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.
Source: construblogspain.files.wordpress.com//09/dsc-large.jpg?w=940&h=704The 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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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.
Source: es.wikipedia.org/wiki/Archivo:Acero_postesado.jpgHowever, 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
Disadvantages of post-tensioned concrete
There are two reasons why builders used post tensions slabs during the construction of a home or building.
A post tension slab is reinforced with stranded steel cables that are tensioned (tightened) after the concrete hardens. The cables are slid inside a plastic sheathing, like a wrapping. This prevents the cables from touching the concrete. When the concrete has sufficiently cured, the plastic sheathing is stretched. Stretching the post-tensioned cables applies significant force to the concrete system, lifting the slab into a compressed state, which reduces shrinkage cracks and cracks caused by difficult soil conditions.
One of the biggest problems with a post tension slab is cracks. The cables laying in the concrete are generally not tightened until at least 7-10 days after the concrete is poured. Since the cables are not stretched or elongated, they cannot provide any crack control in those first few days.
Another reason for cracks is when a vehicle or another large object is placed upon the structure, and its concrete slab undergoes tension. To solve this problem, post tensioned steel tendons are placed when the concrete is poured and tensioned after with conventional reinforced bars.
A second issue with post tension slabs is that the footings anchor the foundation in place. The tension required to bind the additional concrete in the footings plus the dirt between the footings can&#;t be met.
Defects such as slab cracks can be costly to repair after construction is finished. Picture ripping up flooring or carpet to repair a small crack you may never notice. Trying to fix cracks can be time-consuming and expensive. Depending on the extent of damage, a family may have to leave their home while the work is being done. Talk about prices adding up.
Learn more about &#; Concrete slab cracks in Florida home.
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