M. Myint Lwin, Formerly Washington State Department of Transportation
Washington State Department of Transportation (WSDOT) has developed and used high perf o rm a n c e concrete (HPC) mixes containing fly ash and silica fume in several highway bridges since 1992. The concrete has high compressive strength attaining 10,000 psi (69 MPa ) by 28 days, low chloride permeability averaging less than 1,000 coulombs by 56 days, and generally lower shrinkage and creep values than conventional concrete.
In 1995, WSDOT was interested in expanding the use of high perf o rm a n c e concrete and participated in a demonstration project sponsored by the Federal Highway Administration (FHWA). The p roject, known as SR 18 over SR 516 eastbound in King County, consists of a three-span continuous pre s t ressed concre t e bridge. The center span has a length of 137 ft (42 m) and the end spans are each 80 ft (24 m) long. The roadway deck is 38 ft (11.6 m) wide, carrying two 12-ft (3.7-m) lanes and 4-ft (1.2-m) and 10-ft (3.0-m) wide shoulders. The bridge is located in earthquake zone “C” with an earthquake acceleration coefficient equal to 0.25g. The design complies with the new AASHTO LRFD Bridge Design Specifications. Construction of this project started in July 1996 and the bridge was completed and opened to traffic in October 1997.
High Performance Concrete
High performance concrete is used in the WSDOT W74G prestressed concrete I-girders and in the deck. The specified HPC properties for the girders are as follows:
• 56-day compressive strength > 10,000psi (69 MPa)
• Freeze-thaw durability > 80%
• 56-day chloride permeability < 1,000coulombs.
Use of HPC in the prestressed concrete girders allowed a reduction in the number of girder lines from seven to five. In the future , this will result in savings in the superstructure cost.
The design compressive strength of the deck concrete is 4000 psi (28 MPa), but it has enhanced durability due to the use of fly ash and the requirement of a 14-day wet cure.
Lessons Learned
High performance concrete is constructable and can be cost effective in highway bridges for both cast-in-place construction and precast, prestressed applications. Fly ash and silica fume are two key i n g redients that have important effects on the perf o rmance of concrete. Air entrainment is considered essential for freeze-thaw resistance. However, entrained air reduces the compressive strength of HPC. Proper curing is essential for the success of HPC. Water or moisture must be supplied to the concrete surfaces of flatwork or unform e d members soon after finishing and initial set to avoid shrinkage cracks. The cast-in-place concrete surfaces should be kept continuously
wet for 14 days.
HPC containing fly ash and silica fume is very cohesive and has good workability when properly proportioned. With the use of high-range water-reducers, the concrete can be mixed with a low water- cementitious material ratio and placed with a slump as high as 9 in. (230 mm) with no loss of strength or uniformity. The concrete flows laterally with ease in the forms and can be d ropped without segregation. Conclusions
Significant short- and long-term benefits can be realized with the use of HPC in bridges. These include more efficient designs longer spans, fewer beams, and shallower s t ructural depths; improved performance ; faster construction; reduced maintenance; longer service life; and lower life-cycle costs. The successful use of HPC has made it a material of choice by the bridge designers in Washington State. The designers will use HPC whenever and wherever there is benefit in the design.
HPC will help bridge engineers fulfill the vision of “Building Bridges for the 21st Century” to meet traffic and environmental demands with low life-cycle costs.
Further Information
Further information about SR18 is available in Proceedings of the PCI/FHWA International Symposium on High Performance Concrete (1997) available from PCI or by contacting the author at 360-705-8797 or [email protected].
