One of the successes of the I-35W replacement bridge in Minneapolis was the validation that our industry can build structures designed to last 100 years. With renewed interest in durable infrastructure, producers must provide effective solutions to well-documented durability problems. This quest will draw on us to use proven materials and techniques.
For the precast concrete industry, one bump in the road for departments of transportation accepting full-depth precast bridge deck panels has been the challenge of providing durable joints. They suffer great stress because of the constant flexing from vehicular loads. These areas also tend to be out in the open and more subject to corrosion.
In a presentation at the 2007 Concrete Bridge Conference, Vince Perry of Lafarge North America, Calgary, Alberta; P. Scalzo of Cook Engineering, Thunder Bay, Ont.; and Gary Weiss, Ministry of Transportation of Ontario, offered a more durable joint system. They described using ultra-high-performance concrete (UHPC) and glass fiber-reinforced plastic (GFRP) that may help extend joint life.
They reported on a Ministry of Transportation of Ontario (MOT) repair project from 2004. The highway bridge spanned the Canadian National Railway tracks at Rainy Lake, near Fort Frances, Ont. MOT chose to replace the cast-in-place deck with a precast concrete deck and approach slab panels.
The key design element focused on balancing a joint detail. Engineers wanted to provide deck continuity for loads, minimizing traffic disruption, speeding construction, and retaining long-term durability. They used field-cast UHPC to infill portions and to develop continuity in the deck panels. The design required no specific detail or preparation on the face of the precast panels.
Simplifying the process
According to Perry, this simplified design provided MOT with improved tolerances, reduced risk, cost savings, and a more durable joint. The combination of UHPC and GFRP enabled Cook Engineering to greatly simplify the precast panel fabrication and installation.
The design goal was to minimize shrinkage across the joint. Using GFRP bars and UHPC provided this result. And to minimize potential corrosion, Cooke engineers specified a GFRP rebar in the top mat.
Before construction, the UHPC joint fill was thoroughly tested. Along with minimal shrinkage, joint fill material had superior freeze/thaw resistance, extremely low porosity, higher than normal flexural strength, and superior toughness. Combined, these provided improved resistance to climatic conditions and continuous flexing from truck loads across the joints.
Engineers were so encouraged by the results, they also utilized the fill as a bedding substance for the precast slabs (flowing beneath the previously installed slabs). The fill provided a ½- to ¾-inch continuous UHPC interface between the bottom of the precast slab and steel support girder.
Perry says this project demonstrates that current technology used in new ways can help the concrete industry.