Effects of fibres on ultra-lightweight high strength concrete: Dynamic behaviour and microstructures

The use of ultra-lightweight high strength concrete (ULHSC) for prefabricated structures has been recognized as a promising method of construction. Adding fibres commonly enhances the mechanical properties of ULHSC but increases its density. In order to balance the strength and self-weight of ULHSC with different fibres, an extensive investigation was carried out to understand the effects of fibre incorporation on ULHSC made with fly ash cenospheres. This study focuses on the dynamic compressive response and failure mechanism of ULHSC with varied fibre contents. Impact tests were conducted by using a phi 100-mm splitting Hopkinson pressure bar apparatus with different strain rates ranging from approximately 20 s-1 to 120 s-1. The results showed that the compressive properties of ULHSC exhibited a strong strain rate dependency, and the addition of fibres increased the strain rate sensitivity of ULHSC, especially at high strain rates. It was interesting to find that the dynamic compressive strength increased with increasing fibre content, and the dynamic increase factor (DIF) showed the same tendency. These findings indicated that the tested ULHSC with 1 vol% end-hooked steel fibre maintained an excellent balance between the dynamic strength and density. Finally, a recalibrated model considering the effect of fibres was proposed to predict the DIFs of ULHSCs under impact loads.