Spalling of Large-Scale Walls Exposed to a Hydrocarbon Fire

Concrete spalling in fire is a common occurrence which has been attributed to two main mechanisms, thermo-mechanical caused by differential thermal gradients and moisture clog due to the buildup of pore water pressure which act either independently or in combination. Quantifying the magnitude of these variables associated with the spalling mechanisms is the scope of this paper. Nine unloaded large-scale reinforced concrete panels tested in a vertical configuration were exposed to a hydrocarbon fire on one side for a duration of 60 min. Two thicknesses (100 and 200 mm) and two compressive strengths (25 and 80 MPa) were investigated. Specimens were instrumented in situ with thermocouples and pore pressure gauges. The degree of spalling was quantified as mass loss from material dislodgment and mass loss due to water evaporation. No consistency in results were observed between identical samples, particularly for 80 MPa 200 mm thick samples that had total spalling mass losses of 26%, 44%, and 54%, respectively. However, explosive spalling only occurred in the thicker 200 mm specimens. Water pooling was observed across all specimens, which resulted in saturation toward the unexposed side due to the drying front derived from the moisture clog mechanism and the migration of water through the cracks developed by flexural forces. This caused strength deterioration of the concrete that ultimately led to spalling. This investigation, in alignment with other works, showed that despite the specimen size or thickness, explosive spalling occurs when all contributing factors coincide at a converging point during the heat exposure. This explains why explosive spalling occurs randomly in a majority of tests reported, and there is no repeatability between identical samples.