ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/306966
The use of HPC in the Richlands Bridge allowed the number of beams per span to be reduced from seven to five. All photos VDOT unless specified otherwise. 50 | ASPIRE , Winter 2008 S TAT E I n the mid 1990s, the Virginia Department of Transportation (VDOT), in cooperation with the Federal Highway Administration (FHWA), initiated a program focused on the development and implementation of high performance concrete (HPC). Prior to that time, many improvements had been made in design, materials, and construction practices in VDOT, establishing the foundation for the HPC program. Initially, the HPC program entailed work with normal weight concretes and focused on mix designs that yielded high strength, low permeability, and temperature control. Efforts progressed to the development of mix designs using lightweight aggregate to produce lightweight HPC with high strength and low permeability. VDOT has also evaluated the use of self-consolidating concrete. This concrete has very high flow characteristics, enabling consolidation without mechanical vibration. Finally, HPC efforts have led VDOT to investigate ultra-HPC fiber-reinforced concrete in bulb-tee beams with no conventional steel shear reinforcement, very high compressive strength, and negligible permeability. High Performance Concrete Since 1992, Virginia has been requiring protection against alkali-silica reactivity (ASR). If the alkali content of cements is currently more than 0.45 percent, various combinations of slag or pozzolans are required by VDOT to inhibit ASR. The addition of pozzolans or slag leads to low permeability in concretes and protection against chemical attack. In the mid 1990s, in cooperation with the FHWA, new bridge projects were planned under the experimental HPC program. These bridges had high strength concrete beams and/ or a concrete permeability requirement. The first HPC construction project was completed in 1995. This structure carrying Route 40 over Falling River, in Campbell County, consists of four 80-ft-long spans with AASHTO Type IV beams. The beams were fabricated with HPC with a specified compressive strength of 8000 psi and a maximum chloride permeability of 1500 coulombs. The bridge deck was constructed using HPC with a specified compressive strength of 6000 psi and a maximum chloride permeability of 2500 coulombs. The HPC design resulted in a reduction in the number of beams per span from seven to five. Soon after, another HPC structure was constructed carrying Virginia Avenue over the Clinch River in Richlands with beams having 0.6-in.-diameter prestressing strands. The bridge consists of two 74-ft-long spans of AASHTO Type III beams. The specified concrete compressive strength was 10,000 psi at 28 days and the maximum chloride permeability was 1500 coulombs. The deck was constructed with HPC having a specified strength of 5000 psi and a maximum chloride permeability of 2500 coulombs. Again, the use of HPC resulted in reducing the number of beams per span from seven to five. By 1999, VDOT had 76 bridge structures in the HPC program. The specified strength of the concrete ranged from 7000 to 10,000 psi. The low-permeability requirements were a maximum of 1500 coulombs for the prestressed concrete girders, 2500 for the cast-in-place concrete decks, and 3500 for cast-in-place concrete in the substructures. Lightweight HPC The economic benefits of HPC combined with a reduction in dead load make lightweight HPC (LWHPC) a very attractive material choice. In bridge beams, the use of LWHPC results in reduced dead loads that enable longer span lengths and reduced substructure loads. Bridge deck replacement using LWHPC reduces dead load thereby allowing greater lane capacity. The high quality concrete is expected to extend the service life of the structure. The first LWHPC bridge was constructed in 2001. The bridge carried Route 106 over the Chickahominy River near Richmond, Virginia. It was constructed using 84-ft-long AASHTO Type IV beams fabricated with LWHPC with a minimum specified compressive strength of 8000 psi and a maximum chloride permeability of Virginia's Developments in the Use of Concrete in Bridges by H. Celik Ozyildirim, Virginia Transportation Research Council and Julius F. J. Völgyi Jr., Virginia Department of Transportation 10973_ASPIRE_win08.indb 50 12/12/07 3:33:49 PM