# Reliability Study on FRP Composites Exposed to Wet-Dry Cycles
## 1 Introduction
Therefore, most guides for structural design suggest using partial safety factors and an environmental reduction to consider the degradation of the materials. Taking the tensile strength of FRPs as an example, it is one of the most important control parameters for the flexural design of FRP-strengthening concrete beams and the design of FRP-constraining concrete columns. The codes assume certain values for the design loading and the design tensile strength of the materials to assess the effect of a particular limit state on the structure. The variability of tensile is taken into account by assuming a characteristic tensile strength. In the Concrete Society in the United Kingdom (TR-55) [22], the characteristic tensile strength was obtained from the FRP system manufacturer, and calculated by the mean tensile strength of a sample of test specimens minus three times the standard deviation. The design tensile strength is determined by dividing the characteristic tensile strength by an environmental factor, \(C_{\mathrm{E}}\). When CFRP material was used in the cases of exterior exposure (such as retrofitting bridges and piers) or aggressive environments (such as chemical plants), the \(C_{\mathrm{E}}\) was taken as 0.85, and the value of \(C_{\mathrm{E}}\) were 0.65 and 0.50, respectively, for exterior exposure and aggressive environments. Most of these safety factors are determined by the probability analysis method and the reliability theory. Three times the standard deviation for characteristic strength in TR-55 can provide a 99.87% probability that the indicated ultimate tensile strengths are exceeded. The environmental factor is a limit value which can ensure the reliability of the structure is always greater than the allowable minimum reliability during the service period. As stated in the specification, due to the lack of relevant research results about the durability of FRPs, these safety factors can be refined as more research information is developed and becomes available. [PERSON] et al. [23] developed a probability-based limit state standard for the design and evaluation of RC structures strengthened by externally bonded CFRP composites. They found that a separate resistance factor of 0.75 should be applied to the nominal strength of the CFRP to achieve a reliability index of approximately 3.0, and that a partial factor should be applied separately to each strength variable if the strength is represented by a sum of random variables. However, their study of resistance criteria did not take environmental factors into account. With this background, a total of 48 FRP samples, including 24 CFRP samples and 24 GFRP samples were tested to evaluate the deterioration of the FRP properties under long-term wet-dry cycles exposure. The degradation law of mechanical properties of FRP with the number of wet-dry cycles, the possible distribution types and the reliability index \(\beta\) were obtained. Subsequently, a reliability analysis was conducted on the effects of the mean and standard deviation of a load on \(\beta\). The mean values are calculated from the related codes, such as ACI-440 [24], TR-55 [22], GB 50608-2010 [25] and GB 50367-2013 [26]. By comparing the reliability of the FRP composite after many times of wet-dry cycles and the allowable reliability index, it is determined whether the design values of each code can guarantee the long-term safety of the FRP composite.