UK researchers develop sensors to improve protection of concrete coastal structures
More resilient and much longer lasting than traditional corrosion sensors they will make monitoring the safety of structures such as bridges and vital coastal defences much more effective. The carbon steel bars used to reinforce submerged concrete in tidal zone areas are at particular risk of corrosion caused by wet conditions.
The sensors can withstand long-term placement within concrete – unlike any equivalent sensors currently available – they can constantly monitor conditions, enabling a warning to be sent when conditions for corrosion threshold have been crossed.
Notifications can be sent in the form of an email or text to the structure’s maintenance team.
A trio of novel, robust probes is 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. Within the probes are advanced optical sensors specifically designed and built for this project in City’s laboratories. These have been patented for potential commercial exploitation.
Tong Sun, Professor of Sensor Engineering at City and Principal Investigator on the project said: “Key to this successful prototype is our monitoring the variation of the sensor signals of a sample as an indicator of corrosion levels. This means we can use optical sensors made of polymer, which is much more resistant to the high alkaline environments of these structures than sensors currently on the market.”
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 higher than 12. For comparison, domestic bleach has a pH value of between 12 and 13.
“Our design means several probes can be installed semi-permanently in a structure and then connected to a computer data logger, which will 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,” explained Professor Sun.
The sensors within the temperature and humidity probe utilise the fibre Bragg grating. A fibre Bragg grating is 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.
The sensors inside the chloride / pH probe are capable of measuring free chloride concentrations as low as 20 millimoles per litre.
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