Creep-Rupture Limit for GFRP Bars Subjected to Sustained Loads

An American Concrete Institute (ACI) design guide for fiber-reinforced polymer (FRP)–reinforced concrete recommends that, in order to avoid creep-rupture failure, service-load sustained stress should not exceed 0.20 times the design tensile strength $f_{fu}$ of glass-fiber-reinforced-polymer (GFRP) reinforcement, where $f_{fu}$ is equal to the guaranteed tensile strength multiplied by an environmental reduction factor. The 0.20 value, which was put in place prior to 2001, was conservatively chosen due to both limited test data and a lack of standards for both materials and test methods. In this study, values for the creep-rupture strength at a 1,000,000-h (114-year) endurance time were extrapolated from recent creep-rupture tests of GFRP bars based on regression analysis of rupture stress versus the logarithm of endurance time in hours. The bars were made of continuous E-CR or E-glass fibers in a vinyl ester resin and manufactured by the pultrusion process. The analyses indicate that, given a 114-year service life and a 1.67 safety factor as is used by the ACI design guide for FRP-reinforced concrete, a sustained stress limit of $0.30f_{fu}$ is a conservative value to avoid a creep rupture of GFRP reinforcing bars that conform to ASTM standards.