Precast power gives contractors the edge

In an effort to meet tight deadlines, contractors are turning to alternative construction methods. Christopher Sell explores some projects in the UAE that have used precast concrete.

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By  Christopher Sell Published  December 9, 2006

|~|149prod200.gif|~|On the first phase of the Dubai Creek Extension, the quay wall was formed using precast concrete blocks, which varied in weight from 24-34 tonnes.|~|As the sheer demand for construction projects in Dubai shows no sign of abating, contractors are searching for more time-efficient methods that will enable them to meet project deadlines. In the rush to complete contracts on time, and due to technological advances, precast concrete has been used on a range of projects. From beams and wall systems to marina pontoons and tower construction, contractors are turning to more cost-efficient methods that are also less labour-reliant. The emirate’s inclination to use this form of material (of which there is currently a 25% market share and growing) can be seen in a number of high-profile examples, notably on the recently completed Bright Start Tower on Sheikh Zayed Road. The 59-storey building, with a total height of 280m, featured United Precast Concrete (UPC) as the subcontractor for the precast elements of the project. UPC’s remit was to manufacture, supply and erect precast concrete beams, balconies, hollow-core slabs, staircases, solid slabs and panels. It also had to provide pre-stressed solid slabs and hollow-core slabs, precast beams, staircases and balconies for the tower building, and hollow-core slabs and cladding for the car park building. As per the initial programme, UPC had to finish one floor every six days, but finished one floor every three to four days instead, which resulted in time savings of 30% to 50%. According to the company, the speciality of the project was the simultaneous erection of all precast elements, including staircases, in a short period of time. UPC used a special connection system to erect staircases on this project instead of corbels. Each floor was divided into two zones and each took a maximum of 16 hours to erect. According to the company, the tower is one of the tallest in the world to use hollow-core slabs. Hollow-core slabs are 40-50% lighter than conventional reinforced concrete slabs of the same dimensions. It has been calculated that a significant amount of concrete in the slab is superfluous, and so is therefore removed to create a hollow core – reducing the weight and delivering construction cost savings. UPC also recently announced a new facility, which will be three times the size of the current one, and will be the third or fourth biggest precast factory in the world – Amana Contracting & Steel Buildings announced earlier this year it has completed construction of the world’s largest pre-cast factory for Mammut Industries, a leading manufacturer of trailers, steel buildings, sandwich panels and concrete precast panels. Covering a total of 35,000m2, the Amana factory will produce all types of precast panels including new floor and wall panel technologies from Germany and Canada, greatly improving the speed, cost and quality of the construction sector. Jihad J. Bsaibes, general manager, Dubai office, Amana, said: “The structure encompasses a high level of design complexity as it includes extensive machine foundations, networks of underground trenches, batching plants, multi-level conveyor systems and stand-alone ovens for fast curing, among other features,” he says. “Because the factory is fully automated with a network of conveyor systems, it resulted in the need to cast a complex ground floor slab inclusive of various inserts and precise floor casting tolerances suitable to support the robotised processes.” One of the most ambitious construction projects underway in the region is the Dubai Creek Extension. Dutco Balfour Beatty Group was awarded the first phase of the Creek Extension contract, which involved the construction of the initial 7km of canal from Sheikh Zayed Road to Oud Metha Road. In this section of the works, the main canal width varied from 120m to 280m incorporating numerous curved features and smaller off-shoot canals extending from the main body of water. The edge treatment of the canal was formed with precast concrete blocks varying in weight from 24-34 tonnes. The quay walls are placed four courses high on a compacted crushed rock foundation. Geotextile is placed on the sides of the excavated surface, covered with core rock and a filter layer and then turned back onto the top surface to enclose the rock envelope to the rear of the quay wall. A precast concrete capping unit is fixed by dowel bars to the top of quay wall blocks. The daily planned concrete requirement on site, once quay wall block precasting reached full production, was 800m3 per day. To meet this demand a batching plant and precast yard was set up on site. This facility was located within the widest section of the canal so that construction operations would not be affected by developers occupying plots as quay wall construction progressed and future infrastructure contracts. Quay wall blocks were cast in the precast yard in specially developed steel moulds placed on concrete casting beds that were located on either side of elevated ramps to enable concrete placing by direct discharge. According to Dutco Balfour Beatty Group, the main challenge of the project was fast-track nature of the programme, with the initial 24-month duration timeframe reduced upon client’s request during post-tender negotiations. However, with an experienced team in place and due to the repetitive nature of the work, block production increased rapidly from 60 quay wall blocks per day to 100 as new targets were set and shutter rotation and placing cycle times reduced. This optimum rate of progress represented a concrete requirement of 1,400m3 per day. Timescale pressure is not the only driver for precast, however, with increasing legislation also having an impact on this sector, as Abdul Rahman Rashid, managing director, BENA explains: “Yes, there is a huge demand for lightweight concrete and thermal insulated products. Due to the law in the UAE where all new materials have to be thermal, a lot of companies now need these products. Also, the country is facing an acute shortage of labour supply. And the highly automated precast methods will reduce the dependency on labour, give higher accuracy and higher quality.” Using such techniques can cut short a project by almost six months. “We can build a 1,000m2 villa in 30 days. With conventional concrete it will take at least one year to build a villa of similar dimensions,” says Rashid, adding that it would only require six labourers to build a project of this size.” Rashid also points out there are significant material benefits to employing precast concrete that also ensure a better quality of product. “Each piece goes through a laboratory, so there is no chance of the quality of the blocks being diminished,” he says. “And since it is a controlled environment, if the raw material is not up to standard, we’ll know even before the blocks are ready by their appearance and texture, so there is no way any bad quality products will leave the lab.”||**||

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