320. Stone or Brick Facing. A facing of stone or brick is frequently used for reinforced concrete, and is a very satisfactory solution of the problem of finish. The same care is required with a stone or brick facing as if the entire structure were stone or brick. The Ingalls Building at Cincinnati, Ohio, 1G stories, is veneered on the outside with marble to a height of three stories, and with brick and terra-cotta above the third story. Exclusive of the facing, the wall is S inches thick.
In constructing the Harvard University Stadium, care was taken, after the concrete was placed in the forms, to force the stones back from the face and permit the mortar to cover every stone. When the forms were removed, the surface was picked with a tool as shown in Fig. 11S. A pneumatic tool has also been adopted for this purpose.
The number of square feet to be picked per day, depends on the hardness of the concrete. If the picking is performed by hand, it is done by a common laborer; and he is expected to cover, on an average, about 50 square feet per day of ten hours. With a pneumatic tool, a man would cover from 400 to 500 square feet per day.
321. Granolithic Finish. Several concrete bridges in Phila delphia have been finished according to the following specifications; and their appearance is very satisfactory: "Granolithic surfacing, where required, shall be composed of 1 part cement, 2 parts coarse sand or gravel, and 2 parts granolithic grit, made into a stiff mortar. Granolithic grit shall be granite or trap rock, crushed to pass a -1-inch sieve, and screened of dust. For vertical surfaces, the mixture shall be deposited against the face forms to a minimum thickness of 1 inch, by skilled workmen, as the placing of the concrete proceeds; and it thus forms a part of the body of the work. Care must be taken to prevent the occurrence of air-spaces or voids in the sur face. The face shall be removed as soon as the concrete has sufficiently hardened; and any voids that may appear shall be filled with the mixture. The surface shall then be immediate ly washed with water until the grit is exposed and rinsed clean, and shall be protected from the sun and kept moist for three clays. For bridge-seat courses and other horizontal sur faces, the granolithic mixture shall be deposited on the concrete to a thickness of at least 1m, inches, immediately after the concrete has been tamped and before it has set, and shall be troweled to an even surface, and, after it has set sufficiently hard, shall be washed until the grit is exposed." The success of this method depends greatly on the removal of the forms at the proper time. In general the washing is done the day following that on which the concrete was deposited. The fresh concrete is scrubbed with an ordinary scrubbing-brush, removing the film, the impressions of the forms, and exposing the sand and stone of the concrete. If this is done at the right time—that is, when the material is at the proper degree of hardness—merely a few rubs of an ordinary house scrubbing-brush, with a free flow of water to cut and to rinse clean, constitutes all the work and apparatus required. The
cost of scrubbing is small if clone at the right tune. A laborer will wash 100 square feet in an hour; but if that same area is permitted to get hard, it may require two men a day with wire brushes to secure the desired results. The practicability of removing the forms at the proper tune for such treatment, depends upon the character of the structure and the conditions under which the work must be done. This method is applicable to vertical walls, but it would not be applicable to the soffit of an arch. (See Fig. 119.) 322. The Acid Treatment. This treatment consists in washing the surface of the concrete with diluted acid, then with an alkaline solution. The diluted acid is applied first, to remove the cement and expose the sand and stone; the alkaline solution is then applied to remove all of the free acid; and finally the surface is washed with clear water. The treatment may be applied at any time after the forms are removed. It is simple and effective. Limestone cannot be used in the concrete for any surfaces that are to have this treatment, as the limestone would be affected by the acid. This process has been used very successfully. It is said to be patented.
323. Dry Mortar Finish. The dry mortar method consists of a dry, rich mixture, with finely crushed stone. The concrete is usually composed of 1 part cement, 3 parts sand, and 3 parts crushed stone known at the +-inch size, and mixed dry so that no mortar will flush to the surface when well rammed in the forms. When placed, the concrete is not spaded next to the forms; and being dry, there is no smooth mortar surface, but there should be an even-grained, rough surface. With the dry mixture, the imprint of joints of the forms is hardly noticed, and the grain of the wood is not seen at all. This style of finish has been extensively used in the South Park system of Chicago, and there has been no efflorescence apparent on the surface, which is explained by time dryness of the mix and the porosity of the surface." 324. Cast Slab Veneer. Cast concrete slab veneer can be made of any desired thickness or size. It is set in place like stone veneer, with the remainder of the concrete forming the backing. It is usually cast in wooden moulds, face clown. A layer of mortar, 1 part cement, 1 part sand, and 2 or 3 parts fine stone or coarse sand, is placed in the mould to a depth of about 1 inch, and then the mould is filled up with a 1: 2: 4 concrete. Any steel reinforcement that is desired may be placed in the concrete. Usually, cast con crete slab veneer is cheaper than concrete facing cast in place, and a better surface finish is secured.