As the trunk road agent for south east Wales, Capita Symonds has been tasked by the Welsh Assembly to investigate the condition of five concrete cellular slab bridges spanning the M4 and M48 in south Wales.
The bridges, built in the 1960s, have variable depth decks each containing up to 36 rectangular voids 1.3m wide and with depths varying between 750mm and 3m.
Inspection had revealed spalled concrete and corroding reinforcement in the precast sections forming the soffit of the top slab.
However, of the 148 box sections to be inspected, only 13 had entry points.
Capita Symonds asked non-destructive investigation specialist Aperio to recommend, trial and implement an appropriate investigation technique.
Initially perio anned to investigate the concrete from the carriageway using ground penetrating radar (GPR), but a trial core revealed that the waterproofing membrane was made partly of copper, which would reflect all electromagnetic energy.
The solution was to look for spalling in the top slab of the deck using remote visual inspection (RVI) techniques viewed through holes drilled in the bottom slab.
Equipped with RVI tools from Ashtead Technology Rentals, two engineers, one from Capita Symonds and one from Aperio, work under the bridge from a scissor lift during planned lane and bridge closures. Two holes are drilled - for the light source and the endoscope - in advance of the inspection.
The quality of images obtained proved the validity of the technique and allowed the investigation team to assess the optimum arrangement of hole spacing required to attain full visual coverage while minimising intrusive drilling.
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