Synopsis This paper describes two methods that have been used to measure the effective fracture toughness of cement pastes and mortars. The first is a notched-beam technique, combined with compliance measurements to measure the slow crack growth prior to instability. The change of toughness is measured for separate increments of crack growth as the crack propagates. The second method, using a double-cantilever beam, avoids the slow crack growth problem by making a specimen of variable web width such that the length of crack front increases with and exactly compensates for the effect of crack growth. Tests of both pastes and mortars show that the fracture toughness of cement paste is independent of crack growth but that the toughness of mortar increases as the crack propagates. For both materials, the stress intensity required to initiate crack growth was less than that to maintain crack growth at the loading rates used.

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Synopsis The resistance to crack propagation in glass-fibre-reinforced cement paste has been assessed by using a technique developed for measuring the fracture toughness of unreinforced cement pastes and mortars. The values measured with the fibrous material are not true toughness values but can be treated as such for small crack growths or used for relative assessment of the resistance to cracking over a larger range of growth. The material tested was an OPC/pfa matrix with chopped glass-fibre distributed at random. Three different mixes were used (0, 1·1% and 2·3% glassfibre volume fractions) tested 14, 28 and 84 days from casting. The pseudo-toughness of the fihre-reinforced material was no higher than that of the unreinforced matrix at the initiation of crack growth, but thereafter increased linearly with crack growth at a rate proportional to the fibre content. The change of toughness for crack growths up to 12 mm could be measured; at this point, the 2·3% fibre content gave about 5 and the 1·1% content ahout 3 times the toughness of the matrix material, and the toughness was still increasing with crack growth. The psuedo-toughness was not affected by age up to 84 days from mixing.

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