Jerry Weigel, Washington State Department of Transportation
The Washington State Department of Transportation (WSDOT) has been very active in the development of high performance concrete (HPC). WSDOT, as a member of the AASHTO/SHRP Lead States Team, conducted a demonstration project in 1996 through 1998 on the use of HPC to design and construct the three-span bridge carrying State Route 18 over State Route 516.* A showcase on this project was conducted in 1997 to illustrate the use of HPC and to create a mechanism to share the experience with interested parties.
Cost Savings
The project presented an opportunity to compare the standard bridge designs with those made using HPC. The design comparison proved the economic and long-term benefits of using this new technology. HPC allowed the number of girder lines to be reduced from seven to five, realizing a net cost savings of at least $50,000. As a result, WSDOT began using HPC in all its precast, prestressed concrete bridge girders and has used this technology for an average of 20 bridges per year since 1998. When the cost savings is extrapolated to all HPC bridges, significant savings can result. Approximately 41 percent of the current WSDOT bridge inventory, and seven out of ten bridges designed in the past ten years, have precast, prestressed concrete superstructure elements.
Super Girder
HPC technology has also been instrumental in the development of 83- and 95- in. (2.10- and 2.41-m) deep precast, prestressed concrete “super girders” for longer span lengths. This is particularly important with the increasing demand for “rapid construction” (get in, get out, and stay out) and satisfying environmental requirements to keep bridge supports out of wetlands and waterways. High economic value results from the inherent cost efficiency of precast, prestressed concrete girder construction compared to other long span alternatives. Using HPC and the 95-in. (2.41-m) deep section, we are able to build a precast, prestressed concrete girder bridge with a span length of 225 ft (68.6 m).
Design and Specification Changes
Prior to the AASHTO/SHRP Lead States activity, WSDOT required the use of 0.5-in. (12.7-mm) diameter 270 ksi (1.86 GPa) strands and minimum concrete compressive strengths of 4000 to 5000 psi (28 to 34 MPa) at transfer and minimum design concrete compressive strengths of 5000 to 5500 psi (34 to 38 MPa). Since the casting bed turnover rate was of utmost importance to the fabricators, they routinely provided a final strength much higher than the minimum specified value. This was the direct result of their need to acquire high concrete compressive strengths at early ages. To accomplish the high early strength, the mix designs were such that the concrete compressive strengths achieved were far greater than the 5500 psi (38 MPa) required.
Working with industry representatives, WSDOT structural designers found that they could establish higher strengths at transfer and could also require much higher design strengths. During the demonstration project, the final compressive strength was required to be 10,000 psi (69 MPa) with a strength at transfer of 7500 psi (52 MPa). This combination turned out to be very difficult to achieve on a routine basis. As a result, the structural designers reevaluated the strength needs and determined that, for most applications, a design compressive strength of 8500 psi (59 MPa) is structurally adequate and easily attainable.
The following design and specification revisions have been implemented in the state of Washington as a direct result of the AASHTO/SHRP Lead States HPC program research and showcases:
- Accepted the use of 0.6-in. (15.2-mm) diameter prestressing strands, which can provide a much higher compressive force in the limited girder space.
- Use of a compressive strength at transfer of 7000 to 7500 psi (48 to 52 MPa) to allow an improved casting bed turnover rate and an ideal design compressive strength. The strength at transfer can be as high as 8500 psi (59 MPa) for special circumstances, but comes with a higher cost and introduces additional risk.
- Use of a minimum design compressive strength of 8500 psi (59 MPa) at 28 days.
Depending upon section type, the precast, prestressed concrete girder can be shipped after a minimum maturity time of 7 or 10 days, provided that the concrete has attained the required minimum design compressive strength. Thereafter, the specifications allow the girders to be shipped when 95 percent of the specified minimum design compressive strength is achieved. This reflects WSDOT’s recognition that contractors want to ship girders as soon as possible.
Computer Software
WSDOT Bridge and Structures Office has created computer aided design software and has made it available through a mechanism labeled “open source.” One of the more successful programs, PGSuperTM, is a precast girder superstructure design tool. The program features the use of LRFD Bridge Design Specifications and designs and analyzes precast, prestressed concrete girders for flexure and shear; provides camber and deflection analysis as well as long girder stability analysis for lifting and shipping; provides detailed reports to support every calculation; has a fully customizable library for any I- or Ushaped beams; and allows customization of design criteria. Free download of this software is available at http://www.wsdot.wa.gov/eesc/bridge/software/. Additional information on “open source” and help is available by contacting Rick Brice of the Bridge and Structures Office at 360-705-7174 or [email protected].
Editor’s Note
This article is the second in a series that describes how the use of HPC has progressed since it was first introduced into a state’s program. The first article about Texas appeared in Issue No. 30.
*See HPC Bridge Views, Issue No. 2, March/April 1999.