Use of Self-Consolidating Concrete in Precast, Prestressed Girders
Keske, Samuel D.
Type of Degreedissertation
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Self-consolidating concrete (SCC) is a high-performance concrete in the fresh state—because of its highly fluid fresh behavior, it requires no mechanical consolidation during placement. Prior to statewide acceptance of SCC in precast, prestressed bridge member production, the Alabama Department of Transportation (ALDOT) sponsored an investigation of the material to be performed by the Auburn University Highway Research Center. The primary objective of this dissertation is to advance the understanding of the use of SCC in precast, prestressed bridge girders through the synthesis of multiple aspects of that investigation. To this end, material and structural properties were evaluated in a laboratory setting and in full-scale girders in an in-service bridge. In both settings, SCC was evaluated relative to vibrated concrete (VC) and, as importantly, considering existing design standards and construction practices. The laboratory investigation focused on quantification of SCC stability, a unique property of the material that is important to assess during construction. Five fresh concrete stability tests were conducted on twenty SCC mixtures each placed in walls of heights equaling 54, 72, and 94 inches. Fresh test results were then compared to the results of hardened uniformity testing conducted on the concrete walls. Analyses indicate that acceptable mechanical properties can be achieved in a range of mixtures and that some SCC fresh stability tests correlate well with hardened concrete uniformity. Suitable fresh SCC tests and acceptance criteria are recommended, as is a testing protocol for use during implementation of SCC in the production of precast, prestressed elements. The full-scale implementation of precast, prestressed SCC girders consisted of seven BT-54 bulb-tees and seven BT-72 bulb-tees placed in a bridge in rural Alabama. Companion girders were constructed with vibrated concrete. Fresh concrete properties and early-age structural properties were assessed at the plant; measurement of mechanical properties, time-dependent deformations, and elastic responses to applied loads continued until all girders were approximately 1,000 days old (in service for one year). SCC girders exhibited transfer lengths that were approximately 20% longer than those of VC girders, but the difference appeared to be due to differences in elastic material stiffness at transfer. Average transfer lengths in both materials were approximately half as long as predicted using current design provisions. SCC appeared to exhibit a lesser stiffness (5–15% less relative to the square root of its strength) and greater time-dependent deformability (approximately 5–10% greater creep and 30% greater shrinkage) than VC in representative cylinders, but time-dependent prestress maintenance and elastic responses to construction and service loads were practically identical in the SCC and VC girders. Furthermore, full-scale SCC structural behavior was no less predictable than that of VC according to typical AASHTO LRFD methods. All measured behaviors were accurately or conservatively predicted, and the use of design material properties in place of measured values led to distinct under-prediction of structural performance. Based on the results of this laboratory and full-scale testing, it is concluded that SCC is an acceptable alternative to vibrated concrete in the construction of precast, prestressed bridge girders using current design and production procedures.
- S Keske PhD Dissertation.pdf