The Department of Transportation and Department of Energy have clearly specified that all new bridges and steel structures should have a 100 year design life, which is roughly 2-3X the current design life. Researchers are examining each aspect of typical overpass bridges to determine the needs and derive solutions for each area of the bridge, e.g. deck, beams, joints, concrete, steel structure. Corrosion protection needs to be approached from an integrated perspective – if the deck joints do not exist or do not leak, the corrosion protection required on steel girders beneath would change.
The majority of the steel bridges in the US interstate highway system were constructed between 1950 and 1980. Until the mid to late 1970's, virtually all steel bridges were protected from corrosion by three to five thin coats of lead and chromate-containing alkyd paints applied directly over mill scale adherent to the formed steel. These baseline conditions present major materials performance and environmental compliance challenges when maintaining steel bridge structures.
The natural aging of steel bridge structures coupled with harsh environments and exposure to roadway deicing chemicals (salts) has created a growing corrosion control maintenance burden on the nation's bridges. The presence of potentially hazardous materials in existing bridge paint has complicated maintenance processes and created dramatic cost increases for major and routine level bridge paint maintenance. Using the current methods for corrosion protection and after spending about $20 billion every year on maintenance, the bridges and steel structures would achieve only ‘Level D’, the bare minimum in terms of corrosion protection.
Each year the Federal Government and State departments of transportation (DOTs) spend billions of dollars on bridge rehabilitation and maintenance due to corrosion. On bridges, corrosion is most often caused when steel is exposed to atmospheric conditions, such as salt, moisture, and oxygen.
In the past, concrete bridges were built with black steel reinforcing bars and a thin layer of concrete over the top mat of the bars. In states where deicing salts have been used during icy winter storms, such a combination led to premature concrete damage in many concrete bridges, often as early as a few years after their construction. This soon led to the recognition by many that this costly problem is caused by the accumulation of a sufficient amount of chloride ions from the deicing salts in the concrete to initiate corrosion of the steel bars. With this knowledge, two important measures were adopted by state highway agencies in the late 1970s to early 1980s in building new concrete bridges. These measures, which included the use of epoxy-coated bars and the use of a thicker concrete cover over the bars, have led to delays in the onset of bar corrosion in many of the concrete bridges built since then so that the majority of these bridges will likely reach their 50-year design life. The recent adoption of new specifications for relatively less permeable concrete would further ensure that newer concrete bridges would last even longer, perhaps exceeding this design life.
As part of the Virginia Transportation Research Council’s effort to identify cost-effective, corrosion-resistant reinforcing bars that can be used in concrete bridges exposed to heavy salting, a 316L stainless steel-clad bar was tested in a new bridge deck under the Innovative Bridge Research and Construction Program sponsored by the Federal Highway Administration.
This field project was aimed at supplementing a laboratory evaluation that confirmed the excellent corrosion resistance of this potentially cost-effective material. The project revealed no significant technical problem with substituting this type of bar for the black steel and epoxy coated bars currently used. A life-cycle cost analysis indicated that, even though the initial cost of the clad bars is slightly higher, the long-term cost is lower, and the service life expectancy of the structure increases.
Stainless steel-clad bars can be used as direct substitutes for either uncoated black steel or epoxy-coated bars for effective, corrosion-resistant reinforcement of concrete bridge decks that will be exposed to deicing salts. The long-term costs of such structures will be less than those built with either black steel or epoxy-coated bars, which have lower initial costs. This advantage of clad bars becomes more attractive as the expected service life of the structures is raised. As soon as the supply becomes stable, stainless steel-clad bars should be considered an option for reinforcing new concrete bridges that will be part of any major urban and/or heavily traveled highways and will be exposed to heavy salting.
Source: Virginia Transportation Research Council (In Cooperation with the U.S. Department of Transportation, Federal Highway Administration) ‘TRIAL USE OF A STAINLESS STEEL-CLAD STEEL BAR IN A NEW CONCRETE BRIDGE DECK IN VIRGINIA’
Infrastructure Applications:
- Bridges
- Structural Steel
- Rebars