Suitability of recycled concrete aggregate for use in binary cement concrete

The principles of sustainable construction require the prudent use of natural resources and the maximum recycling and reuse of waste. In keeping with this approach, much research was undertaken to increase the use of recycled aggregates derived from construction and demolition wastes as an alternative to primary aggregates in construction. It is now increasingly recognised that the use of coarse recycled concrete aggregate (RCA) in concrete construction represents a further potential outlet for the material. Several investigations have been made to study the effects of coarse RCA on the fresh and hardened properties of Portland Cement (PC) concrete. While these studies suggest the potential use of RCA in a range of concrete applications, issues relating to its suitability in binary cement concrete mixes, relevant to practice, have not been addressed. Against this background, the present study was undertaken to examine the suitability of using coarse RCA in BS 8500 designated concrete mixes produced using binary cements. The binary cements selected were (i) PC/PFA cement, a blend of 70% PC/ and 30% Pulverised Fuel Ash by mass and (ii) PC/SF cement, a blend of 90% PC and 10% Silica Fume by mass. The binary cements were blended in the mixer during concrete production. The effects on the fresh, engineering and durability properties of concrete, of replacing coarse natural aggregates (NA) by coarse RCA by up to 100% in concrete have been established. The RCA mixes were designed to achieve equal 28 day cube strengths as their corresponding NA mixes. The aggregate characterisation results showed that concrete debris obtained from construction and demolition waste can be used to produce clean and properly graded RCA suitable for use in concrete production in accordance with the EN 12620 requirements. The results of the fresh properties of concrete showed that although the slump measurements remained within the allowable tolerances, the use of high RCA contents affected the workability and stability of the mixes. Studies of the hardened concrete properties, comprising the bulk engineering (Compressive cube and cylinder strength, flexural strength, modulus of elasticity, drying shrinkage and swelling deformations) and durability properties (near surface absorption, carbonation, chloride ingress, sulphate attack) showed that RCA concrete mixes made with binary cements had, a comparable or better performance when compared to their corresponding concrete mixes made with PC only. Practical implications derived from the findings of the study are also outlined for the use of RCA in binary cement concrete construction. Overall, the study has shown that RCA is suitable for the production of a wide range of designated mixes made with binary cements with a satisfactory engineering and durability performance, provided that the mixes are designed for equivalent 28 days cube strength