Advantages & Applications of Post-Tensioning

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Post-tensioning, which is a form of prestressing, has several advantages over standard reinforcing steel (rebars):

• It reduces or eliminates shrinkage cracking-therefore no joints, or fewer joints, are needed
• Cracks that do form are held tightly together
• It allows slabs and other structural members to be thinner
• It allows us to build slabs on expansive or soft soils
• It lets us design longer spans in elevated members, like floors or beams

Common Applications for PT

Post-tensioning, or PT, has become increasingly popular over the past 30 years or so as the technology has been perfected. At one time there were problems with corrosion of the cables, especially in deicing-salt-laden parking structures, but better materials and construction methods (plus good training and certification programs) have eliminated most problems.

External post-tensioning is a good method for strengthening or retrofitting concrete structures. www.vsl.net

The applications for post-tensioning are almost endless, but what holds it back is the lack of experience among designers and builders. In an article in Structure magazine (PDF), Gerard Feldman noted that "Most engineers have only a cursory overview of post tensioning in college...this unfamiliarity creates a bias towards conventional reinforced concrete structures."

Some of the more common applications are:

• Slabs on ground: Today, PT is used extensively for slabs on grade where soils are likely to move (expansive soils)-especially in the American southwest. Jim Rogers, editor and publisher of Post Tension Magazine, says that until housing construction ground to a halt last year, about half of all post-tensioning work was slabs-on-ground for homes. Residential Concrete magazine had a good review of this in its September issue.
• Another good application for PT slabs is producing crack-free tennis courts.
• A recently developed application of PT is external post-tensioning for strengthening of existing structures, especially as an upgrade to resist seismic forces. The best review of this is available from the International Concrete Repair Institute, "Guideline for the Selection of Strengthening Systems for Concrete Structures."

Water tanks can be post-tensioned. DYK in El Cajon, CA

• Bridge designers have used PT both for cast-in-place concrete and for precast segmental construction. PT allows longer spans and keeps cracks tight.
• Concrete water tanks are often post-tensioned to reduce crack width and leakage. The companies that make prestressed concrete tanks are Crom, DYK, Natgun, and Preload.
• Masonry walls can be post-tensioned-this is usually done with a solid steel bar fastened to the foundation and stressed with a nut at the wall's top. A good article on PT for masonry is available on Masonry Construction magazine.
• One interesting application is for a concrete countertop that needed to span 6 feet and carry a heavy load.

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

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:

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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:

• 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

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• 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.