New concrete corrosion sensors spot danger signs

Scientists at Queen’s University Belfast have made a major breakthrough in developing sensors that dramatically improve the ability to spot early warning signs of corrosion in concrete.

The sensors are more resilient and much longer lasting than traditional corrosion sensors. Scientists say they will make monitoring the safety of structures such as bridges and vital coastal defences much more effective. The research was carried out over four years in conjunction with researchers at City University London, and was funded by the Engineering and Physical Sciences Research Council.

Dr Su Taylor from the school of planning, architecture and civil engineering at Queen’s said because the sensors could withstand long-term placement within concrete, unlike any equivalent sensors currently available, they could constantly monitor conditions, enabling a warning to be sent when conditions for corrosion threshold had been crossed.

An internet connection meant the notification could be sent as an email or text to the structure’s maintenance team.

“There is a trio of novel, robust probes at the heart of the team’s work: one that monitors temperature, one for humidity while the other senses chloride and pH levels. Changes in these factors indicate the onset of the potentially destructive corrosion,” Taylor said.

“Within the probes are advanced optical sensors specifically designed and built for this project. These have been patented for potential commercial exploitation.”

Tong Sun, Professor of Sensor Engineering at City University and principal investigator, said the researchers’ design meant several probes could be installed semi-permanently in a structure and then connected to a computer data logger, which would constantly collect readings.

“This can be left until the readings indicate conditions have changed enough to warrant a full investigation. Remedial work will be simpler, cheaper and more effective at this stage, rather than waiting until there is visible damage, such as parts of the concrete coming away,” Sun said.

Traditional optical corrosion sensors have only a limited lifetime, usually of several weeks, because of the corrosive alkaline levels within concrete. The new sensors are expected to last for several years, with proper protection, even where pH levels are high.

The integrity of marine structures can face serious risks from a combination of chemical, biological and physical actions presented by the sea. The cost to the owners in the event of a failure can amount to millions of pounds annually for repair or replacement and for removing any sections that have fallen into surrounding water.

In structures such as bridges it is the steel rebar embedded in the concrete that is at risk of corrosion: most at risk are the tidal and splash zones, which experience continual wetting and drying. Seawater contains a cocktail of dissolved inorganic material of which the chloride ion plays a major role in the corrosion of marine structures. This applies to the air above water as well as in the splash zone and below the surface.

The sensors within the temperature and humidity probe use a fibre Bragg grating, a type of reflector made of a length of optical fibre that only reflects specific wavelengths of light while transmitting all others.

This is created usually by ultraviolet laser treatment, which alters the fibre core’s refractive index. A pair of gratings, with one being coated with a polymer layer, was used to separate the temperature and humidity parameters.