Bond strength model for interfaces between nearsurface mounted (NSM) CFRP strips and concrete

Transcription

1 University o Wollongong Research Online Faculty o Engineering and Inormation Sciences - Papers: Part A Faculty o Engineering and Inormation Sciences 2014 Bond strength model or interaces between nearsurace mounted (NSM) CFRP strips and concrete S S. Zhang University o Wollongong, shishun@uow.edu.au J G. Teng Hong Kong Polytechnic University, cejgteng@polyu.edu.hk T Yu University o Wollongong, taoy@uow.edu.au Publication Details Zhang, S. S., Teng, J. & Yu, T. (2014). Bond strength model or interaces between near-surace mounted (NSM) CFRP strips and concrete. In S. T. Smith (Eds.), 23rd Australasian Conerence on the Mechanics o Structures and Materials (ACMSM23) (pp ). ismore, New South Wales: Southern Cross University. Research Online is the open access institutional repository or the University o Wollongong. For urther inormation contact the UOW ibrary: research-pubs@uow.edu.au

2 Bond strength model or interaces between near-surace mounted (NSM) CFRP strips and concrete Abstract This paper presents an accurate bond strength model or carbon-ibre-reinorced polymer (CFRP) strips nearsurace mounted (NSM) to concrete where debonding ailure happens in the concrete layer adjacent to the interace between FRP and concrete. Both bonded joints with a suicient bond length and those with an insuicient bond length are covered by the proposed model. The bond strength model was developed on the basis o an existing analytical solution as well as a recently proposed bond-slip model or such bonded joints. Numerical comparisons between the proposed bond strength model and 51 test specimens collected rom the existing experimental studies as well as the only existing bond strength model or such joints demonstrate the accuracy o the proposed model and its superiority over the existing bond strength model, especially or joints with insuicient bond lengths. Disciplines Engineering Science and Technology Studies Publication Details Zhang, S. S., Teng, J. & Yu, T. (2014). Bond strength model or interaces between near-surace mounted (NSM) CFRP strips and concrete. In S. T. Smith (Eds.), 23rd Australasian Conerence on the Mechanics o Structures and Materials (ACMSM23) (pp ). ismore, New South Wales: Southern Cross University. This conerence paper is available at Research Online:

3 BOND STRENGTH MODE FOR INTERFACES BETWEEN NEAR- SURFACE MOUNTED (NSM) CFRP STRIPS AND CONCRETE S.S. Zhang ecturer, School o Civil, Mining & Environmental Engineering, Faculty o Engineering, University o Wollongong, Northields Avenue, Wollongong, NSW 2522, Australia. shishun@uow.edu.au (Corresponding Author) J.G. Teng Chair Proessor o Structural Engineering, Department o Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China. cejgteng@polyu.edu.hk T. Yu Senior ecturer, School o Civil, Mining & Environmental Engineering, Faculty o Engineering, University o Wollongong, Northields Avenue, Wollongong, NSW 2522, Australia. taoy@uow.edu.au ABSTRACT This paper presents an accurate bond strength model or carbon-ibre-reinorced polymer (CFRP) strips near-surace mounted (NSM) to concrete where debonding ailure happens in the concrete layer adjacent to the interace between FRP and concrete. Both bonded joints with a suicient bond length and those with an insuicient bond length are covered by the proposed model. The bond strength model was developed on the basis o an existing analytical solution as well as a recently proposed bond-slip model or such bonded joints. Numerical comparisons between the proposed bond strength model and 51 test specimens collected rom the existing experimental studies as well as the only existing bond strength model or such joints demonstrate the accuracy o the proposed model and its superiority over the existing bond strength model, especially or joints with insuicient bond lengths. KEYWORDS Near-surace mounted (NSM), carbon-ibre-reinorced polymer (CFRP), strip, concrete, bond strength model, eective bond length INTRODUCTION The near-surace mounted (NSM) FRP strengthening technique (De orenzis and Teng 2007) has attracted signiicant attention worldwide as an eective alternative to the externally bonded FRP strengthening technique. In the NSM FRP strengthening method, grooves are irst cut in the cover concrete o RC members (or the surace layer o other structural members); FRP bars together with adhesive are then placed in the grooves to bond the FRP bars to the concrete. FRP bars used in the NSM strengthening method may have various cross-sectional shapes, including round, square, elliptical and rectangular. In the present study, only FRP strips, as a special orm o rectangular bars with a large section height-to-width (thickness) ratio, are considered. FRP strips are a common orm o NSM FRP reinorcement due to their advantage over FRP bars o other shapes in bond perormance: an FRP strip usually has a much larger perimeter (and hence a much larger area or interacial bonding) or the same cross-sectional area than an FRP bar o other shapes, which leads to a uller utilization o the tensile strength o the FRP material (e.g. El Hacha and Rizkalla 2004). FRP strips made o CFRP are particularly attractive as the high strength o CFRP leads to a small cross-sectional area, which is 1

4 desirable or NSM strengthening applications. The present study is thus ocused on CFRP strips. These CFRP strips are typically procured bars with unidirectional ibres oriented in the longitudinal direction; they oten have a rough surace texture or enhanced bonding with the adhesive. Several debonding ailure modes have been observed in tests on NSM CFRP strip-to-concrete bonded joints. Among these ailure modes, the commonest and most desirable mode is debonding due to cohesion ailure in the concrete near the epoxy-concrete interace. Existing test results have indicated that cohesion ailure-induced debonding can be expected provided that the suraces o concrete and FRP are properly prepared and the adhesive is appropriately selected. A bond-slip model has recently been proposed by the authors (Zhang et al. 2013) or NSM CFRP strip-to-concrete interaces in which ailure occurs in the concrete near the bi-material interace. This paper reports briely on the development o a bond strength model or such bonded joints on the basis o this bond-slip model. A ull description o the study can be ound in Zhang et al. (2014). For simplicity, only CFRP strips are explicitly reerred to in the subsequent discussions, although some o the observations and equations are equally applicable to NSM strips o other FRP materials. BASIC EQUATIONS Following the racture mechanics-based approach used in predicting the bond strength o various bonded joints (e.g. Yuan et al. 2004), the basic equations or calculating the bond strength o an NSM CFRP strip-to-concrete bonded joint take the ollowing orm: P 2G E A C Pt when b e (1) P u ailure 2G E A C u ailure Pt when b e (2) where G (N/mm) is the interacial racture energy; b (mm) is the bond length; e (mm) is the eective bond length; E (MPa) and A (mm 2 ) are the elastic modulus and the cross sectional area o the CFRP strip respectively; C ailure (mm) is the cross-sectional contour o the ailure surace which is taken to be composed o the three sides o the groove surrounding the adhesive layer (i.e. C ailure is the sum o the three side lengths o the groove); β is a reduction actor to account or the eect o insuicient bond lengths and is thus a unction o the bond length, and P t (N) is the ull tensile capacity o the CFRP strip. The interacial racture energy can be calculated using the ollowing equation proposed by the authors (Zhang et al. 2013): G 0.40 c (3) where γ is the groove height-to-width ratio and c (MPa) is the concrete cylinder compressive strength. Eq. 3 was obtained through a regression analysis o numerical results rom a parametric study conducted using a meso-scale inite element model (Zhang et al. 2013). Using Eqs. 1 and 3, the bond strength o an NSM CFRP strip-to-concrete bonded joint with a suicient bond length can be calculated. BOND-SIP MODE The bond-slip model proposed by the authors (Zhang et al. 2013) was adopted in the present study to obtain the eective bond length o NSM CFRP strips and the corresponding reduction actor. This bond-slip model is as ollows: 2B s 2 2B s ( s) A( ) sin( ) with s 2B (4) B 2 B A 0.72 (5) c B 0.37 (6) c where τ(s) (MPa) and s (mm) are the shear bond stress and the shear slip between the NSM CFRP strip and the concrete respectively. Obviously, the interacial shear stress varies across the adhesive layer thickness and around the perimeter o the CFRP strip, so τ(s) is the average shear stress on the deined ailure contour. 2

5 EFFECTIVE BOND ENGTH A parametric study was conducted using a simple inite element (FE) model to obtain the eective bond length and to examine the eect o bond length on bond strength. For this purpose, a simple beam-spring FE model was employed. Part o the beam-spring model near the loaded end is shown in Figure 1. In the FE model, the CFRP strip is simulated using beam elements, the concrete is assumed to be a rigid body, and the CFRP-to-concrete interace is simulated using cohesive elements with a very large normal stiness. The shear stiness o the cohesive elements is deined using the local bond-slip model given by the authors (Eqs. 4 to 6), and no interaction is assumed between normal stresses (strains) and shear strains (stresses) o the cohesive elements. Beam element or the FRP strip Cohesive element or the interace P Figure 1. FE model o NSM CFRP strip-to-concrete bonded joint Table 1. Eect o bond length on bond strength Bond strength (kn) Bond length (mm) C20-t2-h10-E150 C20-t2-h20-E150 C20-t2-h30-E150 C30-t2-h10-E150 C30-t2-h20-E150 C30-t2-h30-E150 C40-t2-h10-E150 C40-t2-h20-E150 C40-t2-h30-E Note: The name o each numerical case starts with a letter C and a two-digit number to represent the concrete strength ( c ), ollowed by a letter t and a one-digit number to represent the thickness o the CFRP strip (t ), a letter h and a two-digit number to represent the height o the CFRP strip (h ), and a letter E and a three-digit number to represent the elastic modulus o CFRP (E ). It has been shown (Zhang et al. 2013) that the groove height-to-width ratio and the concrete strength are the two main parameters or the bond behavior o NSM CFRP strips embedded in concrete, so these two parameters were chosen as the two main variables in the present parametric study. Three values were considered or both the concrete strength (i.e. 20 MPa, 30 MPa and 40 MPa) and the groove height-to-width ratio (i.e. 2.33, 4.00 and 5.67) in the parametric study, leading to nine 3

6 combinations. For each combination, 12 cases were analyzed with the only dierence between them being the bond length which varies rom 25 mm to 300 mm at an interval o 25 mm; or 4 o the 9 combinations, one additional case with a bond length o 350 mm was included to obtain converged results. The details and results o all the numerical cases are summarized in Table 1. Following u et al. s (2005), the eective bond length is deined as the bond length over which the interacial shear stresses oer a total resistance o 99% o the bond strength (i.e. ultimate load) or a joint with an ininite bond length. Then the eective bond lengths or all the 9 combinations can be obtained rom the results summarized in Table 1, through interpolations or values between the discrete data points. Finally a relationship between the eective bond length e and the parameter was ound: 1.66 e (7) 2 2 C ailure max 2G E A (8) REDUCTION FACTOR β By plotting all the results o the parametric study (Table 1), the relationship between and b was obtained. It was ound that this relationship is unique despite the dierent concrete strengths and groove height-to-width ratios. Based on the data in Table 1, the ollowing equation is proposed as a best-it expression or : 99% b b ( ) (9) VERIFICATION OF THE PROPOSED BOND STRENGTH MODE e The predictions o the proposed bond strength model are compared with the results o 51 test specimens collected rom 7 existing studies as well as the predictions o the only existing bond strength model or such joints proposed by Seracino and his co-workers (Seracino et al. 2007; Oehlers et al. 2008; Rashid et al. 2008) (reerred to as Seracino et al. s model hereater). The bond strengths predicted by the proposed model (i.e. Eqs. 1 and 2) and those predicted by Seracino et al. s model are compared with all the 51 collected test results in Figure 2. In general, both models provide reasonably accurate predictions o the test results, but the proposed model shows superior perormance in terms o all the three statistical characteristics (i.e. the average, standard deviation (STD) and coeicient o variation (CoV)). Figure 3 shows a comparison between the predictions o the two models and the test results o specimens with a suicient bond length (32 tests in total). For these tests, the reduction actors predicted by the two models are both equal to 1.0 (i.e. no reduction). In this comparison, the perormance o the two models depends solely on their respective interacial racture energy values. The comparison indicates that both models perorm very well: Seracino et al. s model has a slightly better average value, but the proposed model has slightly better values or the standard deviation and the coeicient o variation. The good average value o Seracino et al. s model is as expected as among the 32 tests used in this comparison, 13 tests were used in the regression analysis o test results by Seracino et al. in developing their model. e e 4

7 Average=0.924 STD=0.109 CoV=0.118 Average=0.885 STD=0.196 CoV=0.222 h g / w g < and 5.67 >5.67 Prediction o the proposed model Prediction o Seracino et al.'s model (kn) (a) between tests and the proposed model (b) between tests and Seracino et al. s model Figure 2. Bond strength comparison or all collected specimens Average=0.913 STD=0.116 CoV=0.127 Average=0.984 STD=0.128 CoV=0.130 h g / w g 2.33 and 5.67 >5.67 Prediction o the proposed model (kn) Prediction o Seracino et al.'s model (kn) (a) between tests and the proposed model (b) between tests and Seracino et al. s model Figure 3. Bond strength comparison or specimens with a suicient bond length Average=1.003 STD=93 CoV=93 Average=0.539 STD=91 CoV=0.168 h g / w g < and 5.67 >5.67 Prediction o the proposed model (kn) Prediction o Seracino et al.'s model (kn) (a) between tests and the proposed model (b) between tests and Seracino et al. s model Figure 4. Bond strength comparison or <1.0 5

8 Figure 4 compares the predictions o the two models and the test results o those specimens with a bond length shorter than that calculated by Eq. 7 (8 tests in total). For these tests, the reduction actors predicted by the two models are both less than 1.0. It is evident rom Figure 4 that the proposed model perorms very well and much better than Seracino et al. s model which signiicantly underestimates the test results. Given that the racture energy in Seracino et al. s model provides reasonably accurate predictions o test results, it can be concluded that the errors seen in Figure 4 arise mainly rom the equation or the eective bond length and the reduction actor. CONCUSIONS A bond strength model or NSM CFRP strip-to-concrete interaces has been presented in this paper. The predictions o the proposed model were compared with the results o 51 test specimens collected rom 7 existing studies as well as the predictions o the only existing bond strength model. These comparisons indicated that or cases o insuicient bond lengths, the proposed model provide more accurate predictions or the eective bond length equation and the bond strength reduction actor. The proposed bond strength model, due to its accuracy, simplicity and applicability to a wide range o realistic cases, can be readily incorporated in design codes and guidelines. ACKNOWEDGMENTS The authors are grateul or the inancial support received rom the National Basic Research Program o China ( 973 Program) (Project No.: 2012CB026201) and or the Niche Area Funding Scheme o The Hong Kong Polytechnic University. REFERENCES De orenzis,. and Teng, J.G. (2007) Near-surace mounted FRP reinorcement: an emerging technique or strengthening structures, Composites-Part B: Engineering, Vol. 38, No. 2, PP El Hacha, R. and Rizkalla, S.H. (2004) Near-surace-mounted iber-reinorced polymer reinorcements or lexural strengthening o concrete structures, ACI Structural Journal, Vol. 101, No. 5, pp u, X.Z., Teng, J.G., Ye,.P. and Jiang, J.J. (2005) Bond-slip models or FRP sheets/plates bonded to concrete, Engineering Structures, Vol. 27, No. 6, pp Oehlers, D.J., Haskett, M., Wu, C.Q. and Seracino, R. (2008) Embedding NSM FRP plates or improved IC debonding resistance, Journal o Composites or Construction, ASCE, Vol. 12, No. 6, pp Rashid, R., Oehlers, D.J., and Seracino, R. (2008) IC debonding o FRP NSM and EB retroitted concrete: plate and cover interaction tests, Journal o Composites or Construction, ASCE, Vol. 12, No. 2, pp Seracino, R., Saiulnaz, M.R.R., and Oehlers, D.J. (2007) Generic debonding resistance o EB and NSM plate-to-concrete joints, Journal o Composites or Construction, ASCE, Vol. 11, No. 1, pp Yuan, H., Teng, J.G., Seracino, R., Wu, Z.S., and Yao, J. (2004) Full-range behavior o FRP-toconcrete bonded joints, Engineering Structures, Vol. 26, No. 5, pp Zhang, S.S., Teng, J.G., Yu, T. (2013) Bond-slip model or CFRP strips near-surace mounted to concrete, Engineering Structures, Vol. 56, pp Zhang, S.S., Teng, J.G., Yu, T. (2014) Bond strength model or CFRP strips near-surace mounted to concrete, Journal o Composites or Construction, ASCE, Vol. 18, No. 3, A