Studies On FRP-Concrete Interface

External bonding of fiber reinforced polymer (FRP) plates or sheets has emerged as a popular method for the strengthening of reinforced concrete (RC) structures. In this strengthening method, the performance of the FRP-to-concrete interface in providing an effective stress transfer is of crucial importance. Indeed, a number of failure modes in FRP-strengthened RC members are directly caused by debonding of the FRP from the concrete. Therefore, for the safe and economic design of externally bonded FRP systems, a sound understanding of the behavior of the FRP-to-concrete interface needs to be developed. In this thesis, the debonding failure behavior of three major FRP-concrete systems is investigated in detail: debonding in direct-pull tests, debonding in flexurally-strengthened beams and debonding in shear-strengthened beams. For each case, a systematic study is presented, which includes the numerical modeling of the FRP-strengthened structure and the detailed clarification of the debonding failure mechanism. Furthermore, the numerical results are verified and combined with available experimental results to establish interfacial bond-slip models and design models for debonding failures in flexurally-and shear-strengthened RC members. The proposed design models represent substantial improvements to existing models as evidenced through comparisons with existing test data. The major contributions of the work presented in this thesis are listed as follows: (1) A finite element (FE) method for the simulation of direct-pull tests based on the non-coaxial rotating angel crack model (RACM) is first presented. FE results obtained using this method provides a preliminary clarification of the debonding process and mechanism. Next, a meso-scale FE model for debonding simulation is presented, which is capable of producing more reliable and detailed numerical results. The interfacial behavior is discussed in detail based on these meso-scale FE element results. In addition, new bond-slip models and bond strength models are proposed based on results from a numerical parametric study. The predictions of the proposed models are shown to be in close agreement with available test results. (2) An interfacial bond-slip model for use in the FE simulation of debonding failures in flexurally-strengthened RC beams (referred to as flexural debonding for brevity) developed from meso-scale FE results is described. A composite crack model based on the meshless method is also proposed in which the conventional smeared crack approach and the discrete crack approach for concrete are combined. Results from the composite crack model provides the basis of a dual debonding criterion for flexural debonding failures. (3) With the new bond-slip model and the novel dual debonding criterion, an interface element for flexural debonding failures is proposed so that flexural debonding can be accurately predicted with the conventional FE approach for concrete. The numerical predictions are compared with the results of 45 test beams and a good agreement is observed. Furthermore, based on the interfacial bond stress distribution from the FE model, and with some simplifications, a design model for flexural debonding is proposed. The predictions of the proposed design model agree well with the test results. (4) Based on both test results and numerical results, the relationship between the stress distribution in the FRP strips for shear strengthening of RC beams and the variation of the width of the critical shear crack is examined. Three typical width variations of the critical shear crack are then proposed, leading to eight simplified computational models for debonding failures in shear-strengthened concrete beams for different strengthening schemes. The debonding mechanism in shear-strengthened RC beams is discussed and a design model for shear strengthening is proposed based on the lower bound predictions of these eight computational models. This design model is compared and shown to agree well with the test results.