Lin Rouxiu BALA HISHAM SHARIF

1.0 introduction

Prestressed concrete may be a sort of structural element made from concrete and high tensile steel used primarily in advanced structures.It became common in Europe after World War II due to the mechanical headways in high-strength concrete and steel.Prestressed concrete is fundamentally concrete in which internal stresses of a reasonable magnitude and distribution are introduced so that the stresses resulting from external loads are neutralized to a desired degree.

2.0 Principle of Prestressing

1.Prestressing is the intentional creation of permanent stresses in a structure or assembly to improve its behaviour and strength under various service conditions.The principle behind prestressed concrete is that compressive stresses initiated by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses imposed within the member during service.

2.High yield strength steel alloys are used to producing permanent pre-compression in areas subjected to tension.

Consider the basic case of a simply-supported beam subjected to a UDL of w kN/m：

A = bd，Mc = wL2 ∕ 8，I = bd3 ∕ 12，Z= bd2 ∕ 6

Therefore the stresses at the centre are：σtop= Mc ∕ Z top;σbot = Mc ∕ Zbot

Therefore the steel stresses required for prestressing is equal to σbot

3.0 How to achieve prestressed concrete that will rejoinder architectural concrete problem

There are two major implementations of prestressed concrete：

Pre-tensioning is the most common form for precast sections in this frame High-strength steel tendons are set between two abutments and stretched in the range of 70 to 80 per cent of their extreme strength.

The duct openings are at that point grouted to protect the tendons from corrosion.Thus this kind of presetressing can be accomplished using two methods：

First Bonded Post-Tensioned Concrete Fortified Post-tensioned Concrete.In this method compression is applied after pouring concrete and the curing process（in situ）.This method produces a great bond between the tendon and concrete，which both protects the tendon from corrosion and permits for direct transfer of tension.

The Second method is the Un-bonded post-tensioned concrete which varies from reinforced posttensioning by giving each individual cable permanent freedom of movement relative to the concrete to attain this，each individual cable is prepared with a coating of lithium-based grease，then given a plastic-based “shell” formed through extrusion.This design gives the capacity to de-stress the embedded tendons earlier to repair.

4.0 Some basic ways that presetress concrete rejoinder architectural concrete problem

Versatility of Design：- Pre-stressing expels a number of design limitations conventional concrete places on span and load and permits the building of roofs，floors，bridges，and walls with longer unsupported spans.

Durability：- Precast concrete does not require painting and is free from corrosion.Its durability extends building life.It moreover resists Fire.Durability and fire resistance mean low insurance premiums and more prominent personnel safety.Longer span lengths increase the usable unhampered floor space in buildings and parking structures.

Minimal maintenance：-Lower maintenance costs over the design life of the structure，since of less joints since joints are the major locus of weakness in concrete buildings.No long-term issues with maintaining the astuteness of the anchor/dead end.

Rapid construction：-Precast concrete construction gets the work done sooner.Early inhabitance gives self-evident benefits to the client

Lower construction cost：- Thinner slabs，which are especially important in high-rise buildings where floor thickness savings can translate into additional floors for the same or lower cost.Also Large reduction in traditional reinforcement requirements

Free of crack：-The prestressed concrete members are free from the cracks under compression.Prestressing is used to counteract the tension stresses in the concrete beam are caused by the weight of members and imposed loads and these loads cause a positive bending moment in a beam，so prestressing is used to introduce a negative bending moment that should counteract the all of the positive bending moment.

Resistance to shear force：-The member of prestressing concrete has offered more resistance against shear force.Prestressed concrete producing the permanent compression in the areas subjected to the tension.The tensile stress portion is counteracted by reducing the cross-sectional area of the reinforcing steel.

Resistance to impacts loads：-It improves the ability of a concrete member to resist the impacts loads more carefully.Such beams having no tensile strength and no bending moment until squeezed or prestressed together.