Sand Blast Finish

Another method of finishing a concrete surface that is used sometimes may be mentioned, although its application is not within the average concrete worker's range. That is the sand blast. No doubt many readers have seen a sand blast machine being used to clean the exterior of some large building. By this method a stream of fine sand is expelled from a jet or nozzle under high pressure created by compressed air, and the small particles striking the surface wear from it a thin film, thus exposing its true color and texture.

Imitating Coursed Stone

In laying up concrete walls the effect of stone courses can be produced by placing V-shaped strips or battens properly arranged horizontally and vertically on the inside face of the forms. Vertical battens should be limited to construction joints rather than used to imitate stone work. The marks or scorings thus produced have somewhat the effect of paneling on the surface, that is, they break the flat monotonous appearance of large surfaces. But while it is perfectly legitimate in concrete work to secure surface finish in the various ways described, it is not desirable to attempt treatments that produce gross imitations of some other material. As an example of such an attempt, the old type of concrete block may be mentioned. Here a broken rock-faced appearance was attempted by the use of different types of face plates in the molds in which the block were formed. It is hard to imagine a more monotonous and, in time, more displeasing surface finish than the imitation stone one obtained in that manner.

Stucco

Nearly everyone has seen houses or other buildings finished by plastering with a cement mortar. This finish is called "stucco."

Stucco is a mixture of portland cement, sand and water, with which there is usually combined a small quantity of hydrated lime, so that the plaster or mortar will work more readily under the trowel. The materials should be proportioned 1 sack of portland cement, 2 cubic feet of sand, and a quantity of hydrated lime equal to one-tenth of the cement. The hydrated lime should first be thoroughly mixed with the cement while the two are dry, then clean sand should be added and the whole mass mixed by turning with shovels several times until of uniform color. Water is added until the required consistency is obtained. If hydrated lime cannot be conveniently obtained, it is necessary to use slaked lime. Hydrated lime is merely a commercial name for lime that has been thoroughly slaked by mechanical means.

Only a small quantity of stucco plaster should be prepared at a time as the cement sets rapidly and if this occurs before the plaster or mortar is applied, the results will not be satisfactory. The stucco mixture should be wet enough to work easily under the trowel but stiff enough to prevent any flowing after applied to the metal lath, expanded metal, or whatever surface is being covered.

In building construction usually two or more coats of stucco are applied. Sometimes a third coat is used for the purpose of securing the required final finish. Before applying a second or third coat the one last placed must be scored or marked on the surface while soft, with some kind of a scratch tool, so that there will be a good bond or key for the next coat. A tool suitable for scratching the surface can be mads from a short piece of 1/2-inch thick board, notched sawtooth fashion on one end and shaved down on the other to form a handle so that the tool will somewhat resemble a small paddle.

A method of securing variety in finish of concrete tile. To the left is shown the plain concrete tile before surface treatment. The next illustration shows a paper mask or stencil laid on the wetted concrete surface. Against this there is then thrown a colored concrete mixture in which selected aggregates are used. The third figure shows this in part and also shows the paper stencil being removed so as to give the finished appearance shown in the view to the right.

Quite a variety of finishes can be given to the final coat. The simplest one is a smooth finish secured by troweling. It is not well, however, to trowel the surface too much, especially with a steel trowel, as the plaster is likely to crack.