Experimental study of the quasi-static and dynamic fracture toughness of notched and repaired concrete using semi-circular bend method

•The dynamic mechanical properties and fracture energy of the NSCB samples is presented.•All fracture modes of concrete exhibit the mode Ⅰ failure in NSCB tests.•The dynamic fracture energy shows a significant dependence on loading rate, increasing with the loading rate.•The repaired specimens consume more energy than the notched specimens and have a lower kinetic energy of flying fragments. Concrete constructions commonly contain cracks or defects during forming and service, repair operations are necessary in many situations. However, limited investigations conducted on the dynamic fracture toughness (DFT) of repaired concrete structures. This paper investigates the cracking behavior of concrete structures through notched dynamic semi-circular bend (NSCB) tests performed using a split Hopkinson pressure bar (SHPB) apparatus with two measurement techniques, namely digital image correlation (DIC) and crack propagation gauge (CPG), to capture the flying half angular velocity and crack propagation speed of NSCB concrete specimens under various loads. The results demonstrate a linear increase in both DFT and crack propagation speed of concrete with the loading rate. Additionally, a threshold value exists beyond which the crack speed stabilizes, and the DFT is not evidently influenced by the notch depths. The cracks propagate along the aggregates and remain mostly intact under a relatively low loading rate. With an increased loading rate, the damage pattern of some otherwise deflected aggregates changes resulting in crack penetration and significantly reducing the fracture surface roughness. Repaired specimens exhibit greater energy dissipation at the same loading rate, thereby producing flying fragments with lower kinetic energy, which can prevent secondary damage to internal personnel and structures.