Mass Concrete Drilled Shafts: Specification Landscape and Recommended Revisions-Paper

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The paper reviews mass-concrete requirements used for drilled shafts across 11 projects and recommends provisions that account for the differences between underground shafts and above-ground concrete. Shaft concrete is surrounded mainly by soil, rock, or water, cannot readily be insulated or cooled with pipes, and must remain workable during demanding tremie placement. Applying conventional mass-concrete rules without adjustment can therefore increase construction risks and costs.

Drilled shafts should not be exempt from mass-concrete provisions, since delayed ettringite formation (DEF) remains a potential durability concern. However, requirements should be tailored to shafts:

- <strong>Maximum temperature:</strong> Follow ACI 201 guidance. A limit of 185°F is appropriate for mixtures with qualifying supplementary cementitious materials (SCMs); otherwise, the limit is 158–160°F.
- <strong>Temperature differential:</strong> Do not impose limits on permanently steel-cased shafts or, generally, on shafts in non-aggressive ground. For uncased shafts in aggressive ground, establish a project-specific limit using crack-control procedures.
- <strong>Monitoring duration:</strong> Do not base monitoring on ambient air temperature. Monitor until at least 24 hours after the center temperature—and, where applicable, the temperature differential—has peaked and is below its allowable limit. Continue monitoring if a limit has been exceeded.
- <strong>Exceedances:</strong> Specifications should clearly state consequences. A differential exceedance should prompt revisions to prevent future exceedances, but should not automatically make the shaft unacceptable. Temperature above 160°F for a non-qualifying mix, or above 185°F for a qualifying mix, generally makes the concrete unacceptable unless the Engineer of Record approves a justified exception.
- <strong>Strength concerns:</strong> Do not use a blanket temperature limit to address possible strength loss. Evidence does not establish 160°F as a reliable threshold. Instead, account for potential strength effects through higher specified strength or targeted trial-batch or match-cured testing.

The authors’ central recommendation is to protect durability while avoiding unnecessary restrictions that conflict with practical drilled-shaft construction.

Keywords

drilled shafts

mass concrete

temperature control

delayed ettringite formation

ACI 201

supplementary cementitious materials

tremie placement

temperature differential

durability provisions

construction specifications