On the other hand, alternate freezing and thawing before hardening is complete, is very apt to injure concrete. Accordingly, if concrete has once been frozen and thawed out before the action of hardening has commenced, it should be protected from freezing again until it has had a chance to harden sufficiently to withstand the action of subsequent frosts.
The freezing of a layer of concrete is very apt to prevent a good bond with another layer placed on top. In sidewalk and pavement work, freezing during construction is apt to cause a thin layer (about inch in depth) to peel off, leaving the wearing surface rough; and a simi lar effect is sometimes seen on concrete walls. The injury, however, will not be deep-seated unless the whole mass has been subjected to repeated freezing and thawing before hardening is complete.
A green concrete mixture which can be easily frozen at a temperature below 32 degrees F. (the freezing point of water), should not be allowed to freeze, if this can be prevented. It is safest, therefore, to avoid mixing and placing concrete in freezing weather.
Sometimes, however, this has to be done, and the work has to be pushed forward irrespective of weather conditions. Fortunately, good re sults can be obtained with almost any degree of cold, by taking proper precautions; but these invariably add to the cost by entailing additional expenses.
The elaborateness of the precautions neces sary to insure satisfactory results on a job of concreting done in freezing weather, will depend upon the class of construction and the impor tance of the work. Plain, massive work, for ex ample, does not require the same degree of care and protection in frosty weather, as is necessary for small reinforced construction.
Lowering the freezing point of the concrete is the simplest and cheapest, but probably not the best method of concreting in freezing weather. This method consists of adding some
substance to the mixing water that will reduce its freezing point; but only those substances that have no effect on the strength and durability of the concrete can be used. Ordinary salt is most commonly used for this purpose; and experi ments indicate that while the addition of a lim ited amount of salt retards the hardening some what, and lowers the initial strength, the ulti mate strength of the concrete is not affected by its use. Salt should be used only in plain con crete work, as its effect on reinforcing metal has not been established. Even when salt is used, it is important that the aggregates be free from lumps of frozen material, as it is impossible properly to mix such materials. Approximately one per cent by weight of salt to the weight of the water is required for each degree Fahren heit below freezing; but more than ten per cent of salt—which is equivalent to about 13 pounds of salt to a barrel of cement—should not be con sidered safe, and this amount is not effec tive for temperatures lower than 22 degrees Fahrenheit.
The use of artificial heat may be accom plished in different ways--by inclosing the en tire work under cover, and keeping it warm by heating the air within the enclosure; or by heat ing the aggregates or water, or both, before mix ing. The former is the most expensive method, and is never employed except in building con struction. The inclosing framework is lightly constructed of wood, covered with canvas or other material.
The best method of concreting in freezing weather is to heat the materials, and to protect the work until it has obtained sufficient strength to withstand the action of _frost. Either the water, the sand and water, or the sand, stone, and water should be heated. The cement is usually not heated. Heating the materials ac celerates the rate of hardening; lengthens the time before the material becomes cold enough to freeze; and, in temperatures but little below freezing, will insure the hardening of the con crete before it can be damaged by frost.