Crack propagation speed in ultra high performance concrete (UHPC)

•Investigated crack propagation in UHPC under wide range of loading rates.•The DIC technique and Canny edge detector technique were adopted.•Established relationships between crack speed and crack initiation strain rate.•Crack speed asymptotically increases as crack initiation strain rate increases.•Crack-velocity dependent dynamic fracture mechanics based model was applied.

This paper investigates crack speed in ultra-high performance concrete (UHPC) using pre-notched three-point bending specimens. The experimental parameters are fiber volume fraction and rate of loading. A hydraulic servo-controlled testing machine is used to apply lower notch tip strain rates, in the range of 0.025–1.0 1/s, while a newly developed impact testing system is used to achieve higher notch tip strain rates, ranging from 6.8 to 41.1 1/s. A high-speed camera is used to record images of the UHPC specimens during testing. Notch tip strain and crack speed are computed from the images, which show that crack speed increases asymptotically as the crack initiation strain rate increases. Crack speeds of up to 514 m/s were achieved at the lower notch tip strain rates and up to 1454 m/s for the higher notch tip strain rates. The achieved relationships are incorporated into a recently proposed crack-velocity dependent dynamic fracture model. The model is validated using published experimental data and used to show that, like conventional concrete, the strain rate sensitivity of UHPC is strongly associated with the characteristics of dynamic crack growth.