According to the Neil Young album title, rust never sleeps. In construction, rust damage can be insidious – especially in infrastructure like concrete bridges where rust can have fatal consequences if the steel in bridges fails. But detecting rust before it’s too late has been an ongoing challenge for engineers and scientists. Experts at the Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg, Germany, have developed an early-warning system for rust. By installing sensor-transponders into in the concrete to measure the extent of corrosion, engineers are being given a vital heads-up.

Concrete bridges, in particular, are subjected to a tough life. They have to be strong enough to withstand frosts, extreme heat, heavy traffic and emissions, which all take their toll on these structures. Then there's various types of road salt used in winter to combat icy roads which are not steel-friendly.

On Germany’s roads, when the salty ice thaws it breaks down into ionic components that penetrate the concrete’s 5cm thick protective alkaline layer. Then, any salt that leaches through to the steel rods used to reinforce the concrete will cause them to rust, causing structural damage and weakening the bridge. Cracks can appear, which can lead to a bridge collapse.

It may seem a little primitive, but until now the most effective tests to determine how deep the ions have penetrated the concrete and what damage they have caused is conducted by construction workers hammering on the reinforced concrete in search of cavities, which are conclusive signs of corrosion damage.

But Fraunhofer Institute experts say the new sensor-transponder can continuously measure and monitor how deep the ions have penetrated the concrete.

The detection equipment is really a two-part operation, comprising a sensor and wireless transponder. The sensor is a criss-cross of very fine iron wires, laid down at even distances inside the concrete.

“If the dissolved salts reach the iron wires, these begin to corrode and break. The number of defective iron wires is an indicator of the extent of corrosion and the depth to which the concrete's protective layer has been penetrated. This allows us to determine when the next repair work needs to be carried out,” says Frederic Meyer, a researcher at the IMS.

The transponder wirelessly transmits the data to a reading device carried by the construction workers – instead of their hammers.

“Our transponder does not get the energy it needs to measure the corrosion from a battery, but from a magnetic field. This means it does not need to be replaced and can remain within the concrete structure permanently,” says Meyer.

A test bridge has been constructed for the purpose of measuring the success of the sensor-transponder and field tests are currently being conducted.