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This paper summarizes the planning and execution stages of a critical mass concrete placement performed during summer months. The subject structure was a critical component of a large heavy industrial facility, consisting of large load bearing elevated flexural members. The planning and execution of this critical mass placement consisted of multiple tasks.

A laboratory study was performed for the purpose of making improvements to the mixture proportions existing and currently in use, admixture dosages and investigating placement temperature options. Adiabatic and semi adiabatic temperature rise was also measured during the laboratory study along with set times. Final proportions and admixture dosages were selected as a result of the laboratory phase. Primary outcome was increase in fly ash percentage from the existing mix design to control heat of hydration.

Based on the findings of the measured adiabatic temperature rise, a thermal control plan was developed adapting the new approach to structural mass concrete placements. A thermal protection/insulation regimen was developed using the mix parameters, expected ambient temperatures following placement, member dimensions and formwork/blanket insulation properties. The pre-placement modifications to the mixture proportions and the delivery temperature requirements protected the concrete against high internal temperatures and potential of Delayed Ettringite Formation (DEF), while the insulation regimen protected the concrete against rapid cooling and occurrence of thermal gradients between core and perimeter.

As part of the thermal control plan analysis, target placement temperatures were recommended to control maximum temperatures to prevent occurrence of DEF, in light of the heat rise of the modified mix. The placement temperature was accomplished by starting the placement at night and the use of ice to draw the temperature down. Upon completion of finishing, a curing compound was applied in lieu of water curing and the placement was insulated.

The thermal control plan simulation predicted a gradual reduction in the temperature of the placement, within limits of maximum internal temperatures and temperature gradients. The actual placement was monitored for core and perimeter temperatures using maturity probes. Monitoring enabled the team to react to abrupt changes in temperature if any was to occur. The placement was completed successfully with internal temperatures and gradients controlled within the desired ranges.

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.

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.

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.

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.