Bond Behavior of Straight and Bent Glass Fiber–Reinforced Polymer Bars at Elevated Temperatures: Pull-Out Tests and Numerical Simulations

Abstract

The bond between concrete and fiber-reinforced polymer (FRP) bars is severely degraded when the glass transition temperature of the FRP (set mainly by the polymeric matrix, typically a thermosetting resin) is approached, and therefore long development lengths are required to enable a proper anchorage in cooler zones of FRP–reinforced concrete (RC) members exposed to fire. In spite of the potential of bent bars to shorten such lengths, and thereby improve the fire resistance of FRP–RC members, very few studies have addressed the effects of elevated temperatures on the bond performance of bent FRP reinforcement. This paper presents experimental and numerical investigations concerning the bond behavior of straight and 90°-bent glass-FRP (GFRP) bars at elevated temperatures. Steady-state pull-out tests were first carried out on bent ribbed bars, from 20°C up to 300°C, and the results were compared with those previously obtained from straight bars. The experiments showed that the hook effect provided by the bend and tail lengths of the bars enabled bond-strength increases of between 30% and 90% compared with straight bars. Three-dimensional finite-element models were then developed to: (1) simulate the pull-out tests; and (2) perform design-oriented parametric studies, aimed at assessing the influence of elevated temperatures on the anchorage strength of straight bars with different surface finishes (sand-coated and ribbed), and of 90°-bent ribbed bars with varying tail and straight development lengths. Temperature-dependent local bond stress versus slip laws were implemented in the models in order to describe the bond interaction along the straight and bent lengths of the bars. The models provided a good agreement with the test data, in terms of load versus slip response, and a reduction in pull-out load and bond stiffness with temperature. The findings were that: (1) the adoption of 90°-bent anchorages with appropriate tail lengths is an effective and practical approach for improving the bond strength of GFRP bars at both ambient and elevated temperatures; and (2) at elevated temperatures, to mobilize the tensile strength of GFRP bars, the development lengths of straight and bent bars designed for ambient temperature must be significantly increased. Finally, optimal anchorage lengths are proposed, as a function of temperature, for beam and slab applications.