Drop-Weight Impact Response of Fibre Reinforced Cement Based Foams

This paper describes the strength and toughness of fibre reinforced cement based foams subjected to bending under variable strain rates. Drop-weight impact tests were conducted on notched beams with cast density between 475 kg/m(3) to 1200 kg/m(3). The foams were reinforced with 0-0.2% polypropylene microfibre by volume and their role in improving the dynamic properties is illustrated. The structure of the cell wall within the foams was engineered through a suitable choice of foaming agent, such that foams with a predominantly closed cell structure were produced. Companion tests were conducted as per ASTM standards in compression and flexure under quasi-static loading. The results were compared to other brittle cellular solids from the literature, and the mechanical properties are described in terms of the relative density, defined as the ratio of the density of the cellular composite relative to that of Portland cement paste in the cell wall. The study shows that under quasi-static loading, the compressive strength, elastic modulus and modulus of rupture of plain mixes scale with the square of the relative density. On the other hand, the flexural toughness factor scaled linearly with it. Fibres were seen to increase the flexural strength at all rates of loading, regardless of cast density. However, the Poisson's ratio remained unaffected by the density of the mix. Further, cement based foams were seen to be strain rate sensitive but, the existing CEB-FIP model for cementitious materials vastly overestimates the dynamic impact factor at lower cast densities.