Paper-Thin Steel Reinvents Bridge Construction
▼ Summary
– Rust in steel rebar causes premature concrete failure, with bridges being particularly vulnerable due to water and salt exposure.
– About one third of U.S. bridges require repair or replacement, estimated to cost nearly $400 billion over the next decade.
– Stainless steel rebar prevents rust but is expensive, so it’s typically reserved for critical bridges, while epoxy-coated rebar is a cheaper alternative with handling drawbacks.
– Allium Engineering offers a cost-effective solution by cladding regular rebar with a thin layer of stainless steel, extending bridge life from 30 to 100 years.
– This stainless-clad rebar reduces the need for extra concrete and allows for greener cement use, potentially matching or undercutting epoxy-coated rebar costs.
Rust poses the single greatest threat to modern concrete infrastructure, particularly for bridges exposed to water and road salt. This corrosion affects the steel rebar embedded within concrete for reinforcement, leading to premature structural failure. Across the United States, approximately one third of all bridges require either significant repair or complete replacement, representing a projected expenditure approaching $400 billion within the next ten years.
Engineers employ various strategies to combat this issue. These include applying epoxy coatings to the rebar or adding extra layers of concrete to delay the moment when moisture reaches the steel. However, these are temporary solutions that ultimately fail. The only definitive method for preventing rust involves using solid stainless steel rebar, but its high cost, roughly five times that of standard rebar, makes it impractical for widespread use.
A new approach from Allium Engineering offers a potential middle ground. The company has developed a composite rebar that features a core of conventional steel enveloped by a thin, protective layer of stainless steel. This design aims to dramatically extend the functional lifespan of a bridge from a typical 30 years to an impressive 100 years or more.
According to Samuel McAlpine, co-founder and CTO of Allium, complete surface coverage is the key. A thin layer is enough stainless steel to where it’s going to resist corrosion for hundreds or thousands of years, he explained, emphasizing that even a minimal amount of the corrosion-resistant metal provides long-term protection when fully encapsulating the rebar.
This innovative material has already been deployed in several infrastructure projects. It was used for bridge deck replacements on Interstate 91 in Massachusetts and U.S. Highway 101 in California, and it also contributed to a commercial boat yard construction in Key West, Florida.
For heavily trafficked, critical bridges, the substantial cost of solid stainless steel rebar is sometimes justified to avoid the massive disruption of closing a major transportation route. For other structures, the more common choice is epoxy-coated rebar, which carries a 25% to 50% price premium over uncoated steel. Yet this option introduces its own complexities, requiring covered storage and careful handling to prevent damage to the coating, which then needs patching.
Allium positions its stainless-clad rebar as a direct and superior alternative to the epoxy-coated variety. Steven Jepeal, the company’s CEO, stated that their goal is to price the product competitively with epoxy-coated rebar, with the potential to undercut it in the future. He noted that installation costs should be lower because the clad rebar does not demand the same delicate handling. Furthermore, its enhanced corrosion resistance can eliminate the need for an extra, non-structural layer of concrete often added to shield rebar from salt.
Jepeal pointed out that this extra concrete serves no structural purpose; it merely acts as a barrier. By removing it, cement usage on a project could be reduced by about 10%. McAlpine added that the rebar’s superior rust resistance could also allow transportation departments to specify more environmentally friendly, lower-alkaline cement mixes that are typically less compatible with standard, corrosion-prone rebar.
The manufacturing process begins with a 7,000-pound steel billet. Wires of stainless steel are welded to its exterior until the surface is entirely covered. This large billet, measuring six to eight inches square and 40 feet long, is then progressively rolled and stretched. It elongates up to 150 times its original length, with the stainless steel cladding thinning proportionally. The final product is rebar with a consistent stainless steel layer approximately 0.2 millimeters thick.
McAlpine asserts that this minute thickness is more than sufficient. You’re not going to corrode through that stainless steel in concrete, basically, he said, confirming that the thin cladding provides a durable, permanent defense against the elements, promising a new era of longevity for vital infrastructure.
(Source: TechCrunch)