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What is Post Tensioning?

Post-tensioning is a method of reinforcing (strengthening) concrete or other materials with high-strength steel strands. The technology reduces the amount of conventional reinforcement and concrete required thus delivering a more economical and environmentally friendly solution.

The benefits post-tensioning brings to a structure include:

• Large open plan column free floor spaces
• Reduced construction costs
• Faster construction
• Water resistant properties
• Floor to floor height reduction
• Reduced carbon footprint
• Greater loading capacity
• Larger spans

Post-tensioning provides active reinforcement and places the concrete structure under compression in those regions where load causes tensile stress.  It is an efficient way of reinforcing concrete while you are building – often allowing the construction of something which might otherwise have been impossible.

Developments in post-tensioning over the past decades means it is now a mainstream component of the construction industry and is applied in various structures including bridges, buildings, arenas, dams, nuclear power stations, wind farms, cryogenic LNG tanks, silos, wastewater treatment plants, water reservoirs, marine structures, retaining walls, towers and tunnels.

There are many key benefits of utilising post-tensioning within buildings, including:

• Greater flexibility of design – PT allows highly creative architectural approaches and very demanding geometry requirements.
• Increased deflection control and improved crack control – virtually crack-free concrete delivers long term durability.
• Faster construction program – early strength stressing promotes a faster cycle time, allowing the structure to progress more rapidly.
• Lower construction material costs – PT can decrease the size of a structural member and reduce the quantity of concrete and steel required for the superstructure.
• Reduced environmental impact – less material creates fewer carbon emissions in terms of production and transportation.
• Reduced construction costs – a faster cycle time and decreased amount of material minimise costs.

A common method of post-tensioning used within buildings is the slab post-tensioning system.  Max Schweiger, Director of the Post-Tensioning Institute of Australia and General Manager of SRG Limited explains how the slab post-tensioning works.

“The Slab Post-Tensioning System comprises high strength steel strands, up to six in number, placed inside flat ducting, typically anchored at one end by deforming the strand and casting it into the concrete then at the other end by means of a steel anchorage casting and anchor block(s) with gripping wedges.”

“After the concrete has reached its transfer strength, the individual strands have specified load applied.  The duct is filled with a high strength cementitious grout mixture to ensure the system is bonded and corrosion protection is maintained in service.”

Applications for slab post-tensioning include:

• Low to high rise residential and commercial buildings
• Industrial floor slabs on grade
• Transfer floor structures
• Carparks
• Water tank bases and walls
• Transverse stressing of bridge decks

Slab post-tensioning was incorporated in the commercial office towers and apartments at Barangaroo South in Sydney for Lendlease.  SRG Limited completed these works which included shop drawings, installation, stressing and grouting of post-tensioning for three multi-storey towers and additional slabs, entailing close to 2,500 tonnes of 15.2mm strand.

The Post-Tensioning Institute of Australia was formed to represent the post-tensioning contracting industry in Australia, seeking to maintain a viable and competitive industry with high standards in design and construction of post-tensioning systems for the benefits of its members, their employees and their clients.

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