Q.: There has been a rash of inquiries in our area about where to purchase heavyweight aggregates. Inquiries have also been made about how to make concrete with a unit weight of 147 pounds per cubic foot (pcf) or 151 pcf. We think some federal agency may be the source of such unusual specifications. We wonder if such exact minimum unit weights are necessary, and if so whether there is any need to use heavyweight aggregate to obtain them. Even the higher unit weight seems almost within the normal range.

A.: Apparently some specifications have been written that call for unusually explicit unit weights. One set of specifications, written for a Department of Energy contract for a power company's experimental flue gas cleanup facility, called for "an in-place concrete density of not less than 147 pcf according to ACI 211.1-81." This applied to all concrete to be used for radiation shielding in various areas that were named in the specifications. A total of only about 200 cubic yards was needed. By using more cement and less sand the concrete producer was able to make concrete with unit weights ranging from 147.5 to 152 pcf without using any imported or special aggregate.

A minimum cured density of 147 pcf was also called for on another construction job, a hospital, in the specifications for the walls and roof of the linear accelerator room and passageways. The concrete supplier decided to design a mix with normalweight aggregate, aiming for a design unit weight of 151 pcf by replacing some of the sand with cement. The actual unit weight was 152 pcf. One sample is reported to have been coated with a sealer, weighed after 1 year, and found to still have a unit weight of 152 pcf.

For significantly higher unit weights heavier aggregates would undoubtedly have to be used, at least in part.