Latex-modified cement mortar reinforced by short carbon fibres XIAOMING YANG and D.D.L. CHUNG (State University of New York at Buffalo, USA) Received 6 October 1991; accepted 13 May 1992 The addition to cement mortar of 0.37 volume% (vol%) of short, pitch-based carbon fibres, together with latex (styrene-butadiene) and antifoam, increased the flexural strength by 54% and the compressive strength by 30% at 28 days of curing; relative to mortar containing latex and antifoam, the fibres increased the flexural strength by 33%, had little effect on the compressive strength and increased the price by 17%. In addition, the fibres increased the flexural toughness and decreased the electrical resistivity. Fibre contents /> 1.1 vol% resulted in substantial degradation of the compressive strength and slight degradation of the flexural strength at 7 days of curing, relative to the strengths at a fibre content of 0.37 vol%. The optimum latex/cement ratio was 0.2. The latex served to disperse the fibres and increase the bonding between the fibres and the matrix. Partial replacement of cement by silica fume in mortar containing latex, antifoam and fibres did not increase the flexural strength. Key words: composite materials; fibre-reinforced cement; latex modifier; flexural strength; compressive strength; composition; curing time; short carbon fibres Latex-modified cement mortars and concretes are attractive because the latex addition substantially increases the flexural strength and the compressive strengthL< On the other hand, fibre-reinforced mortars and concretes are attractive because the fibre addition substantially increases the flexural toughness and, in some cases, it increases the flexural strength as welP 5. It is therefore appropriate to combine these two methods. Among fibres, carbon fibres are attractive because of their chemical stability, low density, high strength, high modulus and rapidly decreasing cost. An increase of 100% in the flexural strength was achieved in mortar at 7 days of curing with the addition of short (5 mm long) pitch-based carbon fibres in the amount of only 0.2 volume% (vol%), together with methylcellulose (a dispersant) and an antifoam5. Larson et al. 6 added short (1.7 mm long) carbon fibres in an amount of 3 vol% to latex-modified mortar and tested the resulting composites at 7 days of curing, but the fractional increase in strength due to the carbon fibre addition was not determined. Soroushian et al. 7 added short (1.5 mm long) carbon fibres in the amount of 3 vol% to latex-modified mortar containing silica fume and tested the resulting composites at 14 days of curing, but the fractional increase in strength due to the carbon fibre addition was again not determined. On the other hand, the latex improved the bonding between the fibres and the matrix6,7, increased the flexural toughness, decreased the compressive strength, had little effect on the flexural strength, increased the freeze-thaw durability and decreased the drying shrinkage, relative to the case without latex. (Both cases with and without latex contained silica fume and carbon fibresL) This work provides a systematic study of the effect of different volume fractions of carbon fibres on the flexural and compressive properties of latex-modified cement mortar at curing ages of up to 28 days. With 0.73 vol% of carbon fibres in latex-modified mortar, a flexural strength of 11.3 (+ 5%) MPa was attained in this work at 7 days of curing; this corresponds to a flexural strength increase of 54% relative to the unreinforced latex-modified mortar. Almost the same 7-day flexural strength was reported in Reference 6 for latex-modified mortar containing 3 vol% of carbon fibres. With 0.73 vol% of carbon fibres in latex- modified mortar, a flexural strength of 12.7 (+ 1.8%) MPa was attained in this work at 14 days of curing; about the same 14-day flexural strength, namely 13 MPa, was reported in Reference 7 for latex-modified mortar containing 3 vol% of carbon fibres. Furthermore, the systematic study of this work showed 0010-4361/92/060453~08 © 1992 Butterworth-Heinemann Ltd COMPOSITES. VOLUME 23. NUMBER 6. NOVEMBER 1992 453