Effect of the Water to Binder Ratio and Ground Fly Ash on Properties of Recycled Aggregate Concrete

Abstract

This research aims to study the effect of water to binder (W/B) ratio and ground fly ash on properties of recycled aggregate concrete. Normal concretes were designed by using Portland cement type I with W/B ratios of 0.45, 0.55, and 0.65. Recycled aggregate concretes were made by using recycled aggregate to fully replace crushed limestone in the mix proportion of the normal concretes. Ground fly ash was also used as a partial replacement of Portland cement type I at 20, 35, and 50% by weight of binder in order to improve the properties of recycled aggregate concrete. Compressive strength and water permeability of recycled aggregate concretes were determined at 28 and 90 days. In addition, the modulus of elasticity of concrete was investigated. The results showed that the use of ground fly ash to replace cement in recycled aggregate concrete could improve the compressive strength to be higher than that of concrete without ground fly ash at the same W/B ratio. Ground fly ash could be used throughout to replace cement up to 35% by weight of binder for recycled aggregate concrete with W/B ratio of 0.45 and should not exceed 20% for recycled aggregate concretes with W/B ratios of 0.55 and 0.65. For the same compressive strength, the modulus of elasticity of recycled aggregate concrete with and without ground fly ash was lower than that of normal concrete by about 16–20%. Moreover, the use of ground fly ash to replace cement in recycled aggregate concrete was more effective for reducing the water permeability coefficient than the reduction of the W/B ratio.