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Out of sight

Thorburn Colquhoun (01234) 349641

Much of the work on strengthening the viaduct which carries the A14 trunk road over the East Coast Main Line railway at Huntingdon is being undertaken without any of the 30,000 drivers who use it daily being aware of anything untoward. And the repair work incorporates a pioneering new acoustic monitoring system which will notify engineers of tendon damage.

Built more than 20 years ago, the six span post tensioned concrete structure has been managed since 1996 by a consortium which includes consultant Thorburn Colquhoun. Inspection and assessment of the structure highlighted several areas of concern. Vertical cracking in the pier tops was discovered to be caused by insufficient reinforcement beneath the bearings. Expansion joints and waterproofing had reached the end of their life. There was also evidence of voids in the grout surrounding the post tensioning tendons, but lack of access made visual inspection largely impossible.

Phase one of the work, which involved strengthening the cracked pier tops, was completed in the first half of 1998. To investigate the performance of the post-tensioning system, a series of 30 sensors was placed in the underside of the post-tensioned section of the bridge. They are connected to a data acquisition unit which filters out surrounding noise and listens for the distinctive sound made by a tendon fracturing.

If the sound is detected, the relevant information is sent via e-mail to the system's developers in Canada for processing, before being passed back to Thorburn Colquhoun.

During most of the work, lane or carriageway closures are implemented at night so traffic disruption is kept to a minimum. However, during the waterproofing and surfacing phase of the £2M project, scheduled for this summer, 24 hour contra-flows will be required.

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