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Cracking story

Lletters

I read with interest your article on the demolition Sir Owen Williams' 1930 Montrose 'New Bridge' (NCE 28 October) and Mr Clements' plea for it to be tested (NCE last week).

Sir Owen Williams' doubts about the quality of Scottish cement (which is normally low alkali) led to his specification of an English cement with higher alkali content. This reacted with the rhyolite in the aggregate to slowly develop alkali aggregate reaction (AAR) damage and cracking.

The work to epoxy up cracks, carried out in the 1970s before AAR was diagnosed, did not halt the expansion, but provided a useful indicator of further cracking.

However, some elements of the bridge with well developed AAR cracking merited more detailed investigation and remedial measures. I assisted with the investigation and comprehensive testing, which is well documented.

Monitoring of the crack growth following IStructE recommendations showed that the expansions and cracking continued even after 70 years.

This and other structures have disproved the widespread and dangerous myth that AAR stops after 10 years.

Jonathan G M Wood, Structural Studies & Design, Northbridge House, Chiddingfold, Surrey GU8 4UU

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