What can be done to prevent blistering?
Usually blistering occurs when the mix contains a high percentage of sand passing the Number 30, Number 50 and Number 100 sieves. This mix will tend to be sticky. Frequently all that is needed is to reduce the quantity of sand in the mix. A reduction of 100 to 200 pounds of sand per cubic yard may be enough. (The sand must be replaced by the smallest size coarse aggregate available.) Entrainment of air in the concrete mix also reduces blistering. If the blistering is being caused by a subgrade temperature that is lower than the concrete temperature, the subgrade should be heated before placing the concrete. Windbreaks and sunshades are helpful also. Using internal vibration followed by low frequency external vibration on each lift can be used to release air. The low frequency can be supplied by an electric hammer fitted with a 2 1/2-inch diameter button over the end. On concretes with slumps from one to 2 1/2 inches care should be taken to keep the number of passes to a minimum when a vibrating screed is used. Floating the concrete a second time helps to minimize blistering and it is helpful to use a magnesium float at least on the first floating. If power floats are used they should be light-duty floats. When high surface friction requires the use of a fog spray for the last machine pass, the nozzle volume for the fog should not exceed one-half gallon per minute.