Scientists at ETH Zurich say that concrete samples are too small to allow for a reliable assessment of the condition of reinforced concrete. That has led them to wonder if reinforced concrete bridges will be able to remain standing for years to come, or has corrosion already set in.

Reinforced concrete is vulnerable to potentially harmful environmental influences, such as CO₂ in the atmosphere and de-icing salt. Over the years, chloride from the salt penetrates the concrete until it eventually reaches the steel reinforcement and the armoring iron begins to rust.

Corrosion in steel reinforcements: here “pitting” has occurred under the concrete. Credit: ETH Zürich/Ueli AngstReinforced steel structures are checked regularly to detect damage early and prevent corrosion, which is the destruction of steel by penetrating chlorides.

“Corrosion is responsible for up to 90% of damage to reinforced steel structures,” says Ueli Angst, professor at the Institute for Building Materials. Estimates from the U.S. suggest that the cost of corrosion could represent 3-5% of GDP, not including indirect costs, such as congestion or loss of production.

Along with visual appraisal and non-destructive examination, extracting concrete samples is another process, says ETH professor Bernhard Elsener. “The chloride concentration in the samples is calculated in the laboratory. If the sample exceeds the critical threshold of 0.4% 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.”

These samples are typically about 5 to 20 centimeters, which makes them a practical size for handling in the laboratory. However, a recent study by the two ETH professors shows that conclusions drawn from these samples often may be 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,” says Angst. Only by anayzing a larger specimen, say one meter long, can a realistic assessment of the condition occur, he says.

This is difficult for practical reasons, so the two ETH materials experts have developed a mathematical formula that allows conversion of the critical threshold in a certain specimen to any other size.

The results of the ETH study do not just apply to laboratory examination of concrete samples, the researchers say. Their findings are also relevant for use of sensors that can be built into reinforced concrete structures to monitor corrosion. These sensors are usually small and have tended to provide overly optimistic data. For more precise forecasts, sensors of a greater size or number are in order.

The scientists say that the only way to prevent corrosion damage entirely is to switch to high-alloy steel, which can be costly.

“This costs about 10 times more than normal reinforcing steel,” says Elsener, “yet when you look at the subsequent costs of regular inspections and repairs it could work out cheaper in the long run.”