521. Another class of rubble concrete differing from the above more in degree than in kind, is formed by placing large stones in the work, and filling the joints between them with a rather wet concrete in which spalls may be rammed if desired. The difficulty of obtaining a compact wall by this method is perhaps a little greater than when smaller stone are used, but in either case if really water-tight work is desired, the inspection must be thorough.

The saving in Cost by the use of rubble concrete depends upon the local conditions, but under ordinary circumstances when broken stone is employed, the Cost of crushing the stone and the cost of cement, for a volume of concrete equal to the volume of the stone imbedded, are practically saved.

522. Joints In Concrete

In the construction of large masses of concrete in place, joints cannot be avoided; that is, it is not possible to make the entire mass monolithic, as force enough could not be employed to carry up the entire structure at once. Even if this were possible, it would not be desirable, since the changes in length of the wall due to changes in temperature would probably result in cracks which would be irregular in outline and mar the appearance of the wall, if they had no more serious effect.

When the concrete is subjected to vertical forces only, as in foundations for buildings, horizontal joints are less objectionable than vertical joints. But in the construction of concrete lock walls, dams, and breakwaters, vertical joints are desirable to confine the cracks to predetermined planes. In the building of such structures, therefore, the method has been adopted of dividing the work into sections of such horizontal dimensions as may be thought best, and completing each section as a monolith. This will sometimes require the continuous prosecution of work for twenty-four or forty-eight hours. Whether this method, involving work at night, which is always more expensive and usually less thorough, is justified by the end sought, depends upon the character of the structure.

523. If this method is not adopted, and a horizontal plane of weakness is a serious defect, special means should be provided for avoiding this plane of weakness. Such provision may be made by iron dowels set in the concrete at the end of the days work and projecting above the surface to be covered by the concrete placed the next day; steps or hollows, or grooves parallel to the length of the wall, may be left to be filled by the next layer. Large stones weighing a hundred pounds or more are frequently imbedded one half their depth in the last layer of a days work to form a bond with the following layer.

In any case special care should be taken to thoroughly wash and clean the surface of hardened concrete before continuing the work, using preferably wire brooms for this purpose and removing any stones at the surface that appear to be loose. A thin layer of rich cement mortar should then be laid upon it, into which the first layer of fresh concrete is well rammed.

If the appearance of the finished face is of importance, special care must also be exercised in joining at this point. Before leaving a layer which is to be allowed to harden before continuing the work, the line limiting the height of the concrete at the face should be made perfectly horizontal, for a slight crack, or at least a noticeable line, may be expected at this point, and if not straight it will be the more unsightly.

524. If for any reason a layer of concrete cannot be carried over the whole area of the wall or foundation, it should never be allowed to taper off to a wedge, but a plank equal in width to the thickness of the layer should be set on edge, firmly secured, and the concrete tamped against it. In the construction of arches, culverts and sewers, such stop planks may well be set normal to the surface of the intrados instead of vertical. In case more than one layer is left incomplete, they should be stepped back, making an offset for each layer of at least one or two feet. The concrete should never be built up on a smooth batter if new concrete is to be joined to it later.

525. Keeping Concrete Moist

All concrete should be kept moist from the time it is in the wall until it has become well hardened. Surfaces exposed to the air should therefore be sprinkled frequently for at least several days after placing. An excellent practice is to cover the surface with burlaps which may be kept saturated, as this not only furnishes the necessary moisture, but protects the work from the direct rays of the sun. The interior of a large mass will probably take care of itself in this regard, but the precaution has sometimes been taken of leaving vertical holes or wells in the mass, which are kept filled with water for some weeks and are then filled with concrete.

526. Finish

Some of the precautions that must be taken to secure a good finish to the face of concrete work have already been mentioned in considering the forms and the methods of deposition. These are usually supplemented, however, by certain special means when the appearance is of much importance.

We must say first, that the application of a plaster of cement mortar to a finished and set concrete face will almost never be permanent. It is seldom that it will adhere with sufficient strength to prevent scaling due to differences in expansion of the materials of different composition and age. If plaster must be used on the face of a wall, it should be applied before the concrete has set, but it is safer to avoid plastering. It is of course advisable to fill with rich mortar any voids that may appear in the face of the work, but such places should be few.

If the molds are removed while the concrete is still moist, the face may be coated with a thin grout and then immediately scraped off with the edge of a trowel. This results in filling the small voids in the face of the work, but does not leave a coat of plaster on the surface to scale off.