Cold-Weather Cast-in-Place Segmental Construction for Long-Span Bridges

An innovative heating and monitoring method was developed and used for wintertime casting of the Wabasha Street Bridge in Saint Paul, Minnesota. The bridge’s twin 384-m (1,260-ft) box-girder structures slope 5 percent from atop Saint Paul’s bluffs on the Mississippi River’s north side down to the lower portion of Saint Paul. Each box girder is composed of two 122-m-long (400-ft-long) center spans and two 70-m (230-ft) approach spans. The deck width of 14.54 m (47 ft 8 in.) contains two 3.66-m (12-ft) travel lanes, two shoulders of 0.92 m (3 ft) and 1.83 m (6 ft) with a 3.36-m (11-ft) sidewalk, and 1.11 m (3 ft 8 in.) to account for the barriers. The superstructure consists of 4.88-m (16-ft) typical-length segments that are 6.10 m (20 ft) deep over the piers and 2.44 m (8 ft) deep at midspan and the abutments. The bridge was constructed in balanced cantilever fashion with form travelers. The contractor, the local concrete supplier, the city, and the Minnesota Department of Transportation worked together to develop an innovative mix that would withstand the frigid temperatures and also achieve 24 115 kPa (3,500 lb/in.²) compressive strength in less than 24 h to allow the stressing of the post-tensioning. To insulate and protect the curing concrete, reinforced plastic enclosures surrounding the form travelers housed three 316 761-kJ (300,000-Btu) propane heaters. A layer of plastic and a double layer of insulating blankets covered the top slab. Thermocouples in the segments provided temperature readings, which the contractor used to monitor the effectiveness of the cold-weather procedures. The forms, reinforcing steel, and previous concrete were heated above 10°C (50°F) by using plastic enclosures, propane heaters, and insulating blankets. The concrete arrived from the batch plant at approximately 21°C (70°F) and was still above 13°C (55°F) when it was pumped into the segments. Multiple thermocouples indicated that the top slab cured above 38°C (100°F) for several days, whereas the bottom slab and webs were about 11°C warmer. The contractor ran the propane heaters for 5 days after each pour or until the segments reached a 28-day strength of 41,340 kPa (6,000 lb/in.²). The segments reached the required 24 115 kPa (3,500 lb/in.²) strength for post-tensioning on the day after each pour, including pours made on days as cold as −28°C (−19°F). Only 3 working days were lost because of the cold, and the bridge was completed on time. The method of heating and protection used at the Wabasha Street Bridge proved that the segmental cast-in-place construction method is a viable option in cold-weather climates on major long-span bridges.