Experimental assessment of explosive spalling in normal weight concrete panels under high intensity thermal exposure

Experiments were conducted to investigate fire-induced explosive spalling in conventionally reinforced normal weight concrete panels that emulate tunnel liners. Nineteen specimens (152.4-mm thick with similar to 40 MPa compressive strength and 38.1-mm cover to reinforcement) were subjected to high intensity single-sided thermal loading from a gas-fired radiant panel. Radiant heat flux intensity was calibrated via standoff to achieve 85% or 60% of the ASTM E1529 hydrocarbon fire exposure. Moisture content by mass (MC) and relative humidity were recorded for each specimen prior to testing. Axial load was applied as a percentage of specified ambient compressive strength at three levels: negligibly low (1%), medium (16.7%), and high (33.3%). Fifteen specimens explosively spalled, and the other four showed post-test discoloration with minor cracking after being heated without spalling for at least 30 min. All specimens tested at the higher heat flux intensity with at least medium axial load spalled regardless of MC (from 2.4% to 4.6%). Temperature measurements through the thickness of those specimens indicated that explosive spalling was likely to occur when the concrete surface temperature reached similar to 450 degrees C and generally penetrated to a depth where the concrete temperature was similar to 150 degrees C. Specimens that did not spall typically had very low axial load or MC.