.8 3 Jacking test results (U.S. customary units) Slab ID Pretreatments Shear Strength at Interface (psi) Statistical Analysis Test 1 Test 2 Test 3 Test 4 Average (psi) Std. Dev. (psi) COV 1A Dry / No Agent 150 168 192 140 162 19.7 12.2% 1B Dry / Wet Agent 175 190 175 150 173 14.2 8.2% 2A SSD / Wet Agent 252 202 224 249 231 20.3 8.8% 2B SSD / Dried Agent 119 120 157 162 139 20.1 14.4% 3A Wet / Wet Agent 217 247 219 193 219 19.3 8.8% 3B SSD / No Agent 172 228 271 221 223 35.4 15.9% Table 4.8 4 (SI units) and Table 4.8 5 (U.S. customary units) list the factored shear strength results for the jac king tests Reference the pull off test results for further explanation of the adjustment factor. Table 4.8 4 Jacking test strength adjustment (SI units) 28 Day Compressive Strength (kPa ) Avg. Interfacia l Shear Strength ( ) (kPa ) Factored Interfacial Shear Strength f'c (min) Slab ID Pretreatments Substrate Overlay 1A Dry / No Agent 42,003 41,031 1,117 5.52 1B Dry / Wet Agent 42,003 47,429 1,189 5.80 2A SSD / Wet Agent 39,179 43,051 1,595 8.06 2B SSD / Dried Agent 39,179 31,888 961 5.38 3A Wet / Wet Agent 43,203 45,388 1,510 7.26 3B SSD / No Agent 43,203 21,160 1,537 10.6 Table 4.8 5 Jacking test strength adjustment (U.S. customary units) 28 Day Compressive Strength (psi ) Avg. Interfacial Shear Strength ( ) (psi ) Factored Interfacial Shear Strength f'c (min) Slab ID Pretreatments Substrate Overlay 1A Dry / No Agent 6,092 5,951 162 2.10 1B Dry / Wet Agent 6,092 6,879 172 2.21 2A SSD / Wet Agent 5,683 6,244 231 3.07 2B SSD / Dried Agent 5,683 4,625 139 2.05 3A Wet / Wet Agent 6,266 6,583 219 2.77 3B SSD / No Agent 6,266 3,069 223 4.02

PAGE 59

48 CHAPTER V D ISCUSSION 5.1 Variation of Compressive Strength In general, the sack concrete used in this study produced relatively consistent 28 day strengths, despite some variability in the measured slump prior to placement. The average 28 day strength for the substrate and first overlay placements was 6,174 psi (42,566 kPa) with a coefficient of va riation of just 6.2%. In contrast, the average 28 day strength for the second overlay placement was just 3,847 psi (26,525 kPa) with a coefficient of variation of 28 .6%. The reason for this significant difference is not entirely certain. The concrete sacks used in both overlay placements were taken from the same shipment, and were kept covered between placements. One possible explanation is that the bags for the second overlay placement experienced increased humidity due to rainfall which may have partially hydrated the cement Figure 5 1 shows the measured 28 day compressive strength for each substrate and overlay slab. Had the variation of compressive streng ths not exceeded the observed differences in the substrate and first overlay placements, they may have been reasonably ignored in the comparison of bond strengths However, the deviations were deemed significant enough to warrant adjustment to the bond str ength results. The adjustment was made by dividing the recorded bond strength in tension or shear strength at the bonded interface by the square root of the concrete strength. The minimum of the substrate and overlay compressive strengths was used in the a djustment factor; it was assumed that the weaker slab would fail first and control the strength result Equation ( 5 1 ) gives the solution for the fact ored bond strength in tension; Equatio n ( 5 2 ) gives the solution for the factored shear strength at the bonded interface

PAGE 60

49 ( 5 1 ) Where: = measured bond strength in tension ( kPa or psi ) c,min = minimum of overlay and substrate compressive strength (psi or kPa) ( 5 2 ) Where: = measured shear strength at interface ( kPa or psi ) c,min = minimum of overlay and substrate compressive strength ( kPa or psi ) Figure 5 1 28 day compressive strength by slab location 0 10,000 20,000 30,000 40,000 50,000 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 Slab 1A Slab 1B Slab 2A Slab 2B Slab 3A Slab 3B Compressive strength (kPa) Ccompressive strength (psi) Substrate Compressive Strength Overlay Compressive Strength

PAGE 61

50 5.2 Effects of Strength Gain on Test Results C ommon engineering practice is to assume that the majority of strength gain in concrete is complete after 28 days of maturity. Accordingly, 28 day compressive strengths were used as adjustment factors to account for the difference in compressive strength between slabs, a process discussed previously in Chapter 5.1 The possible effects o f strength gain after 28 days are discussed in the following paragraphs. Using the strength gain data from the 3, 7, and 28 day tests, a maturity curve ( Figure 5 2 ) was established based on a logarithmic function Although no samples were tested after 28 days, the strength gain after 28 days may be estimated using the logarithmic function. The curve pre dicts a strength of 58 13 psi (40,076 kPa) at 28 days. At 56 days, the concrete is expected to gain an additional 773 psi (5332 kPa) compressive strength, about 13.3% of the 28 day strength. However, between 56 and 72 days, the period in which the majority of the bond strength testing was conducted, the concrete is only expected to gain an additional 280 psi (1933 kPa), or abo ut 4.3% of the 56 day strength. With the exception of the pull off tests, the results from each test were obtained in a single day. Clearly, the strength of concrete did the results of any individual test type are unaffected by strength gain. It is only when the results of one type of test are compared with another that the ef fe cts of strength gain may be of concern However, as was noted above, the estimated difference in compressive strength during the testing period is very minimal and may be reasonably ignored. Therefore, no adjustments were made to the bond strength result s to compensate for the maturity of the concrete at the time of testing. The logarithmic curve used to model strength gain was calculated using a least squares best fit This function is reproduced as Equation ( 5 3 ) in U.S. customary units.

PAGE 62

51 ( 5 3 ) Where: c = compressive strength of concrete (est.) (psi) t = maturity, in days Figure 5 2 Strength gain in concrete 5.3 Bond Strength in Tension Figure 5 3 plots the average unadjusted bond strength in tension for each of the six pretreatment categories Error is displayed as one standard deviation about the mean ( SD). The groupings shown on the horizontal axis represent the different types of pretreatments The first term to overlay placement. The second term indicates whether a cement slurry bonding agent was used. The results of the pull off tests were adjusted for variations in the compressive s trength o f concrete and the average adjusted strength was plotted on Figure 5 4 With the exception of Slab 2B (discussed further below), t h e comparison shows that sim ilar bond strengths were achieved in all f'c = 1115.6 ln(t) + 2095.1 R = 0.5987 0 10,000 20,000 30,000 40,000 50,000 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 0 10 20 30 40 50 60 Compressive strength (kPa) Compressive Strength (psi) Maturity Days

PAGE 63

52 samples prepared with an SSD substrate surface. T he best performance was achieved in Slab 3B, which was prepared solely by prewetting the substrate. No detrimental impacts to strength were observed due to the overwe t substrate surface treatment used in Slab 3A; these samples performed similarly to the SSD samples used in concert with a bonding agent The overlay on Slab 1A was applied directly to the profiled substrate with no prewetting and no bonding agent This b ond exhibited inferior performance relative to the other slabs. This is consistent with the conclusions of Bissonnette (2012) that a carefully controlled amount of moisture within the substrate can produce a better bond Figure 5 3 Unadjusted average bond strengths in tension ( SD) grouped by pretreatment N = 3 samples per pretreatment category The cement slurry on Slab 2B was allowed to dry for a period of several da ys prior to overlay placement. This bond was extremely poor relatively to the other samples tested. This substantiates a 1A 1B 2A 2B 3A 3B 0 500 1,000 1,500 2,000 2,500 3,000 0 100 200 300 400 Dry Dry/Agent SSD/Agent SSD/Dried Agent Wet/Agent SSD Tensile strength at bonded interface (kPa) Tensile strength at bonded interface (psi)

PAGE 64

53 common concern with bonding agents, as expressed in ACI 325 and 345. The dried bonding agent, far from enhancing the bond strength, app ears to act as a bond breaker. It is important that bonding agents remain wet until the moment the overlay concrete is placed. The cement slurry bonding agent applied to a dry substrate (Slab 1B) performed similarly to the slabs prepared by prewetting. It appears that a properly applied bonding agent may benefit strength to a similar magnitude as prewetting, but the effect of both pretreatments in combination (Slab 2A) is not additive. Figure 5 4 Adjusted average bond strengths in tension ( SD) grouped by pretreatment N=3 samples per pretreatment category. 5.4 Shear Strength at the Bonded Interface 5.4.1 Representative shear strengths from previous r esearch values for shear strength s at bonded interfaces vary considerably by source. The AASHTO Br idge Design Specifications permit the designer to use a value of 1.93 MPa 1A 1B 2A 2B 3A 3B 0 2 4 6 8 10 12 0 1 2 3 4 Dry Dry/Agent SSD/Agent SSD/Dried Agent Wet/Agent SSD Adjusted bond strength (SI) Adjusted bond strength (US)

PAGE 65

54 ( 280 psi an amplitude (2014) Felt considered a shear strength of 2.24 MPa ( 325 psi ) (measured in direct shear using a guillotine type jig) to be average; he classified samples that exceeded 2.76 MPa ( 400 psi ) as (Felt, 1956) For adhesive bond strengt h Silfwerbrand found an average value of about 1.0 MPa (145 psi) for surfaces prepared with a pnumatic hammer (Silfwerbrand, 1990) ACI 345 notes that 1.38 MPa ( 200 psi ) is typically sufficent for durability (2006) 5.4.2 Direct shear t est Using typical shear strengths as a guide Figure 5 5 bond strength s were achieved in all slabs prepared with pretreatments (with the exception of Slab 2B, discussed below) The factored average shear strength measurements, adjusted for compressive strength of the concrete, are shown on Figure 5 6 Figure 5 5 Unadjusted average direct shear strengths ( SD) at the bonded interface grouped by pretreatment N=3 samples per pretreatment category. 1A 1B 2A 2B 3A 3B 0 1,000 2,000 3,000 4,000 0 100 200 300 400 500 Dry Dry/Agent SSD/Agent SSD/Dried Agent Wet/Agent SSD Shear strength at bonded interface (kPa) Shear strength at bonded interface (psi)

PAGE 66

55 The adjusted results show that the best performance was obtained from a substrate prepared solely by prewetting (Slab 3B) T he over wet surface on Slab 3A appears to have h ad a detrimental effect on the measured strengths. The surface with no pretreatments (Slab 1A) performed poorly, as did the surface where the bonding agent was allowed to dry (Slab 2B). The results indicate that a surface prepared solely with a bonding agent (Slab 1B) will perform similar to a surface prepared with a combination of prew etting and a bonding agent The good performance of Slab 1B may be in part due to the wetting effect of the bonding agent. The bonding agent may form a barrier to prevent free water from the overlay concrete from being lost into the capillaries of the substrate. It may also encapsulate dust and other particles lef t behind after the cleaning that would otherwise prevent the overlay from bonding to the surface of the substrate (Silfwerbrand & Paulsson, 1998) Figure 5 6 Adjusted average direct shear strengths ( SD) at the bonded interface, grouped by pretreatment N=3 samples per pretreatment category. 1A 1B 2A 2B 3A 3B 0 5 10 15 20 25 0 2 4 6 8 Dry Dry/Agent SSD/Agent SSD/Dried Agent Wet/Agent SSD Adjusted shear strength (SI) Adjusted shear strength (US)

PAGE 67

56 5.4.3 Slant shear t ests The observed failure mo de of the slant shear specimens indicates that the results for the slant shear tests primarily reflect the average compressive streng th of the overlay and substrate. These results do not appear to be influenced by the strength of the bon d or by the pretreatments used. 5.4.4 Jacking t est Measured average jac king test results are shown on Figure 5 7 grouped by pretreatment type. The average results adjusted for compressive strength of concrete are shown on Figure 5 8 The highest adjusted strength results were obtained using an SSD slab with no bonding agent (Slab 3B). A SSD substrate surface used in conjuction with a bonding agent ( Slab 2A) also performed well relative to the other surface preparations, as did the overwet substrate and bonding agent (Slab 3A) though neither performed as well as the prewet only substrate Figure 5 7 Unadjusted average jacking test strengths ( SD) at the bonded interface grouped by pretreatment N=4 samples per pretreatment category. 1A 1B 2A 2B 3A 3B 0 500 1,000 1,500 2,000 2,500 3,000 3,500 0 100 200 300 400 500 Dry Dry/Agent SSD/Agent SSD/Dried Agent Wet/Agent SSD Shear strength at bonded interface (kPa) Shear strength at bonded interface (psi)

PAGE 68

57 The surface with no pretreatmen ts (Slab 1A) obtained similar adjusted strengths as Slab 2B, where the bonding agent was allowed to dry Other tests had obtained significantly lower strengths for th e dried bonding agent relative to all other pretreatments. It is possible that the eccentricity caused by the location of the hydraulic piston during testing may be producing a clamping force at the opposite end of the block. This force may enhance the resistance to shearing due to mechanical interlock, such that even poorly pretreated surfaces exhibit shear resistance corresponding to their roughness. Pretreating with a bondin g agent alone (Slab 1B) did not produce superior shear strength relative to the other preparation methods. This is in contrast to the direct shear test, where the bonding agent alone produced strengths similar to those observed with prewetting. Figure 5 8 Adjuste d average jacking test strengths ( SD) at the bonded interface, grouped by pretreatment N=4 samples per pretreatment category. 1A 1B 2A 2B 3A 3B 0 2 4 6 8 10 12 14 16 0 1 2 3 4 5 6 Dry Dry/Agent SSD/Agent SSD/Dried Agent Wet/Agent SSD Adjusted shear strength (SI) Adjusted shear strength (US)

PAGE 69

58 5.4.5 Comparison of s hear test r esults The direct shear and jacking tests both indicated that the best strength is achieved using prewetting only, with no bonding agent, and that the lowest strengths result from the improper application of a bonding agent (Slab 2B). Poor performance was also o bserved when no pretreatments were used (Slab 1A). The shear tests produced conflicting r esults when comparing the effect of prewetting in conjunction with a bonding agent (Slabs 1B and 2A): the jacking test indicated that prewetting had an beneficial effe ct on bond strength while the direct shear results indic ate d the opposite. The results are inconclusive; it appears that a properly applied bonding agent may benefit in strength to a similar magnitude as prewetting for some testing conditions. However, it can be ascertained that the effect of both pretreatments used in combination is not additive. 5.4.6 Comparison of tension and shear test r esults All three tests agree that the best bond strength in tension and sh ear is obtained when the substrate surface is SSD and no bonding agent is used. When a bonding agent is used in comb ination with a SSD surface, all tests ind icate some reduction in strength in comparison with the prewet only surface The surface prepared with a dried bonding agent performed poorly in all types of tests. Additionally, the surface prepared with no pretreatments performed poorly relatively to the pretreated slabs for both tension and shear tests. The tension and shear t ests responded differently to the overwet substrate surface on Slab 3A. The shear tests both indicated that the overwet surface had a slightly detrimental effect on shear strength, which the pull off test showed no distinct difference in strength between t he overwet surface of Slab 3A and the SSD surface of Slab 2A. The unadjusted direct shear results and pull off te st results are plotted on Figure 5 9 (SI units) and Figure 5 10 (US customary units) Clearly, it is not poss ible to obtain the strength of a single sample using more than one test so the x,y coordinates of the datapoint s are taken from the same test number (Test 1 3) for each of the two tests (pull off and direct shear) T he pull off and direct shear

PAGE 70

59 samples were cored on the same day in alternating order along the length of the test slab. Test sample one for the direct shear test was taken immediately adjacent to test sample 1 for the pull off test, and so on for test samples two and three Figure 5 10 shows that, though there is considerable scatter in the data there may be a linear relationship between shear strength at the bonded interface and bond strength in tension. If a best fit line is calculated using the least squares method and this line is made to go through the origin, Equation ( 5 4 ) is obtained Figure 5 9 Direct shear vs. pull off test results (SI units) y = 0.3911x R = 0.4647 0 400 800 1200 1600 2000 0 500 1,000 1,500 2,000 2,500 3,000 3,500 Pull off test strength (kPa) Direct shear test strength (kPa)

PAGE 71

60 Figure 5 10 Direct shear vs. pull off test results (U.S. customary units) ( 5 4 ) Where: = Tensile strength of bond (est.) ( kPa or psi ) = Shear str ength at bonded interface ( kPa or psi ) If the equation is inverted, th e conversion factor from tensile strength to shear strength is calculated as 2.56. Considering the multitude of fact ors involved in this comparison, this correlates reasonably well with the 2.04 factor determined by Delatte, et al. (2000) and used by Rosen (2016) 5.4.7 Variations in d ata Figure 5 11 plots the coefficient of variation for each pretreatment and test type. Relatively low variation was observed for most of the pretreatment/test combination s Relatively high variation (CV > 30%) in the pull of f test results was observed at Slab s 1A, 2B, and 3B. The variation at S lab 2B is easily explained: the adhesive strengths measured for this slab were so low that they were close to the lower limit o f th e measuring capacity of the device. At low levels of stress, small differences in measurement result in large coefficient s of variation. The variation at Slabs 1A and 3B cannot be as y = 0.3911x R = 0.4647 0 100 200 300 400 0 100 200 300 400 500 600 Pull off test strength (psi) Direct shear test strength (psi)

PAGE 72

61 eas ily explained, as their average shear strength s were comparable to the neighboring slabs. Instead, it appears that the high variation in pull off test results may be attributed to the lack of bonding agent on these slabs The bonding agent, while not significantly inc reasing bond strength decreases th e varia bility of the bond, an effect first noted by Felt (1956) and corroborated by Saucier and Pigeon (1991) The same effect appears to occur in the jacking test results, though it i s not nearly as pronounced. A similar effect was not observed in the direct shear test results. Figure 5 11 Coefficients of variation for each test type, grouped by pretreatment 5.4.8 Statistical s ignificance A statistical analysis of the strength results was und ertaken to further corroborate the observation s regarding the effects of pretreatments A similar analysis was performed by Wall and Shrive (1988) test was performed using datasets gathered from five combinations of pre treatments to determine if the observed differences were statistically significant A significant difference was deemed to occur at the 90 percent confidence interval. The results of the a nalysis are shown in Table 5.4 1 0% 10% 20% 30% 40% 50% Dry Dry/Agent SSD/Agent SSD/Dry Agent Wet/Agent SSD Coefficient of variation (%) Pull-off Test Direct Shear Test Jacking Test SSD/ Dried Agent

PAGE 73

62 Table 5.4 1 Results of statistical analysis Pre treatment Significant Difference (Y or N) (90% CI) Moisture Bonding Agent Pull off Direct Shear Jacking Dry Substrate No Agent vs. Wet Agent N Y N SSD Substrate Dry Agent vs. Wet Agent Y Y Y Dry vs. SSD Substrate No Agent N Y Y Dry vs. SSD Substrate Wet Agent N N Y SSD vs. Wet Substrate Wet Agent N N N Not surprisingly, all three tests indicate a significant difference between th e strength obtained with a wet bonding agent versu s one that has been left to dry The direct shear test indicated a significant difference in strength when bonding agent was applied on a dry substrate, although the jacking test did not substantiate this result. The direct shear and jacking tests both identified that a significant difference in str ength was achieved using an SSD substrate versus a dry substrate in the absence of a bonding agent. The jacking test also identified this difference when a bonding agent was used. None of the tests identified a significant statistical difference b etween a SSD and wet substrate.

PAGE 74

63 CHAPTER VI SUMMARY AND CONCLUSIONS 6.1 Summary Prewetting the substrate and the use of a bonding agent are both common practices in the construction of b onded concrete overlays and other types of repairs yet the efficacy of these pret reatments is often debated The program of research described herein was undertaken to identify best practices with regards to pretreatment. To that end, three substrate and six overlay slabs were constructed, and each substrate surface was prepared with a different combination of pretreatments. Samples from each slab were subjected to a number of tests to evaluate the impact on bond strength in tension and shear st rength at the bonded interface The conclusions, below, are derived from the experimental dat a: 6.2 Conclusions The results indicate that pretreatments can substantially improve the strength of concrete to concrete bond Compared with an overlay constructed with no pretreatments, the overlay placed on the saturated surface dry substrate using a properly applied bonding agent achieved 46 percent greater bond strength in tension and 35 percent greater strength in shear. However, the inappropriate use of pretreatments can hinder the development of bond between substrate and overlay. W hen the bondi ng agent was improperly applied, the bond strength in tension decreased by 62 percent and the shear strength decreased by 31 percent, as compared to the slab constructed with no pretreatments. Prewetting of the substrate generally produced superior bond st rengths, irrespective of the use of bonding agents. For some tests, overlays prepared with a combination of prewetting and a bonding agent performed somewhat better than overlays prepared solely by prewetting. Concern regarding overwetting the substrate su rface appears to be largely unfounded; an overwet substrate surface (one containing small puddles of water) performed better in the tension test and slightly worse in the shear tests, but the overall effect was minor.

PAGE 75

64 The effect on strength of the use of a bonding agent in the absence of p rewetting was inconclusive. Two of the tests indicated superior bond strength while the third saw no strength benefit However, the application of a bonding agent did reduce the variability of the bond strength in the pul l off (tension) test. This effect was not observed in the shear tests. Allowing the bonding agent to dry on the substrate prior to overlay placement had an extreme negative effect on bond strength for all types of tests. In general, reasonably good bond st rengths were obtained using a saturated surface dry substrate alone. Prewetting the substrate substantially increased strength under both tensile and shear stresses, and the risk of overwetting the substrate appears to be minimal. The designer should consi der specifying an SSD substrate whenever good concrete to concrete bond strength is desired. The designer should weigh the increase to bond strength resulting from the appli cation of a bonding agent with the potential drawbacks due to improper application. If the application is carefully controlled, the bonding agent may provide some benefit. In some cases, the pull off, dire ct shear, and jacking tests respond ed differently to combinations of pre treatments. The designer should consider specifying the test which best mimics the loads to whi ch the repair will be subjected. 6.3 Suggestions for Future Research There are numerous ways in which this prog ram of research may be extended. These can be generally grouped into two categories: 1) incorporating other methods of testing bond strength or 2) evaluating other methods of slab preparation and pretreatment In the first category, the literature teems with alternative methods of measuring bond strength in tension and shear strength at the bonded interface. Among thes e are a twist off (torsion) type test described by Whitney, et al. (1992) and a tensile slant shear test described by Austin, et al. (1999) These and other tests could be incorporated into a new program of research to evaluate their sensitivities to the effects of pretreatments.

PAGE 76

65 In the second category, the research discussed herein evaluated the effects of pretreatments on a single ICRI Concrete Surface Profile. It is reasonable to assume that the effects of bonding agents may become more pronounced as the amplitude of surface roughness decreases. A new program of research could be undertaken to determine the interaction between pretreatments and substrate surface rou ghness. Silfwerb rand and Paulsson (1998) note that the scrubbing action during application of a bonding agent may assimilate dust particles that were not removed by cleaning, thereby improving the strength of the concrete to concrete bo nd. A program of research could be undertaken to determine if scrubbing water into the substrate surface would have a similar effect. Certain authors, such as Whitney, et al. (1992) had previously observed that bonding a gents can be very effective in hot weather when the temperature of the substrate is elevated. A program of research could be undertaken to assess the impact of ambient temperature on the effectiveness of pretreatments.

PAGE 77

66 R EFERENCES AASHTO (American Associat ion of State Highway and Transportation Officials). (2014). AASHTO LRFD Bridge Design Specifications, Customary U.S. Units, 7th Edition. AASHTO, Washington, DC. ACI (American Concrete Institute). (2006). Concrete O verl ays for Pavement R ehabilitation, Stand ard ACI 325.13R 06. ACI, Farmington Hills, MI. ACI (American Concrete Institute). (2011). G uide to Highway Bridge Deck C onstruction, Standard ACI 345R 11. ACI, Farmington Hills, MI. ASTM (American Society of Testing Materials). (2015). Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of C on crete Repair and Overlay Materials by Direct Tension (Pull off M ethod), Standa rd ASTM C1583 ASTM, C onshohocken PA. ASTM (American Society of Testing Materials). (2004). Standard Method of Test for Compressive Strength of Cylindrical Concrete Specimens, Standard ASTM C39 ASTM, C onshohocken PA. ASTM (American Society of Testing Materials). (2004). Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of C oncrete, Standard ASTM C42. ASTM, C onshohocken PA. Austin, S., Robins, P., and Youguang, Pan. (1999). Shear Bond Testing of Concrete R epairs. Cement and Concrete Research, 29 (7) 1067 1076. Bis sonnette, B., Vaysburd, A. M., and von Fay, K. F. (2012). Best Pratices for P rep aring Concrete Surface O verlays, Report Number MERL 12 17. U.S. Bureau of Rec lamation, Dept. of the Interior, Denver, CO. Dela tte, N. J., Williamson, M. S., and Fowler, D. W. ( 2000). Bond Strength Development with Maturity of High early Strength Bonded Concrete O verlays. ACI Materials Journal 97(2), 201 207. Djazmati, B. and Pincheira, J. (2004). J oints. ACI Structural Journal, 101(4) 484 493. Felt, E. J. (1956). oncrete. Proceedings of the Thirty Fifth Annual Meeting of the Highway Research Board Vol 35 444 469. Harrington, D. (2008). Guide to Concrete Overl ays: Sustainable Solutions for Resurfacing the Rehabilitating Existing Pavements. National Conc rete Pavement Technology Center, Ames, IA Hindo, K. R. (1990). In Place B ond Testing and Surface Preparation of C oncrete Concrete International, 12(4), 46 48.

PAGE 78

67 Illinois Bureau of Materials and Physi cal Research. (2012). Standard Method of Test for Shear Strength of Bonded Polymer C oncrete ICRI ( International Concrete Repair Institute ) (2013). Selecting and Specifying Concrete Surface Preparation for Sealers, C oatings, Polymer Overlays, and Concrete Repair, Standard 310.2R 2013 ICRI, Rosemont, IL. Julio, E. N., Branco, F. A., and Silva, V. D. (2004). Con crete to Concrete Bond Strength; Influence of the Roughness of the Substrate S urface. Construction and Building Materials 2004 (18), 675 681. Kriegh, J. D. (1976). trength. ACI Journal, July 1976 372 373. Pasko, T. J. (1997 ). Past, Present, and F uture. Public Roads, 62(1) Portland Cement Association (PCA) (1996). Resurfacing concrete floors, Standard IS144.07T PCA, Skokie, IL. Quikrete Companies Quikrete 5000 Concrete Mix Specification Standard Specification 03 31 00 Quikrete Companies, Atlanta, GA Rosen, C. J. (2016). Shear Strength at the Interface of Bonded C onc rete O verlays. M.S. t hesis, University of Colorado Denver Denver, CO R osengarten, W. E. (1919 ). oncrete. Concrete Products Magazine 17( 10), 19 20. Saucier, F. and Pigeon, M. (1991). to Old C oncr ete B ondings. Special Publication 128 43, American Concrete Institute, Farmington Hills, MI. 689 705. Silf ecks C oncrete International, 12(9) 61 66. Silfwerbrand, J. and Paulsson, J. (1998). T he Swedish Experience: Better Bonding of Bridge Deck O verlays. Concrete International, 20 (10), 56 61. Talbot C., Pigeon, M., Beaupre, D., and Morgan, D. (1994). Long term Bonding of S hotcrete. ACI Materials Journal, 91(6), 560 566. Iowa Department of Transportation (IA DOT). (2000). Method of Test for Determining the Shearing Strength of Bonded C oncrete Standard Iowa 406 C. I A DOT, Ames, IA. Vaysburd, A. M. and McDonald, J. E. (1999). An Evaulation of Equipment and Procedures for Tensile Bond Testing of C oncre te R epairs Technical Report REMR CS 61 U.S. Army Corps of Engineers, Washington, DC. Wall, J.S. and Shrive, N.G. (198 8). oncrete. ACI Materials Journal, 85(2) 117 125. Wells, J.A., Stark, R. D., and Polyzois, D. (1999). verlays. Concrete International, 21 ( 3), 49 52.

PAGE 79

68 Whitney, D.P., Isis, P., McCullough, B.F., and Fowler, D.W. (1992). An Investigation of Various Factors Affecting Bond in Bonded Concrete Overlays Report 920 5 Center for Transportation Research, University of Texas, Austin, TX.

PAGE 80

69 APPENDIX A PULL OFF TEST DATA Appendix A contains recorded data from the ASTM C1583 tensile strength (pull off) tests. Specimens are grouped by slab number. The tensile force at failure, measured using the pull off apparatus, is given for each specimen. The tensile stress at failure is calculated based on the tensile force and gross bonded area.

PAGE 81

70 ASTM C1583 PULL OFF TEST DATA Test : ASTM C1583 (Pull off test) Test Date : As Noted Slab I.D. : Slab 1A Moisture : Dry Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 1A1T (6/22 /2016) SI Units Imperial Units Tensile Force at Failure 2.80 kN 630 lbs Tensile Stress at Failure 800 kPa 116 psi SPECIMEN 1A3T (6/21/2016) SI Units Imperial Units Tensile Force at Failure 1.11 kN 250 lbs Tensile Stress at Failure 317 kPa 46 psi SPECIMEN 1A5 T (6/26 /2016) SI Units Imperial Units Tensile Force at Failure 4.23 kN 950 lbs Tensile Stress at Failure 1213 kPa 176 psi

PAGE 82

71 ASTM C1583 PULL OFF TEST DATA Test : ASTM C1583 (Pull off test) Test Date : As Noted Slab I.D. : Slab 1B Moisture : Dry Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 1B5T (6/26/ 1016) SI Units Imperial Units Tensile Force at Failure 3.74 kN 840 lbs Tensile Stress at Failure 1069 kPa 155 psi SPECIMEN 1B7T (6/26/ 2016) SI Units Imperial Units Tensile Force at Failure 4.54 kN 1020 lbs Tensile Stress at Failure 1296 kPa 188 psi SPECIMEN 1B11T (8/4/ 2016) SI Units Imperial Units Tensile Force at Failure 3.65 kN 820 lbs Tensile Stress at Failure 1048 kPa 152 psi

PAGE 83

72 ASTM C1583 PULL OFF TEST DATA Test : ASTM C1583 (Pull off test) Test Date : As Noted Slab I.D. : Slab 2A Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 2A3T (6/26/ 1016) SI Units Imperial Units Tensile Force at Failure 4.41 kN 990 lbs Tensile Stress at Failure 1262 kPa 183 psi SPECIMEN 2A5T (6/26/ 2016) SI Units Imperial Units Tensile Force at Failure 3.83 kN 860 lbs Tensile Stress at Failure 1096 kPa 159 psi SPECIMEN 2A7T (8/4/2 016) SI Units Imperial Units Tensile Force at Failure 3.69 kN 830 lbs Tensile Stress at Failure 1055 kPa 153 psi

PAGE 84

73 ASTM C1583 PULL OFF TEST DATA Test : ASTM C1583 (Pull off test) Test Date : 6/22/2016 Slab I.D. : Slab 2B Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Dried) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 2B1T SI Units Imperial Units Tensile Force at Failure 1.34 kN 300 lbs Tensile Stress at Failure 379 kPa 55 psi SPECIMEN 2B3T SI Units Imperial Units Tensile Force at Failure 1.29 kN 290 lbs Tensile Stress at Failure 372 kPa 54 psi SPECIMEN 2B5T No Photo Available SI Units Imperial Units Tensile Force at Failure 0.45 kN 100 lbs Tensile Stress at Failure 124 kPa 18 psi

PAGE 85

74 ASTM C1583 PULL OFF TEST DATA Test : ASTM C1583 (Pull off test) Test Date : 6/26/2016 Slab I.D. : Slab 3A Moisture : Overwet (small puddles) Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 3A1T SI Units Imperial Units Tensile Force at Failure 3.52 kN 790 lbs Tensile Stress at Failure 1007 kPa 146 psi SPECIMEN 3A3T SI Units Imperial Units Tensile Force at Failure 4.54 kN 1020 lbs Tensile Stress at Failure 1296 kPa 188 psi SPECIMEN 3A5T SI Units Imperial Units Tensile Force at Failure 5.34 kN 1200 lbs Tensile Stress at Failure 1531 kPa 222 psi

PAGE 86

75 ASTM C1583 PULL OFF TEST DATA Test : ASTM C1583 (Pull off test) Test Date : 6/26/2016 Slab I.D. : Slab 3B Moisture : SSD Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 3B1T SI Units Imperial Units Tensile Force at Failure 2.85 kN 640 lbs Tensile Stress at Failure 814 kPa 118 psi SPECIMEN 3B3T SI Units Imperial Units Tensile Force at Failure 3.83 kN 860 lbs Tensile Stress at Failure 1096 kPa 159 psi SPECIMEN 3B5T SI Units Imperial Units Tensile Force at Failure 6.14 kN 1380 lbs Tensile Stress at Failure 1758 kPa 255 psi

PAGE 87

76 APPENDIX B DIRECT SHEAR (GUILLO TINE) TEST DATA Appendix B contains recorded data from the direct shear (guillotine) tests. Specimens are grouped by slab number. The maximum shearing force applied, measured using the MTS equipment, is given for each specimen. The shear stress at failure is calculated ba sed on the recorded shearing force (divided by two) and gross bonded area. The stress strain plot is based on the recorded load displacement data, converted to stress and strain using the gross bonded area and diameter of specimen, respectively.

PAGE 88

77 DIRECT S HEAR TEST DATA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 1A Moisture : Dry Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 1A2G SI Units Imperial Units Applied Load at Failure 16.0 kN 3604 lbs Shear Stress at Failure 2296 kPa 333 psi SPECIMEN 1A 4G SI Units Imperial Units Applied Load at Failure 17.8 kN 3995 lbs Shear Stress at Failure 2544 kPa 369 psi SPECIMEN 1A 7G SI Units Imperial Units Applied Load at Failure 14.4 kN 3233 lbs Shear Stress at Failure 2062 kPa 299 psi

PAGE 89

78 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 1A Moisture : Dry Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 1A2G Maximum Load : 3604 lbs Maximum Stress: 333 psi Strain at Failure: 0.0080 in/in 0 50 100 150 200 250 300 350 0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 Shear Stress (psi) Strain (in/in) Test 1A2G

PAGE 90

79 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 1A Moisture : Dry Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 1A6G Maximum Load : 3995 lbs Maximum Stress: 369 psi Strain at Failure: 0.0086 in/in 0 50 100 150 200 250 300 350 400 0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 Shear Stress (psi) Strain (in/in) Test 1A6G

PAGE 91

80 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 1B Moisture : Dry Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 1B2G SI Units Imperial Units Applied Load at Failure 23.2 kN 5204 lbs Shear Stress at Failure 3316 kPa 481 psi SPECIMEN 1B8G SI Units Imperial Units Applied Load at Failure 21.2 kN 4757 lbs Shear Stress at Failure 3027 kPa 439 psi SPECIMEN 1B10G SI Units Imperial Units Applied Load at Failure 20.4 kN 4580 lbs Shear Stress at Failure 2916 kPa 423 psi

PAGE 92

81 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 1B Moisture : Dry Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 1B2G Maximum Load : 5204 lbs Maximum Stress: 481 psi Strain at Failure: 0.0100 in/in 0 100 200 300 400 500 600 0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 0.0120 Shear Stress (psi) Strain (in/in) Test 1B2G

PAGE 93

82 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 1B Moisture : Dry Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 1B8G Maximum Load : 4757 lbs Maximum Stress: 439 psi Strain at Failure: 0.0113 in/in 0 50 100 150 200 250 300 350 400 450 500 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 Shear Stress (psi) Strain (in/in) Test 1B8G

PAGE 94

83 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 1B Moisture : Dry Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 1B10G Maximum Load : 4580 lbs Maximum Stress: 423 psi Strain at Failure: 0.0084 in/in 0 50 100 150 200 250 300 350 400 450 0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 0.0120 Shear Stress (psi) Strain (in/in) Test 1B10G

PAGE 95

84 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 2A Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 2A2G SI Units Imperial Units Applied Load at Failure 23.1 kN 5200 lbs Shear Stress at Failure 3309 kPa 480 psi SPECIMEN 2A4G SI Units Imperial Units Applied Load at Failure 21.5 kN 4828 lbs Shear Stress at Failure 3075 kPa 446 psi SPECIMEN 2A6G SI Units Imperial Units Applied Load at Failure 16.5 kN 3711 lbs Shear Stress at Failure 2365 kPa 343 psi

PAGE 96

85 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 2A Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 2A2G Maximum Load : 5200 lbs Maximum Stress: 480 psi Strain at Failure: 0.0128 in/in 0 100 200 300 400 500 600 700 0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 0.0120 0.0140 0.0160 Shear Stress (psi) Strain (in/in) Test 2A2G

PAGE 97

86 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 2A Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 2A4G Maximum Load : 4828 lbs Maximum Stress: 446 psi Strain at Failure: 0.0093 in/in 0 50 100 150 200 250 300 350 400 450 500 0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 0.0120 Shear Stress (psi) Strain (in/in) Test 2A4G

PAGE 98

87 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 2A Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 2A6G Maximum Load : 3711 lbs Maximum Stress: 343 psi Strain at Failure: 0.0081 in/in 0 50 100 150 200 250 300 350 400 0.0000 0.0010 0.0020 0.0030 0.0040 0.0050 0.0060 0.0070 0.0080 0.0090 0.0100 Shear Stress (psi) Strain (in/in) Test 2A6G

PAGE 99

88 DIRECT SHEAR TEST DATA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 2B Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Dried) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 2B2G SI Units Imperial Units Applied Load at Failure 4.6 kN 1023 lbs Shear Stress at Failure 655 kPa 95 psi SPECIMEN 2B4G SI Units Imperial Units Applied Load at Failure 10.4 kN 2347 lbs Shear Stress at Failure 1496 kPa 217 psi SPECIMEN 2B6G SI Units Imperial Units Applied Load at Failure 9.9 kN 2234 lbs Shear Stress at Failure 1420 kPa 206 psi

PAGE 100

89 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 2B Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Dried) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 2B2G Maximum Load : 1023 lbs Maximum Stress: 95 psi Strain at Failure: 0.0034 in/in 0 10 20 30 40 50 60 70 80 90 100 0.0000 0.0010 0.0020 0.0030 0.0040 0.0050 0.0060 0.0070 Shear Stress (psi) Strain (in/in) Test 2B2G

PAGE 101

90 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 2B Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Dried) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 2B4G Maximum Load : 2347 lbs Maximum Stress: 217 psi Strain at Failure: 0.0058 in/in 0 50 100 150 200 250 0.0000 0.0010 0.0020 0.0030 0.0040 0.0050 0.0060 0.0070 Shear Stress (psi) Strain (in/in) Test 2B4G

PAGE 102

91 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 2B Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Dried) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 2B6G Maximum Load : 2234 lbs Maximum Stress: 206 psi Strain at Failure: 0.0063 in/in 0 50 100 150 200 250 0.0000 0.0010 0.0020 0.0030 0.0040 0.0050 0.0060 0.0070 0.0080 Shear Stress (psi) Strain (in/in) Test 2B6G

PAGE 103

92 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 3A Moisture : Overwet (small puddles) Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 3A2G SI Units Imperial Units Applied Load at Failure 21.1 kN 4748 lbs Shear Stress at Failure 3027 kPa 439 psi SPECIMEN 3A4G SI Units Imperial Units Applied Load at Failure 16.3 kN 3658 lbs Shear Stress at Failure 2330 kPa 338 psi SPECIMEN 3A6G SI Units Imperial Units Applied Load at Failure 18.2 kN 4088 lbs Shear Stress at Failure 2606 kPa 378 psi

PAGE 104

93 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 3A Moisture : Overwet (small puddles) Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 3A2G Maximum Load : 4748 lbs Maximum Stress: 439 psi Strain at Failure: 0.0087 in/in 0 50 100 150 200 250 300 350 400 450 500 0.0000 0.0010 0.0020 0.0030 0.0040 0.0050 0.0060 0.0070 0.0080 0.0090 0.0100 Shear Stress (psi) Strain (in/in) Test 3A2G

PAGE 105

94 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 3A Moisture : Overwet (small puddles) Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 3A4G Maximum Load : 3658 lbs Maximum Stress: 338 psi Strain at Failure: 0.0087 in/in 0 50 100 150 200 250 300 350 400 0.0000 0.0010 0.0020 0.0030 0.0040 0.0050 0.0060 0.0070 0.0080 0.0090 0.0100 Shear Stress (psi) Strain (in/in) Test 3A4G

PAGE 106

95 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 3A Moisture : Overwet (small puddles) Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 3A6G Maximum Load : 4088 lbs Maximum Stress: 378 psi Strain at Failure: 0.0084 in/in 0 50 100 150 200 250 300 350 400 0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 0.0120 Shear Stress (psi) Strain (in/in) Test 3A6G

PAGE 107

96 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 3B Moisture : SSD Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 3B2G SI Units Imperial Units Applied Load at Failure 18.7 kN 4205 lbs Shear Stress at Failure 2675 kPa 388 psi SPECIMEN 3B4G SI Units Imperial Units Applied Load at Failure 19.1 kN 4290 lbs Shear Stress at Failure 2730 kPa 396 psi SPECIMEN 3B6G SI Units Imperial Units Applied Load at Failure 22.3 kN 5007 lbs Shear Stress at Failure 3192 kPa 463 psi

PAGE 108

97 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 3B Moisture : SSD Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 3B2G Maximum Load : 4205 lbs Maximum Stress: 388 psi Strain at Failure: 0.0086 in/in 0 50 100 150 200 250 300 350 400 450 0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 0.0120 Shear Stress (psi) Strain (in/in) Test 3B2G

PAGE 109

98 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 3B Moisture : SSD Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 3B4G Maximum Load : 4290 lbs Maximum Stress: 396 psi Strain at Failure: 0.0077 in/in 0 50 100 150 200 250 300 350 400 450 0.0000 0.0010 0.0020 0.0030 0.0040 0.0050 0.0060 0.0070 0.0080 0.0090 Shear Stress (psi) Strain (in/in) Test 3B4G

PAGE 110

99 DIRECT SHEAR TEST DA TA Test : Direct Shear (Guillotine) Test Test Date : 7/11/2016 Slab I.D. : Slab 3B Moisture : SSD Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) SPECIMEN 3B6G Maximum Load : 5007 lbs Maximum Stress: 463 psi Strain at Failure: 0.0081 in/in 0 50 100 150 200 250 300 350 400 450 500 0.0000 0.0010 0.0020 0.0030 0.0040 0.0050 0.0060 0.0070 0.0080 0.0090 0.0100 Shear Stress (psi) Strain (in/in) Test 3B6G

PAGE 111

100 APPENDIX C SLANT SHEAR TEST DAT A Appendix C contains recorded data from the slant shear tests. Specimens are grouped by slab number. The maximum compressive force applied, measured using the MTS equipment, is given for each specimen. The shear stress at failure is calculated as the transformed shearing stress on the slanted interface based on the applied uniaxial compression force. The stress strain plot is based on the recorded load displacement data, converted to compressive stress and strain using the area of the specimen and the original specimen length, respectively.

PAGE 112

101 SLANT SHEAR TEST DAT A Test : Slant Shear Test Test Date : 8/1 /2016 Slab I.D. : Slab 1A Moisture : Dry Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) Slant Angle : 45 degrees SPECIMEN 1A1S SI Units Imperial Units Applied Load at Failure 131 kN 2 9360 lbs Comp. Stress at Failure 37.4 M Pa 5425 psi Shear Stress at Interface at Failure 18706 kPa 2713 psi SPECIMEN 1A 2S SI Units Imperial Units Applied Load at Failure 89.7 kN 20148 lbs Comp. Stress at Failure 25.7 M Pa 3723 psi Shear Stress at Interface at Failure 12831 kPa 1861 psi

PAGE 113

102 SLANT SHEAR TEST DAT A Test : Slant Shear Test Test Date : 8/1/2016 Slab I.D. : Slab 1A Moisture : Dry Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) Slant Angle : 45 degrees SPECIMEN 1A3S SI Units Imperial Units Applied Load at Failure 116 kN 26075 lbs Comp. Stress at Failure 33.2 M Pa 4818 psi Shear Stress at Interface at Failure 16610 kPa 2409 psi SPECIMEN 1A4S SI Units Imperial Units Applied Load at Failure 129 kN 28928 lbs Comp. Stress at Failure 36.9 M Pa 5345 psi Shear Stress at Interface at Failure 18430 kPa 2673 psi

PAGE 114

103 SLANT SHEAR TEST DAT A Test : Slant Shear Test Test Date : 8/1/2016 Slab I.D. : Slab 1B Moisture : Dry Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) Slant Angle : 45 degrees SPECIMEN 1B1S SI Units Imperial Units Applied Load at Failure 144 kN 32448 lbs Comp. Stress at Failure 41.3 M Pa 5996 psi Shear Stress at Interface at Failure 20671 kPa 2998 psi SPECIMEN 1B3S SI Units Imperial Units Applied Load at Failure 142 kN 31848 lbs Comp. Stress at Failure 40.6 M Pa 5885 psi Shear Stress at Interface at Failure 20284 kPa 2942 psi

PAGE 115

104 SLANT SHEAR TEST DAT A Test : Slant Shear Test Test Date : 8/1/2016 Slab I.D. : Slab 2A Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) Slant Angle : 45 degrees SPECIMEN 2A1S SI Units Imperial Units Applied Load at Failure 156 kN 35122 lbs Comp. Stress at Failure 44.7 MPa 6490 psi Shear Stress at Interface at Failure 22374 kPa 3245 psi SPECIMEN 2A2S SI Units Imperial Units Applied Load at Failure 132 kN 29586 lbs Comp. Stress at Failure 37.7 MPa 5467 psi Shear Stress at Interface at Failure 18843 kPa 2733 psi

PAGE 116

105 SLANT SHEAR TEST DATA Test : Slant Shear Test Test Date : 8/1/2016 Slab I.D. : Slab 2A Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) Slant Angle : 45 degrees SPECIMEN 2A3S SI Units Imperial Units Applied Load at Failure 135 kN 30215 lbs Comp. Stress at Failure 38.5 MPa 5583 psi Shear Stress at Interface at Failure 19250 kPa 2792 psi SPECIMEN 2A4S SI Units Imperial Units Applied Load at Failure 135 kN 30429 lbs Comp. Stress at Failure 38.8 MPa 5623 psi Shear Stress at Interface at Failure 19381 kPa 2811 psi

PAGE 117

106 SLANT SHEAR TEST DAT A Test : Slant Shear Test Test Date : 8/1/2016 Slab I.D. : Slab 2B Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Dried) Core Diameter: 66.7 mm (2 5/8 in) Slant Angle : 45 degrees SPECIMEN 2B1S SI Units Imperial Units Applied Load at Failure 122 kN 27481 lbs Comp. Stress at Failure 35.0 MPa 5078 psi Shear Stress at Interface at Failure 17506 kPa 2539 psi

PAGE 118

107 SLANT SHEAR TEST DAT A Test : Slant Shear Test Test Date : 8/1/2016 Slab I.D. : Slab 3A Moisture : Overwet (small puddles) Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) Slant Angle : 45 degrees SPECIMEN 3 A2S SI Units Imperial Units Applied Load at Failure 105 kN 23529 lbs Comp. Stress at Failure 30.0 MPa 4348 psi Shear Stress at Interface at Failure 14989 kPa 2174 psi

PAGE 119

108 SLANT SHEAR TEST DAT A Test : Slant Shear Test Test Date : 8/1/2016 Slab I.D. : Slab 3A Moisture : Overwet (small puddles) Tested by : ASP Bonding Agent : Cement Slurry (Wet) Core Diameter: 66.7 mm (2 5/8 in) Slant Angle : 45 degrees SPECIMEN 3 A3S SI Units Imperial Units Applied Load at Failure 96.4 kN 21657 lbs Comp. Stress at Failure 27.6 MPa 4002 psi Shear Stress at Interface at Failure 13796 kPa 2001 psi SPECIMEN 3 A4S SI Units Imperial Units Applied Load at Failure 127 kN 28559 lbs Comp. Stress at Failure 36.4 MPa 5277 psi Shear Stress at Interface at Failure 18195 kPa 2639 psi

PAGE 120

109 SLANT SHEAR TEST DAT A Test : Slant Shear Test Test Date : 8/1/2016 Slab I.D. : Slab 3B Moisture : SSD Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) Slant Angle : 45 degrees SPECIMEN 3B1S SI Units Imperial Units Applied Load at Failure 136 kN 30659 lbs Comp. Stress at Failure 39.1 MPa 5665 psi Shear Stress at Interface at Failure 19533 kPa 2833 psi SPECIMEN 3B2S SI Units Imperial Units Applied Load at Failure 108 kN 24272 lbs Comp. Stress at Failure 30.9 MPa 4485 psi Shear Stress at Interface at Failure 15458 kPa 2242 psi

PAGE 121

110 SLANT SHEAR TEST DAT A Test : Slant Shear Test Test Date : 8/1/2016 Slab I.D. : Slab 3B Moisture : SSD Tested by : ASP Bonding Agent : None Core Diameter: 66.7 mm (2 5/8 in) Slant Angle : 45 degrees SPECIMEN 3B3S SI Units Imperial Units Applied Load at Failure 127 kN 28609 lbs Comp. Stress at Failure 36.4 MPa 5286 psi Shear Stress at Interface at Failure 18223 kPa 2643 psi SPECIMEN 3B4S SI Units Imperial Units Applied Load at Failure 93.0 kN 20909 lbs Comp. Stress at Failure 26.6 MPa 3864 psi Shear Stress at Interface at Failure 13321 kPa 1932 psi

PAGE 122

111 APPENDIX D JACKING TEST DATA Appendix D contains recorded data from the jacking tests. Specimens are grouped by slab number. The maximum shearing force applied, measured using the Simplex hydraulic jack, is given for each specimen. The shear stress at failure is calculated based on the recorded shearing force and gross bonded area.

PAGE 123

112 JACKING TEST D ATA Test : Jacking Test Test Date : 8/13 /2016 Slab I.D. : Slab 1A Moisture : Dry Tested by : ASP Bonding Agent : None Jack Area : 4154 mm 2 (6.44 in 2 ) SPECIMEN 1A1J SI Units Imperial Units Length 130 mm 5 1/8 in Width 14 3 mm 5 5/8 in Applied Load at Failure 19.2 kN 4315 lbs Shear Stress at Failure 1032 kPa 1 50 psi SPECIMEN 1A 2J SI Units Imperial Units Length 133 mm 5 1/4 in Width 152 mm 6 in Applied Load at Failure 23.5 kN 5281 lbs Shear Stress at Failure 1156 kPa 168 psi SPECIMEN 1A3J SI Units Imperial Units Length 140 mm 5 1/2 in Width 143 mm 5 5/8 in Applied Load at Failure 26.4 kN 5925 lbs Shear Stress at Failure 1320 kPa 192 psi SPECIMEN 1A4J SI Units Imperial Units Length 140 mm 5 1/2 in Width 152 mm 6 in Applied Load at Failure 20.5 kN 4605 lbs Shear Stress at Failure 962 kPa 140 psi

PAGE 124

113 JACKING TEST DATA Test : Jacking Test Test Date : 8/13/2016 Slab I.D. : Slab 1B Moisture : Dry Tested by : ASP Bonding Agent : Cement Slurry (Wet) Jack Area: 4154 mm 2 (6.44 in 2 ) SPECIMEN 1B1J SI Units Imperial Units Length 146 mm 5 3/4 in Width 149 mm 5 7/8 in Applied Load at Failure 26.4 kN 5925 lbs Shear Stress at Failure 1209 kPa 175 psi SPECIMEN 1B2J SI Units Imperial Units Length 146 mm 5 3/4 in Width 165 mm 6 1/2 in Applied Load at Failure 31.5 kN 7084 lbs Shear Stress at Failure 1307 kPa 190 psi SPECIMEN 1B3J SI Units Imperial Units Length 146 mm 5 3/4 in Width 133 mm 5 1/4 in Applied Load at Failure 23.5 kN 5281 lbs Shear Stress at Failure 1206 kPa 175 psi SPECIMEN 1B4J SI Units Imperial Units Length 146 mm 5 3/4 in Width 146 mm 5 3/4 in Applied Load at Failure 22.1 kN 4959 lbs Shear Stress at Failure 1034 kPa 150 psi

PAGE 125

114 JACKING TEST DATA Test : Jacking Test Test Date : 8/13/2016 Slab I.D. : Slab 2A Moisture : SSD Tested by : ASP Bonding Agent : Cement Slurry (Wet) Jack Area: 4154 mm 2 (6.44 in 2 ) SPECIMEN 2A 1J SI Units Imperial Units Length 146 mm 5 3/4 in Width 83 mm 3 1/4 in Applied Load at Failure 20.9 kN 4701 lbs Shear Stress at Failure 1734 kPa 252 psi SPECIMEN 2A 2J SI Units Imperial Units Length 146 mm 5 3/4 in Width 152 mm 6 in Applied Load at Failure 31.0 kN 6955 lbs Shear Stress at Failure 1390 kPa 202 psi SPEC IMEN 2A 3J SI Units Imperial Units Length 143 mm 5 5/8 in Width 133 mm 5 1/4 in Applied Load at Failure 29.4 kN 6601 lbs Shear Stress at Failure 1541 kPa 224 psi SPECIMEN 2A 4J SI Units Imperial Units Length 143 mm 5 5/8 in Width 133 mm 5 1/4 in Applied Load at Failure 32.7 kN 7342 lbs Shear Stress at Failure 1714 kPa 249 psi

PAGE 126

115 JACKING TEST DATA Test : Jacking Test Test Date : 8/13/2016 Slab I.D. : Slab 2B Moisture : Dry Tested by : ASP Bonding Agent : Cement Slurry (Dried ) Jack Area: 4154 mm 2 (6.44 in 2 ) SPECIMEN 2 B1J SI Units Imperial Units Length 149 mm 5 7/8 in Width 156 mm 6 1/8 in Applied Load at Failure 19.1 kN 4283 lbs Shear Stress at Failure 820 kPa 119 psi SPECIMEN 2 B2J SI Units Imperial Units Length 152 mm 6 in Width 152 mm 6 in Applied Load at Failure 19.2 kN 4315 lbs Shear Stress at Failure 827 kPa 120 psi SPECIMEN 2 B3J SI Units Imperial Units Length 152 mm 6 in Width 137 mm 5 3/8 in Applied Load at Failure 22.5 kN 5055 lbs Shear Stress at Failure 1081 kPa 157 psi SPECIMEN 2 B4J SI Units Imperial Units Length 152 mm 6 in Width 140 mm 5 1/2 in Applied Load at Failure 23.8 kN 5345 lbs Shear Stress at Failure 1117 kPa 162 psi

PAGE 127

116 JACKING TEST DATA Test : Jacking Test Test Date : 8/13/2016 Slab I.D. : Slab 3A Moisture : Overwet (small puddles) Tested by : ASP Bonding Agent : Cement Slurry (Wet) Jack Area: 4154 mm 2 (6.44 in 2 ) SPECIMEN 3A 1J SI Units Imperial Units Length 159 mm 6 1/4 in Width 108 mm 4 1/4 in Applied Load at Failure 25.7 kN 5764 lbs Shear Stress at Failure 1496 kPa 217 psi SPECIMEN 3A 2J SI Units Imperial Units Length 159 mm 6 1/4 in Width 108 mm 4 1/4 in Applied Load at Failure 29.2 kN 6569 lbs Shear Stress at Failure 1705 kPa 247 psi SPECIMEN 3A 3J SI Units Imperial Units Length 162 mm 6 3/8 in Width 130 mm 5 1/8 in Applied Load at Failure 31.8 kN 7148 lbs Shear Stress at Failure 1509 kPa 219 psi SPECIMEN 3A 4J SI Units Imperial Units Length 162 mm 6 3/8 in Width 140 mm 5 1/2 in Applied Load at Failure 30.1 kN 6762 lbs Shear Stress at Failure 1330 kPa 193 psi

PAGE 128

117 JACKING TEST DATA Test : Jacking Test Test Date : 8/13/2016 Slab I.D. : Slab 3B Moisture : SSD Tested by : ASP Bonding Agent : None Jack Area: 4154 mm 2 (6.44 in 2 ) SPECIMEN 3B 1J SI Units Imperial Units Length 152 mm 6 in Width 159 mm 6 1/4 in Applied Load at Failure 28.7 kN 6440 lbs Shear Stress at Failure 1182 kPa 172 psi SPECIMEN 3 B2J SI Units Imperial Units Length 156 mm 6 1/8 in Width 152 mm 6 in Applied Load at Failure 37.3 kN 8372 lbs Shear Stress at Failure 1571 kPa 228 psi SPECIMEN 3 B3J SI Units Imperial Units Length 159 mm 6 1/4 in Width 130 mm 5 1/8 in Applied Load at Failure 38.7 kN 8694 lbs Shear Stress at Failure 1871 kPa 271 psi SPECIMEN 3 B4J SI Units Imperial Units Length 159 mm 6 1/4 in Width 130 mm 5 1/8 in Applied Load at Failure 31.5 kN 7084 lbs Shear Stress at Failure 1525 kPa 221 psi

PAGE 129

118 APPENDIX E SLANT SHEAR DISCUSSI ON E.1 The slant shear test exerts a combination of compression and shear on the bonded surface resulting from the angle of inclination of the surface. The compression force can be resolved into two N NT Figur e E 1 ). In a roughened surface, the clamping force amplifies the effect of mechanical interlock, in turn greatly increasing the observed failure stress at the bond surface Austin, et al. (1999) plotted the ratio of appl ied compression stress to applied shear on the bond surface using experimentally determined coefficients of friction for three types of surfaces: smooth, medium rough, and rough. The coefficients used were 0.75, 1.0, and 1.25, respectively. The shear stres s experienced by the bond ( c bond ) is determined via Equation (E 1) where nt represents the transformed shear stress on the bond surface, represents the friction coefficient, and nn represents the tra nsformed normal stress. The shear stress on the bond is then divided by 0 ( the applied compressive stress on the sample) ; the resulting ratio is therefore independent of the magnitude of the applied compressive stress. (E 1) Figure E 2 plots the ratio c bond / 0 for the representative roughnesses proposed by Austin, et al (1999) This graph indicates that, for a smooth surface, the optimal slant angle (the angle that is mostly likely to produce a slip failure in shear ) is betwe en 60 and 65 degrees from normal. The optimal angle increases to around 70 degrees for a roughened substrate. Even at the optimal slant angle, the applied shearing stress as a ratio of the compression applied to the specimen decreases as the surface roughn ess increases, starting at 26 percent for a smooth substrate and decreasing to 18 percent for a rough substrate. Therefore, even for specimens prepared at the optimal angle,

PAGE 130

119 significantly greater levels of compression will be needed to produce bond failure in a roughened sample, increasing the likelihood that the sample will fail in compression prior to slip failure in shear Figure E 1 Diagram showing (1) the slant shear specimen under uniaxial compression stress and (2) the resulting str esses on the bond surface Figure E 2 Applied shear stress at bonded interface (c bond ) as a ratio of applied compressive stress ( 0 ), varying surface roughness after Austin, et al. (1999) 0.0 0.1 0.2 0.3 0.4 0.5 40 45 50 55 60 65 70 75 80 85 Ratio of applied shear stress on the bonded interface to applied compressive stress (c bond / 0 ) Slant Angle (Degrees) Smooth Substrate Medium Substrate Rough Substrate ( 1) ( 2)

PAGE 131

120 E.2 Observed Effect of Surface Rough ness on Failure Mode The observed failure mode of all 24 slant shear specimens tested during the research program was one of compression failure. The desired failure mode for the test was failure in shear along the bonded interface, as this re sult would have been indicative of the shear strength of the bonded interface. However, it appears that the clamping force on the roughened bond plane was sufficient to resist th e applied shear force on the 45 degree bond plane. For well prepared bonded sa mples some compressive type failures are to be expected when testing slant shear cylinders (Kriegh, 1976) However, it was noted that even t he weakest bonded surface (Slab 2 B) failed in compression and not in shear failure at the bonded interface Referencing the curves developed in Figure E 2 the shearing stress on the bonded interface at a 45 degree slant is equal to about 15 percent of the applied compressive stress for a relatively smooth substrate surface. For Slab 2 B th e direct shear test produced an average shear strength of 1.19 MPa (173 psi). Applying the curve value of 15 percent, one would expect s lip failure along the bond surface at a compres sive stress of 1.19/0.15 = 7.93 MPa (1,150 psi). Instead, the observed fa ilure of the Slab 2B slant shear specimen occurred at a compressive stress exceeding 35 MPa (5,000 psi). It appears that Concrete Surface Profile 6 used in the preparation of the substrate is sufficiently rough to dramatically increase the frictional slip resistance resulting from the normal (clamping) force when the slant angle is 45 degrees. E.3 Transformed Slant Shear Results Compression stresses on the slant shear cylinders at failure have been included in Chapter 4.7 From these tables, using the stress transformations discussed in Chapter 3.2.5, the maximum shear stress (at compression failure) at the bonded interface may be calculated, as shown in Tables E 1 (SI units) and E 2 ( US c us tomary u nits). A statistical analysis was performed on the data set to compute the average shear strength, standard deviation, and coefficient of variation. The statistical analysis was omitted for Slabs 1B and 2B due to lack of data.

PAGE 132

121 Table E 1 Maximum Shear at Slant Interface (SI units) Slab ID Pretreatments Shear S tress at Interface (kPa) Statistical Analysis Test 1 Test 2 Test 3 Test 4 Average (kPa ) Std. Dev. (kPa ) COV 1A Dry / No Agent 18,706 12,831 16,610 18,430 16,644 2344 14.1% 1B D ry / Wet Agent 20,671 20,284 20,478 2A S SD / Wet Agent 22,374 18,843 19,250 19,381 19,962 1406 7.0% 2B S SD / Dried Agent 17,506 17,506 3A Wet / Wet Agent 14,989 13,796 18,195 15,660 1857 11.9% 3B S SD / No Agent 19,533 15,458 18,223 13,321 16,634 2786 16.8% Table E 2 Maximum Shear at Slant Interface (U.S. customary units) Slab ID Pretreatments Shear Stress at Interface (psi) Statistical Analysis Test 1 Test 2 Test 3 Test 4 Average (psi) Std. Dev. (psi) COV 1A Dry / No Agent 2,713 1,861 2,409 2,673 2,414 340.0 14.1% 1B Dry / Wet Agent 2,998 2,942 2,970 2A SSD / Wet Agent 3,245 2,733 2,792 2,811 2,895 204.0 7.0% 2B SSD / Dried Agent 2,539 2,539 3A Wet / Wet Agent 2,174 2,001 2,639 2,271 269.4 11.9% 3B SSD / No Agent 2,833 2,242 2,643 1,932 2,413 404.1 16.8% E.4 Discussion of Slant Shear Results Based on the observed failure mode, it was suspected that the strength results were affected primarily by the compressive strength of the concrete, and not by the properties of the bond. It was further surmised that both the substrate and overlay compressi ve strengths jointly controlled the failure mode. To test this premise, the shear strength result was divided by the average of the substrate and overlay compressive strengths. Table E 3 lists the adjusted results. Note that c,avg is unitless, theref ore, the table has not been reproduced in SI units.

PAGE 133

122 Table E 3 Slant shear strength adjustment (U.S. customary units) 28 Day Compressive Strength (psi ) Avg. Interfacial Shear at Failure ( ) (psi ) Factored Interfacial Shear at Failure / f'c (avg) Slab ID Pretreatments Substrate Overlay 1A Dry / No Agent 6,092 5,951 2,414 0.40 1B Dry / Wet Agent 6,092 6,879 2,970 0.46 2A SSD / Wet Agent 5,683 6,244 2,895 0.49 2B SSD / Dried Agent 5,683 4,625 2,539 0.49 3A Wet / Wet Agent 6,266 6,583 2,271 0.35 3B SSD / No Agent 6,266 3,069 2,413 0.52 At a slant angle of 45 degrees, the transformed shear stress on the slant surface is equal to one half of the applied compressive stress on the cylinder. Therefore, a failure in compression should result in a c,avg ratio of about 0.50. Indeed, this appears to be the case for the majority of specimens. Slabs 1 A and 3A, with ratios of 0.40 and 0.35, respectively, did fail prior to the anticipated compressive strength of the concrete. However, even for these samples, the observed failure mode was not in shear at the bond surface. This analysis confirms that the r esults are representative only of the compre ssive strength of the concrete. Figure E 3 graphs the results of the slant shear tests with the average compressive strength of the substrate and overlay slabs corresponding to each sample. A best fit line has be en added to illustrate the relationship.

PAGE 134

123 Figure E 3 Compressive strength vs. measured slant shear strength E.5 Conclusions The observed failure mode and analysis of the results confirm that the results of the slant shear specimens are unaffected by the shear strength of the bonded interface and are indicative solely of the compressive strength of the concrete. Although the majority of slant shear samples are prepared in the laboratory using a cylinderical mold and plate, some past research has had good success testing slant shear samples obtained via coring. However, obtaining these cores outside of a laboratory environment presents some unique challenges, since a coring angle of 60 degrees or greater is beyond the capability of the equipment used in thi s test and would have required a special jig and additional footprint on each overlay slab. It is concluded that the slant shear test using cored specimens does not lead to a reliable measure of shear stress at a bonded concrete interface. y = 0.4423x R = 0.615 0 10,000 20,000 30,000 40,000 50,000 0 5,000 10,000 15,000 20,000 25,000 0 500 1,000 1,500 2,000 2,500 3,000 3,500 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 Average compressive strength (substrate and overlay) (kPa) Shear at slant interface at failure (kPa) Shear at slant interface at failure (psi) Average compressive strength (substrate and overlay) (psi)