Q: What is a reliable method of preventing curling in floor slabs?

Vacuum dewatering of concrete slabs can be used to reduce significantly the amount of free water in the concrete. This free water is usually responsible for the uneven shrinkage from top to bottom of the slab that causes the curling at joints and edges. It is claimed that if the subbase is properly prepared, joints properly located, concrete thoroughly consolidated and the slab vacuum processed, curling can be eliminated. The vacuum applied to the face of the slab withdraws a large quantity of water and the atmospheric weight compresses the plastic concrete to densify the mass. This eliminates the interstitial canals otherwise formed during the bleeding process and tends to make the slab less permeable to further water movement.

Proper curing should be done. Vacuum processing will not eliminate engineering mistakes but it will complement other measures taken to produce quality. Properly made, vacuum processed slabs are said not to curl or need uncurling.

Editor's note:

This answer is based on information provided by Jimmy Fikes, of Danco Vacuum Concrete Corporation, Sausalito, California in answer to our inquiry to readers on page 302 of the July issue, "Does anybody know a sure-fire method of preventing curling in floor slabs?" For more information on preventing curling see the article "Curling of floor slabs" on page 391 of this issue.