Perhaps the most surprising threat to modern concrete is rust. Most concrete structures are strengthened with steel rebar, but when this metal corrodes, it can cause the concrete to fail prematurely. Bridges are especially vulnerable because they are constantly exposed to water and road salt. In the United States, about one third of all bridges require repair or replacement, a project that could cost nearly four hundred billion dollars over the next ten years.
Engineers have several methods to fight rust. They can coat the rebar with epoxy or pour extra concrete to delay the time it takes for water to seep down to the steel. However, these solutions are temporary and will eventually fail. The only way to completely prevent problematic rust is to use stainless steel rebar, but this material is very expensive.
Because of the high cost, cities and states reserve stainless steel rebar for only the most critical bridges. A startup called Allium Engineering is proposing a compromise. Their solution involves covering regular rebar with a thin layer of stainless steel. This process can significantly extend the intended life of a bridge from 30 years to 100 years. A company representative explained that as long as the surface is fully covered, this thin layer is sufficient to resist corrosion for an extremely long time.
This stainless steel-clad rebar has already been used in several bridge projects, including replacements on Interstate 91 in Massachusetts and U.S. Highway 101 in California. The technology was also used in a commercial boat yard in Florida.
For high-traffic bridges, engineers sometimes specify solid stainless steel rebar, which costs about five times more than regular rebar. Governments accept this cost to avoid shutting down a major transportation route. For most other bridges, they typically use epoxy-coated rebar, which is only twenty-five to fifty percent more expensive. However, epoxy-coated rebar requires careful handling, covered storage, and any damage to the coating must be patched, adding indirect costs.
Allium is positioning its stainless-clad rebar as a direct replacement for the epoxy-coated type. The company aims to match the price of epoxy coating and potentially undercut it in the future. They state that their rebar should cost less to install because it does not require such delicate handling. Furthermore, using this rust-resistant rebar could eliminate the need for an extra, non-structural layer of concrete that is sometimes added to bridges solely to insulate the rebar and delay corrosion. Removing this layer could reduce cement use by ten percent.
The process begins with a large billet of steel that is clad in a layer of stainless steel. This billet is then fed through a series of rollers that stretch and thin it out into rebar of the desired diameter. During this stretching process, which can make the steel 150 times longer, the stainless steel cladding stretches as well. The final product has a very thin stainless steel layer, approximately 0.2 millimeters thick. Even this small amount is enough to prevent the rebar from corroding within concrete.