Like many bridges built in the 1960s, the original Blackhall Road bridge over Rum Creek in Georgia didn’t meet modern design standards. It was narrow, with a substandard vertical curve, and showing signs of wear. The Georgia Department of Transportation (GDOT) wanted a bigger and better bridge that would meet the community’s needs with minimal disruption to the flow of traffic.
That’s where problems arose. The original plan was to close the road for nine months while a replacement was built. However, that generated a local uproar in the community, causing the project team to rethink their design and reduce the closure time to just two months. They came up with a hybrid precast concrete solution using accelerated bridge construction (ABC) techniques to create the new 195-ft two-lane bridge with 2-ft travel lanes and 8-ft shoulders.
Hybrid Design Solution to Site Constraint Issues
Because the jobsite is tight due to a nearby dam and homes, the design team came up with a plan to use American Association of State Highway and Transportation Officials (AASHTO) precast concrete bulb-tee beams as the base of the bridge, then to cast the bridge deck on top of them at designated staging areas on-site.
“It was the first project in Georgia to use a hybrid concept of using precast concrete beams and precasting the deck in an on-site lay-down area while maintaining the traffic on the existing bridge,” says Dexter Whaley, bridge design ABC liaison for GDOT.
By casting the deck in place at the project site, the common problem of profile management created by differential cambers in precast concrete decked bulb tees was eliminated. False bents that matched the vertical profile and cross slope of the proposed bridge superstructure were fabricated to make construction work.
This method allowed the bridge superstructure to be constructed before closing the road. “It was an innovative use of precast concrete that allowed us to reduce the duration of the road closure and impact to the public from months to weeks,” says Kevin Kahle of CHA Consulting.
Once the roadway was closed, the existing bridge was demolished and the new permanent bridge foundations and substructures were constructed. After the substructure was completed, the deck girders were transported and erected on the permanent bridge substructures using an 800-ton-capacity crane.
Steel diaphragms were used as both temporary bracing for the initial deck pour in the staging yard, then they were reinstalled and the bridge edge beams and end-walls were formed and cast with 24-hour concrete. Wood formwork and foam were used to create block outs for the 9-in. closure pour between the precast concrete decked beam unit.
When the deck was fully cured, the skim pour was chipped out and the foam and wood forms were removed. The skim pour created cross-slope continuity across the deck that would not require additional grinding to achieve a consistent deck surface between each decked beam unit.
In the final phase of construction, 9-in. closure strips were poured using ultra-high-performance concrete (UHPC), and the deck was ground to remove minor undulations. After grooving and completion of the roadway approaches, the road was reopened to traffic on schedule, after a closure of only 60 days.
“The use of precast concrete for this project demonstrated how it is possible to use the standard AASHTO precast concrete beams that were readily available from the Georgia fabricators to develop an accelerated bridge construction solution that allowed the bridge to be completed in under 60 days,” says Jim Aitken of CHA. “This solution did not require any specialty contractors or equipment, and other than the use of a few special details and the ultra-high-performance concrete closure pours, it was constructed like any other bridge in the GDOT inventory with the same durability and maintenance expectations.”