Q: We are considering purchasing concrete from a plant that uses recycled aggregate. This job will continue for some time, so we are concerned about whether the plant will be able to deliver the same quality of concrete throughout the job. What worries us is that the recycled aggregate may contain all kinds of things--retarders, accelerators, water reducers, air-entraining agents--one kind on one day and another kind or two on another. What is this going to do to our concrete? Won't it sometimes be accelerated, sometimes retarded and sometimes air-entrained, depending on where the recycled aggregate comes from?
A.: The effects of the admixtures in the recycled aggregates should be almost negligible. As a matter of fact, two Danish researchers, Torben Hansen and Soren Hedegard, have reported an interesting investigation of this subject in the January-February 1984 ACI Journal (pages 21 and 26). They made some concretes that were identical except that one contained calcium chloride accelerator, one a retarder, one an air-entraining agent, one a superplasticizer and one of them no admixture at all. After these concretes had been cured for 39 days, portions of them were broken up and made into aggregate of about the same gradation as the original. The five aggregates from the five concretes were then used to make concretes of approximately the same mix proportions as the original nonadmixtured concrete. It turned out that the investigators could see little difference among these recycled concretes regarding their setting times or strengths. Nor did the recycled aggregate from the air-entrained concrete have any significant effect on the air content of the new concrete. The air containing calcium chloride did raise the chloride content of the new concrete, but not enough to account for all the original chloride. Further details are given in the article cited.