Next to form or design, the character of the surface has most effect on the appearance of concrete, whether in a building, arch, wall, or abutment; in fact, when the view is had at very close range, or in such structures as retaining walls or pavements, the surface finish may take precedence over proportion.
We shall describe some methods used in trying to obtain satisfactory surfaces in the various classes of concrete work done in the South Park system of the city of Chicago.
The imperfections in the exposed surfaces of concrete are due mainly to well-known causes which may be summed up as follows: 1. Imperfectly made forms.
2. Badly mixed concrete.
3. Carelessly laid concrete.
4. Efflorescence and discoloration of the surface after the forms are removed.
Forms with a perfectly smooth and even sur face are difficult and expensive to secure. Made of wood, as they usually are, it is not practicable to secure boards of exact thickness; joints can not be made perfectly close; the omission of a nail here and there allows warping; and the re sult is an unsightly blemish when least wanted.
Badly mixed concrete gives us irregularly colored, pitted, and honeycombed surfaces, with here a patch of smooth mortar and there a patch of broken stone exposed without sufficient mor tar. Careless handling and placing will produce the same defects.
But, granting we have the best of labor, that all reasonable and care is had in making up forms, in mixing, handling, and placing the concrete, that it is well spaded, grouted, or the farms plastered on the surface, still the results may be unsatisfactory. All these efforts tend to produce a smoothly mortared surface; and the smoother the surface, the more glaring become minor defects. The finer lines of closely-made joints in the forms become prominent, the grain of the wood itself is reproduced in the mortar surface, hair-cracks are liable to form, and, worst of all, efflorescence and discoloration are pretty sure to appear.
It is of doubtful efficiency to line the forms with sheet metal or oilcloth. Imperfections still appear.
Two methods suggest themselves as likely to overcome the defects alluded to above: (1) Treating the surface in some manner after the forms are removed to correct the defects; (2) Using for surface finish a mixture which will not take the imprint of the forms and which will minimize rather than exaggerate every imperfection in the latter, and which will not effloresce.
Methods of treating the surface by bush hammering, tooling, and scrubbing with wire brushes and water, have been described in various published articles, all of which have for their object the removal of the outer skin of mortar in which the various imperfections exist. But the method most used in the South Park work is the acid treatment.
This method of finishing consists in washing the surface with an acid preparation to remove the cement and expose the particles of stone and sand, then with an alkaline solution to remove all free acid, and finally giving it a thorough cleansing with water. The operation is simple and always effective. It can be done at any time after the forms are removed—immediately or within a month or more. It requires no skilled labor—only judgment as to how far the acid or etching process should be carried. It has been applied with equal success to troweled surfaces, like pavements; to moulded forms, such as steps, balusters, coping, flower vases, etc.; and to con crete placed in forms in the usual way. It, of course, means that in the concrete facing, only such material shall be used as will not be affected by acid—such as sand or crushed granite. It excludes limestone.
The treated surface can be made any desired color by selection of colored aggregates or by the addition of mineral pigments. The colors obtained by selection of colored stone are per haps the most agreeable, and doubtless the most durable.
There have been moulded in the South Park shops blocks for buildings, columns, architec tural mouldings, and ornaments with both red and black crushed granite, all treated with the acid to bring out the natural colors of the stone. There has been a large quantity of concrete pavement laid with torpedo sand surface colored a buff sandstone color with a small quantity of yellow ocher and mineral red and treated with acid. The buff color imparted to the surface is a welcome relief from the glare of the ordinary whitish grey concrete pavement in the sunshine; and the etching of the surface adds to the soft ness of the color, at the same time preventing any slipperiness. The same buff color has been used to a large extent in steps, bases of lamp posts, and other moulded articles to be placed on or near the ground. With sand as the aggre gate, thousands of pieces have been moulded for coping, balustrades, concrete seats, drinking fountains, pedestals, etc., which, when treated with the acid, appear like fine-grained, almost white sandstone.