Innovative sensors have been developed that will dramatically improve the ability to spot early warning signs of corrosion in concrete.
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).
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.
Professor Tong Sun
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 for editors
The sensors can withstand long-term placement within concrete
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. [back to story]
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).
Engineering and Physical Sciences Research Council (EPSRC)
The Engineering and Physical Sciences Research Council (EPSRC) is the UK's main agency for funding research in engineering and the physical sciences. EPSRC invests around £800m a year in research and postgraduate training, to help the nation handle the next generation of technological change. The areas covered range from information technology to structural engineering, and mathematics to materials science. This research forms the basis for future economic development in the UK and improvements for everyone's health, lifestyle and culture. EPSRC works alongside other Research Councils with responsibility for other areas of research. The Research Councils work collectively on issues of common concern via Research Councils UK.
City University London
City University London is an international university with a commitment to academic excellence, a focus on business and the professions and a central London location. It is placed in the top 5% of universities around the world by the Times Higher Education World University Rankings 2010-11 and is ranked in the top 30 UK higher education institutions by the Times Higher Education Table of Tables 2011. The University's Student Centre holds the 2011 Times Higher Education Leadership & Management Award for Outstanding Student Services Team. City is a broadly-based university with world leading strengths in business; law; engineering and mathematical sciences; informatics; health sciences and the arts including journalism and music. The University attracts over 21,000 students from more than 160 countries and academic and professional staff from over 50 countries. City University London's predecessor, the Northampton Institute, was founded in 1894. City will celebrate its first half-century as a university in 2016.
Queen's University Belfast
Queen's University Belfast is a broadly-based, research-driven university with a dynamic world-class research and education portfolio and strong international connections. The university promotes the widest possible access to this portfolio of excellence in an environment of equality, tolerance and mutual respect, and it fully embraces its leadership role in Northern Ireland and beyond.
Reference: PN 03/12