An Experimental Study on the Analysis of Factors Influencing Explosive Spalling of Concrete Under Fire

Explosive spalling is the most serious fire-induced deterioration mechanism of high-strength concrete (HSC), yet the influence of the two principal variables—compressive strength and internal moisture content—has not been quantified experimentally over a wide strength range. In this study, 144 cylindrical specimens (ø100 × 200 mm) were produced from four concrete strength grades (21, 50, 80 and 100 MPa) and three curing conditions (air, sealed and high-temperature water curing) together with an oven-drying treatment so as to obtain a controlled variation of moisture content, and were heated for 15 and 30 min following the KS F 2257-1 (ISO 834-1) standard fire curve. The spalling appearance, cracking characteristics, residual compressive strength and pore structure were measured. Normal-strength concrete (NSC) of 21 MPa did not spall under any combination of heating time, curing method and moisture content, whereas every 100 MPa-grade specimen spalled severely, and they were in some cases completely destroyed. For 50–80 MPa HSC, the occurrence of spalling showed no consistent correlation with strength or with moisture content, which indicates that initial imperfections of the material dominate the onset of spalling. The spalling of HSC occurred mainly within the first 15 min of heating and its severity increased with compressive strength and heating time, while the influence of moisture content was threshold-like rather than monotonic. Threshold moisture contents for spalling-free behaviour were estimated as about 3% for 50–80 MPa and about 1% for 100 MPa and above, although these provisional thresholds require confirmation by tests at intermediate moisture contents. The residual compressive strength ratio after 15 min of heating averaged about 88% for the 21 MPa NSC and about 87% and 80% for the 50 and 80 MPa HSC, respectively, but after 30 min it decreased to about 64% for 21 MPa, 53% for 50 MPa, 43% for 80 MPa and 39% for 100 MPa.