Q. We are placing a 4-inch-thick concrete floor for a 93,000-square-foot department store, but the architect has limited the size of each pour to 2,160 square feet to prevent premature shrinkage cracks. This limitation will severely cut our productivity and increase our costs. How can we convince the architect to allow larger pours?
A. It would help to know what the architect means by "premature shrinkage cracks." If this is a reference to plastic shrinkage cracking, the architect may believe that about 2,000 square feet is the maximum surface area you can place and cure without danger of plastic shrinkage cracking. In that case, we'd argue for alternative methods of evaporation control such as the use of evaporation retarders or fog sprays. The architect may be basing the maximum pour size on the reasoning used many years ago to justify checkerboard placing operations. Some designers believed that by placing bays in a checkerboard sequence, with side dimensions of 50 feet or less, the bays cast first would shrink enough to minimize further joint opening. This belief may be why some architects today presume that keeping bay widths under 50 feet will reduce the tensile stresses caused by early drying shrinkage. As noted in the reference, experience has shown that shrinkage of the earlier placements occurs too slowly for this method to be effective. The method also increases forming costs and makes access more difficult when the concrete is placed. Finally, the large number of construction joints makes it more difficult to build a flat and level floor. ACI Committee 302 recommends that the checkerboard sequence of placement not be used.
ACI 302.1R-96, "Guide for Concrete Floor and Slab Construction," American Concrete Institute, Farmington Hills, Mich., 1996, p. 27.