Q.: Please explain flowing concrete. Is it the same as superplasticized concrete? If it flows so well, doesn't it segregate? Is it really as strong as concrete made with a lower slump?

A.: So-called flowing concrete is concrete that is made to a slump that is temporarily higher than normal. It is usually made by adding admixtures after the concrete arrives at the job site at a slump, for example, of 2 inches. The proper amount of admixture is added and thoroughly mixed in. The slump increases to perhaps 6 or 8 inches but then declines over a period of about 1/2 hour to the original 2 inches or less. In the meantime the slump is high enough to greatly facilitate placement operations. Although there have been some reports of segregation, it is said this seldom happens (see "How Super Are Superplasticizers, Concrete Construction, May 1982, page 409).

Admixtures used for flowing concrete are sometimes called high range water reducers and sometimes superplasticizers. Flowing concrete can also be made, however, by using two separate conventional admixtures at the same time; these jointly serve to provide the needed slump and control the set for the particular conditions of the job. The choice of two admixtures is said by some to have advantages over high range water reducers but the particular admixtures used must be selected specifically for the job.

Although flowing concrete should theoretically have about the same strength as normal concrete, reports indicate that it is sometimes a little bit stronger.