Reconsideration of the Environmental Reduction Factor C_E for GFRP Reinforcing Bars in Concrete Structures

Design guides for fibre-reinforced polymer-reinforced concrete (FRP-RC) require that the design tensile strength of FRP bars be determined by multiplying the guaranteed tensile strength by an environmental reduction factor (C_E), to account for the long-term effects of environmental exposure. The recommended C_E values in the American Concrete Institute (ACI) design guide are based on a consensus of the opinion of ACI 440 Committee members and believed to be conservative. The ACI design guide notes the expectation that, with continued research, C_E values will become more reflective of observed environmental performance. In this study, C_E values were estimated for a compiled database of glass FRP (GFRP) bars based on short-term results from accelerated aging tests. The C_E values were calculated with a prediction model considering the effects of seasonal temperature fluctuations, service year, and relative humidity (RH) of exposure. The analysis shows that the calculated values of C_E depend to a certain extent on the mean annual temperatures and RH. In all cases, however, the calculated values were higher than those recommended in the ACI design guide. The authors recommend a C_E value of 0.85 for an assumed service life of 75-100 years to be applied to the guaranteed tensile strength as the basis for determining the design tensile strength. This value is recommended for GFRP bars complying with ASTM Standards, whether exposed or unexposed to earth and weather in cold temperatures and climates encountered in most regions of the world.