Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005 Chen and Teng (eds)

© 2005 International Institute for FRP in Construction

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DEBONDING FAILURES OF RC BEAMS STRENGTHENED

WITH EXTERNALLY BONDED STRIPS

R. Kotynia Department of Concrete Structures, Technical University of Lodz,

Al. Politechniki 6, 90-924 Lodz, Poland, Email: rkotynia@p.lodz.pl ABSTRACT Fibre reinforced polymer (FRP) strips and sheets are commonly used for increasing ultimate strength and ductility of concrete structures in flexure. This paper reports results of experimental tests carried out on reinforced concrete (RC) beams externally strengthened with CFRP strips in flexure. Flexural behavior of the beams and failure modes are discussed. Test results indicated that full flexural capacity of strengthened members can not be achieved due to premature debonding of FRP from concrete. Two main failure mechanisms observed in the tests were classified based on two critical sections. The first was localised in the vicinity of the strip end and the second in the maximum bending moment region. CFRP strain efficiency is discussed with respect to different strengthening modes. Analysis of cross section for the ultimate limit state in bending was used for calculation of load bearing capacity of RC beams strengthened with CFRP. KEYWORDS RC beams, CFRP strip, strengthening, debonding failure, cover separation, strain efficiency, load capacity. INTRODUCTION During last decade a large explosion of real applications and experimental tests has taken place through the use of FRP composite materials for strengthening of reinforced concrete structures. Strengthening of existing reinforced concrete (RC) members with fibre reinforced polymer (FRP) strips, bonded to the tension face of a beam improves its stiffness and ultimate strength. A mount of studies have been carried out to describe stress transfer from the RC member to the externally bonded FRP reinforcement (Triantafillou and Plevris 1992, Talijsten 1997, Chen and Teng 2001, Smith and Teng 2002a, 2002b, Teng et al. 2002). They have shown that the full flexural capacity of the strengthened member can not be achieved due to premature FRP debonding from the concrete member. Most of experimental tests have indicated brittle manner of debonding failure which can be classified on two main types of FRP debonding: end debond and midspan debond (Oehlers and Moran 1990, Blaschko et al. 1998, Kotynia 1999, Pham and Al-Mahaidi 2004). First type of failure appears at the end of the anchorage FRP length and it refers to the plate end debonding together with a concrete cover. Extension of the FRP strip across the entire shear span to the support, providing transverse reinforcement and mechanical anchorage can mitigate the FRP end peel failure. The second type is generally concerned with the intermediate crack debonding characterised as flexural or flexural-shear debond. It initiates in the high bending moment region and then develops towards one of the strip ends. The key of this technique is bond phenomena between composite and concrete. Due to bond characteristic, tension forces can be transferred from concrete into the external FRP reinforcement. In order to explain failure mechanisms and effects of several parameters on bond behavior an experimental program was conducted. EXPERIMENTAL PROGRAM

Experimental overview A test was carried out on three series of twenty single-span, simply supported RC beams with a cross section of 150x300mm and two different spans of 3000mm – for the beams of Series I and II and 4200mm – for Series III. Geometry, steel reinforcement and the strengthening method are shown in Table 1 and in Figures 1 and 2. Average properties of the steel reinforcement and CFRP strips, sheets and L-shaped strips are summarized in Table 2. Detailed study of the tests has been published in Kaminska (2000) and Kotynia (2003).

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Table 1 Beam details and test results Test results Tension steel

ratio, series CFRP-strip

ConcreteBottom CFRP

strain CFRP

utilization Failure mode

Limit load Beam

ρs Series

a/h Type

e (mm)

z (mm)

fc (MPa)

εf (%)

εfu (%)

εf/εfu - 2F

(kN) B-04/S1 0.0039 S812 150 - 28.4 0.25 0.15 ED 120 B-04/S2 0.0039 S812 300 - 28.4 0.25 0.15 ED 90

B-04/0.5S 0.0039 S412 150 - 36.6 0.501.70

0.29 MD 98 B-04/M 0.0039 M1214 250 200 29.7 0.27 1.24 0.22 ED 120 B-06/S1 0.0056 S812 250 350 32.3 0.34 0.20 ED 140 B-06/S2 0.0056 S812 250 500 32.3 0.34 0.20 ED 145 B-08/S1 0.0084 S812 150 - 33.8 0.50 0.29 MD 180 B0-08/S2 0.0084

I 2.67

S812 150 - 36.0 0.55 0.32 MD 180 BF-04/0.5S 0.0039 S412 150 - 33.0 0.58 0.34 MD 481 BF-06/S 0.0056

II 5.00 S812 150 - 32.5 0.54

1.70

0.32 MD 861 B-08M 0.0084 M1214 75 - 37.3 0.51 0.41 MD 140 B-08Mk 0.0084 M1214+L 75 - 32.0 0.56 0.46 MD 150 B-08Mm 0.0084 M1214+UT 75 - 38.2 0.55

1.240.44 MD 152

B-08S2 0.0084 S512 75 - 32.3 0.62 0.36 MD 94 B-08Sk 0.0084 S512 + L 75 - 33.8 0.86 0.51 MD 102 B-08Sm 0.0084 S512+UT 75 - 33.5 0.66 0.39 MD 102

B-08Smb 0.0084 S512+UL 75 - 25.7 0.77 0.45 MD 114 B0-08Smb2 0.0084 S512+UL 75 - 27.4 0.63

1.70

0.37 MD 110 B-083m 0.0084 3 x s 75 - 34.4 0.68 0.45 MD 92 B-083mb 0.0084

III 4.67

2 x s + UL 75 - 25.8 0.841.50

0.56 MD 123

1 Total load for beams BF-04/0.5S and BF-06/S is F (Fig. 1); 2 Beams strengthend under 50% of the ultimate load for non-strengthened beam; Tension steel reinforcement ρs = 0.0039 (2 # 10), 0.0056 (2 # 12), 0.0084 (3 # 12); Shear span-to-depth ratio a/h = 2.67; 5.00; 4.67; UT sheet – U jacket transverse CFRP sheet; UL sheet – U jacket longitudinal CFRP sheet; L-shaped – transverse L-shaped CFRP strip; 3 x s – three layers of CFRP sheet; ED – end debonding; MD – midspan debonding

Figure 1 Geometry and steel reinforcement of tested beams

The following parameters were investigated in Series I and II (see Table 1): internal tension steel reinforcement ratio (ρs), concrete strength (fc), shear span-to-depth ratio (a/h), CFRP strip width (bf), type of CFRP strip (S, M), distance from the CFRP strip termination to the support axis (e), location of additional longitudinally oriented CFRP strips on both lateral sides of the beams in the support region (Figure 2), length of the overlap of the bottom and side CFRP strips (z). The aim of the lateral strips application was to improve the anchorage of the bottom strip and to avoid concrete cover separation.

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Figure 2 Strengthening modes

In order to increase efficiency of strengthening, in Series III following the application of the bottom strips (S5012 or M12014), transverse wet lay-up CFRP sheets or transverse L-shaped CFRP strips (UT) were applied over the middle of the beams (Figure 2). Moreover four beams of this Series were strengthened with the bottom main strip and additional longitudinal U-jacket wet lay-up CFRP sheets (UL), applied over the middle 3100 mm of the beams. Influence of the CFRP type (strips or wet lay-up sheets) and fiber direction of U-jacket composites on failure mechanism, CFRP strain efficiency and ultimate load was investigated.

Table 2 Properties of steel and CFRP reinforcement

Steel CFRP Type ds fsy Es fsu Type bf tf ffu Ef εfu

(mm) (MPa) (GPa) (MPa) (mm) (mm) (MPa) (GPa) (%) #12 (Series I, II) 12 490 195 692 S412 80 #12 (Series III) 12 436 220 662 S512 40 1.2 1.70

#10 (Series I, II) 10 421 199 651 S812 50 2915 172

#10 (Series III) 10 524 209 647 M1214 120 1.4 2742 220 1.24

# 6 6 437 207 501 sheet 300 0.13 2915 170 1.50 L-shaped 50 1.4 2782 155 1.73

TEST RESULTS

Modes of failure and ultimate loads Failure occurred due to abrupt debonding of the bottom strips from the concrete beams. Essentially two main modes of failure were observed. The first appeared by abrupt separation of the CFRP strip end with detached concrete cover below the tension steel reinforcement. The process initiated by the formation of a inclined crack at the end of the strip when the steel reinforcement started to yield. Tension forces could not be transferred then into the external CFRP because of its termination (Figure 3b). The crack propagated to the tension steel level, changed direction to horizontal and progressed along the tension steel reinforcement till the all out concrete cover separation with detached strip (Figure 3a). This concrete cover delamination occurred on a short distance not reached the loading point, at relatively lower applied loads and lower CFRP efficiency. By the strip extending closer to the beam support, the increase in ultimate load was obtained of 25% (Table 1). Higher increase in ultimate load was observed for the beams with lower steel ratio. The initial loading of the beam B0-08Smb, before strengthening had insufficient effect on the ultimate load (Table 2).

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a) b) Figure 3 End strip debonding: a) view of concrete cover separation, b) location of critical section (CS)

In order to delay this kind of failure additional CFRP strips were applied. The strips bonded on both sides of the beams caused the movement of the critical section away from the support (Figure 4). If the overlap length (z) increases the strip debonding delays, but this phenomena is limited by critical length of the side strips, that is explained in Figure 4b. Extension of the overlap length from 350mm to 500mm affected on the ultimate load increase below 10% (Table 1), hence more effective is the end strip extension to the support than the overlap length increasing. The end strip debonding is relatively simple to mitigate by the strip extending closer to the beam support by: applying additional longitudinal and transverse strips or sheets in the anchorage distance of the bottom strip and by increasing the shear span.

a) b) Figure 4 End strip debonding in the beam with lateral strips: a) view of end debonding, b) influence of lateral

strips on location of critical sections (CS) The second type of failure was caused by flexural cracks initiated in the high bending moment region as a result of high width of the flexural crack and then developed towards one of the strip end. The strip debonding occurred partially in the concrete cover and thin adhesive level (Figure 5). Strengthening effect can be increased by using the strips with higher elasticity modulus, pre-tensioned strips and additional composite reinforcement on lateral sides in the pure bending region.

a) b) Figure 5 Midspan debonding: a) view of debonded strip, b) location of critical section (CS)

The aim of the Series III test was to delay the midspan debonding and to increase the CFRP efficiency over 30%. The additional U-jacket strips or sheets were applied on the lateral sides of the beam, together with the bottom main strip (Figure 2, Table 1). Midspan debonding failure occurred in all beams of Series III abruptly, developed to one of the supports (Figure 5). Transverse L-shaped strips or sheet applied on beams delayed the bottom strip delamination but the increase in ultimate load was only of 7%. The application of longitudinal U-jacket sheets was more effected than transverse one, causing the increase in load capacity of 25%. The initial loading of the beam before its strengthening did not affect significantly on the beam stiffness and the ultimate load. Moreover the results of the Series III indicated that CFRP sheets cause the flexural behavior more ductile than stiff strips.

CS

CS

CS

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CFRP strain efficiency The average CFRP strain measured in the pure bending region during end strip debonding was 0.25%, that means only 15% of the CFRP efficiency (Table 1). The strips bonded on both sides of the beams in support region caused the increase in CFRP efficiency of 22%. The strip strain measurements of the I and II Series beams indicated that midspan strip debonding stared when its average strain (measured in the pure bending region) were between 0.5% and 0.6%, that was the CFRP efficiency range of 29-34%. The application of the transverse additional CFRP was ineffective for flexure. The most effective method of strengthening was the application of two bottom sheet layers and one layer of the longitudinal U-jacket sheet (B-083mb). The maximum utilisation of the strip strength was then εt/εtu = 0.56. ANALYTICAL MODEL Analytical model for calculation the load bearing capacity of RC members externally strengthened for flexure with CFRP strips considers the plane section principal and non-linear material characteristic σ−ε for all used materials (concrete, steel and CFRP). The analysis of cross section for the ultimate state in bending concerns two critical sections with respect to two main failure modes: the end strip debonding and the midspan debonding. The first is located close to the end of the strip in the unstrengthened section of the beam and the second in the maximum bending moment region (Figure 6). According to concrete separation failure explained in Figure 3, the limit strain of (non strengthened) RC member were assumed as yielding strain of the tension steel εsy. Based on the midspan failure mode (Figure 5), the limit strip strain was assumed from the test results of 0.55% (as an average value between 0.5 and 0.6%, Table 1). Experimental and calculated ultimate loads of the beams strengthened with the bottom composite are summarized in Table 3, other beams with additional lateral strips are not considered in this analysis.

a)

b)

Figure 6 Internal strain and stress distribution in: a) end strip debonding analysis of unstrengthened, b) midspan debonding of RC beam strengthened with FRP composite

Table 3 Comparison experimental and computational ultimate loads

Test CalculationRcal

RtestFF

Beams

Failure mode FR,test

[kNm] FR,cal

[kNm]

B-04/S1 300 45 40.54 1.11 B-04/S2 150 60 60.88 0.99 B-08/S1 150 90 90,97 0,99 BO-08/S 150 90 90,97 0,99

BF-04/0.5S 150 48 45,30 1.06 BF-06/S 150 86 77,52 1,11 B-08/M 75 70 71,08 0,98 B-08/S2 75 47 48,32 0,97

εsy εsy

εf = 0.55% εf = 0.55%

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The results confirm compatibility of the assumed analytical model for the beams externally strengthened with the bottom CFRP. The average load ratio value is FR,test/FR,cal = 1.02. Hence this model can be used for general analysis of cross section for the ultimate limit state in bending of RC members externally strengthened with prefabricated strips. For the analysis of beams strengthened with hand lay-up sheets the limit sheet strain is suggested of 0.7% (Table 1).

CONCLUSIONS Two main failure modes were observed in the tests: the strip end debonding with concrete cover separation and the midspan debonding. The test confirmed that external strengthening of RC members depended on several parameters: steel ratio and FRP ratio, RC member geometry, distance of the strip end from the support and bending moments distribution over the length of the element. Unfortunately, the ultimate strength of the strip could not be reached. The ratio of composite utilisation range was from 15% to 35% for beams strengthend with only bottom CFRP. The CFRP lay-up sheets affected flexural behavior of RC beams more ductile. The presence of the lateral sheets increased stiffness, load bearing capacity of the strengthened beams and the ratio of exhaustion of the bottom CFRP reinforcement to 56%. Transverse composites were not effected for flexural strengthening. Analysis of two critical cross sections for the ultimate limit state in bending was used for calculation of load bearing capacity of tested beams. The assumption of the limit strip strain based on the test results of 0.55% works very well and gives the average FR,test/FR,cal ratio value of 1.02. ACKNOWLEDGMENTS The author gratefully acknowledge the financial support provided by the Research Grants of the Polish State Committee for Scientific Research No. 7 T07E 030 16 and No. 8 T07E 006 21. REFERENCES Chen, J.F. and Teng, J.G. (2001). “Anchorage strength models for FRP and steel plates bonded to concrete”,

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