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Sat 18 Jul 2009 04:00 AM

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Under pressure

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 guru's tell us where they're at.

Under pressure
VSL Middle East engineer stressing a post-tensioning tendon on a bridge.
Under pressure
Stephen Burke.
Under pressure
Warwick Ironmonger.
Under pressure
Ben Bowsher.
Under pressure
Khalil Doghri.
Under pressure
Tim Peters.

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 guru's tell us where they're at.

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

The concept was first implemented on bridges in Europe and then America and quickly became the norm, but as towers began to reach for the skies over the years, the use of post-tensioning has become quite popular in all areas of the construction industry worldwide including the Middle East.

Post-tensioning is widely believed to have come into existence way back in 1939 when Freyssinet first began to develop the idea. The concept of prestressing already existed but wasn't in wide practice at the time and hadn't been developed as much.

The first prestressed concrete bridges were built a few years before the Second World War and used bars for the purpose of pretensioning. But during the Second World War, because of the shortage of construction materials, the concept failed to pick up and only three prestressed structures were built by post-tensioning.

Anyway, beam back to 2009 and post-tensioning is now coming up as one of the most intelligent technologies in the industry with developments being made on the concept almost every year.

The environment

With Abu Dhabi being chosen to host the International Renewable Energy Agency's permanent headquarters, post-tensioning is set to become a favourite in the industry with its many green advantages.

"We're actually sponsoring some research at Griffith University in Australia regarding the environmental benefits of post-tensioning systems so this is a big interest for us," said Tim Peters, CEO of Australian post-tensioning consultant engineering firm Alliance Design Group. "We've got some numbers where we've actually analysed a typical 8m x 8m slab in both post-tensioned concrete and reinforced concrete, and if it's designed to code, you can have savings of up to 40% of embodied energy in the post-tensioned one, so there are massive environmental benefits.

Again no one has actually put their fingers on it, and most building assessment systems around the world don't truly capture the total cost of embodied energy in constructing buildings. We're taking this research in order to identify that if you utilise more efficient building solutions you'll save more. And then we've ultimately made an accreditation system that properly acknowledges the environmental stamp of the building that's being built as well," he said.

But apart from savings in embodied energy, what is the direct impact of post-tensioning on the environment? Stephen Burke, general manager of VSL Middle East had the answer. It's CO2.

"By using post-tensioning instead of reinforced concrete on a typical high rise tower, you save on structural concrete cross sections, allowing for thinner floors and slabs and thus giving rise to immediate reductions in concrete volumes (and the related CO2 from the cement production) in the order of 25% and reinforcing steel tonnages by up to 65%," explained Burke. "Overall post-tensioned structures have a significantly reduced environmental impact when compared to more traditional construction methods (i.e. the CO2 emission for reinforced concrete is 129.9kg CO2/m2, compared to 95.3kg CO2/m2 for post tensioning," he said.

In addition, Warwick Ironmonger, general manager for Nasa Structural Systems says, "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." 

And if such positive feedback didn't tip the scales, UK-based steel certification body, UK Cares, which has recently increased its focus on environmentally friendly solutions, is also in support of post-tensioning.

"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," says UK Cares executive director Ben Bowsher.

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.

Bowsher also recommends that approved post-tensioning contractors use environmentally certified suppliers, recycled aggregates in concrete, reusable formwork and falsework, and that any waste, such as grout, be disposed of responsibly.

Cost effective

But like most environmentally friendly products or systems that come with a higher price tag, post-tensioning, which is less environmentally harmful than most other construction techniques in the market and is cheaper, seems to be a win-win situation.

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 how cost effective is it really, and when does it fail to be the best option."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, which is typically draped over the span between supports, exerts an upward load that counter-acts the downward (or gravity) loads due to self-weight and super-imposed dead and live loads. This, combined with the axial compression that is imposed on the structure due to the application of the post-tensioning forces, results in an economical solution, whereby significant amounts of concrete and reinforcement, which would otherwise be experienced in a conventionally reinforced structure, are saved."

He also said that construction cost savings are significant as a direct result of less material needing be installed on site at a given period of time.

Peters explains further: "It is cost effective for certain minimum spans, but the rule of thumb in the UAE is that from 7.5m to 8m spans and above, post tensioning will be a cost effective solution over conventional reinforced concrete and steel, or a steel frame, but steel frames aren't the most common solution here anyway so it doesn't matter. "Traditionally it was used in bridges and it's been used in buildings since the 1970s in Australia and South East Asia. In this region, it probably came in about seven or eight years ago in Dubai, and more recently in Abu Dhabi," he added.

According to Peters the total cost savings also depends on design efficiencies but in his experience an average project would save up to 10% to 15% of structural costs. One of the reasons for this would be due to a reduction in the weight of the structure, which would add a further saving including on foundations, columns and walls.

"The beauty of post-tensioning solutions is you can offer better solutions, which mean cleaner and faster delivery, reduced floor to floor height etc, so there are a lot of on savings that aren't very often taken advantage of," he said.

Labour

Post-tensioning is cost effective for buildings as well. The key, like Peters said, is the span - if it's greater than 7.5m it's definitely going to be cheaper than reinforced concrete. One can save, in terms of concrete volume, anywhere from 15% to 25%, depending on the application and obviously the design. There are huge savings on the amount of materials required to go in and that has subsequent savings on labour.

"We did the Motor City project in Dubai," continued Peters, "and [were hired] to do the raft slabs, and they were the first post tension rafts ever done in Dubai and approved by Dubai Municipality, the gangs were 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.

On many projects, the standard local post-tensioning supply arrangement is that post-tensioning contractors provide only the materials and the supervision while the main contractors arrange the labour force. But is this feasible?

"Post-tensioning is a specialty service that requires highly qualified engineers and technicians on sites," says Khalil Doghri, project manager for Freyssinet Middle East. "We manage our sites with experienced staff and we educate our engineers and technicians on a regular basis through a Freyssinet group training programme." Very often in conversation you may hear the term post-tensioning and Freyssinet mentioned in the same sentence. Historically, the French lay claim to the initial concept.

Burke adds: "The issue of training is paramount, in our business where technical excellence is vital both for staff and the project, VSL Middle East has opened a specific training academy in the UAE to instill safety, technical competences and company culture to our operations staff at all levels."

But while Doghri and Burke stress high skills for all, Peters says supervision is key.

"It's more about the supervision," says Peters. "You need skilled supervisors but you can use unskilled labourers to do most of the hack work. Of course there is training involved with the unskilled workers, but primarily it is skilled supervisors controlling unskilled labour. So it's all about the quality of the supervisors really."

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.

Abu Dhabi

Abu Dhabi seems to have become the newest patron to the idea of post-tensioning with many companies that specialise in the technology, now having realised the potential and therefore setting up offices in the UAE capital.

"We're the principal consultant on Al Reem Island and we have in access of 1 million m2 of suspended post-tensioned slabs there, says Peters. "The first buildings on Al Reem have been done in post-tensioned concrete and the tendency for most of them is to go that way. At first in Abu Dhabi, it wasn't the solution of choice but it's definitely happening and the post-tensioning companies are all setting up there so it's certainly on the upswing."

New developments

Apart from research being conducted by AGMC on the effects of post-tensioning on the environment, other developments in the field are currently underway, including VSL's new saddle for its SSI 2000 Bridge Stay Cable System, which allows cables to be installed and later examined on a strand by strand basis independently.Ironmonger says that the recent introduction of plastic ducts in bridge structures offers advantages over the commonly used galvanized metal ducts due to improved sealing of the joints in the ducts, and hence the entire tendon.

"It has been the norm for most prestressing strand in bridge structures to be specified with a 0.6 inch strand and a breaking load of up to 265.5kN per strand, but the recent development of 15.7mm diameter strand of grade 1860MPa of 279kN Breaking Load has effectively enabled a 5% increase in prestressing force to be applied with no change in tendon size," he said.

Challenges

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," explains Doghri.

"A problem, if any, can rise only if this operation is not correctly conducted. It should be achieved as per the good-practice rules, using the adequate material and equipment and qualified personnel to operate. The procedures of grouting and the type of cement used shall 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."

Another question that is begging to be asked is, if post-tensioning, due to its being cheaper, also means a final product of lesser quality and therefore less durable? But Burke vehemently denies this.

"The economies realised that bridge post-tensioning is achieved using internationally recognized codes of practices and state of the art engineering. The fact that many millions of square meters of post-tensioned bridge decks have been constructed safely worldwide is testament to the success of post tensioning technology," he said.

Doghri feels as strongly about the quality of a finished post-tensioned product and says: "Post-tensioning has been behind all the development of modern construction, particularly for bridges. 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.

BioStephen Burke

Stephen Burke is the deputy general manager for VSL Middle East and has been based in the UAE for the past 15 years. Coming from a heavy civils background, Burke is primarily involved in business development for VSL Middle East throughout the Gulf region and has recently been involved in the opening VSL ‘s new permanent office in Doha Qatar.

WarwickIronmonger

Warwick Ironmonger is the general manager of Nasa Structural Systems. He is a structural engineer holding an Honours degree in Civil Engineering and a Masters degree in Engineering Science (Structures) from the University of New South Wales, Australia. Warwick's involvement in post-tensioning stemmed from his studies and thesis at University, and has included 18 years of professional experience providing post-tensioned solutions to the construction industry as a consultant (up until 2000), and then as a specialist contractor. Warwick has been involved in post-tensioning works in Australia, Philippines, UK and the Middle East. His first assignment in the UAE in 1996 included the post-tensioning design management for the Burj Al Arab.

Ben Bowsher

Ben Bowsher is executive director of UK Cares, the UK based certification body primarily concerned with compliance of reinforcing and prestressing steels. He represents Cares on key UK and international certification and accreditation committees, recognising that steel manufacture, supply and use is becoming truly global in nature.

Khalil Doghri

Khalil Doghri is the General Manager of Freyssinet Activities in the Middle East, covering all Freyssinet Group subsidiaries in the region. Holder of Master Degree in Civil Engineering from" Ecole Nationale des Ponts et Chaussees in Paris". Has been with the Freyssinet group since 1987 and established in UAE since 2003. Past experience was with the Head office in the Technical Department in France and at Freyssinet USA.

Tim Peters

Tim Peters is CEO of Australian consulting engineering firm Alliance Design Group. He has extensive experience in numerous large scale building projects in Australia, Asia, the Middle East and the UK.  He has particular expertise in the use of post-tensioned solutions for building structures and is recognised for delivery of budget conscious buildable solutions. He lectures in post tensioning at several universities in Australia and is a member of the Academic Advisory Board at the School of Engineering, Griffith University.

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