# Reliability Study on FRP Composites Exposed to Wet-Dry Cycles
## 1 Introduction  
During the last decades, external bonding of fiber-reinforced polymers (FRP) has been widely used to strengthen or retrofit deteriorated civil engineering structures. This is mainly due to its light weight, excellent tensile strength, resistance to corrosion, durable construction and easy tailoring [1; 2]. However, many questions related to its durability of service life are still unanswered. Previous studies have focused on the short-term characteristics of FRP composite materials (i.e., fibers and resin) [3; 4; 5; 6; 7]. The research data on the rates of deterioration of the properties, such as tensile strength and modulus, under multiple harsh environments exposure (e.g., marine environment, acid rain erosion, high temperature, and sustained load coupling) is insufficient [8]. In the design guidelines of ACI 440.2R-08, it was pointed out that some research related to the durability of FRP materials and standardized test methods with an accent on durability testing, need to be further developed. These facts motivated the development of a study that would address durability issues of FRP composite materials under harsh environments. External bonding FRP in civil engineering may be exposed to harsh environmental conditions during the whole service life including, freeze-thaw cycles, extreme temperatures, solar radiation and aggressive chemical attack, and wet-dry cycles. Freeze-thaw exposure was identified asa severe temperature-related threat by the Civil Engineering Research Foundation (CERF) [9]. Significant loss in the mechanical properties and integrity of FRP was found after freeze-thaw cycles exposure due to fiber/matrix debonding and matrix microcracking [10; 11]. The loss will increase further under the multiple environmental coupling effects of freeze-thaw cycles, salty water, deionized water, wet-dry cycle, or sustained loading. The hygrothermal exposure will significantly accelerate the moisture-induced deterioration process and relaxation process of FRP materials [12; 13]. [PERSON] et al. [14] tested 63 glass FRP (GFRP) or carbon FRP (GFRP) coupons to study the effects of extreme service temperature or freeze-thaw cycles on FRP mechanical properties. The results showed that strength and elastic modulus mildly decreased when the temperature was raised from \(-15\) to \(+36\)\({}^{\circ}\)C; while the ultimate strain remained almost constant. Furthermore, freeze-thaw cycles ranging from 30 to 80 had a negligible influence on the mechanical properties of CFRP laminates, while 210 cycles resulted in slight reductions in tensile strength and an ultimate strain equivalent of 9% and 13%, respectively. [PERSON] et al. [15] found that freeze-thaw cycles under both dry and wet conditions typically have a detrimental effect on the strength and to a lesser degree on the stiffness of carbon and E-glass composites. The studies [16; 17] have demonstrated that wet-dry cycles could decrease the ultimate load of various FRP composites, and combined effects with other environments will be more significant. [PERSON] et al. [18] exposed FRP coupons to wet-dry cycles of using tap water and road salt solutions. It was found that the ultimate failure stress decreased in tap water but increased in the salt solution. When a sustained loading was applied simultaneously, the creep deformation increased the molecular spacing which makes water molecules and NaCl molecules more easily invaded adhesive, thus causing greater deterioration of the reliability and durability of the structural elements [19]. [PERSON] et al. [20; 21] reported that sustained bending strain had no influence on moisture uptake, but could cause severe fiber/matrix interfacial debonding, in addition to fiber pitting and cracking. In summary, when the FRP reinforcement system is in service in a harsh environment, its mechanical properties are continuously degraded. If the design is based on short-term mechanical properties of the FRPs, the structure will have a large failure probability.