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Thermal Issues in High Performance Concrete
John Gajda, CTLGroup
High performance concrete (HPC) is routinely used in all elements of bridge construction from the foundations to the wearing surface. HPC is utilized for various reasons, including the need for high early strength, low permeability, and ease of placement. In most cases, the cementitious materials content in HPC mixes is high. This can result in thermal issues in relatively thin sections, including excessively high internal temperatures and thermal cracking. These are the same issues that are commonly found in mass concrete placements.

Consider the segmental cast-in-place superstructure of the recently completed Benicia–Martinez Bridge described in the previous article. The lightweight HPC mix in the superstructure was designed to achieve a rapid strength gain and meet stringent modulus of elasticity and density requirements. The mix contained approximately 980 lb/cu yd (581 kg/cu m) of cementitious materials. Thermal issues were anticipated in the 40-in. (1.00-m) thick bottom slabs, so these portions were treated as mass concrete and temperatures were measured and tightly controlled. However, after temperatures exceeding 190°F (88°C) were measured in some of the 22-in. (560-mm) thick portions, the entire superstructure was treated as mass concrete. As a result, the concrete was precooled and cooling pipes were installed in portions as thin as about 16-in. to limit the maximum temperature. Temperature differences were also controlled to minimize thermal cracking. While this could be considered an extreme example, it is not all that extreme considering the typical high cementitious materials content of mixes used for high early strengths in segmental construction or for durability in corrosive environments.

Thermal issues result from hydration of the cementitious materials. In most placements, the heat escapes almost as rapidly as it is generated. In thick placements or placements with concrete having a very high cementitious materials content, heat is generated more quickly than it can escape. This results in high internal temperatures within the concrete. The segmental mix used for the Benicia – Martinez Bridge had an adiabatic temperature rise (a measure of the overall heat energy in the concrete) of nearly 150°F (66°C). The temperature rise in the 22-in. (560-mm) thick portions was 117°F (47°C), illustrating how much heat can build up before it escapes.

Maximum Concrete Temperature
Experience has shown that when the internal temperature exceeds 158°F (70°C) during curing, the long-term durability of some concretes can be affected by delayed ettringite formation (DEF). Although DEF is rare and only certain concretes can be affected, it has been identified in bridges and other structures in the United States. When DEF occurs, the concrete paste expands and cracks the concrete with detrimental results. This may not be evident for many years.

Temperature Difference
While the interior of a concrete placement can be quite hot, its surface can be relatively cool. The resulting large temperature difference between the surface and the interior produces large thermal stresses. These stresses add to other stresses such as those from drying shrinkage. Cracking occurs when the stresses exceed the in-place tensile strength of the concrete. This can occur if inadequate measures are used to control the temperature difference, or when these measures are discontinued too soon—before the interior concrete has adequately cooled. Historically, limiting the temperature difference between the interior and surface to less than 35°F (19°C) has been found to prevent or minimize thermal cracking. Certain concretes are more tolerant of thermal cracking than others, and can withstand a higher temperature difference without thermally cracking.

In extreme cases, thermal cracking can be a structural concern; however, in most cases, thermal cracking is a durability issue. When HPC is used for durability, thermal cracking “short circuits” the benefits of the low permeability concrete by providing convenient paths for corrosive agents to readily reach the reinforcing steel.

Recommendations
To minimize or reduce thermal issues, the following guidelines are recommended:
  1. Use a reduced-heat concrete mix, with as low a total cementitious materials content as reasonably practical.
    • If low permeability is desired, increase the percentage of fly ash, silica fume, metakaolin, slag cement, or other supplementary cementitious material.
    • If high early strength is required, consider the use of an accelerator to achieve the high early strength. This will help achieve early strength without greatly exceeding the design strength requirement. Consider the use of silica fume or metakaolin to increase the early age strength. Also, consider the use of maturity- or temperature-matched curing to accurately determine the in-place strength. In placements that get hot, the in-place strength develops more rapidly than that of cylinders cured at lower temperatures.
  2. Limit the maximum temperature in the concrete to 158°F (70°C) by using a reduced-heat concrete mix design (the preferred approach), precooling the concrete, and/or using internal cooling pipes.
  3. Limit the temperature difference to prevent thermal cracking through the use of insulation. Curing methods that artificially cool the concrete should be avoided. The temperature difference must be controlled until the concrete adequately cools to prevent thermal shock. The cooling time depends on the concrete mix and member thickness, and may extend well beyond the normal curing period.
In summary, thermal issues are a concern for HPC placements. If not properly managed during construction, thermal issues can reduce the service life of the concrete. Management of thermal issues can impact construction costs and schedules. These impacts can be minimized or even eliminated with proper planning and the use of an appropriate HPC mix. Consideration of thermal issues is the first step in this process.

More Information
For more information about mass concrete, see PCA Publication EB 547 titled "Mass Concrete for Buildings and Bridges."

HPC Bridge Views, Issue 47, Jan/Feb 2008