The present paper deals with the finite element (FE) analysis of bond slip between concrete and carbon fiber reinforced polymer (CFRP) strips in a single pull-out test under static loads. The commercial software LS-DYNA is used to simulate the test set-up using a plastic damage material model and an elastic material model for the concrete prism and the unidirectional CFRP strip, respectively. The bond interface between the concrete and the CFRP strip is simulated following three different approaches using a perfect bond model, a cohesive bond model and contact algorithms based on recently developed proposed bond slip models. The numerical model is validated based on experimental test results available from literature. The debonding failure mode and the delamination loads of the CFRP strip are predicted. The numerical results show a good agreement with the experimental data using the cohesive bond model. The perfect bond model gives an overestimation of the delamination loads and of the damage distribution in the concrete prism

Belkassem, B.

Lecompte, D.

Maazoun, Azer

Matthys, Stijn

Vantomme, J.

Ghent University Academic Bibliography

* Corresponding author,  Ghent University, Magnel Laboratory for Concrete Research, Technologiepark-Zwijnaarde 904, 9052 Gent, Belgium Tel: +32 4 89 74 16 46, E-mail: maazounazer@yahoo.com  Numerical modelling of the debonding between CFRP strips and concrete in shear tests under static loads using different approaches  A. Maazoun1,2*, S. Matthys1, D. Lecompte2, B.Belkassem2, J. Vantomme2  1 Ghent University, Magnel Laboratory for Concrete Research, Technologiepark-Zwijnaarde 904, 9052 Gent, Belgium  2 Royal Military Academy,Civil and Materials Engineering Department, Avenue de la Renaissance 30, 1000 Brussels, Belgium   Abstract The present paper deals with the finite element (FE) analysis of bond slip between concrete and carbon fiber reinforced polymer (CFRP) strips in a single pull-out test under static loads. The commercial software LS-DYNA is used to simulate the test set-up using a plastic damage material model and an elastic material model for the concrete prism and the unidirectional CFRP strip, respectively. The bond interface between the concrete and the CFRP strip is simulated following three different approaches using a perfect bond model, a cohesive bond model and contact algorithms based on recently developed proposed bond slip models. The numerical model is validated based on experimental test results available from literature. The debonding failure mode and the delamination loads of the CFRP strip are predicted. The numerical results show a good agreement with the experimental data using the cohesive bond model. The perfect bond model gives an overestimation of the delamination loads and of the damage distribution in the concrete prism.     Keywords: Carbon fiber reinforced polymer; cohesive bond model; debonding issues; FE analysis    1. Introduction  Using CFRP strips as externally bonded reinforcement (EBR) in civil engineering has been demonstrated as a very efficient technique for strengthening reinforced concrete structures [1–7]. However, the prediction of the failure mode of the CFRP strips which often happens in a brittle and sudden manner is, still a challenge of this technique. Thus, it’s necessary to study the bond between the CFRP strip and the concrete using bond slip tests. Several bond slip models have been developed in the last few years [8–12]. Nomenclature  Pu  ultimate bond strength  Ep  the young’s modulus of the CFRP strip tp   the thickness of the CFRP strip bp  the width of the CFRP strip bL  the bonded length of the CFRP strip Le  the effective bond length Ec  the young’s modulus of the concrete bc  the width of the concrete prism    fc the compressive cylinder strength of the concrete fct  the tensile strength of the concrete Ga the shear modulus of the adhesive ta   the thickness of the adhesive K0 initial bond stiffness τmax  maximum shear strength at the interface, Gcr  fracture energy   In 1997, Maeda et al. [8] develop an empirical model based on experimental data to predict the ultimate bond strength (Pu) of the bond tests between concrete and CFRP strips, considering an effective bond length:                                                   𝑃𝑢 = (110.2 ∗ 10−6𝐸𝑝𝑡𝑝)𝐿𝑒𝑏𝑝                                                                               (1)   with  𝐿𝑒 =  𝑒6.13−0.580𝑙𝑛𝐸𝑝𝑡𝑝     In 1998, khalifa et al. [7] improve this model by including the concrete properties:  𝑃𝑢 = (110.2 ∗ 10−6 (𝑓′𝑐42)23  𝐸𝑝𝑡𝑝 )𝐿𝑒𝑏𝑝                                            (2) with  𝐿𝑒 =  𝑒6.13−0.580𝑙𝑛𝐸𝑝𝑡𝑝    Moreover, in 2001, Chen and Teng [8] found this model is invalid when L < Le and indicated that the width of the bonded strip to the concrete member is another parameter that affect the ultimate bond strength and that should be included in the bond slip model. They proposed a new model, as follows: 𝑃𝑢 = 0.427𝛽𝑤  𝛽𝐿 √𝑓′𝑐  𝑏𝑝 𝐿𝑒                                                                              (3) Where βL is a coefficient related to the real bond length L and βw is a size factor related to the width of the bonded strip to the concrete prism   In 2005, Lu et al. [9] proposed a new equation for implementation in FE models:   τmax = 1.5βwfct k0 =1.5βwfct0.0195βwfct Gcr = 0.308βw2√fct                                                             (4) However, the numerical model is setup, assuming a plane stress state using a 2D model and neglecting the out of plane mode of the local bond slip model. In 2011, Obaidat et al. [13] developed a 3D finite element model including the adhesive properties in the local bond model and they proposed the  following new formula:  τmax = 1.46Ga0.165fct1.033 k0 = 0.16Gata+ 0.47 Gcr = 0.52fct0.26Ga−0.23                                                                 (5) These previous studies show that some progress has been made to study the interface behavior between the concrete and the CFRP strip. However, the prediction of an accurate bond slip model is still in improvement due the complexity of the problem. This paper presents a 3D FE model of a bond slip test between a CFRP strip and the concrete. The debonding failure mode and the ultimate load are predicted and validated with experimental data from the literature. A comparison is made when implementing different approaches for the bond interaction is the 3D FE model.  2. Description of the experimental tests under static loads Experimental tests of single lap bond shear tests have been performed by Mazzotti et al. [14]. Eight prisms strengthened with a CFRP strip are tested. The concrete blocks are fixed by a steel frame to prevent vertical and horizontal displacement as shown in Figure 1. For more details on the experimental setup see reference [14].   Figure. 1. Experimental setup [14].  3. Numerical analysis  A numerical analysis is conducted using the finite element software LS-DYNA. The concrete and the CFRP strip are modeled using constant stress solid elements. A plastic damage model is used to model the concrete. This material model is a three invariant model that uses three shear failure surfaces and includes damage [15]. An elastic material model is used to model the unidirectional CFRP strip. To model the interface between the concrete and the CFRP strip, cohesive elements are used based on the local bond slip model proposed by Obaidat et al. [12] as shown in Figure 2.    Figure. 2. Local bond slip model  The cohesive model defines surfaces of separation and describes their interaction by defining a bilinear traction displacement softening law [16]. The advantage of using this cohesive model is that the damage of the interface is considered in three different modes; mode I (tensile), mode II (shear) or mode III (out of plane tearing) as shown in Figure 3.  Implementing the cohesive elements, for mode I a bilinear law in relation to the concrete tensile strength and fracture energy is considered with values……………… For the mode II law, the model of Figure 2 is considered with values…………. following [12].    Figure. 3. Mixed mode traction-separation law [16]  The initiation of debonding occurs when the normal and shear tractions attained their respective tensile or shear strength respectively and the interface stiffness is then gradually reduced to zero when the complete fracture occurs at δF [16].   A 3D finite element model of a bond shear test using cohesive elements to model the interface between the concrete and the CFRP strip is shown in Figure. 4.    Figure. 4. FE model of the pullout test    A convergence mesh study is carried out. The delamination load is predicted and compared.  It shows that the simulation converges with a mesh size of 10 mm as shown in Table 1.   Table 1. Convergence mesh study  4. Comparison of the FE models with the experimental data from the literature  During the experimental tests, a load cell is used to measure the applied force until the debonding of the CFRP strips. A comparison between the experimental data of the delamination loads and the numerical results for different bond lengths are shown in Table 2 and Figure 5. The delamination loads found by the numerical model are in a good agreement with the experimental data.  Table 2. Experimental and numerical delamination loads     Figure. 5. (a) Delamination loads vs. bonded length for bp = 50 mm; (b) Delamination loads vs. bonded length for bp = 80 mm  5. Several approaches to model the interface between concrete and CFRP strip To model the interface between the concrete and the CFRP strip, three approaches are used. As a first approach, a perfect bond is assumed between the concrete and the CFRP strip which means that the CFRP strip has perfect and permanent bond to concrete without any failure. For the second approach, the cohesive elements are used as discussed in section 3 based on the Lu et al. equation [11]. The final approach is using tiebreak contact based on the Cheng and Teng equation [10]. The command tiebreak contact allows the modeling of connections which transmits both normal and shear stresses with a failure criteria [16]. Failure of contact between the FRP composite and concrete surface occurs if:   (|𝜎𝑛|𝑁𝐹𝐿𝑆)2+ (|𝜎𝑠|𝑆𝐹𝐿𝑆)2≥ 1   This is a special contact option in which the variables NFLS and SFLS are the tensile strength of the concrete and ultimate bond strength based on the Cheng and Teng equation (see equation 3), respectively. σn and σs are the normal and shear stresses at the interface.   6. Debonding process under static loads  The debonding of the CFRP strip starts from the fixed end to the free end of the CFRP in a brittle and sudden manner. Table 3 summarizes the delamination loads and the failure modes predicted by the different approaches. The perfect bond algorithm cannot predict the debonding of the CFRP strip and should be used only in the ascending branch of the local bond slip model (elastic behavior without prediction of failure). Moreover, the perfect bond gives an overestimation of the delamination loads. The tiebreak contact calibrated with Chen and Teng formula (see equation 3) behaves as a perfect bond until the failure criteria is reached based on the shear and normal stresses implemented in the FE model. An accurate lower bond prediction of the failure load is obtained, the cohesive elements based on the fracture energy of the bond interface gives also an accurate prediction, especially with the bond model of Obaidat et al. [12].   Table 3. Comparison of the delamination loads using several bond slip models    7. Conclusions   This study presents a numerical investigation of the bond behavior between concrete and CFRP strips under shear tests. The numerical models are compared with experimental tests conducted by Mazzotti et al. [14]. The debonding failure mode and the delamination loads are predicted using different contact approaches. Using a perfect bond contact between concrete and CFRP gives an overestimation of the delamination loads and of the damage distribution in the concrete prism. This contact algorithm is not able to predict the debonding. Using cohesive elements to model the interface, the predicted delamination loads based on Lu et al. [11] formula is higher than the experimental data. This is due to the overestimation of the bond shear strength τmax based only on the concrete properties. However, the Obaidat et al. approach [13] is in good argument with experimental results in terms of ultimate load and failure mode. References  [1] Buyukozturk O, Gunes O, Karaca E. Progress on understanding debonding problems in reinforced concrete and steel members strengthened using FRP composites. Constructin and Building Materials 2004;18:9–19. doi:10.1016/S0950-0618(03)00094-1. [2] Mostofinejad D, Kashani AT. Experimental study on effect of EBR and EBROG methods on debonding of FRP sheets used for shear strengthening of RC beams. Composites Part B 2013;45:1704–13. [3] Maruccio C, Basilio I, Oliveira D V, Lourenço PB, Monti G. Numerical modelling and parametric analysis of bond strength of masonry members retrofitted with FRP. Construction and Building Materials 2014;73:713–27. doi:10.1016/j.conbuildmat.2014.09.082. [4] Li G, Zhang A, Jin W. Effect of Shear Resistance on Flexural Debonding Load-Carrying Capacity of RC Beams Strengthened with Externally Bonded FRP Composites. Polymers 2014:1366–80. doi:10.3390/polym6051366. [5] Bencardino F, Condello A, Ombres L. Numerical and analytical modeling of concrete beams with steel , FRP and hybrid FRP-steel reinforcements. COMPOSITE STRUCTURE 2016;140:53–65. doi:10.1016/j.compstruct.2015.12.045. [6] Maazoun A, Matthys S, Belkassem B, Lecompte D, Vantomme J. Experimental Analysis of CFRP Strengthened Reinforced Concrete Slabs Loaded by Two Independent Explosions †. Proceedings 2018:1–6. doi:10.3390/ICEM18-05317. [7] Maazoun A, Belkassem B, Reymen B, Matthys S, Vantomme J, Lecompte D. Blast response of RC slabs with externally bonded reinforcement: Experimental and analytical verification. Composite Structures 2018;200:246–57. doi:10.1016/j.compstruct.2018.05.102. [8] Maeda T, Asano Y, Ueda T, Kakuta Y. A Study on Bond Mechanism of Carbon Fiber Sheet. International symposium; 3rd, Non-metallic (FRP) reinforcement for concrete structures, Sapporo, Japan: 1997, p. 279–85. [9] Khalifa A, Gold WJ, Nanni A, M.I AA. Contribution of externally bonded FRP to shear capacity of RC flexural members. Journal of Composites for Construction 1998:195–202. [10] Chen J, Teng JG. Anchorage Strength Models for FRP and Steel Plates Bonded to Concrete A NCHORAGE S TRENGTH M ODELS FOR FRP AND S TEEL P LATES 2001;9445. doi:10.1061/(ASCE)0733-9445(2001)127. [11] Lu XZ, Teng JG, Ye LP, Jiang JJ. Bond – slip models for FRP sheets / plates bonded to concrete. Engineering Structures 2005;27:920–37. doi:10.1016/j.engstruct.2005.01.014. [12] Obaidat YT, Heyden S, Dahlblom O. The effect of CFRP and CFRP / concrete interface models when modelling retrofitted RC beams with FEM. Composite Structures 2010;92:1391–8. doi:10.1016/j.compstruct.2009.11.008. [13] Obaidat YT, Heyden S, Dahlblom O. Evaluation of Parameters of Bond Action between FRP and Concrete 2013:626–35. doi:10.1061/(ASCE)CC.1943-5614.0000378. [14] Mazzotti C, Savoia M, Ferracuti B. An experimental study on delamination of FRP plates bonded to concrete. Constructin and Building Materials 2008;22:1409–21. doi:10.1016/j.conbuildmat.2007.04.009. [15] Schwer LE, Malvar LJ. simplified cocrete modeling with *MAT_CONCRETE_DAMAGE_REL3. 2005. [16] LS-Dyna keyword user’s manuel. KEYWORD USER ’ S MANUAL VOLUME II. vol. II. 2016.  

Numerical modelling of the debonding between CFRP strips and concrete in shear tests under static loads using different approaches

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Numerical modelling of the debonding between CFRP strips and concrete in shear tests under static loads using different approaches

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