Enhancing the performance of recycled aggregate concrete with microsilica

Recycled aggregate was used as replacement for crushed gravel between 0-100% with an increment of 25%. Synthetic macro fibre and microsilica were added to some of the concrete mixes to improve their mechanical properties. The control mix was designed to have a 28-day characteristic cube strength of 50MPa, water/cement ratio of 0.39 and high workability (60-180 mm). Physical (slump) and mechanical (compressive strength, flexural strength, splitting tensile strength, and modulus of elasticity) tests were conducted on fresh and 660 hardened concrete samples respectively. The aim was to investigate the use of higher percentages of recycled aggregate than the current 20% level recommended by BS 8500. Results show reduction in the physical and mechanical properties with increasing recycled aggregate content. Addition of synthetic macro fibre had no significant effect on the concrete compressive strength. However, the concretes with synthetic macro fibre had higher flexural strength, splitting tensile strength, and elastic modulus compared with those without synthetic macro fibre. Addition of 5% microsilica to the mix with 50% recycled coarse aggregate produced a 28-day compressive strength slightly higher than the target mean compressive strength of 63MPa. This result suggests that there is a potential for increasing the optimum fraction of recycled coarse aggregate in concrete from 20% to 50%.