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 1-1/3 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.