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Study: Rebar Corrosion Testing Needs a Revamp

Monday, August 7, 2017

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Switzerland’s newest concrete span may already be subject to corrosion under the surface—but, say researchers at ETH Zurich, it is hard to know for sure, because the size of concrete samples commonly taken for lab testing on such projects is simply too small.

The Taminabrücke, which opened in June, is the longest reinforced concrete arch bridge in Switzerland. Standing at over 1,500 feet long, and having taken four years to build, the span is intended to last multiple generations, bridging the gorge between Pfäfers and Valens. Like any such structure, it will be subject to regular monitoring to detect whether environmental factors might be speeding corrosion within the bridge structure.

Testing Methods

In addition to visual and nondestructive testing, laboratory testing is one method of determining to what extent chlorides from road salt—which can penetrate the concrete that such bridges are made of, and cause steel rebar to rust—are present in a given structure. But the ETH team studying corrosion in concrete reinforcement says the results of lab tests can be misleading. They've come up with a mathematic formula that they say could help to make lab testing more accurate, and could also be applied to sensors on the structures themselves that serve to detect corrosion.

“The chloride concentration in the samples is calculated in the laboratory,” ETH professor Bernhard Elsener notes. “If the sample exceeds the critical threshold of 0.4 percent relative to cement weight, not just near the surface but in the deeper levels, the assumption to date has been that corrosion could soon set in and that repairs were required.”

ETH Zurich

“Corrosion is responsible for up to 90 percent of damage to reinforced steel structures,” Ueli Angst, professor at the Institute for Building Materials, explains.

Typically, these samples are 5 to 20 centimeters, which allows for easy handling in the laboratory. But the downside is, according to ETH professors, the conclusions drawn from the studies of these samples are often incorrect.

“In our research project, we examined reinforced concrete specimens of various sizes and discovered that corrosive chloride concentration was far more apparent in smaller samples and subject to larger fluctuations than in larger specimens,” Ueli Angst, professor at the Institute for Building Materials, explains.

The professor notes that concrete is not a homogenous material, and that the area of effect of the corrosion can be directly accounted for by these differences.

“Only the analysis of a larger specimen, say a meter long, will allow a realistic assessment of the condition,” Angst says.

A New Formula

In order to address the complications presented by a sample size that large, ETH researchers have devised a mathematical formula that allows conversion of the critical threshold in a certain specimen to any other size. This replaces the fixed critical threshold of 0.4 percent, which had been used until now.

The results of the ETH study also extend to sensors, which are often built into reinforced concrete structures to monitor corrosion. The problem with these lies in their tendency to read data optimistically. For more precise forecasts, larger sensors—in both numbers and size—are needed.

The researchers believe that the only way to prevent corrosion in damage is to switch to high-alloy steel, which is 10 times more expensive than the traditional alternative.

Given the destructive nature of corrosion in these structures—"Corrosion is responsible for up to 90 percent of damage to reinforced steel structures,” Angst explains—it could very well be worth it.

“Yet when you look at the subsequent costs of regular inspections and repairs it could work out cheaper in the long run,” Elsener says.

   

Tagged categories: Chlorides; concrete; Corrosion protection; Corrosion resistance; Research; Research and development

Comment from Mario Colica, (8/7/2017, 4:35 AM)

Another proposal(less expensive and efficient) should be employing Zinc coated rebar for new building and using the Zn spray coating over the rebar danaged


Comment from Martin van Leeuwen, (8/7/2017, 11:55 AM)

For new rebar, hot dip galvanizing or continuous galvanizing are cheap processes to provide the required corrosion protection.


Comment from M. Halliwell, (8/8/2017, 12:10 PM)

I suspect a combination of galvanizing and field spray will be needed....after all, I have yet to see a job site where rebar wasn't cut in the field to make it fit, which exposes rebar below the treated surface.


Comment from BOB HEIDERSBACH, (8/9/2017, 12:36 PM)

The US learned years ago that galvanizing rebar doesn't solve the problem. Zinc corrodes in alkalis (bases) and wet concrete is a base. The researchers should use scanning electron microscopes with energy-dispersive X-ray spectrometers to ID where the salt has penetrated. The picture above shows that MOST of the rebar is unaffected--it is only where air-saturated salty water has penetrated that corrosion occurs. See Chris Haver's article in Materials Performance around 1989 for details.


Comment from Martin van Leeuwen, (8/10/2017, 4:28 AM)

Zinc corrosion is predominant at a pH>13. Zinc coatings are very stable in weaker alkali environments below this pH, with very low corrosion rates. The new NY bridge is built with galvanized rebar for a projected lifetime of 100 years. Cutting edges can be repaired in the field by either thermal spraying of zinc or zinc rich paints.


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