Chopped Fibre Dosage and Material Effects on the Fresh Properties of Normal Strength and Density Concrete

Chopped fibre reinforced concrete (CFRC) is used in many applications, including shotcrete tunnel walls, bridge decks, pavements, and concrete slabs. Cracked concrete’s service performance can be improved using chopped fibres. The influence of chopped fibre type and volume on fresh concrete properties and mixture design was investigated in this study. Trial mixtures were used to optimize a mixture initially developed using the American Concrete Institute’s absolute volume method. Ten normal strength and density test mixtures were then prepared based on these optimized trial mixtures. Mixtures included one control (i.e. no fibres) and nine CFRC mixtures. Mixture parameters included chopped fibre type (steel, glass, and a combination of the two) and fibre dosage (0.5, 1.0, and 1.5%). The bulk fresh, one-day dry room temperature, and 28-day humidity room densities were obtained from 10 cylinders in each mixture. Workability was measured using both slump tests and Ve-Be time tests. Results show significant reductions in slump with increased fibre content, and glass fibre mixtures had less workability than steel fibre mixtures. The hybrid steel and glass fibre mixtures allowed for further study of each fibre’s effect on workability. There was a good correlation between the slump and Ve-Be time results for mixtures with low fibre dosages, but Ve-Be time tests were found to be more appropriate to assess the workability of stiff concretes such as those with a high dosage of glass fibres. Both fresh and dry densities of CFRC cylinders increased due to adding steel fibres, and there was a minor reduction in these densities of glass fibre cylinders. Outcomes of this study will provide concrete mixture designers and structural engineers with additional guidance on mixture design and proportioning for CFRC to obtain desired fresh properties.