FALL 2018

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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Page 41 of 51

F H WA 40 | ASPIRE Fall 2018 e Federal Highway Administration's (FHWA's) National Geotechnical Team has been working with industry partners to better under- stand factors contributing to the performance of concrete when used in underground mass placements such as drilled shafts. Innovations in drilled-shaft construction equipment over the last two decades, combined with increased load- carrying demands for foundation elements, have resulted in significantly larger and deeper shaft excavations that have tested the ability of con- ventional concrete mixtures to meet necessary performance requirements. e construction conditions for drilled-shaft foundation projects are more complex than those for other reinforced concrete structures. erefore, the concrete mixtures used for shaft excavations must be designed to address specific application requirements. e most important requirements are those that contribute to the workability of fresh con- crete during transport and placement operations. Concrete may be transported long distances to remote sites and pumped long distances. It may be required to flow readily through a tremie and through congested reinforcement under slurry. Additionally, concrete may have to remain work- able for 4 to 8 hours, or longer, in wide-ranging ambient temperature conditions. Fresh concrete placed in drilled-shaft opera- tions must also consolidate under its own weight without the assistance of vibration, and it must re- main stable without segregation, excessive bleed- ing, or excessive heat of hydration. ese require- ments for workability are now relatively common in drilled-shaft construction, and balancing them has become a significant challenge to engineers. To address some of these issues, FHWA is cur- rently leading a research effort to study factors that contribute to the performance of concrete placed for geotechnical applications, and spe- cifically drilled-shaft foundations. e primary objective of the research is to develop perfor- mance standards that correlate to basic demands for drilled shafts and allow for better concrete mixture designs. FHWA Guidance for Drilled-Shaft Concrete e basic demands for concrete in drilled- shaft applications are currently summarized in FHWA's Geotechnical Engineering Circular No. 10 as follows: 1 • Workability: e concrete must have the ability to flow readily and fill the shaft exca- vation completely. e concrete must read- ily pass through the reinforcement without blocking to achieve thorough contact with the surrounding soil or rock. e concrete must be self-leveling within the excavation and consolidate under self weight; vibration of concrete in a borehole is not possible or practical. • Workability retention: With underwater tremie placement, or when the casing must be withdrawn after completion of concrete placement, the drilled-shaft concrete must retain workability and have controlled set- ting times suitable for completion of place- ment operations. • Stability: While providing the high degree of workability required, the fresh concrete must still have robust stability and resist any ten- dency to segregate or bleed. e paste within the concrete should have a high degree of cohesion so that coarse aggregate particles are evenly distributed, and water within the mixture should remain distributed without a tendency to bleed and result in nonuniform properties or bleed-water channels. • Durability: e concrete cover on the rein- forcement must provide low permeability so as to minimize the potential for corrosion of the reinforcement. If the subsurface environ- ment is aggressive or may become aggressive during the life of the foundation, the concrete should be designed to have high density and Addressing Concrete Issues for Drilled-Shaft Foundations by Silas C. Nichols, Federal Highway Administration Tremie being used for concrete placement for a drilled shaft. All Photos: Federal Highway Administration.

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