# Reliability Study on FRP Composites Exposed to Wet-Dry Cycles
## 4 Discussion  
### The Safety Factors for FRP  
In common with most guides of practice for structural design, the design methods are based on limit-state philosophy, which means a structure will not become unfit for its intended use. It means the structure will not reach a limit state during its design life. Due to the continuous degradation of FRP composite under a harsh environmental exposure, a safety factor needs to be used in the structural design stage. The designer will need to assume certain values for the design loading and the design  
Figure 6: The parameters change as a function of wet-dry cycles: (**a**) Change of \(\beta\); and (**b**) change of \(P\). strength of the materials to assess the effect of a particular limit state on the structure. The variability of material properties is taken into account by assuming a characteristic strength. Usually, characteristic strength is taken as the value below which not more than 5% of test results lie. A similar approach is used to define the characteristic strength of FRP in the report, but the acceptable failure rate has been reduced to 1%. In the guidelines of the American Concrete Institute ACI-440 [24], the design tensile strength,\(f_{\text{fu}}\), can be calculated by modifying the reported strength,\(f_{\text{fu}}\), by an environmental factor, \(C_{\text{E}}\), as follows:  
\[f_{\text{fu}}=C_{E}f_{\text{fu}}^{*} \tag{12}\]  
where  
\[f_{\text{fu}}^{*}=(\overset{-}{f}_{\text{u}}-3\sigma) \tag{13}\]  
where \(\overset{-}{f}_{\text{u}}\) is the mean ultimate strength and \(\sigma\) is the standard deviation of the test population. 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 \(C_{\text{E}}\) was taken as 0.85, which indicates that the threshold to which the CFRP samples can degrade prior to being considered as below the design allowable is 0.85\(f_{\text{fu}}^{*}\). In comparison, the values of \(C_{\text{E}}\) were 0.65 and 0.50, respectively, for exterior exposure and aggressive environments. A similar restriction exists in the guidelines of the Concrete Society in the United Kingdom (TR-55) [22], the characteristic material properties be defined as follows:  
\[f_{\text{fk}}=(\overset{-}{f}_{\text{u}}-2\sigma) \tag{14}\]  
where \(f_{\text{fm}}\) is the mean ultimate strength of the test population. The design strength is determined by dividing \(f_{\text{fk}}\) by partial safety factors to account for strength at the ultimate limit state, \(\gamma_{\text{mf}}\), and the recommended value of the manufacture factor, \(\gamma_{\text{mm}}\), such that the design value is given by:  
\[f_{\text{fd}}=\frac{f_{\text{fk}}}{\gamma_{\text{mf}}\gamma_{\text{mm}}} \tag{15}\]  
For CFRP material, \(\gamma_{\text{mf}}=1.4\) and \(\gamma_{\text{mm}}=1.4\), and for GFRP material, \(\gamma_{\text{mf}}=1.95\) and \(\gamma_{\text{mm}}=1.05\). In the Chinese Technical Code for Infrastructure Application of FRP Composites (GB 50608-2010) [25], the design value \(f_{\text{d}}\) is suggested to consider the degradation of FRP material properties and is determined by dividing the experimental ultimate strength with guarantee probability \(f_{\text{k}}\) by similar partial safety factors (\(\gamma_{\text{f}}\)) to account for material type and the surrounding environment (\(\gamma_{\text{e}}\)). The equations described in the GB 50608-2010 are as follows,  
\[f_{\text{d}}=\frac{f_{\text{k}}}{\gamma_{\text{f}}\gamma_{\text{e}}} \tag{16}\]  
\[f_{\text{k}}=(\overset{-}{f}_{\text{u}}-1.645\sigma) \tag{17}\]  
where the values of \(\gamma_{\text{f}}\) are taken as 1.4 for CFRP and GFRP materials. The values of \(\gamma_{\text{e}}\) are taken as 1.2 and 1.6 for CFRP and GFRP materials, respectively, under ocean environment exposure. However, in another code in China (Code for design of strengthening concrete structure GB 50367-2013 [26]), the design values of tensile strength of CFRP and GFRP are limited to lower values 2300 MPa and 700 MPa, respectively.