Copyright Goodheart-Willcox Co., Inc. Chapter 18 Concrete Flatwork and Formed Shapes 387 Usually, the side forms of relatively thick sections can be removed in 12 to 24 hours after placing. Testing the concrete to determine hardness is better than relying on an arbitrary age for form removal. The age-strength relationship should be determined from representative samples of concrete used in the structure and cured under job conditions. Figure 18-19 shows the times required to attain certain strength under average conditions for air-entrained concrete. Keep in mind that these are averages, and strength is affected by materials, temperature, and other conditions. If forms are tight and require wedging, only wooden wedges should be used. Do not place a pinch bar or other metal tools against the concrete to wedge forms loose. Start removing forms some distance from a projection. This method relieves pressure against projecting corners and reduces the likelihood of breaking off the edges. Finishing Air-Entrained Concrete Air-entrained concrete has a slightly different consistency from standard concrete and therefore requires a small change in finishing operations. Since air-entrained concrete contains many tiny air bubbles that hold all the materials in concrete in suspension, it requires less mixing water than concrete that is not air-entrained. It also bleeds less. There is no waiting for the evaporation of free water from the surface before floating and troweling. If floating is done by hand, an aluminum or magne- sium float is essential. A wooden float drags and increases the work necessary to finish the surface. If floating is done with a power finishing machine, there is practically no difference in the finishing procedure for air-entrained and standard concrete, except that finishing can begin sooner with air-entrained concrete. Almost all horizontal surface defects and fail- ures in standard concrete are due to finishing opera- tions performed while bleed water or excess surface moisture is present. Better results are usually obtained with air-entrained concrete. Curing Concrete Curing a concrete slab is one of the most impor- tant operations in producing quality work, but also one of the most often neglected operations. Even if the concrete is mixed, placed, and finished properly, poor quality work results if proper curing opera- tions are not followed. Little or no moisture loss should be allowed during the early stages of hardening. If necessary, the concrete should be protected during this critical period. Newly placed concrete should be protected from the sun and not allowed to dry out too fast. This can be accomplished with damp burlap, canvas, or polyethylene film coverings. See Figure 18-20. Coverings can be applied as soon as the surface is hard enough so it will not be marred. Coverings should be kept moist for at least three days. Another method of curing is called ponding. Ponding involves keeping approximately an inch of water on the concrete surface, usually by earth dikes around the edges of the slab. Age-Strength Relationship of Air-Entrained Concrete Strength (psi) Age Type I (normal) Cement Type III (high-early- strength) Cement 500 24 hours 12 hours 750 1 1/2 days 18 hours 1500 3 1/2 days 1 1/2 days 2000 5 1/2 days 2 1/2 days Goodheart-Willcox Publisher Figure 18-19. This chart shows the age-strength relationship of air-entrained concrete that must be considered when removing forms. Portland Cement Association Figure 18-20. Polyethylene film can be used to prevent newly placed concrete from drying out too fast.