Pre-stressed Concrete Structures (02)

Concepts of Pre-stressed Concrete Structures: 

There are three concepts of Pre-stressed Concrete Structures, these are,

1^st concept: Pre-stressing to transform concrete into elastic material,

2^nd concept: Pre-stressing for combination of high strength steel to high strength concrete,

3^rd concept: Pre-stressing to achieve load balancing.

1^st concept: Pre-stressing to transform concrete into elastic material:

During prestressing the concrete; which is a brittle material is transformed to an elastic material by giving precompression. This is done by compressing the concrete generally by steel under high tension so that the brittle concrete would be able to withstand tensile stress.

So that it can be said that Pre-stress involves pre-compression of concrete.

2^nd concept: Pre-stressing for combination of high strength steel to high strength concrete:

Pre-stressed concrete is made of combination of two high quality materials such as high strength concrete and high strength steel in an active member because such active combination results much better behavior of the two materials.

So that it can be said that Pre-stressed concrete is made of combination with high quality materials in an active member.

3^rd concept: Pre-stressing to achieve load balancing:

The application of this concept requires taking the concrete as a free body and replacing the tendons with forces to active on the concrete along the span.

Historical Development of Pre-stressed Concrete:

Figure-01 shows a Flow Chart of Historical Development of Pre-stressed Concrete Structures,

Flow Chart by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-01: Historical Development of Pre-stressed Concrete Structures

Advantages of Pre-stressed Concrete:

The advantages of Pre-stressed Concrete are as follows,

      ---High load carrying capacity,

      ---Pre-tested Structure,

      ---Tension free,

      ---Less deflection,

      ---Relatively economical,

      ---Crack-less structure,

Difference between Pre-stressed Concrete & Reinforced Concrete:

Differences between Pre-stressed Concrete (PC) & Reinforced Concrete (RC) are as follows,

Topic                                          Pre-Stressed Concrete (PC)               Reinforced Concrete (RC)

Steel & Concrete Used          High strength steel with                        Mild steel concrete

   high strength concrete

Anchoring                                          Used                                                            Not used

Economy                                            Economic than RC                                 Expensive

Deflection                                          Less                                                              More

Load Bearing Capacity             High                                                             Comparatively Low

Shock Resisting Ability          High                                                              Low

For Long Span                               Applicable                                                 Not Applicable

Self-Weight                                     Much less than RC                                  Greater than PC

Maintenance Cost                       High                                                              Low

Man-Power Needed                    Skilled Man-Power                                Not Much Skilled Man-Power

Pre-stressed Concrete is Pre-tested or, Pre-certified Concrete:

In producing pre-stressed concrete structures, both pre-tensioning & post-tensioning---this design is based on calculated expected load which are factored to safety. During the pre-stress operation, the steel is subjected to a high tensile stress and when the pre-stress is transformed to the concrete, the concrete is subjected to a high compressive stress. So in one sense, the concrete & steel are subjected to high stresses even before application of any load.

If Pre-stressed concrete cracks, it behaves like a Reinforced Concrete:

In pre-stressed concrete beam, the capacity of the concrete to carry tensile stress gets destroyed as the cracks are develops which is objectionable for any pre-stressed structure where cracking may results in excessive deflections. Hence it can be said that after cracking the pre-stressed concrete beam behaves essentially as an Ordinary Reinforced Concrete Beam.

Deflection is small in case of Pre-stressed Concrete:

When pre-stress is transformed to concrete, compression develops with the concrete as a result of which camber or upward deflection occurs. When the structure is subjected to working loads, the loads cause the upward deflection to decrease and eventually become straight. If the structure is subjected to more extra loads, then it starts deflecting downward. So, it can be said that Pre-stresses Concrete is much stronger and more capable of resisting loads and hence the deflection is small.

Figure-02 indicates the comparison of deflection shape between Pre-stressed Concrete & Reinforced Concrete in different cases,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-02: Comparison of deflection shape between Pre-stressed Concrete (PC) & Reinforced Concrete (RC) in different cases

Pre-stressing Technique of Concrete:

Pre-stressed Concrete is one kind of form of Reinforced Concrete. Pre-stressing technique builds in compressive stress during construction to appose. This can greatly reduce the weight of Beam & Slab also by better distributing the stress in the structure to make the optimal use of reinforcement in the construction.

Flexural Strength of Pre-stressed Concrete:

The strength of a pre-stressed member to resist flexural failure due to bending load/moment is called The Flexural Strength of the Pre-stressed Concrete Member. The flexural strength or, resisting moment is achieved by the coupling effect of the tensile force of the steel & compressive force of the concrete.

Economical comparison between Pre-stressed Concrete & Reinforced concrete based on Shear Carrying Capacity:

The use of curbed tendon in pre-stressed structure helps to carry some of the shear in a member. In addition, pre-compression the concrete tends to reduce the principle tension increasing shear strength. Thus for some external loading; every things else being equal, the shear force in pre-stressed concrete is smaller than RCC. So it is possible to use section in pre-stressed concrete to carry extra amount of external load in a beam. There is also a definite saving in stirrups. These reduce weight will make the pre-stressed member more economic than reinforced concrete member for any construction.

Figure-03 indicates the cracking due to shear failure in Reinforced Concrete (RC) beam,

Figure-03: Cracking due to shear failure in Reinforced Concrete (RC) beam,

In case of long span structure, the main obstacle is the moment, which forms from the self-weight super imposed dead load and live load. As the pre-stressed concrete structures is much more strong to resist load & more slender with less cross section area resulting less amount of dead load. For these reason the long span structures are effectively and economically build using Pre-stressed Concrete.

Pre-stressed Concrete Structures is made of combination of two high quality materials such as high strength concrete and high strength steel in an active member because such active combination results much better behavior of the two materials,

so that it can be said that Pre-stressed Concrete is a major consideration in any construction.

To Be Continued.......... .......... .......... .......... .......... ..........