PRESTRESSES CONCRETE

MADE BY- JAIVEER

HIMANSHU

INTRODUCTION OF PRESTRESS CONCRETE

Prestressed con concrete is method for overcoming concrete’s natural weakness in tension

It con be used to produce beam, floor or bridge with longer span than in practical with ordinary reinforced concrete.

Internal stress are induced to contract external stress.

In 1904, freyssinet attempted to introduce permanent acting force in concrete to resist elastic force under load and was named pre-stressing.

It is a form of reinforced concrete that build stress during construction to oppose those found when in use.

It is combination of steel and concrete that the advantages of both material.

SOME PICTURES OF PRESTESSED CONCRETE

CONCEPT OF PRESTRESSED CONCRETE Prestressed is method in which

compressive force is applied to the reinforced concrete section.

The effect of prestressing is to reduce the tensile stress in the section to the point till the stress is below the cracking stress. Thus concrete does not crack.

It is then possible to treat concrete as elastic material.

The concrete can be visualize to have two compressive force – internal & external.

Stress in concrete when prestressted is applied center of gravity (CG) of the section.

Stress in concrete when prestressed is applied eccentrically with respect to the cg of the section.

In this picture we can easily understand the benefit of prestressed concrete which prevent cracking and deflection.

METHOD OF PRESTRESSED CONCRETE :1. Pre-tensioning2. Post-tensioning

PRETENSIONING – prestressed concrete is cast around already tension tendon. This method produce good bond between the tendon and concrete which both protect the tendon from corrosion &allow for direct transfer of tension.The tendon tries to shrink back to initial length but the concrete resist it through the bond between them. Thus compression force is induce in concrete.

Most pretension concrete element are prefabricated in a factory and must be transported to the construction site which limit their size.

Pretension element may be balcony, lintel, beam or foundation piles.

PRE-TENSIONING:

POST-TENSIONING: In this case , the tendon are tensioned

after concrete has harden. commonly, metal or plastic duct are placed inside the concrete before casting. After the concrete harden and had enough strength, the tendon was placed inside the duct , stressed and anchored against concrete. Grout may be injected into the duct later. This can be done either as precast or cast in place.

ADVANTAGES: The section become thinner & lighter , it

means the materials will less used in prestress concrete.

The chance of deflection and crack becomes less.

The ultimate strength become higher due to the bond between strand and concrete.

It has better shear resistance and prevent from corrosion.

It is more durable than reinforced concrete. It can be used for LONG SPAN as section of

member can be reduce.

DISADVANTAGES: Prestressed concrete required high

strength steel and cement. It required skilled labor and better super

vision It needs specialize construction

machineries like jack anchorage, Overall skilled labor ,special machine

increase the cost of production.