Punching shear strength of reinforced concrete flat slabs subjected to fire on their tension sides

The effect of fire on punching strength of flat slabs is experimentally investigated. An experimental program, consisting of fourteen one-third scale specimens pre-exposed to fire on their tension side and tested under concentric punching, is carried out. The main investigated parameters are the duration of exposure to fire, the concrete cover and the cooling method. Specimens are subjected to direct flame for 1.0, 2.0 and 3.0 h, respectively. Concrete covers of 25 mm and 10 mm are used for test specimens. Two cooling methods are employed; gradual cooling in air and sudden cooling with water applied directly to the heated surface of the slabs. It was found that exposure of slabs to fire resulted in a reduction of up to 18.3% and 43% in cracking loads and ultimate punching loads, respectively. Concrete cover was proven to have a significant effect on level of temperature in tension reinforcement. A reduction in punching strength of up to 14% was observed for specimens with 3 h exposure to fire compared to those with one hour exposure. Sudden cooling was found to reduce punching strength by 25% compared to specimens gradually cooled. A simplified mechanical model for calculating fire effect on punching capacity is proposed and found to be in good agreement with the experimental results.