Effect of high temperature on structural response of reinforced concrete circular columns strengthened with fiber reinforced polymer composites

This research investigates the effect of elevated temperature on behavior of reinforced concrete (RC) circular columns strengthened with different fiber reinforced polymer (FRP) systems. For this purpose, 32 column specimens were prepared. The test matrix comprised: 14 unstrengthened columns, 14 columns strengthened with a single layer of CFRP sheet, and 4 specimens strengthened with a single layer of GFRP sheet. Out of the 14 CFRP-wrapped specimens, 4 columns were thermally insulated with commercially available fire-protection mortar. In addition to control specimens at room temperature, some other columns were subjected to high temperature regimes of 100℃, 200℃, 300℃, 400℃, 500℃, and 800℃ for a period of 3 h. After cooling down, the columns were tested under axial compression until failure. It was indicated that exposure to elevated temperature adversely affected the residual strength, stiffness, and axial/lateral stress–strain response of unstrengthened columns. FRP composites were found effective in enhancing the axial load capacity of exposed columns provided that the temperature at the FRP level does not exceed the decomposition limit of the epoxy resin. The degradation in strength and stiffness was higher in CFRP-strengthened columns compared with GFRP-strengthened columns when exposed to the same temperature level. The used insulation material was found efficient in preventing heat induced damage to CFRP-strengthened columns up to temperature of 800℃ for 3 h duration. Besides this study, the experimental data of 48 uninsulated FRP-strengthened circular concrete specimens subjected to different heating regimes were collected from the literature. The dataset of 55 uninsulated FRP-strengthened specimens was then employed to evaluate the ACI 440.2R-08 model used for assessing compressive strength of FRP-confined concrete. This model was found non-conservative for 48.6% of the data and thus it was revised by the inclusion of an FRP strength reduction factor due to heating, which can be utilized in the design of FRP-strengthened RC columns exposed to elevated temperature.