Getting the inside story on road bridge cracks

The Highways Agency is examining the feasibility of a device that ‘listens’ for newly-formed cracks developing inside the steel of motorway and other road bridges. Developed by Cardiff University engineers and Cambridge company Physical Acoustics, the device enables components to be checked without drilling into the bridge’s structure, thus avoiding the need for scaffolding and lane closures.

In motorway bridges, due to the movements of vehicles, the steel structure is subject to constantly-changing stresses. Internal cracks advance by microscopic amounts, producing ultrasonic waves like miniature earthquakes.

Dr Aled Davies of Cardiff University School of Engineering said the system employs a series of sensors placed on the bridge to listen for the acoustic emissions produced as the cracks form.

Though similar techniques have been employed in the energy and processing industries for some time, monitoring bridges is more difficult due to the random loading of traffic movements and high amounts of background noise.

In general, similar technology has previously only been of use once damage has already been detected, perhaps by sight, often too late to prevent major expense.

The Cardiff/Cambridge technique wanted not only to monitor existing cracks but also to leave the device on the bridge to alert operators to new cracks as they form. Their solution involves ‘tuning’ the sensors to detect active cracks as they occur, but block out background readings from other cracks and benign imperfections.

The technique does not apply only to bridges. Davies envisages applications in the aerospace and car industries, with F1 teams already expressing an interest in using the system for vehicle diagnostics.

His colleague Dr Karen Holford has even used the sensors to analyse students’ knee joints and says she can tell the difference between rugby players and swimmers by the characteristics of cracks in their bones.

Tests of the steel monitoring device are planned for the Avonmouth Bridge on the M5 this summer.