Thomas Drda, Federal Highway Administration and Bryan Spangler, Pennsylvania Department of Transportation
The Pennsylvania Department of Transportation (PENNDOT) has been involved with high performance concrete (HPC) for several years. A performance based specification has been developed and several bridge decks have been constructed. Contractors, material suppliers, producers, and Penn State University have all been active participants in the development of a performance based specification. This approach developed strong support in our contracting community. The Great Bend Bridge on Route 11 over the Susquehanna River is one example of a successful application.
The Great Bend Bridge is a 537-ft (164-m) long, prestressed concrete I-beam bridge with two spans of 129.3 ft (39.3 m) and two of 139.4 ft (42.5 m). Each span consists of five AASHTO Type V beams spaced at 11.2 ft (3.40 m) with an 8-1/4 in. (210-mm) thick HPC deck. The superstructure was made continuous for live load by the placement of continuity diaphragms at the supports. The typical bridge width is 53 ft (16.2 m) with two travel lanes, two shoulders, and a raised sidewalk.
HPC Features
PENNDOT’s objective in implementing HPC is to develop a durable concrete deck that has low permeability, good durability, minimal cracking, and adequate strength. On this project, there were specified requirements for compressive strength, chloride permeability, and shrinkage. Freeze-thaw resistance and abrasion resistance were measured for information only. The 28-day compressive strength was limited to a minimum of 4000 psi (28 MPa) and maximum of 6200 psi (43 MPa). The ratio of 28-day to 7-day compressive strength was required to be greater than 1.33 to help reduce early age cracking by having a more controlled strength gain. The maximum value of chloride permeability was specified to be 1600 coulombs at 28 days.
Construction – Lessons Learned
The prime contractor placed the concrete deck in two separate placements during August 2002. The specifications required that the evaporation rate, as determined using the American Concrete Institute’s evaporation chart for hot weather concreting, not exceed 0.10 lb/ft2/hr (0.49 kg/m2/hr). The first placement commenced at 3:20 a.m. and ended at 4:00 p.m. Weather conditions were cool and humid at the beginning of the placement to hot, dry, and windy at the completion when the evaporation rate was estimated to be 0.24 lb/ft2/hr (1.17 kg/m2/hr). Even under these adverse conditions, the placement went well with the exception of some finishing problems late in the day. Because of the high evaporation rate, the contractor applied an intermediate curing compound, which became a finishing aid even though a finishing aid was prohibited by the specification. The use of the intermediate curing compound in this fashion may lead to scaling problems on the deck in the future. Fogging or wind screens, as allowed by the specifications, should have been used to reduce the evaporation rate.
Fortunately, the only apparent detrimental effects were twelve small areas where the concrete did not finish properly after the finishing machine sat at these locations because of construction delays. These areas were scarified and patched with latex concrete. In the second placement, these problems did not occur because of the smaller placement size and more favorable weather conditions.
Some minor sand balling of the mix was detected during the first placement but was immediately dealt with by reducing the charging rate of the concrete mixers. Cell phone communication between the job site and the concrete plant kept this from being a problem. All sand balls in the delivered concrete were removed at the pump hopper. The deck received a 7-day wet cure followed by the application of a curing compound. Transverse saw grooving was not allowed to begin until three days after the placement of the curing compound.
Results
Follow-up inspections have detected only a few hairline cracks in the deck. The Department anticipates a 75- to 100-year service life for this structure because of the low permeability, other factors contributing to enhanced durability, and prior testing of concrete mixtures at Penn State. Based on our experiences with the Great Bend Bridge, the specifications have been revised towards eliminating the problems that occurred.
Benefits
The use of HPC on bridge decks holds the promise of lower total life-cycle costs by providing bridge decks that are more durable with longer life expectancy. The success of this project is the result of a partnership and cooperation between PENNDOT, the Association of Pennsylvania Contractors, academia, and the FHWA in the development and implementation of an HPC specification.
Further Information
For further information, contact the second author at [email protected] or 717-783-5347.