Spalling phenomenon and fire resistance of ultrahigh-performance concrete

This study provides an empirical analysis of the spalling phenomena and mechanical properties of ultrahigh-performance concrete (UHPC) after exposure to high temperatures. The main purpose of this research was to elucidate the use of polypropylene fibres (PPFs) as an effective method to mitigate UHPC samples’ propensity to explosive spalling while evaluating changes in their mechanical properties after being exposed to different thermal conditions. The effect of the PPFs dose, heating rate and pre-drying conditions of the spalling phenomenon was systematically examined. Adding up to 2 kg/m 3 of PPFs (PP2.0 samples) positively reduced spalling events, with no significant variation in the compressive strength recorded at room temperature (145 MPa to 155 MPa). The incorporation of 2 kg/m 3 PPFs proved an effective measure against spalling but resulted in loss of workability. However, the combination of 0.5 kg/m 3 PPFs with pre-drying at 80ºC for 3 days was also a feasible strategy to mitigate spalling. The PP2.0 samples, which did not undergo spalling no matter what the heating rate or drying cycle was, were selected to investigate their mechanical behaviour when exposed to temperatures of 200ºC, 400ºC, 600ºC, 800ºC and 1000ºC. For each temperature, compressive strength tests were run in hot-tested, air-dried (natural) and water-cooled (forced) samples. The developed UHPC concrete samples’ compressive strength remained relatively stable at up to 400ºC and progressively reduced with higher temperatures, with the best results obtained during air cooling. • Polypropylene fibers (PPF) effectively reduced spalling in UHPC. • Steel fibers alone were insufficient to prevent spalling. • No spalling occurred with 2 kg/m 3 PPF or 0.5 kg/m 3 PPF plus accelerated drying. • Strength reduction of UHPC with rising temperatures. • Best post-fire compressive strength in the air-cooled UHPC.