The economics of post tensioning
POST-TENSIONED slab design is widely used in Australia, with the main benefits over conventionally reinforced concrete being its ability to span greater distances without resorting to thick slabs and beams (and the consequent loss of head height), and reduce deflection and cracking. There are also some associated labour and timesavings.
There are three basic alternatives in insitu concrete framed structure – conventionally reinforced slabs, fully prestressed slabs and partially prestressed slabs. Prestressing involves applying a load to a structure prior to its design load being taken up, and can involve either pre-tensioning or post-tensioning, depending on whether the tension is taken up before or after the concrete sets.
The choice of reinforcing is often a matter of economics, with formwork and steel tying labour costs being a critical factor, but as a rule the breakeven mark between conventional and prestressed solutions is somewhere in the seven to eight metre span range.
Cables are used most commonly for post-tensioning in Australia, although threaded high tensile bar can be used in some applications. Cables are laid inside galvanised ducting, with the ends of the cable protruding through the edge of the slab or through pans set in the surface of the slab, towards the edge. Generally the strands are partially tensioned after 24 hours, to minimise cracking as the concrete sets, with full tensioning after four to five days. Conventional reinforcing is required around the pan because of the forces involved, to develop the forces from the anchor into the structure.
A world perspective
There are a number of companies in Australia who specialise in the design and installation of prestressed solutions, and have an extensive investment in training and specialised equipment.
StrongForce specialises in prestressed solutions, and from its original Brisbane base now competes strongly throughout the eastern seaboard and the Northern Territory. It has developed something of a reputation for providing alternative design, often in conjunction with major builders such as Barclay Mowlem.
Chief design engineer Tim Peters is based on the Gold Coast, but the design office there provides input on projects around the country, as well as overseas projects in Asia and the UK. Irish builder O’Rourke was sufficiently impressed with StrongForce’s contribution on projects they worked together on in the UK that it took a 51 per cent interest in StrongForce in early 2002, and the fruits of this are now seen in regular exchanges of staff between the UK and Australia.
Having been involved in projects around the world, Peters is in a good position evaluate prestressed solutions around the world. His observation is that post-tensioning is accepted in the US, but the cable is generally unbonded, but in the UK it is not widely accepted, and where it is used the cable is generally unbonded (bonding means that the cable is grouted inside its protective tube after it is tensioned, thus providing protection to the cable from accidental severing). Europe favours bonded post-tensioning while in Asia post-tensioning is accepted in most countries other than in Hong Kong, which has followed UK codes.
Peters believes that post-tensioning gained a black mark in earlier times in the UK after failure of a bridge, when road salt used during winter and poor grouting caused corrosion that led to failure. He believes that modern practices have overcome this problem, with practitioners now registered under CARES program in the UK.
Peters finds conventional reinforced slabs are best suited to spans up to eight metres where the spans are uniform and the formwork is simple, so that the form workers can work quickly. This is common in simple residential boxes up to three storeys, although he finds that there is often a wider span required over a driveway to provide parking access. More columns are required for conventionally reinforced slabs, and this is where the labour costs come in.
Conventionally reinforced slabs are also used commonly in basements, where there are concerns about the restraint to the prestressing forces offered by the perimeter walls. However in some cases prestressed beams are used to maximise the head height.
Partially prestressed slabs have become more common in recent times, and are a combination of conventional reinforcing and prestressing. Examples include the Worldmark at Kirra Beach Resort (Gold Coast) built for Trendwest Resorts South Pacific by Barclay Mowlem; Arbor on Grey (Brisbane) by Mirvac; Dockside Towers (Melbourne) by L.U. Simon and Indigo Towers at Burleigh by Matrix.
The partial solution has the advantage of reducing the depth of the slab, the weight of reinforcing, and the volume of concrete (by up to 25 per cent). Partial prestressing has been used for larger spans in buildings where conventional reinforcing is used for the shorter spans.
While generally the partially prestressed solutions have been offered as alternatives to conventionally reinforced slabs, StrongForce successfully offered this as an alternative to a fully prestressed structure to break into the Melbourne market at Dockside. Believing that it would be unable to beat local firms on price with a fully prestressed solution, it used finite element analysis to come up with an economical partial prestressed solution that used conventional reinforcing at the top and bottom of the slab and profiled post-tensioned cable in the critical slabs.
Areas for improvement?
While generally Peters believes that Australian practice in post-tensioned slabs is on par with world’s best practice, he believes that there are considerable benefits in employing the internationally developed practice of using precast edges for flat facades, and tensioning the slabs to the edge through set anchors in these precast units, thus simplifying the process compared to using pans for post-tensioning the full slab, avoiding the extensive reinforcing required around pans and the finishing required to create a flat floor over the pans.
He also believes that Australia could learn from the use of laser screeds overseas in multi-storey construction, where labour is restricted to the operator and a person in front of the screed controlling the flow of concrete; from the use of precast columns; and from labour-saving methods of placing concrete. In general be believes that Australia leads in its ability to control material costs (concrete and reinforcing) but could learn from overseas in buildability and labour control areas.
The Riparian Plaza is the tallest building currently under construction in Brisbane, with structural design work undertaken by Robert Bird & Partners. To maximise the column-free floor area and reduce floor-to-floor heights post-tensioned floor plates achieve the large spans between the central core and perimeter columns. As Riparian Plaza is a mixed use development a variety of structural floor plate options were required to suit the floor function. All floor plates used N40 concrete for high early strength to facilitate stressing at three days.
The carpark floors will be 250mm thick simply-supported, fully post-tensioned flat plates spanning from the core to an 1800mm wide perimeter band beam. An 1800mm wide thickened band beam hugs the core. This architect chose this design to give continuous uninterrupted structural lines around the soffit of the 11 carpark floors.
The office floors use post-tensioned wide flat beams spaced radially around the core at around 5.2m centres and spanning from the core to a perimeter spandrel beam. Recessed haunches at the support locations accommodate services and minimise floor-to-floor heights. A 130mm thick conventionally reinforced two-way slab was adopted between the band beams for structural efficiency, optimised spatial requirements for services and minimised floor-to-floor heights.
The residential floors are 250mm thick simply supported fully post-tensioned flat plates spanning from the core to a wide flat perimeter band beam. A thickened band hugging the core is used for positive connectivity. For balconies a 300mm thick slab will cantilever up to 5m. This design minimises floor-to-floor heights and provides flexibility for residential services fitout.
Robert Bird & Partners also undertook design for Cutters Landing Apartments 1 to 3: a grouping of three 8-storey apartments at New Farm in Brisbane. The design philosophy was to maximise river views through large open living areas and balconies. To achieve these views vertical structure was minimised. A uniform slab thickness was adopted for the design, based on conventionally reinforced slabs for spans to 5.6m. A uniform slab thickness was essential as the concrete soffits doubled as ceilings for the units.
To increase the spans to achieve the design objectives while maintaining constant slab thickness post-tensioning was introduced locally to control deflection, allowing the spacing between columns to be increased from 5.6m to 8m. This had the added benefit of maintaining the minimum floor-to-floor height critical to satisfying council development approval conditions. The cost of this approach is more than justified by the improved market appeal of the sweeping river views.
On the ground
Post-tensioned slabs have equal application on the ground, where they can minimise the use of joints and overcome problems with raft slabs in areas of high hydrostatic load. The long spans provide a better surface for materials handling equipment to operate on in warehouses, and greater strength in applications where heavy equipment uses the slab regularly (a 230mm thick post-tensioned slab at Penang Airport in Malaysia has been designed for three jumbo jets). Peters believes that post-tensioned slabs become the attractive the worse the ground condition or the higher the load becomes. He also believes that by having a joint at one location it becomes economic to install a “Rolls Royce” joint that overcomes traditional problems.