Wired for strength
Post tensioning is a technique for reinforcing concrete and is concealed within many of the iconic structures within Qatar
Post tensioning is used widely in bridges, elevated slabs (parking structures and residential or commercial buildings), residential foundations, walls, and columns.
The system has a number of benefits over pre-tensioned concrete, as it offers a greater scope of flexibility. For example, the use of post tensioned reinforcement to construct slabs results in thinner concrete sections and/or longer spans and thinner decks between supports, as in bridges.
How it works is that post-tensioning tendons – which are prestressing steel cables inside plastic ducts or sleeves – are positioned in the forms before the concrete is placed. Thereafter, once the concrete has gained strength – but before the service loads are applied – the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete.
Most precast, prestressed concrete is pre-tensioned – the steel is pulled before the concrete is poured. Post tensioned concrete means that the concrete is poured and then the tension is applied – but it is still stressed before the loads are applied.
Post tensioning can either be unbonded, with single tendons typically encased in a greased tube and grouted at either end to the anchors, or bonded. The bonded systems are commonly used in all types of building structures.
In this process, several strands of tendons are placed together in ducting systems, with high-strength grouting injected inside the ducting. The unbonded process comprises single strand anchorages and plastic coated strands – the ducts and the grouting process are eliminated. It benefits from being more adaptable on site – the strands can be routed around service holes and curved in plan on more complex floor plates.
Currently constructing over 85km of various pipeline networks on package CP5B on the Lusail City project (as a sub-contractor to Samsung C&T), Mace Qatar is in partnership with OVM Mena (Middle East and North Africa) and offers post-tensioning technology and material services in Qatar.
Majed Bseiso, business development manager for Mace-PTM comments: “Post tensioning is a specialist engineering process and the benefit is that with it there is no need for as many columns along the span of a bridge, to support the structure. It offers wider spans and greater loads.”
Post tensioning is becoming increasingly popular for architectural or highly serviced buildings owing to the advantages it brings over reinforced concrete and it is also well suited to projects with exposed soffits or cement replacements.
According to John Habib, general manager CCL Qatar, “Within the Qatari market we are seeing continued increase in demand for buildings with more open spaces and fewer columns to facilitate alterations to layout in order to accommodate the needs of future tenants.
“The requirement for greater layout flexibility to accommodate future change of use, is reflected in the number of slabs designed by CCL to allow for these architectural modifications requested by building owners and tenants,” he says. Also, by reducing the thickness of each floor in a building, the overall weight of the structure is reduced while decreasing the ceiling to floor height of each level.
Bseiso adds “We use post tensioning in buildings to give us a reduced floor thickness, which is much less than the conventional method.” In belowgrade structures, this can mean less excavation, and in abovegrade structures, it can mean a reduced overall building height.
Bseiso adds, “Through post tensioning you lower the amount of steel used in a structure, namely rebar reinforcement, and you also lower the amount of concrete needed. This in turn has the spin-off that contractors’ costs are lowered.”
Also, post tensioning concrete floor slabs accelerates construction cycles through simplified formwork systems
and reduces crane time, as well as manpower and materials required for residential structures.
This results in substantial savings says Bseiso: “This can amount to between 30 and 40% of materials costs. More and more contractors will therefore use this system,” he adds.