Copyright Goodheart-Willcox Co., Inc. 384 Section 5 Concrete Some surface finishes may not need any further finishing, but most require operations such as edging and jointing, floating, troweling, or brooming. Trade Tip The first operations of placing, screeding, and darbying must be performed before any bleeding takes place. If a finishing operation is performed on the surface while the bleed water is present, serious scaling, dusting, or crazing can result. Edging and Jointing Edging, if necessary, could be the next operation. See Figure 18-14. Edging provides a rounded edge or radius to prevent chipping or damage to the edge. An edger is run back and forth until the desired finish is obtained. Care should be taken to cover all coarse aggregate. Do not leave too deep a depression on the top of the slab, because this indentation could be difficult to remove during subsequent finishing operations. As soon as edging has been completed, the slab is jointed or grooved. See Figure 18-15. The bit (cutting edge) of the jointing tool cuts a groove in the slab that is called a control joint or contraction joint. Any cracking due to shrinkage caused by drying or temperature change is intended to occur at the joint. The joint weakens the slab and induces cracking at the bottom of the joint rather than some other place. These cracks are not noticeable when controlled. It is important that the groove continues across the entire slab to ensure a controlled crack in the joint. In sidewalk and driveway construction, the tooled joints are generally spaced at intervals equal to the width of the slab, but not more than 10′ apart. The joints should be perpendicular (at right angles) to the edge of the slab. The groove is usually made with a 3/4″ or 1″ bit. Use a wood or magnesium straightedge as a guide when making grooves. A 1″ × 8″ or 1″ × 10″ board is ideal. Be sure the board is straight and not cupped or twisted. Large concrete surfaces can be jointed by cutting grooves with a power saw with an abrasive or diamond blade. When grooves are cut rather than jointed, the operation should be performed 4 to 12 hours after the slab has been finished. The cutting must be done before random shrinkage cracks develop but after the concrete is hard enough not to be torn or damaged by the blade. Stanley Goldblatt Figure 18-14. Edging produces a radius on the edge of the slab that prevents chipping. A trowel may be inserted between form and concrete to provide a track for the edging operation shown here. Stanley Goldblatt Figure 18-15. Jointing a slab helps control any cracking due to shrinkage caused by drying out or temperature change.