Fracture properties and reinforcement mechanisms of multi-scale hybrid fiber reinforced ultra-high performance concrete

This study used microscopic and macroscopic fibers to improve the mechanical and fracture properties of ultra-high performance concrete (UHPC) at multiple scales. Thirteen groups of pre-cracked UHPC beams with different fiber content were tested by three-point bending test, and the crack extension process was demonstrated by digital image correlation (DIC). The results indicated that as the content of multi-scale hybrid fiber increased, the flowability of UHPC gradually decreased. The mechanical and fracture properties of UHPC were most improved with the addition of 0.10 wt% multi-walled carbon nanotubes (MWCNTs), 2 vol% calcium carbonate whiskers (CW), and 3 vol% steel fiber. The compressive and flexural strength of UHPC-3-2-0.10 were 191.6 MPa and 39 MPa, respectively, which were 63.20 % and 95.98 % higher compared to UHPC-R (117.4 MPa and 19.9 MPa). With the increase in mechanical strength, the fracture toughness also increased dramatically. The peak load of UHPC-3-2-0.10 was 7.39 kN, which was 291 % higher than that of UHPC-R (1.89 kN). The Kini, Kun, and fracture energy of UHPC-3-2-0.10 were 3.41 MPa m1/2, 166.75 MPa m1/2, and 7.73 kJ/m2, respectively. In addition, the DIC results showed that hybrid fibers increased the complexity of the crack extension path of UHPC, and the mechanism of hybrid fibers reinforced UHPC was explained by using the scanning electron microscope (SEM) technique. The above results showed that multi-scale hybrid fiber reinforced UHPC exhibited excellent ductility, compressive and flexural strength, fracture toughness, and multiple cracking behavior, and had broad application prospects.