Methods of controlling mass concrete temperatures range from relatively simple to complex, and from inexpensive to costly. Depending on a particular situation, it may be advantageous to use one or more methods over another.
Low-heat materials:
Different types of cement (and cements within each type) generate varying amounts of heat. Figure 2 presents the typical heats of hydration of different cement types. Type IV cement is not shown because it is rarely available. Low-heat generating concrete mixtures are always a wise choice for mass concrete to minimize potential thermal problems. Low-heat generating concrete mixes use the maximum allowable level of low-heat pozzolans— such as Class F fly ash or slag—as cement replacements, and the minimum amount of total cementitious materials that achieves the project requirements. Class F fly ash generates about half as much heat as the cement that it replaces and is often used at a replacement rate of 15 to 25%. Ground granulated blast-furnace slag is often used at a replacement rate of 65 to 80% to reduce heat. The reduction in heat generation achieved depends on the concrete temperature, and should be evaluated on a case-by-case basis. Figure 3 illustrates the effect of Class F fly ash and different cement types on the adiabatic temperature rise of concrete. This is the theoretical increase in temperature of the concrete above the placement temperature, if the concrete is not allowed to cool. In Fig. 3, the total quantity of cementitious materials for all mixes is 525 lb/yd3 (311 kg/m3) of concrete.
Precooling of concrete :
The concrete temperature at the time of placement has a great impact on the maximum concrete temperature. Typically, for every 1 F (0.6 C) reduction or increase in the initial concrete temperature, the maximum concrete temperature is changed by approximately 1 F (0.6 C). As an example, to reduce the maximum concrete temperature by approximately 10 F (6 C), the concrete temperature at the time of placement should generally be reduced by 10 F (6 C). Methods to precool concrete include shading and sprinkling of aggregate piles (as appropriate), use of chilled mix water, and replacement of mix water by ice. Efforts to cool aggregates have the most pronounced effects on the concrete temperature because they represent 70 to 85% of the weight of the concrete. Liquid nitrogen can also be used to precool concrete or concrete constituents. This option can significantly increase the cost of concrete; however, it has been used to successfully precool concrete to 34 F (1 C) for highly specialized mass concrete placements.
Post cooling of concrete :
Cooling pipes in mass concrete are sometimes used to reduce maximum concrete temperatures and to quickly reduce interior temperatures. This method can have high initial and operating costs, but benefits can often outweigh these costs if cooling pipe size, spacing, and temperatures are optimized properly. Figure 4 illustrates the reduction in the average temperature of a mass concrete pour with and without internal cooling pipes. Note the reduction in the maximum concrete temperature and the increased rate of cooling. It is important to emphasize again that significant internal and surface thermal cracking can result if post-cooling is improperly designed or performed. However, if properly designed, a post-cooling system can significantly reduce concrete temperatures and the amount of time required for cooling.
Surface insulation:
Insulation or insulated formwork is often used to warm the concrete surface and reduce the temperature difference, which in turn minimizes the potential for thermal cracking. For most mass pours, surface insulation does not appreciably increase the maximum concrete temperature, but it can significantly decrease the rate of cooling. Insulation is inexpensive, but resulting delays from the reduced cooling rate can be costly. Insulation often has to remain in place for several weeks or longer. Removing it too soon can cause the surface to cool quickly and crack. Many types of insulation materials are available, and insulation levels can be optimized to meet required temperature differences and maximize the rate of cooling.
Aggregate:
Thermal properties of the coarse aggregate can have a significant effect on mass concrete. Concretes containing low-thermal-expansion aggregates such as granite and limestone generally permit higher maximum allowable temperature differences than concretes made using highthermal- expansion aggregates, as shown in Fig. 5 (Fig. 5 is similar to Fig. 1, except a second calculated maximum allowable temperature difference is added for concrete with a high-thermal-expansion aggregate). This means that selecting an aggregate with a low thermal expansion will reduce the potential for thermal cracking.
References
1. ACI Committee 116, “Cement and Concrete Terminology (ACI 116R-00),” American Concrete Institute, Farmington Hills, Mich., 2000, 73 pp.
2. ACI Committee 207, “Effect of Restraint, Volume Change, and Reinforcement on Cracking of Mass Concrete (ACI 207.2R-95),” American Concrete Institute, Farmington Hills, Mich., 2000, 26 pp.
3. Portland Cement Association, Design and Control of Concrete Mixtures, 13th Edition, Skokie, Ill., 1988, 212 pp.
4. ACI Committee 207, “Mass Concrete (ACI 207.1R-96),” American Concrete Institute, Farmington Hills, Mich., 1996, 42 pp.
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