Fractal characteristics of fire-damaged ultra high toughness cementitious composite after impact loading

The 80 mm splitting Hopkinson pressure bar was utilized to impact hybrid fiber ultra high toughness cementitious composite (UHTCC) under three sets of impact pressure (0.35MPa, 0.45MPa, 0.55MPa). The specimen was heated up to different temperatures (including ambient temperature, 200℃, 400℃, 500℃, 600℃, 800℃)before the impact loading, and the fragmentations after impact were collected to analyze the fractal characteristics. X-ray CT scanning and SEM (scanning electronic microscope) were utilized to observe the internal structures of the specimens after the high temperature, which could disclose the causes of the fractal characteristics of fragmentation changing with temperature. The results show that, the fractal dimension of UHTCC increases with the impact pressure, and the energy dissipation capacity exponentially increases with the fractal dimension. The fractal dimension of UHTCC decreases with the exposure temperature, which is different from that of concrete. At 800℃, the fractal dimension of UHTCC is close to that of concrete. The PVA and steel fibers in UHTCC have a synergistic crack blocking effect which may change the propagating path of cracks and result in increased roughness of fracture surface and higher fractal dimension. With the increase of temperature, the weakened bridging effect of fibers, the formation of cracks due to temperature and the decomposition of hydration products in UHTCC lead to the decrease of fractal dimension, which gradually close to that of concrete. © 2019, Editorial Office of China Civil Engineering Journal. All right reserved.