Paul V. Liles, Jr., Georgia Department of Transportation
While attending a national conference on high performance concrete (HPC) in Houston, Texas, in 1996, the Georgia Department of Transportation (GDOT) determined that this new material would have significant applications in Georgia to provide longer spans for prestressed concrete beams for highway bridges. An added benefit would be the use of more efficient beam spacings and the possible use of shallower beams for a given span length.
With this in mind, a research program was initiated at the Georgia Institute of Technology in This research project studied HPC mix designs using Georgia’s granite and granite gneiss crushed stone aggregates and determined that HPC mix designs could be developed using local aggregates. Strengths in the 10,000 to 14,000 psi (70 to 100 MPa) range were easily obtained. In addition, these mixes could be produced without difficulty by local precasting plants. Representative samples of prestressed concrete beams were built and tested. Results showed that the current AASHTO specifications conservatively predicted the transfer and development lengths of 0.6-in. (15.2-mm) diameter prestressing strands.
At this point, design was started on Georgia’s first HPC bridge project — the Jonesboro Road bridge over I-75 located in Henry County, south of Atlanta. The bridge has a span arrangement of 53, 127, 127, and 45 ft (16.25, 38.75, 38.75, and 13.75 m) and is 90 ft (27.4 m) wide carrying five lanes of traffic with bike lanes and shoulders. AASHTO Type IV girders are used for the 127-ft (38.75-m) long spans and AASHTO Type II girders for the shorter spans. Beam spacing is 7.60 ft (2.31 m). The specified concrete strength was 10,000 psi (70 MPa) at 56 days. Maximum specified chloride permeability for the beams was 3000 coulombs at 56 days. The deck concrete was specified to have a compressive strength of 7000 psi (50 MPa) at 56 days and a maximum chloride permeability of 2000 coulombs at 56 days. To show the feasibility of placing a concrete deck using 7000 psi (50 MPa) concrete, a demonstration test slab was required to be placed under field conditions adjacent to the bridge.
The bridge was constructed in two stages to handle traffic during construction. In the first stage deck placement, the maximum chloride permeability of 2000 coulombs was exceeded. For the second stage deck placement, Class F fly ash and more silica fume were included in the concrete. Otherwise, the project was very successful and all the HPC goals set out in the program were met.
HPC was critical to the design of the project. The use of 127-ft (38.75-m) long AASHTO Type IV beams minimized the overall depth of the superstructure and avoided the problem of raising the grade with the subsequent need for expensive land purchases. With construction of this bridge, designs of precast, prestressed girders using HPC compressive strengths up to 10,000 psi (70 MPa) were approved by GDOT. The future is bright for the use of HPC in Georgia. We should continue to see increases in concrete strengths with further optimization in the future.