389. It is frequently desirable to use cement in freezing weather, but to ensure good work under these circumstances it is necessary to take certain precautions. If mortar is frozen immediately after mixing, setting cannot take place until it has again thawed. In the practical use of cement it is always gaged with a larger quantity of water than is required for the chemical combination, and if this excess water is frozen after the setting is somewhat progressed, the consequent expansion may be sufficient to disrupt the partially set mortar. By warming the materials or by lowering the freezing point of the water by the addition of salt, glycerine, or some other substance having this effect, it is sought to prevent the mortar freezing until the work is protected by another layer of mortar, or otherwise, and thus to avoid the expansion. Salt is generally used much too sparingly to prevent freezing. The freezing point of water is lowered about one and a half degrees Fahr. for each per cent, of common salt added; thus a twenty per cent, solution would freeze at about two degrees Fahr.

390. The following tests are selected as showing typical results of a large number of experiments made under the author's direction to determine the effect of exposing cement mortars to frost, and to indicate what treatment will alleviate the deleterious effects of low temperature. In making tests with small specimens, it is difficult to approach the conditions existing in the actual use of mortars in freezing weather. A small mass of mortar exposed to the air on all sides sets more quickly than the interior of a large mass; and on the other hand, the effect of frost on a small specimen must be more severe and more quickly apparent. Many of the results are more or less contradictory, and the conclusions that have been drawn are such as appear to be indicated by the majority of the tests. The treatment of the briquets, and the conditions existing, are given in some detail, that the limits of applicability of such conclusions may be seen.

391. Exposure To Frost Of Mortars Already Set

In the tests recorded in Tables 102 to 104 the briquets were allowed to remain one or two days in the laboratory. It is evident that these results are of but limited practical importance, since it is seldom that mortars which are made in winter can be allowed to set in a warm place before exposure; they are given, however, for what they are worth. Tables 102 and 103 give the results obtained with Portland cement briquets exposed to a severe temperature twenty-four to forty-eight hours after made. The most important deduction, and the one most clearly indicated by these tables, is that Portland cement mortar made with fresh water may be subjected to very low temperatures twenty-four to forty-eight hours after molded, without seriously decreasing the tensile strength given at six months to two years. It also appears that solutions containing as much as fifteen per cent, salt are deleterious, and smaller percentages are not advantageous under these conditions.

Table 104, giving the results of similar tests with natural cement mortar, indicates that this brand gives good results if allowed to set in warm air before exposure to frost. Solutions containing more than ten per cent, salt are deleterious for such treatment. Briquets of another brand of natural cement, a one-to-one mortar of which gave about 450 pounds tensile strength at one year, failed entirely when placed, one hour after made, in open air for three days, and then immersed in a tank in the laboratory. A 7.4 per cent, solution of salt used for gaging assisted very materially in preserving the mortar under the same severe treatment, although this amount of salt was not sufficient to lower the freezing point of the water below the temperature to which the briquets were subjected.

Table 102. Exposure Of Portland Cement Mortars To Low Temperatures After Twenty-Four Hours In Laboratory

Notes. — Cement, Portland, Brand R. One part sand to one cement.

Briquets made in laboratory, temp., 64° to 66° Fahr.; materials about 65° Fahr. Temperature, open air, Jan. 16 to Jan. 19, 4° to 15° Fahr. Treatment of briquets: — a — Fresh water, briquets stored in water in laboratory. b — Fresh water, briquets stored in open air after 24 hours. c — Fresh water, briquets alternated, two days in open air and then two days in air laboratory, for fifty-two days, then left in open air. d — Water 5 per cent, salt; briquets stored in open air. e —Water 15 per cent salt; briquets stored in open air. f — Water 25 per cent, salt; briquets stored in open air. g — Water 25 per cent, salt; briquets stored in water in lab.

Table 103. Exposure Of Portland Cement Mortars To Low Temperatures, Twenty-Four To Forty-Eight Hours After Made

Notes. — Cement, Portland, Brand R. Three parts sand to one cement.

Briquets made in laboratory. Temp.: Air and materials, 64° to 67° Fahr. Open air, Jan. 10 to 20,-15° to +18° Fahr.

Treatment of briquets: a, b, c, d and e mixed with water containing 0, 10, 15, 20 and 25 per cent, salt, respectively; a to d, inclusive, air laboratory 24 hours, water laboratory 16 hours, air laboratory 12 hours, then in open air.

f, g, h, i and j, mixed with water containing 0, 10, 15, 20 and 25 per cent, salt, respectively.

e to f, inclusive, put in open air after about 24 hours in air of laboratory.

Table 104. Exposure Of Natural Cement Mortars To Low Temperatures, Twenty-Four Hours After Made

Notes.—Cement, Natural, Brand Gn. Sand, " Pt. aux. Pins" (river sand).

Temp, materials and air of laboratory where briquets were molded, 65° to 68° Fahr. Temp, open air Jan. 21 to 23, = - 1° to +29°.

Treatment Of Briquets