Finish of the Surface of Concrete

FINISH OF THE SURFACE OF CONCRETE. The character of the surface is an important factor in the appearance of a concrete struc ture. It is not wise to neglect entirely the looks of any concrete work; and in some structures the appearance is the most important part of its design and construction. It is really difficult to secure a smooth surface of even grain and uniform color on it concrete struc ture. The imperfections of the surface are usually due to one or more of four causes, viz.: (1) imperfectly made forms; (2) badly mixed or carelessly placed concrete; (3) efflorescence and discolora tion of the surface; or (4) unsightly construction joints.

1. If the joints of the forms are not close, the concrete will run between the planks and leave an ugly fin. If a plank springs out of place, a swell is produced on the face of the concrete. If a very smooth surface is sought, the grain of the wood forms is reproduced in the concrete, and even closely made joints leave a conspicuous line in the finished concrete.

2. If the concrete is not uniformly mixed or is unmixed in the handling, the surface will be irregularly colored and will contain pitted and honeycombed spots.

3. If the concrete is not water-tight, and particularly if there is water or earth against the back of the concrete, efflorescence and discoloration are likely to appear. The efflorescence is due to water percolating through the concrete and dissolving the soluble salts, which are left as a white, powdery substance on the surface when the water evaporates (see li 388).

4. If the placing of the concrete does not proceed continuously clay and night, an unsightly line on the surface is likely to show where the two days' work joined. This defect may be almost entirely eliminated by nailing a triangular strip against the form and finishing the day's work to the inner edge of this strip, thus pro ducing on the face of the concrete a regular groove instead of the usual ragged division line.

There are various ways of correcting the above defects or of securing a surface of better appearance than any left by the most perfect forms. These will next be briefly considered, attention

being given first to the methods of finishing a plain smooth surface and then to the methods of finishing that secure a more ornamental surface.

Mortar Face.

When it was the custom to use a dry or semi-plastic concrete, it was also the custom to make the surface of a richer mixture than the body of the concrete. A 1 : 2 or 1 : 3 mortar was frequently used for this purpose. This facing mortar was sometimes simply banked up against the form a little ahead of the concrete, the tamping of the concrete uniting the two together firmly. A neater and more economical method was to place next to the form a frame into which the face mortar was packed and which was afterwards withdrawn. This frame or form consisted of a --inch steel plate about 8 inches high and 5 feet long, having three 1-inch steel angles riveted transversely to it. This plate was set vertically with the projecting angles against the face form, and the rich mortar was packed between the plate and the wooden form. The plate sometimes had a flare at the top to facilitate the intro duction of the mortar, and sometimes handles were attached to aid in withdrawing it. After the plate was withdrawn the concrete and facing mortar were thoroughly tamped to secure a good union between the two.

This form of face has been abandoned because of its expense and also because of the substitution of a wet for a dry concrete which permits a different procedure.

Although a mortar face is now not much used for a vertical surface, it is customary to finish an exposed horizontal or nearly horizontal surface with a coating of rich mortar. The mortar should be mixed to a plastic consistency and should be put on immediately after the concrete is deposited, care being taken that the surface of the concrete is clean. The facing mortar should he 1 or 1# inches thick, and should be troweled down hard and smooth; but excessive troweling is likely to cause innumerable hair cracks in the finished surface.