Copyright Goodheart-Willcox Co., Inc. Chapter 18 Concrete Flatwork and Formed Shapes 381 cushion for the new concrete, prevents stone pockets, and aids in securing a tight joint. The mortar is generally made of the same materials as the concrete, but without the coarse aggregate. The mortar should have a slump of less than 6″. Pneumatic Application of Concrete Pneumatically applied concrete, frequently called shotcrete, is a mixture of Portland cement, aggregate, and water shot into place by means of compressed air. Aggregate up to 3/4″ in size can be used with some equipment. Shotcrete can be applied by either the wet mix or dry mix process. In the dry mix process, the cement and aggre- gates are mixed in a relatively dry condition. This mix is pumped through a hose to a nozzle where water is added. At least 45 psi air pressure and a nozzle velocity of about 400 feet per second force the dry materials through the hose. Water pressure at the nozzle should be at least 15 psi higher than the air pressure. In the wet mix process, the concrete is premixed before it is applied pneumatically. Water content can be kept to a minimum using the pneumatic process. As a result, high-strength, durable concrete may be obtained. The key element in this process is the person controlling the nozzle. In both processes, the worker directs the nozzle and controls the thickness of the concrete layer and the angle of application. Pneumatic application can be used for new construction and repair work in difficult loca- tions. It also works well where relatively thin sections and large areas are involved. Finishing Concrete Slabs Concrete slabs can be finished several ways, depending on the effect desired and the use of the product. Some surfaces are left rough, while others are broomed, floated, or troweled. Surfaces can be textured, colored, or have exposed aggregate. Screeding Screeding is usually the first finishing operation after the concrete is placed in the forms. It is performed with a tool called a screed. Screeding is the striking off of excess concrete to bring the top surface to the proper grade or elevation. See Figure 18-6. The edge of the screed is either straight or curved, depending on the surface requirements. The screed rests on the top of the form and is moved across the concrete with a sawing motion. The screed is advanced forward slightly with each movement. An excess of concrete should be carried along in front of the screed to fill low places as the tool is moved forward. However, if too much concrete is allowed to build up in front of the screed, hollows may be left behind it. As the screeding process continues, remove the excess concrete in front of the screed and place it in areas that need to be filled. Try to keep a 1/2′ to 1′ of excess concrete in front of the screed as it is pulled forward. Wooden screeds, either 2″ × 4″ or 2″ × 6″, are commonly used. Prior to use, ensure that the screed is straight without twists or warps to produce a quality job. Another screed material is magnesium. The advantage of magnesium is that moisture does not cause warpage or twisting. Magnesium screeds can be purchased in an assortment of widths and lengths. When using either a wood or metal screed, be sure the edges are not damaged, since this can affect the quality of the job. Sharon Meredith/iStock/Thinkstock Figure 18-6. Workers are hand screeding the sidewalk using a wood 2″ × 6″ to strike off the excess concrete.