Heating the Materials, and mixing and placing them warm, has the effect of hastening the hardening processes, and also prevents immediate freezing. If the temperature is but little (:3° or 4°) below the freezing-point, heating the materials and placing the concrete warm may be sufficient to prevent injury from freezing. Protec tion should also be given the concrete to delay freezing as long as possible. The materials should not be at a temperature much above 100' or 110° F. at the time of mixing. The use of hot water is injurious to the cement, and may defeat the object of heating by preventing the cement setting properly.
Having placed the concrete while warm, if the temperature is likely to be more than :3° or 4° below the freezing-point, it is neces sary to have some means of keeping the work from freezing on the surface, which may sometimes be done by enclosing the work in some way and using small stoves, or steam pipes may be available for heating small enclosed spaces. In placing work in large masses, the heat of chemical action will prevent freezing in the body of the work, but exposed surfaces must be protected.
Use of Salt. When the temperature is but little below the freez ing-point, the freezing of concrete may be prevented by dissolving salt in the water used for mixing. A small addition of salt (3 to 5 per cent of the weight of water) lowers the freezing-point of the concrete, and prevents injury from freezing at temperatures perhaps or 6° below freezing. The salt also has the effect of somewhat increasing the rapidity of hardening, which is very slow at such temperatures.
Salt is sometimes used in larger proportion, 10 to 15 per cent of the weight of water, to prevent freezing at lower temperatures. This seems to retard hardening, and is considered by some engineers to be harmful to the concrete.
85. Contraction Joints.—Cement mortar and concrete expand and contract with changes of temperature in the same manner as other materials. They also change in dimension with changes in moisture, expanding when wet and contracting when dry.
Temperature Changes.—The coefficient of expansion of concrete has been found by various investigators to vary from about .0000050 to .0000065 per degree F., the average result being about .00000.55 per degree F., or .0000099 per degree C. If the coefficient of elas ticity of the concrete is 2.000,000, this would be sufficient to produce a unit stress in the concrete of 440 lb.; for a change of temper ature of 40° F. if the concrete be restrained from yielding.
Concrete in large masses frequently reaches a high temperature during the period of early hardening, due to the heat produced by the chemical changes taking place, temperatures of from 95° F. to 150° F. having been observed.' In thin walls this is largely counter acted by the radiation into the atmosphere. The influence of changes of atmospheric temperatures rapidly decreases with the distance from the surface of the concrete. Daily variations of temperature are not felt at depths of more than 2 or 3 feet, while seasonal vari ations may not reach more than one-third the amount of the change in the outside air at a depth of 10 feet.
Moisture Changes.—Variations in moisture conditions are of greater importance than those of temperature in causing mortar or concrete to expand and contract. These changes are of special importance during the time that the concrete is hardening. Experi ments indicate that concrete kept in dry air during the period of hardening undergoes a progressive shrinkage, while that kept in water expands during the same period, but to a less extent. The results obtained by different investigators have varied considerably in the extent of the changes shown. In general, concrete exposed to dry air may be expected to contract .04 to .06 per cent of its length in six months after mixing, while if kept under water it may expand .01 to .02 per cent. The changes for cement mortar are greater than for concrete, the extent of the change being greater as the mortar is richer.
Tests indicate that concrete at any age expands if changed from dry to wet condition, and contracts if changed from wet to dry. Concrete subject to changes in moisture conditions, therefore, alter nately expands and contracts with such changes, unless restrained by its position from such motion. We have no means of estimating the amount of the variations to be expected in concrete work, but under ordinary conditions these effects must be much less than the progressive variations during hardening.
Available data are not sufficient to determine to what extent the progressive expansions or contractions taking pla--e during hardening may he permanent.' Indications are that concrete which has been kept wet during the first month or more and then permitted to dry fot several months, does not shrink to the same extent as that which is kept dry during the whole period. Concrete kept damp during the early period of hardening should not crack when exposed to the air to the same extent as that continuously dry.