Question

QUESTION : describe briefly what is Prestress concrete? what are the advantages and disadvantages of prestress concrete? when it use in construction and why? explain in very briefly  what are the failure in pestress concrete while casting? explain at least 10 failure. give the reason of that failure in detail and provide remediation of that failures in detail.

NOTE: need a very brief information about the above questions.

Answer 1

Prestress Concrete

As the term suggest itself it is generally a concrete in which stress is induced earlier to counteract the strains or deformations which occur in the future. OR

Prestressed concrete is a concrete into which internal stresses are deliberately induced without any form of external loads to improve its performance during its entire life span. The internal stresses induced in the concrete structure is used to counteract the stresses coming from the external load application like due to the selfweight of structure itself or the rolling/Moving load supported by it.

Two types of prestress concrete

1. Pre tensioning

2 .Post tensioning

Advantage of Prestress Concrete

1. Prestressing enhances the shear strength and fatigue resistance of concrete which give an extra edge to concrete in terms of strength and durability.

2. The prestressing of concrete by using high tensile steel improve the efficiency of the materials means in prestressing full property of materials can be utilised upto its ultimate limit which is a plus point.

3. Prestressed concrete remains uncracked even at service load conditions which proves the structural efficiency.

4.Composite construction by using the prestressed concrete can be done by precast which is economical.

5. Prestressing helps to reduce the dead load of the concrete structure.

Disadvantages of prestress

1. Higher material costs because only  high grade steel and high grade concrete can be used for prestressing or poststressing.

2 .Formwork is complex and it required skilled labour.

3. Higher Machinery and manpower is required.

Use in construction

It is used in construction of heavy structures like Rairoad bridge,water tanks,Multistorey buildings,Silos,Nuclear reactors etc because these structures require both structural safety and durability as the cost of construction is very enormous in these type of projects.

Generally normal grade of steel and concrete are not capable to resist very high load for very long span of time.

Failure in prestress concrete

1. Loss of prestress during tensioning due to friction- No loss in pretensioned member

But in post tensioned member there is a loss and due to this loss unbonded concrete may be formed So in order to avoid that high grade steel and concrete shoul be used only.

2.Loss of prestress during anchoring

It is the loss when tendons are tensioned and then concrete is poured but in order to do that we must ensure that tendons are anchored tightly because during pouring of concrete sometime tendons get loosened which causes loss in prestress. So anchoring should be done carefully.

3.Loss of Shrinkage of concrete

As we know high grade concrete should be used so higher the grade of concrete so the evolution of heat is also be high which causes shrinkage of concrete plus external site weather conditions.

In order to avoid that water cement ratio and RMC can be used.

4.Loss due to elastic Shortnening

Tensioning the tendons is the primary step in prestressing So the method of tensioning either one by one or simlutaneously all at a time can be used but tensioning tendons one by one causes loss in prestress so in order to avoid that all the tendons should be tensioned simlutaneously.

5.Loss of stress due to creep of concrete

Creep is the time dependent loss higher the change in external environment higher will be the creep So in order to avoid that either the material properties or external environment should be in controlled manner.

6. Corrosion in tendons

As the tendons are exposed to the external environment water sunlight so to avoid corrosion either greasing or other method should be used to cover the surface of tendons.

7.Diagonal tension cracks in concrete

Sometimes it is seen that during anchoring process the tendons are tensioned upto its ultimate limit causes bonding problems when concrete is set which cause shear cracks in concrete.