Lightweight concrete on a slope

Concrete Construction, June, 2003

Q. I have been told that lightweight concrete cannot be poured to form a sloping surface because it will self-level. What can you tell me about this?

A. The only thing that really matters is the slump of the mix being placed. The same is true of normal concrete mixes. If you use a slump of 5 inches or lower, there is no reason you can't place concrete on the pitches you require (1/8" and 1/4" per foot). If the slump is excessively high, there could be trouble--just as with regular concrete.

There are different ways to produce lightweight concrete. You didn't mention how you intend to do it. One is to use lightweight aggregates. In the past, I've placed considerable amounts of this concrete and didn't notice any differences in placing or finishing. Another is to increase the air entrainment count to 25 or 30%. There are other ways, too, each with its own advantages and disadvantages.

For more about lightweight concrete mixtures and their performance, get the Portland Cement Association's (PCA) recently updated book Design and Control of Concrete Mixtures by Kosmatka, Kerkhoff, and Panarese. Call PCA at 847-966-6200 or visit its Web site at www.portcement.org.