Dynamic fracture toughness of ultra‐high‐performance fiber‐reinforced concrete under impact tensile loading

The fracture toughness and fracture energy of ultra‐high‐performance fiber‐reinforced concrete (UHPFRC) at both static and impact rates (43–92 s−1) were investigated using double‐edge‐notched tensile specimens. Two types of steel fiber, smooth and twisted fiber, were used in producing UHPFRC with different volume ratios of 0.5%, 1.0%, 1.5%, and 2%. The results indicated that UHPFRCs produced very high fracture resistance at impact rates, with first stress intensity factor (KIC) up to 3.995 MPa√m, critical stress intensity factor (KIC*) up to 7.778 MPa√m, and fracture energy (GF) up to 86.867 KJ/m2, which were 2.5, 5.0, and 16.9 times higher than those of ultra‐high‐performance concrete, respectively. The KIC* was clearly sensitive to the applied loading rate, whereas the KIC and GF were not. Smooth fiber specimens exhibited not only higher KIC* and GF at impact rates but also higher dynamic increase factor than twisted fiber specimens. A minimum fiber volume content of 1% should be used in UHPFRC to provide a significant enhancement in crack resistance. The maximum value of UHPFRC crack velocity at impact rates was found to be 527 m/s by using a dynamic fracture mechanic model.