The scourge of modern concrete is, perhaps in a surprising way, rust.
Most concrete structures are covered with steel reinforcement bars to add resistance, but if the metal corrodes, concrete can be broken prematurely. Bridges, exposed to water and salt, are among the most vulnerable. In the United States, around a third of bridges must be repaired or replaced, which could cost nearly $ 400 billion over the next decade.
There are many ways for engineers to fight rust, from the coating of the reinforcement bars with epoxy to the additional concrete flow to save time before the water that infiltrates the reinforcement bars. Finally, these measures also fail. The only way to really prevent problematic rust is to use stainless steel reinforcement bars, which are not cheap.
“It is too expensive to be used on each bridge,” said Steven Jepeal, co-founder and CEO of Allium Engineering. Cities and states will therefore only use it for the most critical periods.
But Allium offers a kind of compromise by covering the ordinary reinforcement bars with a thin layer of stainless steel to extend the scheduled lifespan of a bridge of 30 to 100 years.
“As long as we obtain a complete cover of the surface, a thin layer of stainless steel is enough to resist corrosion for hundreds or thousands of years,” said Samuel McAlpine, co-founder and technical director of Allium.
Startup stainless stainless steel reinforcement bars were recently used to replace the deck of a bridge over the US Highway 101 in the county of Mendocino, California, and another is planned for Interstate 91 in Massachusetts. He also contributed to a commercial shipyard in Key West, Florida, said Allium exclusively in Techcrunch.
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For highly frequented critical bridges, engineers sometimes choose stainless steel, which costs ordinary reinforcements about five times more than ordinary reinforcements. Governments believe that additional costs are worth it if they do not have to close a major artery.
But for most other bridges, they tend to specify reinforcement bars with epoxy coating, which represents only about 25 to 50 % more than reinforcement bars without coating. The reinforcement bars covered with epoxy must be stored in a covered warehouse, and all welded or notch points in the coating must be repaired, which adds additional indirect costs.
Allium offers its stainless steel reinforcement bars to replace epoxy coating reinforcement bars. The company aims to match the price of the epoxy coating and possibly reduce it in the future. Jepeal said that once installed, Allium’s reinforcement bars should cost less because they do not need to be manipulated with so much care. Startup’s reinforcement bars will also not need additional concrete, sometimes added to the bridges to prevent rust.
“This additional layer of concrete is not structural. It is simply intended to try to isolate the reinforcement bars and to delay the time necessary for salt to reach the reinforcement bars,” said Jepeal. Eliminating it could reduce cement consumption up to 20 %. And as the reinforcement bars are not as sensitive to corrosion, they should allow transport services to specify the use of more ecological cements, which tend to be less alkaline than standard mixtures, said McCalpine.
The allium process covers steel billets of 7,000 pounds with a layer of stainless steel, essentially welding threads outside until they are fully covered. This billet, which generally measures between six and eight square inches and 40 feet long, is then introduced into a series of rollers until it reaches the desired thickness, which varies from about a third of a few inches in diameter.
“By covering a smaller surface with a thicker layer and integrating it into the factory process, we can do something much cheaper, much more evolving and much easier to control quality,” said Jepeal.
As the billet is thinning, increasing up to 150 times longer during the process, stainless steel does the same. In the end, each piece of reinforcement bar is found with approximately 0.2 mm of stainless steel coating.
Even with this small quantity, “basically, you are not going to corrode the stainless steel present in the concrete,” said McCalpine.
