United States Department of Transportation
i
Peak Temperature Determination of Drilled Shafts Excluded From Mass Concrete Consideration in Current Specifications
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2023-04-01
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[PDF-34.52 MB]
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Edition:Final Report 01/21 – 01/23
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Abstract:Drilled shafts are reinforced concrete deep foundation elements that typically range in diameter from 3 to 15 feet. Within the past 20 years, drilled shaft installation plans for FDOT projects have gone from requiring no mass concrete information (regardless of shaft diameter) to requiring steps to control temperature for shafts greater than 6 feet in diameter. However, the most recent specifications were in conflict, where all other concrete elements were required to assess temperature for any element with a minimum dimension greater than 3 feet and volume to surface area ratio greater than 1 foot. Shafts supporting miscellaneous (non-bridge) structures until recently required no temperature control regardless of dimensions. While the term mass concrete stems from massive structures that traditionally generated unsafe temperature levels, today concrete mix designs use far more cementitious materials per unit volume. Hence, unsafe temperature levels can occur with nearly any size foundation element if the cementitious materials content is too high. Recently, the American Concrete Institute (ACI) suggested restrictions on peak and differential temperature limits based on a concrete element minimum dimension and the weight of cementitious materials per unit volume. Using the ACI criteria, a typical FDOT drilled shaft with the minimum specified 600 lbs/yd3 of cementitious materials would be restricted to a size no larger than 2 feet in diameter; the minimum FDOT shaft diameter is 3.5 feet. Hence, the ACI criteria, if applied to FDOT projects, requires all shafts to provide a temperature control plan. The disconnect between FDOT shafts and the ACI criteria is two-fold: (1) the curing conditions of underground concrete is not the same as above ground formed and poured elements, and (2) FDOT peak temperature limits are higher than ACI limits. This study did not aim to address which of the two temperature limits is most correct, but rather focused on determining the developed peak and differential temperature in drilled shafts with varied concrete mix designs and from shafts of different diameters. Shaft temperature information was obtained from hundreds of shafts routinely tested using thermal integrity methods and from shafts more thoroughly instrumented to determine the cross-shaft temperature distribution. Results of field data were then used to calibrate numerical models where the internal temperature rise, magnitude, and distribution was verified. Model runs were used to produce predictive methods to better assess when a given shaft size and mix design might be unsafe. However, the threshold of safety is left to the reviewer when using a given acceptance criteria (FDOT, ACI, or other).
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