Salt's Substitute

Jan. 10, 2007

On the 50th anniversary of the U.S. interstate system, the original Transcontinental Highway, the Transcontinental Railroad, celebrates its 140th anniversary by completing a bridge replacement in the middle of the Great Salt Lake using high-performance concrete (HPC) designed for a 100-year service life.

On the 50th anniversary of the U.S. interstate system, the original Transcontinental Highway, the Transcontinental Railroad, celebrates its 140th anniversary by completing a bridge replacement in the middle of the Great Salt Lake using high-performance concrete (HPC) designed for a 100-year service life.

Many of today’s highway and bridge engineering and construction practices have their foundations in the construction of the Transcontinental Railroad, the first federally funded transportation project. The course of the Transcontinental Railroad is virtually unchanged since the original surveyors laid the road in the late 1860s. So fine was the surveyed route that I-80 through Nebraska and Wyoming is almost continuously within sight of the railway. The only exception is a shortcut causeway directly crossing the Great Salt Lake near Ogden, Utah, and bypassing the famous Promontory Point, site of the final golden spike placed to signify the railroad’s completion in 1869. The earthen causeway splits the Great Salt Lake’s water into two bodies. At mile marker 762.71, a 589-ft-long bridge provides an opening to allow fresh water from the mountain streams from the Bear River drainage in the north to mix with the salt water from south of the causeway, maintaining a uniform salinity in the lake, critical to the unique life there.

Originally constructed in 1903, the earthen causeway and a 12-mile wooden trestle bridge carried only the railroad track. Earthen fill replaced the wooden trestle bridge in 1958, essentially creating a dam between the northern and southern portions of the lake. In 1984, a second timber trestle bridge was installed to allow the lake salinity to equalize north and south of the causeway. This new bridge carried both the railroad track and an access service road. The Union Pacific Railroad estimates the service life of railroad timber bridges to be 30 to 50 years.

The single-rail track carries in excess of 20 trains per day and over 45 million gross tons per year, making this bridge a critical east-west link in the Union Pacific railroad system. As the only east-west track crossing the northern portion of the Great Salt Lake, the bridge replacement had to be completed without disruption to the continuous daily traffic. As a result, construction and placement of the new bridge had to be well planned and coordinated with the hourly rail traffic. Union Pacific recognized the importance of this rail bridge, the corrosive environment of the salt lake and the wintry Wasatch Mountains as major factors in the decision to use HPC to extend the useful service life of this new bridge.

Hushing the harsh

The project called for removal of the wood timber bridge and replacing it with a new 14-ft span, steel-reinforced, prestressed concrete bridge structure, supported on drilled shaft piles. Union Pacific chose to upgrade the bridge capacity and increase the dynamic live load to a Cooper E-80 rating, or 80 kips per axle.

In addition to the significant dynamic loads from traffic, the bridge will be subject to severe exposure conditions. Freeze-thaw cycles in the northern Utah Rockies, airborne salt and direct salt-water contact will additionally stress the bridge concrete. To achieve the anticipated 100-year service life, HPC was developed for all the concrete used in both the precast and cast-in-place bridge elements. The concrete mix contained silica fume (ASTM C1240) at 7% by weight of cement to reduce concrete permeability, slowing the rate of chloride penetration while increasing the electrical resistivity of the concrete. Reducing chloride penetration and increasing electrical resistivity are two properties of this HPC well suited to protect the reinforcing steel.

For additional protection, a corrosion inhibitor was used in the concrete to increase the chloride threshold level at which the embedded reinforcing steel would begin to corrode. All reinforcing steel was Grade 60 deformed steel bar with epoxy coating to current ASTM A775 specifications.

A pinch of challenges

A construction site in the middle of the Great Salt Lake was not the only logistical challenge. The project requirements included the need, during the entire bridge reconstruction, for the bridge and causeway to remain open to the major east-west rail traffic. The wooden timber piles were replaced with 24-in.-diam. steel piles, designed to remain in place as a sacrificial layer giving added service life to the piles. The outer steel pipe also served as stay-in-place formwork for the HPC and epoxy-coated steel reinforced piles.

The prestressed box beam bridge girders were manufactured by Enterprise Concrete Products LLC in Dallas and transported by rail to the remote construction site. The 42-in.-wide box-beam girders were in lengths of 35 and 43 ft. The individual girders ranged in weight from 70 to 93.6 kps. Prestressing of the beams with 1/2-in. seven wire, uncoated, low-relaxation strands occurred when the concrete compressive strength reached 4,700 psi. The specified compressive strength of the HPC for the box girders was 7,000 psi at 28 days. The new HPC bridge contains seven beams per span on 15 bents to go along with the 105 piles.

The first phase called for the construction of a new bridge to the north of the old wooden bridge. This first bridge was 21 ft wide incorporating 3- to 7-ft-wide double-box beams on equal piles. When completed, the rail track was moved from the old wooden bridge to this new bridge, and demolition of the old bridge began.

A temporary dock and Flexifloat barge, assembled on site, were used as a base for the hydraulic pile hammer. A hydraulic pile hammer was preferred, according to Union Pacific Bridge Construction Manager Richard Chavez, because, “It’s a lot cleaner operation, environmentally, than diesel.” To accommodate the increased live-load rating, the piles were driven to 200 net tons and approximately 120 ft deep, twice the original pile depth. Once in place, the 24-in.-diam. piles and the formed pile caps were filled with HPC using the same proportions used for the precast bridge beams. The HPC for the piles and caps was produced locally by Jack B. Parsons of Ogden, Utah, and transported for 45 minutes along the causeway access road to a concrete pump that delivered the concrete into the steel pile shells and formed caps. The precast beams were shuttled to site by rail and placed by a Union Pacific track-mounted Ohio crane and pinned at the bents.

The longitudinal joints between the beams were fitted with galvanized steel T strips to prevent the ballast from working its way into the small gaps. After the rail track was moved to the new concrete bridge, the crane and floating barge were moved to the south of the bridge to begin the demolition of the remaining portion of the wooden bridge. The second bridge comprises 4- to 7-ft-wide box beams, making the second bridge slightly wider than the first. When completed, steel plates cast into adjacent beams of the two bridges were welded together to form the one bridge seen today. The rail track then was moved back to the original position, south of the access road.

The Union Pacific prefers box-beam girders for its railroad bridges because the flat top surface of the box beams serves as a good roadbed for the ballast, track and ties, all of which are placed with conventional mechanical track equipment used for all track replacements. The new precast concrete bridge was designed to allow the railroad track to be placed anywhere along the width of the span. This was a critical element in the design that allowed the track to be moved to the service road side of the bridge during removal and replacement of the wooden bridge, maintaining traffic flow. The design also will allow the addition of a second track to accommodate increased freight capacity in the future.

About The Author: Kojundic is a director of the Silica Fume Association and business manager for Elkem Materials, Pittsburgh.

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