Whether tunnels or retaining walls - many Swiss reinforced concrete structures from the 1960s to the 1980s are at risk. Corrosion can make them unstable. ETH start-up Talpa Inspection has now developed a solution to localise corrosion more easily, even in hard-to-reach places.
In brief
- Many reinforced concrete structures have been built in Switzerland and Central Europe since the 1960s. Time leaves its mark: chemical changes in the concrete may result in steel corrosion.
- Researchers at ETH Zurich have now developed an innovative method for precisley locating corrosion, even in difficult-to-access structures such as retaining walls, tunnels and bridges.
- The method is based on electrochemical measurements. This makes it possible to determine the probability of corrosion without having to laboriously chisel into or partially destroy the concrete structures.
"Pull the cable back, please," Lukas Bircher says into his radio. The cable of his probe has become caught in the metal ladder in the four-metre-deep access shaft. Bircher waits; the radio crackles. His colleague Samuel Ballat, who is crouching 30 metres away in the next access shaft of the supporting wall and is holding the other end of the cable, has heard him. The cable jerks and Bircher can release it. Everything is ready for the measurement.
Danger: when the steel in the concrete corrodes
On this cold November morning, Bircher and his team are measuring whether the 200-metre-long retaining wall in Waidbadstrasse in Zurich-Höngg is still holding. More specifically, they are investigating the probability of corrosion in the embedded reinforcing steel, which is designed to make the structure more stable.
The retaining wall on Zurich's Käferberg holds back countless tonnes of earth. Above it are allotments, while a road with a bus stop is below. To ensure that the wall can withstand the earth pressure, the reinforcing steel must be firmly connected to the foundation. If corrosion damages the reinforcing steel, the wall could collapse in a worst-case scenario. To minimise such risks, mechanical engineer Bircher and his colleagues from the research group of Ueli Angst, ETH Professor of Durability of Engineering Materials, have developed an innovative method. This enables them to precisely locate corrosion in the reinforcing steel without having to laboriously chisel into or partially destroy the reinforced concrete wall.
"This has not been possible before," explains Bircher. To check whether the reinforcing steel on the back face of retaining walls was already showing signs of corrosion, in the past there was no way around having to remove the entire layer of concrete above it. "But it's still possible that two metres further on, most of the reinforcing steel has corroded," says Bircher. The new method that he and his colleagues have developed is different: it can be used to determine the probability of corrosion along the entire length of a retaining wall without destroying a single piece of it. This is because measurements are taken through the drainage pipes that run directly next to the critical section of the wall.
50 years after the construction boom - the rust is there
"Most of the infrastructure in Switzerland and Central Europe was built between 1960 and 1980," notes Bircher. Retaining walls, tunnels, bridges: all these reinforced concrete structures can corrode over time. Chemical changes occur in the concrete, and the reinforcing steel begins to corrode. In particular, retaining walls from the 1970s may contain cavities if the aggregates are not sufficiently surrounded by mortar, which might promote corrosion of the embedded steel.
The retaining wall on Zurich's Käferberg holds back countless tonnes of earth. Above it are allotments, while a road with a bus stop is below. To ensure that the wall can withstand the earth pressure, the reinforcing steel must be firmly connected to the foundation. If corrosion damages the reinforcing steel, the wall could collapse in a worst-case scenario. To minimise such risks, mechanical engineer Bircher and his colleagues from the research group of Ueli Angst, ETH Professor of Durability of Engineering Materials, have developed an innovative method. This enables them to precisely locate corrosion in the reinforcing steel without having to laboriously chisel into or partially destroy the reinforced concrete wall.
"This has not been possible before," explains Bircher. To check whether the reinforcing steel on the back face of retaining walls was already showing signs of corrosion, in the past there was no way around having to remove the entire layer of concrete above it. "But it's still possible that two metres further on, most of the reinforcing steel has corroded," says Bircher. The new method that he and his colleagues have developed is different: it can be used to determine the probability of corrosion along the entire length of a retaining wall without destroying a single piece of it. This is because measurements are taken through the drainage pipes that run directly next to the critical section of the wall.
50 years after the construction boom - the rust is there
"Most of the infrastructure in Switzerland and Central Europe was built between 1960 and 1980," notes Bircher. Retaining walls, tunnels, bridges: all these reinforced concrete structures can corrode over time. Chemical changes occur in the concrete, and the reinforcing steel begins to corrode. In particular, retaining walls from the 1970s may contain cavities if the aggregates are not sufficiently surrounded by mortar, which might promote corrosion of the embedded steel.
Today, about 50 years after the construction boom, this risk of corrosion is becoming critical. However, identifying the risk is challenging because the damage is unevenly distributed in a wall. Furthermore, it is located deep underground close to the foundations and at the back of the wall. It is therefore high time that a method is developed to inspect structures systematically, efficiently and cost-effectively for corrosion, even below the surface.
An important early warning system
Bircher's new method is based on what are known as electrochemical measurements. The probe that his team has developed for inspecting retaining walls consists of two inflatable sealing elements at the sides and electrodes in the middle. The probe is connected to the measuring devices via a customised cable featuring not only electrical connections but also pipes for pressurised air and water.
To take a measurement, Bircher and his colleagues pull the probe through the cable into the drainage pipe and inflate the sealing elements so that they fit snugly against the pipe walls. They then pump water into the sealed area. The water flows out through the holes in the drainage pipe and connects the electrodes in the probe to the ground. The moisture in the soil and concrete creates an electrolytic connection to the steel in the retaining wall. This forms a localised electrochemical measuring cell. "We use the measuring cell to record electrical signals that differ depending on whether the reinforcing steel is corroded or not. The electrochemical reactions that take place on the steel in the concrete produce recognisable electrical signals," explains Bircher.
Meanwhile, Bircher is sitting in a van in Waidbadstrasse in Höngg, starting the first measurement on his laptop. Red dots appear on the screen, forming a curve: these are the measured potentials, which will be evaluated later in the office. At Bircher's touch of a button, the seals vent again. "Please move forward 25 centimetres," Bircher instructs his colleague Ballat, who is still in the access shaft moving the cable. "Okay: position is 31.50," he replies. Bircher re-inflates the sealing elements and starts the next measurement.
The team measures the wall every 25 centimetres. This provides information about the condition of the reinforcing steel for the entire section of the wall; it's an early warning system for corrosion. If the team discovers corrosion in a section of the retaining wall, the affected area can be repaired specifically. "In Switzerland, we have over 1,000 kilometres of walls that are potentially only partially corroded and that need to last for another few decades. It is therefore important to identify those sections that pose a risk," explains Bircher.
What was missing: the brilliant idea
The Talpa team is testing its probe in Waidbadstrasse. This works, says Bircher, explaining that it has been tested in pilot projects. However, a lot of the measurement work is still done by hand. "We want to make the measurement more automated in the future and make the inspection probe more robust." The team is already in contact with potential customers and knows that its method is in demand.
What was needed was not an entirely new technology. Electrochemical measurements have been used for some time, but they are not practical for evaluating the front of retaining walls. "However, we realised that we could also determine the state of corrosion of the steel reinforcement in the concrete from the drainage pipes through the moist and conductive soil," says Bircher. All that was needed was a clever new way of getting a measuring cell into the pipes, along with a method of interpreting the measured values. Bircher and his colleagues have now achieved this. If the retaining wall were opened and examined using a random sampling method, it would require several days and a construction site. The Talpa team, in contrast, needs just under a day and does not have to rely on random hits, as would have been the case using the traditional, random sampling method.
The moles are setting up a start-up
Because their new method has proven successful, Lukas Bircher, together with materials engineer Federico Martinelli-Orlando and civil engineer Patrick Pfändler, is currently in the process of founding a start-up called Talpa Inspection. "Talpa" is the scientific name for moles. Bircher not only has the support of Ueli Angst, a professor at the Department of Civil, Environmental and Geomatic Engineering, but has also received an ETH Pioneer Fellowship for his innovative method. This will help him and his team to develop a marketable product and found their own company (see the Short portrait of Talpa Inspection at the Pioneer Fellows ).
