Bond-slip behavior between carbon fiber reinforced polymer sheets and heat-damaged concrete

Abstract

Exposure of concrete to high temperatures results in its deterioration because of the inhomogeneous volume changes of concrete’s ingredients, generation of vapor pressure, and decomposition of cement hydration products beyond a temperature of 550°C. Regaining the structural capacity of severely heat-damaged structural members would be most efficiently achieved using high-strength composite materials such as FRP sheets or plates; especially for flexural elements such as beams and slabs. Therefore, an experimental study was undertaken to investigate the effect of exposing lightweight and normal weight concrete (LWAC and NWAC) to elevated temperatures on their bond behavior with carbon fiber reinforced polymer (CFRP) sheets. Near-end supported and double shear specimens were used for bond evaluation considering key parameters such as concrete grade, and bond area of the CFRP sheets. The significant reductions in residual compressive and splitting strengths at temperatures greater than 400°C was reflected negatively on bond behavior for the various specimens; represented in a reduction in bond strength to 64%, and an increase in slip at failure reaching as high as 254% of control values, respectively. LWAC specimens showed higher residuals for bond strength and slip at failure yet lower peeled-off concrete thickness and width than those of NWAC of similar strength for temperatures greater than 400°C; and regardless of CFRP sheet to concrete width ratio. Use of CFRP at smaller sheet bond area imparted better residual bond strength with NWAC than with LWAC of similar strength grade, and subjected to similar elevated temperatures. Bond failure was categorized by skin peeling for temperatures less than 300°C, yet increased with higher temperatures to much higher concrete thicknesses. Finally, the transfer of tensile stresses from highly loaded concrete elements to the FRP composites may be affected by post-heating damage hence should be accounted for when designing for repair. This can be achieved by the aid of empirical models that are developed in terms in key factors. In this paper, the results from an experimental study are used to develop an analytical model to predict bond strength and slippage between heat-damaged concrete and CFRP in terms of geometric, and material properties. The model was validated against a wide range of data from published literature along with various existing models. Validation indicated superior predictability of present model; hence is recommended for future use for repair design of heat-damaged concrete members.