Optimizing mix proportion of recycled aggregate concrete by readjusting the aggregate gradation

It is necessary to ensure the properties of recycled aggregate concrete (RAC) can meet the requirements of those for the corresponding natural aggregate concrete (NAC), since only in such cases recycled aggregates (RA) may be accepted for structural use. This study aims to propose a modified design method of RAC based on aggregate gradation readjustment (AGR). It tries to maintain the effective amounts of both the coarse and fine aggregates of RAC similar to that of the NAC after concrete mixing as it is well known that due to the weakness of the old attached mortar to RA, the gradation of the aggregates would be undesirably changed. Two series of concrete mixtures were conducted. One was to examine the variations of the compressive strength and porosity along with the aggregate adjustment ratio using a single RA source; while the other one was used to verify the effect of the proposed AGR method through the investigation of concrete properties made with RAs obtained from different sources. The results show that the proposed method was beneficial to improving all the properties of RAC investigated.