Q.: What is wrong with putting flake calcium chloride in the concrete truck at the jobsite to offset cold weather effects on setting and strength gain?
A.: Calcium chloride may not dissolve completely if it's added in dry form, either at the plant or at the jobsite. Also, it may not get mixed uniformly throughout the concrete. Nonuniform mixing can make the concrete set unevenly, causing finishing problems. Undissolved lumps can cause popouts or dark spots in the hardened concrete.
Adding calcium chloride in a water solution prevents these problems. To make the solution, use 1 pound of calcium chloride per quart of water and be sure to put the dry material in the water rather than pouring water on the dry material. If you add water to the calcium chloride, a coating may form and keep the material from dissolving completely.