Use of recycled concrete as a new aggregate

Large urban centres in Canada and the United States are finding it increasingly difficult to locate and develop natural aggregate sources for use in concrete. The reason for this is twofold: first, the increased tempo of construction during the last three decades has exhausted most of the easily accessible sources; second, the pressure from environmentalists has increasingly affected exploration and exploitation of new sources. The study reported herein was therefore undertaken to investigate the possibility of producing aggregates from discarded concrete control test cylinders, normally sent to waste dumps after testing. A series of 2.2-ft3 (0.062-) concrete mixes were made covering the low, medium and high strength levels. Two sets of mixes were made at each strength level, one set consisted of a control mix and a mix made using coarse aggregate prepared from recycled concrete and reference fine aggregate; the second set consisted of a control mix, and a mix using fine aggregate prepared from recycled concrete and reference coarse aggregate. Cylinder and prism specimens were cast from each mix to determine mechanical properties of concretes at various ages and to study the durability of concrete after exposure of freeze-thaw cycling. The analyses of the test results have shown that satisfactory concrete can be made with aggregates prepared from recycled concrete. At higher water: cement ratios, the compressive and flexural strength of concrete made with aggregates prepared from recycled concrete are somewhat lower than the strength of concrete made with reference aggregates; however this can be overcome with suitable adjustments in mix proportions. At lower water: cement ratios, strength of the two types of concrete are comparable. The durability of concrete made with aggregates prepared from recycled concrete is comparable to the durability of concrete made with reference aggregates.