Effects of aggregate packing optimization and cement paste volume on the properties of natural and recycled aggregate concrete

This paper investigates the influence of aggregate packing optimization based on a particle packing method (PPM) and cement paste volume (CPV) on the workability and mechanical properties of both natural aggregate concrete (NAC) and recycled aggregate concrete (RAC). The slump, compressive strength, flexural strength and Young's modulus of concrete were first examined, followed with the characterization of macro-pore structure inside RAC by using X-ray computed tomography (CT). The results indicate that the PPM can help to densify the granular skeleton of aggregates in concrete by reducing the void around the aggregates and the cement paste film thickness, and subsequently enhances the mechanical properties of the concrete with a constant CPV. This enhancement is higher for RAC than NAC, particularly for the Young's modulus. The particle packing optimization can also decrease the number of macro pores in RAC, contributing to the enhancement in the mechanical properties of RAC. Moreover, increasing CPV in concrete with a maximum aggregate packing status considerably decreases the mechanical properties of concrete, whereas increases its workability. This is mainly caused by the increased number of macro pores in the cement paste and at the interfaces between RCAs and hardened cement paste.