Comparison of fire spalling behaviours between ring-restraint and pre-stressed concrete specimens during fire

Abstract

Pre-stressed concrete (PC) structures have higher explosive spalling risk during fires than reinforced-concrete structures, because of the initial compressive stress due to the pre-stressing force. The Japanese Concrete Institute (JCI) has standardised a ring-restraint heating test. Herein, we investigated the explosive spalling of uniaxial PC beam specimens and ring specimens made of concrete with the same mix proportions. A fire test was performed by exposing the specimens at the bottom ends to one-sided heating. RABT 30 and ISO 834 heating curves were used. The spalling was evaluated using spalling indexes specified by the JCI in terms of the maximum spalling depth, spalling area ratio, spalling volume ratio, and maximum spalling depth ratio. The spalling behaviour depended on the shape, degree of restraint, and difference in the pre-stress levels of the specimens. We demonstrated the need to consider the spalling area, depth, and volume in spalling evaluations.