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.
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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:
- 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.
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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.
Pros and Cons of Post-Tensioned Concrete: An Architect's ...
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The early stages of the architectural process are a time to make large-scale decisions related to the building&#;s form and structure. During this time, your design team may present different structural system options, discuss benefits and drawbacks, and make recommendations.
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One recommendation may be post-tensioned concrete. This method introduces tension to the concrete after curing to strengthen the structure, reduce the likelihood of cracks, and allow for thinner slabs.
While this structural system offers many advantages, it can limit flexibility and complicate future modifications. We recommend considering your long-term goals and the potential for renovations and retrofits.
To help you with your decision, this article will define post-tensioned concrete and outline its pros and cons.
What is Post-Tensioned Concrete?
Post-tensioning is a method of reinforcing concrete by introducing tension after the concrete has achieved a specified strength.
Tension is introduced through high-strength steel stands or bars, typically called tendons. During construction, these tendons are placed in forms before the concrete is poured. The tendons are then tensioned with hydraulic jacks after the concrete cures.
While concrete works best under compression, steel works best under tension. Post-tensioning maximizes the use of each material for efficiency. It can be thought of as a &#;sweet spot&#; that provides the strength of concrete and steel without overusing either one.
Often, post-tensioned concrete works well for parking structures and high-rise commercial or residential developments. We have also seen this method used for tennis courts.
In this situation, the tennis courts sat over an unstable site once occupied by a city dump. Post-tensioned concrete&#;s strength prevented cracking or vibrating that would likely occur with a more traditional concrete system.
Pros of Post-Tensioned Concrete
The most significant benefit of post-tensioned concrete is its strength. The combined performance of concrete and steel allows for thinner slabs and longer spans.
Achieving the same strength with another structural system would require more columns and deeper beams. For this reason, post-tensioned concrete best aligns with structures where longer spans are desired, like parking structures.
Post-tensioning's ability to provide longer spans works well for parking garages.
Post-tensioning also allows designers to decrease a building&#;s floor-to-floor height&#;the vertical distance between the finished floors of two consecutive levels. Other systems require deeper beams and thicker slabs, increasing the distance between floors.
As such, post-tensioning can reduce a building&#;s overall height and lower costs for components like façade treatments. In short, post-tensioned concrete improves material efficiency, minimizes waste, and allows for more open interiors. It can help solve multiple challenges&#;from cost concerns to aesthetics and performance.
Cons of Post-Tensioned Concrete
Although post-tensioned concrete offers many benefits, it can complicate future modifications to the building. Vertical penetrations must be planned during the design process due to the tendons, and after construction, you can only cut through these pre-planned areas. If you cut through the tendons, the structure loses its strength.
With these limitations, post-tensioned concrete works best for buildings with stacked layouts, like residential construction and parking garages. Buildings requiring more flexibility, like healthcare facilities, may run into problems in the future. For this reason, some organizations&#;like the General Services Administration (GSA)&#;have rules against using post-tensioned concrete.
Although post-tensioned concrete can be limiting, other structural systems present a degree of inflexibility. A more traditional concrete system requires more columns, which can also complicate modifications. You and your design team should discuss the potential for future changes to determine the right approach.
At the same time, post-tensioned concrete can add complexity to construction. It requires precision and specialized knowledge, potentially limiting your options to a smaller set of contractors.
Is Post-Tensioned Concrete Right for Your Project?
Post-tensioned concrete is a popular structural system for large-scale developments. Depending on your project type and goals, your architect may recommend it in the early stages of the design process.
Combining concrete and steel&#;s strength, post-tensioning can reduce cracking and help achieve longer spans and lower floor-to-floor heights. It can also lower initial costs by improving efficiency and reducing building height.
Despite the benefits, post-tensioned concrete is not ideal for every project. If you desire more flexibility, a more traditional concrete or steel structural system may be a better fit.
For buildings with stacked layouts&#;like parking structures and residential developments&#;post-tensioned concrete may be the right solution. Learn more about these building types by reading about the rules of thumb for parking structure design and considerations for high-rise construction.
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