WALLS OF STONE MASONRY 46. Classification of Masonry.—Stone work is commonly divided into two general classes; ashlar and rubble, depending upon the degree of care exercised in cutting the stone and the closeness of the joints.
Ashlar masonry is that in which the joints are not more than inch thick. The term ashlar is also sometimes extended to include masonry of squared stones in which the joints are not so accurately dressed, but this is not usual.
Ashlar masonry may be divided according to the arrangement of the stones into: Coursed ashlar, sometimes called Range masonry (Fig. 24), arranged in courses of uniform thickness.
Broken ashlar, or Random ashlar, in which the stones are not arranged in courses (Fig. 25).
Broken-coursed or Random-coursed ashlar, in which broken ashlar work is arranged in more or less continuous courses, or masonry laid in parallel but not continous courses.
In the best cut-stone work, as used by architects for public buildings in the cities, the joints may not be more than inch thick, while in first-class masonry in important engineering construction joints from ; to inch are usually allowed. When the thickness of courses and length of stones in ashlar masonry are specified, the work is known as dimension xtonc masonry.
The exposed surfaces of ashlar masonry may be finished by any of the methods in the preceding section, and the masonry is fre quently classified as pitch-faced ashlar, drafted-stone ashlar, or cut stone masonry, which includes all of the more accurate methods of dressing; pointing, bush-hammering, axing, etc. Pitch-faced and drafted-stone work is often called rock ,faced ashlar.
Rubble masonry is that which is not dressed or laid with sufficient accuracy to be classed as ashlar, and may include stones roughly squared or those of irregular shapes.
Rubble masonry is usually uncoursed, but sometimes is leveled off into courses at specified heights, and is then known as coursed rubble. Fig. 26 shows the face of a wall of ordinary uncoursed rubble.
Fig. 27 shows a type of rubble work sometimes used in building con struction, in which hammer-dressed joints are more accurately fitted on the face of the wall. Joints to ', inch may be used in such work.
This is sometimes called " Russian Bond " and is usually rock faced work.
Dry Masonry.—Masonry of rough stone without the use of mortar is sometimes employed and is known as dry masonry. Such walls are frequently used for railway culverts and similar purposes. When stone is roughly placed about the bases of piers or abutments, or on the banks of streams to prevent erosion, it is commonly called riprap.
Masonry is a term frequently used to indicate a class of masonry between ashlar and rubble. When this classi fication is used, it commonly includes masonry of squared stones, with joints from z to 1 inch thick, and the term rubble is limited to the use of irregular and unsquared material.
architectural work, an additional classification is sometimes employed to designate stone used for special purposes, such as moldings, sills, caps, etc. These usually require cutting to specified dimensions and close joints.
47. Parts of a Masonry exposed surface of a masonry wall is called its face, while the interior surface is known as the back of the wall.
Batter is the slope of the surface of a wall, stated as a ratio of horizontal to vertical dimension. The walls of buildings usually have no batter. Retaining walls, bridge piers, and other heavy structures for carrying loads, are commonly given a batter on the face. This gives an appearance of strength and stability to the wall.
Coping is a course of stone on top of the wall to protect it and give a finished appearance. The coping usually projects a few inches over the surface of the wall.
horizontal layer of stones in the wall is called a course, the arrangement of courses in a wall being determined by the character of the material and the appearance desired. When the stone may be readily obtained in blocks of uniform thickness an arrangement in courses, with the thickest courses at the bottom, gives an appearance of stability, and is common practice in engineer ing structures. In architectural work, the arrangement of courses may be made to accord with other features of the design of the struc ture.