Thermal Variation of Mass Concrete Shafts of Different Diameters and Boundary Conditions

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 the generation of heat from hydration of cement and attendant volume change to minimize cracking.” Aside from the qualitative description, there is currently no standardized definition of mass concrete in terms of specific volumes and mix designs. Without a clear definition, state Departments of Transportation (DOTs) vary in their definitions and specifications for mass concrete elements. Georgia Department of Transportation (GDOT) considers mass concrete as any concrete element with a dimension of greater than five feet, or greater than six feet in drilled concrete shafts. Because of this cutoff, mass concrete shafts greater than six feet are avoided by contractors to avert the monitoring aspect and associated higher costs. This avoidance causes additional work on the DOT because they must review changes to the initial design. Because of this additional work, it is important that the specification diameter (i.e., six feet) is truly the dimension at which mass concrete considerations should be made. To determine the conditions that create the mass concrete conditions, an experimental effort was completed. Through field monitoring and laboratory experiments, this study explored the effects of boundary condition (air, soil, water) on the concrete. The research concluded that the boundary conditions of a drilled shaft affect its maximum temperature as well as the temperature distribution. Therefore, it is recommended that these conditions be considered in current mass concrete specifications. Specifically, the consideration of the boundary will better account for drilled shafts that experience more than one boundary condition. Additionally, it is recommended to monitor the temperature gradients closer to the edge of the shaft instead of the currently used linear approximation from the core to the edge region, which does not account for significantly higher local maximum gradients.M.S