Mechanical and environmental suitability of recycled concrete aggregate as a highway base material

The use of recycled concrete aggregate (RCA) materials in highway constructions is economically viable and reduces virgin natural resource demands on the environment. To evaluate their potential use in highway construction, two different RCA materials and four virgin granular aggregate base (GAB) materials and their select mixtures were tested in the laboratory for strength, resilient modulus, permanent deformation, and durability. Laboratory water leach tests and pH-dependent leaching tests were conducted to determine the environmental suitability of the RCA materials. The leaching behavior of Ca, Cr, Cu, Fe, and Zn and the effects of pH, curing time, liquid-to-solid (L:S) ratio, and particle size on metal leaching from the RCAs were investigated. Summary resilient moduli (SM_R) of the RCAs were up to 2.6 times higher than that of the GAB materials. Their stiffness also increased when subjected to freezing and thawing cycles. Consistent trends could not be observed with percent RCA addition and strength or SM_R; however, the RCAs generally yielded higher permanent deformations compared with the GABs. Prolonged curing caused rehydration of cement particles and, in general, yielded a decrease in pH and leached metal concentrations. Increasing the L:S ratio decreased leaching of elements significantly because of the dilution of leached elements in the aqueous solutions. pH excursions yielded cationic leaching patterns for Ca and amphoteric leaching patterns for Cr, Cu, Zn, and Fe.