PRESTRESSED CONCRETE

 

INTRODUCTION

Hardened concrete as the artificial hard rock that could be designed or made with certain desired shape from casted fresh concrete, has some weaknesses that make it alone unfit for major structures which are the following

• ·       Have little or negligible strength against tension
• ·  Brittle property (not ductile), which makes it susceptible to cracking on loading

When we design different major structures, there are two important criteria that are put into consideration

•    Safety of the structure against failure, the structure should be able to carry desired loads for intended period of time.
•   Serviceability or functional of the structure, where by the structure should neither deflect in extent that it affects comfort ability of users nor lead to detaching or breaking of attached materials such as tiles of curtain walls.

Introduction of reinforcements which are ductile or elastic material into concrete, produce reliable composite material against cracks formation and deflection.

In order to avoid excessive deflection on major structures such as bridges, longer spanning beams prestressed concrete were introduced in construction industry.

What is prestressed concrete?

This is a concrete in which internal stress of suitable magnitude and distribution are introduced so that the stresses resulting from external load are counteracted to a desired degree.

Mechanism of prestressing

In this kind of concrete, steel embedded in concrete is stretched or tensioned to certain point not exceeding steel yield point, since it is stretched before yield point (steel with elastic property) will try to regain its original shape.

As high tensile steel try to regain its original shape, it compresses concrete in which it is embedded, so concrete is precompressed before an application of loading desired for the structure.

 

 

Ways of prestressing concrete

Pre tensioning, in this method of prestressing tendons are tensioned before the concrete is casted or poured, this methods mostly is done at factories which produce precast units such as slabs, lintels, and beams.

High tensile strength wire strands, or bundles are set or fixed inside concrete moulds, these strands are stretched or tensioned by using hydraulic jack to a certain point not exceeding yield point of tendons.

These steel strands after being tensioned are anchored tightly against a rigid frame at the head and foot of the setting bed or mould, fresh concrete is poured into the bed or mould in direct contact with the strands or tendons. As the concrete sets, the strands become bonded to the concrete.

 

Post tensioning, .in this method of prestressing tendons or strands are embedded and tensioned after concrete has casted and hardened, this process most of time is done in situ on ready produced precast units.

The concrete is casted around ducts or sheathing in which tendons will be housed, after concrete has been set and cured properly over time, the tendons are inserted in ducts and stretched by hydraulic jack either on one end or both ends.

After tensioning of tendons grout is filled into tendon ducts to assist stress distribution between concrete and tendons, also it prevents corrosion that could be caused by entrapped moisture.

 

 

Two concept in prestressed concrete

1.     Prestressing to transform concrete into an elastic material

This is the first and most common concept among engineers; concrete material is seen to be converted from brittle material to elastic (ductile) material by precompression given to it from tensioned tendons.

This concrete with elastic property is not susceptible to cracking and deflection, as it is believed that no tensile stresses are induced into concrete.

In this point of view concrete is subjected to external and internal forces, where by external force is resulting from tensile force tensioning the tendons and internal force as compression force resulting as tendons try to regain its shape.

 

 

2.     Prestressing for combination of High strength steel with concrete

Here the prestressed concrete is seen as the combination of concrete and reinforcement, similar to reinforced concrete where by reinforcement takes tensile stress while concrete take compression stresses, this behavior also is exist in prestressed concrete.

In prestressed concrete, high tensile steel are tensioned by higher tension to certain strain so steel take all tensile stresses, as high tensile steels tend to regain their shapes compress concrete, so concrete withstand compression stresses form steel.

 

Conclusion

Prestressed concrete is the strong concrete composite that is used in major projects in construction industry such as bridges, high raised buildings, this is due to the use of high strength steel tendons and full utilization of concrete compressive strength also prestressed concrete has lower or negligible deflection when compared to reinforced concrete.