Prestressed Concrete Principles, Need and Advantages

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Process of induction of compressive stresses in the structure before it is put to its actual use is known as Prestressing. Prestressed 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.

Introduction to Prestressed Concrete

• Prestressing 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 prestressed 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. Prestress 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 prestressed concrete member, it is necessary to use not only high strength concrete but also high tensile steel wires.
• Concrete used for prestressed work should have cube strength of 35 N/mm² for post-tensioned system and 45N/m m² for pretensioned system.
• In the design of a prestressed concrete member, the estimated loss of prestress due to shrinkage of concrete and creep of concrete and steel is at the order of nearly 200 N/mm²

Need of Prestressing

• 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 precast components are prefabricated in heated plants.

2. Adaptability

• Precast prestressed 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 precast manufacturers to vastly expand the design variety possible using precast components.
• the inherent plasticity of concrete permits to create precast components in shapes and sizes, which would be prohibitively expensive using other materials

3. Fire resistance

• Prestressed 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

• Prestressed 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 prestressed 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

• Precast concrete components lend themselves to fast construction schedules.
• Precast manufacturing can proceed while site preparation is underway.
• Precast 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

• Precast 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 coloring 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 precast panels at the factory,enabling designers to achieve the expensive look of masonry.

Disadvantages of Prestressed Concrete

Although prestressing 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 prestressing equipment and its installation.
• extra labour and transportation cost for prestressing is also there.
• prestressing is uneconomical for short spans and light loads.

Comparison of Prestressed Concrete Beams with RCC Beams

1. 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 prestressed beams, the entire section is effective.
2. Reinforced concrete beams are generally heavy. Prestressed concrete beams are lighter.
3. 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.
4. In RCC beams, there is no way of testing the steel and the concrete. In prestressed concrete beams, testing can be done while pre stressing.
5. RCC construction does not involve many auxiliary units. But prestressed beams require many auxiliary units.

Assumptions in design of prestressed concrete members

Prestressed 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 Prestressed Concrete

• Large prestressing 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 modulus of elasticity of high strength concrete, the elastic and creep strain are very small resulting in smaller loss of prestress in all steel reinforcement.