Synergistic toughening effect of coarse aggregate and hybrid steel fibres on the evolution of fracture process zone in UHPC under Mode I crack tip deformation

This article investigates the synergistic toughening effect of coarse aggregate (CA) and steel fibres on fracture process zone (FPZ) development in ultra high performance concrete (UHPC). Two specimens, UHPC without fibres, UHPC(NF), and UHPC with hybrid fibres, UHPC(HB), were tested under Mode I crack tip deformation. Digital image correlation (DIC) and acoustic emission (AE) testing were employed to provide insights into the evolution of FPZ. FPZ length ( l FPZ ) was evaluated using the horizontal displacement gap method, while FPZ and fracture core zone (FCZ) widths were determined from AE event density distributions. In addition, micro-CT analysis was conducted to examine porosity and pore size distribution, as well as their influence on FPZ behaviour. It was observed that the positive synergistic interaction between hybrid fibres and CA governed the FPZ evolution and resulted in a substantially superior work of fracture that exceeded the arithmetic sum of reference mixes. This synergy was likely associated with reduced porosity and ultimately a denser microstructure, leading to a stable post-peak phase and a gradual reduction in l F P Z in UHPC(HB) after its full development, in contrast to the rapid decrease in UHPC(NF). This confirmed that the positive synergy is crucial for sustaining residual toughness and stability, thereby encouraging the use of hybrid fibres in UHPC incorporating CA for applications demanding high structural ductility. • Synergistic effect of CA and steel fibres on FPZ evolution in UHPC was studied. • DIC and AE testing were employed to provide insights into the evolution of FPZ. • Positive synergistic interaction was observed between CA and steel fibres. • Enhanced CA–fibre synergy caused a gradual FPZ decrease after full development. • Reduced porosity and tortuous FPZ reinforced fibre–aggregate synergy.