We supply ready-mixed concrete to residential contractors in the Northeast. During cold weather, our customers order concrete with 2% calcium chloride but then add more flake calcium chloride at the jobsite. Are there any harmful effects of this practice? For instance, if the chloride dosage is too high, will the concrete set too fast?
There are several possible harmful effects, and the fact that the customer added the flakes should release you from responsibility for any callbacks. Flake calcium chloride added to the truck at the jobsite isn't likely to get uniformly distributed throughout the concrete, which can cause different portions of concrete flatwork to set at different rates. That makes it tough for the finishers to do their job well. If some of the flakes don't dissolve, they may clump together and float toward the surface, causing visible bumps or popouts. Also, the undissolved lumps may cause dark spots on the hardened concrete. Finally, very high doses of calcium chloride could cause flash setting, although this is less likely in cold weather.