Pumping the I-35W Bridge in Record Time
A new standard for fast track construction was established when replacement of the Minneapolis I-35W bridge was begun after the 40-year old structural steel span collapsed into the Mississippi River on August 1, 2007. The tragedy spawned some great achievements including opening the new bridge only 13 months after the collapse. The project demonstrated the advantages of segmental concrete and the importance of concrete pumping to achieve the contractor’s goals. But the greatest contributions to the record pace of construction may have been the will of the contractors and suppliers to quickly replace a broken major artery (140,000 cars per day) with a safe, attractive bridge incorporating a 100-year design life.
“This was a corporate effort,” explained Mike Began, placing equipment foreman for Cemstone Products Company, the Twin Cities leading ready-mix producer and concrete pumper who won the contract for concrete and placement on the project. He added, “The Becken family (founders of Cemstone) took a personal commitment to get this job done and got all of the employees on board so that everyone in the company had a 24/7 attitude about this project.” In a testament to the versatility of concrete pumping and the imagination and experience of Cemstone’s crews, the project came in months ahead of schedule without the benefit of lengthy planning. Key to the early completion was that Cemstone used virtually every one of their 25-pump fleet on the project including two separate placing booms.
With a 504-ft. main span, the new I-35W bridge (also referred to as the St. Anthony Falls Bridge) combines cast-in-place and segmental concrete. Of four qualified bidders, Figg Bridge (designer) and Flatiron-Manson Joint Venture (general contractor) submitted the lone concrete proposal against steel girder alternates. Minnesota’s freeze-thaw cycle and extensive use of corrosive’s for deicing caused MnDOT to choose the concrete based structure based on projected maintenance as part of a value engineering determination despite the fact that the winning proposal represented the highest bid ($234 million). Cemstone supplied ready mix and pumping for foundations, piers, decks and segmental sections, which were pumped at a temporary casting yard on-site.
Because of the catastrophic collapse of the previous structure where thirteen people died and 145 were injured, extraordinary safety considerations were in place for the replacement effort. “There was such close scrutiny of the project that Flatiron put extra emphasis on safety. Cemstone took on the job of conducting the mandated training of our people by renting classroom space for nearly 300 workers. Everyone that could be involved in the project from pump operators to mixer drivers to mechanics underwent training in case they were called upon to work on the bridge, “Began explained.
Cemstone’s pumping division divided the 1,216-foot long project into three parts. The North approach, the South approach and the casting yard where 178 segmental sections would be pumped. Construction of the approach on the south side of the bridge posed difficult access because the casting yard was also located in this area on a section of the Interstate that was closed during bridge construction. By the end of October 2007, the demolition operation and removal of the old span was substantially complete, enabling construction to begin on the new I-35W bridge on November 1, 2007.
The new bridge is constructed without support in the river to avoid scouring, to extend the design life, allow for barge traffic and speed construction. Drilling and filling of the caissons for the abutments began on opposite shores of the river. Encounters with artesian wells complicated the process and steel casings were left in the holes to form the self consolidating concrete(SCC). The 11-foot diameter caissons were drilled to a depth of 110-feet. A Schwing concrete pump with 34-meter boom pumped the grout around the outside of the casings to stabilize the forming prior to pumping the interiors using a crane to support 60-feet of three-inch pipeline that was lowered into the void.
“This was the first use of SCC on a major transportation project in the U.S., according to Kevin MacDonald, vice-president of engineering for Cemstone and a nationally known figure recognized for his work with concrete mixes. MacDonald had developed a viscosity-based mix design model for another Cemstone bridge project that allowed him to plug in the variables for the I-35W bridge in order to satisfy MnDot’s concerns. “It is a fairly complicated model that deals with plastic concrete including average paste thickness and inter-particle spacing. We worked with Schwing engineers in the U.S. and Germany regarding horsepower, flow rate and the effect of friction in the pipeline so that we now know what it takes to pump a mix a certain distance at a certain slump,” according to MacDonald.
MnDOT required the use of granite aggregates for the entire project. Cemstone’s batch plant conveniently located three miles upriver from the bridge site scrambled to build up the needed stockpiles to be included in the various mixes required, including the SCC for the drilled shafts with a 28-inch spread and a strength requirement of 5,000 psi within 56 days. Footings which rest on the drilled shafts are 13 to 16-feet thick and were pumped with a 5500 psi mix design. Concrete for the deck contained 750 pounds of cementious material per yard and achieved 6500 psi in the first seven days. “Because of the mobility of the pumps, there were no long placements,” noted MacDonald, “But the concrete had to achieve the same settings and flow in temperatures ranging from minus forty degrees to 100 degrees which made it interesting.”
Casting of the box girder segments became a production line that worked steadily for a full year in all weather conditions. The castings are13½ to 16½ feet long, 43- feet wide, and between 11 and 24 feet in depth, depending on their location on the span. The heaviest of the 120 box-girder segments for the main span is approximately 190 tons and the lightest is 105 tons. A heated enclosure was pulled up and down the production line to protect the work in progress. The shelter had a door in the roof to allow the 39-meter boom pump operator to fill the bottom of the form first and then the wall forms on either side to form a U. The boom’s Roll and Fold configuration with 238 degree tip articulation proved invaluable when booming through the top of the temporary enclosure according to Cemstone placing personnel. The deck mix was placed last forming a box that was match cast with the other boxes to ensure perfect alignment. Three different mixes were required for each box which required close communication with the dispatchers.
Cemstone used Schwing boom pumps ranging from a 17-meter to a 61-meter on the project. “The mobility of the 17-meter really paid off when we were working in tight conditions down by the river filling gabions with grout for erosion control. We used the 39-meter on most of the pre-casting. The 61-meter ran for 30 hours straight one time because we were also pumping the new Twins stadium and it was needed over there,” Began reported, “The S 61 SX’s mobility, fast set-up and faster clean-up with less water really helped our utilization. The Overhead Roll and Fold boom combined with Super X outriggers allow the 61SX to set up in difficult situations. We set up close to the pour areas and were able to execute multiple pours each day. And the reliability was outstanding. We never delayed the project because of a pump and easily pumped all five mix designs on the project. Having the versatility of boom types to reach in and around the piers and the tight footprint of these pumps made it all possible.”
“Failure was not an option on this project, “said John Dickey, Cemstone’s vice-president of operations, “And there was no compromise on quality, period. Every load of ready-mix was tested.” Dickey was responsible for coordinating truck mixer drivers, mechanics and pump operators. In order to work under all temperature conditions, the Schwing pumps used synthetic motor and hydraulic oil with insulated pipeline. “We never failed to complete a pour,” Dickey noted, “And there were 65,000 yards delivered and pumped under some tough conditions. The project went seamlessly, but it was tiring.”
As the approaches moved out and up towards the river crossing, production goals would have been compromised with boom pumps from ground level because of the height and 10-lane width of the cast-in-place superstructure. “I’m a separate placing boom kind of a guy, “explained Began, “And that appeared to be the only way to do it.” A veteran of more than 100 separate placing boom set-ups, Began chose to mount his Schwing 39-meter detachable booms – one on either side of the river – on 110 foot tall, free standing lattice towers. Schwing fabricated the adapters to mount the booms to the towers.The new 35W span is actually two, five-lane bridges next to each other and the lattice towers fit in the space between the side-by side approaches. The 114-foot reach of the separate placing booms covered the required distances easily. The truck-mounted pumps (with booms removed) fed the concrete into five-inch pipeline encased in a surge block on ground level. Cemstone’s detachable booms are outfitted with 2023-5 pump kits, which provide higher pressure without sacrificing output. The pumps are rated at 208 cubic yards per hour. The pipeline was bolted to the lattice tower and hooked into the boom. Hydraulic powerpacks allow the four-section Roll and Fold booms to operate through their working range and rotate 550-degrees. Four towers were erected on each side and the booms with four-pin disconnects were flown to placing sites by tower crane.
As the booms finished the placement of floors, walls and decks of the approaches, the pre-cast box girder segments were transported to a location beside the river. Barges were used for the short haul to the bridge. A barge mounted 650-ton ringer crane was used to lift and align the individual sections and move them into position. After a segment was moved into its final position, it was secured and the crane could disconnect and move to the next lifting position. Once underway, four segments were placed in a day. Workers applied epoxy resin to the match-cast faces of the segments to ensure a waterproof connection. After a segment was moved into its final position, it was secured by a series of post-tension bars. At that point the crane could disconnect and move to the next lifting position.
From an adjacent bridge, spectators watched the year-long reconstruction of the bridge and when the last pre-cast section was put in place, there was cause for celebration. Even though site work remained to be completed, including paving of the bridge deck, the broken connection across the Mississippi was now reunited.
For Cemstone, the end of I-35W saga was anti-climatic as their other projects continued. The company had overcome an unexpected burden on their people and equipment while not missing a beat with their normal workload. As dignitaries from MnDOT and the Federal Highway Administration cut the ribbons and dedicated the new structure, it was business as usual for the Cemstone team. As Mike Began observes, “We did what some people said couldn’t be done.”
Project: I-35W Bridge (St. Anthony Falls Bridge), Minneapolis, MN
Owner: Minnesota Department of Transportation
Designer: FIGG, Tallahassee, Florida
General Contractor: Flatiron-Manson Joint Venture
Concrete Pumping and Ready-Mix Contractor: Cemstone Products Company, Mendota Heights, Minnesota
Equipment: Schwing S 17 X, S 31 EZ, KVM 32 XL, KVM 34X, KVM 39 X with detachable placing boom, S 47 SX, KVM 55, S 61 SX