Facing and stones which form the face of the wall are called facing, while those forming the back of the wall are called backing. In the construction of walls, the facing and backing are commonly of different classes of masonry. An ashlar facing is frequently joined to a rubble or concrete backing.
In heavy walls the masonry of the interior of the walls, between the facing and backing, is known as filling, and this may sometimes be different from either the facing or backing. In constructing walls, the facing and backing should always be well bonded, so that the whole acts together in supporting loads or resisting pressures.
Headers and stone whose greatest dimension lies perpendicular to the face of the wall is called a header; one whose greatest dimension is parallel to the face of the wall is a stretcher.
The bond of the masonry in the wall is secured by proper arrange ment of headers and stretchers. The vertical joints in adjoining courses should not be too nearly in the same plane. A stone in any course should break joints with the stones in the course below by a distance at least equal to the depth of the course. The strongest bond is obtained by using an equal number of headers and stretchers in the face of the wall. a header being placed over the middle of each stretcher as shown in Fig. 24.
In the use of coursed ashlar facing, the rubble filling and back ing is usually also coursed at the same height as the ashlar. Some times in massive work the filling may be constructed of irregular uncoursed rubble. Usually, however, concrete would be used in such work instead of rubble.
In the walls of buildings with ashlar facings, the rubble backing is laid in courses with the ashlar and occasional headers are run through the wall as shown in Fig. 2S. Brick backing is commonly used for such work and is usually preferable to rubble. (Fig. 29.) In building work, thin ashlar, 2 to 4 inches thick, is sometimes employed as a veneer on the exterior of a wall of rubble or brick; this is frequently done in marble buildings. The veener is tied to the hacking by iron clamps, and occasional belt courses of wider stones, extending 6 or 8 inches, into the filling give support to the ashlar.
Dowels and Cramps.—For the purpose of strengthening the bond
where great resistance is required, dowels or cramps are often em ployed. A dowel is a straight bar of iron which enters a hole in the upper side of one stone and also a hole in the lower side of the stone above. A cramp is a bar of iron with ends bent at right angles to the length of the bar, the ends entering holes in the tops of adjacent stones.
48. Setting Stonework.—The layers of mortar between stones are called joints. The horizontal joints are commonly called beds or bed joints.
The kind of mortar used in stonework depends upon the character of the work. In engineering structures, 1 to 2 or 1 to 3, Portland cement mortar is usually employed. Cement mortar stains many stones and care must be used in architectural work to prevent injury to appearance of the stone surface from this cause. This may often be effected by keeping the bed and joint mortar back from the face and using non-staining mortar for pointing. The bed joint of ashlar stones should be carefully dressed to a plane surface in order that the stone may bear evenly upon the bed. These joints are sometimes cut slightly concave to make them easier to set with close joints at the surface, which brings the loads upon the edges of the stone with danger of chipping the edges.
The vertical joints in ashlar facing should be carefully dressed to a depth of several inches from the face of the wall, but do not need to be accurately dressed the full depth of the stone. The backs of the ashlar stones may be laid as rubble without cutting. The arrangement of headers and stretchers in the rubble backing should he the same as in the ashlar facing to secure good bond through out the wall.
Placing stones should be set in a full bed of mortar. The mortar bed should be prepared and the stone lowered upon it without disturbing stones already set. The stone must not be slid upon the bed so as to scrape away the mortar. Stones too large to be handled by one man are placed with a derrick, and are settled in place with light blows from a hammer. No cutting or trimming of stone after placing is allowable; the stone must be fitted to its place before spreading the mortar.