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A place in the sun

Concrete Grand mosque Abu Dhabi

A variety of concreting techniques are being used to build a giant mosque.

Antony Oliver reports from Abu Dhabi.

The last time NCEI encountered Sami AlQazzaz, Halcrow's assistant resident engineer at the Grand Mosque, Abu Dhabi, he was in the cold, damp surroundings of London Underground fixing cast iron props beneath Rotherhythe tunnel.

These days he is operating in 40degreesC to 50degreesC temperatures, has switched to working up to 100m above ground and is looking after placement of thousands of tonnes of concrete.

As in London, he still tries to downplay his achievements on site. 'There are a huge number of different types of concrete elements, particularly in the domes, but most of the concreting is fairly straightforward, ' he explains. 'At the end of the day its just reinforced concrete. Very high quality, but it is just reinforced concrete.'

Certainly this is true. But a quick look at a few facts highlights the not so straightforward scale of this project - officially known as the Sheikh Zayed Bin Sultan Al Nayan Mosque.

When complete, the mosque will be the biggest in the United Arab Emirates and with room for more than 30,000 worshippers, the world's third largest.

Spread over an area measuring 1km by 0.5km, its four minarets are each 107m high and the main dome reaches 80m in height at its peak.

In total the mosque will have a large number of separate domes, in seven different sizes. The four largest are precast, the mediumsized are formed using shotcrete and the smallest are cast insitu.

All are designed as structural shell elements which will eventually support marble and gold cladding The whole structure will use close to 150,000m 3of concrete, all of which is batched on site using two purpose built plants.

The contractor Impregilo now has around 1,350 workers on site, but this figure has fallen from a peak of more than 2,500.

Halcrow arrived on site to supervise construction in October 2001, just after work began on the mosque's domes. Project manager John Langdon faced the tough task of picking up the project half way through and working with Impregilo to get the job back on track and structurally complete by February 2003.

'We had to hit the ground running and fortunately it worked, ' explains Langdon, reflecting on the challenges of creating working relationships so that the job did not stall. 'Impregilo has a great attitude. They are here to do a quality job and very happy to work in partnership to achieve results.'

This close working relationship with the contractor has, paid huge dividends in terms of quality and safety on the job. But it has scored even more technically, particularly with the complex precasting required on the dome construction.

The four largest domes are formed using a series of precast panels which are laid in place on formwork and stitched together with reinforcement and insitu concrete. The main dome is made up of 24 segments, each placed and stitched in three rows, the others using just 12 segments, placed in two sections.

'Impregilo has put a lot of effort into planning and temporary works design so that the precast units all fit together first time, ' explains Langdon. 'Sami and the Halcrow structures team have been very involved in the design development process and all the thought and effort has paid off.'

As the main dome construction is a key event on the critical path for the project, it is vital that work goes smoothly.

But similar detailed planning and training has also paid dividends on the quality of the shotcreted domes. All spraying is done by hand, working from fibre glass shutters which form the inside dome surface. Training and constant monitoring ensures that operatives are mindful of the need for consistency when applying the fibre reinforced wet-mix shotcrete.

And with daytime temperatures breaking 50infinityC in the summer and not dropping below 30infinityC in winter, all concreting has had to be done at night, with literally millions of litres of water used to prevent the curing concrete from drying out.

'On this site safety is critical, ' says Al Qazzaz, highlighting the ever present dangers associated with working up to 100m above ground. 'This is not typical for this part of the world, so the labour force has to be taught new ways of working.'

With much of the workforce from India, Pakistan or Asia, the contractor goes to great lengths to look after welfare. Huge camps just off site provide accommodation, canteens, doctors, clinics and a small mosque.

The site operates 24 hours a day, seven days a week.

'The heat and humidity is a problem, particularly in the summer, ' says Al Qazzaz. 'But at least you can plan ahead for your time off. The sun is always out, the sky is always blue and as everybody rents their property, there is no gardening or DIY to disrupt the weekend, ' he jokes.

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