Influence of polypropylene and steel fibres on thermal spalling and physical-mechanical properties of concrete under different heating rates

This study investigates three concrete compositions: a control concrete, a reinforced concrete containing 60 kg/m 3 of steel fibres, and a hybrid concrete containing 60 kg/m 3 of steel fibres and 0.75 kg/m 3 of polypropylene fibres. Cylindrical specimens were manufactured and subjected to two heating rates (10 °C/min and the ISO 834 fire standard) to study their spalling sensitivity. Moreover, unidirectional heat transfer and pressure measurements were performed on slabs heated at 10 °C/min to 600 °C. To understand the influence of fibres on spalling processes more clearly, the thermal properties were studied during heating and cooling: the gas permeability, compressive strength, and Young’s modulus of specimens were measured after a heating and cooling cycle at 0.5 °C/min. The results show that the spalling phenomenon was more severe for cylindrical specimens than for slabs. The addition of 60 kg/m 3 of steel fibres had an unfavourable influence on spalling, especially during the ISO 834 fire test. The addition of 0.75 kg/m 3 of polypropylene fibres allowed the concretes containing steel fibres to avoid spalling, regardless of the thermal loading conditions.