Mark Holloran, Tennessee Department of Transportation
Bridges in Tennessee are most often constructed with precast, prestressed concrete beams and cast-in-place reinforced concrete decks. The majority of these bridges are jointless with integral abutments. The dominance of this type of construction can be attributed to both lower initial costs and lower long-term maintenance costs compared to other bridge systems.
With improved durability and strength characteristics, high performance concrete (HPC) can provide initial and long-term benefits with a reduced number of beams or piers, shallower superstructures, and superior resistance to chloride ion penetration.
HPC Bridge Projects
Two HPC bridge projects are being built in Tennessee to promote this new technology. The first project, which began in the Spring of 1998, involves the reconstruction of the bridge carrying SR 1 over the CSX Railroad in Nashville. This 660-ft (201-m) long bridge is composed of six spans of 63-in. (1.60-m) deep bulb-tee beams with a maximum span of 115 ft 2 in. (35.1 m). The bridge was constructed in stages to maintain traffic flow. The final placement of the bridge deck concrete was completed in January 2000, and the bridge is expected to be fully open to traffic this summer. For this bridge, preliminary designs indicated that using HPC precast, prestressed concrete beams allowed a reduction in the number of beam lines from eleven to nine with no increase in deck slab thickness.
The second project is on a section of SR 840 in Dickson County and began in the Spring of 1999. It includes the construction of two HPC bridges. Both bridges are two-span continuous structures using 72-in. (1.83-m) deep bulb-tee beams. The maximum length from pier to abutment is 159 ft (48.5 m). The bridge deck for the first structure was completed in January 2000, and the second should be completed in Summer 2000. For this project, HPC made this structural type more economical compared to other alternatives.
Generally, the strength and permeability requirements for both projects have been achieved to date. Some finishing problems occurred with the deck due to the stickiness of the fresh concrete. This was attributed to the silica fume used in the HPC mix. As the contractors gained experience, they obtained better finishing results.
For the SR 840 project, TDOT and the FHWA are providing funding for research conducted by Vanderbilt University. Specific research objectives include: characterization of the HPC produced in Tennessee through a materials testing program; instrumentation of the HPC bridges (beams and deck) to allow evaluation of the short-term and long-term performance; and comparison of calculated prestress losses, girder cambers, and deflections to fieldmeasured values. The goal of the latter objective is to evaluate current design practices and to develop modifications for use with HPC, if necessary. The final report for this research is scheduled for completion by the end of this year.
High Performance Partnering
The partnership that developed between the parties involved in these projects is an essential success factor in Tennessee’s HPC construction. In addition to the many individual meetings between particular groups, several meetings were held where all parties attended to discuss ideas and share information. One meeting even included the contractors from both HPC projects.
The following are comments by some of the individuals involved:
Tom Everett, FHWA, Tennessee Office
The FHWA is committed to finding ways to build longer lasting bridges that are economical to construct and maintain. High performance concrete is one material that helps us achieve this goal. Through the Tennessee HPC project, we are not only gaining experience with material properties and construction issues, but also developing a partnership between the FHWA, DOT, industry, and academia that will prove invaluable as the use of HPC grows.
Shay Deason, Project Supervisor, TDOT,
Construction
The experience gained with the HPC on the SR 840 project shows that getting a smooth finish on the bridge deck is dependent on time and moisture conditions. But the improved compressive strengths and reduced permeability outweigh the finishing problems on the project. With more experience and research, the finishing problems can be resolved.
Damon Hogan, Project Supervisor, Bell
Construction, Inc.
The workability of HPC was like any other concrete, but the finishing of HPC proved difficult due to the stickiness of the mix. A pressure fogger and a supply of water had to be maintained during concrete placement. An additional work bridge was necessary to allow early placement of the burlap. With a little more experience with HPC, a good finish can be achieved.
Further Information
For further information, the author may be contacted at [email protected] or 615-741-2416.