Thermal Analysis for Peak Temperature Distribution in Reinforced Concrete Beams after Exposure to ASTM E119 Standard Fire

Abstract

To assess the post-fire or residual strength of fire-damaged reinforced concrete (RC) members, the most detrimental or peak temperature distribution within the members should be perceived. For RC beams, the residual flexural response is strongly influenced by the peak temperature experienced by the steel reinforcements. This paper presents a simplified two-dimensional (2D) nonlinear transient thermal analysis for the peak temperature distribution in RC beams using the finite element method. In the analysis, the thermal loading for heating was the ASTM E119 standard fire. After heating, a linear decrease in temperature was assumed for cooling. Three-sided fire exposure was assumed for rectangular RC beams. The analysis was used to investigate the effects of the heating period (1–4 h), cooling period (1–4 h), concrete cover thickness (30–50 mm) and aggregate type (carbonate or siliceous aggregates) on the peak steel temperature and delayed time (time to attain the peak temperature after heating). The numerical results showed that the temperature inside the beam section continues to rise after heating. The increases in steel temperature after heating and delayed time are influenced by the heating period, cooling period, location of steel reinforcement, and aggregate type. Such increase is significant for the beam with a thick concrete covering subjected to a short heating period followed by a long cooling period. At the longest (4 h) cooling and shortest (1 h) heating periods, the increases in steel temperature after heating in both carbonate and siliceous concrete beams are approximately 35, 50 and 75% for concrete cover thicknesses of 30, 40, and 50 mm, respectively. The carbonate concrete beams are more vulnerable to fire damage than siliceous ones when subjected to long heating and cooling periods.