The Development Would Save Billions Spent in Structural Repair
by Robert Sanders
Berkeley scientists have developed a rust-resistant steel that could save the nation billions of dollars now spent to repair or replace concrete bridges and buildings rendered unsafe because their internal skeletons have corroded.
The steel, trademarked Fermar, was designed specifically to resist corrosion when used as reinforcing bar--known as re-bar--in reinforced concrete. Made from scrap metal, re-bar is among the cheapest steels and is used in virtually every concrete structure in the world.
The process to make Fermar, which was invented and patented by materials scientists here, differs only slightly from that used by American re-bar manufacturers today, yet produces a steel that has remarkable corrosion resistance in concrete when compared to standard re-bar.
"Our research and testing are conclusive. This is no longer a laboratory curiosity," said Gareth Thomas, professor emeritus of materials science and mineral engineering and a research scientist at Lawrence Berkeley Laboratory.
"This steel has got it all--it has far superior corrosion properties, far superior fatigue properties and tensile properties equal to those of traditional re-bar. And it can be made without any major new expense on the part of steel companies."
Thomas and his Berkeley colleagues now are searching for a steel company willing to invest in a few simple alterations in its control of the manufacturing process to obtain enough Fermar re-bar for a full-scale test.
Thomas first patented the steel-making process in 1985, and now holds three patents. Only recently, however, have he and his colleagues shown that re-bar made from this steel is far superior to traditional re-bar in resisting rust when used in concrete. Thomas, Civil Engineering Professor Paulo Monteiro, and civil engineering graduate student David Trejo reported on a year-long corrosion study in the October 1994 issue of the Cement and Concrete Research Journal.
In their tests they submerged concrete reinforced with Fermar in a hot, moist, salty environment for one year. For comparison they also submerged concrete reinforced with traditional "billet" re-bar.
Slicing open the re-bar at the end of the tests, they found extensive rust in the billet re-bar but no evidence of rust in the Fermar re-bar.
"After one year it came out with no visible corrosion--clean as a whistle," said Thomas.
The implications of a rustproof re-bar are staggering, he said. Across the country bridges are being re-examined because in many the re-bar has rusted to the point where the bridges are unsafe.
In 1986, the Federal Highway Administration estimated that more than244,000 bridges in the U.S. were deficient or significantly deteriorating. Recently they estimated the replacement and repair costs at more than $51.4 billion.
The U.S. Navy alone spends $50 million each year repairing and replacing corroded reinforced concrete structures such as ocean piers, Thomas said.
The reaction by the steel industry to improve the durability of the steel has been to coat re-bar with epoxy resin or zinc to delay rusting. But a recent study by the Canadian Strategic Highway Research Program showed this to be relatively ineffective, delaying the process by several years at best.
"At up to two times the cost of uncoated re-bar, coating re-bar isn't worth it for only three extra years, when you design a structure for 50 to 100 years," Trejo said. "Our goal is to develop a steel so that corrosion doesn't occur in the first place."
The basic problem of billet steel is that it contains microscopic fingers of two different components, ferrite and carbide, that in the presence of an electrolyte like a salt solution become a tiny battery.
The minuscule electric currents produced by these "batteries" promote the conversion of solid iron or ferrite into rust.
Concrete is naturally porous so that salts seep in and, upon contacting the re-bar, start the corrosion process at the surface and work inward until the re-bar is eaten away entirely.
The problem is severe enough that richer countries such as Saudi Arabia are beginning to use very costly stainless steel reinforcing bars in concrete, though even stainless steel can eventually rust.
Thomas is the founder of the National Center for Electron Microscopy located at Lawrence Berkeley Laboratory.