Effect of curing conditions on strength development in an epoxy resin for structural strengthening

Abstract Civil structures such as bridges and buildings can be strengthened with prestressed fibre reinforced polymer (FRP) strips to enhance both their stiffness and load-bearing capacity. End anchorage is a crucial issue for prestressed FRP strips. An innovative anchorage procedure, called the “gradient anchorage method” and based on the possible accelerated curing of the epoxy-resin in the end region of the FRP strip, has recently been conceived with the aim of avoiding more invasive mechanical fastening systems. An in-depth knowledge of the actual development of the key mechanical properties of resins under different curing conditions (i.e., in terms of curing temperature) is of paramount importance for employing the above mentioned gradient method in practical applications. This paper presents experimental results and analytical investigations aimed at developing a better understanding of the strength development of a commercial adhesive under different curing times and temperatures. Firstly, direct tensile tests on epoxy specimens were performed at different curing temperatures. It was shown that the necessary curing time to reach the maximum tensile strength can be significantly reduced from several hours at room temperature to approximately 30 min at 90 °C. Furthermore, higher curing temperatures reduced the activation time after which strength starts to increase. The experimental observations are shown graphically with both the activation time and reaction duration at different curing temperatures. Secondly, pull-off bond tests were conducted on 100 mm wide and 1.2 mm thick FRP strips bonded to concrete using epoxy adhesives cured either at 90 °C for different durations or at room temperature. An optical image correlation system (ICS) allowed the load transfer behaviour of the inhomogeneous cured adhesive between the FRP strip(s) and concrete to be studied. Finally, using the experimental measurements, the bond shear stress–slip interface relationships for the different test specimens were identified in order to present the effect of elevated curing temperatures and curing durations.