Effects of Water on the Curing and Properties of Epoxy Adhesive Used for Bonding FRP Composite Sheet to Concrete Lixin Wu, Suong V. Hoa, Minh-Tan Ton-That Concordia Center for Composites, Department of Mechanical Engineering, Concordia University, Montreal, Quebec, Canada H3G 1M8 Received 31 July 2003; accepted 18 December 2003 ABSTRACT: When a concrete surface is contaminated by water due to rain, saline water, ground water, and water jetty treatment, water, alkalis, and other contaminants on the concrete surface may interact with an epoxy adhesive used for bonding fiber-reinforced polymer composite sheets to concrete. This can influence both the curing rate and the degree of cure of the curing reaction. This in turn can affect the time required for field application. It can also influence the mechanical properties and durability of epoxy adhe- sives. In this paper, water effects on the curing and proper- ties of two kinds of commercial adhesives were evaluated. Curing kinetics were studied using isothermal DSC analysis. Results showed that water accelerated the curing reaction. However, excess water offsets part of the accelerating effect. While water is typically considered to be harmful to prop- erties of adhesives, it was seen that a small amount (less than 2%) of water improved degree of cure, mechanical proper- ties, and durability of adhesives. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2261–2268, 2004 Key words: adhesives; epoxy; kinetics; water; reinforcement construction; ageing INTRODUCTION In recent years there has been increasing interest in using fiber-reinforced polymer (FRP) composites to repair concrete structures. Epoxy resins are selected as adhesives for bonding FRP composite sheets to con- crete due to their good adhesive ability to both con- crete and FRP materials. In civil engineering applica- tions, the concrete structures are often contaminated by different materials due to rain, saline water, ground water, and wet jetty treatment, while the con- crete itself has high alkalinity and consists of different compositions. When epoxy adhesive is used to repair a concrete structure, water, alkalis, and other contam- inants on the concrete surface may interact with the epoxy adhesive, and thus influence the curing reac- tion, both the curing rate and the degree of cure. Due to the restrictions of on-site processing, epoxy adhe- sives used in civil engineering applications are usually cured at room temperature. As a result, only a low glass transition temperature (T g ) can be obtained, which limits the service temperature and increases the sensitivity of the bond to environmental factors, such as humidity, temperature, etc. In addition, the me- chanical properties and the durability of epoxy adhe- sives are greatly affected by the degree of cure. As the load is transferred to the FRP material by adhesives, the mechanical properties and the durability of this bond with concrete is critically important for the in- tegrity and safety of the repaired structure. However, so far, most current research has focused primarily on mechanical properties of the whole system of concrete and composite reinforcing sheet. Information on the bond between concrete and the FRP sheet materials is relatively scarce. Chen et al. 1 used near infrared (NIR) spectroscopy to study the curing reaction of two types of epoxy/ hardener systems in the presence of water and found that water accelerated the curing reaction. The reac- tion rate constant increases linearly with water content up to a saturation point (about 0.06%) and then is invariant with the water content. The authors also evaluated the water effect on ultimate tensile strength, shear stress, impact strength, flexural strength, and compression strength and found that water deterio- rated the mechanical strength of adhesives. This work showed that the presence of water can have an effect on the curing of the epoxy adhesives and their prop- erties. However, there are aspects that this paper did not address: 1. The water used in that study was deionized water rather than regular water and concrete pore water, which may have different effects. 2. The quantity of water has an influence on its effect on the epoxy. The curing rate increases with water content for small amounts of water Correspondence to: S. V. Hoa (hoasuon@vax2.concordia.ca). Journal of Applied Polymer Science, Vol. 92, 2261–2268 (2004) © 2004 Wiley Periodicals, Inc.