Experimental Study of Flexural Behavior of Hybrid Beams Exposed to Flame: Normal and Ultrahigh-Performance Concrete

This investigation was conducted to study the response of reinforced hybrid beams exposed to flames and their capacity for carrying loads. At the age of 28 days, concrete samples and beams were exposed to fire temperatures ranging from 25°C to 600°C. Then, the beams were gradually cooled by being left in the air. The testing samples were exposed to flames for a half hour at temperatures of 600°C. All ultrahigh-performance concrete (UHPC) specimens (cubic, cylinder, and prism), upon burning at 600°C, spalled and lost some of their mechanical characteristics. Results indicate a remarkable reduction in compressive strength; the residual compressive strength was 78% and 62% for conventional and ultrahigh-performance concrete, respectively. Therefore, the remaining tensile strength was 77% and 69% for normal concrete and UHPC, respectively, and the residual flexural strength was 41% and 64% for conventional concrete and UHPC, respectively. Four beams consisting of two kinds of concrete, normal concrete (NC) and UHPC, were subjected to flame, and another four reference beams with dimensions of 12 cm width, 15 cm depth, and 170 cm length were used in this study. The results showed that the effect of firing on the UHPC part was larger than on the normal concrete part, and that led to an effect on the beams’ behavior and mode of failure. For reference beams, it can be seen the use of the UHPC layer at the compression zone played a major role by changing the mode of failure from compression (as in beam R1) to flexural failure (as in beams R2 and R3). However, after firing, the mode of failure returned to compression failure because the influence of firing on the UHPC part was greater than on the normal concrete part at 600°C. This study is considered important in buildings that must be highly durable and are likely to be exposed to fires. It is clear when using a layer of UHPC in the NC beam affected by fire that it is possible to benefit from this beam after removing the surface damaged by the fire and making repairs on the surface of the beam.