Three-Phase Evaluation of Fire-Exposed Reinforced Concrete Beams: Methodology and Outcomes

Abstract

This study introduces an improved three-phase evaluation approach to assess the residual strength and structural response of reinforced concrete (RC) beams subjected to fire exposure. Highlighting the potential for RC structures to be utilized again after fire incidents, this research emphasizes the need for comprehensive assessment and rectification to determine their remaining strength. The methodology employs a systematic approach, segmented into three essential phases: pre-fire testing (conducted at room temperature), fire testing including a cooling phase, and post-fire (residual) testing. These evaluations were carried out on two full-scale beam specimens, each 200 mm wide, 300 mm deep, and spanning 2000 mm in length. One RC beam was not exposed to fire, while the other one was subjected to fire on three sides. Each RC beam was then tested under the four-point bending setup until failure to determine the remaining strength. The furnace temperature curve was also compared with BS 476-20 and ASTM E119-00a standard fire curves for reference. The investigation revealed that factors such as thermal degradation of the concrete and pre-loading significantly affect the post-fire performance of the RC beam, leading to irrecoverable plastic deformation. This was evidenced by a 26% reduction in ultimate load and a 51% decrease in secant stiffness for the fire-exposed beam. Additionally, the beam exhibited a 5 mm residual deflection in the unloaded state and a significant 75.8% increase in deflection at ultimate load, rising from 22.3 to 39.2 mm. Further analysis demonstrated the limitations of the 500 °C Isotherm method, which underestimated residual capacity by 16.5%, compared with Finite Element Analysis simulations that closely matched experimental results with only a 1.2% difference. The proposed three-phase evaluation not only deepens the understanding of the structural behavior under fire exposure but also provides a systematic framework for assessing its continued usage throughout its service life.