Environmental suitability of recycled concrete aggregate in highways.
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Environmental suitability of recycled concrete aggregate in highways.

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      The use of recycled concrete aggregate materials in highway constructions as compared to the use of virgin materials reduces virgin natural resource demands on the environment. In order to evaluate their potential use of recycle materials in highway construction, two different recycled concrete aggregate (RCA) materials and two conventional graded aggregate base (GAB) materials and the mixtures of RCA-GAB materials were tested in the laboratory for their strength (California Bearing Ratio), resilient modulus, permanent deformation, and durability. In addition, laboratory water leach tests (WLTs), and pH-dependent leaching tests were conducted to determine the environmental suitability of RCA materials. The leaching behavior of Ca, Cr, Cu, Fe, and Zn from RCAs and the effects of pH, curing time, freeze/thaw cycles, liquid-to-solid ratio, and particle size on leaching of these metals were investigated. The summary resilient moduli (SMR) of RCAs were 2.6 and 2 times higher than that of the natural GAB material and their stiffness increased when subjected to freezing and thawing cycles. Consistent trend could not be observed with CBR and SMR values and percent RCA addition. Overall, GAB yielded lower permanent deformations compared to the RCA materials. In pH-dependent leaching tests, Ca showed cationic leaching patterns, while Cr, Cu, Zn, and Fe showed amphoteric leaching patterns. Leaching tests results indicated that leached Zn concentrations were below the detection limits at pH>5. Increasing curing time caused rehydration of cement particles and generally yielded a decrease in pH and leached concentrations of metals. Freezing and thawing of RCA also led to self-cementing and decreased pH, Ca concentrations while no consistent trends were found between the leaching of Cr, Cu, and Fe concentration and freeze-thaw cycles. Increasing the liquid-to-solid ratio decreased leaching of elements significantly due to the dilution of leached elements in the aqueous solutions.
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