The tragedy of modern concrete is perhaps surprisingly rusty.
Most concrete structures have steel reinforcement woven to add strength, but concrete can fail prematurely if the metal corrodes. Bridges exposed to water and salt are some of the most vulnerable. About a third of the US bridges will need to be repaired or replaced, which could cost around $400 billion over the next decade.
From coating the reinforcement bars with epoxy to pouring extra concrete to buy time before the permeate water reaches the reinforced bars, there are many ways engineers can deal with rust. Ultimately, these measurements also fail. The only way to truly prevent problematic rust is to use stainless steel reinforced bars, which is not cheap.
“It’s too expensive for use on all bridges,” Steven Jepeal, co-founder and CEO of Allium Engineering, told TechCrunch. Therefore, cities and states will only resort to it for the most important span.
However, alliums propose a kind of compromise by covering normal rebars with a thin layer of stainless steel to extend the intended lifespan of the bridge from 30 to 100 years.
“As long as the surface is fully covered, the thin layer is sufficient stainless steel to resist corrosion for hundreds or thousands of years,” says Samuel McAlpine, co-founder and CTO of Allium.
The startup’s stainless steel layer reinforcement was recently used in a replacement for a bridge deck on Interstate 91 in Massachusetts and US Expressway 101 in Mendocino County, California. He also contributed to the commercial boat yards in Key West, Florida, and Allium spoke exclusively to TechCrunch.
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For important bridges carrying a lot of traffic, engineers sometimes specify stainless steel, which takes about five times more than normal reinforcement. The government believes the additional costs are worthwhile if the main arteries are not needed to be closed.
However, most other bridges tend to specify epoxy-coated reinforcement. This is about 25% to 50% more than uncoated rebars. Epoxy coated reinforcing bars should be housed in covered storage, and weld spots or nicks within the coating must be patched, both adding an indirect cost.
Allium throws stainless steel-covered reinforcing bars as an alternative to epoxy coating. The company is aiming to match the price of the epoxy coating and weaken it in the future. Jepeal said installation should reduce costs as Allium rebars do not need to be treated carefully. Also, startup rebars do not require additional concrete, which is sometimes added to the bridge to rust.
“This extra layer of concrete is not structural. It is intended to isolate the reinforcement bars and slow the time it takes for salt to reach the reinforcement bar,” Jepiel said. Eliminates the use of cement that can be trimmed by 10%. Also, since rebars are not affected by corrosion, the transport sector should be able to specify the use of green cement, which tends to be less alkaline than standard mixtures.
Allium’s process covers a 7,000 pound steel billet with a layer of stainless steel and welds the wires outside until they are essentially fully coated. That billet is typically 6-8 inches square to 40 feet long and passes through a series of rollers until it reaches a range of approximately three-thirds to several inches in diameter.
“By covering a smaller surface area with a thicker layer and integrating it into the Mills process, it’s much cheaper, much more scalable, and much easier to control quality,” Jepeal says.
As the billets become thinner and grow up to 150 times longer in the process, stainless steel is done the same. Finally, each part of the reinforcement bar ends with approximately 0.2 mm stainless cladding.
Even with that small amount, “You’re basically not going to corrode that stainless steel with concrete,” McAlpine said.
