Effect of rigid fibres on aggregate packing

Abstract

The addition of rigid fibres, such as steel fibres, to concrete would cause significant decrease in workability, thus rendering the fibre reinforced concrete (FRC) so produced more difficult to compact. Whilst the entanglement of the rigid fibres is definitely one contributing factor, the authors are of the view that another contributing factor is the loosening of aggregate packing, which increases the volume of voids between aggregate particles to be filled with paste. In this research, the effect of rigid fibres on aggregate packing was studied by adding different types of steel fibres at different volumes to aggregates of different sizes and measuring the packing density of the aggregate-fibre mixture. It was found that the proportional decrease in packing density due to the addition of steel fibres increases linearly with the fibre volume but at different rates for different fibre types and different aggregate sizes. Generally, fibres with larger aspect ratios have larger effect and smaller size aggregates are less affected. Lastly, based on the test results, some useful guidelines and a packing model for the mix design of steel FRC were developed.