Ashlar Masonry

ASHLAR MASONRY. This is masonry in which the thick ness of the bed joints is one half inch or less. According to the finish of the face of the stones, ashlar may be divided into either pitch-faced, drafted, or cut-stone masonry (I 545-46); and according to the arrangement of the course it may be range, broken range, or random masonry (I 550).

Ashlar is the best quality of stone masonry, and is employed in all important structures. It is used for piers, abutments, arches, and parapets of bridges; for hydraulic works; for facing quoins, and string courses; for the coping of inferior kinds of masonry and of brick work; and, in general, for works in which great strength and stability are required.

Its strength depends upon the size of the stones, upon the accuracy of the dressing, and upon the bond.

Size of Stones.

The dimensions of the blocks should vary with the character of the stone employed. With the weaker sand stone and granular limestones, the length of any stone should not be greater than three times its depth, as otherwise it is likely to be broken across; but with the stronger stones, the length may be four or five times the depth. With the weaker stones the breadth may range from one and a half to two times the depth; and for the stronger stones from three to four times the depth.

Dressing.

The dressing consists in cutting the side and bed joints to plane surfaces, usually at right angles to each other. The accurate dressing of the bed joints to a plane surface is exceedingly important. If any part of the surface projects beyond the plane of the chisel draft, that projecting part will have to bear an undue share of the pressure, the joint will gape at the what is called an open joint, and the whole will be wanting in stability. On the othar hand, if the surface of the bed is concave, having been dressed down below the plane of the chisel draft, the pressure is concentrated on the edges of the stone, to the risk of splitting them off. Such joints are said to be flushed. They are more difficult of detection, after the masonry has been built, than open joints; and are often executed by design, in order to give a neat appearance to the face of the building. Their occurrence must

therefore be guarded against by careful inspection during the prog ress of the stone cutting.

Great smoothness is not desirable in the joints of ashlar masonry intended for strength and stability; for a moderate degree of rough ness adds at once to the resistance to displacement by sliding, and to the adhesion of the mortar. When the stone has been dressed so that all the small ridges and projecting points on its surface are reduced nearly to a plane, the pressure is distributed nearly uni formly, for the mortar serves to transmit the pressure. to the small depressions. Each stone should first be fitted into its place dry, in order that any inaccuracy of figure may be discovered and cor rected by the stone-cutter before it is finally laid in mortar and settled in its bed.

The entire bed area of a stone should be dressed to a plane; but, unless the wall is so thin that the stones extend clear through, it is not necessary to dress the entire area of the ends of the stones; and it is not necessary to dress any portion of the back side of the stones. The specifications should state the distance back from the face of the stone that the end is to be dressed to a plane surface. This distance is sometimes stated in inches and sometimes as a fractional part of the thickness of the course (see § 23 of Appendix III).

Sometimes specifications permit the vertical joints to be wider than the bed joints. This decreases the cost of cutting, and may not materially reduce the strength of the masonry; but may slightly affect the durability and the architectural appearance.

No cutting should be allowed after the stone has been set in mortar, for fear of breaking the adhesion of the cement.

The thickness of mortar in the joints of the very best ashlar masonry—for example, the United States post-office and custom house buildings in the principal cities—is about k of an inch; in first-class railroad masonry—for example, important bridge piers and abutments, and large arches—the joints are from to * of an inch.