A very handsome surface can be ob tained by removing the forms while the concrete is still friable and scrubbing the surface with water and a brush, and then rinsing with clean water. If the scrubbing is done at the right time, the mortar may be removed to a considerable depth between the stones, giving a decided relief and producing a rough-coarse texture that is very pleasing. The appearance of the finished surface depends, of course, upon the character of the aggregate and upon the uniformity of its distribution. To secure the most artistic effect, the concrete should have a facing an inch or two thick, made with pebbles or crushed stone not exceeding or inch in greatest dimensions, the proportions being 1 : 2 : 2. The facing mixture for different portions of the structure may be made with different-colored or different graded aggregates. The facing of fine concrete may be applied in either of the ways mentioned in § 351.
The time to be allowed for setting before scrubbing depends upon the nature of the cement and the atmospheric conditions; but with portland cement in warm weather the scrubbing can ordinarily be done easily and successfully after the concrete has set for a day, and in cold weather after it has set for a week. The cost is not great, since if done after the concrete has set 10 or 12 hours a man can scrub 100 sq. ft. per hour with a free flow of water from a hose or a sponge; and if the concrete has set one day in warm weather, a man can finish 20 to 30 sq. ft. per hour.
If the wall to be treated is too high to be completed in one day, the face forms must be constructed so as to permit the removal of the planking at the bottom without disturbing the planking or the studding at the top. This can be done by setting the studding 8 or 10 inches away from the face and supporting the planking by small cleats nailed to the studding and to the planking—see Fig. 117, page 527, Fig. 119, page 529, Fig. 120, page 530.
If the concrete has set too hard to permit the application of the method described in the preceding section, the same result can be accomplished by first washing the surface of the concrete with a dilute acid, and then with an alkaline solution, and finally rinsing with clean water. This method cari be applied at any time after the removal of the forms. Hydrochloric acid is preferable, and a solution of 1 part acid and 5 parts of water has been used with success.
The cost of this method of treatment is 2 to 3 cents per sq. ft., exclusive of the extra for the richer face mortar required.
By. cutting into the body of the concrete with a pick or pointed tool, a surface roughening is produced which breaks up the light and gives a pleasing variation of shade and color. This method of finishing the surface can not ordinarily be applied to gravel concrete, as the pebbles will be dislodged before being chipped. This surface is much used by landscape architects; and is produced at comparatively small expense, since a man will dress 40 to 50 square feet of mass concrete per day.
A similar, but less pleasing, finish may be produced at a little less expense by the use of a bush-hammer. If the surface is to be picked or bush-hammered, less care need be taken with the forms, thus compensating in part for the cost of dressing the concrete.
A colored face may be obtained by using a suitably colored aggregate or by mixing mineral pigment in the concrete. The first is the better, since it gives a durable color and does not injure either the strength or the durability of the concrete. Mineral pigments may be secured in almost any shade from any one of several well-known firms; but many of these coloring materials injure the strength of the concrete, and most of them fade in time. Directions for using the pigments may be had of the makers.