Q. We are going to place a slab using shrinkage-compensating concrete. Do we need to change the amount of steel used for “shrinkage” steel, the steel that is used for control of crack widths?
A. We asked Jerry Holland who has worked on a lot of slabs using shrinkage-compensating concrete (SCC). He says, no, “but there are a few other things to consider, some of which are in ACI 223, Standard Practice for the Use of Shrinkage Compensating Concrete. The steel percentage depends on the thickness of the slab and if it is being used structurally (such as in a tank mat foundation that doubles as a secondary containment for a tank farm). Generally, for a typical 6-inch slab the reinforcement would be #4 rebars at 18 to 22 inches on center. It must be placed in the upper third of the slab (thus, making welded wire fabric a poor choice, unless it is the heavy wire ‘Big Foot Mesh') in order for it to properly serve as the ‘rubber band' we seek. Because Wayne Walker and I space our joints at about 150 feet (and thereby get rid of about 90% of the joints), we usually armor the few remaining ones in case any decide to get a little wider than normal and also to minimize the owner's maintenance. Two steel bars 3/8-inch thick by 2 inches wide with anchors works much better than angles—also PNA has a surprisingly low-cost form and armored joint assembly that is very fast and easy to install. We like to use diamond dowels for load transfer across the joints because there is significant differential movement parallel to the joint in the first few hours and days after placing a slab panel next to a previously placed one. However, we space the dowels farther apart than usual because they do not have to work as hard (due to the SCC eliminating curl, if properly done).”