Tooled Surfaces

Another method of finishing the concrete surface is to tool it in several ways similar to the methods of cutting or tooling stone. But in preparing for such a surface finish it is necessary to pay particular attention to selecting the aggregate and still greater attention to proportioning the concrete mixtures so that it will be certain there is sufficient cement used to fill all voids or air spaces in the bulk of aggregates, thus firmly bonding all particles into one solid mass. If mixtures are lean, that is, contain too little cement, there will not be enough bonding or binding material in the mass to prevent the small particles of aggregate from being dislodged or broken out when tooling the surface.

When a concrete surface is to be tooled, it is necessary that the concrete be allowed to harden for a greater length of time than for the washing methods. The concrete must have an almost flint-like hardness so that aggregate particles will not be broken out of the surface. It is the particles themselves that must be tooled to give the desired effect. Usually a month of hardening under proper conditions of protection will be sufficient. Objects that have been steam hardened first are even better for tooled finishes.

Although the concrete surface may be tooled by using chisels just as marble and other stones are carved, the usual tooling consists of hammering with a bush or "pean" hammer, such as is used by stone dressers. When this is used properly the aggregates are slightly cut, thus disclosing their color and giving to the concrete surface an appearance like that of cut stone. Best results, when bush hammering is to be used, come from selecting an aggregate of uniform texture and hardness. Broken stone instead of pebble aggregates are then best, since most natural pebbles vary greatly in hardness and the bush hammered pebble surface has not nearly so uniform an appearance as the bush hammered surface in which stone aggregates of uniform hardness were used.

Polished Surfaces

Concrete surfaces may be given a polish similar to that obtained on granites and marbles. The degree of polish that may be secured depends, however, upon the aggregate used and the thorough grading of it so that the least possible amount of cement necessary to bond the particles will be required. Examples of such finish may be seen in so-called mosaic or terrazzo floors. Such a surface to receive the best polish must expose as great a surface of aggregate as possible-the nearest approach to 85 per cent that can be attained.

Using Color Pigments

Another variation possible in concrete surfaces is secured by adding coloring matter to the cement. This practice also can be combined with that of selecting the aggregates. For example, if a uniform reddish tone is desired for surface finish, then coloring matter such as red oxide of iron may be added to the cement and pink or red granite chips used as aggregate. Such a concrete is then treated by washing and scrubbing or rubbing the surface with carborundum stone.