Going Long on the High Bridge
A local ready-mix firm is producing and pumping a fiber enriched concrete to replace the deck of the St. Paul, MN High Bridge. The bridge is also known as the Smith Avenue High Bridge and carries State Highway 149 across the Mississippi River. The re-decking has closed the bridge, which normally handles 13,900 cars per day, for a year. Cemstone, a diverse ready-mix producer and construction materials supplier, also has a placing service and operates the largest pumping fleet in the Upper Midwest with 25 boom pumps. The company is using its longest boom to handle the deck replacement with weekly pours. The company is a four-generation family owned business with more than 90 years of history.
The bridge design is an Inverted arch and two half-arches for the main span. There are eight plate girder spans on the north side. At 160-feet above the water, the High Bridge deck is closer to the banks on the north side than the south dictating two methods of placement. “We are pumping up from grade and into the hopper of a 600-foot conveyor system on the south side but are able to reach the deck and maneuver the end hose from ground level on the north side placements, “ according to Pat Gagnon, Cemstone placing foreman. The maneuverability of the pump’s placing boom is more efficient than the conveyors. The overall length of the two-lane bridge is 2,770-feet.
After a late start due to large snow totals in April, Cemstone chose their Schwing S 61 SX with 198-feet of vertical reach for all of the pumping chores on the project. “The four-section Overhead Roll and Fold boom provides our operators with good maneuverability going up and then out on the decks,” Gagnon says, “It helps to have the Super X outriggers which allow us to maximize reach by placing the pump close to the pours.” The unique design of the curved outriggers allows a compact footprint.
Weekly pours cover 160×60-foot sections that consume 250 to 280 cubic yards of concrete. ‘The contractor only requires a 60-70 yard per hour rate which is easy to meet with the S 61 SX,” Gagnon adds. The pump is equipped with a 2525H-6 120/85 MPS all-hydraulic pump kit with output to 213 cubic yards per hour. “Even at maximum reach, which is the norm on this project, the boom is nice and steady, which the placing crew appreciates, “ according to Gagnon. The S 61 SX features large diameter 10-inch pumping cylinders operating through a 98-inch stroke that contributes to smooth operation with fewer strokes per minute. Because the boom is feeding a rail mounted conveyor system, Cemstone positions their operator on the elevated deck. Schwing’s two-way Vector Controls allows the operator to monitor important information including strokes per minute and pumping pressure while maintaining his view of the tip hose from up on the deck.
Reducing cracking in the deck is a high priority for the Minnesota Department of Transportation (MNDOT). In the harsh Midwest winters, deicing chemicals are applied to roads and bridge decks to melt snow and ice to help maintain safe traffic operations. Bridges are particularly vulnerable to the use of deicing chemicals that may include chlorides that infiltrate the concrete
deck through cracks and degrade reinforcement within the concrete.
“MNDOT specified a macro-fiber reinforced mix to help control cracking,” according to Kevin Heindel, Senior Materials Engineer at Cemstone. The 4,000 psi bridge mix contains 5 pounds of polypropylene fiber, 1322 pounds of fine aggregate, 1676 pounds of coarse aggregate, and 30-percent fly ash. “The slump specified is two to five inches and we try to keep it as close to five-inches as possible for pumpability,” Heindel adds.
Plastic shrinkage cracking remains a primary concern for placements with high surface/volume ratios that are subjected to early age drying. “The drying of the High Bridge concrete is critical so the contractor has factored in the temperature of the mix, sun angle and humidity and is covering the curing concrete depending on the data,” according to Gagnon. “The high fiber mix has been pumping well,” says Gagnon, “Once we coax it through the hopper grate, the extended housing on the Big Rock valve helps feed it into the pump.”
Testing the concrete is critical in all cases but especially on MNDOT bridge projects. Cemstone has the largest group of certified plant and field technicians in the upper Midwest. They are certified annually by the Iowa, Minnesota and Wisconsin Departments of Transportation for concrete and aggregate testing. In addition, many of these technicians are also certified by the American Concrete Institute for field-testing. The company is servicing the High Bridge project from a St. Paul plant a few miles from the site.
Other certifications that Cemstone maintains are their pump operators, “The ACPA Certification is the only nationally recognized safety certification program that enables our operators to achieve the highest degree of professionalism and maintain the strictest standards for safety. ACPA Certification has become a critical standard for customers looking for a reliable, high quality pumping contractor,” Gagnon notes. The company also maintains their fleet of boom pumps with annual certified boom inspections.
True to its name, the High Bridge remains the highest bridge in St. Paul and was originally built in 1987 at a cost of $20million. The current Smith Avenue High Bridge project at $39million includes improving pedestrian and bicycle safety while extending the lifespan of the High Bridge and providing a smoother road surface. The pumping of the new bridge deck will continue through fall 2018.
Owner: State of Minnesota
General Contractor: Ames Construction, Burnsville, MN
Ready Mix Producer/Pumper: Cemstone, Mendota Heights, MN
Pumping Equipment: Schwing S 61 SX truck-mounted concrete pump with placing boom.