Jan. 09, 2024
Posttensioned Concrete
Designers use post-tensioning as a way to reinforce concrete by prestressing it. In prestressed members, compressive stresses are introduced into the concrete to reduce tensile stresses resulting from applied loads including the self weight of the member (dead load). Prestressing steel, such as strands, bars or wires, is used to impart compressive stresses to the concrete. Pre-tensioning is a method of prestressing in which the tendons are tensioned before concrete is placed and the prestressing force is primarily transferred to the concrete through bond. Post-tensioning is a method of prestressing in which the tendons are tensioned after the concrete has hardened and the prestressing force is primarily transferred to the concrete through the end anchorages.
Designers use post-tensioning as a way to reinforce concrete by prestressing it. In prestressed members, compressive stresses are introduced into the concrete to reduce tensile stresses resulting from applied loads including the self weight of the member (dead load). Prestressing steel, such as strands, bars or wires, is used to impart compressive stresses to the concrete. Pre-tensioning is a method of prestressing in which the tendons are tensioned before concrete is placed and the prestressing force is primarily transferred to the concrete through bond. Post-tensioning is a method of prestressing in which the tendons are tensioned after the concrete has hardened and the prestressing force is primarily transferred to the concrete through the end anchorages.
Post-Tensioning Explained
Unlike pre-tensioning, which can only be done at a precast manufacturing facility, post-tensioning is performed on the jobsite in cast-in-place applications. The concrete component is cast with steel reinforcing strands installed in a way that protects them from bonding with the concrete. This practice gives designers the flexibility to further optimize material use by creating thinner concrete members.
Unlike pre-tensioning, which can only be done at a precast manufacturing facility, post-tensioning is performed on the jobsite in cast-in-place applications. The concrete component is cast with steel reinforcing strands installed in a way that protects them from bonding with the concrete. This practice gives designers the flexibility to further optimize material use by creating thinner concrete members.
The materials used to post-tension concrete members are ultra-high-strength steel strands and bars. Horizontal applications (like beams, slabs, bridges, and foundations) typically employ strands. Walls, columns, and other vertical applications usually utilize bars. Steel strands used for post-tensioning typically have a tensile strength of 270,000 pounds per square inch (psi), are about 1/2 inch in diameter, and are stressed to a force of 33,000 pounds.
Benefits
While concrete is strong in compression, it is weak in tension. Steel is strong under forces of tension, so combining the two elements results in the creation of very strong concrete components. Post-tensioning can help create innovative concrete components that are thinner, longer, and stronger than ever before.
While concrete is strong in compression, it is weak in tension. Steel is strong under forces of tension, so combining the two elements results in the creation of very strong concrete components. Post-tensioning can help create innovative concrete components that are thinner, longer, and stronger than ever before.
Many of today’s “high-performance” concrete structures, including many landmark bridges and buildings, employ some type of prestressing. Parking garages, high-rise residential towers, and many other kinds of structures also employ post-tensioning techniques.
Industry Resources
The Post-Tensioning Institute provides a variety of resources online for architects and engineers.
The Concrete Reinforcing Steel Instituteoffers an extensive collection of publications, including its Ready Reference Reinforcing Steel Resource Guide.
Publication
PCA Notes on ACI 318-05 Building Code Requirements for Structural Concrete with Design Applications, EB705
The ninth edition of this classic PCA resource reflects code changes introduced in Building Code Requirements for Structural Concrete, ACI 318-05. These notes will help users apply code provisions related to the design and construction of concrete structures.
Post-Tensioning - Anchorages
Live VSL type S 6-1 PLUS/Si 6-1 PLUS anchorage,embedded VSL type SF 6-1 PLUS anchorage
It is a very economical system with enhanced corrosion protection.A plastic coat covers the external faces of theanchorage casing in continuity of the plastic sleeve. Anchorages can be used for anchoring single unbonded strands (S 6-1 PLUS) orsingle bonded strands (Si 6-1 PLUS). The prestressing force is transmitted throughwedges to a one-flange cast-iron anchor and then further to the structure. The wedges are protected with plastic grease and equipped with a cap for the unbonded system to provide a watertight seal, the anchorage system certainly solves also the corrosion protection of the point where the strand accesses the anchor. The anchorage is either live, type S 6-1 PLUS/Si 6-1 PLUS, or embedded, non-stressed SF 6-1 PLUS anchorage.
Live VSL type S 6-1/Si 6-1anchorage,embedded VSL type SF 6-1 anchorage, SK 6-1 coupler
This type of anchorage is designed for anchoring single unbonded strands (S 6-1) orsingle bonded strands (Si 6-1). The prestressing force is transmitted throughwedges to a two-flange cast-iron anchor and then further to the structure. The wedges are protected with plastic grease and equipped with a cap for the unbonded system to provide a watertight seal, the anchorage system certainly solves also the corrosion protection of the point where the strand accesses the anchor. The anchorage is either live, type S 6-1/Si 6-1, or embedded, non-stressed SF 6-1 anchorage. Single strands in the structurecan beconnected using the SK 6-1 coupler. New strand is connected once the previous strand is stressed.
Live Ends, Dead Ends, and Intermediates – Anchoring Post-Tension Tendons
Post-tension anchors distribute the force from the tendon into the concrete around the tendon ends. Given the one-tendon-at-a-time nature of unbonded single-strand post-tensioning, each anchor is a plate that is designed to distribute the force from one locked-off tendon. The common form factor is a rectangle about 2 1/2” wide and a little over 5” tall, with a tapered hole in the middle for the tendon and a set of hardened steel wedges that lock it in place. The wedges have serrated teeth to bite into the strand, making a mechanical bond to hold it in place. There are three major applications with slightly different anchorages that are common for elevated decks.
First, there are dead ends. A dead-end is put on in the factory after the tendon has been cut to length. Depending on the manufacturer, a hydraulic press can be used to push the wedges into the cavity, forcing the wedge teeth into the sides of the strand; or a jack grabs the tendon on the bearing side of the anchor and pulls the wedges and tendon into the cavity like a lobster trap. When the wedges are pressed in, the force is significantly larger than the field jack can produce, so the wedges tend to deform the tendon and deform to match the wedge cavity – the bond is basically permanent.
Second, there are live ends. These are nailed to the concrete formwork with a special insert that makes a hole in the concrete for the jack to reach the strand. When the jack has finished stretching the tendon, a hydraulic plunger in the nosepiece pushes the wedges into the wedge cavity, deforming the strand at the wedge teeth. The strand sucks the wedges into the cavity, preventing further movement. Some of the strand does retract during this process, meaning that there is some seating loss between the fully-stressed tendon and the installed condition. If a tendon is over 120’ long, it will usually be shipped with both ends “live” to allow for a larger final effective force.
Third, there are intermediate anchors. Concrete logistics are such that it’s not always possible to pour an entire slab at once, so contractors break them up into smaller pours that are easier to place. In order to reduce the amount of formwork required, it’s common to stress the tendons and then strip the formwork for the bays that are self-supporting. This requires a special type of live-end anchor that can be slid along the tendon and installed in the middle of the slab, which is called an intermediate anchor.
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