Drilled Shafts as Mass Concrete: Requirements and Methods for Satisfying-PPT Presentation

PPT Presentation

Pdf Summary

The presentation examines how mass-concrete requirements should apply to large-diameter drilled shafts. High concrete temperatures can raise two durability concerns: delayed ettringite formation (DEF), associated with peak temperatures, and thermal cracking, associated with temperature differences within the concrete. Although some agencies apply limits developed for above-ground concrete, these may be overly conservative for shafts and can create substantial constructability and scheduling challenges.<br /><br />The presenter notes that drilled-shaft thermal damage has not been documented, but also that it has not been systematically sought and that experience with large shafts is short relative to their intended service life. Thermal effects therefore warrant consideration, while accounting for the shafts’ reinforcement, confinement, and specific durability needs.<br /><br />Research by the University of Missouri and FHWA supports a rational, project-specific process: define the concrete, shaft, and ground input parameters; predict temperatures using an appropriate model; assess DEF potential; establish allowable temperature differentials and compare them with predictions; mitigate excessive differentials; and measure temperatures when needed.<br /><br />For DEF, the proposed approach permits temperatures up to 185°F when specified cement or supplementary cementitious material measures are used; temperatures above that are not permitted. For thermal cracking, allowable differentials should reflect concrete properties, restraint, reinforcement, cover, and acceptable crack width rather than relying solely on generic limits.<br /><br />Mitigation options include adjusting the mix, limiting cement content, cooling materials before placement, restricting placement temperatures, and post-cooling. These measures can affect workability, pumpability, cost, schedules, and tremie placement. The presentation concludes that thermal requirements should be based on durability and shaft-specific conditions, with constructability considered, rather than applying generic limits without modification.

Keywords

mass concrete

drilled shafts

delayed ettringite formation

thermal cracking

concrete temperature

durability

temperature differential

FHWA

construction mitigation

project-specific design