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ACI Materials Journal/January-February 2003 63

ACI Materials Journal , V. 100, No. 1, January-February 2003.MS No. 02-061 received February 7, 2002, and reviewed under Institute publica-

tion policies. Copyright © 2003, American Concrete Institute. All rights reserved,including the making of copies unless permission is obtained from the copyright pro-prietors. Pertinent discussion will be published in the November-December 2003 ACI

Materials Journal if received by August 1, 2003.

ACI MATERIALS JOURNAL TECHNICAL PAPER

Bonded carbon fiber-reinforced polymer (CFRP) composites are

rapidly gaining acceptance as a feasible solution for repair and strengthening of concrete structures. A reliable method of non-destructive evaluation (NDE) is needed to monitor the installation

quality and the long-term efficacy of the repair. This paper presentsresearch into the use of infrared (IR) thermography to evaluatebonded CFRP composites used to strengthen concrete beams.

Reinforced concrete beams were constructed and then strengthened with several configurations of wet layup or precured laminateCFRP composites. Two beam configurations were used to force

either a shear failure mode or a flexural failure mode. IR inspectionswere conducted on the CFRP composite systems before loading to

determine the extent of bond. The beams were then tested staticallyto failure. Periodically during load testing, IR inspections wereconducted to determine the amount and characteristics of bond damage. Results indicated that IR inspections were able to

consistently determine loss of bond as loading progressed.

Keywords: bond; concrete; fiber-reinforced polymer; strength.

INTRODUCTIONStrengthening of concrete members with carbon fiber-

reinforced polymer (CFRP) composites has become increasinglypopular in recent years. These systems are generally surfaceapplied, which prevents the visual inspection of the encapsulated

concrete member. Post-installation detection of unbonded,

debonded, and delaminated areas is critical in evaluating theactual capacity and durability of the CFRP composite, as

well as the underlying concrete and reinforcing. Qualitative

infrared thermography (IR) can be used to detect unbonded,debonded, and delaminated areas under the surface and within

the CFRP strengthening system.

Thermography is commonly used for a variety of industrial

applications including problems associated with bearinglubrication, pipes, valves, seals, motor windings, motor

overloads, electrical circuits and components, and chemical

processes. Thermography has been used in buildings forenergy audits, moisture problems with roofs, and inspectionof reinforced masonry. IR thermography has also been

used for qualitative nondestructive evaluation (NDE) of

bridge piers wrapped with CFRP strengthening systems(Jackson et al. 1999).

RESEARCH SIGNIFICANCE

The Federal Highway Administration estimates that230,000 of the nation’s 575,000 bridges are structurallydeficient or functionally obsolete (Witcher 1996). Many of

these bridges can be repaired or strengthened with CFRP

composites. Much work has been conducted on the structuralaspects of bonded repair systems. The long-term performance

characteristics of the system, however, are still largely

unknown. With the promise of increased use of CFRP systems

in structural applications, it is imperative that a simple andeffective method for NDE be developed. IR thermography can

potentially be used to detect flaws in bonded repair systems,including installation defects and long-term degradation.

The work presented in this paper provides fundamentalqualitative and quantitative data indicating IR inspection

to be a viable method for the NDE of bonded CFRP systems.

CFRP STRENGTHENING AND BOND

Bonded CFRP systems can add flexural strength, shear

strength, and/or confinement to an existing reinforced or

prestressed concrete element. Flexural and shear strengtheningare commonly applied to beam elements while confinementis used to improve the ductility of concrete columns underearthquake loads. When adding flexural strength, CFRP

sheets or laminates are placed so that the fibers are oriented

along the length of the beam in the region of high flexuraltensile stresses. Shear strength is added by orienting the fibers

perpendicular to the beam’s axis, thus providing a stirrupconfiguration. Confinement is typically provided by wrapping

the element, usually a column, with CFRP. In some cases,the column may be wrapped multiple times to ensure sufficient

confinement will be obtained.

Loss of bond in flexural or shear configurations can be

catastrophic. Bond is of lesser importance when consideringconfinement, but it could be an indicator of other more

serious problems.

To avoid confusion, it is necessary to define the four types

of bond loss that will be discussed in this paper. The firsttype is an unbonded area that is not a structural failure butrather an initial lack of bond that occurred during application.

The second type is a debond, which describes the separation

that occurs between the CFRP composite and the underlyingconcrete surface. This type of failure is structural and is

characterized by the fact that there is very little concrete

attached to the composite after pulling away from the concrete.The third type is a delamination, which is the separation of two layers of CFRP composite. This type of bond loss can

occur as a result of faulty installation or as a result of loading.The final type is a substrate failure. The substrate failure is

structural and is characterized by a tensile rupture of the concretebelow the surface of the bond line. This is the preferred mode of

failure because the adhesive has performed adequately and

allows for the maximum transfer of stress between the CFRP

and concrete for that particular concrete strength.

Title no. 100-M8

Nondestructive Evaluation of Carbon Fiber-Reinforced

Polymer-Concrete Bond Using Infrared Thermography

by John M. Levar and H. R. (Trey) Hamilton III


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ACI Materials Journal/January-February 200364

Current applicable limits for unbonded, debonded, and

delaminated areas are given by the ACI Committee 440 report“Guide for the Design and Construction of Externally Bonded

FRP Systems for Strengthening Concrete Structures (ACI440.2R-02).” These guidelines (hereinafter referred to as ACI440.2R-02) recommend the following for general acceptance of wet layup systems:

1. Inspection methods should be capable of detecting

delaminations of 2 in.2

(1300 mm2) or greater;

2. Small delaminations less than 2 in.2 (1300 mm

2) each

are permissible, as long as the delaminated area is less than5% of the total laminate area and there are no more than 10

such delaminations per 10 ft2 (1 m2);

3. Large delaminations, greater than 25 in.2 (16,000 mm

2), can

affect the performance of the installed FRP and should be

repaired by selectively cutting away the affected sheet and

applying an overlapping sheet patch of equivalent plies; and

4. Delaminations less than 25 in.2 (16,000 mm2) may be

repaired by resin injection or ply replacement, depending onthe size and number of delaminations and their locations.

There were four objectives determined at the outset of this

research. The first objective was to determine the overalleffectiveness of qualitative IR thermography for use in

inspecting CFRP strengthening systems for concrete. For IR

thermography to be effective, it must be capable of providingquality control information for both initial application

and long-term performance. The second objective was to

determine the behavior of the CFRP systems as loadingprogressed. Observations of debonded area progression

through failure can be useful in future research and field

detection of strengthening systems that have been overloadedor otherwise damaged. The third objective was to determine

the installation quality that can be expected from the

strengthening systems. These data can be used to developguidelines for initial tolerances of unbonded areas in thestrengthening systems. The flexural and shear capacity of

strengthened beams are calculated based on the assumptionthat a perfect bond exists between the composite and the

concrete. While this assumption is rarely attained in practice,

it is not clear what minimum area of bond is required to ensurethat there is a reasonably low risk of bond failure. The final

objective was to determine the ability of IR thermography

to detect the extent and characteristics of the debondingsurrounding flexural or shear cracks under the CFRP composite.

EXPERIMENTAL PROGRAMSpecimen construction

The specimens used for testing were 4.9 m (16 ft) long

concrete beams, 102 x 305 mm (4 x 12 in.) in cross section.Two 12.7 mm (No. 4) bars with a yield strength of 414 MPa

(60 ksi) were placed in the bottom of the beam with an effective

depth of 250 mm (10 in.). A total of eight specimens were

John M. Levar is President and Co-founder of Advanced Structural Technologies, Inc.,

Minneapolis , Minn. He received his MSCE from the Universit y of Wy oming. He is a

member of ACI Committee 440, Fiber Reinforced Polymer Reinforcement. His researchinterests include the durability and inspection of fiber-reinforced polymer composites and

the effect of reinforcing steel corrosion on composite strengthening systems.

H. R. (Trey) Hamilton III is an associate professor of civil engineering at theUniversity of Florida. He is a member of ACI Committees 222, Corrosion of Metals in

Concrete, and 440, Fiber Reinforced Polymer Reinforcement; and Joint ACI-ASCE

Committee 423, Prestressed Concrete. His research interests include evaluation and strengthening of existing structures.

Table 1—CFRP composite properties

CFRPtype

Fabricweight

System tensilestrength,MPa (ksi)

System modulus,GPA (ksi)

Materialthickness,mm (in.)

Fabric 19.23 oz./yd2 1034 (150) 69 (10,000) 1.0 (0.041)

Laminate na* 2482 (360) 200 (29,000) 1.4 (0.055)

* na = not available.

Fig. 1—Detail of flexural load testing setup.

Fig. 2—Detail of shear load testing setup.

Table 2—Flexural test matrix

Specimen CFRP typeCFRP width,

mm (in.) CFRP layout

F1F5

Fabric 102 (4) Single strip

F2F3F4

Fabric 406 (16) 50% U wrap

F6 Laminate 51 (2) Single strip

CF1CF2

None na na

*na = not available.


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constructed. The concrete strength at the time of testing was

35 MPa (5.0 ksi).

Two different bonded CFRP systems were applied to theconcrete specimens using a wet layup, unidirectional fabric and

unidirectional laminate, each from a different supplier. Each

system was applied using the manufacturer’s instructions. Shearand flexural capacities under the test load configuration were

calculated using the guidelines presented in ACI 440.2R-02.

CFRP system and material properties were taken from themanufacturer’s literature and are shown in Table 1.

Beams to receive the wet layup system (hereinafter referredto as fabric) were prepared by rounding the edges in the

applic ation area to a 12.7 mm (0.5 in) minimum radius with

a handheld grinder. Beams receiving laminate and/or fabricwere lightly sandblasted using 20 grit silica sand in the contactarea. After sandblasting, the beams were cleaned with

compressed air to remove residual sand and laitance.

There were eight flexural specimens with varying con-figurations of CFRP composites as shown in Table 2.

Specimens F1 through F5 used a single layer of fabric foreach specimen. Fabrics used to form the composite wereunidirectional with the strong direction oriented parallel with the

length of the beam. Specimen F6 used a precured laminate

placed on the tension face of the beam. Shear specimens were cut

from the flexural specimens after the completion of the flexural

tests. A total of 10 shear specimens were tested with varying

CFRP composite types and configurations as shown in Table 3.

Test setup and instrumentation

The flexural specimens were loaded in four-point bending

(Fig. 1). The shear test involved a much shorter span with asingle point load to ensure diagonal cracking and a shear failure

mode (Fig. 2). Both test setups were designed so that the

flexural tension face was oriented upward allowing convenient

access for IR inspection (Fig. 3).

The instrumentation included equipment for load-deflection

data collection and IR imaging prior to and during loading.Instrumentation included two 44.5 kN (10 k) load cells, dial

gages, and a multimeter. The load cells were connected to

the multimeter to measure the output and confirm the

support reactions. The dial gages were placed at midspan

to monitor deflection.

The infrared thermometer, infrared camera, an 8 mm VHS

camcorder, and a television formed the IR inspectionequipment. The infrared thermometer was used to take

surface temperature readings for use in scaling the IR images,

and the camera was used to collect IR images during the test.

The IR images were recorded with the VHS camera to provide

the best selection of photos possible. The IR camera had

a 25 mm lens. This particular camera/lens combinationallowed for visualization of defects 320 mm 2 (0.5 in.2)and larger. The display was monochromatic. Detailed

specifications for the IR camera include: detector type/

format; uncooled ferroelectric (320 x 240), spectral response; 7

to 14 microns, thermal stabilization; thermoelectric cooler,

video update rate; 30 Hz-real time, time to operation (typical);

< 25 s at 25 °C. The thermometer specifications included: 8:1target size, back-lit LCD display, temperature range of –32 to

400 °C (–25 to 750 °F), preset emissivity of 0.95, and accuracy

within 0.10 °C.

EXPERIMENTAL PROCEDURES

The experimental program consisted of flexure and shear

tests on the strengthened concrete beams. Control tests were

also conducted on beams that were not strengthened. The

beams were tested statically to failure. IR inspection was

conducted during the testing. The following sections describe

the testing details.

Table 3—Shear test matrix

Specimen CFRP type

CFRPwidth,

mm (in.) CFRP layout

S1S2S3S4

Fabric 406 (16)50% with fiber

longitudinal

S6S7

Fabric 406 (16)

50% with fiberlongitudinal and

50% withtransverse

S5S8

Fabric na

Strip with fiberlongitudinal and50% with fiber

transverse

S9S10

Laminate 51 (2)Strip longitudinal

and stirrup8@ 152 mm o.c.

CS1CS2

None na na


Fig. 3—Fully integrated test setup.


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IR inspectionCreating Temperature Differential—The IR inspections

were conducted using a lagging process to develop the

temperature differential as illustrated in Fig. 4. The specimenstarts with the concrete and CFRP composite at the same

temperature (Fig. 4(a)). When heat is applied to the CFRP

composite surface, its temperature rises (Fig. 4(b)). The un-derlying concrete is now cooler than the composite, creating a

thermal gradient and heat flow from the composite to the

concrete. The rate at which the heat flows is a function of theintimacy of contact between the composite and the concrete.

In areas where the composite is not in contact, the heat transferfrom composite to concrete will be slower than in areaswhere there is intimate contact. In the time following heat

application, this results in a higher temperature and resulting

emittance (or hot spot) at the debonded areas detectable withthe IR camera.

The heat sources that were tested included a heat gun, hair

dryer, kerosene heater, propane torch, and quartz lamp. Hair

dryers and heat guns were unable to heat the surface quicklyenough to create a gradient. The kerosene heater worked

well over large areas. This was especially useful in heatingthe laminate stirrups, as it allowed for the inspection of morethan one stirrup at a time. The propane torch also worked

well on the laminate, but without modification it didn’t provide

enough even coverage for use on the fabric. The most effective

and convenient heat source for the indoor inspection of theCFRP fabric was the quartz lamp. In under 20 s, an area of approximately 0.28 m2 (3 ft2) was heated evenly to 35 to

43.3 °C (95 to 110 °F). They also provided a significant

advantage when working indoors where combustion vaporswere a concern.

The specimen temperature was monitored during testing

using an infrared digital thermometer. Spot temperatures

were taken to aid in control of surface heating. Delayscaused by time-consuming reheating or waiting for the sur-

face to cool after overheating were avoided using this tech-

nique. Monitoring the temperature during heating alsoensured that the CFRP composite temperature was kept wellbelow the glass transition temperature.

The temperature of the specimen relative to the ambienttemperature affected the heat flow from the CFRP composite.

IR inspection was conducted repeatedly on a single specimen

during load testing. Consequently, the average temperatureincreased steadily as the test progressed, resulting in a less

effective IR inspection. The best results were obtained with

the following conditions:

• Ambient temperature below approximately 23.9 °C(75 °F);

• Use a 500 W lamp at a distance of 152 mm (6 in.) from

the surface; and

• Heat surface to 35 to 43.3 °C (95 to 110 °F).

Fig. 4—Lagging method of creating temperature gradient for IR inspection: (a) specimen

at ambient conditions; (b) heat application; and (c) hot spot development after heating.


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Temperatures above this level heated the composite more

than was necessary for a satisfactory inspection. Caution

must be exercised when heating so the glass transitiontemperature of the epoxy, usually 60 to 82.3 °C (140 to180 °F), is not exceeded. Exceeding this temperature limit is

easy to do with available heating methods and can have anadverse effect on the strengthening system.

Qualitative evaluation—IR inspection can be either

quantitative or qualitative. Quantitative IR inspection usesdirect temperature measurement to detect actual temperatures

of an object and requires equipment with temperaturemeasurement capabilities. The results of quantitative inspectionsare influenced by the object’s surface emissivity and texture,

the reflected background, ambient air temperature, cameraresolution, camera accuracy, and operator experience.Qualitative IR does not focus on obtaining the actual surface

temperature but rather the magnitude and configuration of

the temperature gradient. Qualitative IR inspection was usedin this research.

Defect identification and labeling—The need for identifyingand labeling defects is dependent on the intent of the IRinspection. As briefly discussed in the objectives, the purpose of

using IR thermography for NDE is twofold. The first use for

this type of evaluation is quality control of the application

process. The second is continued monitoring of existingrepairs over time to evaluate long-term performance. In the

case of quality control, locating the defects is not necessaryunless continued inspection is required. When continued

inspection is required, debonding, delaminations, and unbonded

areas need to be located for future inspection.

Metallic paint pens were used to mark the boundaries

of the hot spots (Fig. 5). Chalk was used in initial testing

and was found to be ineffective. Chalk was convenient inthat it could be easily removed from a surface, but when

it was needed for longer durations, it was insufficient.

The metallic pens worked well for showing the boundariesand were detectable in the IR camera and visible with the

naked eye. Nonmetallic pens were used for filling in the

areas of progression throughout the load steps. At the endof the test, the areas outlined at each load step were measured

using a transparent 650 mm 2 (1 in2) square grid, resulting in

areas of initial defects as well as damage that occurredduring testing.

Flexural testsIt was impractical to monitor the entire beam with IR

inspection during loading. Consequently, the area of maximum

moment was selected on which to focus the IR inspection(shown as the influence area in Fig. 1). During loading, heat

was applied for 15 to 20 s to an approximately 0.1 m2 (1 ft2)

area on one face of the beam with a 500 W quartz lamp.Immediately after heating, temperature readings were taken

with the infrared thermometer. The heat was removed and

the IR camera operator sketched the outline of the unbondedor debonded area directly on the CFRP composite. IR inspectionwas also conducted prior to loading to identify existing

unbonded areas and then after loading to 60, 80, and100% of the calculated capacity. At each of the selected

load steps, the beam was unloaded and each face was inspected

with the IR camera.

Shear testsThe influence area for the shear specimens consisted of a

762 mm (30 in.) length encompassing the beam within the

load points on the three sides reinforced with CFRP (Fig. 2).

The general IR procedures of inspection with the IR camera

were similar to those used for flexure. The shear specimenswere inspected initially with the IR camera to identify

unbonded areas. The test then progressed with load steps of

25, 50, and 75% of the ultimate capacity, and then ultimate

capacity after initial failure occurred. The ultimate capacitywas chosen over the calculated capacity in the shear tests due

to the variability in CFRP composite bond strength. The

beam was unloaded at each load step followed by IR inspection.

The 500 W quartz lamp was used for the fabric and a 79 MJ(75,000 BTU) kerosene heater was used for the laminate. IR

images were recorded during loading to determine if significant

changes in damage could be detected. The boundaries of

unbonded and damaged areas were marked and measured asin the flexure tests.

DEFECT VERIFICATION

The verification of the debonded, delaminated, and unbondedareas of CFRP was done using several different approaches.Prior to initiating the load tests and IR inspections, several

mockups were constructed with known unbonded areas. IR

inspection was conducted on these known defects to help in

distinguishing areas that have defects or damage. Surfacetemperature variations can occur due to variations in the

thickness of the epoxy. The temperature differential for these

areas was much more subtle than for the areas that have

defects or damage.

A common form of FRP composite inspection is acoustic

sounding (coin tap). The surface is impacted or tapped whilethe inspector listens for hollow sounds. Coin tap was conducted

occasionally during testing to verify results of the IR inspection.

It was found that an estimated 20 to 30% of the defects or

damage found with IR inspection were undetectable with thecoin tap. Unfortunately, it was not possible to verify these

areas following testing due to the destructive nature of the

testing. It was noted, however, that the areas detectable with

IR inspection but not with coin tap increased in size duringthe course of the load test, indicating that the areas were

indeed defects and not variations of epoxy thickness or other

false sources.

The difficulty arises when trying to distinguish at what

depth the anomalies occur. All forms of damage can be

detrimental to the CFRP composite system, but the specific

failure mode is required to determine the capacity of anexisting structural member. Unbonded and debonded areas

appeared differently in the IR image than a delamination due

to the variation in depth between the three types of bond loss.

This difference is not obvious and is difficult to determine

Fig. 5—Outlines created by metallic paint pens representing

hot spots.


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68 ACI Materials Journal/January-February 2003

correctly due to the variability of layer thickness, system

composition, and heating methods. By studying the behaviorof single-layer systems of unidirectional fabric, it was possible

to verify the signatures when layers were combined in different

orientations. In this way, the characteristics of delaminationsin exterior layers were separated from debonding at theconcrete CFRP interface. The more CFRP layers a system

has, the more difficult it is to distinguish between the differenttypes of defects.

The final approach for verification came from physicalremoval of the CFRP from the specimen in an attempt toobserve the debonded/unbonded areas. The CFRP laminate

provided evidence of unbonded areas by a lack of epoxyresidue on the interior surface of the laminate. The fabric did

not provide an obvious indication of subsurface voids when

removed from the concrete. Upon removal from the concretesurface, the CFRP was covered uniformly with concrete

fragments. The concrete fragment also splintered off the

fabric as it was peeled from the concrete substrate. The

remaining concrete substrate looked very similar to the

CFRP and did not provide verification of specific unbonded

or debonded areas.

RESULTS AND DISCUSSIONUnbonded areas

The results of the initial IR inspections prior to load testingare shown in Table 4 and 5 for the flexure and shear specimens,

respectively. Note that the table gives areas as a percentageof the area covered by composite. In addition, the table

gives loss of bond during load test, which will be discussedsubsequently. For the flexural specimens that had a singlelayer of fabric on the tension face (F1 and F5), the bonded

area was found to be 90% while the wrap around configuration

(F2-F4) was less at 82.5%, indicating additional installationdifficulty when wrapping corners. This is true even with the

fiber direction parallel with the beam axis. Note that both of

these measurements exceed the 5% allowed by ACI 440.2R-02.The laminate (F6) has a somewhat better bond coverage

(92%) than the fabrics. The shear specimens using the fabric

with the longitudinal orientation had similar bond coverageas that of the flexural specimens. Somewhat surprising is the80.5 and 85% bond coverage for the shear specimens with

transverse fiber orientation (S5-S8). It is expected thatthere would be a reduction in a bond area caused by having towrap the fabric around the corners of the beam, but none

was apparent.Unbonded areas in fabrics were oriented along the principal

fiber direction regardless of the orientation of the fabrics

with respect to the beam. Figure 6 shows Specimen S1 with

the defects oriented parallel to the beam axis. Figure 5 showsSpecimen S6 in which the fiber direction of the top layer is

oriented perpendicular to the beam axis. This pattern of

unbonded areas is believed to be due to the tension requiredto stretch the fabric to provide a smooth application. This

tension does not allow the fabric to conform to uneven areas

on the surface of the concrete. The fibers tend to pull awayfrom the uneven areas on the concrete surface after application

but before the epoxy cures. IR inspection was performed on

several specimens immediately after CFRP application butbefore full cure was reached. It was found that the detectable

unbonded areas increased as the epoxy cured.Unbonded areas under the laminate were also somewhat

elongated following the axis of the strip (Fig. 7). The laminateis applied by placing a ridge of fresh epoxy along the centerline

of the laminate. As the laminate is pushed into the concrete,

the ridge is flattened and the epoxy is supposed to squeezeout to the edge of the laminate to cover the entire width. If

insufficient epoxy is used, the epoxy is overly viscous, or

inadequate pressure is used, then the entire width may not be

Fig. 6—Orientation of unbonded area aligned with fiber direction.

Table 4—Bonded area of CFRP for flexuraltest specimen

Specimen

Bonded area (% of total composite area)

Initial Average100% of capacity Average Decrease

F1 nana

*na

na 10F5 90 80

F2 na

82.5

na

69.5 10F3 83 78

F4 82 67F6 92 na 86 na 6


Table 5—Bonded area of CFRP for sheartest specimen

Specimen

Bonded area (% of total composite area)

Initial Average75% of capacity Average Decrease

S1 80

80.5

66

66.8 13.7S2 81 64

S3 77 63

S4 84 74

S5 8380.5

5049 31.5

S8 78 48

S6 8685

7573 12

S7 84 71

S9 9291

8685.5 5.5

S10 90 85


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covered, leaving unbonded areas at the edge of the laminate

as shown in the figure.

Load tests

Flexural specimens were tested with three different CFRP

composite configurations. There were several failure modes

exhibited as indicated in Table 6. The failure load of each

flexural specimen is also given in Table 6. The failure load

given in the table is the total load applied to the beam (thesum of the two-point loads). There was only one test in

which the failure mode was debonding (F3). The remainder

of the tests failed by CFRP rupture, concrete crushing, or

shear. When comparing the capacity of the flexural specimen

with the unbonded area of CFRP, the influence of the unbonded

area is not evident. There is a decrease in the experimental

capacity associated with higher debonding during loading.

This may be attributed in part to the unbonded area. The

crushing failure mode exhibited by the strengthened flexural

specimen, however, discredits this relationship. The over-

reinforced sections failed in compression prior to CFRP

rupture and prevented a comparison with the calculated

capacity. For an adequate comparison to be made, thespecimen must be designed such that the failure mode is

CFRP rupture with little chance of compression or shear

failures. The focus of this experimental work was to test

the IR inspection rather than the improvement in capacity

provided by the CFRP.

Although the load test results appear to indicate that larger

unbonded areas did not adversely affect the flexural capacity

of the strengthened beams, there is some doubt cast by the

failure modes. Other work (Jackson et al. 1999) has indicated

high unbonded areas in field applications in which field tests

using infrared inspection on CFRP fabric wrapped bridge

columns showed a total unbonded area after application of

8190 mm2

(12.7 in2

) out of 31,940 mm2

(49.5 in2

) or 26% of the total CFRP area evaluated.

The load test results for the shear specimens are shown in

Table 7. In general, the shear specimens exhibited far more

debonding and delamination than their flexural counterparts.

This is likely due to the lack of shear reinforcement in the

beams, allowing excessive cracking to take place. The

improvement provided by the CFRP composite does not appear

to have been affected by the large unbonded areas. It should

be noted that the shear test configuration provided a rather

short shear span, likely resulting in a shear failure dominated

by compression rather than diagonal tension.

Debonding and delamination

The IR inspections revealed that delamination and debonding

occurred during loading on both the flexural and shear

specimens. In general, debonding and delamination aremanifested as unbonded area growth rather than the appearance

of new areas. This characteristic was common among all of

the fabric and laminate specimens.

One of the objectives of this research was to determine if

flexural cracks under the CFRP composite could be located

and tracked with IR inspection during load testing. It was

thought that the flexural crack would cause local delamination

immediately adjacent to the crack of sufficient size that the

IR inspection could detect. This capability would be a useful

tool in evaluating the CFRP system, particularly to determine if

the system had been overloaded. Unfortunately, cracking

Table 6—Flexural test results

Designation

Failure loadPexp , kN (kips)

Calculatedcapacity Pn ,

kN (kips) Pexp / Pn Failure mode

F1 36.0 (8.1)51.9 (11.7)

0.69 CFRP rupture

F5 48.0 (10.8) 0.92 Shear failure

F2 57.8 (13.0)

51.9 (11.7)

1 .11 Concre te c rushing

F3 48.5 (10.9) 0.93 CFRP debond

F4 45.8 (10.3) 0.85 Concrete crushing

F6 52.9 (11.9) 53.4 (12.0) 0.99 Shear failure

CF1 28.0 (6.3)26.2 (5.9)

1.07 Steel yield

CF2 29.4 (6.6) 1.12 Steel yield

Table 7—Shear test results

Designation

Failure loadPexp ,

kN (kips)

Calculatedcapacity Pn ,

kN (kips) PVexp / PVn Failure mode

S1 257.6 (57.9)

151.8 (34.1)

1.70 CFRP debond

S2 242.9 (54.6) 1.60 CFRP debond

S3 251.8 (56.6) 1.66 CFRP debond

S4 165.9 (37.3) 1.09 CFRP debond

S5 234.4 (52.7) 101.5 (22.8) 2.31 CFRP debondS8 201.0 (45.2) 1.98 CFRP debond

S6 162.8 (36.6)na na

CFRP debond

S7 173.5 (39.0) CFRP debond

S9 188.2 (42.3)157.0 (35.3)

1.20 CFRP debond

S10 162.4 (36.5) 1.03 CFRP debond

CS1 155.2 (34.9)71.2 (16.0)

2.18 Shear failure

CS2 146.8 (33.0) 2.06 Shear failure


Fig. 7—Typical unbonded area alignment on laminate specimens.


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generally shows up as the enlargement and eventual connecting

of adjacent unbonded areas as the load is increased.

There was one occasion when a flexural crack caused

debonding of the adjacent CFRP without the aid of unbondedareas. Figure 8 shows the tension face of Specimen F3. Note

that this specimen failed by CFRP rupture at the location

indicated by the IR inspection. The IR signature was notedduring testing as unusual because it was perpendicular to the

direction of the fabric.

Specimens F1 and F5 had composite applied to the tension

face only, leaving the sides exposed. Visible flexural crackson the sides were traced to confirm the location of the cracking

under the CFRP. The IR signatures for these flexural cracks,

however, were not as distinct as on Specimen F3.

Specimen F6 was reinforced with a single strip of laminatealong the tension face. By having the faces of the beam

exposed, cracking could be tracked to the laminate and

signature locations could be readily anticipated and identified.However, there was no associated IR signature detected

prior to failure.

Specimens S1 through S4 displayed behavior similar to F2through F4 in that debonding resulted in the enlargement of unbonded areas as the load was increased. When 65 to 75%

of the capacity was reached, the debonded areas increased

significantly at the diagonal cracks.

Shear Specimens S6 and S7 were reinforced with two

layers of CFRP fabric. The inner layer was oriented with theprincipal fiber direction parallel to the beam axis. This allowed

for IR inspection of the multilayer systems during load

progression. During load application, most of the damage wasdebonding with little delamination noted. This was verified by

the lack of growth in the vertically oriented signatures inthe external layer of fabric. On further inspection, it wasdiscovered that there were cloudy signatures in some areas

that were not associated with the exterior layer. This cloudy

area was thought to be debonding of the internal layer of CFRP. This was not evident until failure occurred. Once

failure occurred, the area surrounding the failure displayed

the same cloudy signature that was seen during the loadprogression. The status of the cloudy area was unverified as

removal of the fabric layers destroyed the appearance of the

concrete/CFRP interface.

Shear Specimens S5 and S8 were reinforced with a singlestrip of fabric on the tension face and then a perpendicular

wrap encompassing half of each side. Debonding initiated at

the boundaries of unbonded areas. The debonded areas

continued to grow in the principal fiber direction until

failure occurred.

Shear Specimens S9 and S10 used the carbon fiber laminate

in a stirrup configuration. Because of its stiffness, the

laminate did not conform to inconsistencies in the concretesubstrate as well. The increased thickness and density of the

laminate and epoxy substrate also changed the IR signaturefrom that of the fabric. Initial debonding started at unbondedsignatures near the edge of the laminate. The debonded

area usually extended immediately to the top of the stirrup.

Intermediate load levels showed moderate progression of debonding through the stirrup, but most debonding occurred

at the edges. This reinforcing configuration provided capacity

beyond the expected value. The epoxy did not perform well,however, and with better materials and surface preparation

the system may have provided a higher shear capacity. The

stirrup configuration engaged the 45 degree crack as expected.It is evident that the interfacial bond is critical in areas directly

affected by shear.

Area analysisUnbonded, debonded, and delaminated areas were measured

and recorded during the construction and load testing of the

flexure and shear specimens. The IR inspection procedurewas somewhat subjective in that the thermographer identified

and sized the defect while viewing the specimen with the IR

camera. Furthermore, the measured areas were totaledmanually. Even with this low-tech procedure, it was possible

to perform the inspections with remarkable repeatability.

This section presents the results of the area analysis.

Figure 9 shows the loss of CFRP bond as a function of the

applied load for flexural Specimen F3 to F6. The figureshows the loss of bond as a function of the applied load. The

loss of bond is the sum of unbonded, debonded, and delaminated

areas. Hence, at zero load, the loss of bond is equal to theunbonded area determined before the test was initiated. All

of the flexural specimens displayed a gradual increase in the

amount of damage. Specimens F3 and F4 (side wrap specimens)are clustered above F5 and F6 (fabric and laminate strips,

respectively). The close match among the plots is an indication

of good repeatability and may also indicate that bondcharacteristics can be categorized with the configuration of the CFRP composite. No flexural specimen had more than

35% loss of bond at failure.

Table 4 compares the total area bonded at initial inspection

and at 100% of the calculated capacity for the flexural

Fig. 9—Bond damage progression for Specimens F3 to F6.Fig. 8—Crack transverse to fiber direction in flexural specimen.


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specimen. It is interesting to note that the decrease in bonded

area was 10% or less. This indicates that small changes in

bond, when considering flexure, can be an indication that abeam has seen or is under significant load.s

Figure 10 shows the loss of bond for shear Specimens S1to S4. The initially unbonded area for each of the specimens was

very near 20% of the total CFRP area. The repeatability of the

inspection method is confirmed by the lack of variation amongthese similarly reinforced specimens. In addition, the loss of

bond increases at a relatively constant rate up to 75% of theload. There is a drastic increase in debonding and delaminationfrom 75 to 100%. This is due primarily to the explosive release

of energy at failure of the system in shear, which causes a rapid

and complete loss of bond on one side of the shear crack.

Figure 11 shows the loss of bond for Specimens S5 and S8.

Specimens S5 and S8 exhibited consistent changes in thedebonded area as the applied load increased. The two curves

reflect the repeatability of the inspection method. Once

again, the consistency of material and application are alsoevident. The increase in the debonded area at 65% of theactual capacity is also evident in the S5 and S8 specimens.

This configuration provided the highest capacity combinedwith the highest amount of debonded area at failure. This

supports the theory that a higher unbonded area at failure

does not necessarily mean a decreased capacity.


applied load for the S6 and S7 shear specimens. Specimens

S6 and S7 maintained the consistent behavior between

configurations and show a sharp increase in debonded area

near 65% of the actual beam capacity. This configuration

provided multiple layers of CFRP with multiple signaturetypes in the thermogram. The consistency displayed here can

be attributed to the development of signatures in single-layer

systems with variable fiber orientation. Due to the subjectivenature of the inspection process, the ability to eliminate false

signatures is critical in performing an accurate inspection.


applied load for the S9 and S10 shear specimens. The IR

inspection of the specimens reinforced with CFRP laminateshowed initially unbonded areas of 8% of the total CFRP

area for flexural members and 8 to 10% for the shear specimen.

Specimens S9 and S10 displayed behavior consistent withthe other shear specimens. The debonded area increases atapproximately 70% of the capacity for the shear specimens

reinforced with the CFRP laminate. The small, unbondedarea at failure displays a lack of redundancy in the strengthening

system. If significant cracking occurs between the stirrups, a

small percentage of the CFRP is engaged, whereas the fabricencapsulates the surface and engages all of the shear cracks

that occur in the member.

Table 5 summarizes the change in the bonded area when

the load is taken to 75% of the capacity. It is unclear why the

double-layer system of S5 and S8 have such a large changein bond area (31.5%) compared with that of S6 and S7.

Perhaps there are more delaminations on S5 and S8 than

were detected due to the double layer.

Fig. 13—Bond damage progression for Specimens S9 and S10.Fig. 11—Bond damage progression for Specimens S5 and S8.

Fig. 10—Bond damage progression for Specimens S1 to S4. Fig. 12—Bond damage progression for Specimens S6 and S7.


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SUMMARY AND CONCLUSIONSReinforced concrete beams were strengthened with bonded

CFRP composites and tested to failure in both flexure andshear failure modes. The beams were IR inspected for initialapplication quality and periodically during load testing to

record loss of bond with load increase. Inspection procedureswere developed and damage signatures were defined for the

various reinforcing configurations. The following conclusions

have been supported by this research:• Infrared thermography can be used effectively for

nondestructively evaluating CFRP strengtheningsystems for concrete;

• Initially unbonded areas of CFRP composite on concrete

substrates can exceed 20% of the total CFPR area;

• Shear controlled failures can exhibit up to twice as much

CFRP debonding as predominately flexural failures;

• The amount of unbonded area present immediately

after CFRP application can be significantly lower than

after curing of the CFRP has taken place;

• Manual methods of acoustic sounding can leave up to25% of the voids under the CFRP composite undetected.Acoustic sounding is inadequate for detecting small

irregular voids and clearly defining defect and damage

boundaries; and

• Quality control inspections need to be performed on

these systems to establish a baseline for unbonded

areas. Routine inspections must be performed to determinethe status of the bond between the CFRP and the concrete

and prevent unexpected failures.

ACKNOWLEDGMENTSThe authors would like to acknowledge and thank the National Science

Foundation (CAREER Grant CMS-9734227) for funding this project. They

would also like to thank Tom Hurley of Hurley and Associates, Inc., for his

expertise and input regarding infrared thermography. The views expressed in

this paper are those of the authors and not necessarily those of the sponsor.

REFERENCESACI Committee 440, 2002, “Guide for the Design and Construction of

Externally Bonded FRP Systems for Strengthening Concrete Structures

(ACI 440.2R-02),” American Concrete Institute, Farmington Hills,

Mich., 45 pp.

Jackson, D. R.; Islam, M.; Hurley, T. J.; and Alvarez, F. J., 1999,

“Feasibility of Evaluating Fiber Reinforced Plastic (FRP) Wrapped

Reinforced Concrete Columns Using Ground Penetrating Radar (GPR)

and Infrared (IR) Thermography,” Demonstrat ion Project No. 84-2, U.S.

Department of Transportation-Federal Highway Administration,

Washington, D.C.

Mack, J. K., and Holt, E. E., 1999, “The Effect of Vapor Barrier

Encapsulation of Concrete by FRP Composite Strengthening Systems,”

44th International SAMPE Symposium and Exhibition , Long Beach, Calif.

Witcher, D., 1996, “Application of Fiber Reinforced Plastics in New

Construction and Rehabilitation of the Infrastructure,” Intern ationa l

Con ference on Composites in Infrastructure 1996 , (ICCI 96), pp. 774-779.

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