Influence of elevated temperatures on the bond behaviour of ribbed GFRP bars in concrete

Abstract FRP-reinforced concrete (RC) structures operating for several years in chemically aggressive environments have confirmed the good mechanical performance and durability of FRP reinforcement. However, the concerns and lack of knowledge about fire behaviour is still hampering its use in buildings; in fact, even moderately elevated temperatures are reported to cause premature loss of bond in overlapping rebars, potentially affecting the structural safety of load bearing members exposed to fire. This paper presents experimental and analytical investigations on the bond behaviour between glass fibre reinforced polymer (GFRP) rebars and concrete at moderately elevated temperatures. Steady-state tensile and pull-out tests were performed in two types of ribbed GFRP rebars, with different glass transition temperatures (Tg of 104 °C and 157 °C), from ambient temperature up to 300 °C. The results obtained were also compared with those of fibre wrapped-sand coated GFRP rebars (Tg of 98 °C) previously investigated by the authors. As expected, the tensile strength and elastic modulus of the ribbed GFRP rebars and especially the strength and stiffness of the GFRP-concrete interface were severely degraded with temperature. This study highlighted that the key parameters governing the bond to concrete at elevated temperatures are the surface finish and Tg of the rebars – at 100 °C, bond strength reductions between 20% and 34% were obtained for the ribbed rebars, while in the sand coated rebars such reduction had been 81%; most of the reduction of the GFRP-concrete interaction in the ribbed rebars occurred for temperatures above the rebars Tg, contrary to the sand coated rebars in which bond had been almost completely degraded below its Tg. Based on the test results, for each temperature, local bond stress vs. slip models were numerically calibrated for the specific ribbed GFRP rebars used in the experiments. These laws can be used in advanced numerical models to simulate the fire behaviour of GFRP-RC members.