Extremes of heat or cold are both unfavorable to the union of the building material and the mortar.

Masonry Construction During Extremes Of Temperatur 304

Fig. ll.

Extreme heat causes quick evaporation of the water in mortar, leaving the mortar in practically the same condition as before mixture. This effect is intensified by the very dry condition of the stone or brick, which readily absorbs moisture from the mortar. In hot weather the surfaces of the brick and stone are covered with dust, which is very detrimental to adhesion, and should be removed before using the material. The mortar, whether lime or cement, should be thinner than usual, and the brick and stone should be well sprinkled before laying. When built, the wall should be frequently sprayed with water, to lower its temperature, and prevent excessive evaporation.

In making concrete, the sand and gravel should be well washed, and kept moist by liberal spraying; where practicable, a temporary cover over the work, to keep off the sun's rays, should be provided.

In plastering, similar points require attention; the openings in the building should be closed, to prevent hot-air currents from extracting moisture too rapidly from the plaster. Where patent plaster is used, the dry lathing should be well moistened before the mortar is applied, and care must be exercised that the coats do not become dry before the subsequent ones are applied. Hard plasters are not injured by frost after they have begun to set, but should be protected from it for the first 36 hours after application.

Masonry Construction During Extremes Of Temperatur 305

Fig. 12.

Extreme cold prevents the setting of mortar by freezing the moisture in it, and also by forming films of ice on the brick or stone. If lime mortar is to be used during freezing weather, rich, newly slaked lime, thoroughly well mi gives good results, as it retains much of the heat of slaking. With hydraulic cement the set takes place more rapid!y, and, if well begun, frost does not apparently injure the mortar; the particles in combining expel the excess of moisture; a much closer contact is thus secured, and the mortar is denser. Although, in appearance, the work may not have suffered from the action of frost on the cement, many experiments have conclusively shown that cement mortar after freezing loses as much as one-third the tensile strength it possesses when not frozen.

In order to lower the freezing point, a solution containing 2 or 3 per cent, of salt is mixed in the mortar, reducing the temperature of freezing about 5°; that is, until the thermometer registers below, say, 28°, the operations are safe. It has been conclusively proved, however, that the addition of salt to cement is injurious to its strength, as the crystallization of the salt creates a force opposing adhesion, and tends to disintegrate the particles; salt also attracts moisture, thus making the walls constantly wet.

The top of the walls should be kept covered when work is stopped, so that rain may not enter and, freezing, separate the constituent parts of the masonry. Sheets of tarpaulin or roofing felt are most convenient for the purpose. When the temperature has reached the freezing point, it is unsafe to dress-cut unseasoned stone in the open air, as the quarry sap in the stone freezes, and the stone may be fractured during the process of cutting.