Influence of aggregate size and inclusion of polypropylene and steel fibers on the hot permeability of ultra-high performance concrete (UHPC) at elevated temperature

Explosive spalling is one of the most detrimental problems for ultra-high performance concrete (UHPC) in fire condition due to the risks of breaching the integrity of concrete structures. This paper investigates the influence of aggregate size and inclusion of PP and steel fibers on the intrinsic permeability of UHPC at elevated temperature. Hot permeability measurements were performed on UHPCs subjected to elevated temperature up to 300 °C. Microstructure of UHPC samples before and after the exposure to elevated temperature was studied to reveal potential mechanisms responsible for the change of permeability. Results showed that the inclusion of PP fibers or larger aggregates significantly increases the hot permeability while the addition of steel fiber does not contribute to the enhancement of the permeability of UHPC at elevated temperature. The combined use of PP fibers and larger aggregates in UHPC showed synergistic effect and resulted in the significant increase of permeability at elevated temperature, which is mainly attributed to the formation of interconnected micro-crack networks at elevated temperature due to the melting of PP fiber and thermal expansion and mismatch between the aggregate/fiber and matrix.