Copyright Goodheart-Willcox Co., Inc. 380 Section 5 Concrete Trade Tip When working with concrete, never sprinkle or spray water on the surface of the concrete. This may cause the concrete surface to scale or small pieces of concrete to flake off in the future. Placement of concrete in slab construction should be started at the most distant point of the work so each batch is dumped against the previ- ously placed concrete, not away from it. See Figure 18-5. Care should be taken to prevent stone pockets (areas of excessive large aggregate) from occurring. If this happens, some of the aggregate can be moved to areas where there is more cement paste to surround them. Concrete placement in walls should begin at either end and progress toward the center. The same order should be used for each successive layer. Concrete should be placed around the perimeter (outer edges) first in large flat open areas. Whatever method of placement is used, do not allow water to collect at the ends and corners of forms. Compacting the concrete is always necessary. Work the mix with a spade, shovel, or rod to be sure all spaces are filled and air pockets are worked out. This is called “puddling,” “spading,” or “rodding.” This process should help eliminate stone and air pockets and consolidate each layer with the previ- ously placed layer. Compacting also brings fine material to the faces and top for proper finishing. Mechanical vibrators can be used either in the concrete or on the forms. Mechanical vibration does not make the concrete stronger, but it does permit the use of a stiffer mix that is stronger than a wet mix. Too much vibration causes segregation of particles. Indicators of sufficient vibration are the appearance of a line of mortar along the forms and sinking of the coarse aggregate into the mortar. Placing on Hardened Concrete When placing fresh concrete on hardened concrete, it is important to produce a good bond and a watertight joint. To ensure this result, the hard- ened concrete should be reasonably level, rough, clean, and moist. Leaving some coarse aggregate exposed aids bonding. All loose or soft mortar should be removed from the top surface of the hard- ened concrete before placing the fresh batch. For floors that require two courses of concrete, the top of the first course (lower level) can be broomed with a steel bristle broom or stiff bristle broom just as it sets. The surface should be heavily scored and cleaned before the grout coat and top course are placed. The grout coat is a mixture of Portland cement and water. This coat usually has the consistency of thick paint and is scrubbed into the surface of the slab just before the top course is placed. If old concrete is to receive a new topping, it must be thoroughly roughened and cleaned of dust and loose particles, grease, oil, and other materials. The surface can be chipped with pneu- matic tools or sandblasted to expose sound concrete. Hardened concrete must be moistened thoroughly before new concrete is placed on it, but no pools of water should be left standing on the surface of the existing concrete. Where concrete is to be placed on hardened concrete or rock, a layer of mortar 1/2″ to 1″ thick is placed on the hard surface. This mortar provides a Goodheart-Willcox Publisher Figure 18-5. The proper method of dumping concrete when pouring a slab. Proper Placement of Concrete Slab Baffle Chute Proper Placement of Concrete on a Sloping Surface