When it is desired to lay masonry in freezing weather certain precautions must be observed in order that the masonry shall not be damaged. To the water used in mixing the mortar or concrete, salt should be added in quantities varying with the temperature. (See Fig. 21.) The water and sand should be heated. Steam may be used to heat the water in the measuring tank, and coils of steam pipes may be arranged in the sand bin. On discontinuing work for the night, salt should be sprinkled freely over the fresh masonry. In severe weather the masonry should be covered with tarpaulin and live steam blown in under the tarpaulin.

During warm weather water is freely and necessarily used on the wall for cleaning purposes to temper the mortar and to replace water lost by evaporation; during cold weather this must be practically discontinued and steam hose used in place of the water hose. The only necessary purpose in winter is to clean the stone, and this is best done by a steam jet.

One condition and effect worthy of attention, although insusceptible of practical measurement, is the temperature of the large stone. As the large stone comprise 50 per cent, or more of the mass of the masonry, it is obvious that it makes a difference whether their temperature is zero or freezing. It should take some time after any drop in atmospheric temperature before the stone get as cold as the air. If the stone are used immediately after quarrying they may be much warmer than the air, on the same principle as an earth cut is worked in winter. When the cut has some depth and the work is continuous, the frost is negligible on the working face except on the exposed top, i.e., at the original surface.

Use Of Salt

In connection with the accompanying formulae for amount of salt in mortar (see Fig. 21), it should be said that at both the New Croton and Wachusett dams masonry work was not started unless the temperature was as high as 20 deg. Fahr., and either rising or expected to rise. Hence the practice on those dams should not be taken as a precedent for work at a lower temperature.