What is Post-tensioning?

The engineering-best friend of developers, architects, engineers and contractors – post-tensioning enables the construction and refurbishment of concrete structures; improving structural performance while also reducing construction time, costs, materials and environmental impact.

But what is post-tensioning? Post-tensioning is a method of reinforcing concrete. High-strength steel tendons are positioned in ducts or sleeves before the concrete is placed. Once the concrete has gained strength, tension is then applied, pulling the tendons and anchoring them against the outer edges of the concrete, before service loads are applied.

The PT technique is a form of pre-stressing, which differs slightly from post-tensioning as steel tendons are tensioned before the concrete is placed. The tendons are stretched between strong bulkheads which withstand external forces and the concrete is poured around them.

Freyssinet’s founder Eugene Freyssinet successfully developed pre-stressed concrete in the 1930s, after recognising that placing concrete under compression greatly increased its strength. He led the first application of post-tensioning on a marine terminal in France in 1933, and now, 84 years later, the Freyssinet group continues to be at the forefront of PT technology and innovation – making significant advances in its uses, performance and durability.

Post-tensioning uses

Post-tensioning is used extensively in bridges, floor slabs, silos and other forms of concrete construction.

Freyssinet has installed PT systems to a variety of structures, including:

Bridges

Buildings

Tanks

Stadiums

Nuclear containment vessels

Gravity bases for oil rigs and wind turbines

Art structures

Structural repair

The use of post-tensioning in the repair of structures is an area which has seen significant increase, particularly in structures built in the 1960s.

Post-tensioned structures can have issues with corroded tendons due to a lack of grout in the ducts. During an investigation, cores are drilled into the structure to find the duct and then it is opened to reveal the condition of the tendon and the grout. Freyssinet provides a full structural assessment, calculation and consultation in order to determine the most effective and economical repair solution.

Freyssinet uses both mono-strand and multi-strand jacking systems to strengthen structures, and has provided innovative solutions for bridge deck strengthening, column reinforcement and floor slab load capacity on a number of complex projects.

Freyssinet utilised a multi-strand system in different configurations during phase 2 of the Hammersmith Flyover refurbishment and strengthening works in 2015, installing a new post-tensioning system that was significantly more robust than the original system.

The project was the UK’s first and most significant full post-tensioning replacement and Freyssinet developed a number of different innovative methods in order to facilitate the complex requirements of the project.

Working Principle, Components and Construction

Post tension slab is a combination of conventional slab reinforcement and additional protruding high-strength steel tendons, which are consequently subjected to tension after the concrete has set. This hybridisation helps achieve the formation of a much thinner slab with a longer span devoid of any column-free spaces.

In this article we study about the working principle, components, construction and advantages of post tension slab.

Working Principle of Post Tensioning

We all know that concrete has a high compressive strength and steel has a high tensile strength, and when their combination is used to bear loads, the efficiency increases manifold.

Fig 1: Typical Details of Post Tension Slab

When a heavy live load is brought upon a structure, its concrete slab undergoes tension, which leads to the formation of cracks and ultimately deformation occurs. To mitigate this problem, post tensioned steel tendons are inserted at the time of concreting and tensioned after concreting with conventional rebars.

When these post tensioned steel tendons are stressed, the concrete is squeezed, in other terms, the concrete is compacted which increases the compressive strength of the concrete and at the same time the steel tendons that are pulled increase the tensile strength. As a result, the overall strength of the concrete increases.

Components of Post Tensioning Slab

1. Ducts

Thin sheet metal pipes with claw coupling or welded overlapped seam supplied in lengths of 5 and 6 m respectively are used as a standard. Ducts are connected to each other by an external screw coupling and sealed with PE tape. Plastic ducts are also available in the market these days which are water tight , frictionless and fatigue resistant

Fig 2: Type of Ducts used to encase steel tendons.

2. Tendons

The basic element of a post-tensioning system is called a tendon. A post-tensioning tendon is made up of one or more pieces of prestressing steel, coated with a protective coating, and housed inside a duct or sheathing.

Fig 3: Steel tendons used in Post Tensioning of Slab.

The prestressing steel is manufactured as per the requirements of ASTM A-416 and typical strand sizes are 0.50 and 0.60 inch in diameter. A typical steel strand used for post-tensioning will yield about 243,000 psi. In contrast, a typical piece of rebar will yield about 60,000 psi.

3. Anchors

Anchors are used to anchor the tendons into the concrete while terminating or joining two tendons. Main function of anchorage is to transfer the stressing force to the concrete once the stressing process is completed.

Construction of Post Tensioned Slab

The installation of post tensioning tendons in the concrete and stressing it requires skilled labour and a personnel who are certified in doing the tensioning works.

The tendons are laid down along with the conventional rebars. The position of laying of the tendons is decided by the engineer. These tendons are encased in plastic or steel ducts so that they do not come in contact with the water in concrete.

One end of the tendons are anchored with the help of anchor and the other end is left open with plastic pocket former, where the tendons are stressed. Couplers are used in between if any construction joint is formed.

Concrete is poured and the alignment of these tendons are taken care of so as to let their positions unaltered. Once after the concrete has achieved its 75% of strength , that is around 20 - 23 days, these tendons are stressed with the help of stressing jacks.

The tensioning is done to a force equal to 80% of a strand's tensile strength. For a typical ½-inch grade 270 strand, the strand is tensioned to a force of 33,000 pounds. As the tensioning comes into effect, the steel gets elongated, and the concrete is compressed.

When the proper tensioning force is reached, the prestressing steel is anchored in place. The anchors are designed to provide a permanent mechanical connection, keeping the steel in tension, and the concrete in compression.

The extra tendons that are left out at one end are trimmed and non shrink grouting is put in the anchor pocket.

Advantages of Post Tension Slab

1. Architectural Benefits

Post-Tensioned Slab has an advantage over others as it makes a very efficient base for floor design with thin slabs and columnless spaces in larger spans. It provides an architect the freedom to work freely with his designs.

2. Commercial Spaces

Post-tensioning results in thinner concrete slabs making the valuable savings in floor to floor height available as additional floors.This can provide extra rentable space within the same overall building height.

3. Reduces Deadload

As the post-tensioned slabs have lesser thickness, the quantity of concrete and reinforcement used is reduced upto 20% - 30% when compared to conventional concrete slabs.

4. Structural Durability

Post-Tensioned slabs show reduced cracking, improved durability and lower maintenance costs. Their deflection can be controlled by varying the amount of post-tensioning to balance any portion of applied loads immediately after stressing.

5. Popularity

The demand for Post-Tensioned slabs, throughout the world, continues to increase because of the significant benefits for developers, architects, engineers, contractors and end users.

Post Tensioning

Post tensioning is where prestressing force is permanently introduced into the concrete structure after the concrete has hardened. By stressing properly positioned high strength prestressing tendons with the help of Stressing Jack, Post Tensioning System introduces constructive stress into the concrete structure, thus increasing overall productivity and usage efficiency of construction materials. Post Tensioning works covers the whole spectrum of engineering, from bridge construction, buildings, to civil applications, both above & underground.

To understand the advantages of post tensioning, it is important for us to study concrete behaviour. Concrete has high compression capability (the ability to withstand load) yet weak in tension. It will crack under pulling forces. In conventional concrete construction, if a load is applied onto the concrete beam, the beam will tend to deflect under the weight of the load. And it is under this deflection, the concrete that is furthest away from the load will tend to be stretched and get elongated. However small the stretch or elongation, it is enough to cause the concrete beam to crack. And to eliminate this problem, operators will normally embed rebar cages to act as tensile reinforcement to counter this phenomenon. But these rebar are passive by nature! That is to say, these rebar are not carrying any load. Until such a time when the concrete started to deflect and crack, the rebar will then start to carry load and work against the force of the crack. You can say that rebar is a passive reinforcement.

However, in post tensioning, the tendons are active reinforcement. The prestressed force in the tendons will enable the beam to withstand and carry the load. And under proper design and installation, post tensioned beams have minimal deflection and almost no cracking issues, even under the conditions of full load.

These days post tensioning uses low relaxation 7-wire high tensile steel PC strand as prestressing tendons. By using these high tensile steel strand, we can greatly reduce the usage of non-prestressed reinforcement steel, typically with a ratio of 1: 3 or 4. This allows us to have a better utilisation of materials in the course of construction.

Mechanical anchorage devices (commonly known as ‘Anchors’) are used on the 2 ends of these tendons to provide the necessary anchorage function after the stressing of the tendons. Post tensioning are generally classified into 2 category, namely bonded and unbounded.

In a bonded system, the tendons in the ducts are buried intside the concrete and grouted after stressing. The purpose of filling with grout is to introduce compatibility between the prestressed concrete and tendons. This will also make the whole concrete structure as homogenous as possible (that is why the term ‘bonded’). This means that after the bonding, any applied load (or strain) that is experienced by the concrete is also felt by the prestressed tendons, and vice versa.

In the case of an unbonded system, the tendons are only anchored at both ends and bonding with the concrete is deliberately prevented along the length of the tendons. The tendons can be buried inside the concrete or outside of the concrete. Even though it is buried inside the concrete, the tendons are not bonded to the concrete. That is why the term ‘unbonded’. Example of an unbonded system is the stay cable (bridge cable) system for bridges.

Post tensioning introduces deviation forces (curvature profile of tendons), anchor forces and frictional forces into the concrete. These forces are optimally utilised through a variety of tendons profile and positioning, and also through carefully planned stressing sequences to control deflections, which is necessary to counter loads and prevent cracking in the concrete.

The advantages of post tensioning technology in civil constructions are :

Larger spans, i.e. lesser columns and supports.

Cost savings with lesser material usage; thinner concrete member sizes and reduced rebar usage.

Due to decreased dead load, lesser foundations are needed.

Faster speed of construction and thus shorter formwork rotation cycle.

Better crack control; water containment where water tightness is paramount.

Lower overall maintenance and lifecycle cost.

Improved seismic behaviour.

Reduced carbon footprint