THE MORTAR. The functions of the mortar are: (1) to form a cushion to take up any inequalities in the brick and thus distribute the pressure evenly; (2) to bind the whole wall into one solid mass; and (3) to fill the interstices between the brick to keep out water and to prevent changes of temperature. To satisfy the first condition the mortar should be soft and somewhat plastic, and the mortar bed should be thick enough to prevent the bricks from touching each other at any point. To satisfy the second condition the mortar should possess the properties of hardening after a time and of adhering to the brick. To satisfy the third condition the mortar itself should be dense and impervious, and enough of it should be used to entirely fill all the joints and spaces between the bricks. Brick buildings are nearly always built with lime mortar, although occasionally natural cement is added to the lime; but cement mortar, made either of natural or of portland cement, is usually employed in the brick-work of sewers, linings of tunnels, arches, bridge piers, reservoir walls, etc.
If the forces acting upon a wall were only direct compression, the strength of the mortar would in most cases be of comparatively little importance, for the crushing strength of average quality mortar is higher than the dead load which under ordinary circumstances is put upon a wall; but, as a matter of fact, in buildings the load is rarely only that of a direct crushing weight. Thus the roof tends to throw the walls out, the rafters being generally so arranged as to produce a considerable outward thrust against the wall. The action of the wind also produces a side strain which is practically of more importance than either of the others. In many cases the contents of a building exert an outward thrust upon the walls; for example, barrels piled against the sides of a warehouse produce an outward pressure against the walls.
joints are thin, the tendency of the mason is to spread a little mor tar for the bed joint at the back of the brick, and then before lay ing the brick to apply a small quantity of mortar to the front edges of the brick, in which case it will not be well supported and is likely to crack.
Common bricks in exterior walls of buildings are laid with joints varying from } to } of an inch, and for interior walls from I to I inch; and pressed brick with joints from * to of an inch. In engineering structures the thickness of the joints depends upon the quality of the bricks and upon the grade of work desired; and usually the joints are thicker than in buildings, partly because the bricks are harder and therefore rougher, and partly because cement mortar is em ployed, which is not as plastic and can not easily be laid in as thin joints as lime mortar.
Bond is the arrangement of the bricks in successive courses to tie the wall together both longitudinally and transversely. The primary purpose of bond is to give strength to the masonry, but architects employ various longitudinal bonds to improve the appear ance of the wall. Although numerous bonds are employed for artistic effect, in the construction of ordinary brick masonry only three bonds are used, the common, the English, and the Flemish, the first being much the more common.
As in ashlar masonry, so in brick-work, a header is a brick whose length lies perpendicular to the face of the wall; and a stretcher is one whose length lies parallel with the face. Brick should be made of such a size that two headers and a mortar-joint will occupy the same length as a stretcher.