Post tensioned concrete | What is it? | Complete Guide

Post tensioned concrete is an improved version of reinforced concrete. It has a greater resistance to tensile stresses, thanks to its manufacturing method.

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Generally, the construction method used to produce post-tensioned concrete is known as prestressing, which is also used to make pre-tensioned concrete, another improved version of reinforced concrete.

Post-tensioned concrete is more complex than reinforced concrete, which is why it is used in specific structures; mainly those in which heavy loads and complicated geometry are present.

What is post tensioned concrete?

Post-tensioned concrete is a type of prestressed concrete that is subjected to compressive stress after the pouring and setting of the fresh concrete mix, that is, when the concrete is in its early stages of hardening and developing its characteristic resistance.

The compressive stresses are caused by the tensioning of metallic reinforcement steel bars or cables, which are encapsulated in plastic or metallic &#;tubes&#; enclosed and placed during the formwork, carefully following the design specifications.

After the reinforcement cables are tensioned, they are anchored to the structure using of special components.

Post-tensioning, allows concrete to store additional compressive stresses that counteract the tensile stresses caused by heavy loads, and avoids the structural element to noticeably deform and crack.

Types of post-tensioning

Most of the post-tensioned concrete reinforcement plays an active part by creating additional mechanical stresses that interact with those caused by service loads. The remaining part of the reinforcement acts passively, anchoring or fixing the active pieces.

By the time the active post-tensioning reinforcement reaches the desired stress value, the steel cables that compose it are anchored to the structural element by the passive reinforcement. Once this is done, there are two options to finish the post-tensioning process:

Bonded post-tensioning

In this construction method, the active reinforcement conduits are filled by pressure with high-strength mortar masonry to guarantee the adherence of the reinforcement to the conduit, creating a monolithic section. This way, the active steel reinforcement is prevented from seriously deteriorating due to corrosion.

Unbonded post-tensioning

In contrast to the previous method, the active reinforcement conduits are not filled in, which means the tensioned cables are only connected to the concrete by the anchor heads.

Generally, the reinforcement cables are coated with grease and are contained in plastic conduits, to reduce the environmental impact and guarantee their useful lifespan.

Characteristics of post-tensioned concrete

Although post-tensioned concrete is not a recent construction material, since it originated during the end of the XIX century, it currently presents a high level of innovation and optimization that gives it an endless number of characteristics. Among the most relevant we highlight the following:

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• Elements require little maintenance due to their high mechanical strength and durability. However, the reinforcement demands protection against corrosion.
• Effectively controls the appearance of cracks caused by hydraulic shrinkage and flexing. It also reduces deflections and vibrations.
• Elements require little maintenance due to their high mechanical strength and durability. However, the reinforcement demands protection against corrosion.
• Effectively controls the appearance of cracks caused by hydraulic shrinkage and flexing. It also reduces deflections and vibrations.

Use of post-tensioned concrete

As a general rule, post-tensioned concrete is used for structures that receive heavy loads and with large spans in between supports (spans). In these kinds of situations, its use is essential due to the overall cost of materials and weight of the structure, which would lead to cracking, deformation and reduction of the construction&#;s useful life.

However, the prestressed construction system when using post-tensioned concrete is significantly more complicated than using standard reinforced concrete, so usually the profit by gained by saving material costs tends to be minimized by the requirement of using specialized personnel and equipment.

This type of concrete has mainly been used to build cable-stayed bridges and is also widely used to build viaducts and concrete voussoirs.

On the other hand, is also used in construction for making parking lots, industrial buildings, hotels, sports complexes, airports, auditoriums and other structures that have large spans.

Although this concrete is frequently carried out in situ, it&#;s possible to purchase precast post-tensioned concrete structural elements, usually, precast slabs and beams.

Advantages and disadvantages of post-tensioned concrete

Post-tensioned concrete stands out for its high mechanical resistance and the reduced transverse dimensions of the elements that form it. However, these technical advantages are followed by certain aspects that make the construction process difficult to carry out, due to the high level of precision and detail that is required in the design.

Thus, to obtain a complete perspective, its advantages and disadvantages are presented below:

Advantages of post-tensioned concrete

• It allows to reduce the transversal dimensions of structural elements, optimizing structural weight, mechanical resistance and reducing deformations and cracking.
• It enables the construction of structures that cover large spans and have a minimum number of columns.
• It makes the use of construction materials more efficient. In other words, it economizes building materials.
• Structural elements that require little maintenance.
• Increases the average useful life of concrete elements.
• Facilitates the design of concrete structural elements with irregular geometry.
• Post-tensioning is a versatile system that can be adapted to multiple construction situations and to the specific needs of the project.

Disadvantages of post-tensioned concrete

• It requires highly specialized labour and machinery.
• The design calculations are more complex and expensive than the traditional ones used to design reinforced concrete elements.
• Structural elements are susceptible to corrosion.
• The cost of this construction system is higher, due to the fact that the anchors are unrecoverable and that the reinforcement bars need to be inserted correctly inside the conduits.
• Extreme attention is required during the installation of the reinforcement anchors and their coating.

Q: What is post-tensioning?

A: At its most basic level, post-tensioning (PT) is a fiendishly clever way of reinforcing concrete while you are building &#; occasionally even allowing the construction of something which might otherwise have been impossible!

The use of post-tensioning allows thinner concrete sections, longer spans between supports, stiffer walls to resist lateral loads and stiffer foundations to resist the effects of shrinking and swelling soils. Concrete has what engineers call &#;compressive&#; strength &#; meaning that it happily bears its own weight within a structure. As soon as you introduce the &#;live&#; loads of everyday usage, such as vehicles in a car park or on a bridge, the concrete tends to deflect or sag which leads to cracking, thus weakening the structure.

Concrete lacks &#;tensile&#; strength. Alone, it does not always offer the flexibility needed. That&#;s why steel reinforcing bars &#; &#;rebar&#; &#; are often embedded in the concrete to limit the width of cracks. However, rebar provides only passive reinforcement &#; that is, it does not bear any load or force until the concrete has already cracked.

This is where post-tensioning comes in. PT systems provide active reinforcement. The function of post-tensioning is to place the concrete structure under compression in those regions where load causes tensile stress. Post-tensioning applies a compressive stress on the material, which offsets the tensile stress the concrete might face under loading. PT is applied by the use of post-tensioning &#;tendons&#; &#; a complete assembly including the very high strength prestressing steel strands or bar, the sheathing or protective ducting, plus any grout or corrosion-inhibiting coating surrounding the steel strands or bar and the anchorages needed at both ends.

A typical steel strand used for post-tensioning has a diameter of 15.7mm and a tensile strength around four times higher than an average non-prestressed piece of rebar. Sheathing or ducting houses the prestressing steel This allows it to move as necessary when the tensioning force is applied after the concrete cures. The steel stretches as it is tensioned and it is locked into place using an anchoring component, thus maintaining the force in the strand for the life of the structure.

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