Mass concrete is a hot topic. Owners desire long service lives so engineers design concrete mixes for low permeability. These mixtures typically have high cementitious material contents, which results in high temperatures within the concrete. To avoid cracking and other temperature related damage to the concrete, contractors must control the maximum temperature and temperature difference between the interior and the surface of the concrete. This can pit the schedule against the service life. When all involved parties work together, appropriate changes can be made to achieve the desired service life with minimal impacts to the schedule. The key is an understanding of mass concrete.
Mass concrete columns and footings for the James River Bridge. (Courtesy of Fred Parkinson, PB.)
First of all, what is mass concrete?
Mass concrete is defined by the American Concrete Institute (ACI) as:
Any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change to minimize cracking.
While this is a perfect definition, the question is often asked, “so, is this placement considered mass concrete?" As a general rule of thumb, any placement of structural concrete with a minimum dimension equal to or greater than 36 inches should be considered mass concrete. Similar considerations should be given to other concrete placements that do not meet this minimum dimension, but contain ASTM C150 Type III or ASTM C1157 HE cement, accelerating admixtures, or cementitious materials in excess of 600 pounds per cubic yard of concrete.
Now that we know what placements are considered mass concrete, what makes a mass concrete placement any different than a typical placement?
The answer is that temperatures in a mass concrete placement can get high enough to damage the concrete.
All concretes generate heat. Heat is a byproduct of the hydration reactions which gives concrete its strength and durability. In most placements, the heat escapes almost as rapidly as it is generated. In a mass concrete placement, the heat escapes more slowly than it is generated. The result is that temperatures within the concrete can get quite hot. If the internal temperature exceeds (158 degrees Fahrenheit, the long term durability of some concretes can be affected by delayed ettringite formation (DEF). Delayed ettringite formation is rare and only certain concretes can be affected. When delayed ettringite formation occurs, the concrete paste expands and cracks the concrete with detrimental results, which may not be evident for many years. Additionally, while the interior can be quite hot, the surface can be relatively cool. The resulting large temperature difference results in large thermal stresses which can cause cracking of the surface.
Historically, limiting the temperature difference between the interior and surface so that it is less than 35 degrees Fahrenheit has been found to prevent or minimize thermal cracking. Certain concretes are more tolerant of thermal cracking than others, and these concretes can withstand a higher temperature difference without thermally cracking.
How do I prevent high internal temperatures and large temperature differences?
The first step is to select an appropriate mix design. This will reduce other efforts to control temperatures and temperature differences after placement. The temperature rise of concrete is directly related to the types and quantities of cementitious materials in the concrete. An appropriate mix design contains the least amount of cementitious materials needed for strength and durability. Placeability of concrete must also factor into the concrete mix design. This sometimes increases the cementitious content. To reduce heat of hydration, Class F fly ash or slag cement is typically used to replace a portion of the cement. The percentage depends on several factors including environmental exposure and durability requirements.
Concrete insulating blankets on a column.
Once I have a reasonable concrete mix design, do I need to do anything else?
In most cases, the answer is yes; two items must be considered. First, you must ensure that the maximum temperature in the concrete will not exceed 158 degrees Fahrenheit. In placements over about 6 feet thick, the maximum temperature is the sum of the installed concrete temperature plus the temperature rise of the concrete. The temperature rise can be measured or estimated. If the maximum temperature of the concrete is predicted to exceed 158 degrees Fahrenheit, the concrete can be precooled by using chilled batch water, substituting ice for a portion of the batch water, or by liquid nitrogen injection into the fresh concrete. If significant precooling is required, internal cooling pipes can be used to reduce the amount of precooling. Second, the concrete surface will likely also need to be insulated. Insulation limits the temperature difference between the center and surface to minimize or prevent thermal cracking. One or two layers of concrete insulating blankets are often used. Thermal modeling is sometimes done to optimize the amount of insulation and precooling, so that the most cost-effective measures are used.
Do I need to do anything else?
To document the means and methods that are required and will be used, a thermal control plan should be developed. A thermal control plan is similar to a quality control plan, and will allow all involved parties to agree on the measures that will be used, and the expected results. Such measures may include precooling of the concrete, cooling pipe installation and operation, insulation, temperature monitoring equipment and locations.
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Additional and more detailed information on mass concrete can be found in PCA’s publication Mass Concrete for Buildings and Bridges, EB547.