Effect of fly ash and silica fume on compressive and fracture behaviors of concrete

The effects of replacing cement by fly ash and silica fume on strength, compressive stress-strain relationship, and fracture behavior of concrete were investigated. The investigation covered concrete mixes at different watercementitious material ratios, which contained low and high volumes of fly ash, and with or without the addition of small amount of silica fume. It was found that fly ash substantially improved the post-peak compressive behavior of concrete, with a relatively smaller gradient in the descending part of the stress-strain curve. Low volumes of fly ash improved the tensile strength of concrete. High volume fly ash concrete showed slightly lower tensile strength, but higher values of crack tip opening displacement and final mid-span deflection in the fracture tests, with the corresponding KICand GFvalues similar to or higher than the plain cement concrete. A small amount of silica fume had a large positive effect on the cylinder compressive strength and tensile strength but less on the cube compressive strength, while the fracture behavior of the resulting concrete was brittle. Improving interfacial bond between the: paste and the aggregates in concrete had positive effects on KIC, but did not necessarily produce higher GFvalues.