Why read this
Major pier replacement is being undertaken on a decaying road bridge.
New desalination techniques are being carried out.
Whole life costing is guiding the maintenance regime.
Take one ageing multi span concrete viaduct.
Add liberal quantities of de-icing road salt. And the all too common result of this recipe is a chloride attacked structure riddled with corroding reinforcement and spalled concrete.
The 25 year old, 2km long Tees viaduct is typical. Virtually all its carriageway and 69 multi-column pier supports have already been, or must be, extensively refurbished. For the last two decades, traffic cones have rarely been removed from the deck and concrete repair contractors have usually been beneath it.
But the scene is changing at this troubled A19 crossing in Middlesbrough. A radically different repair programme has just started - and it is all down to the new owner.
Long held plans to repair rather than replace most of the corroding columns have been halted. There is now a seemingly more expensive programme totally to rebuild around twice as many piers as before.
'It is all about managing maintenance, ' explains Jeff Wild, general manager for the crossing's new custodian, private finance initiative concessionaire Autolink.
Autolink's 24 year repair timetable, based on a computerised £100,000 state of the art structure deterioration model, calls for a dozen pier units to be refurbished over the next ten years, followed by an intensive 14 year rebuild programme for a further 30 supports. The Highways Agency had proposed totally rebuilding only 16 pier bents and repairing or partially reconstructing the rest.
Within five years of its 1975 opening, the 68 span Tees crossing, with its four lane composite steel beam deck, had started to deteriorate seriously. Now carrying 55,000 vehicles a day over the river, several roads and a railway marshalling yard, it has developed a lethal cocktail of failed deck waterproofing and leaking expansion joints.
Chloride rich rain water seeped down through virtually all the 60 failed joints above pier bents. It attacked both the 1.75m deep crosshead directly beneath and also the integral column supports, up to four of which form each pier bent.
Under the HA's stewardship, the 22m long supported spans were made continuous, halving the number of new expansion joints needed. The deck was rewaterproofed and its concrete carriageway patch repaired.
But beneath lay columns still containing chloride levels of up to 6.5% by weight of cement - over 30 times permitted safe limits - and link reinforcement suffering 90% loss of section.
By 1997, 20 of the worst attacked pier bents had been renewed, and a repair programme established. This made use of a custom built £250,000 temporary works jacking frame flexible enough to surround any pier bent and support the deck while columns within it were rebuilt (NCE 6 June 1996).
But then along came Autolink, a joint venture of Sir Robert McAlpine, Taylor Woodrow and Amey. It had just won the PFI concession to upgrade, and maintain for 30 years, a 120km length of the A19, including the Tees viaduct.
Autolink and its consultant Scott Wilson looked at, but rejected, the inherited HA repair plan, drawn up back in 1988. Its prime driver was not only maintenance over the 30 year concession. 'The structure has to be handed back after the 30 year period in a sufficiently good condition not to need any major maintenance for another 30 years, ' says Wild, conceding that the definition of 'major' has yet to 'fully explored and tested'.
Addressing its 60 year responsibility, Autolink opted to draw up its own maintenance programme based on whole life costing. To determine the optimum time to repair or replace a pier, given that it must then remain maintenance free until at least 2057, Scott Wilson devised an extensive deterioration model analysing the structural capacity of each support.
A 12 strong team of engineers spent 18 months carrying out corrosion surveys on material retained from piers already demolished; reanalysing twenty years of bridge repair data and computing a detailed finite element analysis of the structure.
'The model calculates when the strength of each pier is likely to reduce to its design factor of safety, ' explains Scott Wilson senior engineer Darren Kimberley. 'This will then be the latest date at which the repair or replacement should take place.'
Autolink's philosophy is that although more capital intensive, replacement is better than repair. 'Rebuild is a more robust solution with less of a continuing maintenance risk, ' says Wild.
The resulting decision to rebuild 30 pier bents - double the total suggested by the HA programme - will increase Autolink's capital cost bill by £2M to £12M.
But, explains Wild, 'as most of the replacements take place after 2015, the bridge, at handover date in 2027, will be in a much better condition than under the original plan'.
With the tallest piers - or those difficult to access alongside the river or over the railyard - the whole life costing equation can tip in favour of repair rather than replacement. And it is Scott Wilson's task to determine, in the dozen piers to be refurbished, whether patch repairs, cathodic protection or electrochemical desalination provides the most effective and economic solution.
Desalination is generally favoured over cathodic protection in being a one off remedy requiring no extended maintenance. Its role is to reverse the natural flow of damaging chloride ions as they migrate towards the reinforcement.
A relatively high 1A/m 2current is set up, passing from the steel, as the cathode, to a titanium mesh anode attached to the concrete. The ions are collected in a bath normally containing an electrolyte, calcium hydroxide, so permanently lowering contamination levels in the concrete.
But concrete used in the Tees viaduct contains river gravel which has a high absorbency rate. And it is known that the use of calcium hydroxide produces hydroxyl ions that collect around the reinforcement and can be absorbed into the surrounding concrete. This increases the concrete's alkalinity, creating the risk of another potential problem, alkali silicate reaction.
Use of an alternative electrolyte, lithium borate, reduces the risk of ASR and it is this new technique that is being tested on the viaduct alongside conventional desalination.
'This is the most significant UK trial yet of lithium borate, ' says Kimberley.