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Sat 13 Mar 2010 04:00 AM

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Building tension

Post-tensioning has revolutionised the way the construction industry has begun to build in the region. From bridges to buildings, and from being environmentally friendly to cost efficiency,some of the regional post-tensioning experts tell us where they're at.

Post-tensioning has revolutionised the way the construction industry has begun to build in the region. From bridges to buildings, and from being environmentally friendly to cost efficiency,some of the regional post-tensioning experts tell us where they're at.

Post-tensioning, which has been around since the 1940s gained popularity in the Gulf, particularly in Dubai, only about ten years ago.

It is now widely accepted as one of the most intelligent techniques in construction with improvements being made on the concept almost every year.

What is bonded post-tensioning?

"Bonded post-tensioning describes the method of applying compression after pouring concrete and the curing process," explains VSL Middle East civil engineer, David Grattaux.

"The concrete is cast around plastic, steel or aluminium curved ducts, to follow the area where otherwise tension would occur in the concrete element."

In lay terms, the process involves a set of ‘tendons', similar to steel cables, which are then fished through the duct and concrete poured. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react against the concrete member itself. The tendons are then stretched; they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete. The duct is then (coated, grouted) to protect the tendons from rust.

This method is commonly used to create monolithic slabs for house construction in locations where expansive soil create problems for the typical perimeter foundation. All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure. Post-tensioning is also used in the construction of various bridges, both after concrete is cured and by the assembly of prefabricated sections.

"Post tensioning has become an extremely popular method of construction over the last ten years, as it substantially reduces the amount of raw product [concrete] that is needed for any project, which reduces cost, ground loading, is more environmentally friendly and it allows the architect a free reign," adds Grattaux.

"Post-tensioning generates excellent condition in the structure, enabling efficient use of building materials while controlling deformations under service conditions."

Unbonded vs Bonded post-tensioning

In the Gulf, post-tensioning works tend to be bonded, but unbonded, or external, post-tensioning is also recommended by some contractors depending on the type of construction.

In unbonded systems, the prestressing steel is only bonded to the concrete at the anchorages. In bonded post-tensioning, however, tendons are inserted into a metal or plastic duct, embedded within the concrete, and anchored in a common anchorage device.

"For long bridges," says Najdat Othman, construction manager for Nakheel bridges at Parsons, "it's safe and economical to use the external post-tensioning system.

The advantage is that you can always monitor the ducts for defects. After the bridge has been used for a few years, it may need to be strengthened more, and that can also be done easily."

Working together - contractors and clients

Post-tensioning has revolutionised the construction of infrastructure and is quickly providing subcontractors and their clients with new ways of working together.

Its implementation results in saving building materials, enhancing the performance of concrete structures and simplifying construction - all the elements which bring the goals of contractors and clients together.

A green practice

Despite other advantages, post-tensioning is also environment-friendly.

"If a typical 8m x 8m slab of post-tensioned concrete is designed to code, you can save up to 40% of embodied energy in it, compared to a reinforced concrete slab, so there are massive environmental benefits, said Tim Peters, CEO of Australian post-tensioning consultant engineering firm Alliance Design Group.

But apart from savings in embodied energy, what is the direct impact of post-tensioning on the environment?

Executive director, Ben Bowsher of UK-based steel certification body, UK Cares, which focuses on environmentally friendly solutions said: "Post Tensioning may be considered to be more environmentally friendly than reinforced concrete as it generally allows greater spans of structural elements whilst using thinner structural elements for those spans. This means using less steel and also less concrete.That's less rebar and less cement, which are both very carbon aggressive polluters. Then there's a saving in the cost of transportation and the aggregates like quarrying.

Warwick Ironmonger, general manager for Nasa Structural Systems seconds Bowsher saying: "The implementation of post-tensioning leads to reduced consumption of reinforcement and the cement, water, sand, aggregate and admixtures associated with concrete.  Less material consumption also leads to less transportation, and less pollution."

Save money

But unlike many pricey environment-friendly systems that are currently available in the market, post-tensioning is much easier on the wallet.

We already know that its implementation results in saving building materials, enhancing the performance of concrete structures and simplifying construction.

We also know that the technology strengthens concrete with steel and so we need less concrete for the slabs; and that 90% of bridges and 80% of high rise buildings in the UAE have used post-tensioning.  But exactly how cost effective is it?

"Post-tensioning is more cost effective than traditional reinforced concrete for large span structures given the active nature of the post-tensioning," says Ironmonger.

"The stressing of the high tensile post-tensioning steel, exerts an upward load that counter-acts the downward (or gravity) loads due to self-weight and super-imposed dead and live loads. This results in significant amounts of concrete and reinforcement, which would otherwise be experienced in a conventionally reinforced structure, being saved."

He also said that construction costs are kept to a minimum due to fewer raw materials being needed on site.

According to Peters the total cost savings also depends on design efficiencies but could save up to 10% to 15% of structural costs.

Reduce dependence on labour

In the current cash-strapped climate, savings on labour costs have almost become a necessity. Post-tensioning wins here too.

"We did the Motor City project in Dubai," continued Peters, "and our labour force was reduced from 32 labourers to eight, so there was roughly a quarter of the amount of labour required in any given area at a time."

But because post tensioning uses very complicated machinery, it's very important that the subcontractor has a system to educate and train its engineers and workers.

 "Post-tensioning is a specialty service that requires highly qualified engineers and technicians on sites," says a spokesperson for Freyssinet Middle East.

Ironmonger agrees that supervision as well as other factors are not areas that contractors and consultants can cut costs on.

"Consultants and main contractors should be aware that reducing levels of supervision, supplying GI ducting with reduced wall thickness, ribbing and levels of galvanising; or supplying cheap anchors (that jeopardise safety) to cut costs is not the way forward," he said.

Challenges to overcome

The biggest issue in the post-tensioning industry is widely understood to revolve around grouting with VSL again having invested a lot of research and development in order to ensure that the latest techniques and technologies are applied to this scope of the works. It has even published a comprehensive technical guide for best practice on the issue.

Grouting is meant primarily to protect the post-tensioning steel from corrosion, and secondly, to ensure a proper bonding between the post-tensioning cables and the concrete.

The problem is such only arises if this procedure is not carried out in the proper manner. The procedures of grouting and the type of cement used should be adapted on a case by case basis to take into account different parameters such as the ambient temperature, the length and the size of the cables and the venting installation to expel the trapped air in the ducts.

But whenever one hears of a product being cheaper it is vastly associated with being of lesser quality and therefore less durability.

"Post-tensioning has been behind all the development of modern construction, particularly for bridges," said the Freyssinet spokesperson.

"The technical limits have been pushed to the extent that only post-tensioned structures can suit the needs in all aspects: cost effectiveness, safety, serviceability and durability. All combined together make it the best option."