ADVANCED RADIOGRAPHIC IMAGING FOR ASSESSMENT
OF CIVIL ENGINEERING INFRASTRUCTURE
Donald Andrew Howanick
B.S., University of Colorado at Denver, 1994
A thesis submitted to the
University of Colorado at Denver
in partial fulfillment
of the requirements for the degree of
Master of Science
This thesis for the Master of Science
Donald Andrew Howanick
has been approved
Albert W. Knott
Howanick, Donald Andrew (M.S., Civil Engineering)
Advanced Radiographic Imaging for Assessment of Civil Engineering Infrastructure
Thesis directed by Assistant Professor Kevin L. Rens
Improved radiography (x-ray) imaging techniques are needed that are
capable of helping engineers locate and inspect non-metallic objects buried in
concrete. Many new products are constructed of composite and plastic materials
that are difficult to detect when they are inside of a structure. Of particular
importance is locating fiber optic and other cables inside of concrete slabs. In the
past, communication lines and electrical control cables were made of copper wires
that were relatively easy to detect with standard radiography inspection equipment.
With the increased use of fiber optic cables for computers and control equipment,
the potential for cable severance is present every time a hole is bored into a concrete
slab or wall.
This research project develops a new technique for making the image of
non-metallic objects inside concrete visible and easy to identify by standardizing a
marking system consisting of copper rings at specified spacings on plastic conduits.
A new type of image quality indicator is also developed that allows the depth of the
object within the concrete to be estimated.
Experiments were conducted using plastic pipe 3A inch in diameter placed
inside of concrete slabs that were 4 V2, 6 V2, 8, and 8 V2 inches thick to develop the
system used. The technique developed was tested with two types of radiography
equipment; traditional radiography equipment using a radioactive source of iridium-
192 and x-ray film, and a digital radiography system that uses computer enhanced
imaging. Both types of radiography equipment showed that the new imaging
techniques developed were capable of accurately identifying the non-metallic
objects and estimating their depth.
This abstract accurately represents the content of the candidates thesis. I
recommend its publication.
Kevin L. Rens
Special Thanks to the following:
Kevin L. Rens; Ph.D.; P.E. for being my advisor and mentor for this
Albert W. Knott; Ph.D.; P.E., for helping develop this project.
Craig Hager and the staff at M.Q.S. Inspection Services; Denver,
Colorado, for the assistance with the radiography.
Dave Transue and Atkinson-Noland & Associates for helping get the digital x-
Judith J. Stalnaker; Ph. D.; P.E., for always being kind and helpful.
Dale Hill and the crew at Trans-Teq. LLC; Denver, Colorado, for the
encouragement and use of the fine computer.
The entire Civil and Structural Engineering Department at C. U. Denver.
Thank you Lord Jesus for holding me up when my sandals were weary !
1.1 Identification of Non-Metallic Objects..............................5
1.2 Image Quality Indicators and Verification of Results................5
1.3 Determination of Depth of an Object.................................8
1.4 Questionnaire Sent to Local X-Ray Inspection Company...............10
2. Radiography (X-Ray) Theory.........................................12
2.2 History of Radiography.............................................13
2.3 Sources of Radiation...............................................15
2.3.1 Electric X-Ray Machines............................................15
2.3.2 Radioactive Elements...............................................20
2.4 Measurement of Gamma Radiation.....................................23
2.5 Safety Near Radioactive Sources....................................25
2.6 Quality Control of Radiography
2.7 Applications of Radiography......................................30
2.8 Current Method of Determining Object Depth Using Radiography....32
3. Other X-Ray Methods Available....................................35
3.1 Digital Radiography..............................................35
3.1.1 HRRTR Radiography................................................35
3.1.2 Computed Radiography (CR)........................................37
3.1.3 iiRAD System.....................................................38
3.1.4 CDR Radiography..................................................38
3.1.5 Computerized Tomography (CT).....................................39
4. Experimental Results.............................................40
4.1 Experiments 1,2, and 3 Imaging Using Conventional Radiography ...41
4.1.1 Experiment 1 Minimum Wire Diameter Required for AIQI
4.1.2 Experiment 2 Self Identification System (SIS) Experiment.......46
4.1.3 Experiment 3 Three Dimensional Imaging Using the AIQI
4.2 Experiments 4, 5, and 6 Digital X-Ray Imaging..................60
4.2.1 Experiment 4 Three Dimensional Imaging Using the AIQI
System with a Digital X-Ray System on a Concrete Slab
11.5 cm Thick................................................63
4.2.2 Experiments 5 and 6 Three Dimensional Imaging Using the
AIQI System with a Digital X-Ray System on Concrete
Slabs 16.5 cm and cm Thick....................................68
4.3 Experimental Error............................................71
5. Summary, Conclusions and Recommendations......................72
A. Questionnaire and Cover Letter Sent to a Local
NDT Inspection Company........................................76
B. How Images were Produced for this
1.1 Schematic of Ladder Bar Used as an
Image Quality Indicator.............................................7
1.2 Multiple Exposure Set-Up to Estimate Object Depth
2.1 Electric X-Ray Machine in Test Lab.................................17
2.2 Interior of Electric X-Ray Machine.................................18
2.3 Digital X-Ray System...............................................19
2.4 Truck Used For Field Radiography...................................20
2.5 Interior of Truck Mounted Darkroom.................................21
2.6 Dosimeter Used By Radiography Inspectors to
Monitor Total Amount of Radiation Exposure.........................26
2.7 Audible Alarm used to Detect High Levels of
2.8 Technician Reading Film............................................30
2.9 Double Exposure Set-Up to Estimate Depth
of Object Using Radiography....................................32
4.1 Schematic of Wire Size Layout...................................43
4.2 Arrangement of Test Wires for Experiment 1......................44
4.3 Image of Test Wire Sizes on X-Ray Film
from Experiment 1.1.3..........................................45
4.4 Enhanced Version of Figure 4.3 Showing Location of
3 mm Steel and Copper Wires....................................46
4.5 Plastic Tubes with Copper Rings Used for
Self Identification System.....................................47
4.6 Images of Plastic Tubes with Copper SIS at 25 mm, 50 mm,
and 75 mm Spacing..............................................48
4.7 Schematic of Ladder Bar Used as an
Image Quality Indicator........................................49
4.8 AIQI Ladder Bars Used for 3-Dimensional Imaging.................50
4.9 Arrangement of Experiment 3.....................................52
4.10 Top View Of Experiment 3 Arrangement............................53
4.11 Images of SIS Rings and AIQI Ladder Bars
With a Film to Source Distance of 127 cm (50 in.)..............54
4.12 Images of SIS Rings and AIQI Ladder Bars
With a Film to Source Distance of 63.5 cm (25 in)................55
4.13 Correct Measurement of SIS Rings..................................56
4.14 Method of Estimating Object Depth.................................57
4.15 Measurements form Figure 4.12.....................................58
4.16 Digital X-Ray System Used for Experiment 4........................60
4.17 Digital X-Ray Image of Hex Key Wrench.............................61
4.18 Digital X-Ray Image of Hex Key Wrench and
4.19 Digital X-Ray Positive Image of 11.5 cm Thick Concrete Slab
with AIQI Ladder Bars and Plastic Tube with SIS Rings............65
4.20 Measurements from Figure 4.19.....................................66
4.21 Digital X-Ray Image of 16.5 cm Thick Concrete Slab with
AIQI Ladder Bars and Plastic Tube with SIS Rings.................69
4.22 Digital X-Ray Image of 21.5 cm Thick Concrete Slab with
AIQI Ladder Bars and Plastic Tube with SIS Rings.................70
B. 1 Negative of Plastic Tubes with Copper SIS Rings...................81
B.2 Positive of Plastic Tubes with Copper SIS Rings..................81-
B.3 Negative of SIS Rings and AIQI Ladder Bars........................82
B.4 Positive of SIS Rings and AIQI Ladder Bars........................82
1.1 Responses to NDT Questionnaire....................................11
2.1 Radioactive Elements Used for
NDT Inspection (ASM, 1976)........................................23
New engineering materials are developed every day, and each type of
material brings along new promises and new problems. Specialized plastics and
composite materials are being produced that can replace many products formerly
constructed of steel, aluminum, and copper alloys. Products such as piping,
structural components, automotive parts, and even communication cables may now
be constructed of materials that are more efficient, more corrosion resistant, and
often lighter in weight than the older metal products used in the past. Most new
plumbing, for example, is now made of polyvinyl chloride (PVC) plastic pipe,
which is less expensive than steel pipe, and much lighter, and easier to install.
The increased use of non-metallic products creates a special type of problem
when it becomes necessary to locate these materials inside of concrete slabs after
they have been installed. Holes are often bored through floor slabs to run additional
utility lines or make structural modifications to buildings. When these holes are
bored, it is very important to not cut through existing power or communication
lines. At new airports, the majority of the control tower radar and security systems
are operated through fiber optic lines. A severed fiber optic line at an airport could
result in a major communication dilemma until repairs to the service line could be
completed. Similar conditions exist at banks and office buildings that house
corporate headquarters for large firms.
Locating plastic piping or fiber optic lines inside a concrete floor slab can be
very difficult. Blueprints are not always accurate because last minute changes to
routing of cables and piping are often not recorded. The routing of lines is of little
importance to the installer as long as the equipment is connected to the cable and it
works properly. Furthermore, when concrete is placed, the lines may be moved
around by the weight of the concrete as it flows and is vibrated into place. As a
result, the exact location of the fiber optic cable or piping will be unknown.
When a hole must be bored through concrete, it is often necessary to use
nondestructive testing (NDT) to image the interior of the concrete without actually
cutting into it. Many NDT techniques are available for such inspections. Two of
the most common techniques used for inspecting the interior of concrete are
ultrasonic testing (UT) and radiographic (x-ray) inspection.
With either technique, it is very difficult to detect many of the new
materials. At Denver International Airport (DIA), for example, the location and
verification of fiber optic cables inside concrete slabs has been a difficult task
(Hager, 1999). The fiber optic cables are used for subway controls, automatic
baggage controls, unmanned security doors, and in some cases the control tower.
Now that construction is complete, careful inspection using radiography or other
NDT techniques is necessary to locate these fiber optic lines before new water or
electrical lines are installed. The old style copper wires were much easier to detect
because copper is very dense compared to the surrounding concrete and shows up
well on x-ray film or ultrasonic inspection equipment screens (Hager, 1999). The
new fiber optic cables which are made of nonmetallic materials are often placed
near the bottom of the slab and may be hidden by rebar, seams in metal concrete
forms, or other objects that prevent the cables from being detected by NDT
equipment. Adding to the problem is the fact that cables rarely run in straight lines
between electric components and may curve around utility lines, stairways, or other
objects within the concrete slab.
Although there are several different NDT techniques that may be capable of
locating objects in concrete slabs, x-ray is often the preferred method because it
gives the positive identification of an object on a developed x-ray film. Many
contractors and inspectors trust x-ray inspection more than the other methods
because it has been used for many years with proven results and accuracy. The film
may also be stored for later examination and documentation at some time in the
future. Building and inspection codes consider radiography an acceptable method
for detecting objects in concrete slabs.
Because radiography is such an important and commonly used method of
inspecting the interior of concrete structures, this research project will focus on
improving x-ray imaging of non-metallic objects inside of concrete. The use of
products made from composite materials and plastics is rapidly increasing and new
techniques must be created to locate these objects using NDT methods. This
research will concentrate on three goals:
1. ) Develop a method of standardizing identification markings on non-metallic
objects commonly used inside of concrete slabs so they may be distinguished from
one another when viewed on x-ray film.
2. ) Create an image quality indicator that verifies that the radiography was
conducted correctly. This indicator must also verify that if an object were present,
its image would appear on the x-ray film.
3. ) Create an imaging system that is capable of determining the approximate
depth of an object using only one exposure of radiation on the film. Current
methods of determining depth of an object normally require two or three exposures
to determine depth of an object.
1.1 Identification of Non-Metallic Objects
Many objects are made of the same material, but are intended for different
uses. For example, a small diameter plastic pipe passing through a concrete slab
may be used as a water supply pipe, a conduit for electrical wire, or a conduit for
fiber optic cable. In order to improve x-ray imaging of such a plastic pipe, it is
necessary to establish a method of marking the pipe so it will show up clearly on x-
ray film. These markings should be different for each intended use of the conduit
and allow the inspector to distinguish between a conduit being used for electrical
lines and a conduit carrying fiber optic cable.
For this research project, a series of copper wire rings were located at
different standardized spacings on plastic tubing. The spacing can identify the
intended use of the tubes. The wider spacing was used for less important
applications such as water pipes, and the narrower spacing was used to identify
more important conduits such as fiber optic lines.
1.2 Image Quality Indicators and
Verification of Results
In order to verify the accuracy of results on a developed x-ray film, a device
such as an image quality indicator can be placed within the exposed area of the film.
This device will show up on the film when the radiography is conducted correctly
and will prove whether or not the object inside the concrete slab is in the area
viewed on the exposed x-ray film. If the image quality indicator is not visible on
the developed film, yet is known to be present, then the test area must be re-
inspected and a new film made. For this research project, a ladder shaped bar was
used with a known standardized spacing. This bar allows the inspector to measure
the image of the bar on the developed x-ray film and helps determine the size of
objects viewed on the film. It is also used in helping determine object depth as will
be discussed in section 1.3 of this thesis. Figure 1.1 shows a schematic of a ladder
bar used as an image quality indicator.
50 mm (2 in.)
3 mm (1/8 in.) Diameter
Welded Steel Wire
50 mm (2 in.) Spacing
Figure 1.1 Schematic of Ladder Bar Used as an Image Quality Indicator
1.3 Determination of Depth of an Object
When conventional x-ray techniques are used, it is very difficult to
determine the depth of an object with one exposure of the film. The standard
method of determining depth of an object is to take several exposures on the same
film to show two or more images in different positions. This is done by moving the
source of radiation at measured intervals and using geometry and similar-triangle
calculations to estimate the depth of the object. This method is time consuming and
exposes the surrounding area and potentially the workers to higher levels of
radiation than when using only one exposure. Figure 1.2 is a schematic of a typical
arrangement of x-ray equipment being used to create multiple images on the same
Figure 1.2 Multiple Exposure Set-Up to Estimate Object Depth Using
Xi = Spacing between radiation source locations.
X2 = Spacing between images on x-ray film.
Y2 = Object height above x-ray film.
Y2 = Distance from S2 perpendicular to film .
51 = Location of radioactive source at first exposure.
52 = Location of radioactive source at second
This research project develops a method of using the ladder bar image
quality indicators to estimate the depth of an object with only one exposure. This is
accomplished by placing one ladder bar on top of the concrete slab and another bar
on the bottom of the slab perpendicular to the top bar. When the developed fdm is
viewed, the image of the top bar will have wider spacing than the image of the
lower bar. The image of the spacing of the rings on the cable conduit may then be
scaled in proportion to the ladder bar spacings and the depth of the conduit may be
estimated by scaling the images and knowing the concrete thickness. This
technique is further explained in chapter 4.
1.4 Questionnaire Sent to Local X-Ray
A questionnaire was sent to several branches of a large NDT corporation to
help determine the specific needs of technicians who routinely use radiography for
concrete inspection. The only questionnaires returned were from the local Denver
office, who sent back six responses. The respondents indicated that PVC plastic
pipe and fiber optic cables are hard to locate with radiography inspection. All of
the respondents indicated that they prefer radiography for inspecting concrete slabs
less than 25 cm (10 inches) thick. Most of the respondents indicated that they use
radiography for the evaluation of older existing structures. Table 1.1 shows a
summary of the responses to the questionnaire. Appendix A shows a copy of the
Table 1.1 Responses to NDT Questionnaire
TYPE OF MATERIALS REGULARLY INSPECTED WITH RADIOGRAPHY
STEEL CONCRETE TIMBER MASONRY ALUMINUM AND
6/6 5/6 0/6 2/6 2/6
USE OF RADIOGRAPHY FOR INSPECTION OF OLDER EXISTING STRUCTURES
NDT METHOD PREFERRED FOR CONCRETE LESS THAN 25 cm (10 in.) THICK
ULTRASONIC RADIOGRAPHY OTHER
0/6 6/6 0/6
PROBLEM MATERIALS WHEN USING RADIOGRAPHY
PLASTIC FIBER OPTIC THICK NONE
PIPE LINES CONCRETE
2/6 2/6 1/6 2/6
2. Radiography (X-Ray) Theory
Radiography technology is one of the most common forms of
nondestructive testing because it allows inspectors to see into the interior of the
structure being examined. Radiography is used for a wide range of applications
including medical examination, weld inspection, and to examine the interior of
concrete slabs and walls. This method of non-destructive testing is often used
where failure of a key structural member could result in a life threatening situation
or where failure would result in extensive property damage (ASTM E-94-93,
Radiographic theory is based on two principles:
1. ) Materials with different densities or thicknesses absorb different
amounts of radiation.
2. ) When a photographic film is exposed to a radioactive source, the film
will be darkened. In the new digital x-ray systems, the film is
replaced with a digitized computer screen that darkens after radiation
When a concrete slab with a dense metal object in it is placed in front of a
photographic film or digital x-ray screen, and is exposed to a radioactive source, the
dense object within the concrete will absorb more radiation than the surrounding
concrete and the developed film below that object will be lighter than the
surrounding area. In a similar manner, any large voids in the concrete will allow
more radiation to pass through and the developed film below the void will be
darker. When a concrete slab greater than 20 cm (8 in.) thick is examined using
radiography, a small void such as a 25 mm (1 in.) diameter plastic tube is often very
difficult to see on the developed x-ray film because the difference in density is small
compared to the overall density of the concrete slab. The image of the plastic tube
blends in with the surrounding concrete. (Hager, 1999; Picker, 1991).
2.2 History of Radiography
Radioactivity was discovered in 1896 by French physicist Antoine Henri
Becquerel when he noticed that uranium could darken a photographic plate even
when the uranium was separated from the photographic plate by dark glass or black
paper that blocked sources of light from passing through. In 1898, French chemists
Marie and Pierre Curie determined that radioactivity was associated with atomic
structure, independent of chemical or physical states. Marie Curie also discovered
the radioactive elements radium, polonium, and thorium. In 1899, French chemist
Andre Louis Debienre discovered the radioactive element actinium and British
physicists Ernest Rutherford and Frederick Soddy discovered the radioactive gas
Radon (Funk and Wagnalls, 1993).
It was soon discovered that radioactivity had much more energy than coal or
any other mineral which produced energy during burning. One gram of coal
produces a total of only about 8000 calories during complete combustion. By
comparison, one gram of radium produces 100 calories of energy in the form of heat
every hour for many years. The heat energy associated with the radioactive element
radium is produced by the disintegration of atoms. The unit of measure for rate of
atomic disintegration is called the curie, named after Marie Curie, and represents
3.70 x 1010 disintegrations per second (Funk and Wagnalls, 1993).
Three types of radiation are the emission of alpha, beta, and gamma
particles. Alpha particles, which have a positive charge, have the least penetrating
power and are capable of penetrating aluminum to a depth of only a few thousands
of a centimeter. Beta particles, which have a negative charge, have about 100 times
the penetrating power of alpha particles. Gamma particles, which are uncharged,
can penetrate much deeper than beta particles. The radioactive elements iridium-192
and cobalt-60, which are strong producers of gamma particles, are often used for
NDT inspection (Mix, 1987; Funk and Wagnalls, 1993).
A radiographic inspection system consists of three components; a source of
radiation (the radioactive element or electric x-ray machine), photographic film, and
a darkroom to develop the exposed film. The new digital x-ray systems also consist
of three components: an electric source of radiation, an digitized plate which is used
in place of x-ray film, and a computer which replaces the darkroom. The digital x-
ray system is capable of producing instant images on the computer screen and
eliminates the need to wait for development of the x-ray film in a darkroom
2.3 Sources of Radiation
Radioactive emission can be produced by two primary sources, high voltage
electricity and radioactive elements. High voltage electricity is used by electric x-
ray machines and digital x-ray systems. Radioactive elements are primarily used for
field inspection or where very thick, dense objects are being inspected. NDT
inspections use only gamma rays and x-rays because their relatively short wave
lengths are capable of penetrating dense objects and giving indications of internal
differences. (Golden, 1996; Mix, 1987).
2.3.1 Electric X-Ray Machines
To produce radiation with an electric source, an x-ray tube is used. An x-ray
tube consists of a cathode (negative terminal) connected to a filament (usually a coil
of tungsten wire) and an anode (positive terminal) inside a vacuum chamber. The
filament is heated to incandescence, and begins to emit electrons. High voltage is
then applied between the cathode and the anode to accelerate the electrons across
the tube onto a target. When the electrons smash into the target, x-rays and gamma
rays are produced in a wide range of wave lengths. This is the type of radiation
most commonly used by hospitals and stationary testing laboratories. Higher tube
voltages will produce x-rays that have shorter wave lengths and greater penetrating
power. Typical x-ray tubes have voltages ranging from 150 kV to 1000 kV (Picker,
1991). Due to weight and size, electronic x-ray machines are not used very
commonly for concrete inspection unless small samples can be taken to a testing
laboratory for special examination. Figures 2.1 shows a typical x-ray machine used
in a testing laboratory for NDT inspections. This type of electric x-ray machine is
similar in design and function to x-ray equipment used in hospitals for medical
examinations. The doors and sides of this machine are lined with lead to shield the
surrounding area from harmful radiation.
Figure 2.1 Electric X-Ray Machine in Test Lab
Figure 2.2 shows the inside of an electric x-ray machine used for non-destructive
inspection of welded specimens. This type of machine is also used for examining
small samples of concrete that may be transported to the lab, however, electric
machines usually do not have enough penetrating power for very thick concrete
samples. Figure 2.3 shows a digital x-ray system, one of the newer types of electric
Figure 2.3 Digital X-Ray System
2.3.2 Radioactive Elements
Radioactive elements produce gamma rays that are capable of penetrating
dense objects and producing an image on x-ray film. Unlike electric x-ray
machines, which produce a large spectrum of radiation wave lengths, radioactive
elements produce radiation in specific wave lengths. Radioactive elements are
commonly used to inspect concrete slabs because they are easily transported in
radiation shielded containers, and require no source of electricity. Figure 2.4 shows
a typical truck used for field radiography. This truck has a darkroom mounted in the
bed which is used for developing x-ray film at the job location.
Figure 2.4 Truck Used For Field Radiography
The truck mounting darkroom has chemical tanks, a hanger box to store film, and a
dryer to dry film after it is developed. There are also cabinets that hold extra film
and other darkroom equipment. Figure 2.5 shows the interior of the truck mounted
Figure 2.5 Interior of Truck Mounted Darkroom
The most common radioactive element used for NDT inspection is iridium-
192. This element is widely used because it has adequate penetrating power for
concrete up to 33 cm (13 inches) thick. Iridium-192 has the shortest half-life of all
the radioactive elements used for radiography of concrete. Iridium has a half of
only 74 days and will decay much faster than other radioactive elements if it is
accidentally lost or dropped into a location where removal without exposure to
radiation is difficult. Other radioactive elements used for radiography are thulium-
170, cobalt-60, cesium-137, and radium. Cobalt-60 is widely used by the NDT
inspection industry when iridium-192 does not have enough penetrating power.
Cobalt-60 is capable of penetrating concrete up to 36 inches thick. The
disadvantage of using cobalt-60 for all radiographic inspection is that it may expose
people and equipment behind the concrete section to excessive doses of radiation.
Radium is not used very often because it has a half-life of 1620 years (Mix, 1987).
Table 2.1 shows the commonly used radioactive elements, their penetrating power
through concrete slabs, and their respective half-lifes. (ASM, 1976).
Table 2.1 Radioactive Elements Used for NDT Inspection (ASM, 1976)
Element Half-Life Penetration (concrete) (cm) Penetration (concrete) (inches)
Iridium-192 74 days 30.5 12
Thulium-170 128 days 5.1 2
Cobalt-60 5.3 years 91.4 36
Cesium-137 33 years 30.5 12
Radium 1620 years 50.8 20
2.4 Measurement of Gamma Radiation
Gamma radiation from radioactive elements is measured by source strength,
in units of curies. As stated earlier, one curie is equal to 3.70 x 1010 atomic
disintegrations per second (Mix, 1987). A typical radioactive source of iridium-192
used for field radiography will have a strength of around 100 curries when new, and
will be discarded when the source strength drops to around 25 curries (Hager,
When the source strength is known, the radiation intensity, measured in
roentgens per hour at a distance of one meter (rhm), can be determined by
multiplying the source strength by the radiation output, which is expressed in rhm
per curie. It is important to avoid any unnecessary exposure of workers to the
radioactive source when using radiographic inspection. The amount of radiation
accumulated is a function of dose rate and exposure time as given in Equation 2.1.
D = Drxt (2.1)
D = Total Dose of Radiation in Roentgens.
Dr = Dose Rate in Roentgens per Second.
t = Time in Seconds.
Radiation moves radially outward from the source. As distance from the
source doubles, radiation intensity decreases by a factor of four. This is known as
the inverse square law, which can be expressed by Equation 2.2 (USNRC, 1982).
D = D0
D ~ Calculated Dose Rate at a Distance, R, from the
D0 = Known Dose Rate Near the Source.
R0 = Distance from Source Where Dose Rate is
2.5 Safety Near Radioactive Sources
Radiation exposure is dangerous and radioactive sources require special care
to ensure that they are used in a safe and effective manner (USNRC, 1982). One of
the biggest disadvantages of radiography is the emission of harmful radiation to the
surrounding area. The entire inspection area should be blocked off from access by
unauthorized personnel using special rope and cones with radiation warnings on
them. The surrounding areas should be examined to make sure people do not have a
way to enter the radiation zone. Electronic equipment may have to be removed
from the vicinity before radiographic inspection is done to prevent damage to the
sensitive instruments inside.
Radiographic inspection companies require special state and federal licenses
to use radioactive sources at job sites. This may require fees, permits, and records
of all inspections made with radioactive elements. Any accidents involving
radiation leakage are carefully documented and reviewed by the U.S. Nuclear
Regulatory Commission and local authorities. Negligence in safety may result in
large fines and loss of permission to use radiographic equipment (USNRC, 1980).
All personnel working around radiographic equipment are required to carry
radiation monitoring equipment with them. This includes a dosimeter which is a
film badge that measures the total amount of radiation a technician is exposed to
during a time period of several weeks, as well as radiation monitoring alarms which
sound an alarm when harmful amounts of radiation are detected. Figures 2.6 and
2.7 show a film badge dosimeter and an audible alarm, respectively. When a
structure is about to be examined by radiography, the entire area must be evacuated
such that no one in the area is exposed to dangerous levels of radiation. This
requires many inspections to be done at night when workers have gone home.
Figure 2.6 Film Badge Dosimeter Used By Radiography Inspectors to
Monitor Total Amount of Radiation Exposure
Figure 2.7 Audible Alarm used to Detect High Levels of Radiation Exposure
Special training and certification is required before technicians are allowed
to conduct commercial radiography. Certification and licensing requires 79 hours of
classroom time, 2100 hours of field experience under close supervision of a licensed
inspector, and passing specified tests. Many companies have additional
requirements as well (ASNT, 1984).
2.6 Quality Control of Radiography
It is important to ensure that accurate results are obtained on the developed
x-ray film. All inspection laboratories must calibrate the film with a device called a
penetrameter which serves as an image quality indicator. The penetrameter consists
of thin brass or lead plates with holes in them or wires of known sizes and is placed
on the test specimen when it is exposed to the radioactive source. If the hole or wire
is visible on the developed film, then it assures that an object of the same size or
larger will be indicated on the film. When radiography is used to inspect concrete
slabs, the penetrameter is placed on the side of the slab closest to the radioactive
source so that the image of the penetrameter must be projected through the entire
slab thickness. This ensures that if the image quality indicator is visible on the film,
then an object inside the slab would also be seen. This helps verify that the
procedure for the radiography has been followed correctly. The image quality
indicator also helps verify the nonexistence of an object if the indicator is visible on
the film, but the object is not. Penetrameters are often made of lead and are small
enough to be placed in the field radiographer's tool kit (Picker, 1991; ASTM E-
1025, 1993; ASTM E-747, 1993).
Film selection is also important for best results when using radiography to
inspect concrete. The film used for radiography is similar to photographic film used
in a camera, but film size is typically larger. Film used to inspect concrete is
typically 356 mm x 432 mm (14 in. x 17 in.) and may be larger for some special
applications. The ability of a film to indicate a certain size of object is called
sensitivity. There are many different sensitivities of film available. Smaller objects
require more sensitive film. Highly sensitive films allow shorter exposure times,
but are much more expensive than standard films. The fast films are only used for
special applications (Hager, 1998; ASTM E-142, 1993; ASTM E-999, 1993; Hull
and John, 1994). Figure 2.8 shows a technician examining a developed x-ray film
of a weld by holding it up to a light source.
Figure 2.8 Technician Reading Film
2.7 Applications of Radiography
Radiography may be used to image any dense material and is commonly
used to examine steel, concrete, bones, machine parts, castings, or other materials in
which the presence of internal flaws is suspected. Objects that are inspected by
radiography must be at right angles to the lines of radiation. A thin object parallel
to the lines of radiation may not show up on the developed x-ray film. Objects with
very similar densities and thicknesses may be hard to distinguish from each other on
x-ray film (Picker, 1991).
Welds are often inspected using radiography to determine the presence of
voids or cracks within the weld. This method of NDT also allows the inspector to
determine if the mating parts were not properly aligned, or if there is a lack of
complete fusion in the weld zone. Radiography is commonly used to inspect
critical welds on pipelines, skyscrapers, and in shipyards (AWS D1.1-96, 1996;
ASME-Section IX, 1992).
Reinforced concrete is often inspected using radiography to determine the
location of rebar, electrical conduit, and prestressing cable (ACI-311, 1983). Areas
with voids in the concrete or missing rebar may be located with radiography
inspection methods before a slab is loaded to its full capacity. This is especially
important for large bridges and stadiums where thousands of pieces of rebar may be
placed and it is easy to leave some out (ASTM E-94-93, 1993).
One of the greatest difficulties encountered when inspecting concrete slabs is
locating nonmetallic materials. When it is necessary to locate fiber optic lines or
small plastic conduit, the much greater density of the surrounding concrete tends to
block out the image of the smaller, less dense object. Radiography is less effective
on non-metallic materials, and no viable non-destructive alternative technique is
2.8 Current Method of Determining
Object Depth Using Radiography
The current technique most commonly used to estimate the depth of an
object using radiography is described in the ASME Boiler and Pressure Vessel Code
(ASME, 1992). This method uses two exposures of radiation to create a double
exposure on the film. Figure 2.9 shows how the double exposure radiography
experiment is set up.
Figure 2.9 Double Exposure Set-Up to Estimate Depth of
Object Using Radiography
From Figure 2.6, above, Equation 2.3 to determine object depth:
Xx = Spacing between radiation source locations.
X2 = Spacing between images on x-ray film.
Yl = Distance from S2 perpendicular to film .
Y2 = Object height above x-ray film.
Sj = Location of radioactive source at first exposure.
S2 = Location of radioactive source at second
When this method is used to estimate object depth, it requires two exposures of
radiation to the film. This may be time consuming and expensive. This method
also requires evacuation of the area during radiation exposure and may result in lost
time at a job site while the radiographic inspection is taking place. However, this
method of determining depth of an object (or void in the test specimen)
has been used for many years with reliable results.
Other X-Ray Methods Available
Many different types of radiographic NDT methods exist and there are
several new systems being currently developed. This chapter will discuss some of
the methods that are, or may become, applicable to imaging an object inside a
3.1 Digital Radiography
One of the newest methods of NDT imaging is digital radiography. The
present day trend is to make everything, including radiography, digital and
computerized. Some of the more important advances in this field will be discussed
as will applications of these techniques. The advantage of digitized radiography is
that it requires much lower levels of radiation and produces images on a computer
screen instantly. The disadvantage of digital systems is that they have less power
and require electrical connections. These systems eliminate the time spent
developing x-ray film in a darkroom and allow the inspector to determine if the
image produced is acceptable as soon as the image appears on the screen.
Although digital x-ray systems use an electric source to produce x-rays,
these systems only emit about one tenth to one twentieth of the radiation as a
conventional electric x-ray system. This allows the operator to remain in the work
area much longer without being exposed to unsafe levels of radiation. The lower
radiation output also reduces the risk of people in the vicinity of x-ray equipment
3.1.1 HRRTR Radiography
High-Resolution Real-Time Radiography (HRRTR) is a system being
developed by Lockheed Palo Alto Research Laboratories of Palo Alto, California. It
uses a digital imaging system to enhance x-ray images of aircraft wings when they
are being inspected for fatigue cracks. Aircraft wings develop fatigue cracks from
being stressed as the wings oscillate during flight. This new system can give
instant high-resolution images of the wings during inspection. (ME-CIME,1994).
The HRRTR system uses a fiber optic plate and a charge-coupled device
(CCD) for high-resolution digital imaging. This system uses a robot to place the
radioactive source at the desired location with the CCD and an image of the wing
being inspected is instantly produced. This image is produced in the same way as a
picture on a video camcorder. The digital system gives the inspector an instant
picture of the interior of the wing without waiting for the x-ray film to be developed.
This system turns x-rays into photons that show up on the surface of the
CCD plate using a proprietary material developed by Lockheed. Defects as small as
25 microns have been detected with this system, and very minute amounts of
corrosion have been observed before the wings began to develop fatigue cracks in
the affected area. Lockheed is trying to develop procedures that can detect second
layer corrosion which is undetectable with common NDT methods. Lockheed is
currently developing a portable unit that may be used on flight lines to inspect
aircraft in the field. (ME-CIME,1994).
3.1.2 Computed Radiography (CR)
Fuji NDT Systems has developed a system called Computed Radiography
(CR) that uses a new type of film, called a photo-stimulable plate, which is scanned
by a laser to convert the image into a digital form. This type of x-ray film is much
more sensitive than conventional film and allows the use of smaller amounts of
radiation for imaging. This system is very useful in the automotive industry, where
NDT inspection is used extensively to inspect components. The digital image can
be computer enhanced to let inspectors examine thick and thin parts on the same x-
ray image, something that has not been possible in the past. This is because the
amount of radiation required for inspecting thick and thin parts is different for
CR technology was first introduced for the medical field in the 1980's, and is
used at many major medical facilities today. The latest developments are being
used for industrial applications and NDT inspection. CR imaging is used in the
automotive industry to inspect clutch assemblies, air bag squibs, and many other
components critical to the proper performance of the vehicle (Demmler, 1997).
This system is capable of producing up to 70 images per hour with easily
portable equipment. The CR system has many advantages over conventional
radiography including; (1) up to 20 times less radiation is required compared to
conventional x-ray equipment, (2) images are produced on a digitized plate and
eliminate the need for a darkroom, (3) unlike film the imaging plate may be reused
hundreds of times, and (4) the images stored on the digital plate remain sharp and
clear and do not fade away as x-ray film images tend to do over a period of time.
3.1.3 iiRAD System
This system uses digitized radiography and the DirectRayTM flat viewing
panel ( a digital screen) to produce images within a few seconds after exposure to
the radioactive source. This system is currently being tested for the medical
industry, but may develop into some industrial applications in the near future.
Similar to other digitized radiography systems, the iiRAD system produces less
radiation, creates less waste because the film is replaced by the digital screen,
produces instant images, and eliminates the need for a darkroom. (P.R. Newswire,
3.1.4 CDR Radiography
CDR digitized radiography is a system being developed by the Schick
Company for dental x-rays. This system uses approximately 10 percent less
radiation than conventional dental x-ray equipment and produces high resolution
images that can help the dentist determine the condition of a patients teeth to a much
higher degree of accuracy. CDR technology is also being developed for the
assessment of bone mineral density. Like other digitized radiography systems, CDR
uses a digitized screen instead of a conventional x-ray film, and produces an image
in a matter of seconds. (P.R. Newswire, 1998)
3.1.5 Computerized Tomography (CT)
The computerized tomography (CT) system is a type of digitized
radiography that views an object as a series of thin "slices" rather than as a whole
object. The image produced is similar to a catscan image used in the medical field.
This system is being developed by the Aerojet Corporation to inspect large rocket
motors built for NASA and other military applications. Using this system allows
the inspection of critical high stress areas on these rocket motors while they are fully
loaded and placed on strategic rockets. This system allows high speed inspection
using lower levels of radiation, and produces pictures of higher resolution than
conventional x-ray film.
This system is used after conventional radiographic methods or digital
radiography when it is desired to get a close up look at a questionable cross section
of a high stress area. The CT system is currently being used to evaluate older
Minuteman missiles that may need to be refurbished. It helps inspectors decide
whether to replace or rebuild the existing missiles. (Materials Evaluation, 1994).
4. Experimental Results
In order to verify the theory of the proposed imaging system presented in
pages 5 and 6 of Chapter 1, six experiments were conducted. Three experiments
were done at a remote field location with a portable radiography truck and the
assistance of a professional NDT inspector. These three experiments used iridium-
192 as a source of radiation to expose conventional x-ray film. The film was then
developed at the site using the darkroom in back of the truck. The other three
experiments were at a laboratory using one of the new digital x-ray imaging
systems. This x-ray system used a digitized screen instead of x-ray film,
eliminating the need for a darkroom and development of the film. When digital x-
ray equipment is used, the image is visible instantly. The imaging screen has
controls to change the brightness and contrast of the image, allowing the operator to
create many different views of the image. The image can then be stored on a
computer disk for review in the future.
4.1 Experiments 1, 2, and 3 Imaging Using
The first experiment determined the required size of wire that should be used
for the Advanced Image Quality Indicator (AIQI) and Self Identification System
(SIS). The AIQI is used to verify the existence or nonexistence of an object within
a concrete slab and to verify that the radiography was conducted properly. If the
image of the AIQI is visible on the developed x-ray film (or the screen of a digital
x-ray system), this verifies that the proper radiography procedure was followed. If
the image of the AIQI shows up on the film (or digital x-ray screen) and no image
of the object is observed, then the object must be at another location. The second
experiment determined if the SIS was capable of positively identifying non-metallic
objects that were inside of a concrete slab. The third experiment tested the three
dimensional depth finding capability of the AIQI system. Each of these three
experiments are described and analyzed in the next several sections of this chapter.
4.1.1 Experiment 1 Minimum Wire Diameter
Required for AIQI and SIS
For the first experiment, five copper and one steel wire varying in diameter
from 0.5 mm (0.02 in.) to 3 mm (1/8 in.) were placed diagonally through a test
section of concrete slab. The wires were aligned from the top of one side of the
concrete mold to the bottom of the other side so that their depth of cover would
vary. A metal frame was built to hold the wires in place and each end of the wires
was secured by a large steel washer that would show up on the developed x-ray
film. This allowed the location of the ends of the wires to be known even if the wire
diameter was so small that the wire itself did not appear on the developed film.
Figure 4.1 shows a schematic of the system, and Figure 4.2 is a photograph of the
STEEL FRAME IMAGING WIRES
Figure 4.1 Schematic of Wire Size Layout
The concrete was then carefully placed in the mold to form a slab of
concrete 20 cm (8 in.) thick. The test slab was then imaged using conventional
radiography with a source to film distance of 203 cm (80 in.). After the film was
developed, the smallest wires that showed up best on the film was observed to be
solid copper or steel wire 3 mm (1/8 in.) in diameter. This wire could be identified
at all depths of cover. This was the size and type of wire used for the SIS rings
around the test specimens and the AIQI ladder bars in Experiment No. 2. Figure 4.3
shows the positive image produced from the wire frame inside of the concrete slab.
Appendix B explains how positive images were produced from negatives to enhance
Figure 4.3 Image of Test Wire Sizes on X-Ray Film from Experiment 1.1.3
Figure 4.4 shows an enhanced version of Figure 4.3 showing the location of the 3
mm steel and copper wires.
Figure 4.4 Enhanced Version of Figure 4.3 Showing Location of 3 mm Steel
and Copper Wires
4.1.2 Experiment 2 Self Identification System
The second experiment involved casting in place three hollow 22 mm (0.87
in.) diameter plastic tubes with SIS rings inside a concrete slab and checking
detectability. The SIS rings were spaced at 25 mm (1 in.) on the first tube, 50 mm
(2 in.) on the second tube, and at 75 mm (3 in.) on the third tube. The tubes were
then placed in a concrete slab mold on top of 7.5 cm (3 in.) of concrete and
covered with an additional 12.5 mm (5 in.) of concrete. Figure 4.5 shows the three
tubes used for this experiment.
Figure 4.5 Plastic Tubes with Copper Rings Used for
Self Identification System
The slab was then imaged using conventional radiography and the developed
film showed that the plastic tubes could easily be determined from each other.
Hollow plastic tubes this small are normally very difficult to see on x-ray film. The
SIS rings made these tubes visible and allowed the different tubes to be
distinguished from each other as shown in section 4.1.3. These images were
produced with a source to film distance of 203 cm (80 in.). Figure 4.6 shows the
positive images produced by the SIS Rings.
Figure 4.6 Images of Plastic Tubes with Copper SIS at 25 mm, 50 mm, and
75 mm Spacing
4.1.3 Experiment 3 Three Dimensional Imaging
Using the AIQI System
The third experiment determined the ability of the Advanced Image Quality
Indicator (AIQI) system (the ladder bars) to determine the depth of an object within
a concrete slab.
The AIQI bars were made of steel wire 3 mm (1/8 in.) in diameter, the diameter
determined in Experiment 1 as the minimum diameter wire that would produce a
visible image on the x-ray film throughout the depth of the concrete slab. Figure 4.7
shows a schematic of the ladder bars. Figure 4.8 shows a photo of the ladder bars
used for the AIQI system.
.4----------50 mm (2 in.)
3 mm (1/8 in.) Diameter
Steel Round Bar
50 mm (2 in.) Spacing
Figure 4.7 Schematic of Ladder Bar Used as an Image Quality Indicator
Figure 4.8 AIQI Ladder Bars Used for 3-Dimensional Imaging
Experiment No. 3 involved placing two hollow plastic 22 mm (0.87 in.)
diameter tubes with SIS rings having a spacing of 50 mm (2 in.) at different depths
inside the slab. The center of the first tube was placed 7.5 cm (3 in.) deep in the
slab. The center of the second tube was placed 12.5 cm ( 5 in.) below the center of
the first tube, at the bottom of the slab. The AIQI ladder bars were placed on the top
and bottom of the slab and the slab was imaged with conventional radiography. The
developed film showed that there was a slight difference, about 0.75 mm (1/32 in.),
in the size of the images produced by the SIS rings and the AIQI ladder bars at the
top and bottom of the slab when a source to film distance of 127 cm ( 50 in.) was
used. This size difference was too small to allow accurate measurements needed to
calculate the estimated object depth. By shortening the source to film distance to
63.5 cm (25 in.) a much greater difference in size of the images was produced on the
developed film. This made it possible to calculate the estimated depth of the object
imaged on the developed x-ray film with reasonable accuracy. Figure 4.9 shows the
arrangement of the source of radiation, the film, the plastic tube with SIS rings, and
the AIQI ladder bars.
LOWER TUBE & SIS RINGS
Figure 4.9 Elevation View of Experiment 3
Figure 4.10 shows a plan view of the AIQI ladder bars and the plastic tube with SIS
Figure 4.10 Plan View Of Experiment 3 Arrangement
Figures 4.11 and 4.12 show the images produced on the x-ray film by using
a film to source distance of 127 cm (50 in.) and 63.5 cm (25 in.) respectively.
Figure 4.11 Images of SIS Rings and AIQI Ladder Bars With a Film to
Source Distance of 127 cm (50 in.) Note That There is Little Difference
Between the Spacing of the Upper and Lower Ladder Bars
Figure 4.12 Images of SIS Rings and AIQI Ladder Bars With a Film to
Source Distance of 63.5 cm (25 in.) Note The Greater Difference Between the
Spacing of the Upper and Lower Ladder Bars Compared to the 127 cm (50 in.)
Film to Source Distance
To estimate the depth of the tube with the SIS rings the following procedure is
followed. First the images on the developed film are carefully measured. The SIS
ring spacing is determined by comparing the images of the rings to the images of the
AIQI bars which have a standard spacing of 50 mm. This will determine if the SIS
rings are spaced at 25 mm, 50 mm, or 75 mm. Then the image of the SIS rings is
compared to the image of the lower AIQI bar, which is at the film and represents a
spacing of 50 mm. The depth of the object may then be determined by using
Equation 4.1. Figures 4.14 and 4.15 show an illustration of this technique. To be
sure the depth of the plastic conduit is measured at the center of the tube, the outside
edge of the SIS ring image must be measured as shown in Figure 4.13.
Figure 4.13 Correct Measurement of SIS Rings
Xi = Actual Spacing Between Rings
X2 = Width of Image on Film
h = Distance From Film to Object
H= Film to Source Distance
Figure 4.14 Method of Estimating Object Depth.
Figure 4.15 Measurements from Figure 4.12
From Figure 4.15, Equation 4.1 may be used as follows.
Xi = 1.5 in.
X2= 1.88 in.
H = 25 in.
h = 5.05 in.
The actual depth was 5.0 inches. This experiment estimated the object depth
very near to the actual depth, with an error of about 1 %.
4.2 Experiments 4, 5 and 6 Digital X-Ray Imaging
Experiments 4, 5 and 6 were conducted using the digital radiography system
shown in Figure 4.16, that produced images on a computer screen the instant the x-
ray machine was turned on. This system eliminated the need for a darkroom to
develop x-ray film. Controls on the system computer allowed the operator to
enhance the images by changing the brightness and contrast as soon as the image
appeared on the computer screen.
Before the experiments were performed, two test images were produced
using the digital x-ray system. The first test produced an image of a hex key wrench
placed on top of a small concrete slab 11.5 cm (4.5 in.) thick. Figure 4.17 shows the
image produced by the digital x-ray system.
Figure 4.16 Digital X-Ray System Used for Experiment 4
Figure 4.17 Digital X-Ray Image of Hex Key Wrench.
The second test involved placing the hex key wrench on another concrete
slab 11.5 cm (4.5 in.) thick that had a void inside of it. The goal was to be able to
see the image of the hex key wrench and to determine the location of the void.
When the image of the second test was examined, the hex key wrench and the void
were plainly visible. Figure 4.18 shows the image produced by this test.
Figure 4.18 Digital X-Ray Image of Hex Key Wrench and Void Region
Experiments 4, 5 and 6 were conducted after the two test images indicated
the system was ready to be used for imaging concrete slabs. Experiment 4 tested the
ability of the AIQI system to be used to estimate the depth of an object inside of an
11.5 cm (4.5 in.) thick concrete slab using digital radiography. Experiment 5 tested
the AIQI systems ability to help estimate the depth of an object inside of a 16.5 cm
(6.5 in.) thick concrete slab. For Experiment 6, a concrete slab 21.5 cm (8.5 in.)
thick was tested. It was found that the digital x-ray system used did not have
enough penetrating power to penetrate the slabs in Experiments 5 and 6 and produce
a clear image of the upper AIQI ladder bar. To ensure accurate results, both the
upper and lower AIQI bars must be visible. If only the lower AIQI bar is visible it
is very difficult to determine if the image of SIS rings on the screen are from an
object close to the bottom of the slab or the projected image of a smaller SIS ring
spacing closer to the top of the slab. For an image quality indicator system to be
reliable, all indicators must be clearly visible on the digital screen or developed x-
4.2.1 Experiment 4 Three Dimensional Imaging
Using the AIQI System with a Digital X-Ray
System on a Concrete Slab 11.5 cm Thick
Experiment 4 involved placing a hollow plastic 22 mm (0.87 in.) diameter
tube with SIS rings spaced at 50 mm (2 in.) inside of a concrete slab 11.5 cm (4.5
in.) thick. The center of the plastic tube was located at a point 35 mm (1-3/8 in.)
above the bottom of the slab. The AIQI ladder bars were placed on the top and
bottom of the slab and the slab was imaged using the digital x-ray system. The film
to source distance used was 63.5 cm (25 in.), the same distance used for the best
imaging in experiment 3. The image produced showed much more detail than a
similar image produced with conventional radiography. The plastic tubes could
clearly be identified on the screen. The size difference between the images of the
AIQI ladder bars on the top and bottom of the concrete slab was very similar to the
size of the images produced in experiment 3. The size of the spacing of the SIS
rings on the plastic tube was between the size of the spacing of the upper and lower
AIQI ladder bars indicating that the tube had SIS rings with a 50 mm ( 2 in.)
spacing. The estimated depth of the tube could then be calculated using geometry
and similar triangle equations. Figure 4.19 shows the image produced by the digital
x-ray system used for Experiment 4.
Figure 4.19 Digital X-Ray Positive Image of 11.5 cm Thick Concrete Slab
with AIQI Ladder Bars and Plastic Tube with SIS Rings. Note How
Sharp and Clear the Images Are.
From Figures 4.19 and 4.20, the depth of the tube with the SIS rings may be
estimated by using Equation 4.1.
Figure 4.20 Measurements from Figure 4.19
From Figure 4.15, Equation 4.1 may be used as follows.
Xi = 1.25 in.
X2= 1.31 in.
H = 25 in.
25'" (l.31'" -1.25' ) 1ir.
------51-----:--------L = 1.15 in,
h = 1.15 in.
The actual depth of the object was 1.375 inches. This experiment estimated
the object depth within 20% of the actual depth.
4.2.2 Experiments 5 and 6 Three Dimensional
Imaging Using the AIQI System with a
Digital X-Ray System on Concrete Slabs
16.5 cm and 21.5 cm Thick
Experiments 5 and 6 were very similar to experiment 4 except that the
thickness of the concrete slabs were increased to 16.5 cm ( 6.5 in.) and 21.5 (8.5
in.). As the thickness of the slab increased, it became harder to distinguish objects
on the computer screen of the digital x-ray system. A lot of computer enhancing to
brighten and contrast the image was required before the objects could be viewed.
The image of the lower ladder bar was plainly visible on the screen, however, the
image of the upper ladder bar was very difficult to see. The image of the upper AIQI
ladder bar was very blurred because the density of the concrete blocked a lot of the
radiation from the x-ray machine and prevented a clear image from appearing on the
digital x-ray viewing screen. Figures 21 and 22 show digital x-ray images of the
16.5 cm (6-1/2 in.) and 21.5 cm (8-1/2 in.) slabs with the SIS rings and AIQI bars in
place. The depth of the object could not be determined because the objects farthest
away from the digital screen could not be measured accurately.
Figure 4.21 Digital X-Ray Image of 16.5 cm Thick Concrete Slab with AIQI
Ladder Bars and Plastic Tube with SIS Rings. Note How the Image of the
Upper AIQI Ladder Bar Begins to Fade Away and Blend into the Dark
Figure 4.22 Digital X-Ray Image of 21.5 cm Thick Concrete Slab with AIQI
Ladder Bars and Plastic Tube with SIS Rings. Note How the Image of the
Upper AIQI Ladder Bar Begins to Blur and Fade from View.
4.3 Experimental Error
Some experimental error occurred during these six experiments. All efforts
were made to eliminate as much experimental error as possible. The tubes may
have moved when the concrete was placed into the molds and vibrated in to place.
However, the tubes were secured by placing them on supports that rested on the
bottom of the concrete molds to help eliminate experimental error. The concrete was
placed into the molds very slowly and carefully. The molds were kept level during
settling of the concrete to help prevent the tubes from being pushed to the side. As a
final precaution, the molds were set aside to allow the concrete to cure in an area
where no one would disturb them. The results of this research appear to be accurate
and there is no reason to believe that any large errors were present during the
imaging of these objects. When using the radioactive source and a portable truck
with x-ray film and a darkroom, it took several attempts before the images became
clear on the film. The digital x-ray equipment was much easier to use because the
on-board computer allowed the operator to enhance the images with the touch of a
5. Summary, Conclusions and Recommendations
After the experiments were conducted, the estimated depths of the tubes with
the SIS rings were compared to the actual depths of the objects before the concrete
was placed into the molds. The experimental results were very close to the actual
depths. Experiment 3 estimated the object depth to be 5.06 in. and the actual depth
was approximately 5.0 in. This represents a difference of slightly over 1%.
Experiment 4 estimated the object depth to be 1.15 in. and the actual depth was
approximately 1.375 in. This represents a difference of slightly less than 20%.
In order for this system to be used effectively, it is necessary to standardize
the spacing of the SIS rings so that a specific spacing always identifies a tubes
intended use. The tube may then be positively identified as a fiber optic conduit,
electrical conduit, or water pipe on the x-ray film or digital screen. The spacing of
the AIQI ladder bars must also be standardized so all inspectors have the same tool
to work with when estimating the size and depth of objects they are viewing. With
complete standardization of this system, it is a very effective and useful tool for
engineers and inspectors to use as a NDT method. As more non-metallic materials
are introduced into the construction and manufacturing environment, it becomes
increasingly important to develop new techniques that allow identification and
inspection of these products.
These results show that this system is capable of helping inspectors locate
objects, identify them and estimate the depth of objects inside of concrete slabs.
This method may also be used to determine the thickness of a concrete slab if the
film to source distance is known. The difference in size between the images of the
upper and lower ladder bars may be measured, and the thickness of the slab may be
determined by using Equation 4.1 where Xi is the spacing of the image of the lower
ladder bar, X2 is the spacing of the image of the upper ladder bar, H = film to source
distance, and h is the slab thickness. Using the measurements from Figure 4.12:
X, = 1.5
X2 = 2.1
H = 25 in.
h = ^ x 25in. = 7.14in.
h = 7.14 in.
Actual Thickness = 8 in.
In the future, the techniques developed by this research may be used with
some of the other NDT methods and may not be limited to finding objects inside
concrete slabs. Other important products made of non-metallic materials that may
be difficult to inspect include structural aircraft parts made of composite materials
or plastics, critical mechanical fasteners made of composite materials, even new
guns made from composite plastics that may evade metal detectors at airport
security gates (Welbom, 1986). Some of these products could begin to incorporate
wire mesh or similar metal in their interior that would make them easy to detect and
inspect. With careful design, the desirable mechanical properties of these products
such as corrosion resistance, light weight, and resistance to fatigue failure would not
be affected. Although the cost of manufacturing plastic conduit with SIS rings built
in may expensive, the increase in price would certainly be justified if it prevented
fiber optic cables at airports from being severed accidentally.
Continued research on identification and location techniques for NDT
methods is the long term goal of this research project. These techniques could
easily be adapted to manufacturing of non-metallic products. The SIS rings or a
similar device could be installed directly into plastic conduit at standardized
spacings during production. Firms that build structural aircraft parts could install a
metal mesh between composite material layers. Advanced imaging systems could
help detect and locate fatigue cracks and other failures before they become large
enough to create dangerous conditions.
Appendix A. Questionnaire and Cover Letter Sent
to Local NDT Inspection Company
The following pages show copies of the questionnaire and the cover letter
sent to a local NDT inspection company. The inspectors who filled out these
questionnaires have many years of experience with radiography techniques and
other methods of NDT. Their answers and comments helped focus this research
project toward a goal that was very important and useful.
The Civil Engineering Department of the University of Colorado at Denver is currently
involved in a research project that involves radiographic imaging. This project will improve and
expand the use of radiographic inspection for evaluating civil engineering structures. This research
is primarily focused on concrete structures; however, the techniques developed may be applied to
any material that could be examined by radiographic methods. Responses from people in the N.D.E.
inspection industry are very valuable to this project and it would be greatly appreciated if you could
take some time to fill out the enclosed questionnaire / survey.
The comments returned will help determine areas that would benefit the most by this
research. In addition, the responses will also provide direction to further focus the research scope.
The goal of this project is to increase the use of nondestructive inspection technology for use in
structural evaluation. Many structural engineering firms may be interested in using the new
techniques developed in this research program. All respondents to this questionnaire will be
included in any further correspondence. Please return in the enclosed self addressed envelope or you
may send the questionnaire by FAX to 303-556-2368
Your comments are valuable to this project.
Kevin L. Rens, Ph.D., P.E.
Assistant Professor of Structural Engineering
Don Howanick, E.I.T.
Graduate Research Assistant
UNIVERSITY OF COLORADO AT DENVER
CIVIL ENGINEERING DEPARTMENT
1. ) On what types of materials do you regularly use radiographic inspection technology ?
steel concrete timber masonry other______________
2. ) Do you ever utilize radiography for evaluation of older, existing structures ? (please circle)
3. ) Which method do you prefer for inspecting concrete slabs less than 10 inches thick? (please
ultrasonic methods radiography other______________________________
4. ) Do you encounter any problem materials that may benefit from improved imaging systems ?
5. ) We would appreciate any examples of case studies where detection of objects was difficult or
any comments about routinely encountered problems.
( Please use back of page for additional comments )
Thank you very much for your participation, please return by January 31,1999 in the enclosed
self addressed envelope or FAX to: (303) 556 2368, Attn: Dr. Kevin Rens.
Appendix B. How Images were Produced for this
X-ray film will darken when it is exposed to a source of radiation. If an
object that is more dense than the surrounding material is placed in the path of the
radiation, this object will block some of the radiation and produce a light spot on the
developed x-ray film. This image is called a negative of the object. It is often
difficult to identify the negative of the object when the object size is small compared
to the surrounding material.
The object is visible on the developed x-ray film as a light spot that tends to
fade and blur into the surrounding dark background. It is difficult to take accurate
measurements of the objects image on a negative and estimate the depth of the
object within the surrounding material using the SIS and AIQI systems. It is
possible to make a positive out of the negative on the x-ray film and obtain a clearer
picture of the image of the object. The image of the object will appear as a dark
spot on the film and the surrounding more dense material will show up as a light
background. This makes the object easier to identify and measure on the picture of
the x-ray. The photographs in this research project were produced by making
positives out of the negatives on the developed x-ray film.
Two methods were used to obtain positives of the x-ray images. When
conventional radiography was used, the negatives on the x-ray film were taken to a
photo lab and developed in the same manner as typical camera film. When the
digital x-ray equipment was used, the on board computer had the capability of
producing a positive of the images on the viewing screen with the touch of a button.
The digital x-ray machine allowed the operator to switch instantly between
negatives and positives and also store the images on a computer disk. The
following pages show some examples of negatives and positives in figures B.l
through B.4. The use of positives greatly enhanced the imaging capability of the
SIS and AIQI systems.
Figure B.l Negative of Plastic Tubes with Copper SIS Rings
Figure B.2 Positive of Plastic Tubes with Copper SIS Rings
Figure B.3 Negative of SIS Rings and AIQI Ladder Bars
Figure B.4 Positive of SIS Rings and AIQI Ladder Bars
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