Modern Masonry: Brick, Block, Stone, 8th Edition Page 389 (407 of 512)

Copyright Goodheart-Willcox Co., Inc. Chapter 18 Concrete Flatwork and Formed Shapes 389 accelerators, or a combination of the three. Type III cement is similar to ordinary Type I cement, but it is almost 50% stronger after 24 hours because the particles are finer. Type III cement may not be readily available from smaller companies. Chemical accelerators provide the same benefits as adding an extra 100 pounds of cement per cubic yard or using Type III cement. Accelerators (Type E and Type C) are available in chloride-based or nonchloride-based formulations. Calcium chloride can increase the potential for corrosion of steel rein- forcing if water is present. Air-entrained concrete is not as susceptible to damage caused by freezing and thawing as standard concrete because ice crystals form in the tiny air spaces in the mix. For this reason, air-entrained concrete is generally a good choice for exterior concreting that will be exposed to freezing and thawing. Cold Weather Protection Fresh concrete should be protected from freezing, and curing conditions should be maintained to ensure adequate strength of the concrete. Two methods are generally used to protect concrete and maintain curing conditions—insulating the concrete and providing a protective cover with heat. The most common way to protect concrete against low temperatures is by insulating it. Unless the temperature is too low, the heat of hydration can provide enough heat to protect the concrete. However, 6″ of straw held in place with polyeth- ylene sheeting or tarps provides extra protection. An alternative method is to use insulating or curing blankets. Insulating blankets are easy to install and can be secured with weights or ties. Most blankets are made from fiberglass or closed-cell polypro- pylene insulation that is laminated to canvas or some other material. To be effective, insulating blan- kets must lie flat on the concrete surface with edges secured to prevent air movement under the blanket. In very cold weather, a tent or other heated enclosure can be used to protect the concrete. Be sure to use vented heaters or electric heaters. Heaters that produce carbon dioxide can cause a soft, chalky layer to form on the surface of the concrete. Safety Note Carbon monoxide is a toxic gas formed when combustion is incomplete. Carbon monox- ide poisoning can result from unvented heaters. Hot Weather Construction There is a good reason not to place concrete in hot, dry, windy weather. The concrete sets much faster and may result in cracked or poorly finished slabs. The American Concrete Institute (ACI) states the following: “If the initial setting time for a concrete mix is 2 1/2 hours at 60°F (16°C), that time is likely to be reduced to about an hour or less at 95°F (35°C).” Concrete loses its slump faster in hot weather and can become unworkable before a large load can be placed. In addition, moisture evaporates rapidly from the slab surface, which reduces finishing time. Plastic shrinkage cracking is a potential problem for any placement done in hot, dry, windy weather. These cracks can appear after the slab has been placed, screeded, and bull floated. The only thing that can be done in this situation is to continue sealing the cracks by troweling. To prevent rapid drying in extremely hot weather, avoid high temperatures in fresh concrete. The temperature of the mixing materials can be cooled by using chilled water or ice. The ice should be melted by the time the concrete leaves the mixer. When concreting in hot weather, get an early start while it is cooler, break big placements into smaller sections if possible, and shade the operation if practical. Some contractors use additives like set retarders and superplasticizers to reduce the problems of rapid set or slump loss. These additives are not complete solutions, however. Although superplasticizers can make a stiff mix flow more easily, the effect wears off suddenly—sometimes before the finishing process is completed. The use of set retarders requires experi- enced cement finishers due to the possibility of unex- pected results with these chemicals. Subgrades should be saturated some time in advance and sprinkled just ahead of placing the concrete. Wood forms should be treated or wetted thoroughly. Placing should not be delayed, and the concrete should be screeded and darbied immedi- ately after placing. Covers, such as burlap, which are kept constantly wet, should be placed over the concrete as soon as it is darbied. When the surface is ready for final finishing, a small section should be uncovered immediately ahead of the finishers and recovered as soon as possible. The purpose of curing is to maintain conditions in the setting concrete that encourage complete hydration. The water in the concrete must be kept from evaporating. Moisture must be present for at least seven days. Longer than seven days is

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Copyright Goodheart-Willcox Co., Inc. Chapter 18 Concrete Flatwork and Formed Shapes 389 accelerators, or a combination of the three. Type III cement is similar to ordinary Type I cement, but it is almost 50% stronger after 24 hours because the particles are finer. Type III cement may not be readily available from smaller companies. Chemical accelerators provide the same benefits as adding an extra 100 pounds of cement per cubic yard or using Type III cement. Accelerators (Type E and Type C) are available in chloride-based or nonchloride-based formulations. Calcium chloride can increase the potential for corrosion of steel rein- forcing if water is present. Air-entrained concrete is not as susceptible to damage caused by freezing and thawing as standard concrete because ice crystals form in the tiny air spaces in the mix. For this reason, air-entrained concrete is generally a good choice for exterior concreting that will be exposed to freezing and thawing. Cold Weather Protection Fresh concrete should be protected from freezing, and curing conditions should be maintained to ensure adequate strength of the concrete. Two methods are generally used to protect concrete and maintain curing conditions—insulating the concrete and providing a protective cover with heat. The most common way to protect concrete against low temperatures is by insulating it. Unless the temperature is too low, the heat of hydration can provide enough heat to protect the concrete. However, 6″ of straw held in place with polyeth- ylene sheeting or tarps provides extra protection. An alternative method is to use insulating or curing blankets. Insulating blankets are easy to install and can be secured with weights or ties. Most blankets are made from fiberglass or closed-cell polypro- pylene insulation that is laminated to canvas or some other material. To be effective, insulating blan- kets must lie flat on the concrete surface with edges secured to prevent air movement under the blanket. In very cold weather, a tent or other heated enclosure can be used to protect the concrete. Be sure to use vented heaters or electric heaters. Heaters that produce carbon dioxide can cause a soft, chalky layer to form on the surface of the concrete. Safety Note Carbon monoxide is a toxic gas formed when combustion is incomplete. Carbon monox- ide poisoning can result from unvented heaters. Hot Weather Construction There is a good reason not to place concrete in hot, dry, windy weather. The concrete sets much faster and may result in cracked or poorly finished slabs. The American Concrete Institute (ACI) states the following: “If the initial setting time for a concrete mix is 2 1/2 hours at 60°F (16°C), that time is likely to be reduced to about an hour or less at 95°F (35°C).” Concrete loses its slump faster in hot weather and can become unworkable before a large load can be placed. In addition, moisture evaporates rapidly from the slab surface, which reduces finishing time. Plastic shrinkage cracking is a potential problem for any placement done in hot, dry, windy weather. These cracks can appear after the slab has been placed, screeded, and bull floated. The only thing that can be done in this situation is to continue sealing the cracks by troweling. To prevent rapid drying in extremely hot weather, avoid high temperatures in fresh concrete. The temperature of the mixing materials can be cooled by using chilled water or ice. The ice should be melted by the time the concrete leaves the mixer. When concreting in hot weather, get an early start while it is cooler, break big placements into smaller sections if possible, and shade the operation if practical. Some contractors use additives like set retarders and superplasticizers to reduce the problems of rapid set or slump loss. These additives are not complete solutions, however. Although superplasticizers can make a stiff mix flow more easily, the effect wears off suddenly—sometimes before the finishing process is completed. The use of set retarders requires experi- enced cement finishers due to the possibility of unex- pected results with these chemicals. Subgrades should be saturated some time in advance and sprinkled just ahead of placing the concrete. Wood forms should be treated or wetted thoroughly. Placing should not be delayed, and the concrete should be screeded and darbied immedi- ately after placing. Covers, such as burlap, which are kept constantly wet, should be placed over the concrete as soon as it is darbied. When the surface is ready for final finishing, a small section should be uncovered immediately ahead of the finishers and recovered as soon as possible. The purpose of curing is to maintain conditions in the setting concrete that encourage complete hydration. The water in the concrete must be kept from evaporating. Moisture must be present for at least seven days. Longer than seven days is

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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.

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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.

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.

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Page 2Section 1 Introduction to Masonry click to expand contents

Page 76Section 2 Materials click to expand contents

Page 178Section 3 Techniques click to expand contents

Page 260Section 4 Construction Details click to expand contents

Page 330Section 5 Concrete click to expand contents

Page 408Section 6 Succeeding on the Job click to expand contents

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