Q: How do the properties of recycled-aggregate concrete compare to those of concrete made with natural aggregates?

A: The Portland Cement Association's website (www.cement.org) has lots of good information about concrete technology. They answer this question as follows: Recycled aggregates typically have higher absorption and lower specific gravity than natural aggregate and produce concrete with higher drying shrinkage and creep. These differences become greater with increasing amounts of recycled fine aggregates. Too many recycled fines also can produce a harsh and unworkable mix.

Many transportation departments have found that using 100% coarse recycled aggregate but only about 10% to 20% recycled fines works well. The remaining percentage of fines is natural sand. The chloride content of recycled aggregates is of concern if the material will be used in reinforced concrete. However, the quantity of chloride typically found in old concrete pavement is usually below critical threshold values.