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Beneficial use of buffed rubber as a fiber mesh in concrete mixtures

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Title:
Beneficial use of buffed rubber as a fiber mesh in concrete mixtures
Creator:
Jones, Karaline Melinda
Publication Date:
Language:
English
Physical Description:
xiii, 176 leaves : illustrations ; 28 cm

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Subjects / Keywords:
Concrete -- Additives ( lcsh )
Fibers ( lcsh )
Rubber ( lcsh )
Concrete -- Additives ( fast )
Fibers ( fast )
Rubber ( fast )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Includes bibliographical references (leaves 172-176).
General Note:
Department of Civil Engineering
Statement of Responsibility:
by Karaline Melinda Jones.

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Source Institution:
|University of Colorado Denver
Holding Location:
|Auraria Library
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
757515426 ( OCLC )
ocn757515426
Classification:
LD1193.E53 2011m J66 ( lcc )

Full Text
BENEFICIAL USE OF BUFFED RUBBER AS A FIBER MESH IN CONCRETE
MIXTURES
by
Karaline Melinda Jones
B.S. Civil Engineering, University of Colorado Denver, 2007
A thesis submitted to the
University of Colorado Denver
In partial fulfillment
of the requirements of the degree of
Master of Science, Structural Engineering
Civil Engineering
2011


This thesis for the Master of Science
degree by
Karaline Melinda Jones
has been approved
by
Dr. Stephan A. Durham
Dr. Brian Brady


Jones, Karaline Melinda (M.S. Structural, Civil Engineering Department)
Beneficial Use of Buffed Rubber as a Fiber Mesh in Concrete Mixtures
Thesis directed by Dr. Stephan A. Durham
ABSTRACT
The research investigates the effects that buffed rubber fiber mesh has on the fresh
and hardened properties of concrete. Nine different concrete mixtures containing
varying amounts of the two fiber meshes were designed and batched for this research.
Only the fiber contents and sand volume changed for the nine mixtures. All of the
other materials (cement content, fly ash content, coarse aggregate, and water) and
properties (water to cementitious ratio (w/c)) remained the same. Three concrete
mixtures were prepared with PP blended fibers in the amounts of 0.5%, 1.5% and 0.5
cubic feet (cf) (0.014kg/m ). Five mixtures were tested with buffed rubber. The
concretes contained amounts of 0.5%, 1.5%, 0.5 cf (0.014 kg/m2), 1.5 cf (0.042
kg/m ), and 2.5 cf (0.071 kg/m ). A control mixture containing no fiber mesh was
also prepared. A direct comparison between the 0.5%, 1.5% and 0.5 cf (0.042
kg/m2) mixtures was made and statistically analyzed. The fresh concrete tests
included slump, unit weight, and air content. The hardened concrete properties
examined were compressive strength, flexural strength, splitting tensile strength and


permeability. The slump and workability results varied with the addition of fibers.
The air content was increased and the unit weight was decreased with fibers. It was
determined that the compressive strength was decreased and the flexural strength was
increased with the addition of fibers, but the test data was found to be not
significantly different. The fiber concretes yielded a residual stress capacity after the
initial failure as the Control concrete did not.
This abstract accurately represents the content of the candidates thesis. I recommend
its publication.
Signed
Dr. Stephan A. Durham


TABLE OF CONENTS
Figures.....................................................................ix
Tables......................................................................xii
Chapter
1. Introduction......................................................1
2. Literature Review.................................................5
2.1. Preface...........................................................5
2.1.1. History of Fiber Mesh Reinforcement............................. 6
2.2. Synthetic Fiber Reinforced Concrete...............................7
2.2.1. Production and Properties.........................................7
2.2.2. Mixture Proportioning.............................................9
2.2.3. Effects on Fresh Concrete Properties.............................11
2.2.3.1. Slump............................................................11
2.2.3.2. Air Content and Unit Weight......................................12
2.2.3.3. Plastic Shrinkage................................................12
2.2.4. Effects on Hardened Concrete Properties..........................14
2.2.4.1. Strength.........................................................14
2.2.4.2. Permeability.....................................................16
2.2.5. Summary..........................................................18
v


.18
.18
.20
.23
.23
.23
.24
.24
.25
.25
.29
.29
.29
.31
.31
.34
.34
.35
,35
,37
39
Rubber Fiber Reinforced Concrete
Production and Properties....................
Overview of Tire Retread Manufacturing Process
Mixture Proportioning........................
Effects on Fresh Concrete Properties.........
Slump........................................
Air Content and Unit Weight..................
Plastic Shrinkage............................
Effects on Hardened Concrete Properties......
Strength.....................................
Permeability.................................
Summary......................................
Conclusion...................................
Problem Statement............................
Statement....................................
Experimental Plan............................
Design Summary...............................
Material Properties..........................
Portland Cement..............................
Fly Ash......................................
Course and Fine Aggregates...................
vi


.40
.41
.41
.42
.43
.45
.50
.51
.52
.54
.55
.55
.55
.55
.56
.56
.57
.57
.58
.58
.61
Chemical Admixtures
Air Entraining Admixture (AEA)......................
High Range Water Reducing Admixture (HRWRA).........
Polypropylene (PP) Fibers...........................
Buffed Rubber Fibers................................
Mixture Designs.....................................
Mixture Batching....................................
Curing..............................................
Concrete Testing....................................
Summary.............................................
Experimental Results................................
General.............................................
Problems with this Study............................
Spreadsheet/user error with percent vs. CF replacements..
Poor consolidation for 0.5 CF PP Mixture............
Needed more PP Fibers than the Quantity on Hand ....
Not Enough Beam Molds...............................
Day 56 Permeability Tests were not performed on Day 56
Fresh Concrete Properties...........................
Slump...............................................
Air Content.........................................
VII


5.4.3. Unit Weight....................................................62
5.5. Hardened Concrete Properties...................................65
5.5.1. Compressive Strength...........................................66
5.5.2. Flexural Strength..............................................89
5.5.3. Splitting Tensile Strength.....................................97
5.5.4. Permeability..................................................104
5.6. Summary.......................................................110
6. Statistical Analysis..........................................113
6.1. Introduction..................................................113
6.1.1. Compressive Strength Analysis.................................114
6.1.2. Flexural Strength Analysis....................................114
6.1.3. Splitting Tensile Strength Analysis...........................115
6.1.4. Permeability Analysis.........................................116
6.2. Summary.......................................................117
7. Conclusion....................................................118
7.1. Summary.......................................................118
7.2. Recommendations for Future Studies............................121
Appendix
A. Concrete Mixture Design.......................................123
B. Material Product Data Sheets..................................133
viii
Bibliography
172


FIGURES
Figure
2.1 Photo of Buffed Rubber Samples...............................20
2.2 Photo of Tire Retreading Stages..............................21
2.3 Diagram of behavior of RAC under compression (Courtesy of
Nehdi and Khan, 2001)........................................27
4.1 Photo of Polypropylene Fibers................................43
4.2 Photo of Buffed Rubber Particles.............................44
5.1 0.5 CF PP Fresh Concrete Mixture Photo.......................56
5.2 Mixture Slump Results........................................60
5.3 Unit Weight and Air Content..................................64
5.4 Compressive Strength Gain, Control and Buffed Rubber
Concrete.....................................................68
5.5 Compressive Strength Gain, Control and PP Fiber Concrete.....68
5.6 Normalized Compressive Strength Gain for Concrete Mixtures...69
IX


5.7 Normalized Compressive Strength Gains for Concrete
Mixtures....................................................70
5.8 Compressive Strength Gain for 0.5 % by Volume Fiber
Mixtures....................................................75
5.9 Compressive Strength Gain for 1.5% by Volume Fiber
Mixtures....................................................77
5.10 Compressive Strength Gain for 0.5 CF Fiber Mixtures.........79
5.11 Compressive Strength Gain for 0.5, 1.5, and 2.5 0.5 CF
Mixtures....................................................81
5.12 28 Day Normalized Compressive Strengths compared to
CDOT Class P Requirements...................................82
5.13 28 Day Compressive Cylinder Test Photos....................83
5.14 28 Day Compressive Test Loading tests for Buffed Rubber
Concrete....................................................84
5.15 28 Day Compressive Test Loading tests for PP Fiber Concrete.85
5.16 56 Day Compressive Test Loading tests for Buffed Rubber
Concrete....................................................85
x


5.17 56 Day Compressive Test Loading tests for PP Fiber Concrete....86
5.18 Flexural Beam Loading Diagram (Courtesy of ASTM C78,
2002).........................................................90
5.19 28 and 56 Day Flexure Strengths Line Graph.....................92
5.20 28 and 56 Day Flexure Strengths Column Graph...................92
5.21 Flexure Strength Compared to CDOT Class P Requirements...........95
5.22 28 Day Flexural Beam Test Photos...............................96
5.23 28 and 56 Day Splitting Tensile Strengths Line Graph...........99
5.24 28 and 56 Day Splitting Tensile Strengths Column Graph........100
5.25 28 Day Splitting Tensile Test Photos From Cylinder End........102
5.26 28 Day Splitting Tensile Test Photos From Side of Cylinder....103
5.27 Abnormal Control Cylinder Failure Photo........................104
5.28 Results of RCPT Testing compared to CDOT Class H
Requirements..................................................107
XI


TABLES
Table
2.1 Range of Mixture Proportions for Normal Weight Macro Fiber-
reinforced Concrete (ACI 544.3R-08, Table 4.1, 2008).............10
4.1 Colorado Department of Transportation (CDOT) Class P
Concrete specifications and field requirements...................34
4.2 Holcim Type I-II Cement Physical and Chemical Properties..........36
4.3 Boral Class F Fly Ash Physical and Chemical Properties............38
4.4 Coarse and Fine Aggregate Properties..............................40
4.5 Concrete Mixture Design Matrix....................................45
4.6 Fresh and Hardened Concrete Property Tests........................54
5.1 Fresh Concrete Properties.........................................58
5.2 Measured Unit Weights, Theoretical Unit Weights and Air
Contents..........................................................63
5.3 Average Compressive Strengths.....................................66
5.4 Average Normalized Compressive Strengths..........................67
xii


5.5 Average Flexure Strengths...........................................90
5.6 Average Splitting Tensile Stresses..................................98
5.7 ASTM C 1202 RCPT Permeability Classifications......................105
5.8 Results of RCPT Testing and ASTM Classification....................107
6.1 Compressive Strength Paired t-test Results.........................114
6.2 Flexural Strength Paired t-test Results............................115
6.3 Splitting Tensile Strength Paired t-test Results...................116
6.4 Permeability Paired t-test Results.................................117
xiii


1.
INTRODUCTION
Tires and rubber are a large part of our everyday lives. Tires are used for our
daily commute and as we tend to other daily business. Everything we use as
consumers is delivered or effected by the trucking industry. But, what happens to
these tires when they have been worn past the point of safe driving? Some tires,
namely truck tires, are recycled and given a new life through re-treading.
Millions of other tires end up in landfills, polluting the earth. It is not a new idea
to reuse old products and goods in new ways, so that is why in recent years scrap
tires have been used in the production of concrete.
Rubber has been used in concrete in many forms in the past. Waste tire
rubber has been ground into small particles and used as a small aggregate to
replace a portion of sand in the concrete mixture. Large pieces of recycled tire
have been used as large aggregate and filler in concrete. Rubber has even been
used to create insulation and sound barriers in concrete block construction.
However, this thesis specifically looks at scrap rubber from the re-treading
process used as a fiber mesh in concrete.
Truck tires that are reused go through a re-treading process to prepare the
casing for the new treads. During this process the old tread is removed and the
casing is scraped down such that there is a smooth surface for the new tread to
adhere to. There is a significant amount of rubber that is removed during this
process and discarded. This rubber is called buffed rubber. The buffed rubber is
1


removed from the tires and then stored in onsite trailers until it can be disposed of.
Currently the only market for this discarded rubber is in the manufacturing of
floor mats and outdoor tracks. The rubber particles vary in size and shape, but
they are mostly long and slender. Today it is common practice to recycle. It is
present in the home as well as in industry. What can we do to improve our planet
and find greener ways to live is on the forefront of everyones minds and is
reflected in the way that our lives are changing daily. Concrete is the most
commonly used construction material used today. Scientists and concrete
manufacturers have been looking for ways to make concrete more
environmentally friendly for years. One of the potential ways to do this is by
using recycled rubber fiber mesh to increase the flexural capacity of concrete.
This thesis focuses on the inclusion of these fibers for concrete pavement and
slabs.
There are different types of fiber meshes currently on the market. These
mainly consist of fiber made from steel, glass, plastics, and carbon. Most of these
products are made from manufacturing virgin materials into final products rather
than using recycled materials. It is difficult to justify the use of products made
from virgin materials when there is so much waste that has the potential to be
incorporated into new goods. However it is important to understand the effects
that commercially available materials have on concrete and allow a basis in which
to compare the experimental test data. The fibers used for comparison with the
2


buffed rubber fibers for this research are a blend of polypropylene (PP) macro and
micro fibers.
The research investigates the effects that buffed rubber fiber mesh has on
the fresh and hardened properties of concrete. Nine different concrete mixtures
containing varying amounts of the two fiber meshes were designed and batched
for this research. Only the fiber contents and sand volume changed for the nine
mixtures. All of the other materials (cement content, fly ash content, coarse
aggregate, and water) and properties (water to cementitious ratio (w/c)) remained
the same. For fresh and hardened concrete properties all the mixtures were
compared to a control mixture with no fiber mesh. Prior to batching, the specific
gravity of the rubber was tested.
Three concrete mixtures were prepared with PP blended fibers in the
amounts of 0.5%, 1.5% and 0.5 cubic feet (cf) (0.014kg/m2). Five mixtures were
tested with buffed rubber. The concretes contained amounts of 0.5%, 1.5%, 0.5 cf
(0.014 kg/m2), 1.5 cf (0.042 kg/m2), and 2.5 cf (0.071 kg/m2). A control mixture
containing no fiber mesh was also prepared. A direct comparison between the
0.5%, 1.5% and 0.5 cf (0.042 kg/m ) mixtures was made and statistically
analyzed. The fresh concrete tests included slump, unit weight, and air content.
The hardened concrete properties examined were compressive strength, flexural
strength, splitting tensile strength and permeability.
3


The following chapters include a literature review, problem statement,
experimental plan, experimental results, and statistical analysis. A literature
review was performed to investigate past research completed regarding rubber
particles being used as a fiber mesh, PP fiber mesh use in concrete, and concrete
properties associated with these additions. The problem statement includes
specific data that was tested for including the different concretes and fiber
replacements that were examined. The specific tests performed, including testing
procedures are discussed in the experimental plan. This section also includes the
properties of the materials used and concrete batch design tables. The
experimental results chapter includes test data on fresh and hardened concrete
properties. The fresh concrete test results include slump, air content, and unit
weight. The hardened test results include information on compressive, flexural,
and splitting tensile strength tests as well as permeability results. A statistical
analysis was performed for concretes with like fiber replacements and the results
are examined in the statistical analysis section.
4


2.
LITERATURE REVIEW
2.1. Preface
Fibers have been used to reinforce concrete, adobe, mortars and other brittle
building materials since ancient times. Since the 19th century it has been a rather
common construction practice, first with the use of asbestos and then with the use
of plastics and metals that we use today. In the 1960s and 1970s commercially
made products have been readily available (ACI 544.1R-96, 2002). With such a
long usage time within the construction industry there is no doubt that there has
been much research done on the addition of commercially available fibers in
concrete. While much research has been done on manufactured fibers being
added to concrete, the addition of rubber to concrete is a relatively new idea.
Adding large rubber chunks as large aggregate replacement (Tantala et al, 1990;
Bakri, 2007; Cairns, 2004) and smaller, crumb rubber particles as fine aggregate
replacement has been researched a fair amount with plenty more research still
needing to be done (Nehdi and Khan, 2001; Kaloush, 2004). Little research has
been performed on rubber being added as fiber mesh reinforcement in concrete
mixtures. The fiber mesh particles are produced when a tire is retread. The
modem process that these particles come from is relatively new and has only been
around fifty or so years. However the uses of these scrap particles is limited to
very few applications and more and more tires are being retread everyday.
5


2.1.1. History of Fiber Mesh Reinforcement
As far back in time as ancient Egyptian fibers were being used in building
materials. As early as 1200 BC straw fibers were being used in combination with
clay to make bricks in Egypt. For this mudbrick, the straw was added to a
mixture of clay, mud, sand, and water and left in the sub to dry (Wikipedia,
2010) . Adobe is a type of mudbrick also used today to save energy and is an
environmentally safe way to insulate a house. This type of house tends to stay
cool in the summer and warm in the winter (Wikipedia, 2010). In ancient times
horse hair was a common additive in mortars (Wikipedia, 2011).
In the 19th Century asbestos fibers were used in the manufacturing of
concrete bricks, concrete, and fireplace cements until the 1950s (Wikipedia,
2011) . The Hatschek process was invented in 1898 and lead the way for mass
production of asbestos fibers for construction usage (ACI544.1R-96, 2002). Also
until the 1950's horse hair was a common fiber used in the construction of lath
and plaster walls. The horse hair was used to reinforce the keys on the backside
of the lath layer so that the plaster would not fall away from the lath. After the
1950's however the lath and plaster method of wall finishing was replaced with
drywall (Wikipedia, 2011).
In the 1960's the concept of composite materials came into light.
Composite materials are engineered or naturally occurring materials made from
two or more constituent materials with significantly different physical or chemical
6


properties which remain separate and distinct at the macroscopic or microscopic
scale within the finished structure. It was at this point where steel, glass, and
synthetic fibers started to be produced. Synthetic fibers include plastics, such as
polypropylene and polyethylene, carbons, nylon, and other fibers (Wikipedia,
2011). The first experiments done on the addition of synthetic fibers were largely
unsuccessful (Monfore, 1968 and Goldfein, 1963). However, better
understanding of the concepts behind fiber reinforcement, new methods of
fabrication, and new types of organic fibers have led researchers to conclude that
both synthetic fibers can successfully reinforce concrete (Krenchel, 1985 and
Naaman, 1982).
2.2. Synthetic Fiber Reinforced Concrete
2.2.1. Production and Properties
Synthetic fibers for reinforcing concrete are made by a variety of companies for
use in concrete. Synthetic fibers can come as macro fibers, micro fibers or a
blend of the two. The fibers are generally made from plastic materials, such as;
acrylic, nylon, aramid, polyester, carbon blends, polyethylene, or polypropylene
(ACI 544.1R-96, 2002). For many of these fibers, there is little reported research,
while others are found in commercial applications and has been the subject of
extensive reporting. This thesis will concentrate on research done and the usage
of polypropylene fibers only.
7


ACI 544.1R-96 has found that cast-in-place concrete will accommodate
up to 0.4 percent by volume of synthetic fibers with minimal mix proportion
adjustments. Fiber length and fiber configuration are important factors in
determining fiber content. Macro-synthetic fibers are larger particles that are
linear in shape and range from about 0.5 to 2.5 inches (1.3 to 6.4 cm) in length.
They can be sinusoidal or flat in shape. A dimpled shape however allows for a
better bond (ACI 544.1R-96), this type of shape is called fibrillated. Macro-
synthetic fibers are made of PP which is a hydrophobic material. Because of the
hydrophobic nature of the plastic, the concrete has been shown not to chemically
adhere to the fibers but to bond mechanically. This makes these fibers more
similar to steel reinforcing. In concrete these fibers lay in an irregular, multi-
directional pattern creating a matrix of fibers that overlap and intertwine. With
these fibers in place, the concrete bond to the fibers fails first, as it is the weakest
link in the concrete matrix. Although not as strong as steel these fibers are
effective for reducing cracking and binding concrete together. These fibers are
produced by an extrusion process (ACI 544.1R-96, 2002).
Micro fibers are made from synthetic hair like fibers that are 0.25 to 0.5
inches (0.6 to 1.3 cm) in length. The purpose of the micro fibers is to address
early age cracking problems (plastic shrinkage) (ACI 544.1R-96, 2002). Micro-
synthetic fibers prevent 80-100% of all cracks when concrete is in the plastic state
8


(Concrete Solutions by Propex, 2011). Concrete has more of a tendency to crack
while it is in the plastic state.
Hybrid fibers are a blend of macro and micro synthetic fibers. They take
the benefits of both the fiber types and use them together. Synthetic fiber mesh
can be used in lieu of welded wire fabric reinforcing in concrete or in addition to
it (ACI 544.3R-08, 2008).
2.2.5. Mixture Proportioning
Recommendations for the percentage of macro fiber mesh replacement are given
in the range of 0.2% to 2.0% by ACI 544.3R-08. Depending on maximum
aggregate size ACI gives varying ranges of other ingredients for producing the
most ideal concrete, see Table 2.1. Specifically, it is stated that low dosages of
micro synthetic fibers, less than 3 lb/cy (~0.16%), can generally be added without
changing conventional concrete mixtures. For higher volumes, adjustment of the
fine-to-coarse aggregate ratio may be required.
9


Table 2.1 Range of Mixture Proportions for Normal Weight Macro Fiber-
reinforced Concrete (ACI 544.3R-08, Table 4.1, 2008)
Maximum aggregate size, in (mm) 3/8 (9.5) 3/4 (20) 1-1/2 (38)
Mixture Parameters
Cementitious material, lb/cy (kg/m3) 600 to 1000 (356 to 593) 500 to 900 (297 to 534) 470 to 700 (279 to 415)
w/c 0.35 to 0.45 0.35 to 0.50 0.35 to 0.55
Percent of fine to coarse aggregate 45 to 60 45 to 55 40 to 55
Fiber content, volume % 0.3 to 2.0 0.2 to 0.8 0.2 to 0.7
Proportioning macro synthetic in fiber reinforced concrete (FRC) is similar to
proportioning steel fibers. Both types of fibers follow the recommendations of
Table 2.1. As the fiber volume increases to greater than 0.5% by volume,
adjustments to aggregate ratios and use of water-reducing admixtures become
more necessary (ACI 54.3R-08, 2008). Hybrid fibers can follow the general
aforementioned recommendations. Additional information can be found in the
manufacturers product data for each type of PP fiber. For each fiber blend there
is given a recommended minimum and maximum replacement; for macro, micro
and blended types of fiber mesh. For hybrid fibers lower dosages have been
added to concrete mixtures (0.1%) compared to concrete mixtures when only a
macro fiber has been used (0.25%) (Concrete Solutions by Propex, 2011).
According to ACI tests, data has been compiled for composites containing PP
10


fibers at volume percentages ranging from 0.1 to 10.0%. The material properties
of these composites vary greatly and are affected by the fiber volume, fiber
geometry, method of production and composition of the matrix. Optimum
mixture proportions should be obtained by trial mixtures when using higher fiber
volumes. Smaller volumes of replacement were shown to be more effective
(Vondran et al, 1990).
2.2.6. Effects on Fresh Concrete Properties
2.2.6.I. Slump
ACI reports (ACI 544.1R-96, 2002) that workability and slump still remain within
a satisfactory range even when up to 2% by volume replacement of PP fibers were
used. To maintain workability high-range water reducers were used to maintain
the water to cement ratio (w/c). In another ACI publication (ACI 544.2R-89) it is
stated that slump is not a good indicator for the workability of concrete, but
should be used instead only to determine the consistency between batches. In
general, the slump for FRC determined from ASTM C143/C143M should be at
least 1 inch (2.5 cm), but not greater than 7 inches (17.8 cm). Factors that
influence the measured slump include the fiber type, length, aspect ratio, amount
of fibers, cement content, fine aggregate content, aggregate shape, and grading
(ACI 544.1R-96, 2002).
Another method to test workability has been developed specifically for
FRC. The time of flow through inverted slump cone test (ASTM C 995) takes
11


into the account the mobility of the concrete when subject to vibration. The
slump test when this alternate test is used is merely for monitoring the consistency
of concretes from batch to batch. It has been noted that even at very low slump,
FRC mixtures respond well to vibration.
2.2.6.2. Air Content and Unit Weight
In general air content and unit weight are specified and tested for a FRC the same
way that they would be for a plain concrete. Due to the low specific gravity of the
PP mesh the unit weight is expected to be lower than that of plain concrete. In
tests performed by ACI the air content has been shown to be higher than expected
in FRC mixtures that have had 2% volume replacement with PP (ACI 544.3R-08).
At smaller percentage replacements the air content has been shown to be
unaffected. As PP volume replacement is increased unit weight has been shown
to decrease with up to a 2% replacement by volume during fresh concrete testing.
At smaller percent replacements the unit weight is shown to remain unaffected by
adding synthetic fibers. When more fibers are added more clumping, segregation
and bleeding have been reporting leading to increased air content and a decreased
unit weight (ACI 544.3R-08). Tests are to be run per ASTM C 138, C 173, and
C231 just as with a non fiber reinforced concrete.
2.2.6.3. Plastic Shrinkage
As noted above different types of fibers have different functions within concrete.
Micro fibers primary purpose is to decrease cracking when concrete is in it plastic
12


state. The plastic state of concrete occurs immediately after placement of the
concrete prior to the concrete obtaining its rigidity. Plastic shrinkage will occur
when the bleed water at the concrete surface evaporates at a faster rate than
supplied from within the concrete. If this phenomenon occurs, the concrete will
begin to crack due to tensile stresses created at the concrete surface. Past research
has shown that the finer the fiber, the more effective it is at reducing plastic
shrinkage cracks. (Qi and Weiss 2003; Banthia and Gupta 2006). Naaman et al.
(2005) evaluated the effect of several fibers on the plastic shrinkage cracking
characteristics of concrete and found that for a given percentage volume of fibers,
changing the fiber length or aspect ratio did not have a noticeable effect on plastic
shrinkage cracking. However, decreasing the fiber diameter and thus the number
of fibers crossing a section of concrete did significantly improve the control of
plastic shrinkage cracking. At a percentage volume of 0.2%, most fine-diameter
synthetic fibers tested provided a reasonable control of plastic shrinkage cracking,
reducing the number of cracks to approximately 10% of the control.
Najm and Balaguru (2002) studied the effect of polymeric fibers on the
reduction of crack width caused by plastic and drying shrinkage. The percentage
of fiber content ranged from 7.5 to 45 lb/cy (4.4 to 26.7 kg/m3). It was found that
the addition of fibers increased the strain capacity and reduced the crack width
compared to the control sample. At 15 lb/cy (8.9 kg/m3) the crack width was
reduced by 60%. In addition, it was found that adding half the amount of
13


polymeric macro-fibers provided the same reduction in width of cracks as steel
fibers.
Ma et al. (2002) studied the shape that different micro fibers have on
plastic shrinkage. The test results showed that PP micro fibers with circular cross
section had a more pronounced effect on the resistance to plastic shrinkage
cracking than PP micro fibers with rectangular cross section made by a fibrillated
film technique. And better yet the PP micro fibers with a Y-shape cross section
were slightly more effective than the round or the rectangular shaped fibers.
Qi et al. (2003) used image analysis to systematically characterize the size
of the plastic shrinkage cracks that developed. They found it crucial to use a large
set of measurements to obtain statistically reproducible results because of the
large variation typical in crack width measurements. In addition to a reduction in
the width of the plastic shrinkage cracks, fibrillated fibers were effective in
reducing the rate of corrosion.
2.2.7. Effects on Hardened Concrete Properties
2.2.7.I. Strength
Compressive strength tests have been inconsistent thus far in the research
performed with FRC. ACI 544.1R-96 states that in general compressive strength
has not been affected by adding PP fibers. The minor differences noticed are
expected variation in experimental work. In addition, the differences can be due to
14


variations in the actual air contents of the hardened concrete and the differences in
their unit weights.
At higher volume replacements (greater than 2.0%) the compressive
strength has been shown to decrease (Ramakrishnan, 1989). This decrease is due
to the fibers clumping, mixture segregation and bleeding of concrete. The
addition of PP fibers has shown a significant effect on the mode and mechanism
of failure of concrete cylinders in a compression test. Plain concrete cylinders
typically shatter due to an inability to bounce back to its original state after initial
failure and load release. The FRC fails in a more ductile mode and can continue
to sustain load and endure large deformations without shattering into pieces
(Nagabhushanam et al, 1989). In an investigation by Vondran et al (1990) it was
found that there was no reduction in compressive strength when 0.1 to 1.0% by
volume of fibers was added. The same compressive strength tests used for non-
FRC are to be used for FRC (ASTM C31, C39, and Cl92).
Compressive tests indicate the FRC fails in a more ductile way, thus this
type of concrete should be tested for flexural strength as well. Flexure beams will
be tested per ASTM C78, 0018, or C293. Similar to compressive strength
results, there is no consensus in the published literature about the effect of adding
PP fibers on the modulus of rupture. Zollo (1984) found that there was a slight
increase (0.7 to 2.6%) in the flexural strength when PP fiber was at 0.1%
replacement. At 0.2 to 0.3% replacement there was a slight decrease. Other tests
15


have reported that from a 0.1 to 0.3% replacement that there was an increase in
the modulus of rupture (MOR) at 28 days of age when compared to plain concrete
(Ramakrishnan et al 1987). In a later experiment performed by Ramakrishnan in
1989 the MOR decreased as the amount of fiber was increased from 0.1 to 2.0%
and all other mixture proportions remained the same. There has been an incorrect
direct comparison to low compressive strengths indicating low flexural strengths.
For the 1.0, 1.5 and 2.0% volume replacements in the experiment above the
compressive strength was low due to the higher air content, when flexure strength
was tested is was also low. It was easy to jump to the conclusion that these two
failure mechanisms might be related. In Vondrans (1990) experiment, the
mixture proportions were optimized by trial mixtures for higher quantities of
fibrillated PP. The compressive and MOR tests showed no changes at higher
volume percentages. The modulus of elasticity of PP fibers is between 217,000
and 290,000 psi (1496.2 and 1999.5 MPa) (Wikipedia, 2011).
2.2J.2. Permeability
Permeability is an important aspect to concrete. With time, concrete is subject to
degradation through corrosion, alkali-silica, sulfate attack, freeze thaw, and other
problems caused by water infiltration. Permeability is less likely in a dense
concrete. With a w/c of about 0.45 the permeability is nearly negligible in an
uncracked condition (Wang et al, 1997). Permeability, however, is vastly
increased by the introduction of cracks and increases in crack width (Wang et al.
16


1997). Since fiber reinforcement influences the way cracks develop, has a great
deal of influence on crack growth, and a greater likelihood for crack branching
and multiple crack development, the addition of fiber reinforcement may be used
to significantly reduce the permeability of cracked concrete.
To directly investigate the effects fibers have on the durability of concrete
structures, Sanjuan et al. (1997) included PP fibers in the top portion of a
conventional steel reinforced slab. Concrete was cast around steel bars in
specimens that were then exposed to restrained shrinkage conditions immediately
after casting. After curing, the specimens were ponded with a chloride ion-rich
solution while the rate of corrosion in the steel was monitored. The FRC cracked
less and demonstrated lower corrosion rates showing the direct impact of fibers on
permeability. Concrete materials that contain cracks of approximately 0.004 inch
(0.1 mm) in width show a substantial increase in water permeability both for plain
concrete and FRC (Wang et al., 1997). At this type of cracking limit the
durability of concrete could potentially be maximized. Further work is required
to confirm a relationship between crack width and concrete durability, but FRC
has proved to be a valuable tool in reducing cracking. As noted above, however,
with larger percentages of fiber the concrete will begin to segregate and clump,
this will inherently lead to more porous concrete. As the amount of fiber mesh is
increased in the mixture the permeability will also increase.
17


2.2.8. Summary
Fiber mesh can be added at small percentages, less than 1.0% until it starts
decreasing its effectiveness. Higher quantities of replacement should be subject to
trial batches to determine the effectiveness of the mixture. Concrete slump,
uniformity, workability, and strength all decrease with the addition of more fiber
mesh from a concrete with no fiber mesh. Permeability will decrease with the
addition of fibers up to a certain point. All synthetic fiber meshes are made from
plastics and can be made up of micro fibers, macro fibers, or a combination of the
two, called hybrids. Strength has been shown to increase compared to plain
concrete in mixtures.
2.3. Rubber Fiber Reinforced Concrete
2.3.5. Production and Properties
Rubber has been used in concrete in a variety of ways. Larger chunks have been
used as a large aggregate replacement (Tantala et al, 1990; Bakri, 2007; Cairns,
2004). Smaller processed pieces of rubber have been used as fine aggregate
replacement (Nehdi and Khan, 2001; Kaloush, 2004, Kardos, 2011). This thesis
however is interested in shredded rubber particles that can be used as fiber mesh
reinforcement. In an effort to be more sustainable and preserve resources for
many generations to come, recycling and reusing have been a hot topic in recent
years. Tires are used in our everyday lives to deliver us, our food, our clothing
and other resources. Tires wear out over time. Many truck tires are re-treaded to
18


give them new life. During this process the excess tire is scraped off and
discarded leaving a new surface for the new tread. This scraping is precisely
where the rubber fiber mesh is intended to come from for this research. ASTM
defines this type of rubber as buffing rubber (ASTM D6270-08).
The particles that come from this process vary in length and width, as
shown in Figure 2.1. Most of the particles are between 0.25 and 0.75 inches (0.64
to 1.9 cm) in length. They are very similar to shredded cheese in appearance.
Prior to the particles being added to concrete the only extra step that should be
made is to discard any abnormally large particles and any extra debris that may be
in the mix. Occasionally there are pieces of metal in the rubber from the steel belt
that is in the tire core. These pieces sometimes get scrapped off in the same
manner that the rubber did. The specific gravity of rubber is approximately 1.1
(Reade Advanced Materials, 2006).
19


Figure 2.1 Photo of Buffed Rubber Samples
2.3.5.I. Overview of Tire Retread Manufacturing Process
Figure 2.2 shows the overview of the tire retreading process. This process was
witnessed firsthand by the author thanks to a local Bridgestone Bandag Tire
Solutions sales representative. The process begins with a new tire being used and
worn down until it is no longer safe to drive on (Far left tire, Figure 2.2). At this
point it is brought to the retreading shop where it is subject to inspection for hole
or any other type of damage. Three different steps take place during the initial
inspection of the tire. First the tire is visually inspected to determine if the tire
casing is even suitable for retreading. The areas that are damaged are marked for
repair at a later stage. Next the tires are subjected to electrical impulses that
20


detect nails and nail holes that may not be visible to the naked eye. During the
third part of the inspection process the tire is placed in a vacuumed pressurized
machine and the use of lasers and shearographic technology is used to map the
tire casing. This process maps the entire thickness of the casing at varying
vacuum pressures. If at any point the tire fails one of the inspection it is removed
from the assembly line and deemed unsafe for retreading.
Figure 2.2 Photo of Tire Retread Stages
After the tire has passed all of the inspections it is inflated to a normal running
pressure and all the old treading is removed. The casing is scraped smooth until it
has the exact measurements that it needs to adhere the new casing in the most
21


effective way (Second tire from left, Figure 2.2). This is where the buffed rubber
for this study comes from. During this stage in the process any holes that were
found earlier are repaired using liquid rubber and reinforced patches on the casing
interior.
During the next phase process a cushion gum is applied to the outside of
the tire where the new tread is going to go. This gum is uncured rubber that acts
as an adhesive for the new tread. Directly on top of this uncured rubber is where
the new tire tread is applied. The new tire treads are made of cured rubber that is
formed with the new tread design pattern. This new tread is precut so that it fits
around the casing perfectly (Third tire from left, Figure 2.2). The ends of the new
tread are secured to the casing using staples that will later be removed.
After the tire has been assembled the entire casing gets placed inside a
flexible rubber envelope. This envelope allows for an equal amount of pressure to
be applied to all tread and tire surfaces. The enveloped tires are pressurized to 72
psi and then placed into a pressurized curing chamber. The pressure in the
chamber is 85 psi, this is 13 psi greater than the inside pressure of the tire to
ensure that there is an even pressure distribution on all surfaces of the tire. The
temperature in the chamber is only 210F. The low curing temperature allows for
the tire casing to be retreaded several times. The envelope is removed after the
tire is done curing and a final inspection is done on the tire to make sure that all
22


holes have been repaired and that the tire can be properly inflated for safe driving
(Last tire, Figure 2.2).
2.3.6. Mixture Proportioning
There is no specific data given about mix proportioning rubber fiber mesh in
concrete. Buffed rubber seems to fit the criteria for a macro fiber according to
ACI 544.3R-5. The given recommendation for adding a macro fiber mesh is
0.2% to 2.0% depending on maximum aggregate size, see Table 2.1 (above).
Since there is no data specifically related to the percentage of rubber fiber mesh it
is assumed that the recommendations for macro fiber mesh should be used. In a
study done by Martins and Akasaki they added 3% volume replacement of
shredded rubber, well above the assumed recommendations.
2.3.7. Effects on Fresh Concrete Properties
2.3.7.I. Slump
The few tests that could be found that have been performed on buffed rubber as a
fiber mesh have shown that a decreased slump is expected with the addition of
adding rubber. In one test (Martins, Akasaki, 2005), with a 3% addition of
rubber, slump was seen to go from 5.5 inches (14 cm) for the control to a little
under 2 inches (5 cm) for the rubber mixture. In another study conducted on the
effects of coarse vs. fine rubber replacement it was found that the slump
decreased as more rubber was added and became virtually zero at a 40% by
volume replacement. Finely ground crumb rubber retained its workability much
23


better than the larger chucks or a combination of small and large pieces, but still
decreased in slump as rubber content was increased (Nehdi and Khan, 2001).
2.3.1.2. Air Content and Unit Weight
Due to the lower specific gravity of rubber versus aggregate the overall unit
weight of a rubber added concrete (RAC) will be lower than that of a plain
concrete. Nehdi and Khan (2001) found that air content is expected to increase
with the addition of rubber, but is almost negligible with rubber replacements less
than 10-20% of total aggregate volume replacement. Rubber is a hydrophobic
material and could account for the increase in air content. Hydrophobic means
that rubber repels water, and as a result of that action it also attracts air. Due to
the air now being in closer vicinity to the rubber the air is then trapped in the
rough surfaces of the buffed rubber. This series of conditions is expected to
account for the increase in air content. Due to more air the unit weight is
expected to decrease.
2.3.1.3. Plastic Shrinkage
The addition of buffed rubber in concrete has been shown to decrease the amount
of shrinkage cracks. While no specific tests have been found on the shrinkage of
RAC, there have been studies performed on mortar. Mortar is a mixture similar to
concrete made with the same basic materials with a different end purpose in mind.
In a study done by Raghaven et al (1998) cracks in mortars with buffed rubber
were compared to plain mortars. The mortar with rubber had more frequent
24


smaller cracks than the control mortar that had one main shrinkage crack. The
average crack width in the control mortar was 0.035 in (0.9 mm) while the
average crack width in the mortar with 5% buffed rubber was 0.015 to 0.024 in
(0.4 to 0.6 mm). The smaller more frequent cracks were found to have a smaller
total area of cracks than the control mortar. Even with the low bond between the
rubber and concrete the addition of rubber will still decrease the amount of
shrinkage cracks that occur. In the same study it was shown that adding rubber to
the mortar actually delayed the onset time of cracking by approximately 30
minutes allowing the concrete to gain more strength prior to the first crack
occurring. The higher the content of buffed rubber, the smaller the crack length
and crack width, and the more the onset time of cracking was delayed. Although
additional studies are necessary to confirm these observations, it appears that the
addition of buffed rubber could be beneficial for reducing plastic shrinkage cracks
of mortar and probably of concrete.
2.3.8. Effects on Hardened Concrete Properties
2.3.8.I. Strength
Numerous studies that have been conducted on a wide variety of rubber sizes in
concrete have all shown that with the addition of rubber the concrete does not
have brittle failure, but does have a decrease in compressive strength. In a 2011
study conducted by Kardos on the substitution of crumb rubber for fine aggregate
replacement it was found that compressive strength was decreased with the
25


addition of crumb rubber particles. At 28 days of age the compressive strength
was decreased by 35% from the control with a 10% by volume substitution and by
260% for concrete with 50% substitution.
Plain concrete when subject to compression has a very brittle failure, often
breaking apart the entire specimen and shooting pieces everywhere. With the
addition of rubber a more gradual failure has been observed. It is argued that
since the cement paste is much weaker in tension than in compression, the RAC
would start failing in tension before it reaches its compression limit. The
compressive stress results in many tensile micro cracks that form along the length
of the tested specimen. These cracks will propagate in the cement paste until they
encounter rubber particles. At these locations the rubber is able to withstand a
large tensile deformation prior to failing, thus acting like little springs holding the
concrete together (Figure 2.3). This spring action delays the widening of cracks
and prevents the full mass from breaking apart. The continuous application of
compressive load will cause more cracks to form and enlarge the existing ones.
During this process the sample will be able to sustain large deformations without
full disintegration. Eventually the stressed will be overcome and the bond
between the cement and rubber will fail or the rubber itself will fail.
26


Compressive Stress Compressive Stress
' > ' > ' i > f \ K i C 3 f ' f V '
Tensile Stress 25 25 Tensile Cracks 1 1 1 1 I 1 1 1 Tiny Springs JWL _M/V_ _/W\_ _/W\_ _AAA_ -WA_ JWL _j\AA_ ^WV_ _MA_
k A A A Jfk A 1 k J J
Compressive Stress Com prassive Stress
Figure 2.3 Diagram of behavior of RAC under compression (Courtesy of
Nehdi and Khan, 2001)
In a 1996 study evaluating the addition of fine aggregate replacement with crumb
rubber, Biel and Lee reported that the failure of plain concrete cylinders resulted
in explosive conical separations of cylinders, leaving the specimens in several
pieces. The severity and explosiveness of the failures decreased as the rubber
contents increased. The failure in the specimens occurred as a gradual shear that
resulted in a diagonal shear failure plane when 30 to 60% by volume of the fine
aggregate was replaced with crumb rubber (Biel and Lee, 1996). The cylinders
also did not separate and continued to sustain load after the initial failure. The
cylinders rebounded back to near their original shape after the load was removed.
For the samples exceeding 75% fine aggregate replacement the compression
samples failed through a gradual compression that appeared like a true crushing.
The resulting material that was sponge-like and elastic in nature.
27


It was similarly found in a study done by Khatib and Bayomy (1999) that
as the rubber content was increased the RAC specimens tended to fail gradually in
either a conical or columnar shape failure mode. Both the fine and coarse
aggregates were replaced at varying amounts in this study. At 60% total rubber
replacement the samples showed significant elastic deformations which was
retained upon unloading. Below 60% total replacement the samples sustained
much higher deformations than the control mix without rubber. In the end it was
found that the flexibility and ability to deform at peak load were increased
significantly by rubber addition.
Due to the very elastic failure mode of the compressive tests, flexure tests
are found to be very interesting with RAC. In an experiment by Cairns, Kew, and
Kenny (2004) tests containing different percentage by volume replacements of
fine aggregate with crumb rubber were tested for MOR. The results show that the
flexural strength increased compared to the control mix for rubber aggregate
contents up to 25% replacement. Whereas the concrete with rubber aggregate
replacement contents greater than 50% the flexural strength was shown to
decrease. This test indicates that improvements in flexural strength are limited to
relatively small rubber aggregate contents. In a similar studies by Kaloush, Way,
and Zhu (2004) and Kardos (2011) all mixtures with rubber added to replace fine
aggregates were shown to have a decreased flexural strength and MOR. The data
found about flexural strengths has been inconclusive thus far. The modulus of
28


elasticity of rubber is between 1500 and 15000 psi (10.3 and 103.4 MPa)
(Wikipedia, 2011).
Splitting tensile strengths have been shown to generally decrease with the
addition of crumb rubber as a fine aggregate replacement (Kardos, 2011). No
additional data was recovered regarding tensile strength the addition of rubber.
2.3.8.2. Permeability
In a study conducted by Kardos (2011) the permeability was shown to increase up
to a 30% replacement of crumb rubber for fine aggregate. Above 30%
replacements the permeability decreased. No other research was located on the
effects that rubber has on permeability in concrete.
2.3.9. Summary
Many studies have been performed on the inclusion of coarse and fine rubber
particles as aggregate replacements, whereas not many studies have examined the
use of buffed rubber. From the available information it can be expected that the
buffed rubber concrete will have a more elastic failure in the compressive strength
test. It can also be expected that the concrete strength and the total crack area will
decrease with the addition of more rubber. The workability of the concrete is also
expected to decrease as rubber content is increased.
2.4. Conclusion
Previous research has shown that workability will decrease as fiber is increased.
The air content will increase and unit weight will decrease as fibers content is
29


increased. The effectiveness of added PP fiber mesh is shown to decrease with an
addition greater than 1.0%. There has been no studies done on varying amounts
of rubber mesh to determine a level of effectiveness. Both the PP fiber and buffed
rubber concretes are expected to have a greater flexural strength than a plain
concrete. The PP fiber concretes and rubber fiber concretes have shown less
explosive failures when subject to compressive loads than plain concretes. Less
plastic shrinkage has been noted for all of the PP and rubber added concretes.
Permeability will decrease with the addition of fibers up to a certain point and
then begin to increase due to poor consolidation.
30


3.
PROBLEM STATEMENT
3.1. Statement
Why are all the roads being paved in concrete now? Concrete roads are shown to
have a much higher life-span than traditional asphalt paved roadways. Concrete
paved roads can last up to forty years whereas asphalt roads will only last about
ten years (Suvo, 2010). Concrete roadways are more durable and do not require
as much repair and maintenance as asphalt roads during the service life of the
road (Suvo, 2010). Concrete is able to withstand chemical and oil spills better
than asphalt, which deteriorates when subject to such chemicals. In addition, it
has also been shown that cars are 15-20% more fuel efficient when driving on
concrete roads (Suvo, 2010). Today, many concretes are made by incorporating
recycled materials producing more sustainable green concrete mixtures.
Asphalt is made from petroleum which is very expensive and toxic to the
environment.
These are all great reasons for why roads are now paved with concrete.
However with every great system there are always downfalls. The initial cost of
concrete paving is significantly more expensive than traditional asphalt paving.
Concrete road slabs have to be replaced in their entirety if damage occurs. If
damage occurs to an asphalt paved road the repair is more localized to the
problem area. How can these problems be fixed to make concrete paving a more
realistic option?
31


Many improvements have been made in the design of concrete over the
past decade, but one fact still remains that concrete is a brittle material and is
guaranteed to crack. The significance of this thesis is to examine the use of
recycled rubber mesh from re-tread tires to produce a concrete with increased
flexural capacity while finding a beneficial use for a waste -stream material
(buffed rubber). Rubber is a flexible material and will be added to the concrete as
fiber mesh reinforcement.
A total of nine different concrete mixtures were produced to evaluate the
potential for buffed rubber to be used in concrete mixtures. The test matrix
included a control mixture without fibers, five mixtures containing the recycled
rubber mesh at 0.5 cubic feet (cf), 1.5cf, 2.5cf, 0.5%, and 1.5% (0.014 m3,
0.042m3, 0.071m3, 0.5% and 1.5%) of the total volume of the concrete mixture,
and three mixtures containing a commercially available polypropylene fiber mesh
at 0.5cf (0.014m3), 0.5%, and 1.5% of the total volume of the concrete mixture.
The mixtures were tested for compressive strength, modulus of rupture, and
permeability. The performance of the concrete mixtures containing the two types
of fibers was compared to one another as well as the control mixture (no fibers).
The results of this study were used to determine whether the use of recycled
rubber mesh is (1) beneficial to improving the flexural performance of the
concrete mixture and (2) similar to commercially available fiber mesh currently
used in the concrete industry in terms of structural performance. A statistical
32


analysis was performed to determine whether the recycled rubber mesh produces
structural and durable results significantly different than that of commercially
available material.
The recycled rubber mesh used in the experimentation was obtained from
a local tire re-treading plant. During the tire re-treading process the rubber is
buffed off the existing tire casing and discarded. This is an essential part to the
re-treading process because it preps and shapes the tire casing for the new tread.
This discarded rubber is then used by other industries to make products such as
rubber floor mats and other various products. Using this product in concrete will
only add to the recycled content making concrete an even more eco-friendly
material.
33


4.
EXPERIMENTAL PLAN
4.1. Design Summary
The purpose of this thesis was to investigate the potential benefits of using
recycled buffed rubber as a fiber mesh in concrete and compare the results to
commercially available fiber mesh materials. In order to investigate and compare
the effects the buffed rubber would have on the concrete, nine mixtures were
designed and batched. These mixtures were designed based on a Class P,
Colorado Department of Transportation (CDOT) concrete mixture. A Class P
CDOT concrete is designated as a pavement concrete. Requirements for a Class P
CDOT concrete are listed in Table 4.1
Table 4.1 Colorado Department of Transportation (CDOT) Class P
Concrete specifications and field requirements
28 Day Compressive Strength 28 Day Flexural Strength Minimum percent Pozzolan Replacement Total Cementitious Range Air Content Range Maximum Water to Cementitious Ratio Minimum Coarse Aggregate by Weight
(psi) (psi) (%) (lb/cy) (%) (ratio) (%)
4200 650 10 520-660 4-8 0.44 55
The total cement, fly ash, and w/c were held constant for all the mixtures. All
mixtures had the same percentage of coarse aggregate added. The only variables
were the amount of fiber mesh that was added and the amount of fine aggregate as
a result of replacing a portion of it with the fiber meshes (absolute volume
method). The research began by designing a control mixture and mixtures with
varying amounts of fiber mesh replacements. The control concrete consisted of
34


100% fine aggregate and no fiber mesh. Polypropylene (PP) fiber mesh was
added at an addition rate of 0.5% and 1.5% replacements of total volume and also
at an addition of 0.5 cubic foot (cf) (0.014m ). The buffed rubber mesh mixtures
were designed for an addition rate of 0.5% and 1.5% of total mixture volume as
well as 0.5, 1.5 and 2.5cf (0.014, 0.042, and 0.017m3) additions.
4.2. Material Properties
4.2.1. Portland Cement
The cement used for this research was a Type I-II portland cement. The cement
was supplied by Holcim, Inc. located in Florence, Colorado. The cement was
tested in accordance with ASTM C 150 and the results are shown in Table 4.2.
35


Table 4.2 Holcim Type I-II Cement Physical and Chemical Properties
Holcim Type I-II Portland Cement
Chemical and Physical Properties Test Results ASTM C 150 Specifications
Si02 (%) 19.6
AI2O3 (%) 4.7 6.0 max
Fe203 (%) 3.2 6.0 max
CaO (%) 63.4
MgO (%) 1.5 6.0 max
S03 (%) 3.4 3.0 max
C02 (%) 1.4
Limestone (%) 3.7 5.0 max
CaCC>3 in Limestone (%) 84.0 70.0 min
C3S (%) 59.0
C2S (%) 11.0
C3A (%) 7.0 8.0 max
c4af (%) 10.0
C3S + 4.75 C3A (%) 92.0 100.0 max
Loss of Ignition (%) 2.6 3.0 max
Blaine Fineness cm2/g 414 260 430
Air Content of PC Mortar (%) 6.3 12 max
Specific Gravity 3.15
36


4.2.2. Fly Ash
Boral Class F Fly Ash was used in all the concrete mixtures. Boral Fly Ash is a
pozzolan for concrete, consisting of the finely divided residue that results from
the combustion of ground or powdered coal as defined by ASTM C 618. A
pozzolan, as defined by ASTM, reacts chemically with calcium hydroxide
produced by the hydration of portland cement to form additional cementitious
compounds. Boral Material Technologies, Inc is located in San Antonio, Texas.
The material properties are included in Table 4.3. Actual test reports are included
in Appendix B.
37


Table 4.3 Boral Class F Fly Ash Physical and Chemical Properties
Boral Class F Fly Ash
Chemical and Physical Properties Test Results ASTM C 618 Specifications
Si02 (%) 52.8
AI2O3 (%) 24.11
Fe2C>3 (%) 5.62
Total of Si02, A1203, Fe203 (%) 82.53 70.0 min
CaO (%) 9.75
MgO (%) 2.48
S03 (%) 0.32 5.0 max
Na20 (%) 0.14
K20 (%) 1.17
Total Alkalies (as Na20) (%) 0.91
Moisture Content (%) 0.06 3.0 max
Loss of Ignition (%) 0.78 6.0 max
Amount Retained on No.325 Sieve (%) 18.62 34 max
Loose, Dry Bulk Density lb/cf 70.07 --
Autoclave Soundness (%) 0.03 0.08 max
SAI, with portland cement at 7 days (%) of control 87.1 75 min
SAI, with portland cement at 28 days (%) of control 99.5 75 min
Water Required (%) of control 96.3 105 max
Specific Gravity 2.39
38


4.2.3. Coarse and Fine Aggregates
Both coarse and fine aggregates were obtained by the University of Colorado
Denver from Bestway Concrete. The aggregates used in this study were supplied
by the South Patte River near Brighton, CO. The material properties and
gradations of these aggregates were performed by WestTest Laboratories in
February of 2010. The coarse aggregate was tested in accordance with ASTM C
33 No. 57 and 67 Coarse Aggregate and ASTM C 127. The aggregate was found
to meet these requirements. The maximum nominal aggregate size (NMAS) was
determined to be 0.75 inches (1.9cm). The fine aggregate was tested in
accordance with ASTM C 33 Fine Aggregate and ASTM C 128. The fine
aggregate was found to meet the requirements. The material properties and
gradations for both aggregates are included in Table 4.4 and also in Appendix B.
39


Table 4.4 Coarse and Fine Aggregate Properties
Coarse Aggregate Properties Fine Aggregate Properties
Sieve Size (%) Passing ASTM No.57 Specification ASTM No.67 Specification (%) Passing ASTM C 33 Specification
1-1/2 100 100
1 100 95-100 100 -- --
3/4 91 90-100
1/2 49 25-60 --
3/8 30 20-55 100 100
#4 6 0-10 0-10 100 95-100
#8 2 0-5 0-5 98 80-100
#16 2 78 50-85
#30 2 48 25-60
#50 1 18 10-30
#100 1 4 2-10
#200 0.9 0-1.5 0-1.5 0.9 0-2
Fineness Modulus -- 2.55 2.3-3.1
Bulk Specific Gravity 2.58 2.61
Bulk Specific Gravity (SSD) 2.60 2.63
Apparent Specific Gravity 2.63 2.66
Absorption 0.70% 0.70%
Bulk Density 102 lb/cf
4.2.4. Chemical Admixtures
To satisfy the Class P CDOT concrete mixture parameters chemical admixtures
were used. Chemical admixtures are used to increase workability and entrain air
in to concrete. An air entraining admixture and a high range water reducing
admixture were used in all the concrete batches.
40


4.2.4.1. Air Entraining Admixture (AEA)
BASF MB-AE 90 air entraining admixture was used in all mixtures to aid in
achieving 6.0% air content. The recommended dosage is between 0.25 and 4 fl
oz/cwt (16-260 mL/100kg). This AEA meets ASTM C260 / C260M 10a
Standard Specification for Air-Entraining Admixtures for Concrete. BASF
Construction Chemicals, LLC is the manufacturer and is located in Ohio. The
chemical properties and material data sheets are included in Appendix B.
4.2.4.2. High Range Water Reducing Admixture (HRWRA)
A HRWRA was used for all mixtures due to the low w/c and decreased
workability of concretes with added fiber mesh. Glenium 3030 NS was the
HRWRA used. It is manufactured by BASF Construction Chemicals, LLC and
meets ASTM C494 / C494M 10a Standard Specification for Chemical
Admixtures for Concrete for both Type A, water reducing, and Type F, high-
range water reducing, admixtures. The recommended dosage rate is up to 3 fl
oz/cwt (195 mL/100 kg) for Type A applications, 3-6 fl oz/cwt (195-390 mL/100
kg) for midrange use and up to 18 fl oz/cwt (1,170 mL/100 kg) for Type F
applications. The dosage range is applicable to most mid to high-range concrete
mixtures using typical concrete ingredients according to the manufacturer
recommendations, but that alternate dosage may be needed do to the use of
alternate cementitious materials. The chemical properties and material data sheets
are located in Appendix B.
41


4.2.5. Polypropylene (PP) Fibers
The PP fibers that were used were a blended fiber mesh manufactured by Propex
Concrete Systems, Corp located in Chattanooga, Tennessee. The Novomesh 950
consists of a blend of polypropylene/polyethylene high performance macro
monofilament fibers with sinusoidal deformations and 100% virgin polypropylene
collated-fibrillated fibers containing no reprocessed olefin materials. The
manufacturer requires that Novomesh 950 is not added at less than 5 lb/cy (2.97
kg/m3) as it is produced in a disposable 5 lb (2.27 kg) bag. The bag is meant to
disintegrate in the concrete while mixing. This allows all materials to be
deposited directly into the mixture with no additional measurements required.
Novomesh 950 complies with ASTM C 1116 Standard Specification for Fiber-
Reinforced Concrete and Shotcrete, Type III 4.1.3. Figure 4.1 shows a photo of
the PP fibers that were used.
42


Figure 4.1 Photo of Polypropylene Fibers
4.2.6. Buffed Rubber Fibers
The buffed rubber particles were obtained from a local Tire Distribution Systems
re-treading plant in Commerce City. The rubber composition varies from tire
brand to brand and also from model to model. The tires that are retread in this
shop are made of pure rubber compositions as well as synthetic blends. With the
quantity and varying tire brands that come through the shop there is no way to tell
what sort of rubber composition the buffed rubber is made from. No product data
or gradation was available for this material as it is not a readily available or
manufactured good. The sizes of the particles vary in length and thickness,
ranging from sand sized granular particles to 1 inch (2.54 cm) long string like
particles. Most of the particles however are between 0.25 and 0.75 inches (6.3 to
43


19mm) in length x approximately 0.125 inches (3.1mm) in width. A specific
gravity (SG) of this material needed to be evaluated in the lab. The SG was found
using a volume displacement method. The weight of a graduated cylinder plus
water was weighed. Then the rubber was added and the increase in volume was
noted as well as the new weight. SG was found by taking the weight over the
volume and then dividing by 62.41b/cf (weight of water). This gave a value of
1.11 for the SG of buffed rubber (Kardos, 2011). The rubber was left in the
stoppered graduated cylinder for two months. During this time the volume of the
water did not change at all. This additional test shows that the absorption
capacity of the rubber is extremely small to negligible.
Figure 4.2 Photo of Buffed Rubber Particles
44


4.3. Mixture Designs
A total of nine mixtures were designed and batched for this research. The
purpose of this study was to investigate the use of buffed rubber as a fiber mesh in
concrete and to compare it to concrete made with commercially available PP fiber
mesh. A mixture design matrix is shown in Table 4.5.
Table 4.5 Concrete Mixture Design Matrix
Material Mixture Identification
Control i 0.5% SR 1.5% SR 0.5 CF SR 1.5 CF SR 2.5 CF SR 0.5% PP 1.5% PP 0.5 CF PP
Cement (Type I-II) (lb/cy) 561 561 561 561 561 561 561 561 561
Fly Ash (Class F) (lb/cy) 99 99 99 99 99 99 99 99 99
Cementitious Material (Cement + Fly Ash) (lb/cy) 660 660 660 660 660 660 660 660 660
Coarse Agg. (lb/cy) 1781 1781 1781 1781 1781 1781 1781 1781 1781
Fine Agg. (lb/cy) 1081 1059 1015 1000 837 674 1059 1015 1000
PP Fibers (lb/cy) - - - - - - 8 23 28
Rubber Fibers (lb/cy) - 9 28 35 104 174 - - -
Water (lb/cy) 264 264 264 264 264 264 264 264 264
w/c (ratio) 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40
Air Content (%) 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0
AEA (fl oz/cwt) 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
HRWRA (fl oz/cwt) 5.0 7.0 7.0 5.0 7.0 7.0 7.0 7.0 7.0
All the mixture design quantities are based on the CDOT Class P concrete design
criteria. The total cement content was based off of the maximum amount of
45


cement that can be added for this class of concrete. The lower range of
cementitious material is based on a flexural acceptance criteria alone, which is not
applicable here as this study is interested in the compressive strength for each
mixture as well. For all mixtures the total amount of cementitious material that
>
was used in each mixture was 660 lb/cy (391.56 kg/m ). Given a minimum
pozzolan replacement of 10% a value of 15% fly ash replacement was selected.
The increased fly ash content will contribute to higher long-tem compressive and
flexural strengths. All mixtures used 561 lb/cy (332.8kg/m ) of Type I-II portland
cement and 99 lb/cy (58.7 kg/m ) of Class F fly ash. The w/c was chosen based
on a maximum value of 0.44 for the Class P concrete. A 0.4 w/c was chosen and
seemed reasonable to work with when incorporating a HRWRA. Each mixture
included 264 lb/cy (156.6 kg/m ) of water. All mixtures incorporated AEA to
achieve the 6.0% desired air content. The air content value was chosen from the
acceptable range of values given by the CDOT Class P concrete. The AEA added
to the mixtures was held constant at 3.0 fl oz/cwt. With the cement, fly ash, air,
and water constant in all mixtures the volume of cement paste in each mixture
was the same at 34% of the total volume of each mixture. The amount of water
was decreased with the addition of HRWRA and AEA to adjust for the quantity of
liquid that was added to each mixture. Each mixture contains 1781 lb/cy (1056.5
kg/m3) of coarse aggregate, which is approximately 41% of the total volume and
47% weight of the mixture. As the amount of fiber mesh is increased the ratio of
46


coarse aggregate to total aggregate is increased. For the Control mixture the
percent of coarse aggregate to total aggregate is 62%, 63% for the 0.5% fiber
mixtures, approximately 64% for the 1.5% and 0.5 cf mixtures, 68% for the 1.5 cf
replacement, and 73% for the 2.5 cf mixture. All of these values meet the
minimum course aggregate content criteria for a CDOT Class P concrete. A
HRWRA was added to all the mixtures to increase workability. In the first two
mixtures 5 fl oz/cwt was added, for all the following batches the HRWRA was
increased to 7 fl oz/cwt as the workability of the first two mixtures was not in a
desired range. Both of these additions of HRWRA were within the manufacturers
recommendations.
The absolute volume method was used to design these mixtures. Through
ACI recommendations as well as CDOT design criteria a coarse aggregate weight,
w/c, and cementitious content were selected. These contents were converted into
volumes and the remaining volume in the cubic yard was left to be filled by fine
aggregate. All PP and rubber mesh replacement fibers are based on a total
volume or percent volume of the concrete mixture (cy) and the amount used
lessens the amount of fine aggregate required in the mixture.
Fibers were added to the mixtures at both a percentage replacement and by
a cubic foot replacement. Typically fibers are added by a percentage volume
replacement, however for this study there was a mistake when producing the first
several mixtures. The first mixtures were produced using a cubic foot
47


replacement by the chance of user error in the mixture design spreadsheet.
However, when tested the mixtures gave interesting results with good 1 day
compressive strengths, so the cubic foot replacement was attempted to be
duplicated using the PP fibers. The PP fiber mixtures turned out to be too
difficult to consolidate, so typical percentages were then used for the remaining
mixtures so as to have good comparison concrete mixtures. Fiber mesh is
typically added up to 2.0% for concretes with a small max nominal aggregate size
(0.375 inch, 0.95 cm) and up to 0.7 or 0.8% for larger aggregate sizes. For this
research the envelope from previous tests that had been done need to be pushed so
that the true effectiveness of the buffed rubber could be determined. According to
ACI 544.3R-5 the recommended fiber addition is between 0.2 and 0.8% for a 0.75
in (1.9 cm) (Table 2.1). The values of 0.5% and 1.5% were chosen because 0.5%
is centered within the recommended values given by ACI and 1.5% is
approximately double the recommended value. Previous testing has been done on
concrete with replacement values inside ACI recommendations; however the
point of experiment is to travel new ground and that is why a value exceeding the
recommendations was chosen.
The mixture designated as Control was the control mixture for this
research. As noted above all mixtures contained the same amount of cement, fly
ash, water and air content. The control mixture contained no fiber mesh
reinforcement giving a basis to compare all the mixtures with fiber mesh added.
48


The weight of sand in the mixture was approximately 1081 lb/cy (641.2kg/m )
and 24.6% of the total volume of the mixture.
The mixture designated 0.5% SR contained a 0.5% of total mixture
volume replacement with the buffed rubber particles. The total weight of buffed
rubber added was 9 lb/cy (5.3 kg/m ). The sand content is approximately 24% of
total volume weighing approximately 1059 lb/cy (628 kg/m3).
The mixture designated 1.5% SR contains a 1.5% of total mixture volume
replacement with the buffed rubber particles. The total weight of buffed rubber
added was 28 lb/cy (16.6 kg/m ). The sand content is approximately 23% of total
volume weighing approximately 1015 lb/cy (602.1 kg/m3).
The mixture designated 0.5 CF SR contains a 0.5 cubic foot (0.014 m3)
replacement with the buffed rubber particles. The total weight of buffed rubber
added was 35 lb/cy (20.8 kg/m3). The sand content is approximately 22.7% of
total volume weighing approximately 1000 lb/cy (593.2 kg/m ).
The mixture designated 1.5 CF SR contains a 1.5 cubic foot replacement
with the buffed rubber particles. The total weight of buffed rubber added was 104
lb/cy (61.7 kg/m3). The sand content is approximately 19% of total volume
weighing approximately 837 lb/cy (496.5 kg/m ).
The mixture designated 2.5 CF SR contains a 2.5 cubic foot replacement
with the buffed rubber particles. The total weight of buffed rubber added was 174
49


lb/cy (103.2 kg/m3). The sand content is approximately 15% of total volume
weighing approximately 674 lb/cy (399.8 kg/m3).
The mixture designated 0.5% PP contains a 0.5% of total mixture volume
replacement with the PP blended fibers. The total weight of the Novomesh 950
-i
added was 7.7 lb/cy (4.6 kg/m ). The sand content is approximately 24% of total
volume weighing approximately 1059 lb/cy (628.2 kg/m3).
The mixture designated 1.5% PP contains a 1.5% of total mixture volume
replacement with the PP blended fibers. The total weight of the Novomesh 950
added was 23 lb/cy (13.6 kg/m ). The sand content is approximately 23% of total
volume weighing approximately 1015 lb/cy (602.1 kg/m3).
The mixture designated 0.5 CF PP contains a 0.5 cubic foot volume
replacement with the PP fibers. The total weight of the Novomesh 950 added was
28 lb/cy (16.6 kg/m3). The sand content is approximately 22.7% of total volume
weighing approximately 1000 lb/cy (593.2 kg/m3).
4.3.1. Mixture Batching
All mixtures were batched in accordance with the guidelines stipulated in ASTM
C 192 Standard Practice for Making and Curing Concrete Test Specimens in the
Laboratory and ASTM C 1116 Standard Specification for Fiber-Reinforced
Concrete and Shotcrete. Prior to batching the concrete the moisture contents were
taken of the aggregates. To accomplish this aggregates that would be needed
were placed in covered buckets, to maintain moisture content over night. A
50


sample of each aggregate was taken the day prior to batching. The samples initial
weight was measured and then placed into an oven overnight. On the day of
batching the sample was weighed again. Procedures were followed per ASTM C
566 Standard Test Method for Total Evaporable Moisture Content of Aggregate
by Drying
The batching procedure was the same for all concrete mixtures. The AEA
and HRWRA were added to the batch water. First all the aggregates and a portion
of the water were added to the mixer, where applicable the fiber mesh was then
added and allowed to disperse. The cementitious materials and the remaining
water were then added alternately and all ingredients were allowed to mix
together per ASTM specifications.
4.3.2. Curing
After preparing the specimens all mixtures were placed in the humidity controlled
room over night. The humidity controlled room is supposed to be held at a
constant humidity and temperature of 40% and 70 F respectively. However the
humidity controlled room was not functioning properly at the time of batching so
all the specimens were covered with plastic and then placed in the closed
humidity room overnight. The following day all the samples were removed from
their molds and submerged in water until the time of testing.
51


4.4. Concrete Testing
Fresh concrete properties were evaluated immediately after batching. The fresh
properties that were examined included; slump, unit weight, and air content.
Slump was measured using the provisions in ASTM C 143 Standard Test Method
for Slump of Hydraulic-Cement Concrete. The slump test is used to measure the
workability of concrete. As the amount of fiber was increased the slump of the
concrete was expected to decrease. Unit weight was tested in accordance with
ASTM C 138 Standard Test Method for Density (Unit Weight), Yield, and Air
Content (Gravimetric) of Concrete. The unit weight test is performed to see if the
actual weight is the same as the expected unit weight. During the mixture design
process the unit weight is determined from the volume and weights of the
different concrete components. As the fiber content is increased the unit weight is
expected to decrease due to more air voids and worse consolidation. Air content
was measured by the pressure method using ASTM C 231 Standard Test Method
for Air Content of Freshly Mixed Concrete by the Pressure Method. The concrete
mixtures were designed per the absolute volume method. In this method the
expected air content takes up a certain volume. The air content is then tested to
ensure that the actual volume is close to the expected volume. Air content is
expected to increase the with the inclusion of more fibers. The air content has a
large effect on the hardened properties of concrete.
52


The hardened concrete property tests included; compressive strength,
flexural strength, splitting tensile strength, and permeability. Compressive tests
were done in accordance with ASTM C 39 Standard Test Method for
Compressive Strength of Cylindrical Concrete Specimens. Flexure tests followed
the provisions outlined in ASTM C 78 Standard Test Method for Flexural
Strength of Concrete (Using Simple Beam with Third-Point Loading). ASTM C
496 Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete
Specimens was followed for samples tested for splitting tensile stresses.
Permeability or rapid chloride penetration (RCPT) tests were conducted in
accordance with ASTM C 1202 Standard Test Method for Electrical Indication of
Concretes Ability to Resist Chloride Ion Penetration. Hardened property tests
began when the concrete was one day of age and concluded at 56 days of age.
Table 4.6 summarizes the tests and dates that different properties were evaluated.
53


Table 4.6 Fresh and Hardened Concrete Property Tests
Fresh Concrete Tests Time of Testing
Slump When Batched
Unit Weight When Batched
Air Content When Batched
Hardened Concrete Tests Time of Testing (Days)
Compressive Strength 1, 7, 28, and 56 (3 cylinders/day)
Flexural Strength 28 and 56 (2 beams and 1 beam)
Split Tensile Strength 28 and 56 (2 cylinders/day)
Permeability (RCPT) 28 and 56 (2 cylinders/day)
4.5. Summary
There were nine mixtures that were batched and tested for fresh and hardened
concrete properties. Slump, air content, and unit weight will be tested with the
fresh concrete. Slump is expected to decrease as fiber content is increased. Air
content is expected to increase and unit weight is expected to decrease as more
fibers are added. Compressive, flexural, and splitting tensile strength tests will be
performed on the hardened concrete. With the addition of more fibers the
compressive strength is expected to decrease. The flexural and splitting tensile
strengths are expected to increase to a certain effectiveness and then to decrease
with the addition of fibers.
54


5.
EXPERIMENTAL RESULTS
5.1. General
This chapter contains the presentation of the results and any significant
observations noted during the research. The first section includes problems that
were encountered during testing. The fresh concrete properties are presented next
followed by the hardened concrete results. The mixture designations from
Chapter 4, Table 4.3 will be used throughout the results section.
5.2. Problems with this Study
5.2.1. Spreadsheet/user error with percent vs. CF replacements
At the beginning of this experiment it was established that concretes were to be
batched with 0.5%, 1.5% and 2.5% fiber replacements for both the buffed rubber
and PP fibers. After batching what was thought to be the control mixture, the
0.5%, 1.5%, and 2.5% mixtures of the buffed rubber it was determined that the
actual amounts that had been added were 0.5cf (0.014kg/m3), 1.5cf (0.042 kg/m3),
and 2.5cf (0.071 kg/m3) replacements, the mixture designations are 0.5 CF SR,
1.5 CF SR, and 2.5 CF SR respectively. Based on the 1 day compressive strength
tests the 1.5 CF SR and 2.5 CF SR mixtures did not look like they would achieve
the CDOT compressive strength that was the goal compressive strength. An
apples to apples comparison was needed so whatever concretes were batched next
needed to correspond to the buffed rubber concretes that had already been made.
Going forward the next batch that was mixed was a mixture containing 0.5cf
55


(0.014 kg/m ) of PP fibers (0.5 CF PP mixture designation). This concrete was
found to be very hard to work with a nearly impossible to consolidate. From this
mixture it was determined that the PP fiber could not be added at any higher
dosage, so the 1.5 CF and 2.5 CF PP fiber mixtures were never made. The
mixture set was reevaluated again to include concretes with lower percentages of
fiber and the final mixtures that were settled upon are listed in Table 4.3.
5.2.2. Poor consolidation for 0.5 CF PP Mixture
As mentioned above the 0.5 CF PP mixture was extremely hard to consolidate.
Figure 5.1 shows a picture of the concrete as it is being put into a beam mold.
Figure 5.1 0.5 CF PP Fresh Concrete Mixture Photo
5.2.3. Needed more PP Fibers than the Quantity on Hand
After the final set of mixtures was settled on it was determined that approximately
5.1 pounds (2.31 kg) of the PP fibers were needed. The quantity in the lab was a
56


little less than 5 pounds (2.27). A sales representative from Propex Concrete
Solutions was contacted and he shipped a case of the Novomesh 950 that had
been requested. This process delayed the final batch of concrete by more than a
week and subsequently the rest of the testing schedule after that.
5.2.4. Not Enough Beam Molds
When preparing the mixture designs it was determined that two flexure tests
would be run at both day 28 and day 56. This totaled four beams for each
concrete mixture. On the first day of testing it was determined that there were
only a total of six beam molds in the laboratory. Batching two concrete mixtures
per day there were only enough molds to make three beams of each concrete
mixture. This left a dilemma for deciding when to break the limited number of
test beams. The determination was to break two beams on day 28 and only one
beam on day 56. This decision was made based on the fact that there were flexure
requirements given by CDOT for day 28, deeming this the more important day to
test.
5.2.5. Day 56 Permeability Tests were not performed on Day 56
When it came time to test the permeability of the concretes at day 56 there was
another student conducting tests at that time. The tests for the Control, 0.5CF SR,
1.5 CF SR, 2.5 CF SR, 0.5% PP, and 0.5 CF PP were delayed by 2 days to allow
the other testing to be completed. During preparation of the 0.5% and 1.5% SR
concretes for the permeability tests the laboratory door was locked and access
57


could not be gained over a holiday weekend to remove the samples from the water
and test them. Another round of preparation and testing occurred after the door
was unlocked the following week. These two mixtures were tested when the
concrete was 62 days of age. The 1.5% PP concrete was tested on the actual day
56.
5.3. Fresh Concrete Properties
The fresh concrete properties tested included slump, air content and unit weight.
These results are presented in Table 5.1.
Table 5.1 Fresh Concrete Properties
Mixture Identification Slump (in) Air Content (%) Measured Unit Weight (pcf)
Control 0.25 5.9 146.6
0.5% SR 0.00 6.7 147.4
1.5% SR 0.00 5.7 147.2
0.5 CR SR 0.75 6.5 143.5
1.5 CF SR 1.00 6.8 139.4
2.5 CF SR 1.00 7.1 134.5
0.5 % PP 0.50 5.0 147.0
1.5% PP 0.00 5.7 144.6
0.5 CF PP 0.00 6.5 144.0
5.3.1. Slump
CDOT Class P concrete does not have any specific requirements for the amount
of slump that should be obtained. Due to the low w/c values of these mixtures
and an expected decrease in the slump as rubber and PP fibers were increased a
58


HRWRA was used for all mixtures. The HRWRA was added to the mixture
water and subsequently to the mixture alternately with the cementitious material.
There were several mixtures with zero slump. As expected as the PP fiber
content increased the slump decreased. The two mixtures containing the highest
amount of PP fiber showed zero slump. Alternately the workability of the
concrete containing buffed rubber did not decrease as the amount of rubber was
increased. The two mixtures containing 0.5% and 1.5% buffed rubber, the least
amounts, had zero slump. The concrete containing 1.5 CF and 2.5 CF SR, the two
mixtures with the highest amount of buffed rubber, had the largest amount of
slump. The control mixture had less slump than the 0.5% PP, this is the only PP
relationship shown in the experiment to not conform to the expected outcome.
The 0.5% and 1.5% SR mixtures had less slump than the Control mixture. These
relationships are the only buffed rubber concretes that follow the hypothesis.
In general a slump of 3 inches (7.62cm) is believed to be a reasonable
value to achieve consolidation of the test specimens. None of the mixtures
obtained a reasonable amount of slump and were very dry and hard to work with.
The amount of HRWRA was not adjusted from the original design to obtain more
workable concrete as it was being batched. This was done so that the slump for
each mixture would give a true comparison to the others. HRWRA is often added
liberally to the concrete mixture if desired workability is not obtained through the
initial design process. The results from the slump test are shown in Figure 5.2.
59


1.20
Control 0.5%SR 1.5%SR 0.5CR 1.5CF 2.5CF 0.5% 1.5%PP 0.5CF
SR SR SR PP PP
Mixture Identification
Figure 5.2 Mixture Slump Results
What can be deduced from these results is that the results obtained were both the
same and different than the expected results of the literature review. In the
literature review it was noted that as PP fibers were added to concrete the slump
was decreased. This is the result that was obtained within this research.
However, the literature review noted that the same decrease in slump should
result as more rubber is added. This was not the case for this research. The
slump was seen to increase with the addition of buffed rubber. The concretes
with 0.5% and 1.5% buffed rubber displayed zero slump. As the rubber content
increased beyond the 1.5% SR mixture the slump was shown to increase with the
addition of buffed rubber and a constant HRWRA dosage rate. These results are
60


directly opposite of the expected slump outcome. The 0.5 CF SR mixture had a
slump of 0.75 inches (1.91cm) and the greatest slumps were achieved by the
concretes with the highest inclusion of buffed rubber, the 1.5 CF and 2.5 CF
mixtures. In general the concretes with buffed rubber had better slumps than the
concretes with PP fibers. Vibration would be needed to provide adequate
consolidation for all the concrete mixtures.
5.3.2. Air Content
The use of AEA was used in all the concrete mixtures at a dosage rate of 3.0 fl
oz/cwt. The target air content was 6.0% based on CDOT Class P concrete.
Although the dosage was measured correctly and remained constant for all
mixtures the actual air contents varied. The lowest air content of 5.0% and the
highest air content of 7.1% were measured, however these only vary from the
targeted air content by about 1%. Both mixtures with 1.5% replacements of
buffed rubber and PP fibers were found to be the same at 5.7%, just shy of the
6.0% target.
In the literature review it was noted that the air content is expected to
increase with the addition of both buffed rubber and PP fibers, due to the concrete
being harder to consolidate and more air voids being present. It was observed that
the air content of the PP fiber added concretes increased as the amount of PP
replacement was increased. The 0.5% PP, 1.5% PP, and 0.5 CF PP concretes had
air contents of 5.0, 5.7 and 6.5% respectively. Figure 5.1 shows the 0.5 CF PP
61


concrete and the poor consolidation due to the fibers and low w/c. This could
account for the increase in air content for this mixture (6.5%).
The 2.5 CF SR concrete mixture had the highest air content of 7.1%. In
the literature review it was noted that an increase in air content could be
contributed to rubber being hydrophobic and trapping the air on the surface of the
rubber. This seems to be the general trend for the concretes with rubber added to
them. As the rubber contents increased from the 1.5% SR, 0.5 CF SR, 1.5, CF
SR, to the 2.5 CF SR mixture the air content is also shown to increase from 5.7,
6.5, 6.8, to 7.1% respectively. The only concrete containing rubber that does not
fit this trend is the lowest rubber mixture, 0.5% SR. The 0.5% PP mixture
recorded the least amount of air content, even lower than the Control mixture.
This low air content could be accounted for by user error. While it is believed to
have been added correctly, the amount of AEA could have been measured
incorrectly. Differences in all the concrete mixtures from the targeted air content
of 6.0% could be accounted for by a difference in mixing time. ACI notes an
increase in air content with mixing time, up to a certain point.
5.3.3. Unit Weight
The unit weights of each mixture were tested immediately after batching
according to ASTM C 138 procedures. The theoretical unit weights were
between 140.0 and 149.2 pcf (2242.5 to 2389.9kg/m3) decreasing with the
addition of PP fibers and buffed rubber particles. This decrease was expected
62


since the SG of the both the rubber and the PP fibers (1.11 and 0.91 respectively)
are lower than the SG of the sand (2.61) that the fibers are replacing. The w/c
ratio was held constant for all mixtures and therefore did not play a role. Table
5.2 shows the theoretical unit weights, measured unit weights and noted
difference from the theoretical to measured unit weight values.
Table 5.2 Measured Unit Weights, Theoretical Unit Weights and Air Contents
Mixture Air Content Measured Unit Theoretical Unit Difference from
Identification (%) Weight (pcf) Weight (pcf) Theoretical (pcf)
Control 5.9 146.6 149.2 -2.6
0.5% SR 6.7 147.4 148.7 -1.3
1.5% SR 5.7 147.2 147.7 -0.5
0.5 CR SR 6.5 143.5 147.3 -3.8
1.5 CF SR 6.8 139.4 143.7 -4.3
2.5 CF SR 7.1 134.5 140.0 -5.5
0.5 % PP 5.0 147.0 148.6 -1.6
1.5% PP 5.7 144.6 147.5 -2.9
0.5 CF PP 6.5 144.0 147.1 -3.1
63


Control 0.5%
SR
1.5% 0.5 CR 1.5 CF 2.5 CF 0.5% 1.5% 0.5 CF
SR SR SR SR PP PP PP
Mixture Identification
I Measured Unit Weight Air Content
Figure 5.3 Unit Weight and Air Content
Air content plays a major role in the measured unit weights. Air content and unit
weights are inversely proportional, so as the air content is increased the unit
weight will decrease. In the literature review it has been observed that as fibers
increase the air content is also increased resulting in a decrease in unit weight. In
the design process the target air content of 6.0% by volume was taken into
account when the theoretical unit weights were calculated. All the mixtures have
a measured unit weight lower than the theoretical unit weights. The 0.5% SR, 0.5
CF SR, 1.5 CF SR, 2.5 CF SR, and 0.5CF SR follow the expected pattern of
increased air and decreased unit weight. The decreases in unit weight vary from
1.3 pcf to 5.5 pcf (20.8 to 88.1kg/m3). The maximum difference is for the 2.5 CF
64


SR mixture that had the greatest air content of 7.1 %. If the theoretical unit weight
is adjusted to account for a 7.1% air content the new theoretical unit weight would
be 139.7 pcf (2237.7 kg/m3) and the difference between the measured value
would decrease to 5.2 pcf (83.3 kg/m3). This difference in the theoretical values
only accounts for a 0.2% difference in the actual unit weight. This is a negligible
amount in variation. The more likely difference in this case probably occurred
due to the consolidation in the mixture. Refer to Figure 5.1 for consolidation
photo.
The Control, 1.5% SR, 0.5% PP, and 1.5% PP mixtures all had decreased
air content and a decrease in the unit weight. This does not follow the expected
trend, but overall the values of difference are pretty negligible. The differences in
measured values range from 0.5 pcf to 2.9 pcf (8.0 to 46.5 kg/m3), which accounts
for only a 2% difference from the expected value. Several factors may have
played a role in these discrepancies. It is hypothesized that improper
consolidation may have caused the most variation in the results. As noted above,
all of the mixtures were extremely dry and difficult to consolidate.
5.4. Hardened Concrete Properties
Hardened concrete testing was performed on all mixtures at various ages after
batching. The tests that were conducted were compressive tests, flexure tests,
splitting tensile tests, and permeability tests.
65


5.4.1. Compressive Strength
The compressive strength for all mixtures was tested is accordance with ASTM C
39 at 1, 7, 28, and 56 days of age. A total of three 4 inch x 8 inch (10.16cm x
20.3cm) cylinders were tested at the specified age. The compressive strength
(stress) is calculated by dividing the ultimate load at failure by the cross sectional
area of the cylinder.
Concrete strength and air content are inversely proportional. For every
1% increase in air content the compressive strength is decreased by 5% (Mindess,
2003). With the varying air contents for this research it is necessary to normalize
the results and compare the strengths as if all the mixtures had 6.0% air content.
The values before normalization are shown in Table 5.3 and the normalized
results are shown in Table 5.4.
Table 5.3 Average Compressive Strengths
Mixture Identification Average Compressive Strengt th (psi)
1 Day 7 Day 28 Day 56 Day
Control 3119 5276 6822 7096
0.5% SR 1994 5235 5991 5978
1.5% SR 2838 5238 5572 6281
0.5 CF SR 2245 4646 5413 5764
1.5 CF SR 1445 2967 3475 3991
2.5 CF SR 1064 1905 2435 2710
0.5% PP 2624 5423 6451 6769
1.5% PP 2640 4675 5145 6063
0.5 CF PP 1652 3407 4840 4404
66


Table 5.4 Average Normalized Compressive Strengths
Mixture Identification Actual Air Content (%) Normalized Compressive Strength (psi)
1 Day 7 Day 28 Day 56 Day
Control 5.9 3204 5420 7008 7290
0.5% SR 6.7 2057 5400 6180 6167
1.5% SR 5.7 2912 5375 5718 6446
0.5 CF SR 6.5 2313 4788 5578 5940
1.5 CF SR 6.8 1491 3062 3587 4119
2.5 CF SR 7.1 1100 1969 2517 2801
0.5% PP 5.0 2683 5545 6597 6922
1.5% PP 5.7 2709 4798 5280 6222
0.5 CF PP 6.5 1702 3511 4988 4538
Strength gain curves are plotted in Figures 5.4 to 5.6. First a plot of average
normalized compressive strengths for the control mixture and all the buffed
rubber concretes. Next, a graph of the normalized control mixture and all PP fiber
mixtures in shown in Figure 5.5. The combined normalized strength curves for
all mixtures are shown in Figure 5.6 and a bar graph of the same data is presented
in Figure 5.7.
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Compressive Stress (psi) 22 Compressive Stress (psi)
8000
7 14 21 28 35 42 49 56
Concrete Age (Days)
> Control
0.5% SR
41.5% SR
- 0.5 CF SR
- 1.5 CF SR
----2.5 CF SR
gure 5.4 Compressive Strength Gain, Control and Buffed Rubber Cone.
Control
0.5% PP
1.5% PP
0.5 CF PP
Figure 5.5- Compressive Strength Gain, Control and PP Fiber Concrete
68


Figure 5.6 Normalized Compressive Strength Gain for Concrete Mixtures
Control
0.5% SR
1.5% SR
0.5 CF SR
1.5 CF SR
2.5 CF SR
0.5% PP
1.5% PP
0.5 CF PP
Concrete Age (Days)


8000
Control
SO.SKSR
1.5%SR
1*0.5 CF SR
1.5 CF SR
P2.5CFSR
0.5%PP
C1.S96PP
O.SCTPP
Figure 5.7 Normalized Compressive Strength Gains for Concrete Mixtures
When comparing the results from Tables 5.3 and 5.4 it is apparent that air content
had some factor in the strengths of the concretes. This normalization however is
very minimal due to the air contents varying by only 1 %. When referring to a
4000 psi (27.6 MPa) concrete, 5% is only 200 psi (1.4 MPa). However the need
to normalize the data still exists and all values mentioned in the future will be
referring to the normalized data.
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The Control mixture achieved the greatest strengths at 1 day of age
exceeding 3000 psi (20.7 MPa). Even at this early age it is expected that concrete
with fibers added will be of less strength than a mixture with no fibers. The 1.5%
SR mixture produced he next highest one day strength just below 3000 psi (20.7
MPa). Curiously the 0.5% SR mixture had a one day strength lower that that of
the 1.5% replacement. However, as greater amounts of buffed rubber are added
beyond the 1.5% replacement the compressive strength is shown to decrease. The
same strange situation occurs with the 1.5% replacement of the PP fibers being
stronger than the 0.5% replacement. The 0.5 CF SR concrete showed a lower
strength than both the other two concretes with PP fibers added. The lowest
compressive strength for one day tests was the 2.5 CF replacement of buffed
rubber.
The Control, 0.5% SR and the 1.5% SR mixtures produced strengths of
approximately 5400 psi (37.2 MPa) at 7 days of age. The concrete containing the
least amount of PP fibers was the highest strength at day seven at over 5545 psi
(38.2 MPa). The concrete with the least amount of strength was again the 2.5 CF
SR mixture. As expected, the strengths at day seven decreased as the amount of
fiber added to the mixture increased. The RAC showed a decrease from 5400 psi
to 1969 psi (37.2 to 13.6 MPa) as the rubber content was increased from 0.5% to
2.5 CF. The concrete with PP fibers added was shown to decrease from 5545 psi
to 3511 psi (38.2 to 24.2 MPa) as the amount of PP fibers increased from 0.5% to
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0.5 CF. From 1 to 7 days of age the control mixture increased an average of 2216
psi (15.3 MPa). The 0.5%, 1.5%, 0.5 CF, 1.5 CF and 2.5 CF buffed rubber
mixtures increased 3343, 2463, 2475, 1571, and 869 psi (23.0, 17.0, 17.1, 10.8,
and 6.0 MPa) respectively. The highest increase in strengths was shown by the
0.5% SR mixture and lowest was shown by the 2.5 CF SR mixture. For the
concretes with PP fibers the strength was increased by 1862, 2089, and 1809
(12.8, 14.4, and 12.5 MPa) for the 0.5%, 1.5% and 0.5 CF mixtures respectively.
All of the rate gains for the PP fiber mixtures are relatively similar for the 6 day
period between the first and seventh day.
All the mixtures had less gains from 7 to 28 days as they did from 1 to 7
days of age. This type of rate gain is typical of most normal concrete mixtures.
At 28 days of age the control mixture once again had the highest strength of
approximately 7000 psi (48.2 MPa). The 2.5 CF SR concrete still produced the
lowest strength just over 2500 psi (17.2 MPa). For the buffed rubber concretes as
the amount of rubber was increased the compressive strengths were decreased.
As the rubber content increased from 0.5% to 1.5%, 0.5 CF, 1.5 CF and 2.5 CF
the compressive strength decreased from 6180 to 5718, 5578, 3587, and 2517
(42.6, 39.4, 38.4, 24.7, and 17.3 MPa). The increase in compressive strength
from day 7 to day 28 was less than 1000 psi (6.9 MPa) for all the rubber added
concretes. The same rate was observed for the PP fiber concretes. As the amount
of PP fiber increased the strength decreased from 6597 to 4988 psi (45.5 to 34.4
72


MPa). The rate of strength increase from 7 to 28days of age for the PP fiber
concretes were higher than the increases seen from the buffed rubber concretes.
The 0.5%, 1.5%, and 0.5 CF PP fiber concretes were shown to increase by 1052,
482, and 1477 psi (7.2, 3.3, 10.2 MPa) respectively. It is curious as to why all the
PP fiber concretes did not increase by over 1000 psi (6.9 MPa) each or by values
closer to the strength gain seen in the RAC, but the difference might lie in the
consolidation of each of the different concretes.
At 56 days the Control mixture increased by 282 psi (1.9 MPa) yielding a
concrete strength of 7290 psi (50.2 MPa). The Control was the highest strength
mixture from this study. The 0.5% SR mixture had the second highest strength at
56 days. The strength for the 0.5% SR mixture was taken as the average of only
two cylinders. The third cylinder that was tested had a stress much lower than all
the other cylinders due to inadequate consolidation. For this mixture three test
cylinders were broken at 56 days of age with each of the tests strengths being,
6420, 4071, and 7442 psi (44.2, 28.0, 51.3 MPa). The lowest cylinder break is
obviously bringing the average strength down and the data was discarded. From
28 to 56 days of age the 0.5% SR mixture increased by 940 psi (6.5 MPa) or
15.7%. The 1.5% SR concrete increased by 728 psi (5.0 MPa) bringing the
concrete stress to just over 6400 psi (44.0 MPa). The 0.5 CF, 1.5 CF and 2.5 CF
SR concretes increased by 362, 532, and 284 psi (2.5, 3.7, and 2.0 MPa) bringing
the total compressive strengths to 5940, 4119, and 2801 psi (40.9, 28.4, and 19.3
73


MPa) respectively. For the PP fiber mixtures the 0.5% and 1.5% mixtures
increased by 325 and 942 psi (2.2 and 6.5 MPa) bringing the compressive
strengths to 4119 and 6222 psi (28.4 and 42.9 MPa) respectively. The 0.5 CF PP
mixture was also taken as an average of only two mixtures at 56 days of age.
When all three cylinders were averaged a decrease in strength was shown from 28
to 56 days. The three test cylinders broke at 2772, 5664, and 4777 psi (19.1, 39.0,
and 32.9 MPa). The lowest cylinder data was discarded resulting in an average
increase of 381 psi (2.6 MPa) or 7.9%. This concrete was extremely hard to
consolidate and had several voids within some of the cylinders. The low
compressive strength in a few of the cylinders is completely valid and shows how
non-uniform and difficult that this mixture really was.
The results of the concretes containing the same amount or PP fiber or
buffed rubber were compared to one another. Figures 5.8 through 5.10 show the
graphs of the concretes as strength is gained over time for the same proportions of
SR and PP fibers. Figure 5.8 shows both the concretes that have 0.5% fiber
replacements. For strengths at each day the PP fiber concrete is shown to have a
higher compressive strength. At 1 day of age the PP fiber concrete is 626 psi (4.3
MPa) or 30% higher than the buffed rubber concrete. Over the six day period
between day 1 and day 7 the PP concrete increased 2862 psi (19.7 MPa) or 107%,
while the buffed rubber increased 3343 psi (23.0 MPa) or 162.5%. The greater
increase in the RAC brought the compressive strengths for day 7 close to each
74


other with only a 145 psi (1.0 MPa) difference. From day 7 to day 28 the strength
gain was significantly lower, as noted above, with the PP concrete only gaining
1052 psi (7.2 MPa) or 18% and the RAC gaining 780 psi (5.4 MPa) or 14.4%.
The separation between the 28 day strengths of the two concretes was greater at
417 psi (2.9 MPa) or 6.7%. At 56 days of age the PP fiber mixture surpassed the
SR mixture in strength. The PP mixture gained 325 psi (2.4 MPa) or 4.9% from
the 28 day, while the SR fiber concrete gained 970 psi (6.7 MPa) or 15.7%, thus
making the SR mixture 228 psi (1.6 MPa) or 3.7% stronger than the PP at 56 days
of age.
* 0.5% SR
0.5% PP
7 14 21 28 35 42 49 56
Concrete Age (Days)
Figure 5.8 Compressive Strength Gain for 0.5 % by Volume Fiber Mixtures
75


Figure 5.9 shows both the concretes that have 1.5% fiber replacements. Different
from the 0.5% replacements the buffed rubber concrete is stronger at each day
than the PP fiber concrete. At day 1 the RAC is 203 psi (1.4 MPa) or 7.4% higher
than the PP concrete. From day 1 to day 7 the PP concrete increased 2089 psi
(14.4 MPa) or 77%, while the buffed rubber increased 2463 psi (17.0 MPa) or
84.5%. With a greater strength gain from the RAC, it remains even further above
the PP fiber concrete by 577 psi (4.0 MPa) or 12%. From day 7 to day 28 the
strength gain was significantly lower for both concretes, as noted above compared
to the strength gain to day 7. The PP concrete however gained more strength
during this stage with a 482 psi (3.3 MPa) or 10% gain than the RAC which only
gained 343 psi (2.4 MPa) or 6.4%. At 28 days of age the RAC was 438 psi (3.0
MPa) or 8.2% stronger than the PP concrete. From 28 days to 56 days the PP
fiber concrete still gained more strength than the RAC. The PP concrete gained
942 psi (6.5 MPa) or 17.8% while the RAC gained only 728 psi (5.0 MPa) or
12.7%. At 56 days the buffed rubber concrete is only 224 psi (1.5 MPa) or 3.6%
greater than the PP concrete. Since the 7 day tests the PP fiber concrete has
shown more of a strength gain compared to the RAC, so it is reasonable to believe
that if the concrete was again tested at 90 days that the PP fiber might exceed the
buffed rubber concrete. It is unknown why the PP fiber concrete would be
gaining more strength over time than the SR concrete.
76


8000
7000
1.5% SR
1.5%PP
Concrete Age (Days)
Figure 5.9 Compressive Strength Gain for 1.5% by Volume Fiber Mixtures
Figure 5.10 shows both the concretes that have 0.5 CF fiber replacements. Like
the 1.5% replacements the RAC is shown to be stronger at each day than the PP
fiber concrete. This could be due to the fact that the SR concrete had a much
better consolidation than the PP concrete. At day 1 the RAC is 611 psi (4.2 MPa)
or 36% higher than the PP concrete. From day 1 to day 7 the PP concrete
increased 1809 psi (12.5 MPa) or 106%. The RAC had about the same
percentage gain at 107%, increasing the strength by 2475 psi (17.1 MPa). At day
7 the buffed rubber concrete exceeds the PP fiber concrete by 1277 psi (8.8 MPa)
or 36%. These differences in the strengths are much higher than was seen for the
77


1.5% replacement concretes, comparing 36% difference for the 0.5 CF concretes
and only 12% for the 1.5% concretes at 7 days. From day 7 to day 28 the strength
gain was significantly lower for both concretes, but greater than any of the
increases previously discussed. The PP concrete gained more strength during this
stage with a 1477 psi (10.2 MPa) or 42% gain than the RAC which only gained
790 psi (5.4 MPa) or 16.4%. At 28 days of age the RAC was 590 psi or 12%
stronger than the PP concrete.
From 28 days to 56 days the PP fiber concrete decreased in strength while
the buffed rubber concrete still gained 362 psi (2.5 MPa) or 6.4%. The PP
concrete decreased by 450 psi (3.1 MPa) or 9%. At 56 days the buffed rubber
concrete is 1402 psi (9.6 MPa) or 31% greater than the PP concrete. As
mentioned above the decrease in strength is most likely due to this mixtures non-
uniformity and difficult consolidation.
78


8000
7000
6000
5000
0.5 CF SR
g 4000
0.5 CF PP
| 3000
o
u
2000
1000

f
0 #-
14 21 28 35
Concrete Age (Days)
42
49
56
Figure 5.10 Compressive Strength Gain for 0.5 CF Fiber Mixtures
With a decreased workability for the 0.5 CF PP mixture it was impractical to try
and increase the amount of PP fibers any more. This is the reason why there is no
direct comparison of the PP fiber concrete to the buffed rubber concretes for both
1.5 CF and 2.5 CF fiber replacements. Figure 5.11 shows the strength gain curves
for 0.5, 1.5, and 2.5 CF replacements of buffed rubber and the 0.5 CF replacement
of PP fibers. Both the 1.5 CF and 2.5 CF mixtures show less strength than both
the 0.5 CF fiber replacement concretes. At 1 day the concrete strength is shown
to decrease 822 psi (5.7 MPa) or 35.5% from the 0.5 CF to 1.5 CF buffed rubber
replacements. And decrease again from the 1.5 CF to 2.5 CF by 391 psi (2.7
MPa) or 26.2%. The 2.5 CF SR mixture has been shown to have the lowest
79


concrete strength over all. At 7 days the 1.5 CF and 2.5 CF mixtures are 36% and
58.9% lower than the 0.5 CF SR mixture. From day 1 to day 7 the 0.5 CF SR
mixture gained 2475 psi (17.1 MPa) or 107% of its strength, the 1.5 CF SR
mixture gained 1571 psi (10.8 MPa) or 105% and the 2.5 CF SR mixture only
gained 869 psi (6.0 MPa) or 79% more strength. The 2.5 CF is obviously not
gaining strength as fast as any of the other concretes during this stage. From 7 to
28 days the 2.5 CF SR mixture gained 548 psi (3.8 MPa) or 27.8%, the 1.5 CF SR
mixture gained 525 psi (3.6 MPa) or 17%, and the 0.5 CF SR mixture gained 790
psi (5.4 MPa) or 16.4%. While the 2.5 CF mixture did not gain as much strength
as the other two mixtures it did have a higher percentage of gain during this
period than did the other two. At 28 days the 1.5 CF SR and 2.5 CF SR mixtures
had strengths 1991 psi (13.7 MPa) or 35.7% and 3061 psi (21.1 MPa) or 55%
lower than the 0.5 CF SR mixture. The 2.5 CF SR mixture was 1070 psi (7.4
MPa) or 29.8% lower than the 1.5 CF SR concrete. These are pretty significant
differences in strength with the additional replacement of 1 cf (0.028 m3) of
material. From 28 to 56 days the 0.5 CF SR gained 362 psi (2.5 MPa) or 6.4%,
the 1.5 CF SR gained 532 psi (3.7 MPa) or 14.8%, and the 2.5 CF SR gained 284
psi (2.0 MPa) or 11.2%. At 56 days the 1.5 CF SR and 2.5 CF SR mixtures had
strengths 1821 psi (12.5 MPa) or 30.7% and 3139 psi (21.6 MPa) or 52.8% lower
than the 0.5 CF SR mixture. The 2.5 CF SR mixture was 1318 psi (9.5 MPa) or
32% lower than the 1.5 CF SR concrete. The 1.5 CF concrete only got to be over
80


4000 psi (27.6 MPa) at 56 days, while the 2.5 CF mixture never surpassed 3000
psi (20.7 MPa). If the 2.5 CF SR mixture continued to gain strength at 11.2%
over a 28 day period it would in theory be able to reach 4000 psi (27.6 MPa) after
an additional 357 days. See Chapter 6 for statistical analysis.
Concrete Age (Days)
Figure 5.11 Compressive Strength Gain for 0.5,1.5, and 2.5 0.5 CF Mixtures
Figure 5.12 shows the 28 day compressive strengths and compares them to the
minimum compressive strength value of 4200 psi (28.9 MPa) defined by CDOT
for Class P concrete. The 1.5 CF SR and 2.5 CF SR mixtures are the only not to
meet the compressive criteria set forth by CDOT. The Control and the 0.5% PP
mixtures are both well above 6000 psi (41.3 MPa). The 0.5% SR, 1.5% SR, 0.5
CF SR, and the 1.5% PP mixtures all exceeded 5000 psi (34.5 MPa) at 28 days of
81


age. Lastly the 0.5 CF PP mixture came in just below 5000 psi (34.5 MPa) at
4788 psi (33.0 MPa).
0.5 CF PP
Control
3000 4000 5000
Compressive Stress (psi)
6000 7000 8000
Figure 5.12 28 Day Normalized Compressive Strengths compared to CDOT
Class P Requirements
The failure mechanism of the fiber mesh concretes was addressed heavily in all
the previous research done on fiber reinforced concretes. It should be noted that
the results found in this study were similar to those of past research. The concrete
with fiber mesh when subjected to a compressive load did not break and fracture
apart like the control mixture did. For the 1.5 and 2.5 CF mixtures the concrete
82


appeared to be crushing instead of breaking. Photos of the compression tests for
28 days are shown in Figure 5.13.
Figure 5.13 28 Day Compressive Cylinder Test Photos
83


Due to the fact that the concretes containing fiber mesh did not shatter many of
these concretes had residual strength. A residual strength is the load that is able to
be maintained on an element even after failure or the original loads have been
removed. See Figures 5.14 through 5.17 for a sampling of loading diagrams
showing residual strength for the fiber reinforced concretes at 28 and 56 days of
age. Each of the graphs only shows one out of the three compressive tests that
were performed at each day of testing.
8000
Time (sec)
Control
------0.5% SR
-----1.5% SR
- - 0.5 CF SR
1.5 CF SR
---- 2.5 CF SR
Figure 5.14 28 Day Compressive Test Loading tests for Buffed Rubber
Concrete
84


8000
Time (sec)
Control
0.5% PP
1.5% PP
0.5 CF PP
Figure 5.15 28 Day Compressive Test Loading tests for PP Fiber Concrete
8000
Control
-----0.5%SR
----1.5% SR
-----0.5 CF SR
1.5 CF SR
2.5 CF SR
Time (sec)
Figure 5.16 56 Day Compressive Test Loading tests for Buffed Rubber
Concrete
85


8000
Time (sec)
Control
------0.5% PP
-----1.5% PP
--- 0.5 CF PP
Figure 5.17 56 Day Compressive Test Loading tests for PP Fiber Concrete
As seen in the figures above, the Control mixture has a much different failure than
the concretes with fiber added. In the graphs once the peak stress has been
reached the Control mixture drops off to zero stress almost immediately. This
indicates that the concrete cannot carry any type of load after the initial failure.
When the concrete was observed during failure the concrete was shattered apart
and the failure was very sudden and violent with shards of concrete flying
everywhere. As for all of the concretes with fiber mesh the graphs tell a very
different story. In general the load is increased to the peak stress and then drops
rapidly to a certain point and then the concrete starts to carry the load again. The
stresses in the concrete again are increased until the concrete has a second failure.
86


This pattern can repeat itself and continue to hold load after each failure. The
concretes ability to carry load after the initial break is due to the concrete not
shattering as the Control mixture had. As the fiber concretes were observed
during failure the concrete remained intact for the most part, failing and breaking
apart only in localized zones.
Referring to Figure 5.14 all of the mixtures containing buffed rubber
exhibited residual strength capacity after the initial concrete failure. In addition,
as shown in the graph the Control mixture shows no residual capacity after the
initial failure. The 0.5% SR mixture has a very high residual strength of about
5500 psi (37.9 MPa) or 89% of the ultimate concrete strength. This capacity well
exceeds the minimum strength requirement given by CDOT for compressive
strength. The 1.5% SR shows a residual strength capacity of about 3500 psi (24.1
MPa) or 61% of the ultimate 28 day capacity. The 0.5 CF SR concrete shows
very good residual capacity in the 4500 psi (31.0 MPa) range or approximately
80% of the 28 day strength. The 1.5 CF SR mixture had a 28 day compressive
strength that did not exceed 4000 psi (27.6 MPa). Yet this concrete still displays
the ability to hold load after the initial failure close to 3000 psi (20.7 MPa) or
83% of the total strength. The 2.5 CF SR concrete had 28 day compressive
strength that did not exceed 2500 psi (17.2 MPa) but showed a residual strength
above 2000 psi (13.8 MPa). This is a residual strength of 79% of the total
capacity. At 28 days of age all of the mixtures containing the PP fibers showed
87