Copyright Goodheart-Willcox Co., Inc. 390 Section 5 Concrete desirable, because the curing process can continue for 28 days. The longer the curing process, the stronger, harder, and more durable the finished product will be. Several techniques are used to trap moisture in fresh concrete. The most common method is to spray on a liquid curing compound that forms a thin film over the slab. Some contractors use ponding sprin- kling or fogging covering the concrete with sheets of plastic or applying wet sand, hay, or burlap to the surface after the finishing is completed. All of these methods work, but each has its advantages and disad- vantages depending upon the specific situation. Air-entrained concrete requires special exper- tise, especially during hot weather. It develops a rubberlike surface in hot weather if finishing is delayed. This concrete is then very hard, if not impossible, to surface smoothly. Joints in Concrete Three basic types of joints are frequently used in concrete construction. These joints are as follows: • Isolation joints. Isolation joints, sometimes called expansion joints, separate different parts of a structure to permit both vertical and horizontal movement. This type of joint is used around the perimeter of a floating slab floor and around columns and machine foundations. See Figure 18-23. • Control joints. Control joints provide for movement in the same plane as the slab or wall is positioned. They compensate for contraction caused by drying shrinkage. Control joints should be constructed in such a way that they permit the transfer of loads perpendicular to the plane of the slab or wall. If control joints are not used in slabs or lightly reinforced walls, random cracks occur due to drying shrinkage. Control joints are sometimes called “contraction joints” or “dummy joints.” See Figure 18-24. • Construction joints. Construction joints provide for no movement across the joint. They are only stopping places in the process of casting. In a construction joint, the two surfaces are not to be allowed to bond together. This can be accomplished by applying oil, paint, or curing compound to one surface of the joint to allow the two surfaces to move independently. Construction joints, however, may be made to perform as control joints. See Figure 18-25. Goodheart-Willcox Publisher Figure 18-23. Isolation joints separate different parts of a structure. A—Isolation joint material was placed against a building wall to separate the wall from the floating slab. The floating slab was then poured against the joint material. B—Detail of an isolation joint. The joint material can be flush in areas where no safety hazard from tripping exists. Concrete Gravel base Earth Isolation joint Building A B Decorative and Special Finishes A variety of patterns, textures, and decorative finishes can be built into concrete during construc- tion. Color can be added to the concrete. Aggregates can be exposed. Textured forms can be used. Concrete can be ground to produce a polished appearance. Geometric patterns can be scored or stamped into the concrete to resemble stone, brick, or tile. Divider strips can be used to form interesting patterns. The possibilities are unlimited.