M. Myint Lwin, Federal Highway Administration
Photos: Virginia Department of Transportation.
Self-consolidating concrete (SCC) has generated worldwide interest in research and applications in precast and cast-in-place highway construction. SCC has many engineering, architectural, economic, and environmental benefits. Eliminating the need for concrete vibration reduces the noise level in the plants and at the construction sites, resulting in improved environmental and working conditions. SCC cuts down on the labor needed and wear and tear on equipment and formwork because internal and external vibrators are not used. This, in turn, accelerates concrete placement and results in cost savings, less traffic disruption, and safer work zones. SCC has the ability to flow into and completely fill intricate and complex formwork and to pass through and bond to congested reinforcement without segregation. The formed surfaces of SCC take on the textures of the formwork with little or no defects to repair. SCC contributes to the construction of “Green Bridges.”
SCC facilitates the use of creative ways to prefabricate precast, prestressed concrete elements to achieve high production, while increasing plant safety and workers’ health and job satisfaction. Prefabricated SCC elements of different shapes and sizes have been produced economically and successfully in plants and then brought to the job site for safe and speedy construction.
Cast-in-place SCC was used in the construction of the inclined pylons of a cable-supported pedestrian bridge in Virginia. Because of the congestion caused by the heavy reinforcement and the cable anchorages, it would have been very difficult to obtain quality concrete in the inclined pylons with conventional concrete.
FHWA’s Role
The Federal Highway Administration (FHWA) continues to work with the states, researchers, concrete industry, admixture suppliers, and other partners in advancing SCC technology through research, development, deployment, and construction projects. Many states are using SCC in beams and girders, bridge piers and pile caps, columns, walls, and drilled shafts. SCC is also used in repair of concrete members with limited access for conventional placement and vibration techniques.
The FHWA offers a one-day SCC workshop to the state departments of transportation (DOTs). The workshop is generally held at the request of a state and at a state-furnished facility. The states are encouraged to have projects ready to use the technology before requesting workshops. The FHWA finds that it is most cost effective to use the information and lessons from a workshop right away. For example, the Nevada DOT requested a workshop on SCC for an awarded project, which specified SCC for drilled shafts. The Nevada DOT involved the state laboratory technicians, construction quality control and quality assurance personnel, and the contractor’s crew in the workshop. The SCC workshop laid the groundwork for successful application for SCC in drilled shafts. For more information on the SCC workshop program, contact [email protected] or [email protected].
The Innovative Bridge Research and Deployment (IBRD) Program is the successor to the Innovative Bridge Research and Construction (IBRC) Program. The main objectives of the IBRD program are to promote, demonstrate, evaluate, and document the application of innovative designs, materials, and construction methods in the construction, repair, and rehabilitation of bridges and other highway structures. Under the IBRC and IBRD programs, over 20 SCC projects have been awarded and constructed. The projects cover a wide range of applications, including beams and girders, bridge piers and pile caps, columns, abutment walls, retaining walls, drilled shafts, traffic barriers, bridge rails, bridge repairs, and prefabricated elements and systems. Many states, including Florida, Illinois, New Jersey, Nevada, Ohio, and Virginia, have developed SCC construction specifications that allow SCC as an option to the contractors.
Research
States, in collaboration with the universities, have conducted studies to determine the feasibility and practicality of using locally available materials for developing SCC mixes for construction of highway projects. The studies provide guidance for the design and testing of SCC mixes, prediction of time-dependent properties, and constructability considerations.
A National Cooperative Highway Research Program (NCHRP), Project 18-12 Self-Consolidating Concrete for Precast, Prestressed Concrete Bridge Elements is scheduled for completion in 2008. The main objectives of this research are to develop guidelines for the use of self-consolidating concrete in precast, prestressed concrete bridge elements and to recommend relevant changes to the AASHTO LRFD Bridge Design and Construction Specifications.
Drilled Shafts
Researchers at Auburn University, AL, investigated the use of SCC in drilled shafts in 2003 and put SCC into practice in 2005 with the construction of drilled shafts for the U.S. 76/SC 9 bridge replacement over Lumber River in South Carolina. The project started out with the experimental use of SCC in one of the drilled shafts under the IBRC program. The experiment was so successful that the South Carolina Department of Transportation decided to use SCC for production of the five remaining drilled shafts.
The high flowability through congested reinforcement, reduced amount of bleed water, more resistance to segregation, and extended workability (6 to 10 hours) combine to make SCC ideally suitable for drilled shaft construction. SCC can overcome some of the problems encountered in the past in using high-slump conventional concrete.
Under an IBRC grant, the Hawaii DOT is conducting a pre-construction study on the use of SCC in drilled shafts for the replacement of the North Kahana Bridge. The study is targeted to develop SCC specifications and mix design guidelines for using locally available aggregates in Hawaii. The Alabama DOT is conducting a similar study on the use of SCC in drilled shaft construction.
More recently, the contractor for the I-35W replacement bridge in Minneapolis, MN, selected SCC for the drilled shaft construction. Test shafts were constructed to gain valuable lessons in planning, designing, and placing the SCC in over 80 drilled shafts with 7 and 8 ft ( 2.1 and 2.4 m) diameters. In addition, SCC drilled shafts have been constructed in New Jersey and Virginia.
Recommendations
SCC has many technical, economical, and environmental advantages. It can help solve some difficult and costly field problems and should be given consideration when the concrete work involves:
- Intricate and complex formwork;
- Congested steel reinforcement;
- Architectural features;
- Precast elements;
- Need for quality, speed, and high productivity; and
- Limited access for proper vibration.
Overall, SCC demands more diligent planning, engineering, and execution than conventionally vibrated concrete. The beginning users of SCC will benefit greatly from the construction of mockups prior to the production of the final members. Mockups are cost effective ways for owners and contractors to gain experience and assure quality and speed in the final production work.
Conclusion
SCC has high potential for wider applications and larger payoffs. By requiring little or no vibration, its use results in less noise, less labor, faster construction, healthier working conditions, improved concrete quality, and better durability.
Further Information
For further information, contact the author at [email protected].