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Iver South Tanks in the fast lane

New Concrete Engineering - Changing the concrete mix has accelerated construction at the Iver South sludge treatment works. Bernadette Redfern went on site.

Heathrow Terminal 5 might be the biggest construction project in west London at the moment but it is not the only one. Just 10km away from Heathrow, at Iver South, contractor Costain is four months ahead of schedule on the construction of a £106M sludge dewatering plant. The site will take sludge from the nearby Modgen sewage works, put it through a centrifuge to dry it out then store it before it is taken off for use on farmland.

Building a new sludge treatment works was a direct consequence of the construction of the new terminal at Heathrow. The T5 site is currently home to the Perry Oaks dewatering plant, where sludge left over from the sewage treatment at Modgen sewage works, is treated and stored. Thames Water was forced to find a new home for the plant which handles the sludge produced from the waste products of 1.8 million people.

Surprisingly there were not many objections to Thames Water's choice of site at Iver South. 'We are not close to houses and Thames already owns the access road into the site, ' says Costain project manager Colin Howe. Until two years ago, the site housed a small local sewage treatment works, before it was decommissioned and the flows redirected to Mogden.

The impressive four month lead, on a project for which the contract was awarded in late 2001, is partially due to the contractor's decision to change the concrete mix used on the walls of the eight sludge holding tanks.

Each of the 20m diameter, 7m deep pits will provide storage for the sludge before it is dewatered.

'Sludge from Mogden will enter two feed tanks, the other six will provide additional storage as required, ' says Thames Water project manager's representative Paul Barrow.

The new mix gained strength earlier than the standard mix used elsewhere on the site, enabling the team to remove shutters after two days rather than four. 'Typically we achieved cube strengths of 40N/mm 2after seven days and 50N/mm 2after 28 days. Concrete was poured at a maximum rate of 2.5 vertical metres per hour, ' says Costain section agent Rob Sharpe.

'The important difference here is the cement type. The main mix used CEM I (Ordinary Portland Cement) with pulverised fuel ash (PFA) added at a 30% replacement level to form a CEM IIB cement. On the slipformed walls of the holding tanks, strength was needed earlier, so the cement was changed to Sulphate Resistant Portland Cement (SRPC), ' says supplier London Concrete technical manager, Sean Roach.

The C28/35 concrete resulting from a SRPC also has a better chemical resistance than its CEM IIB counterpart, desirable when storing sewage sludge. It does, however, require more water in the mix. But this greater water/cement ratio, which would normally imply lower durability and reduced strength, is more than compensated for by the greater reactivity of the SRPC at early ages.

As concreting of the tanks began in winter, the cold temperatures threatened the construction rate. 'On a couple of very cold days we opted to use hot water to mix the concrete to raise the temperature, ' says Costain project manager Colin Howe. This increased the reaction rate and ensured that the project stayed ahead of itself.

Two teams have been working simultaneously on the tanks over a five month period. One 10strong team would move in and construct the 2m deep base and two days later the second eightman team started on the walls.

Each tank took five separate pours to create and the accelerated pour rate meant that the shutters could be moved every other day, rather than every third day, to pour the next 7m deep, 0.5m thick, section of wall.

'During the wall pours it was demonstrated that all five sections could be poured in one week. It had been programmed to take two weeks, ' says Sharpe.

Sealing the joints between the concrete walls was done using a water bar. 'Retarder was applied to the stop ends after striking the shutter. The concrete was jetwashed off to expose the aggregate then the water bar was fastened to the vertical surface by silicone sealant. A Hilti nail was used on the top of the wall to hold the end, ' explains Sharpe.

Constructing the 32,000m 2dry sludge storage area, used 20,000m 3of concrete and combined with the tanks, 27,000m 3ofconcrete was used on site.

'By next April we will start running the plant and diverting the sludge. From August we will use it fully but we will keep Perry Oaks operational until March 2006. BAA expects us to hand over the site in June 2006, ' says Barrow.

However, at the rate the team is going, BAA might get the site much earlier than that.

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