Performance of FRP-strengthened beams subjected to elevated temperatures.

Abstract

Fiber reinforced polymer (FRP) systems have been widely used for strengthening and rehabilitation of reinforced concrete structures. They can provide significant improvement in static load carrying capacity of concrete members. However, one main obstacle which hinders FRP from becoming more widely used is the very limited information on the behavior of FRP-strengthened members under elevated temperatures.

This research presents test results regarding the structural behavior of FRP-strengthened RC beams after subjecting them to elevated temperatures. The investigation on different fire protection systems as well as the effect of sustained loadings serves as useful reference for future work. An analytical method is also proposed to predict the failure load and failure mode for FRP-strengthened RC beams.

The experimental investigation composed of two main test programs. The first program was carried out using small prism specimens strengthened with glass FRP systems with various fire protection systems and basalt FRP systems without any protection. The specimens were subjected to elevated temperatures in a small electrical furnace. Subsequently a second program was carried out on prototype beams strengthened with carbon or basalt FRP systems using a larger chamber. The effects of elevated temperatures and sustained loading were investigated. Two other protection systems were examined in the test program.

Subjecting the beam specimens to elevated temperatures of up to about 600 celsius degrees led to a decrease in ultimate strength. For carbon FRP strengthened beams, the ultimate strength decreased but the initial beam stiffness is not affected after subjecting to temperatures ranging from about 300 celsius degrees to 600 celsius degrees. The failure mode changed from flexural debonding to flexural rupture after subjecting to elevated temperatures due to the deterioration of the materials. Sustained loading applied on prototype beam specimens during heating did not however affect in the beam stiffness and strength.

Among all the protective systems, mortar overlay had limited effectiveness on prototype beams. Other coating systems were effective in protecting the FRP systems but further improvements are needed if the specimens are subjected to elevated temperatures higher than 600 celsius degrees.

The analytical model is based on strain compatibility and force equilibrium, and predicts the ultimate strength and failure mode of FRP-strengthened reinforced concrete beams using the deteriorated material properties. The analytical predictions compared with test results well. However further improvement is needed before the model can be used in a fire design of FRP strengthened beams.