Copyright Goodheart-Willcox Co., Inc. Chapter 18 Concrete Flatwork and Formed Shapes 385 Safety Note When sawing dry, wear an appropriate res- pirator. Concrete may contain silica, which is a severe health hazard. When sawing either wet or dry, wear safety glasses and a face shield to protect from flying particles. Wear ear protection to protect your hearing from the noise created by sawing. Floating There are three reasons why concrete is floated: • To embed aggregate particles just beneath the surface. • To remove slight imperfections, waves, and voids. • To compact the concrete at the surface in preparation for other finishing operations. After concrete has been edged and jointed, it should be allowed to harden enough to support a person and leave only a slight foot imprint. Floating should not begin until the water sheen has disap- peared. Refer to Figure 18-13. When all bleed water and the water sheen have left the surface, the concrete has started to stiffen. The surface is then floated with wood or metal floats or with a finishing machine using float blades. See Figure 18-16. Trade Tip Aluminum or magnesium floats work better than a wood float, especially on air-entrained concrete. Air-entrained concrete can be placed with less water in the mix. Because of this, the concrete mix may be more tacky or sticky and have a tendency to adhere to wood. Metal floats reduce the amount of work required by the finisher. Drag is reduced because the float slides more readily over the surface. A wood float tends to stick to the surface and produces a tearing action. A light metal float also forms a smoother surface texture than a wood float. Be sure not to overwork the concrete. Overworking brings excess water and fine aggregate material to the surface, resulting in surface defects. Floating can be done to provide a coarse texture as the final finish. If this is done, then a second floating may be necessary after the concrete has partially hardened. Troweling When a smooth, dense, hard surface is desired, steel troweling is performed after floating. Cement masons frequently float and trowel an area before moving their knee boards. See Figure 18-17. Troweling can be done by hand or with a trow- eling machine. During the first troweling, whether by hand or machine, the trowel blade must be kept as flat against the surface as possible. If the blade is held at too great an angle, a washboard effect results. A new trowel is not recommended for the first trow- eling. An old trowel that is broken in can be worked quite flat without the edges digging into the surface. As the concrete progressively hardens, it can be troweled several times to obtain a very smooth and hard surface. Usually, smaller size trowels are used for successive applications so sufficient pressure can be exerted for proper finishing. If necessary, tooled joints and edges can be rerun after troweling to maintain uniformity. Brooming Steel troweling produces a very smooth surface that is often slippery when wet. Sidewalks, drive- ways, and other outside flatwork frequently are broomed or brushed to produce a slightly rough- ened surface. The broomed surface is made by drawing a soft-bristled push broom over the surface Stanley Goldblatt Figure 18-16. Floating a concrete surface with a wood float.