Indian Institute of Technology Madras Prestressed Concrete Structures

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Indian Institute of Technology Madras B.Tech Civil Engineering: ‘Prestressed Concrete Structures’ Notes:

Introduction
This section covers the following topics.
Basic Concept
Early Attempts of Prestressing
Brief History
Development of Building Materials

Basic Concept
A prestressed concrete structure is different from a conventional reinforced concretestructure due to the application of an initial load on the structure prior to its use.The initial load or ‘prestress’ is applied to enable the structure to counteract the stressesarising during its service period.

The prestressing of a structure is not the only instance of prestressing. The concept ofprestressing existed before the applications in concrete. Two examples of prestressingbefore the development of prestressed concrete are provided.

Force-fitting of metal bands on wooden barrels
The metal bands induce a state of initial hoop compression, to counteract the hooptension caused by filling of liquid in the barrels.

Pre-tensioning the spokes in a bicycle wheel
The pre-tension of a spoke in a bicycle wheel is applied to such an extent that there will always be a residual tension in the spoke.

For concrete, internal stresses are induced (usually, by means of tensioned steel) for the following reasons.
The tensile strength of concrete is only about 8% to 14% of its compressivestrength.
Cracks tend to develop at early stages of loading in flexural members such asbeams and slabs.
To prevent such cracks, compressive force can be suitably applied in theperpendicular direction.
Prestressing enhances the bending, shear and torsional capacities of the flexuralmembers.
In pipes and liquid storage tanks, the hoop tensile stresses can be effectivelycounteracted by circular prestressing.

Brief History
Before the development of prestressed concrete, two significant developments ofreinforced concrete are the invention of Portland cement and introduction of steel inconcrete. These are also mentioned as the part of the history. The key developmentsare mentioned next to the corresponding year.

1824 Aspdin, J., (England)
Obtained a patent for the manufacture of Portland cement.

1857 Monier, J., (France)
Introduced steel wires in concrete to make flower pots, pipes, arches and slabs.

The following events were significant in the development of prestressed concrete.
1886 Jackson, P. H., (USA)
Introduced the concept of tightening steel tie rods in artificial stone and concretearches

1888 Doehring, C. E. W., (Germany)
Manufactured concrete slabs and small beams with embedded tensioned steel.

1908 Stainer, C. R., (USA)
Recognised losses due to shrinkage and creep, and suggested retightening therods to recover lost prestress.

1923 Emperger, F., (Austria)
Developed a method of winding and pre- tensioning high tensile steel wires around concrete pipes.

1924 Hewett, W. H., (USA)
Introduced hoop-stressed horizontal reinforcement around walls of concrete tanks through the use of turnbuckles.

Indian Institute of Technology Madras B.Tech Civil Engineering: ‘Prestressed Concrete Structures’ Notes:

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