Effect of the maturity of recycled aggregates on the mechanical properties and autogenous and drying shrinkage of high-performance concrete

Abstract

The high cement content of high-performance concrete (HPC) results in improved strength, but also in large shrinkage. The substitution of natural aggregates (NA) with recycled aggregates (RA) notably affects these properties in conventional concrete. This study intends to analyse the effect of the content of RA and their maturity (time elapsed between casting and crushing of the parent concrete from which RA are obtained) on these properties of HPC. To this end, five mixes were manufactured with 0%, 25%, and 100% of coarse and fine RA of different maturities, 7 days (early-age RA) and 6 months (matured RA). The mechanical properties and the autogenous, drying, and total shrinkage of all mixes were determined. Both the increase of RA content and the lower stiffness and strength of early-age RA relative to matured RA worsened the mechanical behaviour of HPC. Regarding shrinkage, the lower stiffness and higher water absorption of matured RA compared to NA decreased autogenous shrinkage by 20%, but increased drying and total shrinkage. The lower stiffness of early-age RA and their shrinkage amplified all types of shrinkage of HPC around 10–20%. Nevertheless, the hydration of their unhydrated cement particles during the mixing process reduced those increases from what was expected. Total shrinkage of early-age recycled aggregate HPC could be estimated from the shrinkage of both a HPC mix of identical composition but with matured RA and the parent concrete. Overall, it can be concluded that RA’s maturity affects the mechanical performance and, especially, the shrinkage of HPC, so it should be considered when using this type of aggregate.