What is pre-tensioned concrete? 

The article aims to answer the question, “What is pre-tensioned concrete?”. It will also explain how pre-tensioning of concrete is done:

What is pre-tensioned concrete? 

Pre-tensioned concrete is also known as pre-stressed concrete. As a structural material, prestressed concrete allows for the application of engineering stresses to members before loading. 

These engineering stresses reduce the stresses that would otherwise develop in the members due to the applied load. Combining the great compressive strength of concrete with the high tensile strength of steel, it is a solid material.

Regular reinforced concrete relies on steel reinforcement to bear the load, whereas prestressed concrete uses induced stresses to distribute the force evenly throughout the whole element. Long, thin structures with substantially smaller sectional areas to carry equal loads may be formed because of this material’s increased resilience to shock and vibration compared to traditional concrete.

San Francisco engineer P.H. Jackson developed prestressed concrete in 1886. Still, it wasn’t until after World War II when a lack of steel and technical advances made prestressed concrete the material of choice for rebuilding in Europe.

Structures including bridges, water tanks, roofs, runways, floor beams, piles, and train sleepers, are often used currently. Prestressed concrete isn’t always required for walls and columns, but it’s a cost-effective option for tall columns and retaining walls subject to significant bending strains.

To a span of 6 meters or less, conventional reinforced concrete is often the most cost-effective option. Spans more significant than 9 meters benefit economically from the use of prestressed concrete. The optimal choice between the two possibilities is between 6 and 9 m, and several factors determine this range.

What is the pre-tensioning of concrete?

Pre-tensioning of concrete is where the wires or cables, up to 120 m in length, are anchored at the end of a metal form and subjected to tension. Hydraulic jacks stress the wire to the specified tension, plus about 10% to account for creep and other pre-stress losses. 

Once the wires are taut, the side molds can be fastened, and the concrete may be poured in. As the concrete cures and shrinks, it grips the steel throughout its length, transmitting the strain from the jacks to create a compressive force in the concrete.

Wires are unhooked from jacks after the concrete has hardened to the correct consistency. By adding chemicals and steam curing the concrete for 24 hours, the normal concrete strength of 28 N/mm2 may be attained.

Shorter members may be made by inserting dividing plates at various points along the member and then removing them to expose the wires for cutting. In post-tensioning, the concrete component is cast first, and the prestressing happens after the concrete has been set, the process is reversed from pre-tensioning. 

This technique is often used when on-site stressing must be accomplished after the casting of an in-situ component or after the assembly of many precast concrete pieces. Before concreting, the wires, cables, or bars may be placed in the unit, but they won’t attach to the concrete if they’re encased in a flexible duct or rubber sheath that can be deflated and removed afterward.

After the concrete has hardened, hydraulic jacks at each end of the member are used to apply stresses. It is usual practice to include a helical (spiral) reinforcement because of the high local stresses at the anchoring sites. 

The wire or cables are anchored after the desired stress has been achieved. Cement mortar is used to seal the unit’s ends to help distribute tension and prevent corrosion from retained moisture.

Whether the tendons are being strained singly or collectively determines the anchorages employed in post-tensioning. Most of these systems use a set of split-cone wedges or jaws that push on a bearing or pressure plate.

How pre-tensioning of concrete is done?

Continue reading the article to learn how pre-tensioning is done. 

Tendons are wires or strands stressed to a certain extent by stretching them between two anchorages before concrete is poured. When the concrete is poured, the tendons adhere to it over their length. 

Tendons are usually severed at their anchor points once the concrete has been set. As the tendons shrink to their original length, they transmit compressive stress to the concrete through the bond. Hydraulic jacks are often used to provide tension to the tendons. 

Tendons are anchored to abutments that may be as far apart as 200 meters to keep the tension in the tendons constant while the concrete cures. Prestressing benches or beds refer to the abutments and other formwork utilized in this process. 

Although the fundamental idea used by all pre-tensioning building methods is universal and well-known, most of the methods themselves are patented.

What is post-tensioning?

Post-tensioning is an alternative to pre-tensioning. A post-tensioned beam is one in which the tendons are strained, and each end is fastened to the concrete section after the concrete has been formed and has reached a strength where it can safely bear the prestressing force. 

The post-tensioning technique protects the tendons from the concrete by covering them with grease or a bituminous compound. 

Putting the tendons inside a flexible metal tube before inserting them into the forms is another approach to prevent the tendons from adhering to the concrete during the pouring and curing. The metal tubing, now called a duct or sheath, will be left in place.

After the tendon has been under tension, grout is inserted into the space between the tendon and the sheath. This prevents the tendons’ steel from corroding by forming a bond with the concrete. On-site prestressing using post-tensioning is feasible. 

Some situations may call for or benefit from this technique. Transporting a member from a precasting factory to a project site might be challenging if it has to support a significant weight or span a considerable distance in a structure or bridge. 

However, pre-tensioning may be used in both precast and cast-in-place building methods. The end of the post-tensioning tendons must be attached to or anchored to the concrete portion using some mechanism. End anchorages are a common term for these kinds of tools. 

There is a wide variety of anchoring kinds, each with its patent. Their constructional particulars may likewise vary. Examples of standard post-tensioning techniques for prestressed concrete include:

Conclusion 

Pre-stressed concrete is a sort of concrete where initial compression is delivered in the concrete before applying the external load so that stress from external loads is counterbalanced appropriately over the service term. This first compression is created by high-strength steel wire or alloys (called ‘tendon’) positioned in the concrete portion.

Frequently asked questions (FAQS): What is pre-tensioned concrete? 

What is pre-tensioned concrete? 

Pre-tensioned concrete is also known as pre-stressed concrete. As a structural material, prestressed concrete allows for the application of engineering stresses to members before loading. 

These engineering stresses reduce the stresses that would otherwise develop in the members due to the applied load. Combining the great compressive strength of concrete with the high tensile strength of steel, it is a solid material.

What is post-tensioning?

Post-tensioning is an alternative to pre-tensioning. A post-tensioned beam is one in which the tendons are strained, and each end is fastened to the concrete section after the concrete has been formed and has reached a strength where it can safely bear the prestressing force. 

The post-tensioning technique protects the tendons from the concrete by covering them with grease or a bituminous compound. 

What is the pre-tensioning of concrete?

Pre-tensioning of concrete is where the wires or cables, up to 120 m in length, are anchored at the end of a metal form and subjected to tension. Hydraulic jacks stress the wire to the specified tension, plus about 10% to account for creep and other pre-stress losses. 

Once the wires are taut, the side molds can be fastened, and the concrete may be poured in. As the concrete cures and shrinks, it grips the steel throughout its length, transmitting the strain from the jacks to create a compressive force in the concrete.

Bibliography

Mithun Ray. What is Pre-stressed Concrete? How Does It Work? Retrieved from: https://civiltoday.com/civil-engineering-materials/concrete/225-pre-stressed-concrete

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