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Reinforcing the right image

Concrete Refurbishment

One of Britain's busiest car parks is undergoing a major repair and renovation programme. NCE reports from the heart of Heathrow.

The forest of cranes at London Heathrow Terminal 5 may be catching the eye at the moment, but further to the east a multi-million pound project to upgrade one of the airport's vital central car parks is just as relevant to the millions of passengers who use the airport every year. Built in the 1960s to hold up to 1,900 cars, the six storey Car Park 1A/B, to give it its official title, is suffering from all the usual ills to be found on concrete parking structures from that era.

Chloride ions from de-icing salts dripping from newly parked cars have got through to the steel reinforcement. The insitu concrete decks are cracking and spalling, water is leaking through the deck joints. And 20 years of localised patch repairs have given the structure a woebegone appearance.

Demolition and replacement was unthinkable. There was no realistic alternative site for short term car parking. The only option was a major engineering led refurbishment, one that would guarantee at least another 20 years of operational life.

'It was also vital to minimise disruption and the loss of parking spaces during the project, ' explains construction manager at Taylor Woodrow Construction (TWC), Dave Cullen.

BAA appointed TWC's airport division as construction manager for the scheme in 2003. In turn, TWC called on its specialist in-house expertise, and its Corrosion Engineering & Concrete Repairs subsidiary was appointed as the first tier supplier.

TWC adopted a two phase structural strategy. All previous patch repairs would be ruthlessly removed and replaced; all new defects discovered would be repaired. Existing movement joints would be replaced.

'But this alone would not guarantee another 20 years, ' says Cullen. 'The extra reassurance came from the installation of an impressed current cathodic protection system, which should stop further reinforcement corrosion.'

Belt and braces thinking also led to the specification of a liquid applied Triflex DFS waterproofing membrane to repaired decks.

This £8M first phase contract will upgrade just one quarter of the whole car park. More phases will follow over the next three years. But three years of jackhammer racket, vibration and dust was not an attractive option. Then there were the health and safety issues, not least hand-arm vibration syndrome, better known as vibration white finger.

TWC had to consider structural sensitivities as well. 'The floors are unusually slim, only 152mm thick, and up to 50mm of decayed concrete had to be broken out in places, ' Cullen reports. 'This doesn't leave much room for error, and there's always the risk of reinforcement damage. What we needed was a more precise technique.'

Using jets of high pressure water travelling at more than 1,000km/h is an established alternative to conventional methods of breaking out concrete. Vibration and dust are almost eliminated; there is little risk of damage to the bond between the concrete and the reinforcement. And it has the added advantage that the same operation removes both defective concrete and reinforcement rust.

But there are some practical and safety problems with the technique. Hand held lances have ferocious recoil, debris can be ejected at high velocity, and operators need to be both skilled and sensible. On columns and other vertical surfaces TWC had to accept these limitations. For the hundreds of square meters of defective deck concrete, however, TWC opted to go down a higher technology route.

Southampton-based Doornbos Rental supplied a Conjet Robot 322 hydro-demolition machine. Operated remotely, the Robot 322 scores over hand lances on a number of fronts, not least that its self-weight and cowl deal with both reaction forces and debris ejection.

'The machine can be preset to remove only defective concrete, leaving good concrete undamaged and the steel rust free, ' says Cullen. 'We must have at least a one floor buffer zone between any hydro-demolition and live parking areas, but the benefits outweigh the drawbacks.'

One particular problem at 1A/B is that the generations of patch repairs are generally a lot stronger than the surrounding original slab concrete - even when the slab concrete is in perfectly acceptable condition.

In practice this means the Conjet has to be followed up by hand lances, which are used to take out the patch repairs. Whichever system is used, TWC has to dispose of large volumes of water containing suspended, highly alkaline concrete particles.

'This can have a pH as high as 10, ' Cullen reports. 'We put a lot of effort into this problem. In the end the Environment Agency agreed to a system in which the contaminated water runs through the existing drainage system to a series of settlement tanks on the first floor.

'Once it's particulate free it can be discharged into an existing stormwater drain, where it's eventually diluted by run off from other car parks, roads and runways.'

This first phase involves nearly 13,000m 2of roof and intermediate decks, of which 650m 2has to be broken out.

Fosroc's LA55 repair system is used throughout. Some 1,500m of new movement joints are needed, plus 2,000m of new safety barriers.

The result, BAA believes, is a customer friendly facility that will be cost effective and reliable.

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