Effect of Elevated Temperatures on the Mechanical Properties and Relaxation of CFRP Strands

Abstract

Although prestressing carbon fiber–reinforced polymer (CFRP) strands outperform steel strands on different levels, such as strength and durability, their performance under elevated temperatures remains a susceptible design issue that requires careful evaluation. Moderate increase in the temperature of prestressing CFRP strands takes place during construction due to concrete curing. CFRP strands can also experience increase in temperature if the CFRP-prestressed structural element is subjected to fire during service. This paper addresses the effect of increasing the temperature on the strength of prestressing CFRP strands as well as the level of prestressing force. Two sets of CFRP strand specimens with two different diameters were prepared and evaluated for strength degradation triggered by the increase in temperature to 350°C (662°F). Two more sets of prestressed CFRP strands were evaluated for prestress loss due to increase in temperature to 204°C (400°F). The prestress loss due to temperature increase was verified by constructing and monitoring half-scale decked bulb T-beams prestressed with CFRP strands. Test results showed that tensile strength of CFRP specimens decreased with the increase in temperature. In addition, first heating cycle of prestressed CFRP strands led to a slight permanent strand relaxation and a corresponding prestress loss. Subsequent cycles of heating and cooling did not seem to generate additional relaxation of the strands as long as the temperature of the first cycle was not exceeded. Furthermore, CFRP specimens subjected to heating and cooling cycles showed no reduction in the strength when tested at ambient conditions afterwards.