Flexural specimens of hardened cement paste, fibre reinforced concrete, and conventionally reinforced concrete were tested in an instrumented dropweight impact machine, employing a 345 kg mass impact hammer dropped from a height of 500 mm. The crack development in the specimens was monitored using high speed motion picture photography, carried out at about 10,000 frames per second. It was found that, for all three specimens, some crack branching occurred. The cracks did not propagate in a continuous fashion; they appeared to arrest occasionally, and then began to grow again. However, it appeared as if the crack velocities reached a maximum value very soon after the impact occurred; they then decreased, and finally increased again just prior to failure. The average crack velocities were in the range of about 75 to 115 m/s.