Comparing performance of modified two-stage mixing approach for producing recycled aggregate concrete

Exhaustion of landfill areas coupled with the extensive redevelopment programme in Hong Kong has prompted the use of recycled aggregate. However, the inferior quality of recycled aggregate (RA) has restricted its use to low-grade applications such as roadwork sub-base and pavements, while its adoption for higher-grade concrete is rare because of the lower compressive strength and higher variability in mechanical performance of RA. A new concrete mixing method, that is the two-stage mixing approach, was advocated to improve the quality of RA concrete (RAC) by splitting the mixing process into two. The current paper describes two modified mixing methods with some alteration to the two-stage mixing approach by proportioning ingredients of cement and water with the percentage of RA added in the first mix, referred to as the two-stage mixing approach((proportional-1)) (TSMA(p1)), and proportioning the cement content (without water) with the percentage of RA used in the first mix, referred to as the two-stage mixing approach((proportional-2)) (TSMA(p2)). Based on experimental works and results, improvements in strength to RAC were achieved with both TSMA(p1) and TSMA(p2). This can be attributable to the porous nature of RA and the pre-mixing process that fills up some of its pores and cracks, resulting in a denser aggregate and concrete. An improved interfacial zone around RA with lower water/cement ratio generated from TSMA(p2) without changing the ultimate water/cement ratio gives a higher strength than the normal mixing approach (NMA) and TSMA(p1).