Stephen Mary, Hamilton County, Ohio and Richard A. Miller, University of Cincinnati
Many bridges in the USA are designed and maintained by city and county engineers. These bridges must meet the same strength, serviceability, and durability requirements as state-owned bridges. County engineers, like their state counterparts, have found that high performance concrete (HPC) can be beneficial for both strength and durability.
In Ohio, HPC has been used for stateowned bridges for almost a decade. In the early 1990s, Ohio Department of Transportation (ODOT) created an HPC specification for bridge deck concrete. In 1997, ODOT installed their first HPC precast, prestressed concrete bridge as part of the Federal Highway Administration Showcase program. This bridge superstructure consisted of adjacent box girders. Availability of 10,000 psi (69 MPa) compressive strength HPC enabled the span of the Ohio B42-48 section [42 in. deep by 48 in. wide (1.07 m by 1.22 m)] to be extended to 116 ft (35.4 m).
Since that initial installation, three Ohio counties have built precast, prestressed HPC bridges. Columbiana County built a 120-ft (36.6-m) long box girder bridge, again using an Ohio B42-48 section and 10,000 psi (69 MPa) concrete. Mercer County has a 130-ft (39.6-m) long adjacent box girder bridge under construction. This bridge uses 8000 psi (55 MPa) concrete with a modified Ohio B42-36 section. A regular B42-36 has a 5-in (127-mm) thick bottom flange, which allows for only one bottom row of 17 strands, a second row of four strands, and then rows of two strands in the webs. The modified girder has a 6-in. (152-mm) thick bottom flange to allow for two full rows of 17 strands. The completed girder has 38 bottom strands.
In Hamilton County, HPC is used to increase durability of precast, prestressed concrete elements. Over 20 HPC bridges have been built in the last ten years. The Hamilton County specification for precast concrete allows the fabricator to use the regular bridge girder concrete mix, but requires 7 percent silica fume by weight of cement, either as a replacement or as an addition. HPC designed for durability normally has a water-cementitious materials (w/cm) ratio of 0.40 or less. Since the precast industry tends to use low w/cm ratios in order to get high early strengths, the w/cm ratio is usually less than 0.36. Hamilton County requires a release strength of 4000 psi (28 MPa) for box girders and 4500 psi (31 MPa) for other girders. The fabricators must submit the proposed mix design for approval before casting the beams. Although not designed for high strength, the precast elements often have concrete compressive strengths approaching 9000 psi (62 MPa) at 28 days.
Hamilton County now uses a performance-based specification for HPC in bridge decks. Previously, the county used the ODOT Class S specification. The concrete used was a prescriptive mix with a standard aggregate gradation and a cement content of 715 lb/cu yd (424 kg/cu m). It was felt that this mix was prone to increased shrinkage, which could cause full-depth deck cracking. The county now requires the contractor to submit a mix design for approval. The mix design must have a w/cm ratio less than 0.40, maximum slump of 6 in. (150 mm), minimum compressive strength of 4500 psi (31 MPa) at 28 days, and 2 lb/cu yd (1.2 kg/cu m) of polypropylene fibers not less than 3/4 in. (19 mm) long to minimize plastic shrinkage cracking. Thirty days prior to deck placement, a test placement must be made on the project site to check air, slump, workability, and compressive strength. The deck must be cured using a combination of liquid membrane curing compound and seven days of water curing.
HPC has also been used for bridge deck overlays. These overlays are specified to have 7 percent silica fume by weight of cement, a maximum water/cement ratio of 0.36, and a bonding agent in addition to the above requirements for bridge decks.
County Engineer Bill Brayshaw has been pleased with the HPC specification. Three large full-depth decks have been placed with very little or no apparent cracking. Some cracking has occurred on two of the bridges that received an HPC overlay. “Overall, HPC has been well worth the additional material cost due to the quality of the final product. It has superior durability due to increased density and lower chloride permeability. This office will continue to provide the highest quality bridges to the traveling public through the use of innovative methods and materials,” said Mr. Brayshaw.
Further Information
For further information, contact Stephen Mary at [email protected] or 513-761-7872.