Building tension

Post-tensioning is used on 90% of bridge projects in the UAE.

New developments
Apart from research being conducted 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.

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,” says Warwick Ironmonger, general manager for Nasa Structural Systems.

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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.”