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Causing a stir

Bridges : Corrosion prevention

Use of an American developed concrete additive to prevent reinforcement corrosion in a Newcastle bridge is being heralded as a time saving success. David Hayward reports from Tyneside.

Over 200 rare wild celery plants plus otters and their riverbed mud habitat, lie right beneath the main structure and all are protected and untouchable. Such restrictions would be more than enough for any contractor.

But Edmund Nuttall had more than that to contend with on its scheme to construct a precast concrete arch bridge in Newcastle. It also had to programme in a minimum 28 day delay before it could spray a corrosion protecting silane coating.

With arch erection on the critical path of a contract already threatened with foundation piling delays, a rethink was needed.

Silane spraying was scrapped and replaced by a chemically based corrosion inhibitor added to the concrete mix. This allowed instant arch erection plus the bonus of reduced long term maintenance with no silane to recoat after 15 years.

But it also added costs for a relatively new technique that some engineers say has yet to be proven.

'For us it meant 100 arch sections arrived ready for erection the next day, ' recalls Nuttall agent David Murphy. 'And it removed the access and pollution protection hassles of having to spray the beams.'

Although banned by the Highways Agency, the chemical additive is thought to have proved so beneficial at Newcastle that the client is likely to contribute half the £9,000 extra cost for the arches. It has also permitted its use for protecting new concrete bridge parapets and capping beams for riverside sheetpiling.

It is fortunate for Nuttall that its client for the £6.7M upgrade of the city's Scotswood Road, including a new two span arch bridge providing access to a riverside industrial estate, is not the Highways Agency but Newcastle City Council. The planned development lies on a promontory of the River Tyne separated from the shore by a narrow muddy inlet - hence the need for a 50m long bridge.

Speed of construction was a primary factor in the choice of a row of 1m wide curved precast beams to create each arch span, with bridge spandrels, superstructure and embankments formed of reinforced earth.

Although still relatively unusual in bridge construction, placing the up to 9m high reinforced earth walls proved little challenge. It was the arches that posed the problem.

Erected in 13m long half arch sections, the 450mm thick exposed concrete beams were classic candidates for chloride attack both from road salts above and an aggressive tidal flow river below. Taking a lead from the Highways Agency, the client had specified a sprayed silane coating to protect the beams from salt ingress and subsequent rebar corrosion.

But such coatings cannot be applied until the concrete has cured for 28 days and only then if dry weather can be guaranteed for 24 hours.

Nuttall planned to bring in an expensive 400t mobile to erect all 50 arches in one go. This meant the last section had to be sprayed and ready before placing could start. The only alternative was to spray them all insitu after erection - a task demanding access scaffolding and sheeting to prevent river pollution from the organic compound.

Murphy had not been relishing the: 'hassle and grief of this time delaying, weather dependent, pollution risky, health and safety led task'. So it was receptive to Nuttall's suggestion to use a calcium nitrite corrosion inhibitor instead, added at the concrete batcher.

'Our corrosion inhibitor has a 30 year proven record in the US where a vast number of structures are protected in this way, ' says Bruce Perry, engineering services manager for supplier Grace Construction Products, the company brought in to do the persuading. 'We claim it will prevent virtually all chloride induced reinforcement corrosion, so concrete spalling should not occur over the full 120 year design life of this bridge.'

But, Perry adds: 'The UK construction industry's conservatism, ignorance and resistance to any form of concrete additive leaves only a handful of British structures treated with inhibitors.

Such UK resistance is understandable as the technique - with no contractual guarantees on offer - involves a complex chemical reaction with the calcium nitrite enhancing the reinforcement's own natural protection against corrosion (see box).

TRL has completed a three year test programme of this type of inhibitor. And, although laboratory results show the chemical to be 'effective in delaying the onset of chloride induced corrosion' the Agency's senior technical advisor, Neil Loudon, claims: 'Its use in practice - with varying concrete and environmental conditions - has yet to be proved.'

But a revised Agency advice note, due out ' within a month or so' will allow its use to protect against particularly aggressive salt attack, but only in addition to - rather than replacing - a silane coating.

Back at Newcastle the client is less reticent.

'There is a risk, but it could be a good one, ' says the council's chief bridges engineer Nigel Burn. 'We would be struggling for the cash to recoat the silane later, so believe the considerable time saving on this contract is worth that risk.'

The arches did go up the next day and similar plans to silane coat 400m of precast bridge parapet, plus 600m of river edge capping beam, were also scrapped in favour of the corrosion inhibitor.

'Its use increased the cost of our concrete by 30%, but time savings and less hassle made the total £14,000 extra bill worth it overall, ' sums up Murphy.

'Adding an inhibitor just cannot delay us, so we would be willing to use it again.'

Making barriers

Grace Construction Product's 30% calcium nitrite, 70% water DCI.S corrosion inhibitor - added as a 10 litre dose per m 3of concrete - works by ensuring that the reinforcement's own natural resistance to corrosion remains effective. In the alkaline matrix of fresh concrete, the steel surface oxidises to create a ferric oxide 'passivating' layer around each bar - an efficient barrier to attack by salt-invading chloride ions.

But this oxidising reaction sometimes stops short, creating instead surface imperfections of less effective ferrous oxide, and it is here that rusting starts. The added calcium nitrite ensures these weak patches continue oxidising to fully protective ferric oxide. This provides, claims Perry; 'a total near permanent barrier against corrosion'.

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