New Design of Beam FRP Reinforcement for Fire Performance

Abstract

More widespread use of fiber reinforced polymer (FRP) as reinforcement for concrete is hampered by a lack of proven performance in fire. The performance of the FRP reinforcement at elevated temperatures is dominated by the polymer resin that is used as the FRP’s matrix and forms its outer surface. The resin softens at relatively low temperatures – the glass transition temperature of the polymer is typically in the range 60-180°C – and whilst this depends on the resin and cure conditions used, these temperatures will almost certainly be exceeded within a concrete element during a fire. As a consequence, bond will be lost between the reinforcement and the concrete. This paper investigates a new arrangement of internal FRP reinforcement that does not rely only upon bond for load transfer. Instead of using straight, separate, FRP bars as reinforcement, closed FRP loops are used as the longitudinal reinforcement. When the interlock, adhesive, and friction mechanisms of bond force transfer are lost due to softening of the resin, tensile forces in the reinforcement (FRP fibers) can still be carried by the loops. The paper describes the results from an initial series of 4-point bending tests on beams subjected to heating using gas radiant panels under sustained applied load. The tests demonstrate the initial potential of the FRP loop reinforcement system compared to similar FRP-reinforced specimens containing straight bars; however, the performance of the loop-reinforced system was limited by concrete shear failure in the lap area, indicating that a better understanding of the lap length requirements is needed