Prestressed Concrete Basic Concept, Principle and Material Used, advantages and disadvantages, applications etc. detailed blog

                          Prestressed Concrete

Introduction

1. What is Prestressed Concrete ?

        prestressed concrete is a method of overcoming concrete natural weakness in tension. it can be used used to produce beams, floors and bridges with a longer span than practical with ordinary reinforced concrete in simple words ,Prestressed concrete is a form of concrete used in construction. It is substantially "prestressed" (compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service. the prestress is commonly introduced by tensioning high tensile steel reinforcements known as tendons 

2. Basic Concept of Prestressing

                    It is meant for improving the behavior and capacity of the system is expected to have tensile stresses due to external loads, Pre-compression can be given to the system to a desired level even before the application of the external loads. Then the tension developed due to the external loads, will get largely initially applied compression.

                    this principle was used long back in our civilization. it is reflected in the design of barrels to contain liquid or wheel of a bicycle. Metal strip wound around wooden boards of barrel are tightened on the barrels.

3. Principle of Pre-stressing

• Pre-stressing is a method In which compression force is applied to the reinforced concrete section.
• the effect of pre-stressing is to reduce the tensile stress is below the cracking stress. thus the concrete does not work.
• It is possible to treat concrete as elastic material.
• The concrete can be visualized to have two compressive force

                1. Internal pre-stressing force

                2. external forces

• These two forces must counteract  to each other

4. Materials of Prestressed concrete

        High strength steel and high grade concrete are the basic materials of Prestressed. All the materials is used for preparing (cement, mineral, admixtures, aggregates, water and chemical admixtures) and steel used for pre-stressing should be according to IS:1343 code

NOTE: IS:1343-2012 "Prestressed concrete code of practice" Specifies the materials, workmanship, inspection and testing, general design requirements and structural design: Limit state method. 

Methods of Pre-stressing

   1. Pre-tensioning

   2. Post-tensioning

1. Pre-tensioning System 

In which the tendons are tensioned before the concrete is placed, tendons are temporarily and tensioned and the prestress is transferred to the concrete after it is hardened 

The different stages involved in pre-tensioning are explained in figures

1. In the first stage, the tendons are starched by using to the rigid supports known as bulk heads.
2. then the concrete member is cast around develops between the tendon and concrete.
3. when the concrete attains sufficient strength, the tendons are released the bulk heads.

immediately after release, the tendons try to return to its original length. But the bond between steel and concrete prevents this movement thereby forcing the concrete into compression. since in pre tensioning method the prestressing from the tendon is transferred to concrete by bond pre tensioning members usually have large numbers of prestressing steel smaller diameter as increase the area of contact

Post-tensioned system 

The difference stages involved in post-tensioning process

1.  In the beginning metal ducts or corrugated high density polyethylene ducts are placed in the required tendon profile by tying them to the reinforcement cage and concrete is cast around the duct.
2. Once the concrete attains sufficient strength, the tendons are stretched against the hardened concrete from one end with the other end anchored (known as dead and anchored) using external anchorage systems. during the stretching process, the concrete  gets compressed and this pre-stress is maintained by the tendons anchored at the ends. Hence in post-tensioned is transferred to concrete at the anchorage ends.
   
   
   

Application of Prestressed Concrete

        construction of structural members with large values of span to depth ratio using reinforced concrete is almost impractical. in such cases, prestressed concrete can be used to achieve economical, durable and elegant looking structures, therefore prestressed concrete is used for long span beams and bridge

            Prestressed concrete are used in the construction of floor slabs in buildings where large column free space are required. they are also widely used in the construction of huge structures like containment walls of unclear reactors large capacity liquid retaining structures, cement silos, chimneys towers etc.

Advantages of Prestressed Concrete

1. The cracks in the members can be avoided.
2. The smaller section can be designed.
3. The prestressed concrete member can be used for longer spans and heavy loads and impacts.
4. There will be reduction in self-weight of the member.
5. The pre-stressing in member are reduces the diagonal tension.
6. Prestressed member are tested before use.
7. Pre-stressing eliminates corrosion of steel when structure exposed to weather.
8. Prestressed concrete members show less deflection.
9. Precast members like electric poles and railway sleepers are produced easily in factors using simple pre-stressing.

Disadvantages of Prestressed Concrete

1. It uses high strength concrete and steel.
2. Cost of materials in prestressed concrete is very high.
3. Prestressed concrete requires specialized tensioning equipment's and devices which are very costly.
4. Prestressed sections are brittle.
5. Prestressed concrete sections are less fire resistant.
6. It requires a better quality control.
7. It may requires complicated form work.

Main source; wikipedia