Anthony N. Kojundic, Silica Fume Association
The course of the Trans-Continental Railroad is virtually unchanged since surveyors originally selected the route more than 140 years ago. The only exception is a short-cut causeway that crosses the Great Salt Lake near Ogden, Utah. The earthen causeway splits the Great Salt Lake’s water into two bodies. At mile marker 762.71, a 500-ft long bridge provides an opening to allow fresh water from mountain streams in the north to mix with the salt water, maintaining a uniform salinity in the lake. This is critical to the unique life there.
The causeway is a wide landfill carrying both the railroad track and an access road. The previous timber bridge was constructed in 1958. Estimated life spans of timber bridges are 30 to 50 years. For the railway bridge on the causeway, daily traffic averages 20 trains or 45 million gross tons per year. The conclusion was obvious—a replacement was needed and the decision was made to build the new bridge using
concrete.
Environmental
Conditions Call for High Performance Concrete The project called for the removal of the timber bridge and replacing it with a 14-span, prestressed concrete bridge with a 100-year design service life, using high performance concrete (HPC).
Besides the significant dynamic loads from traffic, the bridge would be subject to severe exposure conditions. Freeze-thaw cycles in northern Utah, airborne salt, deicing salts, and salt water would all affect the concrete and these factors played a major role in the decision to use HPC.
The HPC incorporated silica fume at 7 percent by weight of cement to reduce concrete permeability, slow the rate of chloride penetration, and increase electrical resistivity of the concrete. A corrosion inhibitor was used to increase the chloride threshold level at which reinforcing steel would begin to corrode. The entire Grade 60 deformed reinforcing steel was epoxy coated.
Challenging Site Logistics
A construction site in the middle of the Great Salt Lake wasn’t the only logistical challenge. The project requirements included the need for the bridge and causeway to remain open to the major east-west rail traffic during the entire bridge reconstruction.
The road bridge was removed first and replaced. The track was then moved to the road bridge and the old track bridge removed and replaced. Finally, the track was moved back to the original alignment.
The wooden timber piles were replaced with 105 24-in. (610-mm) diameter steel piles, left in place, and filled with a locally produced HPC. The fifteen pile caps used cast-in-place concrete. The same mix proportions of the HPC, as shown in the table, were selected for use in the piles, pile caps, and box beams.
Precast, prestressed box beams for the superstructure were manufactured in the Dallas, TX, area and transported by rail to the construction site. The dual cell box beams with a width of 7 ft (2.0 m) ranged in length from 35 to 43 ft (10.7 to 13.1 m). Individual beams ranged in weight from 70 to 93.6 kips (32 to 42.4 Mg). The road bridge used three beams and the railroad bridge used four beams for total widths of 21 and 28 ft (6.4 and 8.5 m), respectively. The beam design allowed for track placement anywhere on the member, and was capable of supporting a Cooper E-80 live load with a maximum 30 in. (760 mm) depth of ballast. The prestressing strands were straight 1/2-in. (13-mm) diameter low-relaxation seven wire strands. The specified compressive strength of the HPC for the box beams was 4700 psi (32 MPa) at detensioning and 7000 psi (48 MPa) at 28 days.
Ready for the Next 100 Years
The causeway replacement project started in 2003 and was completed in early 2006. The Union Pacific inspects its bridges every two years, and expects that it will be 50 inspections from now until the end of the next century before another replacement bridge may be needed. This expected long life will be credited primarily to the use of HPC.
