By: Scott Reeve
Eight Mile Road Bridge, a short-span concrete structure built in 1940, needed to be replaced in Hamilton County, near Cincinnati, Ohio. In addition to replacing the bridge deck slab, the abutments required rehabilitation to extend the life of the structure. In 2000 and 2001, Hamilton County engineers chose to rehabilitate three bridges on Five Mile Road (Bridges B-0071, B-0087 and B-0171) with fiber-reinforced polymer (FRP) bridge decks on precast, pre-stressed concrete beams. An inspection conducted in July 2008 revealed only minor surface cracking in the decks. The decks were cleaned and sealed. Testing confirmed deck deflection was unaffected and corrosion nonexistent.
For Hamilton County engineers, the positive performance of the Five Mile Road composite bridge decks is building a track record for the technology’s capability to reduce maintenance and extend service life through the noncorrosive nature of the material. Innovative Bridge Research and Construction (IBRC) program funding supported the county’s request to use composite technology for Eight Mile Road bridge, but county engineers had a more aggressive goal for the aging bridge.
The county’s bridge department wanted to eliminate the horizontal construction joints used in the previous composite deck designs. And, instead of a product that just provided a composite deck on reinforced concrete beams, engineers were looking for a total bridge superstructure application. Their search led them to Composite Advantage’s (CA) integrated drop-in-place FRP bridge system. Unlike conventional FRP decks on steel beams, CA’s drop-in-place system is prefabricated with integral beams and deck and incorporates the entire superstructure.
“Eight Mile Road was the first site in the United States to receive this drop-in-place composite bridge system,” said Steve Mary, engineer for Hamilton County. “Typically, failure or structural issues occur where materials are joined together. This system eliminates the connection joints between deck and beams. Our office has always had a special interest in using innovative materials and products because our objective is to continually improve our infrastructure and extend service life. This drop-in-place composite bridge system was a logical next step.”
Quick drop off
Bridge parameters called for a 22-ft span and a width of 62 ft. Other specifications included the standard AASHTO HS 20 loading, an alternate military truck loading; L/800 deflection criteria; an integral concrete diaphragm; skew, cross-slope kick across the bridge to control asphalt thickness and an asphalt wearing surface. CA designed the FRP bridge superstructure while LJB Engineering designed the concrete approaches leading up to the bridge’s entrance.
CA used its unique resin infusion process to mold eight panels that covered the bridge’s full span length (22 ft) and were just under 8 ft wide. Panel width was dictated by shipping size. A joint beam was bonded and bolted to join adjacent panels. The bridge skew was molded into the ends of net-shape panels. Beams and the deck were integrally molded together. The deck facings, beam shear webs and beam caps consisted of multi-axial fiberglass fabrics. In order to handle the high shear loads and crushing loads on the deck, a fiber-reinforced internal core was used. The core has multiple shear webs in both longitudinal and transverse directions to minimize deck deflection. The entire lay-up of beams and deck were infused with a corrosion-resistant vinyl ester resin with pigment and UV inhibitor. Eight panels were molded to cover the width of the bridge. A structural test program was conducted to validate the design. Specimens and panels were tested in the following areas:
- Laminate material properties;
- Deck bending and shear;
- Integral deck/beam section static bending (full scale);
- Fatigue of the integral deck/beam section for 2M cycles;
- Residual strength of the integral section; and
- Joint beam section static bending.
Traffic volume for Eight Mile Bridge, located southeast of Cincinnati near the Ohio River, was considered low–about 2,000 vehicles a day.
“Eight Mile’s short span and low traffic volume made the bridge a good candidate for this new system,” Mary said. “Traffic downtime was close to 100 days with weather a partial contributor. Emergency vehicles and school buses made arrangements for alternate routing. The detour route was not lengthy. As a result costs were minimal.”
After the old bridge was removed, new abutments were poured. Bearing pads were located on the abutments at each FRP beam location. The panels were dropped in place starting with the east side of the bridge. To integrally tie the FRP bridge to the abutments, rebar was slid into pre-drilled holes through the FRP beams. After the next FRP panel was fit-checked, adhesive was applied to the joints and the panels were bonded together. Bolts were used at the joint to clamp the mating surfaces together to ensure a consistent bond line and to provide a redundant shear connection between panels. All eight panels were installed in one day.
A poured concrete diaphragm was used to connect the FRP bridge to the abutment. The diaphragm was poured through holes cored through the deck to ensure even support under the deck and the beams. An asphalt wear surface was applied to the bridge. Trend Construction Co. acted as the prime contractor. Rod-Techs tied the steel; Security Fence installed the guardrail; Bernard Concrete Sawing sawed the abutments; and J.K. Meurer paved the roadway.
According to Mary, traditional construction methods and materials for the superstructure would have required more labor resulting in additional time and higher costs for construction. Since CA prefabricated the bridge panels and preassembled them on their manufacturing shop floor, installation of the system in the field went quickly and smoothly.
“For this project in particular, the goal was to construct a total superstructure out of composites that could deliver a service life capable of exceeding 100 years,” said Mary. “In addition to a long life cycle, the noncorrosive nature of composites greatly reduces maintenance costs. The speed of construction combined with the ease of installing the lightweight drop-in-place composite panels reduced the amount of time the road was closed and generated a savings to the traveling public.”
Trend Construction also found an advantage in working with the drop-in-place system. “It was very convenient,” said Andy Kloenne, vice president for Trend Construction. “We did not need a large crane to position the panels, a piece of equipment that would have been expensive to use and taken more time. The bridge was much easier to install using equipment already on site.” Because the drop-in-place system prefabricates more of a bridge’s components into one piece, the cost premium for using high-performance composite material is reduced. The overall cost of the integrated system is less than a conventional FRP bridge deck and separate beams because fabrication and assembly of the panels takes place under one roof to minimize production costs.
“Choosing FRP for higher-volume roads should reduce closure time and the costs of detour due to the fact the integrated bridge can be manufactured, delivered and placed quicker than traditional methods,” said Mary.
Hamilton County also is looking at applications for longer-span bridges. CA has integrated drop-in-place designs up to 50 ft.
Testing was performed on Eight Mile Bridge in 2010. The Bridge Department expects that the service life of its bridge structures has increased through the use of FRP decks and that reducing maintenance costs will provide real cost savings in future.
