LAYING CONCRETE IN FREEZING WEATHER. The effect of cold is to retard the setting of cement; and if a mass of concrete is repeatedly frozen and thawed before it has dried out or has set hard enough to resist the expansive action of the frost, the bond between the cement and the coarser materials may be destroyed. However, with large masses of concrete there is usually little probability of the moss's freezing much, if any, before the cement has set sufficiently to resist the expansive action of the frost, since the temperature of the concrete when freshly mixed is considerably above freezing, and since it is protected by the forms. Further, the chemical action of setting causes a considerable rise of temperature. The temperature at the center of a 1-foot cube of neat portland-cement mortar may be 200° F., and that of natural cement 100° F.* The rise of tem perature of concrete is less than that of neat mortar, but the effect of this increase of temperature in concrete must considerably retard its freezing. Most natural cements are seriously injured by freezing, but portland cement will stand a moderate amount of freezing with out material damage.
Foundations or heavy walls whose face appearance is unimportant, may be laid with portland-cement concrete in freezing weather without any further precaution than to keep the upper surface of the concrete free from ice and frozen dirt and to cover the work with cement bags or straw or manure or some such material when stopping work at noon or night. If it is not permissible to allow the concrete to freeze, it may safely be laid in freezing weather by taking one or more of the four following precautions: 1. Use dry concrete, as it will set quicker than a wet mixture; but care must be taken to tamp it well.
2. Lower the freezing point of the water used in making the concrete, by adding common salt. To prevent water from freezing absolutely until the temperature reaches 0° Fahr., add salt equal to 1 per cent of the weight of the water for each degree below freezing. But for the two reasons stated in the first paragraph of this section, it is not necessary to use in concrete the full amount of salt required by the above rule. A common rule, which has long been in use with suc cess for temperatures of 10° to 15° F. below freezing, is: "Dissolve 1 pound of salt in 18 gallons of water when the temperature is 32° F.,
and add 3 ounces of salt for each 3° of lower temperature." This rule gives proportionally an excess of salt at temperatures near zero. A more scientific rule, and one more easily remembered, is: "Add s of 1 per cent of salt for each 1° F. below freezing." Except be tween 32° and 28°, this rule gives more salt than the common rule above. Salt up to 10 per cent does not weaken the concrete.* Several other substances could be used to lower the freezing point of the water, but salt is much the cheapest. The only objection to salt is that it is liable to cause a white, .powdery deposit on the surface, which, however, is likely soon to be washed off or blown away. Dissolving the salt in the water rather than mixing it with the cement lessens the probability of efflorescence. Concrete to which salt has been added dries out more slowly, and hence retains its dark color longer, than that containing no salt; but both are likely finally to have the same color.
3. Warm the ingredients, which accelerates the setting of the cement and also lengthens the time before the mixture becomes cold enough to freeze. The water may be heated with a jet of steam, and in extreme cases the sand and the stone may be heated on a steel plate placed upon two brick walls with a fire between.f With proper care it is possible to get the concrete into place at 60° F. • 4. Protect the concrete by surrounding the work with a canvas or wood covering, and heating the interior with steam pipes, stoves, or open charcoal fires.$ In temperatures only n few degrees below freezing, it is sufficient to nail building paper on the outside of the forms. A single thickness of tarred paper, well tacked and so put on as to prevent a free circulation of air, has raised the temperature of the air under it 20° F.
349. From the above it is seen that concrete can be safely laid in freezing weather with reasonable precautions; but nevertheless it should not be done unless really necessary, since there is then more danger through carelessness, and also since the concrete freezes to the tools and forms, which adds considerably to the expense. Concrete has been laid comparatively frequently when the atmos phere was 10° or 15° F. below zero,—sometimes by simply heating the water, but usually by heating all the materials for the concrete.