Development of Accelerated Aging Test Methodology and Specimen for Bonded CFRP Systems
ABSTRACT: Determining long-term behavior of bonded CFRP systems requires developing an accelerated aging test method for CFRP applications. This paper examines the development of test methodology and specimen for both flexure and direct tension behavior of bonded CFRP materials using a specimen submerged in a water bath subject to elevated temperature. Test results of three commercial CFRP systems are presented. A discussion of accelerated aging is included in the developmental effort.
KEYWORDS: accelerated aging, CFRP, bond strength, flexural strength, tensile strength, strength reduction factor, durability
In recent years, fiber-reinforced polymer (FRP) composites have been increasingly used in various fields such as aerospace, automotive, athletic, recreational equipment, military, and infrastructure facilities. Glass, carbon, and aramid fiber polymers are currently the three most commonly used advanced polymer composite materials. Compared to glass and aramid fiber polymers, carbon fiber polymers exhibit superior resistance to harsh environmental exposure and fatigue load, making it more practical for civil engineering applications [1-3]. A
great deal of research, including laboratory tests and short-term field applications, has identified that externally bonded CFRP composite systems could efficiently improve load carrying capacity [4-6]. The short-term behavior of CFRP composites has been studied, and construction specifications for bonded repair have been developed under NCHRP Project 10-59A . Despite this research, the long-term performance of CFRP applications is not fully quantified and durability issues remain unanswered. The lack of real-time test data has made it difficult for investigators to completely understand the deterioration mechanism of FRP bonding in composite systems. This knowledge is necessary for establishing a uniformly accepted durability assessment criterion for externally bonded CFRP applications. Lack of a uniform test method and variability of CFRP systems further complicates this situation.
Moisture and temperature are commonly regarded as two predominant factors affecting the bond performance of FRP composite systems [8-11]. Lefe – bvre et al.  investigated adhesion loss at interfaces between adhesive and inorganic substrates. The adhesive was air-cured at room temperature with a high relative humidity. They found a critical relative humidity (RH) value, which was an intrinsic property of the adhesive. When the environmental RH was higher than the critical RH, some permanent changes in the internal state of the material took place and an abrupt adhesion drop occurred. RH values exceeding the critical threshold humidity resulted in moisture accumulation at the bond line. Ultimately this resulted in a loss of adhesion between the adhesive and substrate. Au et al.  used interface fracture toughness as the quantification parameter of CFRP-adhesive resin-concrete systems to investigate bond deterioration mechanisms by peel and shear testing accelerated by moisture conditioned specimens. They obtained a similar conclusion as Lefebvre, that there existed a threshold value of moisture accumulation beyond which the fracture toughness could decrease by 60 %. Malvar et al.  conducted three separate investigations to evaluate short-term effects of temperature, moisture and chloride content on the CFRP adhesion using pull-off tests and had strength reduction results similar to Lefebvre’s results.
Karbhari and Engineer  investigated short-term environmental exposure effects on the bond performance between composite laminates and the concrete substrate. They concluded that the use of low Tg resin systems resulted in deterioration and loss of efficiency of the CFRP retrofit. The selection of appropriate resin systems was critical to successfully retrofitting and reha – biliting infrastructure applications. Further research performed by Abanilla and Karbhari [12-14] indicated that moisture uptake and other environmental factors could appreciably deteriorate the strength characteristics at the matrix and interface levels. They reported that increasing the number of CFRP layers of samples increases the rate of degradation at the matrix and interface levels and concluded that exposure to aqueous solutions results in deterioration in interlaminar and intralaminar characteristics. Toutanji and Gomez  studied the effect of harsh environmental conditions such as wet/dry cycling using saltwater on the performance of FRP-bonded concrete beams and on the interfacial bond between the fiber and the concrete. A pronounced bond strength reduction in specimens subjected to wet/dry cycling was observed; furthermore, fibers did not break but rather the adhesive resin debonded at the fiber-
TABLE 1—Properties of CFRP materials.
concrete interface. They postulate that the strength reduction may be attributed to deterioration in the interface and the bond between the fiber and the concrete.
In practical applications, engineers are concerned with how to select an appropriate CFRP composite system under specific environmental conditions and make proper predictions regarding service life. Accelerated aging has long been used to determine the suitability of new materials for structural engineering applications to characterize long-term behavior. In CFRP bonding applications, accelerated aging is typically attempted by increasing the temperature or concentration of conditioning agents to speed up chemical or physical processes in an effort to study changes in bond strength . The objective of this study is to investigate the effects of elevated temperature water baths on bond strengths of externally bonded CFRP concrete beams, and to develop a conservative accelerated aging test specimen for bonded CFRP systems. Three commercial CFRP composite systems were bonded to saw-cut plain concrete beams and exposed to different elevated temperature water baths. These specimens were tested at regular intervals by three-point bending and direct tension tests.