Post tension slab labeling - Structural Inspections

I am one of those that have not seen this type of tensioning and therefore looking into finding more information like all else.
I thought it would be appropriate to share what I find so we can all learn.

If you want to learn more, please visit our website post tension anchor.

What is Post - Tensioning?

Simply put, Post-Tensioning is a method of reinforcing concrete, masonry, and other structural elements. Post-Tensioning is a method of prestressing. Prestressed concrete or masonry has internal stresses (forces) induced into it during the construction phase for the purpose of counteracting the anticipated external loads that it will encounter during its lifecycle.

There are two methods of prestressing. One is called pre-tensioning. This method consists of stressing the reinforcing inside of large steel buttresses, and then casting the concrete around the reinforcing. This method can only be done at a precast manufacturing facility and requires the completed prestressed concrete members to be trucked out to the job site and then assembled.

The other method of prestressing is called post-tensioning. Instead of stressing the reinforcing inside of large steel buttresses at a manufacturing plant, the reinforcing is simply installed on the job site after the contractor forms up the slabs or constructs the walls. The reinforcing steel is housed in a sheathing or duct that prevents the steel from bonding to the concrete so that it can be stressed after the concrete cures (hardens). Using the post-tensioning method of prestressing enables a builder to get all the advantages of prestressed concrete or masonry (described below) while still enabling the freedom to construct the member (slab, wall, column, etc,) on the job site.

**WHY DOES CONCRETE AND MASONRY NEED TO BE REINFORCED **

Concrete, masonry, and most cement based products are very strong in compression, or, in other words, they have a high capacity to resist compressive forces. Compressive forces can be described as crushing forces. Concrete has a very high compressive strength. It can be anywhere from 2,500 pounds per square inch, in most residential foundations, to 4,000 psi in suspended slabs and walls in buildings, to even higher strengths in bridges. However, concrete is relatively weak in tension, i.e. it doesn&#;t resist tensile forces very well. Tensile forces are the forces that pull an element apart.

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***Tensile forces pull apart the bottom of this concrete slab when it bends ***

Conversely, steel is very strong in tension. It has a high capacity for resisting the forces that pull apart or bend it. Therefore, combining reinforcing steel with concrete or masonry results in a product that can resist both compressive forces and tensile forces. Additional, substantial benefits can be obtained by using the reinforcing steel to &#;squeeze the concrete together&#;, or place it in compression. Compressing the concrete increases it tensile (bending) strength. By increasing the tensile strength of the concrete itself (making the concrete slab or masonry wall stiffer), a designer can achieve longer spans with thinner concrete sections.

Putting the concrete into compression also helps to resist the development of shrinkage cracks. Shrinkage cracks, while typically not detrimental to the performance of the structure, can be unsightly, and can allow the passage of moisture or termites. Shrinkage cracks will develop in most cement based products as the water combines with the cement and the concrete cures (hardens). The more the concrete is &#;squeezed together&#;, the less likely it is that shrinkage cracks will develop or open.

**WHAT KIND OF MATERIALS ARE USED IN POST-TENSIONING **

Post-Tensioned reinforcing consists of very high strength steel strands or bars. Typically, strands are used in horizontal applications like foundations, slabs, beams, and bridges; and bars are used in vertical applications like walls and columns. A typical steel strand used for post-tensioning has a tensile strength of 270,000 pounds per square inch. In comparison, a typical non-prestressed piece of reinforcing (rebar) has a tensile strength of 60,000 psi . Strands typically have a diameter of ½ in., and are stressed to a force of 33,000 pounds using a hydraulic jack.

The prestressing steel is housed in a sheathing or duct to allow it move as 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 that forms a mechanical connection and keeps the force in the strand for the life of the structure.

**USES AND ADVANTAGES **

Post-Tensioned reinforcing has been used for many decades in bridges, elevated slabs (parking garages and residential or commercial buildings), residential foundations, walls, and columns. The use of post-tensioned reinforcing can result in 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. The additional advantage of putting the concrete into compression can be used to construct slabs and walls that have fewer visible cracks that can allow the passage of moisture and termites.

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**The above details are common for post-tensioned foundations. These are located and noted on the foundation plan below. In normal post-tension construction, footings are 12 inches wide, but when deep brick shelves are inserted, footings must be at least 18 inches wide. **
http://www.posttensionslabs.com/images/foundation_plan_sm.gifTypical Foundation Plan Showing Effective Bearing Width
**The interior footings and depth of slab are all noted on these details. Since details change, following the submitted engineering plan provided to you for your particular project is advised. **

**This plan and details are typical post- tensioned. If you notice, the footings (ribs) run from the front to the back and from the left to the right of the foundations. The current building code and PTI (Post-Tension Institute) recommend that footing spacing for one and two family dwellings be at approximately 17 feet on center each way. **
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**A typical post-tensioned slab contains very little conventional steel. There may be sections that are too short to put our tendons in, so there may be instances where some conventional rebar reinforcement may be utilized. There are inside corners where concrete cracking would possible occur, so as you will see on the plan layout, there are some corners that will have three #4 (1/2") rebar 10 feet long laid in the corners to minimize cracking. The x pattern indicates an elevation change. Reference the architectural details for all drops, offsets, elevation changes, etc. **

One also notices, that our tendons run from the front to the back and also left to right throughout the slab thickness and footing areas. Arrowheads denote the tendons. One arrowhead indicates one tendon and two arrowheads indicates two tendons, one below the other. Normal post-tensioned footings are 12 inches wide, but when deep brick shelves are inserted, footings must be at least 18 inches wide.

**Never seen this in Maine and most likely will not. Sure is fun learning though. **

**Well, this help me a little, so I hope it dose the same to some. ha. ha. **

**Marcel **

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?

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Contact us to discuss your requirements of wholesale post tension wedges. Our experienced sales team can help you identify the options that best suit your needs.

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

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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.