Copyright Goodheart-Willcox Co., Inc. 392 Section 5 Concrete that the color mixes uniformly in the cement prior to other ingredients being added to the mix. Use clean tools and a separate mixer to prevent streaking. When using the one-course method, uniform moistening of the subgrade is important for good color results. The only difference between the one-course and two-course methods is that the two-course method uses a base coat of conventional concrete. The surface of the base coat is left rough to produce a good bond. The top coat may be placed as soon as the surface water disappears. The top coat of colored concrete is generally 1/2″ to 1″ thick. In the dry-shake method, a commercially prepared dry color material is applied over the concrete surface after floating, edging, and grooving. Apply two coats of the dry-shake to the surface of the slab. Perform the finishing operations after each application of dry-shake. The color must be thor- oughly worked into the concrete. Green Note Concrete Sealers Concrete sealers protect concrete from sur- face damage, staining, and corrosion. Some of these sealers are considered “green,” but many are not. Many types of sealers release volatile organic compounds (VOCs) as vapors into the atmosphere. Toxic chemicals in sealers can produce adverse health effects in humans and affect wildlife when used outdoors. Water-based sealers are considered environmentally safe because they do not contain VOCs. Local, state, and federal governments are increasing regulation of VOC emissions. To check whether a product meets VOC guide- lines, consult your regional EPA office or local government environmental office to learn the VOC limits in your area. Manufacturers are reformulating their prod- ucts to conform to new limits. A common change is a reduction in xylene, which produces VOCs, and an increase in acetone, a non-VOC pro- ducer. The performance of sealers and coatings is not affected by this change, but other differ- ences may be noted—difficulty in rolling out, faster drying times, surface blistering, and an increase in cost. Low-VOC coatings often require a change in application techniques. Exposed Aggregate A popular decorative concrete finish is the exposed aggregate finish, which provides an unlim- ited color selection and a broad range of textures. Not only are exposed aggregate finishes attractive, but they can be rugged, they are slip resistant, and they are immune to weather. See Figure 18-27. Exposed aggregate finishes can be produced in several ways. One of the most common is called the seeding method. The procedure is to place, screed, and bull float or darby the concrete in the usual Kathie Nichols/Shutterstock.com Wilm Ihlenfeld/Shutterstock.com Figure 18-27. Exposed aggregate finishes are attractive and slip-resistant. A—Exposed aggregate finishes provide a wide range of colors and textures. B—An attractive walkway of concrete with exposed aggregate has been created for a garden. To accentuate the exposed aggregate, a course of pavers was placed around the outer edge of the wandering walkway slab. A B