Where there are projections or marks left by the moulds or forms, they are tooled or rubbed down before treatment; and where it is neces sary to plaster up rough places or cavities in the surface, it may be done after treatment, and can not be detected.
These various classes of work have in all cases proved satisfactory.
The second method of preventing or minimiz ing surface defect has also been tried in the South Park work with quite a measure of success.
During the years 1904, 1905, and 1906, groups of concrete buildings were erected in nine differ ent parks, costing with their accessories, from $65,000 to $150,000 for each group. These buildings are all monolithic structures, with occasional expansion-joints, the exposed sur faces of walls being of concrete composed of one part cement, three parts fine limestone screen ings, and three parts crushed limestone, known as the one-fourth-inch size. This was thoroughly mixed quite dry, so that no mortar would flush to the surface, and well rammed in wooden forms made in the usual manner. The result was an evenly grained, finely-honeycombed sur face, of a pleasing soft grey color, which grows darker with time and blends admirably with the park landscape. In placing, the concrete was not spaded next the forms; it was too dry to cause any flushing of mortar; so there is no smooth mortar surface, the imprint of joints be tween the boards is hardly noticed, and the grain of the wood is not seen at all. There is no efflorescence apparent on the surface anywhere, and cannot be on account of the dryness of the mixture and the porosity of the surface. The buildings are used as gymnasiums, assembly halls, reading and refreshment rooms; and, as a rule, the same grey concrete finish is given the interior walls as the exterior. In some cases a little color has been applied on the interior walls, and the walls of shower and bathrooms have been waterproofed with plaster. The porosity of the surface makes it well adapted to receive and hold plaster.
This sort of surface is not capable of treat ment with acid as is a smooth mortared surface; nor is such treatment desirable. Consequently the only color obtainable is the natural color of the cement-covered stone, which is softer and far more agreeable than the grey of the usual mortar-finished surface. It is not suited for the
surface of a pavement, and is not impervious to water. Although it is evident that water enters the poures to a considerable extent, there is no evidence of injury from the frost during the winter.
The same finish has been used for retaining walls, arch bridges, fence-posts, walls enclosing surface yards, etc. In the buildings the thin walls were made entirely of this mixture, while in the heavier structures it has been used only as a facing. Two reinforced arches of 60 feet span were faced with this mixture, but the steel was imbedded in a wetter, more impervious con crete. The same dry mixture can be used for moulded stones when the mould is open enough to permit tamping, and of course it is eminently suited to block machines.
With the finely crushed stone, a sound, smooth surface was obtained (when the sides of the boxes were removed) where it was man ifestly impossible to plaster or grout the surface, and where spading a mixture of coarse stone simply washed the cement away from the surface stones.
Stucco. Stucco-work is cement plastering, and, in one form or another, has been in use for ages. It is durable, artistic, and impervious to weather. For veneering new buildings, or pro tecting old structures, and wherever the cost of solid concrete is prohibitive, Portland cement stucco has qualities that very highly commend it.
As a rule, two coats are used—the first, a scratch coat composed of 5 parts Portland ce ment, 12 parts clean, coarse sand, and 3 parts lime, with a small quantity of hair; the second, a finishing coat composed of 1 part Portland cement, 3 parts clean, coarse sand, and 1 part slaked lime paste. Should only one coat be de sired, the finishing coat is used. Some masons prefer a mortar in which no lime is used, but this requires more time to apply.
In applying stucco to brick or stone struc tures, clean the surface of the wall, and, after thoroughly wetting, plaster inches thick. For a finish, either smooth with a wooden float, or rough by rubbing with burlap.