A Novel Mix Design Method for Mixed Recycled Coarse Aggregate Concrete

Abstract

Recycled aggregates extracted from the construction and demolition waste of brick and concrete building are mixed recycled aggregates (MRA), which comprise concrete recycled aggregate (CRA) and brick recycled aggregate (BRA). The components of coarse aggregate used in concrete incorporated with MRA are natural aggregate (NA), CRA, and BRA. Due to the complex component of coarse aggregate used in mixed recycled aggregate concrete (MRC), the conventional mix design method is not suitable for MRC. The increase of coarse aggregate heterogeneity, larger total mortar volume of MRC, and the weaker strength of CRA and BRA are the primary differences between MRC and natural concrete (NC). There is no literature to propose a mix design method suitable for MRC with regard to the defects mentioned previously. To address this issue, a novel mix design method was proposed. First, the minimum density difference (MDD) method was proposed to minimize the density difference between fresh mortar and the MRA of MRC, so as to improve the homogeneity. And then the mortar volume in MRC is equal to that of NC by using the equivalent mortar volume (EMV) method. Finally, the minimum paste theory is combined with MDD and EMV methods to determine the parameters of mix proportion, so as to compensate for the defect of too little cement paste content caused by the EMV method. The experimental results showed that when the water:cement ratio is 0.5, compared with the conventional concrete mix design method, the proposed mix design method can increase the compressive strength and elastic modulus by 5% and 22.8%, respectively, and reduce the heterogeneity degree and shrinkage value (90 days) by 8.5% and 23.4%, respectively. When the water:cement ratio is 0.65, the compressive strength and elastic modulus increased by 4% and 33.5%, respectively, and the heterogeneity degree and shrinkage value (90 days) decreased by 28.44% and 23.7%, respectively. The proposed mix design method also consumed a lesser quantity of cement.