Q.: We're building a slab on grade for a large warehouse. The engineers want us to place 5,000 square feet maximum for each pour and use a checkerboard sequence. We're able to place 10,000 to 15,000 square feet daily if we're allowed to use the long-strip method. Is there any literature that we can use to convince the engineer that our method will produce as good a floor?

A.: There's a widely held belief that checkerboarding eliminates or minimizes shrinkage cracking because earlier placements shrink before infill panels are placed. This supposedly reduces shrinkage stresses, shrinkage cracking, and joint widths. But according to the Portland Cement Association, the reasoning behind this idea is incorrect. Shrinkage of earlier placements occurs too slowly for the method to effectively reduce shrinkage and joint widths. Because drying shrinkage takes place over a long period, lengthy delays would be required between casting of adjacent bays to gain much benefit from checkerboarding. The American Concrete Institute's Guide for Concrete Floor and Slab Construction (ACI 302.1R-89) recommends using long-strip construction. Checkerboard construction is not recommended.