Use of Recycled Concrete as Aggregate.

Supplies of natural mineral aggregates are diminishing as their usage increases. Disposal problems exist because of steadily increasing accumulation of solid wastes. In light of these two situations an investigation was made to evaluate the use of crushed waste concrete as concrete aggregate. If such use is practical, it will help to alleviate both problems. A discarded concrete driveway that contained siliceous aggregates and a laboratory concrete beam that contained limestone as coarse aggregate and natural siliceous sand as fine aggregate were selected. Portions of each kind of concrete were processed into aggregate sizes. Three test mixtures and two control mixtures were made. Specimens from each round of each mixture were tested for compressive strength at different ages up to six months, for resistance to accelerated freezing and thawing, and for volume changes due to temperature changes or to moisture effects at a constant temperature. The aggregate particles produced by crushing concrete had good particle shape, high absorption, and low specific gravity by comparison with conventional natural mineral aggregates. Other results revealed that use of crushed concrete as aggregate a had no significant effect on mixture proportions or workability, b resulted in slightly lower compressive strengths, c had no significant effect on volume response of specimens to temperature or moisture effects, and d resulted in a significant increase in frost resistance when the crushed driveway concrete was used, but essentially no effect on frost resistance when the crushed beam was used.