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Sat 17 Jul 2010 04:00 AM

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Post haste

Post-tensioning reinforcement now dominates many areas of GCC construction, though innovation hasn't stopped. Ben Roberts reports.

Post haste

Post-tensioning reinforcement now dominates many areas of GCC construction, though innovation hasn't stopped. Ben Roberts reports

Adownturn in any market rarely prevents innovation among its players - indeed, the best ideas often arise when money is tight and needs to be conserved.

Advocates of post-tensioning work may attest to this, on the back of the growth in this method of building reinforcement over the last ten years. It has provided stiff competition for other methods of using steel to strengthen concrete, such as the use of reinforcing bars.

Today it is ubiquitous in bridge building and for structures that will carry a heavy load, such as multi-storey car parks - and innovations within post-tensioning are continuing apace.

Warwick Ironmonger, general manager for the Middle East Region at Dubai-based NASA Structural Systems, the local trading name of Structural Systems, says an educative drive by the company 14 years ago targeting consultants, authorities and contractors, has paid dividends.

"Nowadays, most consultants and contractors in the UAE are familiar, at least to some extent, with the efficiencies that can be achieved through the introduction of post-tensioning, but the concept needs further explanation to consultants and contractors in neighbouring Gulf states where there is interest in, but little awareness of, the technology," he says.

To begin at the beginning, post tensioning prevents concrete from developing, or at least minimises the development, of cracks, allowing the structure made from that concrete to be able to hold heavy weight without ‘deflecting' under the pressure. Deflection causes a concrete beam to elongate, causing cracks.

The process involves feeding high-strength steel strands - usually referred to as tendons - through drilled holes in the concrete. These tendons are stretched by hydraulic jacks and held in place by anchoring devices that look like wedges. The tautness of the permanently-stretched tendon essentially transfers to the strength of the concrete.

Post-tensioning engineers say that it reduces the amount of concrete needed. This allows the same beam, slab or wall to be thinner, with increased strength, allowing for all kinds of elaborate and intricate building designs to come into reality. Plus, less concrete means less cost.

Anish Thomas, technical manager at Middle East Prestressing (MEPS), says financial districts, where tall towers are constructed in close proximity to each other, also call for post-tensioning of the foundation walls. "Traditionally walls would be supported by anchors, but when you have buildings which are very close together, the client won't allow you to drill on another's land, so in that case you can't have a ground anchor support," he explains.

MEPS is distinctive in the Middle East by offering this service of reinforcing underground walls, also known as diaphragm walling. In each case, the walls can be made thinner than with traditional methods. "The strand acts as a stay cable as if for a bridge," explains Thomas.

"It's also quite a fast process too, and there is less concrete needed. For example, for the wall for Dubai Marina, where the concrete blocks could be as wide as eight metres, the thickness can be cut down, as well as the added benefit of better finishes and higher durability."

VSL, the engineering giant that has post-tensioning as its core business, also conducts diaphragm walling but through its foundation subsidiary Hong Kong Intrafor. For standard post-tensioning, its recent projects in the area include the Saadiyat Road Link, one of the UAE's largest civil infrastructure projects, which sees VSL provide strands for seven bridges with a total deck length of 1.6km. VSL was also involved with the Adnec Capital Gate Tower, a 35-floor mixed-use development with a total area of 50,000m2, and completed post-tensioning works, as well anchorage sets, for the Palm Jumeirah's Trunk Spine Bridge, in Dubai.

Expansion seems to be the key for all post-tensioning engineers, and the education of the industry must continue. Ironmonger adds that relatively strong demand in Dubai should not affect the drive into surrounding markets.

"There is definitely a need to scour adjoining emirates and GCC countries for infrastructure opportunities, as no significant bridge projects such as the Nad Al Sheba Racecourse Development Access Bridges and the Al Qudra Bridge both recently completed by Structural Systems, are currently being tendered by the RTA in Dubai."

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