Best Practices and Specifications for Massive Concrete Drilled Shafts
Sponsor: Georgia Department of Transportation
Project Information
PI: Lauren Stewart, Co-PI: Larry Kahn
Students: Diwakar Singh
Dates: September 25, 2018 - July 21, 2021
Funding: $258,000
Summary
American Concrete Institute (ACI) Guide to Mass Concrete defines massive (mass) concrete 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 the cement and attendant volume change to minimize cracking (ACI, 2006).” Specifically, in the case of drilled shafts, Georgia Department of Transportation defines mass concrete to be any shaft with diameter greater than six feet (GDOT, 2013). The choice to consider an element as mass concrete is meant to prevent damage from an excessive maximum temperature, and to minimize thermal cracking, which occurs because of an excessive temperature difference within the placement (Gajda, 2016). Once a drilled shaft has been classified as mass concrete, additional general GDOT specifications for mass concrete apply. These specifications detail temperature, material selection, thermal control, placing, and curing (GDOT, 2013). An example of a massive concrete drilled shaft is given in Figure 1. Because the current specification lacks details specific to drilled shafts, a multitude of methods can be used, which can increase the cost of the of the structure because contractors are unsure how to satisfy the mass concrete requirement and put the additional risk into the bid. Additionally, because the specifications are vague, the contractor and designer often avoid the specification by installing multiple smaller (less than or equal to six-foot diameter) shafts in place of one single larger shaft. This practice of adding multiple smaller shafts often adds additional cost and the possibility for less effective designs.
This project seeks to develop best practices and specifications for mass concrete drilled shafts to reduce the hesitance for use in practice and promote better overall construction practice. Specifically, this project will: (1) understand the major deterrents from utilizing mass concrete for drilled shafts in practice, (2) determine and specify the appropriate thermal conditions, monitoring, and other requirements for drilled shafts specific to Georgia, (3) utilize ongoing GDOT research for temperature control to determine best practices for application on drilled shafts, and (4) make recommendations to GDOT for specifications to advance best practices.