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Sat 5 Jun 2010 04:00 AM

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On the road again

A crushing plant in Abu Dhabi has placed itself at the quarry-face of sustainable construction, recycling concrete into aggregate for use in road building.

On the road again
The plant processes construction and demolition waste, recycling it into aggregate.
On the road again
On the road again
Mark Chandler is the C&D recycling manager at the Thiess Services site in Al Dhafra.
On the road again

A crushing plant in Abu Dhabi has placed itself at the quarry-face of sustainable construction, recycling concrete into aggregate for use in road building.

Construction and demolition in the emirate of Abu Dhabi has changed and there's more to come. You may not know it yet, but soon you will have to recycle all construction and demolition waste.

A new recycling plant in Al Dhafra, on the very outskirts of Abu Dhabi's future urban sprawl, has been developed by Thiess Services Middle East - a joint venture between Al Habtoor Leighton Group and Australia's Thiess Services - for this very purpose.

Last year, the company won an exclusive concession to build and operate the recycling plant for 15 years. For now, disposal of recyclable concrete spoil is free for contractors, though the door remains open for charges to be introduced at a later stage.

The plant has two distinct purposes. Firstly, is stops a lot of useful construction material simply heading straight to landfill. Secondly, it takes this material and turns it into aggregate, ready to be reused in suitable construction projects.

"So it's free to get rid of waste, from there we value add it and produce a high-grade material," said Mark Chandler, C&D recycling manager.

Right now the process is just for concrete, which is being turned into aggregate for road building. Gradually, aggregates for other construction uses will be developed. The site already has a stockpile of asphalt, which will soon be added to the plant's recycling programme.

Having been officially opened about a month ago, production is gradually building to full capacity. However, the business case for the plant and its products is a strong one. The Center of Waste Management Abu Dhabi - the government client behind the project - has developed regulations that will mean all construction and demolition waste will have to be recycled. Further regulation will require projects in Abu Dhabi, which have a use for the products the site produces, to use a minimum of 40% of the recycled material in their construction.

"With the regulations they want to put in place, waste has to come here," said Chandler. "We can deem it unsuitable, but it has to come through our fingers, or it is illegal dumping.

"The regulations will mean that everything that is suitable for recycling can no longer go to landfill."

This marks a change in the emirate, where much waste has been used either as landfill, or for land reclamation. Now the recycling will realise the full value of the materials, which until now have frequently been thrown away. The product

"The beauty of recycled concrete is that you can actually recycle it endlessly, it doesn't wear out," said Chandler. "Concrete can continuously go around in circles; it's a wonderful resource for recycling. Some plastic can be recycled once or twice; glass can only be done if it's sorted correctly. So when you look at concrete, colour doesn't matter, as long as you get the specifications right, you can recycle it as many times as you like."

The cost for each tonne of the end product will be the same as the market rate from any quarry. But, compared to the usual sources of raw materials, such as Oman or Ras Al Khaimah, traveling distances to projects in the emirate will be greatly reduced. This will help to lower the carbon footprint of projects under construction in the emirate and reduce associated haulage costs.

Thiess has high-hopes for the product it will make from the construction waste.

"We believe we can produce a product that is not only as good, but actually better, than the virgin material," said Chandler.

The company claims a better degree of compaction, in part generated by the even size of the aggregate produced through recycling. For now the output will be suitable for the different base layers used in road building, literally everything beneath the asphalt.

"We can build a road from scratch, all the way up to asphalt, out of concrete," said Chandler. "There's no reason why any road can't be built out of recycled concrete."

With road projects on the scale of the Mafraq-Ghweifat highway, a 327km strip of tarmac that provides access to Abu Dhabi's Western Region and the industrial centre of Ruwais, there should be plenty of opportunity to put the material to the test. Volume shouldn't be a problem either. With a capacity to handle more than a hundred truck loads of waste a day, the site can take in 5,000 to 16,000 tonnes per day. The facility has an output design load of more than 7,000 tonnes per day. »» The process

Meeting this capacity means having some big kit on site. Dump trucks leave piles of concrete at the top of a hill. A couple of Caterpillar 988 wheel loaders are used at the face to drop this material into the feeders. These wheel loaders are the second biggest in range from Cat, but the third biggest, the 980, is also working on site.

The concrete is fed into two Lippmann Milwaukee primary jaw crushers, to start its journey through the processing plant, which features several screens, a number of powerful magnets - which are known to break people's watches if they get to close - to pull out unwanted steel, and picking stations, all of which are linked by a series of conveyors.

The screens and picking stations allow contaminants, such as plastic and cardboard, to be manually removed from the mix. The conveyors are up to six feet wide, and designed to put an even layer of material all over the surface, so the pickers can see it clearly. Picking is considered one of the plum jobs on site, as it actually takes place inside air-conditioned cabins.

"There's a massive screening capacity," said Chandler. "In my view, its the best equipment you can put in this type of plant for our volumes."

The type of crushing employed is low speed compression crushing. While impact crushing is a common alternative and will be used for recycling asphalt, big hunks of concrete can cause devastating wear and tear on such machinery.

"Something like a lift weight can go through the machine without busting anything," said Chandler. "In an impact crusher it's like a grenade going off."

A control system allows operators to monitor the process from one spot. Parts of the crusher can be stopped and started, feed rates can be adjusted, and if there are any material quality issues, software allows the team to identify where the resulting product is in the stockpile.

Once extracted by the magnets, any steel is sorted, cleaned and sent elsewhere for recycling. The only rubbish to go back to landfill is anything taken out that is unsuitable for recycling.

Although it's early days in the operation as yet, Chandler estimates that less than 1% of the material from every tonne of concrete recycled will go to landfill.

"There's a massive saving in landfill space," he said. "General rubbish will compact 5:1, but you can't do that with concrete." Safety first

What's very noticeable is the clean and orderly lay out of the site. A one way system keeps everyone moving in the same direction and reduces risk of vehicle accidents. The site team are in steady radio communication with the machine operators as they move around. Conveyors and other fixed plant were raised an additional two metres above the height normally classified as ‘plant clearance'. This offers workers in loose clothing and headgear added safety from entanglement in moving parts, since they are well beyond reach.

This is just one of the areas where Thiess has made an investment in safety. And it's an investment that has paid dividends in the set up of the plant prior to production, which went so smoothly that there wasn't a single lost-time injury.

"There has been a massive investment in safety in the plant. It doesn't matter how safe you make it, there is always a risk somewhere, so the investment in safety was enormous," said Chandler. "It was key that we focused on making the plant as safe as possible in the design stage."

The safety investment will be a valuable asset when plant volumes ramp up to capacity. When the materials produced become a common addition to road works and other sites around the emirate, the construction industry will have taken a very tangible step toward being sustainable.

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