Mechanical behaviour and impact response of ultra-high-performance concrete incorporating steel and polypropylene fibers

This work delivers a head-to-head, dosage-resolved evaluation of steel fibers (SF) versus polypropylene fibers (PPF) in a UHPC matrix under repeated ACI-544 drop-weight impact, coupling fresh properties, non-destructive indices (density, UPV), static mechanics, and energy-based impact metrics (blows to first crack/failure, absorbed energy, toughness, ductility index). Fiber type and volume (0–1.5 %) were systematically varied at an interval of 0.5 %. Findings revealed that increasing dosage reduced flowability; SF enhanced density while UPV decreased with increasing fiber content. SF showed improvement in compressive and flexural strengths and the largest improvements in all impact parameters, markedly increasing blows to first crack/failure, energy absorption, toughness, and ductility. PPF slightly reduced compressive strength but enhanced flexural strength up to 1.5 % and improved impact ductility, offering a crack-arrest/weight-neutral option. The integrated dataset establishes novel links between NDT indices and impact capacity and provides dosage-response envelopes that distinguish when SF-reinforced UHPC (impact hardening) or PPF-reinforced UHPC (ductility/crack control) is preferable.