EXPERIMENTAL STUDY ON BOND BEHAVIOR BETWEEN CFRP PLATES AND
CONCRETE UNDER COMBINATION OF FRACTURE MODE

1

Wei Zhang 1 and Toshiyuki Kanakubo 1
Department of Engineering Mechanics and Energy, University of Tsukuba, Japan
Email: wei@rcs.kz.tsukuba.ac.jp

ABSTRACT
Diagonal tension cracks in reinforced concrete members adversely affect the performance of flexural members
strengthened by externally boned fiber-reinforced polymer (FRP) materials. The interface between the CFRP
plate and concrete is influenced by both shear bond and peeling for such conditions. The objective of this study
is to understand bond behavior in CFRP plate and concrete systems under the combination of fracture mode
(peeling and shear) by special double-face shear bond test specimen. The specimens were designed for different
initial angles at the middle of the specimen to ensure that the interface acts for both shear and peeling conditions.
From the test results, the bond behavior under the combination of fracture mode can be observed using special
specimens. It is confirmed that the tensile load decreases as the peeling angle increases. The relationshipbetween peeling angle and tensile load is determined to evaluate the bond strength under the combination of
fracture mode. Based on the test results, a new calculation method under the combination of fracture mode isproposed. The proposed method makes good prediction of the bond strength between CFRP plate and concrete
for shear-peeling fracture conditions.
KEYWORDS
Concrete, CFRP plate, double face, shear and peeling, combination fracture, bond strength
INTRODUCTION
In recently, many rehabilitation or strengthening of existing concrete structures using external attached CFRP
(Carbon Fiber-Reinforced Plastic) plate have been developed to extend their service life. And in the strengthened
concrete structures, the diagonal tension cracks failure may adversely affect the performance of flexuralstrengthened members by fiber-reinforced polymer (FRP) materials. In many past studies, which the FRP
materials were considered to act in plane with the member surface, and the interface between the FRP materials
and concrete was subjected to shear bond stress (called Mode II in fracture mechanics) have been investigated alot. In the case of flexural strengthened concrete beams, however, when delamination is induced by the openingup (Mode I) of a flexural shear crack and a shear crack, a relative vertical displacement exists between the twosides of the crack as shown in Fig. 1. Hence, the bond performance of this type structure, strengthened by FRP
materials, depends not only on the shear bond strength of the interface, but also on the combined effect of shear
and peeling. The objective of this study is to investigate bond behavior of CFRP plate and concrete bondingsystem in peeling and shear fracture at interface simultaneously using special double-face shear bond specimen.

Fig.1 Combination fracture mode

EXPERIMENTAL INVESTIGATIONSpecimens and CFRP Plates
Basic concept of this study is to obtain bond strength directly using double shear bond specimen. A total of 12specimens, which have curved surface to connect two different cross sections for each specimen to allow an
initial angle attached, were prepared to test by uniaxial static loading. The details of specimen are shown inFig.2.The level difference between two different cross sections of specimen is called step height. The M24 steel
bars have no connection at the middle of the specimen, and the pre-crack is introduced before loading. Thatmeans two concrete blocks just connect with CFRP plates. The specimen list is shown in Table 1. All thespecimens were designed with "pre-unbond" length setting near the center region due to cause the combinationof fracture failure clearly at the bonding part. The variables factors are initial angles, which designed for 2 and 4degrees (little initial angle), 10 and 20 degrees (large initial angle), and concrete compressive strength (targetstrength is 13.5MPa and 21MPa). High strength type CFRP plate with 1 mm thickness is utilized in thisexperiment.

Fig.2. Specimen details (mm)Table 1 Specimen list
Specimen

Concrete 1/3 Designed
Pre-unbond
Step height
secantinitial angle
CFRP plate
lengthmodulusθConcretecompressivestrengthConcrete
splittingstrength

MPa

MPa

GPa

18.4

1.97

20.3

S13-2-1
S13-2-2
S13-4-1
S13-4-2
S13-10
S13-20
S21-2-1
S21-2-2
S21-4-1
S21-4-2
S21-10
S21-20degreemm

28.8
2504
16.9

1.89

19.6

31.0

2.87

25.4

1020

17.5
20022.46

23.0

35.0
72.5
8.8

2504

30.8mm

1020Highstrengthwith 1mmthickness

17.5
200

35.0
72.5

Test Setup and Procedure
All the specimens were subjected to tensile force to cause shear and peeling debonding clearly at the interface ofspecimen. Fig.3 shows the test setup by universal testing machine. In data acquisition, for each combination oftest variables, one specimen was instrumented with 8 strain gauges on the CFRP plate surface at one side. Thesegauges were spaced at 50 mm from the center of the CFRP plate to the load end. On the opposite side, one gauge
was used at the center of the plate. In addition, the total displacements and crack widths at the specimen center
were measured by using linear variable displacement transducers (LVDTs) and Pi gauges, respectively.

Fig.3. Test setup

RESULTS AND DISCUSSIONS
Failure Progress
The experimental results are shown in Table 2. Calculated bond strength is given by previously proposed
calculating method (Matsunaga et al. 2008) for shear bond strength of CFRP plate. All the specimens were
subjected to tensile force until debonding failure happened on either side. Typical failure surfaces fromexperimental results are shown in Fig.4. From the post-failure photographs, it is apparent that the failure surface
of initial angles with 10 and 20 degrees specimens are between CFRP plate and adhesive and the initial angles
with 2 and 4 degrees specimens debond in the concrete surface. It is observed that the large initial angle
specimens show clearly peeling of the plate and the little initial angle specimens’ debonding progress are almost
controlled by shear debonding manner. Nevertheless, it is considered that all the debonding is due to thecombination of fracture mode (peeling and shear).
Table 2 Experimental results
Specimen

Maximumload PkN

S13-2-1
S13-2-2
S13-4-1
S13-4-2
S13-10
S13-20
S21-2-1
S21-2-2
S21-4-1
S21-4-2
S21-10
S21-20

20.8
20.3
15.5
16.0
5.1
5.5
23.4
24.3
20.8
17.0
7.3
6.1

Calculatedbond strength
kN

38.7

41.4

P/P cal

0.54
0.53
0.40
0.41
0.13
0.140.61
0.63
0.54
0.44
0.19
0.16

Failure face

Concrete
Epoxyresin
Concrete
Epoxyresin

Failure faceFig.4 (a) Typical failure face of little initial angle specimen

Failure faceFig.4 (b) Typical failure face of large initial angle specimen
Tensile Load vs. Displacement Relation and Strain Distributions
Some selected load-displacement curves are shown in Fig. 5. It is observed that CFRP plate peeled off toward
the loaded end of the specimen with gradual increase of tensile load in the little initial angle specimens. However,in the large initial angle specimens, it can be found that as the delaminating of the CFRP plate, the 10-degree
specimens and 20-degree specimens show the different behaviors. One is with a short decrease progressively
after up to the maximum tensile load and gradually increased to failure. Another is up to the peak load andgradually decreased to the failure. The sudden drops in the load appeared in curves represent the propagation ofdelaminating of CFRP plate.

100.4
0.6
Displacement (mm)0.80180.2

0.4
0.6
Displacement (mm)0.80124000.2

0.40.6
Displacement (mm)

0.8

0.2

0.40.6
Displacement (mm)0.8

1

S21-2064018422211

S21-4-1
S21-4-2

S21-10

Load (kN)

Load (kN)

40.8

0.8

66

0.4
0.6
Displacement (mm)0.40.6
Displacement (mm)S13-20

6

0.2

0.288
S13-10010

Load (kN)

0.2

20

S21-2-1
S21-2-2

10100

Load (kN)

20

Load P (kN)

Load P (kN)

Load P (kN)S13-4-1
S13-4-220

Load P (kN)S13-2-1
S13-2-220

0.20.40.6
Displacement (mm)

0.8

100.2

0.4
0.6
Displacement (mm)0.8

1Fig.5 Load-displacement relationshipThe observed strain distributions, with the tensile load versus crack width diagram for some selected specimens
are shown in Fig.6. The strain distributions correspond to mark plots on the tensile load versus crack widths
diagrams. As expected, some negative strains are observed due to bending of CFRP plate during debonding
progress in the large initial angle specimens as same as the case of FRP sheet (Alam et al. 2012). As thedebonding progress, the position of negative strain also moves to the load end. However, there is no negative
strain appeared in the debonding progress of little initial angle specimens. It can be found that curves trend inlittle initial angle specimens are almost same to the shear bond behavior. This phenomenon supports thedifferences of debonding failure face shown in the previous photos.
S21-2-1

S13-2-2

100kN
8.1kN
15.0kN
19.3kN
20.1kN

0.2

0.4
0.6
Crack width (mm)

0.8

10

0.2

0.8

100kN
8.1kN
15.0kN
19.3kN
20.1kN

0.15

0.2

0.4
0.6
0.8
Crack width (mm)S21-4-1

0kN
10.1kN
17.3kN

10.2

0.2

S13-2-2

S21-2-1

10.4kN
15.5kN
20.0kN
23.4kN
22.7kN0.15

0.15

19.8kN
15.9kN
19.4kN
20.2kN

0.05

0.05

0.05

050
100 150 200 250 300 350
Location of strain gauges from specimen center (mm)8050
100 150 200 250 300 350
Location of strain gauges from specimen center (mm)8050
100 150 200 250 300 350
Location of strain gauges from specimen center (mm)8Strain(%)

0.1

Strain(%)

Strain (%)

0.40.6
Crack width (mm)0kN
10.1kN
17.3kN
19.8kN
15.9kN
19.4kN
20.2kN

1010.4kN
15.5kN
20.0kN
23.4kN
22.7kN

0.2

S21-4-1

Load P (kN)

Load P (kN)

1002020

Load P (kN)

20

0.1

S13-10

0kN
3.5kN
3.4kN
2.6kN
2.8kN
2.5kN

20.2

0.4
0.6
Crack width (mm)

0.8

40kN
2.93kN
2.6kN
2.47kN

2100.20.40.6
Crack width (mm)

0.8

100

0.10kN
4.4kN
3.5kN
2.7kN
3.1kN
3.0kN

0.20.40.60.8Crack width (mm)10.2
S13-200kN
2.93kN
2.6kN
2.47kN

0S21-10Strain (%)

0kN
3.5kN
3.4kN
2.6kN
2.8kN
2.5kN

Strain (%)

S13-10420.2

0.2

Strain (%)

6
Load (kN)

Load (kN)

Load (kN)40.1

S21-10

S13-20660

0.1

0.10kN
4.4kN
3.5kN
3.0kN
4.2kN
2.7kN

0-0.1

-0.1

-0.1

050 100 150 200 250 300 350
Location of strain gauges from specimen center (mm)050 100 150 200 250 300 350
Location of strain gauges from specimen center (mm)050 100 150 200 250 300 350
Location of strain gauges from specimen center (mm)Fig.6 Strain distributions correspond to mark plots load-crack width relation

Bond Strength Evaluation
Bond strength affected by combination of fracture mode is evaluated as same method for the case of FRP sheet(Alam et al. 2012). That means it can utilize the relation of tensile load ratio (P/Pcal) and peeling angle (tanθ) toevaluate the combination of fracture bond strength. A detailed delamination process is shown in Fig. 7. As theload increases, the delamination of the plate occurs due to the peeling effect. Simultaneously, the bond lengthand the peeling angle (tanθ) between the CFRP plate and the concrete face decreases. The delaminated region isdefined as the distance from where the pre-unbond region starting on test part of the specimen to the strain
gauges where the minimum strain (negative strain) is observed in case of large initial angle specimens. The restof the CFRP plate length is the bonded region, which gradually diminishes to failure. The peeling angle isobtained as the angle between the axial direction and tangential line taken from the specimen surface to theposition of minimum strain appeared in CFRP plate. The load P corresponding to this minimum strain wasrecorded from the data to fit the relation of tensile load ratio and peeling angles. In the case of little initial angle
specimens, the values at sudden drop of tensile load are selected because of no recorded negative strains.
Tensile load P ratio to the calculated bond strength Pcal using Eq. (1)-(5) (Matsunaga et al. 2008) is plottedagainst the peeling angle (tanθ) as shown in Fig. 8. It can be seen that the tensile load decreases with theincreases in the peeling angle. The relationship between the tensile load ratio and peeling angle can be obtained
as shown in the figure.

Fig.7 Details of delamination process
1.2
S13-2-2
S21-2-1
S13-4-2
S21-4-1
S13-10
S21-10
S13-20
S21-20P/P cal

0.6
0.4
-1.46

y=0.003x

0.2
0Calculated shear bond strength

10.8

S13-4
S21-4P/Pcal

0.8

0.6

0.1
tanθ

0.15

0.2

Fig.8 Effect of peeling angles on bond strengthS13-20
S21-200.4
0.2 S13-2
S21-2

0.05

S13-10
S21-100y=0.003x -1.46

0.02 0.04 0.06 0.08 0.1 0.12 0.14
tanθ

Fig.9 Evaluation of combination fracture bond strength b ,m ax  2.5   B 0.23 .................................................................................................(1)le 

2  t fp  E fp  s e
k e   b .max........................................................... .................................(2)Pb  k e   b .m ax  b fp  le ( lb  le )..............................................................................(3)Pb  k   b .max  b fp  l e ( lb  l e )................................ ...............................................(4)k  l  1  ke
1 - ke.................................................................................(5) cos  b  22
 le Where, maximum local bond stress τb,max, concrete compressive strength σB in MPa, effective bond length le,width bfp and thickness tfp and elastic modulus Efp of CFRP plate, local slippage of the effective bond area se(0.234mm), stress coefficient of EBSB (Equivalent Bond Stress Block) in case of effective bond length ke(0.428), bond length lb, bond strength Pb, stress coefficient of EBSB k (Kanakubo et al. 2003).

Plots of calculated shear bond strength varying peeling angle (is estimated by bond length) and the evaluatedtensile load ratio are shown in Fig. 9. The calculated bond strength using formulas (1)-(5) is obtained withdifference by bond length (lb) to effective bond length (le). In this experimental investigation, it is considered thatthe tensile load reached the shear bond strength with all the plate delaminated. In other words, as the platedelaminated, the bond length decreased. That means the bond length is shorter than effective bond length.
According to above analysis, the trend of calculated shear bond strength curves are shown in the figure. Theintersection points represent the bond strength with combination of fracture mode. Therefore, using the values oftanθ at these intersections, the corresponding values of P/Pcal can be calculated. The values of P/Pcal, tanθ andthe calculated bond length for each specimen are tabulated in Table 3. It can be seen that the predictions are
better comparing of test results.
Table 3 Comparing of test results
CalculatedExperimental
Calculatedbond length Experimentaltanθ/Specimen
P/PP/P cal(cross point)
Calculated
calmm
values
S13-2-1
0.54
0.026
0.62
0.87
61.5
S13-2-2
0.53
0.026
0.62
0.86
S13-4-1
0.40
0.038
0.36
1.13
39.5
S13-4-2
0.41
0.038
0.36
1.15
S13-1013.9
0.13
0.060
0.18
0.71
S13-20
7.9
0.14
0.092
0.10
1.43
S21-2-1
0.61
0.026
0.62
0.99
61.5
S21-2-2
0.63
0.026
0.62
1.02
S21-4-1
0.54
0.038
0.36
1.52
39.5
S21-4-2
0.44
0.038
0.36
1.24
S21-10
13.9
0.19
0.060
0.18
1.04
S21-20
7.9
0.16
0.092
0.10
1.64
CONCLUSIONSThe bond behavior under the combination of fracture mode is observed using special specimens which have stepheight in surface. It is confirmed that the tensile load decreases as the peeling angle increases. The relationshipbetween peeling angle and tensile load is determined to evaluate the bond strength under the combination offracture mode. Based on the test results, a new calculation method under the combination of fracture mode isproposed. The proposed method makes good prediction of the bond strength between CFRP plate and concrete
bonding system for shear-peeling fracture conditions.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the support of Constec Engineering. Co., Japan.
REFERENCES
Alam M.S., Kanakubo T., Yasojima A., (2012)."Shear-Peeling Bond Strength between Continuous Fiber Sheetand Concrete", ACI Structure Journal, V.109. No.1, pp.75-80;
Kanakubo, T., Furuta, T., Fukuyama, H., (2003). "Bond strength between Fiber-Reinforced Polymer Laminates
and Concrete", Proceeding of 6th International Symposium on Fiber-Reinforced Polymer (FRP) Reinforcement
for Concrete structures, Vol.1, pp133-142;
Matsunaga K., Kanakubo T., et al., (2008). "Study on Bond between CFRP Plate and Concrete", Summaries ofTechnical papers of Annual Meeting of Architectural Institute of Japan, C-2, pp.307-310, (in Japanese).