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Concrete cancer kills bridge

A once-controversial Scottish bridge is one of the few UK structures to be demolished as a result of untreatable 'concrete cancer'. Dave Parker reports.

TWO WEEKS ago the distinctive concrete trusses of Montrose Bridge on the A92 still spanned the swirling tidal waters of the South Esk estuary in Angus.

Today it lies, still almost intact, on the town's South Quay.

'Semi-serious' attempts by main contractor Balfour Beatty to sell the historic structure on internet auction site eBay came to nothing, so now the jack hammer brigade is moving in.

Balfour Beatty has the £7.5M contract to demolish the bridge and replace it.

Perhaps the contractor will be able to sell some chunks of the 75 year old concrete to universities and research bodies, because it has one of the worst cases of alkali-silica reaction (ASR) - 'concrete cancer' - ever seen in the UK (see box).

Angus Council director of roads Ronnie McNeil said the 158m long bridge had been deteriorating for a number of years and various repair attempts had failed to slow its rate of decline. 'We were increasingly concerned about the risk of water penetrating the cracks - which were all over the superstructure - and corroding the reinforcement, ' he said.

'In an aggressive marine environment this is a serious problem. We were advised in 1996 that it would be uneconomic to maintain the bridge for more than 10 years, and that replacement was the only option.'

Investigations had revealed that the aggregates used to build the crossing in the late 1920s contained the reactive silica minerals felsite and rhyolite.

Local folklore suggests that the source of the natural gravels was in fact beach deposits from further up the coast - which could have contained salt water, another possible source of alkalis.

Replacing a major bridge could have resulted in considerable disruption. So to keep traffic on the busy A92 flowing, a temporary crossing was constructed alongside. On 23 September the old bridge was ceremoniously closed. A scant three weeks later Balfour Beatty was ready to remove it - in three pieces.

'We won the contract on the basis of conventional 'crunch and munch' techniques, ' said Balfour Beatty project manager Keith Bowman. 'But I wasn't happy about the safety and environmental implications of working above the estuary on a structure that was already in a pretty damaged condition.

'So one day, I entered 'big cranes' into internet search engine Google. Several companies came up, we started talking, and after a year's planning and preparation the 'Rambiz' arrived at Montrose.'

Owned and operated by Antwerp-based Scaldis Salvage, the Rambiz has a theoretical lifting capacity of 4,000t - and a draft of 4.5m. This imposed tidal working on the project.

But even at high tide, a large sandbank under the southern span posed problems.

'We started with the central span, which only weighed 300t and had no problems, ' Bowman said. 'The northern truss over the deepwater channel only took two hours to lift out and transfer to the quay, despite weighing 2,200t - including 250t of cables, shackles and spreader beams.

'But we had to lift the southern truss off its supports and drop it onto the riverbed on one high tide, then come back on the next high tide to pick it up again and transfer it to the quay.

Balfour Beatty now has to remove the existing piers, which turned out to be made with high alumina cement concrete, before construction of the replacement bridge can begin.

This is a conventional composite concrete/steel design and is due to open next September.

Professor Les Clark of Birmingham University said it was highly unusual for UK structures suffering from ASR to be demolished. 'Normally the structural effects have not been serious, and extensive international research has shown that ASR isn't as much a threat as was first thought.

'But cracking on a structure in a marine environment is always serious, and there doesn't seem to be any practical or economic way of sealing such structures.'

Chemical reaction

Reinforced concrete made with Portland cement depends on the alkalinity of the hardened cement matrix to passivate the steel and inhibit corrosion. Sometimes if a very alkaline cement is used, or if some of the aggregate is not chemically inert as is normally assumed, then undesirable and potentially disastrous chemical reactions can occur.

The most common is a reaction between the soluble alkalis in the pore water and the semiamorphous silica in some types of aggregate. In the UK these are typically flints and cherts.

A hydrophilic gel is produced, which can absorb large quantities of water, expand, and crack the matrix.

Alkali silica reactivity is now only a problem with older structures. Risk can be 'specified out' by limits on the total alkali content of the mix and restrictions on the types of aggregate used for new construction. Even on older structures the progress of ASR can be halted by keeping water out of the concrete, usually by some form of waterproof membrane.

Concrete controversy

A local landmark for more than 70 years, the distinctive Montrose Bridge over the South Esk is believed to be one of only two similar structures in the world. It replaced an earlier suspension bridge, and some believe its unusual form was conceived to continue the same visual impression. Others see it as a homage to the world famous Forth Bridge, as, despite initial impressions, Sir Evan Owen Williams' design linked two balanced cantilevers with a short suspended central span, just like the Forth Bridge.

Nevertheless, the curved top chords of the complex concrete trusses that formed the balanced cantilevers were in tension, normally anathema to the concrete engineer. To cope with this, each chord contains no less than 76, 40mm (1 1/2 in) mild steel reinforcing bars, a massive amount for the time.

Main span was 66m, backspans 46m, and the roadway was 11m wide. The suspended span was only 6.4m long.

The new bridge created little interest in the UK when it opened in 1930, but it triggered a prolonged and acrimonious correspondence on the letters page of US construction weekly 'Engineering News Record'. US engineers claimed that Owen Williams' choice of concrete was whimsical, inappropriate and much more expensive than a steel alternative.

Williams responded by asserting that a detailed comparison with a steel alternative had shown his design was cheaper, and that concrete 'dispensed with the need for rivets.'

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