Introduction

Post-tensioning is a method of producing prestressed concrete, masonry and other structural elements. The term prestressing is used to describe the process of introducing internal forces (or stress) into a concrete or masonry element during the construction process in order to counteract the external loads that will be applied when the structure is put into use (known as service loads). These internal forces are applied by tensioning high strength steel, which can be done either before or after the concrete is actually placed. 

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.

 When the steel is tensioned before concrete placement the process is called pre-tensioning. When the steel is tensioned after concrete placement the process is called posttensioning. The advantages of utilizing prestressed concrete and masonry have long been recognized by engineers. When a designer wants to take advantage of those benefits, they must determine whether the structure is to be constructed using the pre-tension method or the post-tension method. Pre-tensioning is generally accomplished at a manufacturing facility where concrete members are constructed in special casting beds with steel bulkheads that hold the steel in place while tension is applied. Concrete is then placed around the pre-tensioned steel and allowed to harden. The steel is then cut loose from the bulkheads and the entire precast concrete member is transported to the project site for assembly. This process may be limited to the use of standard shapes, and sizes that can be easily transported. Post-tensioning is done at a project site and requires little to no modifications of the same forming system that would be used to construct non-prestressed concrete. The systems used to post-tension concrete and masonry consist of prestressing steel that is housed inside a duct or sheath, which allows the prestressing steel to be placed inside the typical job site formwork at the same time rebar and other reinforcing is placed. Concrete is placed in a typical manner and allowed to reach a predetermined strength before the steel is tensioned. Since the prestressing steel is housed in the sheathing or duct, it will be free to move inside the concrete during the tensioning operation, and since the steel is tensioned after concrete placement, the tensioning is done against the hardened concrete instead of relying on large steel bulkheads. Using the posttensioning method of prestressing enables a builder to get all the advantages of prestressed concrete or masonry while still enabling the freedom to construct the member (slab, wall, column, etc,) on the job site in almost any shape or configuration imaginable.

Post-Tensioning Concept

 The technical concept of NOOR is to provide maximum efficiency with minimal cost. Post-Tensioned concrete provides and optimization of all the desirable properties of high strength concrete and steel. NPT can provide the technical ability to give your project the advantages of Post-Tensioned concrete. The synthesis of these ideas leads to maximum economy.

Post-Tensioned concrete means that internal stresses have been introduced in the concrete of such magnitude and distribution that stresses caused by external loadings can be counteracted to any desired degree. By Post-Tensioning maximum advantage is taken of concrete’s desirable properties (rigidity and compressive strength) while minimizing its undesirable property (low tensile strength).

 Diagram 1

Diagram 2

Another feature of Post-Tensioning is the concept of load balancing (see diagram 1 and diagram 2): the tendons are typically installed in the members with a curved parabolic profile. After the concrete has reached strength, tensioning of the tendon causes it to try to straighten out. This straightening effect of the Post-Tensioned tendon actually “balances” the load and carries it directly to the supports, be they columns (flat plate or beam) or beams (one way slab). The effect allows much thinner members to span longer distances.  

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.

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.

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.

The basic element of a post-tensioning system is called a tendon. A post-tensioning tendon is made up of one or more pieces of prestressing steel, coated with a protective coating, and housed inside of a duct or sheathing. A tendon will have anchors on each end to transmit the forces into the structure. Long tendons may also have intermediate anchors along their length. The prestressing steel can be a high strength steel strand (typical in horizontal applications) or a high strength steel bar (typical in vertical applications).

Prestressing steel is manufactured to applicable ASTM requirements. Typical strand sizes are 0.50 in. and 0.60 in. diameters, and bar sizes can typically range from 1 in. to 2.5 in.

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