The influence of silica fume on the fracture properties of paste and microconcrete

The Paper reports a series of fracture tests on Portland cement pastes and microconcretes with and without silica fume (SF). Water-cement (w/c) ratios of 0·3, 0·5 and O·7 were used. All specimens were tested at 28 days, and 3 day and 7 day tests were also carried out for the 0·3 w/c microconcretes. The fracture tests were based on ISRM Method I, the specimens being chevron-notched cylindrical beams. In comparison with ordinary Portland cement (OPC) materials, the effects of SF were to produce linear pre-peak load-displacement relationships, to strengthen the microconcretes but not the pastes, to embrittle the pastes, to embrittle the low w/c ratio microconcretes (to a small degree), and to toughen the high w/c ratio microconcretes. The observed effects can best be explained in terms of strengthening of the interface bond between paste and aggregate and densifying of the interfacial zone by silica fume.