Scc Matches Conventional Mixes’ Engineering Values

Bridge members cast with self-consolidating concrete can be designed using the same methods, assumptions, and limiting values appropriate for conventional

Bridge members cast with self-consolidating concrete can be designed using the same methods, assumptions, and limiting values appropriate for conventional concrete members. That’s the conclusion of a report, Bulb-T Beams with Self-Consolidating Concrete on Route 33 in Virginia, Virginia Transportation Research Council’s H. Celik Ozyildirim and Rodney Davis presented eariler this year to the Transportation Research Board, Washington, D.C.

The Virginia Department of Transportation routinely uses high performance concrete (HPC), which is defined as any concrete providing enhanced performance characteristics for a given application. Incorporating HPC with high strength-above 6,000 psi-could result in more economical structures by allowing increased prestress force and section shear capacity. Improved economy is realized though increased span lengths, a reduction in the number of beam lines, and lower substructure, transportation , and erection costs, Ozyildirim and Davis contend.

Virginia DOT has built bridges containing standard AASHTO beams, the authors report. However, these beams are not efficient for long spans or wide beam spacing. Therefore, engineers have introduced prestressed bulb-T beams with large bottom flanges that include more prestressing strands per foot of bridge width. As compared to standard AASHTO beams, bulb-T beams generally are more efficient for longer spans using high-strength concretes (exceeding 6,000 psi). The bulb-T beams have the potential to increase prestressed concrete girder span limits to 175 feet and provide a competitive alternate design to steel-plate girders in the 120- to 175-ft. span range.

Standard high-strength concrete mixtures have a low w/cm and, therefore, require high range water-reducing admixtures. Slump loss problems and stiff concretes can lead to consolidation problems, considering the placement of many strands and conventional reinforcement in tight areas.

By contrast, self-consolidating concrete (SCC) mixes provide a high degree of workability, the authors emphasize. Such mixes easily fill congested spaces between reinforcement (both mild reinforcement and prestressing steel) and in formwork, under the influence of their own mass without applying additional consolidation energy. Easy-flowing SCC permits convenient and fast placement of concrete in beams, they say. Eliminating the consolidation problem enhances strength and reduces permeability of concretes, which is essential for longevity.

According to Ozyildirim and Davis, SCC has been used advantageously in Japan and Europe since the early 1990s. Its benefits include less labor and increased construction speed, improved mechanical and durability characteristics , flowability for heavily reinforced and congested areas, consolidation without vibration, and reduced noise-level at plants and construction sites. However, there are some concerns with its use, the researchers add, including segregation, poor air-void system, shrinkage, and reduced bond between strands and concrete.

The purpose of the study was to evaluate overall SCC performance in bulb-T beams constructed for a bridge on Route 33 over the Pamunkey River in Virginia. Field evaluations included fabrication and placement of SCC and conventional concrete components, instrumentation of bridge beams with strain gauges and thermocouples, testing of specimens cast during placement, and measurement of strain and camber over time.

Prior to fabrication of the bridge beams, two full-scale test specimens were prepared and tested for transfer and development length, as well as shear and flexural strength. Fresh and hardened SCC properties were also determined.

[The beams] were tested for transfer length, development length, flexural strength, and shear strength, note Ozyildirim and Davis. The test beams demonstrated that SCC members can be designed using the same methods, assumptions, and limiting values applied to normally consolidated concrete beams. Based on the positive results, beams with SCC were cast and placed in the bridge.

The authors conclude:

  • SCC can be prepared yielding adequate slump flow without segregation and with satisfactory strength and permeability.
  • SCC is highly sensitive to water content.
  • Low slump flow does not lead to self-consolidation, and concrete intended to be SCC may require mechanical vibration. Slump loss over time and a short height of concrete discharge, as in the top flange of the beam, may necessitate minimal consolidation.
  • High slump flow may lead to segregation.
  • Low-air content concrete containing polyc arboxylate-based HRWRA may not demonstrate proper resistance to freezing and thawing when tested by the severe procedure A of ASTM C 666. Satisfactory freeze-thaw resistance is critical in saturated elements, since water expansion during freezing generates high stresses. Nonetheless, satisfactory field performance of concrete investigated in the study is expected due to the lack of critical saturation.
  • Strand slip was minimal, indicating satisfactory bond between concrete and strand.