Jerry Weigel, Washington State Department of Transportation
Since our 1997 HPC Showcase, state and local agencies in Washington State have constructed eight bridges with high performance concrete (HPC) girders, have ten ready to be advertised, and have eight being designed. Environmental requirements to keep piers out of waterways and the necessity of providing for future widening to accommodate increasing traffic demands are creating an ever-growing need for longer spans. The use of HPC improves construction economy by providing for longer spans, increased girder spacings, and shallower girders. Experience gained through the design and fabrication of HPC girders has shown that release strength is the critical parameter. A specified release compressive strength of 7500 psi (52 MPa) and a specified design compressive strength of 8500 psi (59 MPa) result in an optimum design economy. While compressive strengths of 10,000 psi (69 MPa) are possible, the extended in-form curing time and design mix complexities are uneconomical and difficult.
Super Girders
Washington State Department of Transportation (WSDOT), partnering with industry, has developed two deep precast, prestressed concrete I-girder sections called “Super Girders.” An article in the July-August 1998 PCI Journal provides technical data and describes the development of these sections. These new sections, using HPC and 0.6-in. (15.2-mm) diameter prestressing strands will span up to 225 ft (69 m). WSDOT has a project, Twisp River Bridge, under contract using “Super Girders” with a single span length of 197 ft (60 m). The girder concrete has a specified release compressive strength of 5000 psi (34 MPa) and a design compressive strength of 8000 psi (55 MPa). Because of their weight, these girders were fabricated in three sections for transportation to the site and will be post-tensioned together on site.
Due to constructibility concerns, the girder sections will be placed on temporary falsework and the girder wet joints completed after the placement of the deck concrete. The inability to consistently produce high strength concrete at a remote project site was one of the major concerns. With this construction sequence, the required compressive strength of the joint concrete is 4400 psi (30 MPa). If the joints were completed and the girders stressed before deck concrete placement, the joint concrete compressive strength would need to be 7500 psi (52 MPa) and the design compressive strength of the girder concrete would need to be 10,000 psi (69 MPa). However, when the deck concrete is placed before the girders are stressed, the falsework must support the weight of the girders and the deck. Consequently, the construction sequence has a significant impact on the design requirements and falsework costs.
Materials
Except for microsilica and corrosion inhibitors, HPC uses similar materials and admixtures as conventional concrete. The amount and type of each component are selected in order to achieve the required durability and strength properties. In addition to prestressed concrete girders, WSDOT has used HPC for bridge deck concrete; cast-in-place piling concrete; and deck overlays of latex modified concrete, microsilica modified concrete, and fly ash modified concrete.
The Future
Through our effort as a member of the “Lead States Program,” our participation in a showcase, and working with HPC, we have learned a great deal and are convinced that the future of high performance concrete is very bright. This learning experience has confirmed that we can build bridges that are durable, cost effective, and will require minimal maintenance.
Further Information
For further information about HPC in Washington State, contact the author at (360) 705-7207 or [email protected].
Editor’s Note: An article about Washington State’s first HPC bridge was included in HPC Bridge Views No. 2.