Sensor sensibility: Better protection for 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 (see note below).

The breakthrough has been made by researchers based at City University London and Queen's University Belfast following several research projects funded by the Engineering and Physical Sciences Research Council (EPSRC).

Because 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.

Thanks to an internet connection, the notification 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 says: "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," said Professor Sun.

Notes

The four-year project 'Corrosion Monitoring Systems for Structures in Extreme Marine Environments', began on 31 March 2006 and concluded on 30 March 2010. It received EPSRC funding of £243,145 to City University London and £244,788 to Queen's University Belfast.

The success of the sensor development and its potential value to industry means the research team were also awarded a follow on grant by the EPSRC (£110,125 to City and £65,318 to Queen's University Belfast) to explore the commercial potential of these sensors. This project will see further improvement on the sensor design, especially chloride sensors and their subsequent testing of the probes which will then be put forward for the patent update before being made available for next-stage commercial exploitation.

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, 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 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.

Due to the commercial sensitivity of the design, details of the sensors inside the chloride / pH probe are not being disclosed. But it can be revealed that it is capable of measuring free chloride concentrations as low as 20 millimoles per litre (a mole is a unit measuring the concentration of a substance by counting the number of atoms or molecules in it).