Several concrete bridges in Philadelphia have been finished according to the following specifications; and their appearance is very satisfactory:

"Granolithic surfacing, where required, shall be composed of 1 part cement, 2 parts coarse sand or gravel, and 2 parts granolithic grit, made into a stiff mortar. Granolithic grit shall be granite or trap rock, crushed to pass a 1/4-inch sieve, and screened of dust. For vertical surfaces, the mixture shall be deposited against the face forms to a minimum thickness of 1 inch, by skilled workmen, as the placing of the concrete proceeds; and it thus forms a part of the body of the work. Care must be taken to prevent the occurrence of air-spaces or voids in the surface. The face shall be removed as soon as the concrete has sufficiently hardened; and any voids that may appear shall be filled with the mixture. The surface shall then be immediately washed with water until the grit is exposed and rinsed clean, and shall be protected from the sun and kept moist for three days. For bridge-seat courses and other horizontal surfaces, the granolithic mixture shall be deposited on the concrete to a thickness of at least 1 1/2 inches, immediately after the concrete has been tamped and before it has set, and shall be troweled to an even surface, and, after it has set sufficiently hard, shall be washed until the grit is exposed."

The success of this method depends greatly on the removal of the forms at the proper time. In general the washing is done the day following that on which the concrete was deposited. The fresh concrete is scrubbed with an ordinary scrubbing-brush, removing the film, the impressions of the forms, and exposing the sand and stone of the concrete. If this is done at the right time - that is, when the material is at the proper degree of hardness - merely a few rubs of an ordinary house scrubbing-brush, with a free flow of water to cut and to rinse clean, constitutes all the work and apparatus required. The cost of scrubbing is small if done at the right time. A laborer will wash 100 square feet in an hour; but if that same area is permitted to get hard, it may require two men a day with wire brushes to secure the desired results. The practicability of removing the forms at the proper time for such treatment, depends upon the character of the structure and the conditions under which the work must be done. This method is applicable to vertical walls, but it would not be applicable to the soffit of an arch. (See Fig. 119.)

Fig. 118. Pick for Facing Concrete.

Fig. 118. Pick for Facing Concrete.