Both glazed and enameled bricks are Impervious to moisture and are excellent for the facing of halls, courts, or wherever a light, clean and waterproof surface is desired. In addition to these, special bricks are made for special purposcfi, as fire bricks for furnace linings, etc., which are open-grained or porous to admit of a rapid loss of heat, and paving bricks which are burned to vitrification to with stand the wear of travel and the action of frost.
Sizes. The sizes of bricks vary with the maker and with local customs. Common bricks in New England average about 2 X 3 x 71 inches; in the Western States about 2 X S? inches. Face bricks are more uniform in size and average about 21 X X 8 inches. Pressed bricks are also trade in a thinner pattern 1 X 4 X 12 inches. This style is known as the Roman brick.
Freezing of Brickwork, To obtain a satisfactory result, bricks should never he laid in freezing weather. If the temperatnte is much below F. during the day, so that it is likely to freeze daring the night, salt may be mixed with the mortar and the top of the wall well covered with hoards and straw, and if the upper courses are found to have been frozen over night they must be taken down and re-laid, as the alternate freezing and thawing will materially damage the strength of lime mortar and will entirely ruin mortar made of natural cement of the Iloscndale type. "Mortar made of one part of Portland cement and three parts of sand is entirely unin jured by freezing and thawing." If it is absolutely necessary that brickwork should be laid in freezing weather with natural cement mortar, it may be done by mixing the mortar "with water to which salt has been added in the proportion of one pound of salt to eighteen gallons of water, when the temperature is at 32° F. and, for each degree of temperature below 32°, add three additional ounces of salt. Mortar mixed with such a solution does not freeze in ordinary winter heather, and hence is not injured by frost." In addition to this, the bricks should be warmed to remove any ice or frost.
These methods may he used in emergencies, but the laying of bricks in freezing weather is not to he recommended, if it can be a ; oiled.
Arches. In the erection of brick walls, especial care 511(011(1 be given to the construction of arches whieh will he necessary to span the openings. Arches, in general, should be laid in cement mortar. The two principal forms of arch in brickwork are the rowlock arch, where the bricks are laid in concentric rings of headers, Fig. 114, and the ganged arch, where the bricks are cut and bonded on radial lines, as in Fig. 173. For arches of large spans the bricks arc often laid in rings of stretcher,, Fig. 11(i, and, if the span is very large, these may be strengthened by bonding in headers as in Fig.
117. An important point in the use of arches is to see that each abutment contains sufficient masonry to support the thrust of the arch; tie rods should be freely used in case of any doubt, and arches of large span should be sprung from stone skewbacks caefnlly cut to radial lines, as in Fig. 118.
liortar. All mason-work below ground should be laid in cement mortar, and all arches or heavily loaded piers as well. In many city laws, the use of cement mortar is required to a certain proportion of the height of wall. Cement mortar of some kind should be used for setting cornice stones and wherever the work is especially exposed. For all work under water, for large arches and for toys of chimneys, Portland cement should he used, and Portland cement may be mixed with Rosench.le to make a strong, quick-setting mortar. Portland cement told lime may he mixed to give a strong light-colored mortar which is much used in England for face-brick work.
Sand. The proportion of sand used in mortar should vary with the kind of cement and the use for which the work is intended. With Rosendale cement, the proportion of sand should not exceed three to one, and for piers two to one. Portland cement may contain sand in proportion of four to one, and lime tar three to one. The object of using sand in mortar is to prevent too much shrinkage, and as a cheaper material than lime or cement, its functions being mechanical rather than chemical. Tt strengthens lithe mortar, by supplying a base to which the particles of lime adhere more firinlv than to each other, Ian its presence in cement mortar is a weakness. In all cases a thorough mixing is essential, the object being to so thoroughly mix the particles that no two grains of sand shall lie against each other without a film of cement between. The cement and sand should be mixed dry, the process being to spread them in layers and their turn and work the mass until it is thoroughly mixed, when water is added in sufficient quantity to obtain mortar of the desired stiffness.
If too much sand is added the mortar will stick to the trowel and will not work easily. The superintendent should become, as soon as possible, familiar with the appearance of good mortar so that he can tell readily whether too much or too little sand has been added. If the mortar slides easily from the trowel it is usually of the right proportion. In ordinary practice, the lime and sand are mixed as soon as the lime is slaked and allowed to stand until needed, but it is better not to mix the sand until ready for use. Coarse sand makes a stronger mortar than fine sand, and a fine, loamy sand, although it works easier than sharp coarse sand, does not make a strong mortar.