Rubble Masonry

When carefully executed with good mortar, rubble possesses all the strength and durability required in structures of an ordinary character, and is much less expensive than either ashlar or squared stone masonry. But it is difficult to get rubble well executed. The most common defects are (1) not bringing the stones to an even bearing; (2) leaving large unfilled vertical openings between the several stones; (3) laying up a considerable height of the wall dry, with only a little mortar on the face and back, and then pouring mortar on the top of the wall; (4) using insufficient cement, or that of a poor quality. The only way to prevent the first defect is to have an inspector on the job all of the time. The second and third defects can be detected by probing the wall with a small pointed steel rod. To prevent the fourth defect it is customary for the owner to furnish the cement to the contractor. Apparently it is commonly believed that the rougher the stones and the poorer the grade of masonry, the poorer the cement or the leaner the mortar should be. The principal object of the mortar is to equalize the pressure; and the more nearly the stones are reduced to closely fitting surfaces, the less important is the mortar. Consequently, when a substantial rubble is required, it would not be amiss to use a first-class cement mortar, particularly if the stones are comparatively small. The extreme of this rule is the use of a first-class cement mortar with crushed stone to make concrete; and owing to the success of this practice and the decrease in the cost of portland cement in recent years, concrete has been largely substituted for all kinds of masonry, particularly rubble.

Amount of Mortar Required.

The amount of mortar required varies greatly with the character of the surfaces with which the stone quarries out. If the stone is stratified sandstone or limestone yielding flat-bedded stones with good end surfaces, the rubble may not require much if any more mortar than ashlar built of the more refractory stones; but if the rubble is built of stone that quarries out in irregular chunks and is difficult to dress, a very large per cent of mortar may be required. The amount of mortar required can be considerably reduced by packing spalls into the vertical spaces between the stones,—a proceeding that is always economical since spalls are always much cheaper than cement mortar. How ever, when the cement is furnished by the owner, the mason is apt to fill the joints entirely with mortar since it requires less time.

If

rubble masonry is composed of small and irregularly shaped atones, about one third of the mass will consist of mortar; and if laid in 1 : 2 mortar will require about 1 barrel of portland cement per cubic yard, and if laid in 1 : 3 about 0.8 barrel. If the stones are large and regular in form, one fifth to one quarter of the mass will be mortar; and the rubble will require about 0.7 barrel per cubic yard for 1 : 2 mortar and 0.6 barrel for 1 : 3.

For the amount of cement and sand required for mortar of various compositions, see Table 22, page 120.

Specifications.

The following requirements, if properly complied with, will secure what is generally known among railroad engineers as superior rubble.* Rubble masonry shall consist of coursed rubble of good quality laid in cement mortar. No stone shall be less than six inches in thickness, unless otherwise

directed by the engineer. No stone shall measure less than twelve inches in its least horizontal dimension, or less than its thickness. At least one fourth of the stone in the face shall be headers, evenly distributed throughout the wall. The stones shall be roughly squared on joints, beds, and faces, laid so as to break joints and in full mortar beds. All vertical spaces shall be flushed with good cement mortar and then be packed full with spalls. No spalls will be allowed in the beds. Selected stones shall be used at all angles, and shall be neatly pitched to true lines and laid on hammer-dressed beds. Drafts lines may be required at the more prominent angles.

The top of parapet walls, piers, and abutments shall be capped with stones extending entirely across the wall, and having a front and end projection of not less than four inches. Coping stones shall be neatly squared, and be laid with joints of less than one half inch. The steps of wing-walls shall be capped with stone covering the entire step, and extending at least ix inches into the wall. Coping and step stones shall be roughly hammer-dressed on top, their outer faces pitched to true lines, and be of such thickness (not less than six inches) and have such projections as the engineer may direct. • The specifications for rubble masonry will apply to rubble masonry laid dry (see 1555), except as to the use of the mortar.

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RUBBLE. Sometimes in building large structures, as dams, the rubble is made of cyclopean blocks, and wet concrete is used instead of mortar. The large stones are placed in the wall by means of a derrick, and concrete is deposited from a dump bucket. This form of construction is a recent innovation; and is specially applicable in building a dam, in which the faces are laid in ashlar, squared-stone, or rubble, and serve as forms in which to place the concrete-rubble filling. This form of masonry has several obvious advantages over either ordinary rubble or concrete. Rubble Concrete. This is ordinary concrete in which large irregular stones, sometimes called plums, are embedded. This form For specifications of rubble masonry for railroad work, see Appendix III.

of masonry is adapted to moderately massive construction. The "plums" decrease the cost of crushing the stone and also decrease the amount of cement required, and increase the density of the mass. The "plums" are usually limited to about 40 per cent of the entire volume, to Insure that they shall be surrounded by concrete. If the concrete is wet, there is little or no trouble in getting the large stones thoroughly bedded, and consequently this form of masonry is as good as or better than ordinary concrete.

Unfortunately, there is no uniformity as to the terms employed to designate either of the two above types of masonry. Each is frequently referred to as concrete rubble, and also as rubble concrete. It will add to clearness if the term concrete rubble be reserved for that form of masonry which consists chiefly of large stones sur rounded by concrete, and the term rubble concrete for that type which consists of concrete in which are embedded a comparatively few large stones. Of course the two forms shade one into the other.