Copyright Goodheart-Willcox Co., Inc. Chapter 18 Concrete Flatwork and Formed Shapes 393 manner, keeping the level of the surface about 3/8″ to 1/2″ lower than the forms. This space will be filled with the extra aggregate. When these finishing operations have been completed, spread the aggregate uniformly with a shovel or by hand so the entire surface is completely covered with a layer of stone. Next, embed the aggre- gate by tapping with a wood hand float or straight- edge. Work the surface with the hand float until the surface is similar to that of a normal slab after floating. Starting the next operation requires accurate timing. When the slab can bear the weight of a worker on kneeboards with no indentations, brush the slab with a stiff nylon bristle broom to remove the extra mortar over the stones. Next, apply a fine spray of water along with brushing. If the aggregate becomes dislodged, stop the operation for a while. Continue washing and brushing until the water is clear and there is no noticeable cement film left on the aggre- gate. A surface retarder can be used for better control of the exposing operations, but is not necessary. Another method of producing an exposed aggre- gate finish is to expose the stone in conventional concrete. No extra stone is added, but a high propor- tion of coarse to fine aggregate is necessary. The coarse aggregate should be uniform in size, bright in color, closely packed, and uniformly distributed. The slump of this concrete must be between 1″ and 3″. A third method is to place a thin topping course of concrete containing special aggregates over a base of regular concrete. This technique is used for terrazzo construction. Terrazzo toppings on outdoor slabs are generally 1/2″ thick and contain marble, quartz, or granite chips. Random cracking is eliminated by using brass or plastic divider strips set in a bed of mortar. This type of terrazzo is called rustic terrazzo or washed terrazzo. Textured Finishes A wide variety of interesting decorative textures can be produced on vertical concrete surfaces by using textured form materials. See Figure 18-28 and Figure 18-29. Special procedures may be required for these finishes. Textured surfaces on slabs can be achieved by brooming and the use of mortar, dash coat, or rock salt, or other materials. Brooming can be executed in a fine or coarse straight line pattern or a wavy texture. The procedure for brooming a surface was discussed earlier in this chapter. A travertine finish, sometimes called a keystone finish, is produced by applying a dash coat of mortar over freshly leveled concrete. The dash coat is mixed to the consistency of thick paint and usually contains a yellow pigment. It is applied in a splotchy manner with a dash brush. Numerous ridges and depressions are formed by this procedure. After the coating has hardened a little, the surface is troweled slightly to flatten the ridges and spread the mortar. The resulting finish is smooth on the high spots and coarse grained in the low areas. This effect resem- bles travertine marble, from which it gets its name. Another texture can be produced by scattering rock salt over the surface after hand floating or troweling. The salt is pressed into the surface so that the top of each grain is exposed. When the concrete has hardened, the surface is washed and brushed. The salt pellets dissolve, leaving holes in the surface. Trade Tip Neither the rock salt nor travertine finish is recommended for areas that experience freez- ing weather. Water frozen in the recesses ruins the surface. Goodheart-Willcox Publisher Figure 18-28. This pattern in the pier was developed using thin strips attached to the form. Goodheart-Willcox Publisher Figure 18-29. Cast concrete textured to imitate stone.