Q. I have to place an 8-inch structural slab on top of piles and grade beams around the slab perimeter. The weather is cold, and the ground is frozen. Is there a problem with placing slab concrete on top of the frozen subbase?

A. Placing concrete on a frozen surface may cause the lower layer of concrete to freeze or greatly prolong setting of the concrete. Several American Concrete Institute documents prohibit or warn against placing concrete on a frozen subbase. ACI 306.1-90, '''Standard Specification for Cold Weather Concreting,''' requires thawing the subgrade to a depth specified in contract documents before beginning concrete placement. ACI 301-96, '''Standard Specification for Structural Concrete,''' requires the subgrade to be free from frost or ice before concrete placement. ACI 306R-88, '''Cold Weather Concreting,''' states that concrete should not be placed on frozen subgrade material.

You can try thawing the subbase by covering it with insulating material a few days before concrete placement, but in many cases external heat has to be applied.

Check your project'''s specification requirements for any details on required subbase temperature or depth of subbase thawing required.