SECTION X. TEMPERATURE EFFECTS
5-76. Concreting in hot weather poses some special problems such as strength reduction and cracking of flat surfaces due to too-rapid drying.
5-77. Concrete that stiffens before consolidation is caused by too-rapid setting of the cement and too much absorption and evaporation of mixing water. This leads to difficulty in finishing flat surfaces. Therefore, limitations are imposed on placing concrete during hot weather and on the maximum temperature of the concrete; quality and durability suffer when concrete is mixed, placed, and cured at high temperatures. During hot weather take steps to limit concrete temperature to less than 90°F, but problems can arise even with concrete temperatures less than 90°F. The combination of hot dry weather and high winds is the most severe condition, especially when placing large exposed slabs.
EFFECTS OF HIGH CONCRETE TEMPERATURES
5-78. Three common things affect high concrete temperatures.
Water requirements. Because high temperatures accelerate hardening, a particular concrete consistency generally requires more mixing water than normal. Figure 5-23 shows a linear relationship between an increase in concrete temperature and the increase in mixing water required to maintain the same slump. However, increasing water content without increasing cement content results in a higher W/C ratio, which has a harmful effect on the strength and other desirable properties of hardened concrete.
Compressive strength of concrete. Figure 5-24 demonstrates the effect of high concrete temperatures on compressive strength. Tests using identical concretes having the same W/C ratio show that while higher concrete temperatures increase early strength, the reverse happens at later ages. If water content is increased to maintain the same slump (without changing the cement content), the reduction in compressive strength is even greater than that shown in Figure 5-24.
Cracks. In hot weather the tendency for cracks to form increases both before and after hardening. Rapid water evaporation from hot concrete can cause plastic shrinkage cracks even before the surface hardens. Cracks can also develop in the hardened concrete because of increased shrinkage due to a higher water requirement, and because of the greater difference between the high temperature at the time of hardening and the low temperature to which the concrete later drops.
Figure 5-23. Concrete mix water requirements as temperature increases
Figure 5-24. Effect of high temperature on concrete compressive strength at various ages
COOLING CONCRETE MATERIALS
5-79. The most practical way to obtain a low concrete temperature is to cool the aggregate and water as much as possible before mixing. Mixing water is the easiest to cool and is also the most effective, pound for pound, in lowering concrete temperature. However, because aggregate represents 60 to 80 percent of the concrete's total weight, the concrete temperature depends primarily on the aggregate temperature. Figure 5-25 shows the effects of the mixing water and aggregate temperatures on the temperature of fresh concrete. Lower the temperature of fresh concrete by:
Figure 5-25. Mixing water temperatures required to produce concrete of required temperature
5-80. High temperatures increase the hardening rate, thereby shortening the length of time available to handle and finish the concrete. Concrete transport and placement must be completed as quickly as possible. Take extra care to avoid cold joints when placing it. Proper curing is especially important in hot weather due to the greater danger of crazing and cracking. But curing is also difficult in hot weather, because water evaporates rapidly from the concrete and the efficiency of curing compounds is reduced. Leaving forms in place is not a satisfactory way to prevent moisture loss when curing concrete in hot weather. Loosen the forms as soon as possible without damaging the concrete and cover the concrete with water. Then frequent sprinkling, use of wet burlap, or use other similar means of retaining moisture for longer periods.
COLD WEATHER CONCRETING
5-81. Do not suspend concreting during the winter months. Take the necessary steps to protect the concrete from freezing in temperatures of 40°F or lower during placing and during the early curing period.
5-82. In your prior planning, include provisions for heating the plastic concrete and maintaining favorable temperatures after placement. The temperature of fresh concrete should not be less than that shown in lines 1, 2, and 3 of Table 5-6. Note that lower temperatures are given for heavier mass sections than thinner sections, since less heat dissipates during the hydration period. Because additional heat is lost during transporting and placing, the temperatures given for the freshly mixed concrete are higher for colder weather. To prevent freezing, the concrete's temperature should not be less than that shown in line 4 of Table 5-6 at the time of placement. To ensure durability and strength development, further thermal protection will need to be provided to make sure that subsequent concrete temperatures do not fall below the minimums shown in line 5 of Table 5-6 for the time periods given in Table 5-7. Concrete temperatures over 70°F are seldom necessary because they do not give proportionately longer protection from freezing, since the heat loss is greater. High concrete temperatures require more mixing water for the same slump and this contributes to cracking due to shrinkage.
Table 5-6. Recommended concrete temperatures for cold-weather construction
Table 5-7. Recommended duration of protection for concrete placed in cold weather (air-entrained concrete)
EFFECTS ON LOW CONCRETE TEMPERATURES
5-83. Figure 5-26, demonstrates which temperature affects the hydration rate of cement; low temperatures retard hardening and compressive strength gain. The graph shows that the strength of concrete mixed, placed, and cured at temperatures below 73°F is lower than concrete cured at 73°F during the first 28 days, but becomes higher with age and eventually overtakes the strength of the concrete cured at 73°F. Concrete placed at temperatures below 73°F must be cured longer. Remember that strength gain practically stops when the moisture required for hydration is removed. Figure 5-27 shows that the early strengths achieved by Type III or high-early-strength cement are higher than those achieved by Type I cement.
Figure 5-26. Effects of low temperature on concrete compressive strength at various ages
Figure 5-27. Relationship between early compressive strengths of portland cement types and low curing temperatures
COLD WEATHER TECHNIQUES
5-84. When heating concrete ingredients, the thaw-frozen aggregate makes proper batching easier and avoids pockets of aggregate in the concrete after placement. If aggregate is thawed in the mixer, check for too much water content. Aggregate in temperatures above freezing seldom has to be heated, but at temperatures below freezing, the FA used to produce concrete may need to be heated.
Heating aggregate. Use any of several methods to heat aggregate. One method for small jobs is to pile it over metal pipes containing fires or stockpile aggregate over circulating steam pipes. Cover the stockpiles with tarpaulins to both retain and distribute the heat. Another method is to inject live steam directly into a pile of aggregate, but the resulting variable moisture content can cause problems in controlling the amount of mixing water. The average temperature of the aggregate should not exceed 150°F.
Heating water. Mixing water is easier to heat because it can store five times as much heat as solid materials having the same weight. Although aggregate and cement weigh much more than water, water's stored heat can be used to heat other concrete ingredients. When either aggregate or water is heated above 100°F, combine them in the mixer first before adding the cement. Figure 5-28, shows how the temperature of its ingredients affects the temperature of fresh concrete. This graph is reasonably accurate for most ordinary concrete mixtures. As shown in Figure 5-28, mixing water should not be hotter than 180°F so that, in some cases, both aggregate and water must be heated. For example, if the weighted average temperature of aggregate is below 36°F, and the desired fresh concrete temperature is 70°F, heat the water to its maximum temperature of 180°F and heat the aggregate to make up the difference.
Using high-early-strength cement. High-early-strength cement produces much higher hydration temperatures which can offset some of the cold water effects. Other benefits include early reuse of forms and shore removal, cost savings in heating and protection, earlier flatwork finishing, and earlier use of the structure.
Using accelerators. Do not substitute accelerators for proper curing and frost protection. Also, do not try to lower the freezing point of concrete with accelerators (antifreeze compounds or similar products), because the large quantities required seriously affect compressive strength and other concrete properties. However, you may use smaller amounts of additional cement or such accelerators as calcium chloride to speed up concrete hardening in cold weather, as long as you limit it to no more than 2 percent of calcium chloride by weight of cement. But be careful in using accelerators containing chlorides where an in-service potential of corrosion exists, such as in pre-stressed concrete or where aluminum inserts are planned. When sulfate-resisting concrete is required, use an extra sack of cement per cubic yard rather than calcium chloride.
Preparing for placement. Never place concrete on a frozen sub-grade because severe cracks due to settlement usually occurs when the sub-grade thaws. If only a few inches of the sub-grade are frozen, thaw the surface by burning straw, by steaming or, if the grade permits, by spreading a layer of hot sand or other granular material. Be sure to thaw the ground enough to ensure that it will not refreeze during the curing period.
Figure 5-28. Effect of temperature of materials on temperature of fresh concrete
5-85. Concrete placed in forms or covered by insulation seldom loses enough moisture at 40oF to 55oF to impair curing. Forms distribute heat evenly and help prevent drying and overheating. Leave them in place as long as practicable. However, when using heated enclosures during the winter, moisten curing concrete to offset the drying effects. Keep the concrete at a favorable temperature until it is strong enough to withstand both low temperatures and anticipated service loads. Concrete that freezes shortly after placement is permanently damaged. If concrete freezes only once at an early age, favorable curing conditions can restore it to nearly normal, although it will neither weather as well nor be as watertight as concrete that has never frozen. Air-entrained concrete is less susceptible to freeze damage than nonair-entrained concrete. (See TM 5-349 for details of cold weather concreting.) Three methods for maintaining proper curing temperatures are described below.
Live steam. When fed into an enclosure, live steam is an excellent and practical curing aid during extremely cold weather, because its moisture offsets the rapid drying that occurs when very cold air is heated. Use a curing compound after removing the protection if the air temperature is above freezing.
Insulation blankets or bats. The manufacturers of these materials can usually provide information on how much insulation is necessary to protect curing concrete at various temperatures. Because the concrete's corners and edges are the most likely to freeze, check them frequently to determine the effectiveness of the protective covering.
Heated enclosures. Use wood, canvas, building board, plastic sheets, or other materials to enclose and protect curing concrete at below-freezing temperatures. Build a wood framework and cover it with tarpaulins or plastic sheets. Make sure enclosures are sturdy and reasonably airtight, and allow for free circulation of warm air. Provide adequate minimum temperatures during the entire curing period. The easiest way to control the temperature inside the enclosure is with live steam. Unless enclosures are properly vented, do not use carbon monoxide-producing heaters (salamanders or other fuel-burning heaters) when curing concrete for 24 to 36 hours after curing.
|David L. Heiserman, Editor||
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Revised: June 06, 2015