Study On The Bond-slip Mechanism Of CFRP-concrete Interface

The outstanding characteristics of fiber reinforced polymer (FRP) are their high strength, good corrosion resistance, fatigue resistance, etc. which make FRP reinforced concrete strutures be widely applied in the field of civil engineering. The reinforce effect and failure mechanism of FRP reinforced structures have the close relationship with FRP-concrete interface, Therefore, the mechanical behaviors of FRP-concrete interface are the most basic and critical issues for FRP reinforced technique. The study here focuses on the bond-slip mechanism of CFL-concrete interface under static load and fatigue load, based on the carbon fiber laminate(CFL) invented by our research group, and this research provides the experimental and theoretical basis for the design and research of CFL reinforced concrete strutures. The conclusions are summarized as followed:(1) The damage mechanism, transmission of interfacial stress and evolution of interfacial slip are discussed here by the results of static experiments of CFL-concrete interface and the analysis of damage process and failure modes. The bond-slip constitutive relationship of CFL-concrete interface based on Popovics model, hyperbolic model, bilinear model and trilinear model are introduced, and thereafter the formula for characteristic parameters of curves are also given here.(2) By the analysis of the bonding mechanism of interface, the failure mechanism for bond-slip of CFL-concrete interface is described here from the perspective of damage. The damage variables and threshold are given for describing the damage process of interface and damage-slip models for bond strengthen and bonded stiffness are put forward based on the bond-slip constitutive relations of CFL-concrete interface.(3) The bi-shear analysis model for CFL-concrete interface is constructed based on the elastic theory of elasticity. Comparing the results calculated by linear theory of elasticity with the experimental results, it is found that the end stress is much larger than the interfacial bond strength and the transmission region of interfacial stress is much less than the region from tests. Therefore, the stress concentration of interface calculated by the linear theory of elasticity is greatly exaggerated.(4) The bond-slip constitutive relationship of concrete interface based on the bi-shear analysis model is introduced here. Note that the interface is composed of bond area, damage area and debonding area in general. Four stages are experienced through the process of interfacial bond failure: a) elastic stage, b) elastic-damage stage, c) elastic-damage-debonding stage, d) damage-debonding stage. The evolution of the interfacial stress is deduced by the model and the effect of the different material parameters on interfacial stress is estimated here.(5) Through the fatigue experiment of CFL-concrete interface, the fatigue life of interface decrease with the increasing amplitude of fatigue load. TheΔP~N andΔσ~N curve are derived from this experiment. There is a threshold, sd, for fatigue damage in CFL-concrete interface: when s < sd, the fatigue damage of interface have D→0; When the fatigue load Pmax >Ps (Ps is the load correspond to sd), the fatigue damage will occur in the interface.(6) The strain evolution of CFL presents three developing phases under fatigue load, The debonding of CFL-concrete experiences three stages: concrete crack, stable interfacial debonding, and unstable debonding. The relative slip of specimens shows three developing phases: initial rapid development, stable development, and unstable development. The stable development accounts for more than 99% of whole fatigue life. An approximate linear relationship is found between the relative slip of specimen and the ratio of life cycle to fatigue life. According to these features, we developed a three-phase fatigue damage model based on a stiffness coefficient Kf to describe the fatigue damage process on the CFL-concrete interface.(7) Based on the integrate analysis on fatigue failure process of CFL-concrete interface under fatigue load, we put forward a bond-slip analysis model on the CFL-concrete interface. We presented the evolvement of interfacial bond strength and stiffness Bn with cycle number. Thereafter we developed a CFL-concrete interfacial bond strength and stiffness damage model under fatigue load.