Sat, 07/24/2010 - 18:27 — Decknash

Pre-Stressed Concrete – Introduction

process of induction of compressive stresses in the structure before it is put to its actual use is known as Pre-stressing. Pre-stressed Concrete member is a member of concrete in which internal stresses are introduced in a planned manner, so that the stresses resulting from the superimposed loads are counteracted to a desired degree.

Pre-stressing is the intentional creation of permanent stress in a structure or assembly, for improving its behavior and strength under various service conditions.
In ordinary reinforced concrete, consisting of concrete and mild steel as basic components, the compressive stresses are born by concrete while tensile stresses are born entirely by steel. The concrete only acts as a binding material; it does not take part in resisting the external forces.
In pre-stressed concrete, compression is induced prior to loading in the zones where external loads would normally cause tensile stresses.
In the case of long beams, where large shear forces exist, the beam sizes have to be large to limit the diagonal tensile stresses under certain limits. Pre-stress decrease diagonal tensile stresses. This has led to adopt modified I-section and T-section in which there is substantial reduction in web area.
In order to get the maximum advantage of a pre-stressed concrete member, it is necessary to use not only high strength concrete but also high tensile steel wires.
Concrete used for pre-stressed work should have cube strength of 35N/m m2 for post-tensioned system and 45N/m m2 for pre-tensioned system.
In the design of a pre-stressed concrete member, the estimated loss of pre-stress due to shrinkage of concrete and creep of concrete and steel is at the order of nearly 200N/m m2

Need of Pre-stressing

To offset the deficiency of tensile strength in concrete, steel reinforcement is provided near the bottom of simple beams to carry the tensile stresses.

ADVANTAGES OF PRESTRESSED CONCRETE

1.Durability

As this technique eliminates weakness of concrete in tension, such members remain free from cracks; hence can resist the effects of impact, shock, and reversal of stresses more efficiently than R.C.C. structure.
They provide reliable long-term performance in extremely harsh conditions that could destroy lesser materials.
They are resistant to deterioration from weather extremes, chemical attack, fire,accidental damage and the determined efforts of vandals.
Winter construction can proceed with few weather delays as pre-cast components are prefabricated in heated plants.

2. Adaptability

Pre-cast pre-stressed concrete products can be designed and manufactured for any application, ranging in size from short span bridges to some of the largest projects in the world.
Permits pre-cast manufacturers to vastly expand the design variety possible using pre-cast components.
the inherent plasticity of concrete permits to create pre-cast components in shapes and sizes, which would be prohibitively expensive using other materials

3. Fire resistance

Pre-stressed concrete bridges are not easily damaged by fire. Have excellent fire resistance, low maintenance costs, elegance, high corrosion resistance, etc.

4. impacts local economy directly

Pre-stressed concrete is produced by local small business – employing local labour.
Most of its raw materials are also locally purchased and the health of the local pre-stressed concrete industry directly impacts further on the local economy.
Due to smaller loads, due to smaller dimensions being used, there is a considerable saving in cost of supporting members and foundations.
standard structural shapes such as hollow core, double tees, beams, columns and panels can be mass-produced at low cost.

5. Fast and Easy Construction

Pre-cast concrete components lend themselves to fast construction schedules.
Pre-cast manufacturing can proceed while site preparation is underway.
Pre-cast units can be delivered to the jobsite and installed the moment they are needed in any weather.
Fast construction means earlier completion and the resulting cost savings.
Saves the cost of shuttering and centring for large structures.

6. Aesthetics

Pre-cast components can be delivered with a wide range of shapes and finishes ranging from smooth dense structural units to any number of architectural treatments.
Strikingly rich and varied surface textures and treatments can be achieved by exposing colure sands, aggregates, cements and colourings agents using sandblasting and chemical retarders.
custom form liners can be used to introduce reveals, patterns and other architectural effects.
Stone, tile brick and other materials can be cast into pre-cast panels at the factory,enabling designers to achieve the expensive look of masonry.

DISADVANTAGES OF PRESTRESSED CONCRETE

Although pre-stressing has many advantages, there are still some drawbacks of this process.

The unit cost of high strength materials being used is higher as mostly high tensile steel is used.
extra initial cost is incurred due to use of pre-stressing equipment and its installation.
extra labour and transportation cost for pre-stressing is also there.
pre-stressing is uneconomical for short spans and light loads.

COMPARISON OF PRESTRESSED CONCRETE BEAMS WITH RCC BEAMS

In RCC beams, the concrete in the compression side of the neutral axis alone is effective and the concrete in the tension side is ineffective. But, in the pre-stressed beams, the entire section is effective.
Reinforced concrete beams are generally heavy. Pre-stressed concrete beams are lighter.
RCC beams being heavy and massive are more suitable in situations where the weight is more desired than strength. Pre stressed beams are very suitable for heavy loads and long spans.
In RCC beams, there is no way of testing the steel and the concrete. In pre- stressed concrete beams, testing can be done while pre stressing.
RCC construction does not involve many auxiliary units. But pre-stressed beams require many auxiliary units.

Assumptions in design of pre-stressed concrete members

Pre-stressed concrete members are analysed and designed on the basis of the following assumptions given below:

A transverse plane section of the member will remain a plane after bending also.
Within the limits of the deformation taking, Hook’s law is applicable to concrete and steel components.
The stress in the reinforcement does not change along the length of the reinforcement. Stress changes take place for the concrete component only. Variation of stress in the reinforcement due to changes in the external loading is ignorable.

Principles of pre-stressing: -

Large pre-stressing force are applied to the member by the tendons, high bearing stresses are developed at the ends by the anchoring devices. The anchorages are generally designed to be meant for use only for high strength concrete work.
Busting stresses liable to at the ends of the beam cannot be satisfactorily resisted by low strength concrete work.
When stress transfer to concrete has to take place by bond action, the concrete should have a high strength concrete.
Shrinkage cracks will be very little when high strength concrete is used.
Due to the high modules of elasticity of high strength concrete, the elastic and creep strain are very small resulting in smaller loss of pre-stress in all steel reinforcement.

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