Detecting rusted metal encased in concrete structures

We wrote the other day (13 January 2009 HS Daily Wire) that when experts discuss the state of U.S. infrastructure, they all appear to be choosing from a rather narrow menu of adjectives. These adjectives are “aging,” “dilapidated,” and the favorite “crumbling” (see HS Daily Wire’s three-part series, “The Crisis of U.S. Infrastructure”: Pt. 1, 10 July 2008; Pt. 2, 14 July 2008; Pt. 3, 15 July 2008). One of the major reasons for the sorry state of America’s infrastructure is rust. There is hope: Rusted metal in concrete structures can be detected through new technology that uses electromagnetic fields to measure corrosion through non-ferrous material. This technology is now being tested in the United Kingdom.

The non-destructive technology from SciSite, a spin-out company from Keele University, is being used to survey stretches of motorway such as the Hammersmith Flyover in west London and car parks across the country. The equipment includes a probe with a passive 3D field sensor and a device that emits a proprietary combination of electromagnetic signals to energize the rust and steel in a concrete structure. The Engineer reports that in the technology’s current form, the probe is pulled along a surface in a frame like a golf trolley to gain an initial scan of the steel structure. The energizing device trails behind the probe. The probe is then sent along the surface path again to take another scan. “We compare the before-energized scan with the after-energized scan and analyze it to find anomalies between the two,” said Matthew Hocking, SciSite operations director.

If the probe detects an anomaly such as corrosion or a break in the steel structure, a rise in signal amplitude is displayed on a screen. If the structure is normal, the signal remains flat. The probe also contains a data logger and its information can be uploaded to a computer to give users a more detailed view of the structure, with color graphs.

The scanning process is fast and it can be done through thick layers of tarmac, concrete, or any non-ferrous material. The probe, which is a few centimeters in diameter, fits into expansion gaps and many inaccessible structures and can give a resolution of a few millimeters.

The technology needs to be pulled manually along a surface but the company was recently asked by Transport for London to incorporate the probe sensors and energizer device into a vehicle that could be driven over a stretch of road. The vehicle would have a line of 20 3D field sensors at the front, a large energizer device in the middle and 20 more 3D field sensors at the back. Hocking said although this a good idea, the sensors are expensive and it would require a lot of funding.

The SciSite equipment is relatively inexpensive to hire. Hocking said its upfront cost is the same as any other test kit but claimed the total expense is much less than other tests that require sections of the motorway to be closed off while the tarmac and under layers are ripped off. “It costs £10,000 a week in traffic management to close the lanes of a motorway,” he said, adding that weeks could be knocked off the timescale of a road works project if operators knew exactly which areas needed work and which did not. Hocking admitted that despite these advantages, some aspects of the technology require improvement.

One of the limitations of the technique, he said, is that users often have to infer the cause of an abnormal signal reading when scanning a structure. So his team is working on a second, add-on technique that will give users more information. The new technique, Hocking said, is based on a ground penetrating radar system that uses ferromagnetic resonance. “It would be used for the same purpose of detecting corrosion but it is slightly different because with radar, you can get the exact location of structural information,” he said. “An expert user can spot steel bars and whether there is delamination in the structure.”

SciSite has global patents on this new technique, which will be launched and added onto the current technology next year. The development will be significant, Hocking said, and will allow his company’s technology to be used in areas other than construction. He said radar has the potential to detect corrosion non-destructively at a distance — something that would be useful in steel production. SciSite is now testing this technique with a leading international steel manufacturer. “The radar could monitor the scale thickness on the outside of a billet of steel as it’s going through a reheat furnace, which nobody can do at the moment,” he claimed.