Asia-Pacific Conference on FRP in Structures (APFIS 2007) S.T. Smith (ed)

© 2007 International Institute for FRP in Construction

811

INTERLAMINAR PEELING IN CONCRETE BEAMS STRENGTHENED WITH CFRP

PLATES - EXPERIMENTAL STUDY

P. Rusinowski 1,2 and B. Täljsten 1,3

1 Technical University of Denmark, Lyngby, Denmark

2 Norut Technology Ltd., Narvik, Norway, 3 Luleå University of Technology, Luleå, Sweden

ABSTRACT FRP composites are becoming a material of choice in an increasing number of rehabilitation and retrofitting projects around the world. Depending on the design objectives, these materials can be used to improve one or more of the structural member characteristics such as the load capacity, ductility and even durability. Design of structural strengthening applications using externally bonded FRP composites is usually based on conventional design approaches with improvement to account for the presence and characteristics of the FRP material. Non-conventional design issues that are specific to the type of application require special considerations for their proper inclusion in the design process. One such design issue is the debonding problems in externally bonded FRP strengthening applications that have been a concern and a research challenge since the initial development stages of the strengthening method. End-peeling has governed a large interest and several debonding models have been presented. However, interfacial peeling at flexural cracks has not attained the same focus – even though this debonding failure is most likely more common. This paper presents laboratory tests of concrete beams strengthened in flexure with CFRP epoxy bonded plates. In order to localize failure a notch in the middle section of the beams was made. Furthermore wrapping with CFRP sheets was applied in order to prevent debonding on one side of the notch. Optical equipment allowed capturing the moment of failure and measuring crack openings and debonding propagation. KEYWORDS FRP, RC beams, strengthening, optical measurements. INTRODUCTION Externally bonded FRP plates and sheets are a more and more common mean of strengthening. Although handbooks and codes provide advanced guidelines for externally bonded strengthening, there is still a need for improvement of bonding models. The most recognised and understood debonding failure is end peeling, see for example Täljsten (1997) or Smith and Teng (2001) where analysis of interfacial stresses for uncracked sections has been developed. Much more complex is the situation for cracked sections. While a crack in concrete beam is being initiated, concentration of stresses may cause debonding in the interface between FRP and the concrete. A number of publications describe and analyse experiments of FRP-strengthened beams where interlaminar peeling was obtained. Sebastian (2001) showed significance of midspan debonding by performing an experimental study on RC CFRP plated RC beams with pre-localized flexural crack in the mid-section. He observed that midspan debonding initiates at flexural cracks and propagates ripping off the mortarcrete layer of the beam. Since flexural cracks act on the CFRP-concrete interface, it might be supposed that simple slip-bond tests, such as in Lu et al. (2005), can represent this type of failure. However, experiments performed by Teng et al. (2003), where strengthened cantilever beams were tested, show some divergence in results. This may be the effect of uncertainty of debonding mechanisms, curvature of the beams and presence of multiple cracks. The last phenomenon causes debonding between two adjacent cracks, studied by Chen et al. (2007).

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A number of authors emphasize more significance of shear-flexural than flexural cracks. Transverse displacement of inclined crack edges causes tension stresses in the interface layer which eases debonding initiation and propagation. This type of debonding was analysed numerically by Niu et al. (2006). Much earlier, Karbhari and Engineer (1996) examined peeling off CFRP sheets from concrete with varying peeling angles. Wu et al. (2005) studied peeling and spalling resistance of CFRP sheets by applying loads directly on sheets through a notch. Pan and Leung (2007) simulated mid-span debonding by applying pulling and shear force at the same time. Their analytical model solution compared with experimental results showed good agreement. There are only some examples of investigation of midspan debonding mentioned here, while a more extensive literature review was performed by Buyukozturk et al. (2004). AIM AND SCOPE The aim of the study presented in this paper was to obtain a better understanding of crack formation as well as debonding initiation and propagation in CFRP plated RC beams. The data collected during the experimental program creates a background for further analytical and numerical studies. Compared to other studies in this field, the use of optical techniques to measure crack widths and debonding propagation is an innovation. EXPERIMENTAL STUDY

Specimens In the experimental study 7 RC beams were tested. The specimens were prepared with varying parameters: concrete quality, epoxy adhesive type, adhesive thickness, amount of tensile reinforcement and CFRP plate type. Reference beam (no. 1) and strengthened beams 2-6 were cast in one batch using high-strength concrete while beam 7 was cast with standard concrete of lower quality. In order to localize failure a 20-mm-deep notch was made in the centre section of the beams prior to strengthening. Furthermore, a 20-mm-long section on one side of the notch was unbonded to initiate debonding while the other side was prevented from debonding by wrapping with CFRP sheet. Steel reinforcement section reduction was not performed. Due to optical measurements, the side face of adhesive and CFRP plates had to be applied in one plane with the beam side face. For this reason two 50-mm-wide CFRP plates were applied. Prior to strengthening the concrete surface was sand blasted and primer was applied. The geometry of the beams is shown in Figure 1 and the test matrix is presented in Table 1.

Figure 1. Geometry and static system of strengthened beam

Table 1. Test matrix

Concrete quality CFRP type Adhesive type Adhesive

thickness Tension

reinforcement Beam 1 (Ref) High - - - 2φ10 Beam 2 High StoFRP Plate E 50 C StoBPE Lim 567 1 mm 2φ10 Beam 3 High StoFRP Plate M 50 C StoBPE Lim 567 1 mm 2φ10 Beam 4 High StoFRP Plate E 50 C StoPox Sk 41 1 mm 2φ10 Beam 5 High StoFRP Plate E 50 C StoBPE Lim 567 1 mm - Beam 6 High StoFRP Plate E 50 C StoBPE Lim 567 2 mm 2φ10 Beam 7 Low StoFRP Plate E 50 C StoBPE Lim 567 1 mm 2φ10

Material Properties Reinforcing steel properties were evaluated with tension tests performed on 6 samples and stress-strain relationships are presented in Figure 2a. Average concrete properties for beams 1-6 (batch 1) were evaluated from 18 cylinders (100×200mm) and 11 wedge splitting samples (100×100×100mm). Average concrete properties for beam 7 (batch 2) were evaluated from 3 cylinders and 3 wedge splitting samples.

100 900 900 100 150

170

120

50 4×50 15×100

4φ10

φ6

StoFRP Sheet S300 M200

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In order to obtain relatively smooth curves, wedge splitting tests were conducted with crack opening control, see Figure 2b. The output of the tests was in form of P – CMOD curves. Evaluation was done with inverse analysis, described by Østergaard (2005), which consists in finding such material properties which can reproduce output curves from the tests. According to Skoček and Stang (2007), who investigated efficiency of multi-linear softening curves, tri- and four-linear curves appear to give most accurate results. For the needs of this paper, tri-linear curves were evaluated for two batches (high and low concrete quality). Figure 2c shows the general shape of softening curves and all evaluated concrete properties are presented in Table 2. Modulus of elasticity in compression, Ec, was assumed to be equal to modulus of elasticity in tension, Et, evaluated in inverse analysis. Epoxy adhesive and CFRP properties were not evaluated and data provided by the manufacturer, Sto Scandinavia AB, was used in further consideration, see Table 3.

0 2 4 6 8 10 12Strain [%]

0100200300400500600700800

Stre

ss [M

Pa]

a1

a2

a3

b1

b2

b3

σ/ft

CMOD a) b) c) Figure 2. a) Stress-strain curves for reinforcing steel, b) Scheme of wedge splitting test, Østergaard (2003),

c) General shape of softening curve

Table 2. Concrete properties E [GPa] fc [MPa] ft [MPa] a1 b1 a2 b2 a3 b3

Beams 1-6 33.7 55.8 3.83 15.18 1 2.30 0.43 0.17 0.08 Beam 7 28.0 40.0 3.64 18.29 1 4.85 0.59 0.48 0.19

Table 3. Adhesives and CFRP properties Adhesives CFRP Plates BPE 567 Pox SK 41

E 50 C M 50 C

Comp. Strength [MPa] 93 100 Tensile strength [MPa] 2000 2500 Flex. Strength [MPa] 46 30 Young’s modulus [GPa] 155 260 Young’s modulus [GPa] 7 11 Thickness [mm] 1.4 1.4 Failure strain [‰] ~15 ~8

Test Setup and Instrumentation All specimens were subjected to 3-point-bending tests, with the load applied in the mid-point, see Figure 3a. Tests were performed with displacement control at a rate of 0.7 mm/min and 0.5 mm/min for reference and strengthened beams respectively. Deflections and support settlements were measured with LVDTs. Due to practical reasons, deflections were measured 250mm from the midspan on both sides of the notch, see Figure 3b. In the region close to the notch strain gauges were glued to one of the CFRP plates in order to measure strain variation along the plate, see Figure 3c. Furthermore, strain gauges were mounted in three sections on reinforcing steel and CFRP in order to measure strain distribution through height. Crack distribution and crack widths on the unwrapped side of the notch were measured using optical equipment Aramis, provided by GOM mbH. The system consists of two digital cameras which take photos in specified intervals. The measured area must be covered with a spatter pattern so that each point can be recognized by the software. Each picture is compared with the initial state and detected movements are recalculated into strains. A more detailed explanation of the process can be found in Pease et al. (2006), where cracking of concrete and steel-to-concrete interface is analysed. Such methods of optical measurements are not common in experimental studies of strengthened structures. Ali-Ahmad et al. (2006) used a digital camera system in slip-bond tests to measure strains in CFRP sheets, not to study crack formation and debonding. Failures of specimens were captured using a high-speed camera Photron APX RS at a rate of 1500 frames/s.

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a)

b)

c)

250 250

Notch

4×20

Unbonded region

LVDT

Section I II III

Figure 3. a) Beam subjected to 3-point-bending test, b) Location of LVDTs and strain gauges in three sections,

c) Location of strain gauges on CFRP plate near the notch

EXPERIMENTAL RESULTS

Only two beams (no. 2 and 6) failed with expected failure mode, debonding at flexural crack, see Figure 4b. Beams no. 3 and 4 failed by concrete cover separation. In case of beam 3 the failure was caused by application of a stiffer laminate. Beam 4 showed better bonding due to application of different epoxy adhesive, StoPox Sk 41. Beams no. 5 and 7 failed by debonding at shear crack on the wrapped side. Comparing load-deflection curves in Figure 4a, it can be deduced that despite different failure modes, beams 2, 3, 4 and 6 carried similar load. It is worth noting that even in beams 2 and 6 multiple cracks in the concrete cover were created during the test which indicates that cover separation was also likely to occur. Table 4 shows ultimate loads and failure modes of each tested beam.

0 10 20 30 40 50Deflection [mm]

0

10

20

30

40

50

60

70

80

Load

[kN

]

Ref. beam

Beam 2

Beam 3

Beam 4

Beam 5

Beam 6

Beam 7

a) b)

Beam 2 Beam 3

Beam 4

Beam 5

Beam 6 Beam 7 Figure 4. a) Load-deflection curves, b) Failures of specimens and their location

Table 4. Failure modes and ultimate load comparison Beam 1 Beam 2 Beam 3 Beam 4 Beam 5 Beam 6 Beam 7

Failure mode Steel yield.

Peeling at flex. crack

Cover separation

Cover separation

Peeling at shear crack

Peeling at flex. crack

Peeling at shear crack

Ultimate load [kN] 29.1 72.6 68.8 69.3 30.7 69.7 58.2

Figure 5 shows strain contours at final stage plotted from the optical equipment Aramis. The data is used only for crack visualization. It can be seen that regardless of failure mode debonding at flexural crack was initiated in all strengthened specimens. It can also be noticed that debonding initiated at shear-flexural cracks rather than flexural cracks at the notch.

Beam 2 Beam 3 Beam 4

Beam 5 Beam 6 Beam 7 Figure 5. Crack distribution at the ultimate load

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Further discussion of the results is limited to beam no. 2, where debonding at shear-flexural crack was the cause of failure. Figure 6 presents crack distribution at different load levels. Initially, vertical flexural cracks were being created. In further stages of the test the presence of CFRP strengthening caused shear cracks in the concrete cover which were bridging with vertical cracks. From that time vertical cracks in the concrete cover became less significant than new diagonal cracks. The failure of beam 2 was caused by shear-flexural cracks located approximately 150 mm from the notch.

10 kN 20 kN 30 kN 40 kN

50 kN 60 kN 70 kN 72 kN Figure 6. Crack distribution in beam 2 at different load levels

The application of Aramis in the study provided not only crack distribution but also crack widths. The measurement of crack widths was performed by the comparison of displacements of two points adjacent to a crack. Figure 7a shows the load-crack width relationship for the diagonal crack and associated debonding. The results extracted from Aramis were not filtered; the noise seen in the diagrams may be an effect of brittleness of concrete or vibrations from the setup. In Figure 7b strain distribution at different load levels on the CFRP plate is shown. At the lower loads, higher strains can be noticed near the notch. In further stages strains are more even due to cracking and debonding. For the uncracked region a drop in strains can be observed. The results obtained from the strain gauges, however, were not accurate enough to obtain reliable results of shear stresses.

-100 -80 -60 -40 -20 0 20Distance [mm]

-2000

0

2000

4000

6000

8000

10000

12000

Stra

in [μ

m/m

]

0 0.1 0.2 0.3 0.4Crack width [mm]

0

20

40

60

80

Load

[kN

]

10 kN

Debonding

70 kN

Crack

Figure 7. a) Crack and debonding widths, b) Strains along CFRP plate

High-speed cameras, used to capture the last phase of testing, appeared to be an efficient tool in localizing failure. Looking at the picture of beam 2 in Figure 4b, it is difficult to point out the exact failure location. Figure 8 presents a sequence of stages from a high-speed camera. It can be noticed that debonding propagated from the diagonal crack until anchorage failure. Debonding of the plates near the notch was caused by energy release after failure.

0.0 s +0.00067 s +0.00133 s +0.002 s Figure 8. Failure of beam 2 captured with high-speed camera

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CONCLUSIONS The main aim of the study was to provide knowledge about crack formation and debonding propagation in CFRP strengthened beams. Optical equipment was found to be an efficient tool in crack detection and crack width measurement. It was shown that in the case of three-point-bending test debonding occurs initiating from flexural-shear crack. Although only 2 specimens failed by interlaminar peeling, in all strengthened beams debonding initiation was detected. Interlaminar peeling and concrete cover separation were failures most likely to occur. The latter failure mode occurred in the case of a stiff CFRP plate and finer epoxy adhesive. The lack of tension reinforcement in beam no. 5 caused debonding at shear crack. In case of application of lower quality concrete, debonding of shear crack was combined with concrete cover separation. ACKNOWLEDGEMENTS The research work presented in this paper was performed at the Technical University of Denmark and financed by the Norwegian Research Council through the strategic institute program RECON at Norut Technology Ltd. The optical equipment was provided by Ph.D. Lennart Østergaard and sponsored by the Villum Kann Rasmussen foundation. REFERENCES Ali-Ahmad, M., Subramaniam, K. and Ghosn, M. (2006). “Experimental investigation and fracture analysis of

debonding between concrete and FRP sheets”, Journal of Engineering Mechanics, 132(9), 914-923. Buyukozturk, O., Gunes, O. and Karaca, E. (2003). “Progress on understanding debonding problems in

reinforced concrete and steel members strengthened with FRP composites”, Construction and Building Materials, 18(2004), 9-19.

Chen, J.F. and Yuan, H. (2007). “Debonding failure along a softening FRP-to-concrete interface between two adjacent cracks in concrete members”, Engineering Structures, 29(2007), 259-270.

Karbhari, V.M. and Engineer, M. (1996). “Investigation of bond between concrete and composites: use of a peel test”, Journal of Reinforced Plastics and Composites, 1996; 15; 208.

Lu, X.Z., Teng, J.G., Ye, L.P and Jiang, J.J. (2005). “Bond-slip models for FRP sheets/plates bonded to concrete”, Engineering Structures, 27(6), 920-937.

Niu, H., Karbhari, V.M. and Wu, Z. (2006). “Diagonal macro-crack induced debonding mechanisms in FRP rehabilitated concrete”, Composites: Part B, 37(2006), 627-641.

Pan, J. and Leung, C.K.Y. (2006). “Debonding along the FRP-concrete interface under combined pulling/peeling effects”, Engineering Fracture Mechanics, 74(2007), 132-150.

Pease, B., Geiker, M.R., Stang, H. and Weiss, J. (2006). “Photogrammetric assessment of flexure induced cracking of reinforced concrete beams under service loads”, Proceedings of the Second International RILEM Symposium : Advances in Concrete through Science and Engineering, ISBN: 2351580036. - 2006.

Sebastian, W.M., (2001). “Significance of midspan debonding failure in FRP-plated concrete beams”, Journal of Structural Engineering, 127(7), 792-798.

Skoček, J. and Stang, H. (2007). “On the inverse analysis of the wedge splitting test”, submitted to J. Engineering Fracture Mechanics.

Teng, J.G., Smith, S.T., Yao, J. and Chen, J.F. (2003). “Intermediate crack-induced debonding in RC beams and slabs”, Construction and Building Materials, 17(2003), 447-462.

Smith, S.T. and Teng, J.G. (2001). “Interfacial stresses in plated beams”, Engineering Structures, 23, 857-871. Täljsten, B. (1997). “Strengthening of beams by plate bonding”, Journal of Materials in Civil Engineering, 9(4),

206-212. Wu, Z., Yuang, H., Kojima, Y. and Ahmed, E. (2005). “Experimental and analytical studies on peeling and

spalling resistance of unidirectional FRP sheets bonded to concrete”, Composite Science and Technology, 65(2005), 1088-1097.

Østergaard, L (2003). “Early-age fracture mechanics and cracking of concrete”, Ph.D thesis, Department of Civil Engineering, Technical University of Denmark, Lyngby, Denmark.