Tarif M. Jaber, Jaber Engineering Consulting, Inc. and Frank J. Nelson, Oregon Department of Transportation
Imagine designing bridges with an expected service life of 100 to 120 years. That is what the Oregon Department of Transportation (ORDOT) is considering as it looks towards the future. Those who plan, design, and build bridges and highways think of 40, 50, or maybe 75 years for the expected service life of structures. But why not consider longer time frames?
Investing in the Future
ORDOT is undertaking a bold investment in their future highway infrastructure. Using a time horizon of 100 plus years, a design must pass two tests – it must be economically justifiable and technically feasible. Building a bridge for 40 years and then coming back and building a second bridge for another 40 years, and a third bridge for 40 years, involves the cost of building that bridge three times, interruption in service, public safety, and the maintenance costs. Therefore, it makes good engineering and financial sense to build the bridge one time for a life of 120 years, by investing the money up front.
Durability
The key technical challenges to extending the life of bridges are corrosion resistance, freeze-thaw durability, and surface abrasion. Most of Oregon’s highways have two-lane bridges, and the climate requires extensive use of ice melting chemicals. In addition, ORDOT highways receive significant studded tire traffic during the snow season.
Silica fume concrete was found to have the best resistance to abrasion. It also offers reduced concrete permeability and improved corrosion protection for reinforcement. ORDOT believes, based on the number of bridges that they have built and the comfort level that the contractors have working with silica fume concrete, that there is no real additional cost in adding silica fume to the concrete. In fact, all ORDOT bridge deck concretes have silica fume at 3 to 4 percent of the total cementitious materials. For bridges near the ocean, silica fume is used in the concrete of all bridge components.
Corrosion Protection
ORDOT decided to look at an engineered approach for corrosion protection and to face the corrosion problem head-on. Two alternative approaches are used:
- Completely mitigate the corrosion issue by using non-corrosive steel reinforcement and a very low permeability concrete
- Use regular uncoated reinforcing steel and introduce a cathodic protection system when corrosion starts
For complete mitigation of corrosion, ORDOT uses silica fume concrete to reduce chloride penetration and solid stainless steel reinforcement to prevent corrosion. Corrosion takes place at the steel. With one mat of stainless steel at the top and one at the bottom of the deck, the stainless steel reinforcement will not corrode, even if the concrete becomes contaminated with chlorides. For bridges near the ocean, stainless steel is used in both the deck and the concrete girders. For bridges further inland, stainless steel is used only in the decks.
The cost of using stainless steel reinforcement in bridges ranges from 10 to 15 percent of the total cost of the bridge depending on whether the steel is used in the girders as well as the deck. But that cost yields a major jump from a 40-year service life of the bridge all the way to 120 years and beyond because stainless steel is never going to corrode, regardless of how much chloride is in the concrete.
ORDOT’s priority is return on investment, public safety, and reduced maintenance. It realizes the need to invest more money up front to get a longer service life. The combination of silica fume and stainless steel reinforcing bars gives tremendous long-term economic advantage.