Enhancement of fracture toughness and mechanical properties of ultra-high performance concrete with ZnO nanoparticles under mixed-mode loading conditions: a multi-scale numerical investigation

This study is the first to thoroughly examine the effects of zinc oxide (ZnO) nanoparticles (NP) on fracture toughness, particularly in mixed-mode (I-II) conditions. It also pioneers the measurement of the impact of ZnO NP on the mechanical and fracture properties of Ultra-High Performance Concrete (UHPC). In order to simulate the behavior of ZnO nanoparticles in the UHPC matrix while taking into account different fracture modes (Mode I, Mode II, and mixed-mode I-II), the study presents a novel multi-scale finite element (FE) model. In order to improve the durability and performance of cementitious materials under complex loading scenarios, the study determines the ideal dosage of ZnO NP for increasing tensile strength and fracture toughness. The UHPC matrix behavior was simulated by the Concrete Damaged Plasticity (CDP) model, which was calibrated using experimental uniaxial compression and direct tension data. The fracture behavior of the UHPC specimens was examined using the Cracked Straight-Through Brazilian Disc (CSTBD) test configuration with different central crack inclination angles (β). The numerical model’s validity was confirmed against experimental results for β = 0° and β = 45°. A subsequent parametric study assessed the impact of ZnO NPs volume fraction and crack inclination angle on Mode I, Mode II, and mixed-mode (I-II) fracture toughness. Findings indicate that an optimal 0.4 wt% ZnO NPs addition significantly enhances mixed-mode fracture toughness, particularly at critical inclination angles dominated by combined tensile-shear mechanisms. While the optimal dosage for maximizing compressive strength was 0.6 wt% (yielding a 20.6 % increase), 0.4 wt% ZnO proved optimal for enhancing tensile strength (up to 40 % increase) and fracture toughness. Notably, at 0.4 wt%, the pure shear (Mode II) fracture toughness increased by over 2.1 times. This research highlights the substantial potential of ZnO NPs to improve UHPC mechanical properties and fracture resistance under complex, mixed-mode loading conditions.