Q.: Our standard flatwork mix is six-bag, 3/4-inch limestone, with 6% air and 1 1/2 pounds of polypropylene fibers. It is placed at a slump of 4 inches. When ambient temperatures dictate, we use a maximum of 1% calcium chloride. Adding the chloride at the ready mix plant ensures adequate mixing, but results in the need to add water at the jobsite to maintain slump; something we are reluctant to do. Since adding chloride flakes to the mixer at the site carries its own potential problems, we premix the flakes in a bucket, then pour the solution into the mixer. Mixing the flakes in a bucket beforehand creates tremendous heat; energy that seems to be lost before the flakes are mixed with the concrete. Should we adjust the quantity of the chloride flakes to compensate for this, and does our calcium chloride solution have a shelf life before it is mixed into the concrete?
A.: We asked Guy Detwiler of Chicago's Material Service Corp. to respond to your question. Detwiler says you are following the proper procedures for adding calcium chloride. Adding flake calcium chloride directly to the mixer can lead to discolored concrete as well as soft spots in the surface. Although the heat of reaction for calcium chloride dissolving in a bucket seems high, it wouldn't have much effect on a truckload of concrete. The calcium chloride solution doesn't have a shelf life, but be sure to agitate the solution before mixing into concrete.