BROOMING

A nonskid surface can be produced by brooming the concrete before it has thoroughly hardened. Brooming is carried out after the floating operation. For some floors and sidewalks where scoring is not desirable, a similar finish can be produced with a hairbrush after the surface has been troweled once. Where rough scoring is required, a stiff broom made of steel wire or coarse fiber should be used. Brooming should be done so that the direction of the scoring is at right angles to the direction of the traffic. GRINDING

When grinding of a concrete floor is specified, it should be started after the surface has hardened sufficiently to prevent dislodgement of aggregate particles and should be continued until the coarse aggregate is exposed. The machines used should be of an approved type with stones that cut freely and rapidly. The floor is kept wet during the grinding process, and the cuttings are removed by squeegeeing and flushing with water.

After the surface is ground, air holes, pits, and other blemishes are filled with a thin grout composed of one part No. 80-grain carborundum grit and one part portland cement. This grout is spread over the floor and worked into the pits with a straightedge. Next, the grout is rubbed into the floor with the grinding machine. When the filings have hardened for 17 days, the floor receives a final grinding to remove the film and to give the finish a polish. All surplus material is then removed by washing thoroughly. When properly constructed of good-quality materials, ground floors are dustless, dense, easily cleaned, and attractive in appearance.

Figure 7-50.-Mechanical troweling machine.

SACK-RUBBED FINISH

A sack-rubbed finish is sometimes necessary when the appearance of formed concrete falls considerably below expectations. This treatment is performed after all required patching and correction of major imperfections have been completed. The surfaces are thoroughly wetted, and sack rubbing is commenced immediately.

The mortar used consists of one part cement; two parts, by volume, of sand passing a No. 16 screen; and enough water so that the consistency of the mortar will be that of thick cream. It may be necessary to blend the cement with white cement to obtain a color matching that of the surrounding concrete surface. The mortar is rubbed thoroughly over the area with clean burlap or a sponge rubber float, so that it fills all pits. While the mortar in the pits is still plastic, the surface should be rubbed over with a dry mix of the same material. This removes all excess plastic material and places enough dry material in the pits to stiffen and solidify the mortar. The filings will then be flush with the surface. No material should remain on the surface above the pits. Curing of the surface is then continued.

RUBBED FINISH

A rubbed finish is required when a uniform and attractive surface must be obtained. A surface of satisfactory appearance can be obtained without rubbing if plywood or lined forms are used. The first rubbing should be done with coarse carborundum stones as soon as the concrete has hardened so that the aggregate is not pulled out. The concrete should then be cured until final rubbing. Finer carborundum stones are used for the final rubbing. The concrete should be kept damp while being rubbed. Any mortar used in this process and left on the surface should be kept damp for 1 to 2 days after it sets to cure properly. The mortar layer should be kept to a minimum thickness as it is likely to scale off and mar the appearance of the surface.

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