Thermal incompatibility between CFRP tendons and concrete

Results are presented from a comprehensive experimental study to assess the occurrence of heat-induced longitudinal splitting cracks in concrete specimens reinforced with carbon fiber-reinforced polymer (CFRP) or steel when exposed to severe heating from one side, as would likely occur during a fire in a building. Tests were performed on large- and medium-scale precast CFRP-reinforced or prestressed specimens. Large-scale specimens were tested in a standard fire-resistance test, while medium-scale specimens were tested using a novel Heat-Transfer Rate Inducing System (H-TRIS) that controls thermal exposure by imposing a time-history of incident heat flux at a specimen's exposed surface. The formation of thermally-induced longitudinal splitting cracks and failure of the concrete cover to provide sufficient confining action, and thus sufficient bond strength, is shown to be more likely for FRP-reinforced or prestressed concrete elements than for those reinforced or prestressed with steel. This appears to be at least partly due to thermomechanical incompatibility between CFRP reinforcement and concrete; the formation of heat-induced longitudinal splitting cracks is related to rapid thermal expansion of CFRP tendons relative to the surrounding concrete. Many aspects of bond performance at elevated temperature remain poorly understood, and these require additional investigation before FRP-reinforced or prestressed elements can be used in fire-rated applications with confidence.