By: Mark Bintzler, P.E.
Designated a National Scenic Riverway, the Saint Croix River carves out a deep valley, gliding and rushing through a lush green landscape surrounded by wooded bluffs and historic towns until its confluence with the Mississippi River.
About a half hour from the Twin Cities of Minneapolis and St. Paul, the historic two-lane, 23-ft-wide Stillwater Vertical Lift Bridge crosses the river only a few feet above the water to connect Minnesota to Wisconsin. Opened in 1931, the historical steel bridge was deemed structurally deficient by inspectors in 2008. Traffic often is disrupted because of bridge deck lifts, flooding and maintenance. In addition, the narrow widths and functional deficiencies of the crossing cause safety and congestion issues because traffic volume has grown far more than the bridge was designed to support.
In spring 2013, construction got underway to replace the 85-year-old lift bridge with a new St. Croix River Crossing designed to reduce congestion, improve roadway safety and provide an adequate level of service for traffic volumes forecast for 2030. A joint project of the Minnesota Department of Transportation (MnDOT) and the Wisconsin Department of Transportation (WisDOT), the new mile-long, four-lane segmental concrete structure, which opened to traffic in August 2017, spans 140 ft above the water to connect expressways on both sides of the river. The bridge’s main span was built with 650 massive precast concrete box girders to provide structural integrity, resiliency and long-term durability.
The proper blend
The $646 million St. Croix River Crossing project has been decades in the making, partly because of the many historic, cultural and environmental features along the St. Croix National Scenic Riverway. Because the bridge is in a National Park and the river is designated a National Waterway, the complexities of the area and environmental sensitivities were significant factors in deciding what type of bridge to design.
Terry Zoller, project manager for MnDOT, explained that a lot of environmental impact issues needed to be mitigated. “The stakeholders did not want the bridge to be the focal point, but rather blend into the valley,” said Zoller. “They did not want a tall structure that was too imposing, like a cable-stay bridge, and they did not want a lot of piers in the water, which would be required with a box girder bridge design.” The stakeholder cooperative group finally compromised on an extradosed, segmental concrete bridge—a hybrid between a pre-stressed box girder bridge and cable-stay bridge—because of its unique design to achieve a low profile and mitigate environmental impact.
The extradosed, segmental concrete design uses significantly shorter towers compared with a traditional cable-stay bridge and uses stays that act as exterior post-tensioning tendons on the box girder. Unlike cable-stay bridges, extradosed bridges do not carry the entire deck load through the stays, but rather through both the stays and the deck. It is a design that allows for longer spans and minimizes the number of piers in the water, which improves the navigational aspects of the river. With the new crossing, the heights of the bridge towers are below the tree line on the Wisconsin side, and the span lengths are the longest that could be made for this type of structure.
The new bridge is entirely structural concrete. In total, it contains approximately 140,000 cu yd of concrete: approximately 65,000 yd for cast-in-place concrete applications, such as the substructure foundations and piers, and approximately 76,000 yd (60,000 yd for the main river spans and 16,000 yd for approach bridges) for producing the 650 precast concrete girders that form the driving surface of the main span.
Right on a variety of levels
The Lunda/Ames Joint Venture and Aggregate Industries teams held meetings to discuss the unique concrete performance requirements for the crossing. The goal was for the superstructure’s precast concrete bridge girders to provide a 100-year service life before needing major maintenance.
According to Dale Even, P.E., project manager for the Lunda/Ames JV, one of the biggest challenges was coming up with a concrete mix design that would perform on a variety of levels. “The precast girders’ large size and radial exterior webs required concrete that provided high early strength, durability and workability—something fluid enough that it could be placed in the uniquely shaped casting formwork but achieve a high early strength and gain strength over time,” said Even.
The standards required an early strength of 3,000 psi at 16 hours for stripping the girders from the casting beds, 4,000 psi at 20 hours for post-tensioning and match casting, 7,000 psi at 14 days for moving the girders into the storage yard, 8,000 psi at 56 days for barging and erecting the girders, and 9,000 psi at 90 days for final design strength. In addition to achieving these unique early and late strength parameters, the concrete had to meet stringent performance requirements for flow through a very tight reinforcement mat, permeability, durability and shrinkage.
Aggregate Industries collaborated with the Holcim Technical Services Group in evaluating the composition of various mix designs. More than 30 trials and three months of testing were performed to ensure the final mix would meet the expectations of the state and general contractor. Performance evaluation included different blended cement mixes with numerous aggregates and admixture combinations. The analysis included tests for compressive strength, permeability, durability, flowability and shrinkage.
Upon successful completion of the performance assessments, which were validated by an independent third-party testing laboratory, the project team decided on a high-performance concrete (HPC) mix containing a blend of Type I/II portland cement, slag and fly ash. HPC mixtures incorporate supplementary cementitious materials (SCMs)—such as slag cement and fly ash—to make concrete stronger, more durable and longer lasting. Blended cements, which are a mix of portland cement and one or more SCMs, can significantly extend the life of concrete because they can reduce the permeability of concrete to water, chlorides and other aggressive agents. In concrete structures, permeability is generally considered the critical factor affecting durability.
The new St. Croix River Crossing is a four-lane segmental concrete span designed to last more than a century; it spans 140 ft above the river, which is a designated National Waterway.
Leasing and lifting
Minnesota’s plan for the bridge span called for lifting the 650 large precast bridge girders from barges to heights of 140 ft. Considering each segment weighed 180 tons and measured 10 ft long, 48 ft wide and 18 ft high, logistics posed a real challenge, as transporting the finished girders from the casting yard via truck and then trans-loading them onto barges for float-out would be very difficult, time-consuming and expensive.
The contractor determined that the best approach would be to set up casting operations on the shores of the river system, load the finished girders on barges and use the waterway for direct transport to the jobsite. “We looked for possible precast sites around the bridge area, but there were height and weight restrictions,” said Zoller. “We also looked at different locations along the St. Croix River, but because it is a National Park there were very few land options.”
By leasing 20 acres of land at Aggregate Industries’ Nelson Sand & Gravel Plant on Grey Cloud Island in the Mississippi River, the Lunda/Ames JV could set up its casting facilities only 33 river miles away from the bridge site. “The site was very convenient as it provided us with easy access to raw materials needed to produce the concrete and allowed us to barge the completed girders down the Mississippi River and then up the St. Croix River,” said Even. “We avoided major permitting issues and time constraints associated with truck transport, and river access allowed us to move up to 12 girders at a time instead of just one.”
In February 2014, the project team began transforming the assigned parcel of land into a full-fledged segment-building operation. Work included the construction of a 1.5-acre climate-controlled facility for segment production, a holding area for completed segments and project-specific equipment, and a barge loading slip. The casting yard also was fortified with 60,000 tons of base material to support the weight of the segments, gantry crane and segment lifters.
Nothing goes to waste
To ensure a high degree of consistency, reliability and quality, concrete was delivered “on demand” to the segment fabrication facility using a portable ready-mix concrete plant. This Erie Strayer Central Mix Plant could produce 300 yd of concrete per hour, and contained four aggregate storage bins and three cement silos. Following strict quality-control guidelines, every batch of concrete was tested to ensure all performance requirements were met.
During construction, segments were numbered and built using a match-cast system. In the warehouse, five specially designed casting beds sat on tracks. Wooden pre-tie jigs that act as a pattern for assembling the reinforced steel framework of each segment also were built and stored inside the warehouse. When it was time for a segment to be built, workers used a crane to pick up the steel rebar cage from the correlating pre-tie jig and set it into place on the appropriate casting bed. After installing hollow plastic ducts that allow space for the post-tensioned (PT) steel strands that connect the segments together, workers then placed the concrete into the casting bed, where it cured to the specified 3,000-psi strength at 16 hours. When the concrete reached a strength of 4,000 psi at 20 hours, workers inserted the PT cables transversely in the slab and match-cast.
The beds were lined up one after another to allow the segments to be built in sequence, which is necessary for accurate placement of the post-tensioning ducts and a perfect match of adjoining segments. Once a segment was measured to precise specifications, tested and deemed ready for construction, segment “lifters” transported the finished segments to outside storage areas, where the concrete continued to strengthen before being barged to the project site.
Each segment contained more than 85 cu yd of concrete and 25 tons of rebar. To maintain good flowability characteristics, a slightly lower slump concrete was used for the bottom and vertical wall pours, and a slightly higher slump was used for the top pour. Three times for each segment, crews conducted on-site testing for slump, air entrainment and temperature. Test cylinders also were cast and tested to ensure the hardened concrete achieved its required strength at the specified age. In total, almost 25,000 concrete test cylinders were cast throughout the segment production process.
“Of the 650 segments produced, we didn’t throw away one segment,” said Even. “That speaks volumes about the quality of the mix and tremendous dedication of the team to maintaining strict quality-control measures throughout the casting operation.”
The driving surface on the main span of the St. Croix River Crossing is comprised of 650 precast concrete bridge girder segments.
Properly placed
Upon reaching a strength of 8,000 psi, the segments were loaded by gantry cranes onto the barges. After traveling a short distance down the Mississippi River, the barges turned north up the St. Croix River to the project site for placement on the pier tables.
Once in position, each girder was hoisted off the barge using a segment-lifting system, aligned to within about a foot of the previously erected girder, and locked off in that position. Epoxy was then applied to the mating surfaces of both segments and pulled together. Twelve 1.75-in. PT bars were installed next and stressed to 175,000 lb each. Then, 25 strand cantilever PT tendons were installed, two per segment. The tendons anchor near the top of the segments and continue to its mate on the other side of the pier. Each tendon was stressed to a total force of approximately 1.1 million lb.
Subsequent segments were installed in both directions moving outwards from the pier to form the 600-ft spans between each pier location. After all segments were secured with cantilever tendons, a small cast-in-place mid-span segment was cast to connect adjacent piers. Then, continuity PT tendons were installed in the bottom of the segments and stressed to make the span continuous. Each extradosed stay cable contained 76 individual strands that were anchored to the exterior edges of the box girders and stressed to approximately 33,000 lb for a total force of approximately 2.5 million lb.
A standing tribute
After four years of construction, the new bridge opened to vehicle traffic on Aug. 2, 2017. The historic Stillwater Lift Bridge, located 2 miles north, closed shortly after the new bridge opened and is scheduled to reopen in two years as part of a pedestrian-bicycle loop trail between the two bridges.
MnDOT and WisDOT worked hard to minimize the bridge’s negative effect on the environment while making the most of its usefulness as a transportation connector between Minnesota and Wisconsin. The result is a beautiful bridge that fits well into the river gorge, is built to last a century or more, and is anticipated to be a boon to economic development in the region.
“The new St. Croix River Crossing will provide significant benefits to those who live in, work in and visit this beautiful river valley,” said Zoller. “This highly aesthetic structure will meet the needs of our local transportation system long into the next century while mitigating environmental impact. It will be a tribute to the tremendous collaboration and contribution of the many stakeholders involved.”
About The Author: Bintzler is a technical services manager for Aggregate Industries.
