nondestructive evaluation of rc structures strengthened with frp laminates containing...

Ekenel, M. and Myers, J.J. 1

Nondestructive Evaluation of RC Structures Strengthened with FRP Laminates

Containing Near-Surface Defects in the form of Delaminations

Mahmut Ekenel1 & John J. Myers2

Abstract. Fiber reinforced polymer (FRP) composites are being utilized in a wide range

of application areas in structural rehabilitations because these materials are less affected

by corrosive environmental conditions and known to provide longer life with less

maintenance. However, there are still some concerns about FRP strengthened reinforced

concrete (RC) structures, such as the presence of near-surface defects. Currently limited

data exists regarding this issue. The presence of near-surface defects in the form of

delaminations between the fiber reinforced polymer (FRP) laminate and concrete

substrate can significantly affect the structural integrity and stiffness of the structural

section. These defects should be properly detected and accurately located to access if

injection or replacement is warranted. The control and assessment of these defects still

require improvements in detection techniques and standardization in these assessment

methods. The latest advances in non-destructive evaluation (NDE) techniques including

technologies in microwave, acousto-ultrasonic, impact-echo and thermography are

providing promising results in detecting such defects and are discussed in further detail in

this paper.

1 Senior Evaluation Specialist, International Code Council-Evaluation Service, 5360 Workman Mill Road, Whittier, California, 90601 [email protected] (Corresponding Author) 2 Associate Professor, Department of Civil, Arch. & Env. Engineering, University of Missouri-Rolla, Rolla, Missouri, 65401 [email protected]


Keywords: Non-destructive evaluation, FRP strengthening, delaminations, surface

defects, microwave, acousto-ultrasonic, impact-echo, thermography.

INTRODUCTION

The strengthening and rehabilitation of existing structures with new material systems like

fiber reinforced polymers (FRP), especially carbon fiber reinforced polymer (CFRP), are

being used in more instances under a wide range of applications in recent years. FRPs are

mainly formed of fibers and matrix; fibers are the main load carrying constituents while

the matrix is the medium for stress transfers among fibers. FRP laminates have gained

importance over steel plate retrofits in rehabilitation applications because they offer

superior performance with respect to high stiffness-to-weight ratio, being lightweight,

easy to handle, and low installation costs which can offset the higher material costs.

Moreover, these materials pose enhanced corrosion resistance as compared to

conventional materials such as steel plates; hence they are expected to provide a longer

service life and require less maintenance. Meier and Kaiser (1991) reported that material

costs are approximately 20% of the overall rehabilitation cost, and the remaining 80%

consists of labor costs [1]. Hence easy handling and rapid installation even with higher

initial material costs could be expected to reduce the total retrofit costs significantly.

Even though FRP laminates are produced and utilized in different applications, the most

common form is built-up fabric that is externally bonded to a structural element by the

wet lay-up method. The wet lay-up method consists of an initial primer application after

the concrete surface has been roughened. Following this step a saturant is applied then

the FRP fabric. A final impregnation of FRP fabrics by a second layer of saturant


application is the final step. This method is easy and cost effective; however, it has the

disadvantage of field mixing the components. FRP laminates can easily bond externally

to RC beams, slabs, and columns using the wet lay-up method and has the added

advantage of resulting in a minimum affect on the structural aesthetics. Because the FRP

strengthening provides additional flexural or shear reinforcement, the behavior of the

entire structure depends on the combined action of the whole system. Therefore, success

of the system depends on the integrity of the bond between FRP laminate and concrete

substrate.

Ideally designers desire a FRP laminate that is perfectly bonded to substrate concrete

without any type of near-surface defects such as delaminations (disbonds and air-

blisters). Delaminations are largely formed during the installation process at the job site.

Delaminations between FRP and concrete substrate can be caused by several reasons

such as the presence of moisture in the concrete or it may occur due to significant

changes in temperature during curing and improper application (see Fig. 1 and Fig. 2).

Delaminations can also be formed between multiple plys of FRP laminates during

installation. Another form of delamination is sagging of FRP fabric, which can be caused

due to improper application of resin and inadequate rolling during installation to develop

good bonding. Sagging can cause bond loss between the composite and substrate and

reduce the stress transfer capabilities [2]. Any type of delamination between FRP

laminate and substrate may effect and significantly weaken the structural integrity and

performance of the system depending on the size of the defect and location; moreover it

may limit the life expectancy of the structure [3, 4, 5, 6, 7 and 8]. An undetected

delamination may also cause fracture of the material [8].


Because of these issues, the American Concrete Institute (ACI) document 440.2R-02

requires the evaluation of delaminations and air voids between multiple plies or between

the FRP system and the concrete with a minimum detectable size of 1300 mm2 [9]. Some

other criteria of ACI 440.2R-02 may be noted as: total delamination area should be less

or equal to 5% of the total laminate area and no more than 10 delaminations are permitted

per 1 m2, large delaminations (greater than 16,000 mm2) should be repaired by cutting

away and applying a new patch, and delaminations smaller than 16,000 mm2 may be

repaired by resin injection or ply replacement.

The early determination of above mentioned problems are to ensure the safety of the

rehabilitated structure. Since quality of the FRP products are very consistent for this type

of externally bonded system because of the quality assurance systems of manufacturers,

the main remaining issue is the quality of work performed during rehabilitation or

strengthening at job site. Therefore, these defects should be properly detected and

accurately located before delamination growth occurs due to cyclic service loads, thermal

effects, or environmental conditioning. Since destructive testing may have adverse effects

on sound structures, a test method without compromising the structural integrity is

required. Consequently, there is a need for efficient, reliable, cost effective and user-

friendly non-destructive test (NDT) methods to detect and characterize near-surface

defects in the form of delaminations at the FRP fabric-concrete substrate interface. The

NDT method should be able to detect not only the smallest defect, but also capable of

inspecting large areas as well. It should be lightweight, preferably hand held or portable

and easy to carry. The outcome of NDT technique must be interpreted easily by the users.

The method should also be capable of verifying that the delaminations have been


properly and fully injected upon repair. Even though there is a great interest to develop

such a procedure, presently there is no standard method available to assess the efficiency

of FRP bonded civil structures [10 and 11].

The most common conventional methods in delamination detection involve visual

inspection and tap-test. Even though visual inspection is fast, inexpensive and can detect

large areas, it is obvious that it cannot provide quantitative and objective information and

highly susceptible to proper judgment of human vision. Another common conventional

testing method is the tap-test (or acoustic impact test) in which a small hammer or a steel

bar is used to tap the FRP surface (after curing) to detect air voids. A perfectly bonded

FRP generates a different audible sound than a delaminated one (see Fig. 3). This method

is low technology, simple and has low equipment cost, but more subjective and less

reliable since it needs an operator with a discerning hearing ability [8 and 12]. In

addition, the hitting or striking of the FRP surface can damage the interface of concrete-

FRP by initiating delamination in those regions.

The majority of NDT equipment output is the comparison of changes of the measured

parameters (voltage, microwave, sound …etc) as compared to an initial measurement.

Hence, a baseline is needed among these NDT techniques. The most current NDT

techniques to detect these defects consist of ultrasonic, microwave, infrared

thermography, acousto-ultrasonic, impact-echo, to name a few.

The discussion presented herein is intended to provide an overview among the most

current NDT techniques in detection of near-surface defects such as delaminations and air

blisters between concrete surface and FRP laminate. This paper also presents several


validation tests in detecting delaminations on a RC bridge structure and RC samples

produced in a laboratory setting.

OVERVIEW OF NONDESTRUCTIVE TESTING METHODS ON

DELAMINATION DETECTION

In this section, the NDT techniques of thermography, ultrasonic C-scan, acousto-

ultrasonic, impact-echo, microwave, ground penetrating radar, eddy current and laser

shearography have been introduced and their applications in the literature have been

summarized.

Thermography

Thermographic NDT is based on observing heat flow, which is altered by the

discontinuities presented in a structure, such as delaminations or blisters between FRP

fabrics and concrete substrate. Principally, thermography measures the local temperature

changes. The changes in the heat flow provide localized energy differences on the surface

of the specimen, and these changes can be viewed using an infrared detector.

Thermography can be classified into two categories as active and passive [5]. Passive

system utilizes the natural heat of a body; whereas active system works by injecting heat

flow into the test element using sources such as photographic flash lamps, hot gas jets

and heat guns. Active thermography is the most common method used for the inspection

of delaminations in the composite systems, defects in polymers and delaminations in

honeycombs and laminars. Two methods of observation are available in thermographic

inspection: reflection and transmission [13]. The thermal source and the detector are

located on the same side of the sample in reflection thermography; whereas, transmission

thermography utilizes thermal source and detector located on opposite sides of the


sample. Thermography can be fully applied to detect air blisters because trapped air has

low thermal conductivity; it displays a localized radiant heat. Fundamentally, the

presence of air voids or delaminations reduces the heat diffusion rate, which is reflected

to the infrared imaging as different temperatures with respect to surrounding sound areas.

One of the important advantages of thermography is that there is no necessity for contact

between the testing equipment (i.e. probing camera) and the object under inspection [14].

Other advantages are: fast inspection rate (up to a few m2 at a time), security of

personnel, results are easy to interpret; whereas, some disadvantages are: difficult to

obtain a quick and uniform thermal simulation, cost of equipment, ability to inspect a

limited thickness of material under the surface, capability of detecting only defects

resulting in a measurable change of the thermal properties [15]. Moreover, this technique

is sensitive to ambient temperature conditions as well as the thermal characteristics of the

target [16].

Defects in a FRP reinforced panels such as unbonded areas (disbonds) and impact

damages have been inspected by Newman and Zweben (2002) using thermal excitation

[5]. Delaminations were formed by placing two layers of polyethylene film with a

powdered material between FRP laminate and concrete substrate. They have concluded

that the thermographic NDT was able to detect engineering disbonds as small as 12 mm,

even if three plies of CFRP fabrics are bonded to a cementitious panel. Testing of CFRP

strengthened RC beams with near-surface defects in the form of delaminations under

fatigue loading was studied by Senthilnath et al. [17]. They monitored delaminations

(approximately 150 mm in diameter) using thermography. The defects were

photographed by an infrared camera during the fatigue cycling process (see Fig. 4 and


Fig. 5). They reported an insignificant growth in delamination sizes after fatigue testing

for 2-million cycles. They measured the delamination areas via software and reported that

the increase in delamination area was approximately 5%. Hence, they noted that ACI

Committee 440.2R-02 requirements about permissible delamination sizes appeared to be

conservative.

Duke and Horne tested hybrid glass and carbon fiber reinforced vinyl ester pultruded

“double box” I beams that are installed in a small bridge via infrared thermography (IT)

[18]. The structural members were heated using quartz halogen heat lamp. They

concluded that a region of severe delamination in one of the beams were detected

successfully using this method. They also noted that the infrared thermographic

examination offers potential for detecting absorbed moisture in composite material

infrastructure components. Miceli also reported that infrared thermal imaging can be

adopted to monitor the health of the FRP structures where disbond (delamination) type

defects are present on a fiber reinforced polymer composite deck [10]. Wong et al.

applied lock-in thermography and ultrasonic C-scan in detecting near-surface defects

within a fiber reinforced composite material [13]. The difference of lock-in thermography

was using a continuous sine heat wave rather than a pulsed heat source. They have stated

that lock-in thermography provided nearly equivalent sensitivity to C-scan ultrasonic

testing for near surface defects.

Shih et al. examined artificially induced air-voids between CFRP laminate and concrete

substrate [6]. They evaluated the influence of air-voids on the structural performance of

FRP strengthened RC elements. They conducted an active thermographic approach,

which was utilized by providing a heat source via electric resistance heating elements to


establish a surface temperature variation along the FRP laminate. Shih et al. concluded

that the infrared thermography successfully detected the air voids in every case,

independent of FRP fabric type. Moreover, the air-voids can be identified remotely, from

distances of up to 20 m. They have also stated that small blisters (2.04% loss of area)

have tendency to influence the flexural performance of the RC structures; whereas, large

blisters (5.29% loss of area) appeared to have greater impact. Another investigation was

conducted on detecting disbonds located between CFRP laminate and concrete substrate

[11]. The disbonds were created artificially by placing different materials (such as air and

plastic, air and wax, Styrofoam…etc) at the interface. The dimensions of the

delaminations were approximately 25 mm x 25 mm. The results showed that all

simulated flaws were successfully detected. Jackson et al. applied infrared thermography

(IT) on detecting delaminations and entrapped moisture behind the FRP wrap [16]. The

FRP wrap surfaces were heated up with a portable propane heater to apply active IT

technology. The IT technology successfully detected the delaminations and blisters

between the FRP wrap and concrete substrate, which were also confirmed by application

of tap-tests. Entrapped moisture between the FRP wrap and concrete could also be

detected. Hag-Elsafi used IT technique to check the bond quality between FRP fabric and

concrete substrate in reinforced concrete T-beam bridge two years after installation [19].

Test results were compared with strain compatibility, by observing FRP and concrete

strains at similar locations. Both test methods did not reveal any serious debonding in the

system laminates.


Ultrasonic

Ultrasonic inspection is essentially based on generation and detection of mechanical

pulses that propagate through the materials. The pulse frequencies are usually more than

20,000 Hertz. The wave types are primarily longitudinal (compression) and transverse

(shear). Fundamentally, ultrasonic pulses are transmitted into the material by a probe; the

pulses are reflected back if there is a defect exists on their path and received by a receiver

probe. The outcome can be displayed in various ways such as A-scan, B-scan and C-scan.

A-scan can provide pulse travel time and distance from surface to location of

discontinuity or defect, whereas a C-scan can provide plan view of the defects detected.

Hence mapping is only available on C-scan. On the other hand, A-scan and B-scan have

higher mobility. The disadvantage of the ultrasonic technique, especially as compared to

thermography, is that it requires intimate and continuous contact with the specimen under

evaluation. However, using C-scan ultrasonic imaging can produce images with reduced

noise and clearly define the characteristics of the defects as compared to thermography,

which can only provide the general shape of the defect [20]. Ultrasonic C-scan testing is

one of the most common applications for defect detection in FRP materials. However, it

should be noted that the acoustic impedance of the defect should be different from the

surroundings; otherwise ultrasonic method may fail to detect it [21]. Gros and Takahashi

reports that delamination detection with ultrasonic is very limited due to the fact that

delamination is a planar defect with high ultrasonic reflectivity in one direction [8]. Also,

it is often assumed that the delamination has an infinite attenuation of ultrasonic signal,

so calibration of the equipment is difficult and small changes in stiffness or thickness can

create important differences. However, as reported by Duke and Horne, ultrasonic C-scan


can be successfully used to detect concentrated areas of porosity, resin starved regions, or

matrix cracking [18].

Franklin et al., applied ultrasonic longitudinal waves in through-transmission along the

radial direction of FRP wrapped concrete cylinders, which were subjected to

environmental conditions such as freeze-thaw, alkaline solution, dry heat and water

immersion [22]. The ultrasonic testing was conducted with ten cycle sine pulse of 400

volts peak-to-peak amplitude to excite the 250 kHz central frequency transducers. They

have reported that ultrasonic testing in the radial direction captured the debonding effect

between the wrap and concrete.

Kaczmarek conducted a research to determine delaminations in a carbon-epoxy

composite material under static and fatigue loading [23]. They have applied transmission-

mode ultrasonic testing with high-frequency longitudinal waves. The ultrasonic results

were compared by X-ray radiography results. The results of ultrasonic were comparable

to those observed by radiography, which shows the feasibility of the system used to

detect delaminations in composite materials. Castaings performed delamination detection

tests on carbon fiber composite plates via a single sided, air coupled ultrasonic NDT

system using Lamb waves [24]. The air-coupled transmitter is oriented at a specific angle

to produce pure Lamb waves. The advantage of Lamb waves over normal incident is that

they are capable of traveling for longer distance. They concluded that delaminations have

been successfully detected within layers using air-coupled ultrasonic transducers. Kundu

et al. used two ultrasonic techniques to detect delaminations between the GFRP plate and

the concrete surface [7]. These techniques consisted of utilization of longitudinal waves

and Lamb waves (L-scan) using relatively low frequencies (below 1 MHz).


Delaminations were formed by applying no epoxy the some certain areas of about 50 mm

diameter. Their conclusion was that both Lamb wave scanning (L-scan) and longitudinal

wave scanning (pulse-echo and tone bust C-scan) can detect the delaminations between

GFRP/concrete interfaces. They have recommended Lamb wave scanning technique to

be the superior technique due to the fact that C-scan technique is sensitive to the glue and

GFRP plate property variations; whereas L-scan image is insensitive to the small

variations in the glue and plate properties.

Acousto-ultrasonic

Acousto-ultrasonic (AU) evaluation method has also been implemented to detect

damages in FRP materials used in structural applications. The AU is a combination of

acoustic-emission monitoring and ultrasonic characterization. AU method functions by

inducing acoustic pulses using a transducer, a second transducer to detect the portion of

these pulses that has propagated away from the point of excitation [18]. These acoustic

pulses travel through the material and are reflected by the different interfaces inside the

sample. Travillion et al. applied a theoretical model and experimental work to

demonstrate the feasibility of using the acousto-ultrasonic technique in the inspection of

FRP/concrete interface with surface defects [25]. Surface defects were generated by

inserting Teflon wafers between the concrete substrate and the FRP laminate. They used

rubber-tire differential rolling sensors. Rubber tires acted as a dry couplant between the

sensor and the surface and maintained a constant distance which reduced the reflected

amplitude changes due to contact variation between the sensor and the member under

test. Delaminations were viewed via A and C-scan imaging. Test results and field

applications revealed that AU technique can detect disbonds of 6 mm in diameter. System


is capable of generating A and C-scan images of defects. Two of the limitations they have

encountered during the field testing were unable to scan wide areas due to half-scale

model they have used and the splintered fibers at the edge of the FRP strips which also

limited the scanning area. Godinez et al. applied AU technique successfully in detection

delamination between fiber reinforced polymer fabric and concrete at a retrofitted

structural member [26]. Guided wave generation, produced by using an array of angled

sensors, is the technology behind their system. They have concluded that the

delaminations can be detected using AU technology independent of fiber type, such as

glass or carbon. This has also been validated in the field by Ekenel et al. through

comparative NDT techniques (see Fig. 6) [27]. Acousto-ultrasonic assessment of

composites and laminates were standardized by ASTM E1495-02: “Standard Guide for

Acousto-Ultrasonic Assessment of Composites, Laminates and Bonded Joints” [28].

Impact-echo

Impact-echo method is another NDE method used for damage detection in FRP materials.

The impact-echo methods works just like majority of NDE methods by sending stress

waves (impact-generated stress waves such as sound) across the material under

investigation and observing the reflected waves at any property chance throughout the

material (such as delaminations). ASTM C1383-04 (Standard Test Method for Measuring

the P-Wave Speed and the Thickness of Concrete Plates Using the Impacy-Echo Method)

defines impact-echo method as an NDT method based on the use of a short-duration

mechanical impact to generate transient stress waves and the use of a broadband

receiving transducer placed adjacent to the impact point [28]. The advantage of this

technique is that it is a single-sided test; only one side of the material should be


accessible for testing [29]. The impact-echo method can be used successfully in concrete

bridge decks and slabs to locate cracks, voids, honeycombing and delaminations [30].

The feasibility of this method has also been validated in the field by Maerz et al. through

comparative NDT techniques [31].

Microwave

Delamination detection in layered composites or at the FRP-concrete interface has been

successfully implemented via near-field microwave technology in recent years.

Microwave frequency may change between 300 MHz and 300 GHz with wave lengths

between 1 mm and 1 m. Microwave ND testing functions by transmitting microwaves

and capturing the reflected waves like most of the NDT techniques (see Fig. 7). The

transmitted and reflected waves are first plotted and superposed by fixing maximum and

minimum points, which is called as standing wave. A defect between layers can be

detected if the standing wave shifts by changing its phase angle and amplitude.

Microwave NDE is also proven to provide satisfactory results in thickness measurements

in non-metallic materials, monitoring degree of polymer cure and concrete curing,

concrete moisture and water-to-cementitious ratio detection, monitoring density of

ceramics besides delamination detection between FRP fabric and concrete substrate.

However, microwaves are not suitable for internal inspection of metals due to complete

reflection of all waves (penetration is possible to only a micro millimeter in distances).

Hughes et al. performed tests on Microwave detection of delaminations between carbon

fiber reinforced polymer composite and concrete [4]. Delaminations between CFRP

fabric and concrete were formed by placing thin slices of foam which were attached to

the concrete surface by a thin layer of Teflon tape. Two major aspects of their research


were determination of orientation of the waves with respect to fiber orientation and stand-

off distance between probe and the concrete surface. Fiber orientation is important

because the reflection of signal totally depends on it; if the field polarization is parallel to

the fiber orientation, then the signal will be completely reflected. Stand-off distance was

important to eliminate surface roughness problems and increase sensitivity (see Fig. 7).

The tests were carried out with a waveguide between 10.5 GHz and 24 GHz. Results

proved that such defects can be successfully detected using near-field microwave

techniques. Kim et al. monitored delaminations (disbonds/air voids) between an FRP-

jacket and the concrete substrate via electromagnetic imaging technology with frequency

between 8.2 GHz and 12.4 GHz, because air voids produce reflections of electromagnetic

wave [3]. The depth of delaminations was obtained by microwave subsurface imaging

technology with a frequency of 10 GHz. Delaminations were obtained via inserting a

small block or a small strip of Styrofoam in the bonding interface between the FRP and

concrete substrate, because the Styrofoam has same dielectric property as that of the air.

They reported that the locations and areas of these delaminations were successfully

detected using microwave-based surface scanning. Akuthota et al. investigated disbonds

and delaminations between CFRP fabric and concrete substrate using near-field

microwave technique [32]. The experiments were conducted using custom made

reflectometers at 10 GHz and 24 GHz. Test results indicated that delaminations as small

as 1 cm2 can be detected, which are much smaller than the required minimum detection

size referred by ACI 440.2R-02. Moreover, it has also been demonstrated that this

technique can successfully evaluate the quality of repair of a delamination by epoxy

injection, which is also a requirement of ACI 440.2R-02.


Ground penetrating radar

Ground penetrating radar (GPR) functions by emitting electromagnetic wave of

frequency ranging between 500 MHz and 1.5 GHz from an antenna; part of which is

reflected back if there is a discontinuity exists in an object [21]. It has been used widely

in civil applications for many years such as inspecting wood, concrete and masonry. It is

highly mobile and no intimate contact is required; however, there is little information

available on its validity if applied to composite materials. It is generally not

recommended for thin materials because near-surface defects are very difficult to detect

due to strong interference between signal reflected off the front surface and signals

originated from near-surface depths [21]. Jackson et al. applied ground penetrating radar

technique to inspect the delaminations between the FRP and concrete substrate [16].

They concluded that the GPR technique can easily detect defects within concrete;

however, in order to detect very shallow defects such as delaminations between the FRP

and concrete substrate, a high frequency antenna (such as 2 GHz) is required.

Foucault currents (Eddy current or Induced current)

The eddy current method requires a source of varying magnetic field (often caused by an

AC coil) and a sensor to detect small changes in magnetic field. Defects can be detected

if there is a change in magnetic field. It should be noted that defects should have adequate

dimensions to affect the magnetic field; otherwise they remain undetected. Even though

most composites are not suitable for eddy current testing, carbon fiber reinforced

polymers can be inspected because electrons can move along carbon atoms, which limits

this method for inspection of only carbon fiber related problems. Kaiser et al. reports that

eddy current testing of composite materials do not require any intimate contact as a plus


side; on the other hand, it is difficult to interpret and only sensitive to conductive

materials, hence matrix-related defects remain undetected [12]. Gros and Takahashi

applied Foucault currents (eddy current) to detect delaminations between carbon

fiber/epoxy laminates and reported that this technique provides successful application in

delamination detection between plies if correctly applied, unlike what most of the

researchers who stated as Foucault currents are insensitive to delaminations [8]. Mook et

al. also reported that eddy-current scanning system based on special probes can visualize

fiber orientation, local imperfections like fiber fraction, resin rich zones, delaminations

and impact damages [33].

Laser shearography

Laser shearography is a large area non-contact inspection method, which is faster than

contact scanning via ultrasonic C-scan. It can record images digitally and can be used

with a variety of excitations such as heating and acousto-loading [5]. Kaiser states the

advantages of shearography as having high detail even if operated at full-field level, no

intimate contact required and having excellent sizing capabilities [12]. He lists the

disadvantages as having relatively high environmental stability and the structure must be

exposed to controlled loading conditions. It was reported by Newman and Zweben that

shearography NDT is able to detect delaminations (disbonds) between carbon fiber

reinforced FRP layers and concrete substrate under thermal excitation [5].

EXPERIMENTAL VERIFICATION

The Center for Infrastructure Engineering Studies (CIES) at the University of Missouri-

Rolla (UMR) has studied the effect of delaminations between FRP fabric and concrete

substrate on structural performance of FRP strengthened structural members. First, three


RC beams were prepared, pre-cracked under four point bending loading and strengthened

with CFRP fabrics [34]. The CFRP fabric was 203 mm wide and 0.165 mm thick and was

applied as single ply throughout the length of the test span. CFRP fabrics were attached

to the tension face of test specimens. Delaminations were produced in all three beams.

Delaminations were formed right after the final layer of epoxy application when the

epoxy was still in fresh state. Three spots on the laminate were pinned and a small

amount of pressured air compressed beneath the CFRP sheet, which created

delaminations (see Fig. 8). A spike-roller rolled around the perimeter of delaminations

was used to form the final shape of the delaminations. The delaminations were formed

with one at mid-span, and two directly outside the constant moment zone as shown in

Fig. 1. The total delamination areas were measured and divided by the total CFRP fabric

area to obtain the percent of total delamination area. The percent delamination areas were

3.85, 3.00 and 4.20 for each beam; which were consistent with ACI 440.2R-02

requirements (less than 5%). All beams were tested under fatigue loading.

All of the delaminations were tested via near-field microwave measurement system at

initial, 500000, 1-million and 2-million cycles. Test set-up is illustrated in Fig. 9. They

have also been inspected visually and by applying tap-test (see Fig. 3). Tests were

terminated after 2-million cycles. It was observed that the flexural stiffness of CFRP

strengthened beams were significantly affected because of the presence of the

delaminations under fatigue loading [35]. The scanned images showed that the

microwave technology can successfully detect and image the delaminations. Several

unintentional images have also been detected. The microwave images showed that the


area of the delamination did not change significantly as a function of increased loading

[34]. These beams are still under conditioning for long-term study.

Two additional beams were prepared to observe the effect of delaminations on flexural

strength. One beam contained three delaminations with the percent delaminations area of

5%, which was also consistent with ACI 440.2R-02 requirements (less or equal to 5%).

Delaminations were formed same as described previously (see Figs. 1 and 8). The beams

were tap tested in order to determine whether they contain delaminations and the

locations of the delaminations. One of the delaminations beneath one of the loading

supports was also equipped with two strain gages; one placed at the center of

delamination, another one attached to the disbond interface of delamination-concrete to

observe the behavior of delamination. These two beams were tested under static four-

point bending. Fig. 10 shows the load versus mid-span displacement curves of the two

beams tested. As Fig. 10 shows, the beam with delaminations exhibited 9% reduction in

the ultimate strength as compared with the beam with no delaminations. The reduction in

the ultimate displacement was 6% (Fig. 10) as compared to the beam with no

delaminations. Fig. 11 shows the strain readings on CFRP fabric and steel at mid-span for

the beams with and without delamination, in which an insignificant difference can be

observed among strain readings. Fig. 12 shows the readings which were recorded over

CFRP fabric at mid-span and at the corner and center of one of the one of the

delaminations located beneath one of the loading supports. As seen in this figure, the

strain reading at the center of the delamination at the load level of 65 kN was 37% lower

than the strain reading at the corner of the delamination at the same load level.


Figs. 10 and 11 demonstrate that the displacement measurements of CFRP strengthened

structures using nondestructive-load-testing methods performed at service-load levels

(usually 40% of the predicted ultimate strength) cannot clearly indicate the presence of

delaminations since these test results did not exhibit any distinctive difference among

strain readings in steel and CFRP, or in displacement readings; however, more research is

warranted.

Another verification application was performed in a bridge structure which was

sponsored by Missouri Department of Transportation (MoDOT), USA [36]. Fifteen

CFRP laminates with dimensions of 508 mm x 610 mm were installed on various spots of

abutment and bents for this investigation. These locations were sacrificial places which

were specially selected to not affect the structural integrity of the bridge. Delaminations

were formed on several spots of these CFRP panes by applying pressurized air beneath

CFRP sheet while epoxy was in fresh state (see Fig. 2 and Fig. 13). Two of the

delaminations were placed near a splash zone area where the water level in the river

fluctuates to investigate the effects of freeze-thaw and excessive moisture (see Fig. 14).

Moisture migration may fill the air-pockets and then could be subjected to freeze-thaw

cycles during the winter; ultimately causing failure in the repair system. Moisture can

significantly affect the bond strength of the composite, although it does not influence the

strength and modulus of carbon fibers [2]. All delaminations at these locations were

scanned using microwave technology, impact-echo and acousto-ultrasonic (AU) as well

as visual inspection and tap tests. The following items have been observed:

1. Even though visual and tap test were fast, detected large areas in a short period of

time and located some of the delaminations, they could not located many small


blisters and they did not provide quantitative and objective information such as depth

and size of the delaminations.

2. The AU system demonstrated its ability of detecting delaminations between FRP and

concrete substrate without being influenced by the interior reinforcement, coarse

aggregate or the surface orientation (whether vertical or horizontal) [27]. Prior

marking of CFRP panel was required to provide scan-guidelines during inspection.

AU testing was performed in a matter of minutes to scan a 305 mm x 305 mm area.

The calibration of apparatus took minimal time with little expertise, and test results

were easy to interpret. The delaminations were imaged instantly on the display of the

apparatus. However, there was not sufficient information available about the depth of

the delaminations.

3. The CFRP panels have to be marked with a mesh of 25 mm x 25mm before testing

with impact-echo testing. The impact-echo tests also successfully identified the

artificial delaminations; however, delamination measurements were time consuming

[31].

4. Microwave measurements were also very successful in detecting intentional and

unintentional delaminations [37]. It was feasible to have a close idea of the depth and

the dimensions of the delamination [37]. However, the current microwave test

apparatus needed skilled personnel and tests were time consuming.

Comparative studies of these techniques for this field related case study will continue

through May 2008 to monitor any changes in the defects as well as provide additional

NDE system comparisons.


SUMMARY

The results of this literature review and investigation shows that currently there is no

available NDT method which can scan large scale FRP strengthened areas for

delaminations between FRP fabric and concrete substrate in a short period of time; even

though the non-destructive evaluation methods of thermography, microwave, ultrasonic,

acousto-ultrasonic and impact echo showed promising results in successfully detecting

and locating delaminations (air-blisters) between FRP fabric and concrete substrate. The

current microwave NDT appeared to give more detailed information about the defect

location and dimension even though the test apparatus requires trained and skilled

personnel and consumes relatively more time. Acousto-ultrasonic and impact-echo

showed a lightweight and more user-friendly apparatus; however, it cannot provide

detailed information like microwave technology. Laser shearography, ground penetrating

radar and eddy current test methods have also been tested in detecting such defects,

especially between FRP plies; however, more work is required in these fields. The

authors believe that the above mentioned technologies can be very useful defect detection

methods where the bond property is extremely critical and where the tap-test and visual

inspection provides signs of delamination in a FRP fabric strengthened structural

member. These tools also hold great promise as a confirmation tool for defect repair and

injection verification. Table 1 shows the overall comparison among NDT techniques

presented in this paper.


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Table 1 – Comparison of NDT techniques used for delamination detection between

CFRP fabric and concrete substrate. NDT Technique Advantages Disadvantages

Visual Testing

• Fast inspection rate • Ability to detect large areas • No equipment expense • Low labor cost • No intimate contact required • Single-sided test

• No quantitative and objective information

• Highly susceptible to human misconception

Tap Testing

• Fast inspection rate • Ability to detect large areas • Low labor cost • Low equipment cost • Single-sided test • Highly secure for personnel

• No quantitative and objective information

• Highly susceptible to human misconception

• Intimate contact required, hence can damage FRP if misused

Thermography

• Fast inspection rate • Ability to detect large areas • Low labor cost • Easy to interpret the outcome • No intimate contact required • Highly secure for personnel • Single-sided test • Independent of FRP fabric type

• High equipment cost • Sensitive to ambient

temperature • Low material thickness is

required • Can detect only defects with

measurable thermal property change

Ultrasonic and

Acousto-Ultrasonic

• High mobility • Can be single-sided • Ease of interpretation of

outcome depending on type of imaging

• Highly secure for personnel • Imaging can be provided

during scanning

• Calibration required for thickness change or FRP fabric type

• Requires skilled personnel • Moderately high equipment

cost • May require intimate contact or

prior marking of inspection area

(Continued)


Table 1 – Comparison of NDT techniques used for delamination detection between

CFRP fabric and concrete substrate (continued). NDT Technique Advantages Disadvantages

Impect-Echo • Moderately slower inspection rate

• Lightweight and user-friendly equipment

• Ease to interpret outcome • Single-sided test • Low labor cost • Safety of personnel

• Moderately high equipment cost

• Prior marking of inspection area may be required

• Less effective for small delaminations (< 1 in2)

• Imaging cannot provide informative outcome

Microwave • Can detect any size delamination successfully

• Can provide highly detailed images such as depth and the area of the delamination

• Safety of personnel • Single-sided test • Imaging can be provided

during scanning

• High equipment and labor cost, requires highly skilled personnel

• Intimate contact may not required; however, requires determination of stand-off distance

• Slower inspection rate and time consuming testing

• Prior marking may be required

Fig. 1 – Test beam with delaminations

between CFRP fabric and concrete substrate [34].

Fig. 2 – Delaminations between CFRP fabric and concrete substrate on a bridge

structure [36].

Fig. 3 – Application of tap-test with a steel bar.

Fig. 4 – Thermographic image of a

delamination prior to fatigue cycling [17].

Fig. 5 – Thermographic image of a delamination after 2 Million fatigue cycles

[17].


Fig. 6. Acousto-ultrasonic detection of near surface defects artificially produced on a bridge pier as described by Ekenel et al. [27].

Delamination or air blister

CFRP sheet

Concrete

Microwave probe

Epoxy

Stand off distanceTension reinforcement

Fig. 7 – Schematic of a near-field microwave NDT system [34].

Fig. 8 – Forming intentional delaminations on a RC beam.


Fig. 9 – Microwave NDT of delamination on a RC beam as described by Ekenel et al. [34].

0 10 20 30Mid-span Displacement (mm)

0

20

40

60

80

Load

(kN

)

Without DelaminationWith Delamination

Fig. 10 – Load vs. mid-span displacement curves of beams with

and without delaminations.


0 0.4 0.8 1.2Strain Readings (%)

0

20

40

60

80

Load

(kN

)

Steel-No DelaminationCFRP-No DelaminationSteel-DelaminationCFRP-Delamination

Fig. 11 – Mid-span strain readings of the beams with and without delaminations. Strains were measured over CFRP fabric and Steel

reinforcement of the beams with and without delamination.

0 0.4 0.8 1.2Strain Readings (%)

0

20

40

60

80

Load

(kN

)

Over DelaminationCorner DelaminationMid-span

Fig. 12 – Strain readings of the beam with delaminations. Strain

readings were measured at the middle of the delamination, corner of the delamination, and at the mid-span of the beam over perfectly bonded

CFRP fabric. The delamination that readings were performed was located beneath one of the loading supports.


Fig. 13 – Forming intentional delaminations on a bridge structure [36].

Fig. 14 – CFRP fabric with near surface defects applied on a

bridge structure near splash/water zone [36].

nondestructive evaluation of rc structures strengthened with frp laminates containing...

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