Copyright Goodheart-Willcox Co., Inc. 388 Section 5 Concrete Membrane curing compounds are frequently sprayed on concrete surfaces. See Figure 18-21. Uniform coverage is necessary. Two coats are often required to provide adequate protection. Curing Temperatures The rate of chemical reaction between cement and water is affected by temperature. Therefore, temperature affects the rate of hardening, strength, and other qualities of concrete. Cold Weather Construction According to the American Concrete Institute (ACI), cold weather is considered to be air tempera- ture that averages less than 40°F (5°C) and is below 50°F (10°C) more than half of each day for three weeks in a row. In cold weather, concrete placement may require heated materials, a covering for the fresh concrete, or a heated enclosure. See Figure 18-22. Under cold weather conditions, concrete sets up more slowly, takes longer to finish, and gains strength more slowly. The hydration of the cement generates some heat, but this may not be enough. If the concrete freezes before it hardens, the damage done by freezing can reduce the final compressive strength by as much as 50%. Ice starts to form in plastic concrete when the concrete temperature approaches 27°F (–3°C). The freezing point can be as low as 20°F (–7°C) if there are admixtures in the mix. Ice requires more space than water, and this expan- sion in wet concrete weakens the product by creating void spaces that disrupt the bond between the cement paste and the aggregate. The temperature of concrete at the time of placing should generally be 50°F to 70°F (10°C to 21°C). The materials should never be heated to the point that the fresh concrete temperature is above 70°F (21°C), or strength will be reduced. Concrete should never be placed on a frozen subgrade. When subgrades thaw, uneven settling and cracking of the slab usually results. Snow and ice in the form takes up space intended for the concrete. Thawed subgrade should be recompacted before placing concrete to avoid uneven settlement and cracked concrete. Forms, reinforcing steel, and embedded fixtures should be free of ice when the concrete is placed. A thin layer of warm concrete should be placed on cold, hardened concrete when an upper layer is to be poured. The thick upper layer shrinks as it cools, and the lower layer expands as it warms. Failure of the bond results if care is not taken. In cold weather, moisture for curing is still very important. Keep the concrete moist, especially near heating units. Maintain the temperature of normal concrete at 70°F (21°C) for three days or at 50°F (10°C) for five days. Keep the temperature of high early-strength concrete at 70°F (21°C) for two days or at 50°F (10°C) for three days. Do not allow the concrete to freeze for the next four days. It is estimated that for every 10°F drop in concrete temperature, set time increases by approximately one-third. Hydration stops completely at 14°F (–10°C), but resumes when the temperature rises. Set time can be shortened and the early strength of the concrete increased by ordering the concrete with extra cement, Type III cement, chemical Portland Cement Association Figure 18-21. Membrane curing compounds can be sprayed on a concrete surface immediately after the concrete has had its final finishing operation. Jack Klasey Figure 18-22. Straw is used to protect newly placed concrete in cold weather.