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This paper summarizes the planning and execution stages for a large mass concrete placement. The subject structure was a component of an industrial facility, consisting of a large mat foundation on grade. The planning and execution of this critical mass placement consisted of multiple phases:

For the first phase of the work, a laboratory study was performed for the purpose of developing a concrete mixture that will perform satisfactorily meeting the mass concrete objectives. The laboratory development phase consisted of conventional strength based design of a mix meeting additional specification requirements for control of heat of hydration. The project specifications required the use of 35% ash and imposed a cap on total cementitious materials content. The selected proportions were then batched and placed in a mock-up consisting of a 3 ft (91 cm) by 3 ft (91 cm) by 3 ft (91 cm) cube for the purposes of observing peak temperatures exhibited by the mixture and the temperature differentials. This member size was selected for the mock-up as it is typically the delineating minimum member dimension for mass concrete.

As part of the next phase of the preparatory work, a thermal simulation of the actual placement was performed using public domain software. Based on a review of the findings from these efforts, the specifics of the thermal control plan for the actual placement were finalized. As part of the thermal control plan analysis, target placement temperatures were recommended to control maximum temperatures that prevent occurrence of Delayed Ettringite Formation (DEF), in light of the heat rise of the mix.

The actual placement of nearly 760 m³ (1000 CY) was performed in early fall weather conditions over 9 hours. Concrete was chilled to meet the delivery temperatures and insulated per the thermal control plan specifics. The placement temperature was accomplished by starting the placement at night and with the use of chilled water, ice, and liquid nitrogen (as needed) to lower the placement temperature during the day. Upon completion, the placement was insulated using three different insulation regimes. The resulting concrete temperatures were monitored and enabled observation of differences between each insulation regime. This phase also served as a confirmatory phase of the specific insulation attributes indicated by the thermal analysis in the previous phase. The placement was completed successfully with internal temperatures and gradients controlled within the desired ranges.

The specific selections in mixture proportions and the delivery temperature requirements protected the concrete against high internal temperatures and potential of DEF, while the insulation regimen protected the concrete against rapid cooling of the surface and occurrence of thermal gradients between core and perimeter. The multiple insulating regimens implemented during actual placement were instrumental in confirming the effects of insulation on peak temperature and loss of heat, as indicated by the analytical simulation.

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.