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Q.One of the contractors I'm interviewing to install a driveway surfaced with interlocking concrete pavers has suggested using recycled concrete rather than processed stone as a base material. He claims he's used it already on several driveways and that it has held up well, but I'm not very familiar with this material. Is this a good or a bad idea?

A.Bill Palmer, editor of Concrete Construction magazine, responds: Recycled concrete has been used extensively as aggregate in highway work around the country. A recent study by the Federal Highway Administration's Pavement Recycling Team looked at the use of recycled concrete aggregate (RCA) in five states; all five reported that RCA used as a base material performs as well as or better than virgin material.

As an added bonus, using RCA as a base keeps it out of landfills. The Texas Department of Transportation recycles nearly 100 percent of its concrete rubble. In the Denver area, about 6 million tons of concrete from the old Stapleton Airport runways has been recycled for various purposes.

Recycled concrete has been used in many areas as aggregate in making new concrete, although there have been problems with the workability of the mix. Generally, these have been overcome by making sure the RCA has consistent moisture content when it goes into the mix.

When used as a base material, RCA should be placed wet — nearly saturated — and compacted with a steel roller. But this could create a problem if it's being used underneath pavers, because RCA can contain some unhydrated portland cement.

Interlocking pavers need a base that allows drainage; if there is free cementitious material in the RCA, it could set into a low-strength mass that doesn't drain well. The likelihood of this problem occurring depends on the source of the recycled concrete, which typically is produced by state DOTs or by specialized companies doing big jobs like runways or roadways.

Thus, the RCA should be tested prior to use.