Mind the gap

The burgeoning transport network in Dubai is presenting opportunities for a number of specialist contractors. The emirate's expanding road system requires bridge experts for its numerous overpasses and the Metro project, as well as large developments such as the Garhoud Bridge extension and Business Bay Bridge. Companies are therefore prioritising workloads and pushing new technologies into the market.

Stephen Burke, manager, VSL Middle East, explains. "Innovation and cutting edge technology is something VSL is always pushing and we are currently involved in the Metro superstructure works, using launching girders with a pre-cast concrete segmental system, which is a first in Dubai."

A post-tensioning specialist, VSL's dedicated R&D team is based at the company's head office in Switzerland and continues to explore ways of improving the post-tensioning technology in bridge construction. In this case, VSL has been pioneering post-tensioning plastic duct system and electrically isolated tendons, as well as innovations to reduce wind drag loads on cable-stayed bridge structures.

Burk adds that in pursuing the plastic route, Dubai is, surprisingly, not leading the way, but showing interest in adopting the technique being shown by its neighbour. "In Abu Dhabi they are moving away from steel ducts and specifying plastic as the norm. For once, Abu Dhabi is ahead of Dubai on this," adds Burke.

VSL is working on a number of high profile projects in Dubai and the neighbouring emirates. It is carrying out work on the Garhoud Bridge extension, Cultural Village bridge, the double-decking of Doha road, the Jebel Ali airport bridge and a number of other projects in Al Ain and Sharjah.

The technology of post-tensioning is commonly used for the construction of bridges. It is a method of applying compression after pouring concrete and the curing process. Concrete is cast around a duct, before sets of tendons are placed and the concrete is poured. Once this has hardened, the tendons are tensioned by hydraulic jacks transferring pressure to the concrete. The duct is then grouted to protect the tendons from corrosion. VSL has developed its own particular type of grout to inject into the ducts. According to Burke, it is a high performance product which is tailor-made for different types of cement, making the job a lot easier.

This technique affords a number of benefits over conventional concrete, namely it allows the bridge deck to be lighter and thinner, hence you can increase the span of the bridge without requiring so many columns or piers. It saves time and money during construction and in the long run there is less maintenance to be done.

Khalil Dhogri, general manager, Freyssinet Middle East, says: "The nature of new bridges in the UAE deserves to see new worldwide technologies used in their construction. These technologies relate to different construction methods such as segmental span by span [Dubai Metro], balanced cantilever [cast in situ or precast] and incremental launching of decks." Doghri adds that it is worth noting that the largest infrastructure project currently taking place in the UAE - the Dubai Metro - is having its elevated deck built using the segmental ‘span by span' method avoiding any scaffolding, traffic diversions and complications, thereby ensuring a very high speed of construction.

Freyssinet carries out post-tensioning of building slabs as well as bridges, where the company also covers the technologies of constructions of decks, bridge bearings and expansion joints, and supply and installation of post-tensioning cables. The firm is currently involved in a number of projects in Dubai including the Dubai Metro, Arabian Ranches interchange and Dubai Bypass Three.

Technology recently seen in Iraq during the rebuild programme has been implemented in Dubai on a slightly smaller scale. At the beginning of this year, Dubai Municipality began a US $4.8 million (AED17.6 million) project to construct five pedestrian bridges within the city to improve flows and safety.