Richard A. Miller, University of Cincinnati
Adjacent box girder bridges are frequently used in Ohio and other eastern states. These bridges have a favorable span-to-depth ratio—an important benefit when vertical clearance is a design consideration. Furthermore, with noncomposite sections the bridge can be constructed quickly because there is no need to form, cast, and cure a separate deck. With HPC adjacent box girders, savings can also be realized by using longer spans and eliminating piers.
The Ohio HPC Showcase bridge is located on U.S. 22, near Cambridge, Ohio. The existing structure was a 70-ft (21.3-m) long steel stringer bridge over a river. The Ohio Department of Transportation (ODOT) decided to widen the channel at this point and to provide sloping sides, rather than the existing vertical sides. The original replacement structure was designed as a three-span bridge using 21-in. (535-mm) deep, simply-supported boxes.
To save the cost of constructing the piers and to provide better flow characteristics by having an unobstructed channel, the bridge was redesigned as a single span. Thus, the new structure needed to span 116 ft (35.4 m). The largest box girder available is an ODOT B42-48, which is 42 in. (1.07 m) deep and 48 in. (1.22 m) wide. ODOT box girders have a 5-in. (125-mm) thick bottom flange rather than the 5.5 in. (140 mm) flange used in standard AASHTO sections. The ODOT section can only accommodate a single full row of 23 strands and several partial rows of 2 or 4 strands. With all possible strand positions used, this section can only span 105 ft (32 m) with conventional strength concrete and 0.5-in. (12.7-mm) diameter strands. With 0.6-in. (15.2-mm) diameter strands, the maximum span can be increased to 135 ft (41.1 m). Because the 0.6 in. (15.2 mm) strand provides a much higher prestressing force, high strength, high performance concrete is needed to resist the higher compressive stresses.
High performance concrete was also used in the girders because of its lower permeability. This provides better corrosion protection to the prestressed and nonprestressed reinforcement. This is
particularly important with adjacent box girders as salt-laden water tends to penetrate into any longitudinal cracks that may form between adjacent girders.
One of the first steps in the design was to check the probable transportation route for weight and size restrictions and to ensure that the beams could be safely transported to the site. Next, mix designs were developed for both the beams and abutment concretes. Once the beam mix design was selected, two heavily instrumented test beams were cast. These beams provided the fabricator with production experience for the concrete mix. The two test beams were then subjected to cyclic loading and tested to destruction. This testing showed that the beam behavior was predictable using standard analysis techniques and met the provisions of the AASHTO Standard Specifications. With satisfactory results from the test beams, the production beams were cast.
Just a few weeks before site construction was due to begin, the aggregates for the abutment HPC were found to contain too much carbon. The abutment mix had to be redesigned in a very short time. After casting, differential shrinkage cracking occurred in some areas of the abutments because the wet burlap was not maintained in contact with the concrete surface. In later pours, the concrete surface was maintained completely wet and the cracking was greatly reduced.
The bridge was constructed in two phases to maintain traffic. In Phase I, one half of the old bridge was removed and seven HPC beams installed for the first half of the new bridge. Traffic was then diverted to the new half of the bridge while the remaining five beams of Phase II were installed. After both phases, the bridge was load tested by parking up to four dump trucks, each weighing 32 kip (142 kN), in various patterns on the deck. In both phases, the maximum deflection under the worst case static loading was only 0.5 in. (13 mm). Subsequent measurements of deflection under normal, moving traffic loads showed a maximum deflection of 0.25 in. (6 mm). There was concern that a bridge this long and slender would vibrate excessively, but the measured vibrations were not excessive and they damped out quickly.
This HPC structure is expected to provide a long service life, with minimal maintenance.
Further Information
Further information about the U.S. 22 HPC bridge may be obtained by contacting the author at 513-556-3744 or [email protected].