Early-warning sensors could help reduce roadwork delays by alerting authorities to concrete corrosion in Britain’s ageing bridges and overpasses.
Researchers in the UK are developing forms of fibre-optic sensors that can measure temperature, humidity and chemical changes in concrete structures and — unlike most existing equivalent devices — can withstand long-term placement.
The sensors could enable authorities to spot corrosion and prevent more serious damage to the UK’s concrete infrastructure such as the Hammersmith Flyover in London, which was recently forced to close for emergency repair work.
‘Given the amount of our infrastructure that was built in the 1950s and 1960s, there is the potential for an awful lot of these [problems] to crop up over the next few years,’ said Prof Ken Grattan of City University London, who has worked on the sensors with colleagues at Queen’s University Belfast.
‘And what we want to be sure of is that these types of very important arteries into our cities can be monitored, and then there’s some chance at least that you can take corrective action at an early stage.’
Problems occur when corrosion allows water to enter the concrete, weakening the structure by causing the carbon steel support rods inside it to rust. Submerged tidal defences are at particular risk.
Fibre-optic sensors are already used to monitor physical changes in structures. They tend to be small, lightweight, don’t become saturated with water and can be linked into large networks more easily than alternative electrical resistance sensors.
The researchers’ sensors are also made from polymers that are more resistant to high pH levels than existing optical devices and so can remain in place for several years instead of just months.
Optical sensors work by monitoring the wavelength of light travelling down the fibre, which changes as the fibre stretches due to physical movement. The researchers applied coatings to the fibre that react to changes in temperature and humidity, also causing it to stretch.
The devices that detect certain chemicals rely on other coatings that absorb the light or produce their own fluorescence when the chemicals are present. The pH detector uses an indicator that changes colour in acid or alkali conditions, again altering the reflect light’s wavelength.
Grattan said the team was looking to develop the sensors commercially, but that there was a certain amount of conservatism in the industry.
‘Our aim is to produce a suite of sensors that will enable us to measure physical and chemical changes, and we’re at various stages along that process,’ he said.
‘There are a fair few tests being done that we’ve been involved with, but there isn’t the level of actual installation that one would hope to see because it’s still regarded as pretty new technology.’
Transport For London (TfL) is responsible for the maintenance of the Hammersmith Flyover and other road infrastructure in the capital, but doesn’t currently use fibre optic sensors.
Dana Skelley, director of roads at TfL, said: ’The monitoring equipment used at Hammersmith Flyover comprises acoustic emission sensors, movement transducers, temperature sensors, inclinometers, total station measurements and strain gauges.
’We are also working with Cambridge University as part of a research project they are undertaking to trial wireless sensing technology. As members of the Bridge Owners Forum, we are aware of the latest innovations in structural health monitoring and are therefore able to ensure that TfL is using the best and most cost effective technology available at the time to monitor our structures.’