When the I-35W Mississippi River Bridge in Minneapolis collapsed in August 2007, very few could have imagined that a new bridge would be open to traffic just 13 short months later. Flatiron-Manson, Joint Venture, took charge and used many time-saving techniques to finish bridge construction in September 2008, three months ahead of schedule. Among the tools used were four E3000 Ground Heater units from Wacker Neuson to cure concrete during the cold winter months.

The tragic I-35 Bridge collapse meant that much of the initial construction would take place during the frigid Minnesota winter. The bridge offers direct access to Downtown Minneapolis for more than 140,000 vehicles per day. With MN/DOT’s estimated road-user cost set at $400,000 for every day the bridge is unavailable, an aggressive schedule had to be established. Groundbreaking for the new bridge was October 15, 2007 with a completion date of December 24, 2008.

MN/DOT accelerated delivery of the new bridge by employing a design-build procurement process. This allowed Flatiron and Manson’s crews to begin working on the $234 million bridge before all of the design details were final.

The two side by side bridges of the new I-35 Mississippi River Bridge carry five, 12-ft lanes of traffic in each direction, two more lanes in each direction than the previous bridge. The new bridges feature a two-box-girder design, offering an estimated 100-year life. Precast concrete segments used to construct the 504-ft mainspan over the river were poured off site concurrently with bridge substructure work to minimize total construction time.

Winter Cure
The accelerated schedule meant pouring most of the concrete footings and 70-ft tall piers would take place in the heart of winter. Daytime highs rarely rose above freezing and nighttime lows often fell below zero, making it impossible to cure concrete without an alternate heating source. “Ideally a contractor would wait until spring, but there was a lot of money on the line,” says Brad Backlin, outside sales representative for United Rentals.

Flatiron-Mason, JV’s solution: four E3000 Ground Heater units to aid in concrete curing. “They had the units on rent for about four months,” adds Backlin. The crews used the heat from the hydronic hoses to keep the forms at optimum curing temperature.

As the contractor poured the footings and pier segments, the formwork was wrapped in plywood. Bent nails on the outside of the plywood held the E3000’s hoses in place (about one foot apart) during the curing process. After the hoses were positioned, the crews covered everything with insulating blankets.

The mobile E3000 hydronic heating system features 385,000 BTU/hr gross input and delivers 83% heater efficiency, the highest in the industry. One positive displacement pump per hose loop delivers more BTUs/hr and allows Heat Transfer Fluid to be pumped up to 250 ft from the base unit. No other systems are capable of providing flexibility like the E3000 did while heating the piers.

A digital temperature controller with pushbutton adjustment affords quick temperature changes in the Heat Transfer Fluid. “Optimum concrete curing temperature is 68° F, so you need to adjust the outflow temperature higher or lower based on ambient temperature in order to have the concrete maintain its 68° F curing temperature,” says John Schemmel, metro jobsite specialist for Wacker Neuson.

The E3000s did their job and helped to properly cure the concrete during the cold winter months. They were just some of the innovative tools that allowed Flatiron-Manson, JV, to finish the bridge project more than three months ahead of schedule, allowing the contractor to realize a bonus of more than $17 million.