Post-tensioning: Tight deadlines
As construction efforts ramp up throughout the region, post-tensioning is proving its value in helping to deliver top-quality structures in rapid time
As construction efforts ramp up throughout the region, post-tensioning is proving its value in helping to deliver top-quality structures in rapid time. Jamie Knights reports.
With all systems go regarding construction projects of all shapes and sizes in the region, the post-tensioning sector is seeing an upswing in demand. Its cost-effective techniques, coupled with growing concern about the availability of resources, means interest is only likely to get stronger.
So how does this specialist area of construction fit into the wider picture?
CCL, with a robust worldwide presence, has completed more than 6 million m2 of post-tensioning in Qatar alone over the last 10 years. Included in this are projects with Qatar Foundation, Qatar Petroleum, shopping malls, office complexes, hotels and residential developments.
Alberto Issa, technical manager at CCL’s Qatar arm, argues that high-profile international events such as World Cup 2022 and Expo 2020 not only depend on critical special structures such as stadia and conference venues, but they “also result in a ripple effect with added demand from governments and private developers for new construction of both infrastructure and high/low rise buildings to accommodate expected tourists”.
“The build-up phase will be on a large scale and within a short period of time, making timely, cost-effective, material-saving project design and technology necessary,” he continues. “This is where post-tensioning comes in.”
Post-tensioning technique and design allows for long, flat spans, with minimal reinforcement required, making the construction process simpler and faster.
It also ensures a fast redeployment of shuttering after a slab has been poured.
Issa says: “One of the many advantages of the post-tensioning technique is that it allows for reduced slab thickness, and therefore requires smaller quantities of concrete (cement, sand, gravel) and passive reinforcement (normal rebars), insulating it from the adverse effects of a possible shortage of materials.
He continues: “The?expected high demand for materials and the limited supply in the region, especially of materials such as passive reinforcement, concrete ,aggregates, etc, will make any savings on materials crucial.”
Rachmad Irwanto Ichsan of Ichsan Post-Tensioning concurs and believes that major events will provide a good platform on which post-tensioning can showcase its capabilities. Ichsan’s main business comes from private land owners “willing to take a risk in appointing us for their investment,” he says. “But we’ll pay them back well with special designs and excellent execution”.
Recent post-tensioning projects for Ichsan include multi-storey buildings, but a gap in the market has opened up for certain industry techniques in the construction of luxury villas.
“With good knowledge from both the engineers and the clients, post-tensioning will make a significant contribution to major upcoming projects,” he adds.
VSL is currently providing post-tensioning solutions to a number of large bridge infrastructure projects in the UAE, Qatar, KSA and Oman. Cable Stay Hodariyat Bridge in Abu Dhabi, which VSL delivered for TDIC as a main contractor in a joint-venture with AST, has been a recent highlight.
And as the adoption of external post-tensioning technology for bridge works is gathering favour, Stephen Burke, deputy general manager VSL says it is possible for the client’s preventative maintenance team to easily monitor the condition of the post-tensioning over the lifetime of their infrastructure assets.
“Regarding Dubai’s Expo 2020, local press has announced that the Dubai Metro will be extended to the new site, the elevated concrete super-structure sections would employ post-tensioning as did the original Red and Green lines that VSL was proud to be a part of,” he asserts.
“For the World Cup in Qatar post-tensioning techniques will be implemented for the construction of the large stadia which are currently under design.”
Post-tensioning experts have been noting changes, especially with the use of new materials and techniques.
Irwanto says he has been intrigued by reports of post-tensioning on a timber structure and believes it will be “interesting if local municipalities allow that”.
For Burke, advances in the field of post-tensioning research and development “could ultimately see the replacement of steel-based components with carbon fibre elements, which have the strength of steel but are much lighter and very durable”.
“We have seen carbon fibre replacing steel in the aircraft industry over recent years, therefore a progression to the adoption of carbon fibre with respect to the post-tensioning industry is not science fiction,” he adds.
Combining post-tensioning services for precast elements is a recent development and CCL has provided post-tensioning for precast structural elements such as beams and slabs, according to Issa.
“Even though these two technologies have been available for quite some time now, combining the two techniques has helped our clients solve many of the challenges they faced in transporting heavy, long-span beams from precast factories to site,” he continues.
“Casting yards were built and post-tensioning was used in order to stress the beams on site and then install them as required.”
Furthermore, use of plastic ducts for post-tensioning bonded systems is now used by the post-tensioning industry in severe environmental conditions such as areas subject to high chloride levels ? bridges over sea water stretches, for example ? and areas beneath paths for metro rails and at stations.
“Choosing the right plastic duct and duct manufacturer is vital,” warns Issa.
“Specifications have been developed to ensure the correct material is used, for instance the plastic should be from a pure source and the duct should be stable against UV.”
While advancements are creating opportunities, there are, of course, issues to contend with. Irwanto specifies three major challenges facing the post-tensioning sector in the region.
“Challenges happen when engineers – especially hired foreigners – who represent the government set their own interpretations as the rules for approvals of building permit applications,” he begins.
“Secondly, clients in the Middle East should be aware of the benefits of having a specialised post-tensioning consultant, instead of using their own general managers.
“The third point is municipalities should encourage land owners to employ cost-optimised design through official seminars and advertisements in order to educate them for basic technical awareness.”
Taking the argument forward, Issa states that a lack of a regulatory authority for the post-tensioning technique is a challenge.
“The design of post-tensioned slabs, beams and even columns and walls is regulated in Europe and in the United States by organisations such as the PTI, in which CCL is very active, ACI, and in Europe by FIB, and by the implementation of Euro Codes,” he explains.
“The region has yet to develop the necessary research funds and regulatory authority to produce specific codes and specifications for the design and implementation of the post-tensioning technique.
“It continues to rely on international standards which may not be suitable for its specific environment.”
Having said that, Qatar has taken a step in this direction by including part of the post-tensioning specifications as a section within the Qatar Construction Standards Manual.
And Issa continues that to ensure proper application of post-tensioning techniques, several consultants have already set up minimum quality requirements which must be met before a post-tensioning specialist is approved on a project.
“The post-tensioning system should be fully compatible (following ETAG 013) and from the same brand, the design should be carried out using the latest finite element software, and original software versions must be used,” he asserts.
While, as Issa tells Construction Week that post-tensioning technology was introduced primarily for bridges and later in slabs, “over time, as architects and engineers developed more complicated structures, and demanded longer spans and more open spaces, post-tensioning technology presented itself as a solution”.
This prompted a wider use of the technology and a rise in demand for using post-tensioning in almost every type of construction, from single family houses, to low- and high-rise buildings, hospitals, malls and offshore oil platforms has been noted.
“Post-tensioning has the potential to be used in any structural elements such as high retaining walls, silos [and] core walls of skyscrapers,” Issa continues.
“In addition, the use of external post-tensioning, especially in the construction of bridges, has led to the introduction of this innovative technique in the strengthening of concrete slabs.
“Where deflection is an issue, introducing external post-tensioning cables to stress the slabs and reduce deflection can provide an effective and easy-to-apply solution to this problem.”
And Irwanto believes that post-tensioning in the Middle East “will be good for the next 10 to 15 years”.
However, he leaves a warning that “development – in terms of technology - on the other hand would distract the business if that happens too early within that period”.