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To minimize thermal cracking, specifications for mass concrete often state a maximum allowable temperature difference, ΔT, between the hottest interior location (usually the center) and surface of the mass concrete section in the days following placement. Section 8 of ACI 301-05, “Specifications for Structural Concrete,” did not set such a limit, but the recent revision, ACI 301-10, sets a default value of 35F for the difference (see box). The 35F difference is based on experience with unreinforced mass concrete dams where the consequences of cracking and subsequent water leakage were critical. For mass concrete in mat foundations, large piers, and thick walls, 35F may be unduly conservative.

Mass concrete mixtures used in transportation-related construction often have large percentages of portland cement replaced by supplementary cementitious materials (SCMs), including slag cement, fly ash, or both. The principle benefit for using SCMs in mass concrete is to create a concrete mixture, which has a low temperature rise. The development of the maturity concept focused primarily on the study of concrete without SCMs. Many of the concrete mixtures being utilized today incorporate considerable amounts of SCMs. This paper investigates the relationship of equivalent age and physical properties of different mass concrete mixtures containing portland cement and SCMs.

ACI defines mass concrete as:

Any volume of concrete in which a combination of dimensions of the member being cast, the boundary conditions, the characteristics of the concrete mixture, and the ambient conditions can lead to undesirable thermal stresses, cracking, deleterious chemical reactions, or reduction in the long-term strength as a result of elevated concrete temperature due to heat from hydration.

While this definition provides an excellent description of the characteristics of concrete to consider for the purposes of defining mass concrete, it does not provide clear and uncontestable requirements for determining whether a particular placement must be treated as mass concrete. The purpose of this paper is to better define what placements should be treated as mass concrete and to provide the reasoning behind the definition. This paper serves as a guide to provide specification writers, owners, engineers, and contractors a way to better identify the need to treat (or not treat) a particular concrete placement as mass concrete.

The exothermic reaction of the heat of hydration in concrete can lead to problematic temperature differences between the surface and the core of mass concrete elements, which can lead to thermal cracking. This problem has led many engineers to create maximum temperature differential specifications, as well as maximum temperature specifications in response to concerns over producing conditions which may lead to delayed ettringite formation (DEF). In general, there are two solutions to meet this specification: design a mix that has low or an extended heat of hydration or cool the mass element internally as it cures. Regardless of the method, many engineers require that the mass elements’ temperatures be predicted for the mix design, dimensions of placement, day of placement, placing temperature, and construction methods including the use of insulation. Therefore, mass concrete mix designs are tested experimentally for heat of hydration and thermal properties, and those values are used in a mathematical model. The following is a description of using Isothermal calorimetry to generate information about a mix design, which was used to input into the thermal modeling.

As a part of the 15 m [49 ft] raise of Hinze Dam, the existing 33 m [108 ft] high mass concrete spillway structure was raised an additional 12.5 m [41 ft] by using conventional mass concrete placed on the top and downstream side of the existing spillway to form a new monolithic structure. Heat generated by the hydration of the cement and fly ash would raise the peak temperature in the body of the new concrete relative to the stable and relatively uniform temperature within the existing concrete, resulting in a potential for tensile strains to develop along the interface that are large enough to cause cracking through the body of the composite dam and potentially compromise the interface bond. Two-dimensional transient coupled thermal-structural finite element (FE) analyses were used to predict thermal deformations and stresses within the body of the spillway in the weeks and months following placement. These analyses formed part of the basis for establishing pre-cooling placement requirements for the mass concrete. The concrete mix was designed to greatly minimize the evolution of heat by using a higher than usual percentage of fly ash. Laboratory measured mechanical and thermal properties of the concrete and local boundary climatic data were input to the analyses. This paper presents the assumptions, methods, and criteria used in the finite element method (FEM) analyses; the results of the mix selection process and laboratory thermal testing program; and the results and conclusions drawn from the analyses. A discussion on the concrete mix design trials recently completed on site is also included.