THE USE OF SALT.
The use of salt is objectionable, as it forms a white efflorescence on exterior surfaces and is liable to corrode the steel in reinforced concrete work. The quantity of salt required varies with the temperature, but it should not exceed 10 per cent of the weight of the water used in mixing. A 10 per cent solution is eight-tenths (approximately 13 ounces) of a pound of salt per gallon of water and will prevent freezing at a temperature of 22° F. Lower temperatures would require a greater proportion of salt, which would impair the strength of the concrete, and hence is not practicable.
A rule, frequently advocated, for varying the percentage of salt is to use 11/3 ounces per gallon of water for each degree Fahrenheit below freezing. Since it is impossible to foretell the exact drop in temperature, the exact quantity of salt can not be predetermined, so that provision should be made for several degrees lower than anticipated. The salt should be dissolved in the mixing water, and in order that the proportion be correct the amount of water required for each batch should be determined by trial and this quantity used throughout the work.