PRESTRESSED CONCRETE

Submitted By :-Azhar Ayyub

Ashish PandeySaurav Juyal

Siddharth Pathak

Pre stressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete

Basic Concept :- Pre stressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree.

Terminology The terms commonly used in prestressed concrete are explained.1. Wires :-Pre stressing wire is a single unit madeof steel.

2. Tendon: A stretched element used in a concrete member of structure to impart pre stress to the concrete.

3. Cable A group of tendons form a prestressing cable.

4. Bars A tendon can be made up of a single steel bar. The diameter of a bar is much larger than that of a wire.

Types of Pre Stressing Steel

Pre Stressing Systems :-

Pre Tensioning System

Post Tensioning System

Pre Tensioning :- In the pre-tensioning systems, the tendons are first

tensioned between rigid anchor-blocks cast on the ground or in a column or unit –mould types pre tensioning bed, prior to the casting of concrete in the mould. The tendons comprising individual wires or strands are stretched with constant eccentricity or a variable eccentricity with tendon anchorage at one end and jacks at the other. With the forms in place, the concrete is cast around the stressed tendon.

Effect of due to Pre Stressing

POST - TENSIONING

WHAT IS POST-TENSIONING?Post-tensioning- is a method of reinforcing (strengthening) concrete or other materials with high-strength steel strands called tendons.Post-tensioning allows construction that would otherwise be impossible due to either site constraints or architectural requirements.Requires specialized knowledge and expertise to fabricate, assemble and install.After adequate curing of concrete, reinforcing tendons (placed in side the voids of the structure) are tensioned/stretched by jacks on the sides & grouts filled with appropriate mix.Applications – a) Structural members beams, bridge-deck panels, Roof –Slabs, Concrete Silos Etc.

BENEFITS Concrete is very strong in compression but weak in tension,This deflection will cause the bottom of the beam to elongate slightly & cause cracking. Steel reinforcing bars (“rebar”) are typically embedded in the concrete as tensile reinforcement to limit the crack widths.Rebar is what is called “passive” reinforcement however; it does not carry any force until the concrete has already deflected enough to crack. Post-tensioning tendons, on the other hand, are considered “active” reinforcing. Because it is prestressed, the steel is effective as reinforcement even though the concrete may not be cracked . Post-tensioned structures can be designed to have minimal deflection and cracking, even under full load.

Post-tensioning allows longer clear spans, thinner slabs, fewer beams and more slender, dramatic elements. Thinner slabs mean less concrete is required. It means a lower overall building height for the same floor-to-floor height. Post-tensioning can thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors reducing the foundation load and can be a major advantage in seismic areas. A lower building height can also translate to considerable savings in mechanical systems and façade costs. Another advantage of post-tensioning is that beams and slabs can be continuous, i.e. a single beam can run continuously from one end of the building to the other. Reduces occurrence of cracks .

ADVANTAGES/APPLICATIONS

Methods:-

Freyssinet system Gifford-Udall system Magnel blaton system Lee-McCall system

Comparative Study: Pretension Vs Post-tensioned Member

Extended Range of Application

Advantage of Pre stressed Concrete ● Take full advantages of high strength concrete and high strength.● Need less materials● Smaller and lighter structure● No cracks● Use the entire section to resist the load● Better corrosion resistance● Good for water tanks and nuclear plant● Very effective for deflection control● Better shear resistance Improved performance of concrete in “ordinary” design situations (compared to R/C). Extended range of application of structural concrete (longer spans). Innovative forms of structures.

• Bridges• Slabs in buildings• Water Tank• Concrete Pile• Thin Shell Structures• Offshore Platform• Nuclear Power Plant

Applications of pre stressed concrete

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