BOND MODELLING OF FRP REBARS IN FRC

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1 BOND MODELLING OF FRP REBARS IN FRC S. Sólyom, G. L. Balázs and S. G. Nehme Department of Construction Materials and Technologies, Budapest University of Technology and Economics, Muegyetem rkp.3, H-1111 Budapest, Hungary ABSTRACT Existing bond models for FRP reinforcements are based on limited number of parameters only. Present study includes the description of an extensive experimental work, as well as an analytical part for modelling the bond behaviour of FRP rebars in plain and in fibre-reinforced concrete. Experimental parameters consisted of: (1) type of FRP rebar (carbon, glass or basalt fibres) including different surface characteristics and moduli of elasticity, (2) concrete strength and type (four different grades of conventional concrete and two SCC mixes), (3) type of fibres in concrete mixes (no fibres, steel, synthetic micro or synthetic macro polymer fibres), resulting 216 pull-out tests and 144 additional material property tests (concrete compressive strength tests and concrete splitting tensile strength tests). All bond test results were analysed for possible ways to refine bond modelling. Conclusions are drawn on bond behaviour and modelling by using the studied parameters. KEYWORDS FRP, carbon, glass, basalt, FRC, short fibres, bond behaviour, modelling of bond. INTRODUCTION Mild steel rebars have been extensively used for reinforced concrete structures as internal reinforcements owing to their numerous advantageous properties such as: ductility, high tensile strength, bendability etc. However, they present some disadvantages too being the most significant one is that steel reinforcement is susceptible to corrosion. One of the main advantages of Fibre Reinforced Polymer (FRP) reinforcing materials is their excellent corrosion resistance (Baena et al and Borosnyói 2014). Numerous studies can be found in literature underlining the significance of the bond behaviour of reinforcement in concrete. Yet, in case of FRP rebars there are still open issues owing to the high number of parameters which affect the bond. During the last decades, numerous experimental studies have been conducted to investigate the bond strength of FRP rebars in concrete. The studies also investigated the influence of the parameters, such as rebar diameter, concrete strength, concrete cover, embedment length, surface treatment, etc. on the bond characteristics of FRP rebars (Achillides and Pilakoutas 2004, Baena et al and Haffke et al. 2015). On the other hand, less attention was payed to determine analytically the bond stress-slip constitutive law for FRP rebars, which is essential for finite element analysis of FRP reinforced concrete structures. Therefore, only few FRP bond stress-slip models have been reported so far, and the suitability and capability of these models for numerical modelling of FRP reinforced concrete structures have not been confirmed yet. One of the numerous bond affecting parameters that still calls for further investigation is the effect of short fibres, mixed into the concrete matrix. Mixing short fibres into the concrete matrix represents a possible way to confine the concrete in compression zone and provide it with additional strain capacity. The placement of fibre reinforced concrete can be difficult due to clumping of the fibres. Moreover the use of poke vibrator can be difficult as well. These problems may be overcome by using self-compacting concrete which contains short fibres (Sólyom and Balázs 2016). Available studies show that FRC has improved tensile strength, strain capacity and ductility over normal concrete (Czoboly and Balázs 2015). 136

2 Combining fibre reinforced self-compacting concrete with FRP reinforcing bars holds major benefits, because the additional strain capacity allows an increase in flexural strength over the equivalent conventionally reinforced beams (Ibell et al. 2009). An overview of the combined use of FRP rebars with FRC is provided in Sólyom et al. (2015) and will be briefly summarized herein. In the research by Belarbi and Wang (2004) Glass Fibre Reinforced Polymer (GFRP) and Carbon Fibre Reinforced Polymer (CFRP) rebars were used. GFRP rebars had 12.7 and 25.4 mm in diameter, surface wrapped with helical fibre strand and additionally sanded afterwards. CFRP rebars were 12.7 mm in diameter, with smooth surface. The fibres added to the concrete mix were commercially available polypropylene short fibres, with maximum length of 57 mm. From the experimental data (27 pull-out specimens) it was concluded that the addition of polypropylene fibres did not increase the bond strength, but larger slips were recorded. The large slip values made the bond behaviour more ductile and the failure mode changed from splitting to pull-out. This is in contradiction with the results reported by (Ding et al. 2014). The authors claim, that the addition of short fibres to concrete can enhance the bond strength of GFRP rebars and also reduces the slip corresponding to the bond strength. Others (Won et al. 2008) have found also that the addition of short fibres to concrete increases the bond strength of FRP bars. It can be concluded from the above review, that there are inconsistencies in the available results and further research is needed. In this paper two bond stress versus slip models will be assessed. They show how the parameters of the models change according to the studied bond influencing factors, namely: FRP fibre type and surface characteristics, short fibre type and concrete compressive strength. EXPERIMENTAL STUDIES The pull-out test is the most frequently chosen method for comparing the bond behaviour of different FRP rebars in various concrete compositions. This test is a powerful tool to study the effect of different parameters on bond strength, owing to its simplicity and ease of application. 150 mm cubic moulds were used to manufacture the pull-out specimens. The bars were vertically placed in the centre of the moulds with 5Ø bond length in the lower part of the moulds (Ø - rebar diameter). Following the pouring, the specimens were left in the moulds under laboratory ambient conditions for one day. Thereafter, the concrete cubes were demoulded, marked and placed under water for 6 days. After this period of time the concrete specimens were taken out of water and kept in laboratory air until testing (mixed curing was applied). The concrete pull-out specimens were placed into a metal frame and the FRP rebars were gripped by the test machine. This gripped portion was considered as the loaded end of the test specimen and the relative displacement between the FRP rebar and concrete was measured with three Linear Variable Differential Transducers (LVDT). At the other end, usually referred to as unloaded or free end, the slip was measured by one LVDT. Displacement controlled test was selected on the loading machine to capture post-peak behaviour. The load was applied to the rebar at a rate of 1 mm/min and measured with the electronic load cell of the testing equipment (Instron 600 kn). An automatic data acquisition system was used to record the data transmitted by LVDTs. Three nominally identical specimens for each configuration were tested. In order to study the influence of concrete strength on bond development between FRP bars and concrete six different concrete grades were used (mean values of concrete compressive strength ranging between 27.7 and 91.9 MPa measured on three 150 mm cubic specimens). Three different type of FRP bars were used, carbon, glass and basalt. CFRP and GFRP bars had similar surface characteristics (sand coated) and diameter (9.5 mm) as well. The Basalt Fibre Reinforced Polymer (BFRP) bars were 12.7 mm in diameter and had a surface with helical wrapping. Three different short fibres were added to the concrete matrix: steel, synthetic micro and synthetic macro polymer fibres. The symbols of concrete mixes consist of 3 characters, the first C or S stands for traditional and self-compacting concrete, respectively. The second character represents the concrete strength, the higher value of the character, the higher the concrete strength is. The third character mix refers to the type of short fibres in concrete, namely: 1 represents that no short fibres are mixed to concrete, while 2 refers to synthetic macro, 3 to steel and 4 to synthetic micro fibres. For example C11 represents traditional concrete prepared according to composition C1 without short fibres, while S23 stands for self-compacting concrete prepared according to composition S2 with steel fibres. 137

3 The main objective of this experimental series was to examine the effect of the short fibres if added to concrete matrix on the bond strength and bond behaviour of FRP bars. Nevertheless, other additional factors that need further research to better understand their final influence on bond behaviour were also considered. The parameters taken into consideration during this study are: short fibres, type of FRP rebar fibre (modulus of elasticity), rebar surface characteristics and concrete strength. RESULTS AND DISCUSSION The results of the experimental part of this study along with the conclusions is presented in the authors previous paper (Sólyom and Balázs 2016). In this paper authors focus on analytical modelling, more specifically on curve fitting parameters of two already available analytical models: modified BPE (mbpe) (Cosenza et al. 1997) and CMR (Cosenza et al. 1995) models. These models are discussed in some papers (e.g. Baena et al. 2009), therefore only the necessary equations will be presented herein. To perform numerical and analytical analysis of the behaviour of reinforced concrete members and structures including the interaction between concrete and reinforcement to determine for example the anchorage length, the crack width, etc. an analytical model of the bond stress - slip constitutive law is necessary. Mix α Table 6 Parameter fitting of modified BPE and CMR models mbpe (double branch) model 138 CMR model Ascending branch Descending branch Ascending branch α SD SE p p SD SE β SD s r s r C C C SD SE C C C C C C C C C C C C C S S S S S S S S Avg Table 1 shows the results of curve fitting procedure for the selected two analytical models in case of BFRP rebars. The accuracy of each analytical model was evaluated by calculating the discrepancy (standard error) between the experimental bond stress values (τ exp ) and the corresponding analytical predictions (τ) for each registered slip value (s). Results, calculated by equation (Eq. 1), are presented in columns: 5, 9 and 16:

4 SE = 139 n (τ i τ exp,i ) 2 i=1 n where n is the number of experimental values registered during the pull-out test in case of one specimen (ascending or descending branch), while τ is calculated as a dependant of the assessed analytical model. In case of the Double Branch (mbpe) model (Cosenza et al. 1997) the equations for calculating the bond stresses are as follows (Eqs 2 and 3): ascending branch (0 s s m, where s m is the slip corresponding to the maximum bond stress, τ max ) descending branch (s m s) τ = τ max ( s s m ) α (2) τ = τ max [1 p ( s 1)] (3) s m where α and p are parameters to be calibrated on the basis of curve fitting of the experimental data. the third branch, where the bond stress is constant, it is not taken into consideration because it was not observed on the experimental bond stress vs. slip diagrams (most probably due to the limitation of LVDTs). In case of CMR model (Cosenza et al. 1995) the bond stress can be calculated by using Eq. 4: τ = τ max [1 e ( s sr ) ] where β and s r are parameters to be calibrated based on curve fitting of the experimental data. In case of each concrete mix three specimens were tested, their mean obtained test data is presented in Table 1, columns: 2, 6, 10 and 13 (defined by curve fitting of the analytical model). In column 4 the standard deviation of the three values (in case of parameter α) are calculated using Eq. 5. SD = n (α α avg) 2 i=1 (5) n 1 Similarly, the same equation (Eq. 5) can be used to calculate the values in columns 8, 12 and 15, only the studied parameter needs to be changed accordingly. To be able to study the effect of concrete strength on the curve fitting parameters, the mean values of each concrete composition are defined in columns 3, 7, 11 and 14. Similarly to Table 1, tables for GFRP and CFRP rebars were prepared. However, because of the limitation of the paper they are not presented herein, only a part of the results is given in Figure 1. Analysing Table 1 and Figure 1, the following observations can be made if attention is given to BFRP rebar, which is relatively new on the market. mbpe model (ascending branch), parameter α: slightly increases with the addition of short fibres, which in turn represents a less stiff bond stress vs. slip diagram. As α increases the ascending part of the diagram is approaching to a linear function. the highest increase is in case of synthetic micro and steel fibres, Table 1, column 2. slightly increases with the increase of the strength of concrete, which in turn represents a less stiff bond stress vs. slip diagram, Table 1, column 3. mbpe model (descending branch), parameter p: increases with the increase of the concrete strength, which represents a steeper descending branch of bond stress vs. slip diagram, Table 1, column 7. no clear tendency for the effect of short fibres can be observed, Table 1, column 6. CMR model (ascending branch), parameter β: slightly increases with the increase of the strength of concrete (C0 and S2 are exceptions), which represents a negligible stiffness increase of the ascending branch of bond stress vs. slip diagram, Table 1, column 11. no clear tendency for the effect of short fibres can be observed, Table 1, column 10. CMR model (ascending branch), parameter s r : increases with the increase of the strength of concrete, which represents a less stiff ascending branch of bond stress vs. slip diagram, Table 1, column 14. decreases with the addition of short fibres, which represents a stiffer ascending branch of bond stress vs. slip diagram. The highest decrease is achieved with synthetic macro fibres, Table 1, column 13. Since the scatter of these parameters in case of BFRP rebars is relatively low, generally valid parameters for this specific BFRP rebar can be estimated as the resultant of these values, which are presented in the last row of Table 1: α=, p=0.30, β=0.49 and s r =1.81. However, if we do not take into consideration the values of C0 concrete β (1) (4)

5 Bond stress (MPa) Bond stress (MPa) α (-) composition, which seem to be outsider, they become: α=0.29, p=0.32, β=0.41 and s r =2.12. These values offer a satisfactory agreement with the results reported by (Baena et al. 2009) α=0.18 to 0.23, β=0.36 to 0.84 and s r =0.04 to 0.06 for GFRP rebar with similar diameter and slightly lower modulus of elasticity. The surface treatment consisted of helical wrapping and sand coating. The significant difference in s r can be explained by the fact that different slip values (at loaded and unloaded end, respectively) were considered in different studies. Slightly different results were reported by (Yoo et al. 2015) for these parameters: α=0.18, β=0.49 and s r =0.22 for a GFRP rebar with similar diameter and helically wrapped surface. Analysing Figure 1 the following observations can be made. Values of parameter α in case of CFRP rebars (filled symbols) are the highest which leads to the conclusion that the higher the modulus of elasticity, the higher the parameter α is, since CFRP and GFRP have similar surface characteristics and diameters. Furthermore, parameter α is higher in case of rebars with sand coated surface treatment when compared to rebars with helically wrapped surface. BFRP and GFRP rebars have similar modulus of elasticity and diameters, however α parameters are lower in case of BFRP rebars (unfilled symbols) than in case of GFRP rebars α-c-re α-c-ma α-c-st α-c-mi α-g-re α-g-ma α-g-st α-g-mi α-b-re α-b-ma α-b-st α-b-mi CFRP GFRP BFRP Concrete compressive strength (MPa) Figure 1 Parameter α for mbpe model (each symbol represents an average value of three specimens). C, G and B stand for CFRP, GFRP and BFRP, respectively. Re reference concrete (no short fibre) and concrete with synthetic macro (Ma), steel (St) or synthetic micro (Mi) fibres C13 concrete mix Modified BPE C13 CMR-C Loaded-end slip (mm) C14 concrete mix 4 Modified BPE C14 2 CMR-C Loaded-end slip (mm) Figure 2 Experimental and analytical bond stress vs. slip diagrams, C13 and C14 concrete mixes In Figure 2 comparisons between experimental and analytical results are presented. It is straightforward that both mbpe and CMR models are in good agreement with the experimental results (Figure 2, left), however CMR have higher accuracy (see also Table 1, columns 5 and 16). Nevertheless, in Figure 2, just where the local maximum point in the bond stress vs. slip diagram is more accentuated, the similarity between experimental and analytical results is smaller, though it is still acceptable. At low slip values, the analytical models slightly overestimate the bond stresses, while as the slip increases this tendency changes. It needs to be highlighted, even though there are 140

6 only two of the numerous studied bond slip laws are presented here, the rest of the results are similar to the ones presented in Figure 2. The local maximum points are fairly typical for this type of helically wrapped BFRP rebars. More details on this phenomenon and the possible reasons of it can be found in the authors previous paper (Sólyom and Balázs 2016). Finally, please note that, the experimental values were taken for τ max and s m, alternatively these parameters could have been estimated by formulae available in literature. Table 2 Values for parameters defining the relation between α and concrete compressive strength a b CFRP GFRP BFRP Studying the relationship between the concrete compressive strength and parameter α it was concluded that it can be best described with a polynomial function (Eq. 6): α = af c 2 + bf c (6) where a and b are parameters which need to be defined by a curve fitting procedure (Table 2). It is worth mentioning, that the values in Table 2 are based on a limited number of specimens and further data is needed for the validation of this equation and these parameters. CONCLUSIONS In the present paper the interfacial bond behaviour between three types of FRP rebars (Glass, Basalt and Carbon fibres) in six different concrete compositions have been analysed. Since the experimental results have been presented in a previous paper, here the focus was given to analytical modelling, more specifically parameter fitting of two already available analytical models, namely: modified BPE and CMR models. Results of 24 mixes (216 pull-out tests) were considered. Studied parameters: concrete compressive strength, FRP fibre type and surface characteristics and short fibre type. Based on the results of the presented study the conclusions can be presented as follows. Parameter α (mbpe model) increases with the increase of the compressive strength of concrete in case of all the investigated FRP rebars (basalt, carbon and glass). However the effect of short fibres is not straightforward, but it seems that α increases with the addition of short fibres, which in turn provides a less stiff bond stress vs. slip diagram. Values of α are higher in case of sand coated FRP rebars when compared to helically wrapped surface. Furthermore, increase of modulus of elasticity of FRP rebars results in an increase in the values of parameter α. Similarly, parameter β (CMR model) slightly increases with the increase of the compressive strength of concrete in case of all investigated FRP rebars. Values of β are higher for sand coated rebars when compared to helically wrapped ones. Moreover, increase of modulus of elasticity of FRP rebars results in an increase of the values of β. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of European Network for Durable Reinforcement and Rehabilitation Solutions (endure) by the Marie Curie, Seventh Framework Programme (Grant: PITN-GA ). REFERENCES Achillides, Z. and Pilakoutas, K. (2004). "Bond behavior of FRP Bars under direct pullout conditions", Journal of Composites for Construction, 8(2), Baena, M., Torres, L., Turon, A. and Barris, C. (2009). "Experimental study of bond behaviour between concrete and FRP bars using a pull-out test", Composites Part B: Engineering, 40(8), Belarbi, A. and Wang, H. (2004). "Bond-slip response of FRP reinforcing bars in fiber rein-forced concrete under direct pullout", In Int. Conf. on Fiber Comp., High Perform. Conc. and Smart Mat. Chennai, India. Borosnyói, A. (2014). "Use of corrosion resistant Fibre Reinforced Polymer (FRP) reinforcements for the substitution of steel bars in concrete", Korróziósfigyelő, 54(1), (in Hungarian) Cosenza, E., Manfredi, G. and Realfonzo, R. (1995). "Analytical modelling of bond between FRP reinforcing bars and concrete", In L. Taerwe (Ed.), Nonmettalic (FRP) reinforcement for concrete struc.(pp ). Cosenza, E., Manfredi, G. and Realfonzo, R. (1997). "Behavior and Modeling of Bond of FRP Rebars to 141

7 Concrete", J. of Comp. for Cost., (May), Czoboly, O. and Balázs, G. L. (2015). "Can too long mixing time negatively influence properties of FRC?", In 11th CCC Conference. Hainburg, Austria. Ding, Y., Ning, X., Zhang, Y., Pacheco-Torgal, F. and Aguiar, J. B. (2014). "Fibres for enhancing of the bond capacity between GFRP rebar and concrete", Construction and Building Materials, 51, Haffke, M. M., Veljkovic, A., Carvelli, V. and Pahn, M. (2015). "Experimental investigation of the static bond of GFRP rebar and concrete", In SMAR The Third Conference on Smart Monitoring, Assessment and Rehabilitation of Structures (pp. 1 8). Ibell, T., Darby, A. and Denton, S. (2009). "Research issues related to the appropriate use of FRP in concrete structures", Construction and Building Materials, 23(4), Sólyom, S., and Balázs, G. L. (2016). "Influence of FRC on bond characteristics of FRP reinforcement", In 11th fib International PhD Symp. in Civil Engineering - paper accepted (pp. 1 8). Tokyo: Accepted for publ. Sólyom, S., Balázs, G. L. and Borosnyói, A. (2015). "Bond behaviour of FRP rebars parameter study", In SMAR The Third Conf.on Smart Monitoring, Assessment and Rehabilitation of Structures (pp. 1 8). Won, J. P., Park, C. G., Kim, H. H., Lee, S. W. and Jang, C. I. (2008). "Effect of fibers on the bonds between FRP reinforcing bars and high-strength concrete", Composites Part B: Engineering, 39(5), Yoo, D. Y., Kwon, K. Y., Park, J. J. and Yoon, Y. S. (2015)."Local bond-slip response of GFRP rebar in ultrahigh-performance fiber-reinforced concrete", Composite Structures, 120,

1. INTRODUCTION. (a) Sand/ Fabric-coated (b) Sand-coated deformed. (c) Helical wrapped/ribbed Fig.1 FRP anchors with different outer surfaces

1. INTRODUCTION. (a) Sand/ Fabric-coated (b) Sand-coated deformed. (c) Helical wrapped/ribbed Fig.1 FRP anchors with different outer surfaces S2B3 Interface Bond Strength of Helical Wrapped GFRP Ground Anchors Weichen Xue Prof., Department of Building Engineering, Tongji University, Shanghai, China Yuan Tan PhD candidate, Department of Building