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Major structures face new concrete cancer threat


BRIDGES, DAMS, tunnels and other major structures are threatened by the return of 'concrete cancer', a leading specialist claimed this week.

The phenomenon, properly known as alkali aggregate reactivity (AAR), causes massive cracking in affected structures and was blamed for the recent demolition of Montrose Bridge in Scotland (NCE 28 October).

It was a headline grabbing threat in the late 1970s and 1980s, but most concrete specialists believed the problem was under control. Many structures were repaired and research resulted in changes to materials specifications to limit alkali content.

But this week Dr Jonathan Woods, director of specialist consultant Structural Studies & Design, said some of the tests into potential aggregate reactivity were 'not rigorous enough for major strategic structures'.

'Most developed countries thought they had succeeded in 'specifying out' the problem, and research virtually ceased, ' he said.

'Now in countries like France and the Netherlands AAR has become a major problem again. It could soon happen here, ' he said. Woods is particularly concerned about the use of recycled aggregates like glass.

'Finely ground glass may have some value as a cement additive, but it shouldn't be used as an aggregate, ' he said.

'And there is evidence that alkali release from some forms of aggregate can cause AAR to develop relatively late in a structure's life.'

Four hundred motorway bridges in northern France are believed to be suffering from substantial AAR attack.

The Netherlands has similar problems, especially where no deck waterproofing is used.

And in Japan viaduct crossheads affected by AAR have significantly reduced seismic resistance.

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