Prestressing Beam Concrete

Location of project: One Menerung, Bukit Bandaraya, Bangsar, KL

Principles ~ the well known properties of concrete are that it has high compressive strength and low tensile strength. The basic concept of reinforced concrete is to include a designed amount of steel bars in a pre determined pattern to give the concrete a reasonable amount of tensile strength. In prestressed concrete a precompression is induced into the member to make full use of its own inherent compressive strength when loaded. The design aim isto achieve a balance of tensile and compressive forces so that the end result is a concrete member which is resisting only stresses which are compressive. In practice a small amount of tension maybe present but providing this does not exceed the tensile strength of the concrete being used tensile failure will not occur.


Materials ~ concrete will shrink whilst curing and it can also suff sectional losses due to creep when subjected to pressure. The amount of shrinkage and creep likely to occur can be controll by designing the strength and workability of the concrete, his strength and low workability giving the greatest reduction in both shrinkage and creep. Mild steel will suffer from relaxation loss which is where the stresses in steel under load decrease to minimum value after a period of time and this can be overcome increasing the initial stress in the steel. If mild steel is used for prestressing the summation of shrinkage, creep and relaxatilosses will cancel out any induced compression, therefore spec alloy steels must be used to form tendons for prestressed work.


Tendons … these can be of small diameter wires (2 to 7 mm) in aplain round, crimped or indented format, these wires may be individual or grouped to form cables. Another form of tendon isstrand which consists of a straight core wire around which is helically wound further wires to give formats such as 7 wire (6over 1) and 19 wire (9 over 9 over 1) and like wire tendons strand can be used individually or in groups to form cables. The two main advantages of strand are:-

1. A large prestressing force can be provided over a restricted area.

2. Strand can be supplied in long flexible lengths capable of being stored on drums thus saving site storage and site fabrication space.


Post-tensioning ~ this method is usually employed where stressing is to be carried out on site after casting an in situ component or where a series of precast concrete units are to be joined together to form the required member. It can also be used where curved tendons are to be used to overcome negative bending moments. In post-tensioning the concrete is cast around ducts or sheathing inwhich the tendons are to be housed. Stressing is carried out after the concrete has cured by means of hydraulic jacks operating from one or both ends of the member. The anchorages which form part of the complete component prevent the stressed tendon from regaining its original length thus maintaining the precompression or prestress. After stressing the annular space inthe tendon ducts should be filled with grout to prevent corrosionof the tendons due to any entrapped moisture and to assist in stress distribution. Due to the high local stresses at the anchorage positions it is usual for a reinforcing spiralto be included in the design.


Anchorages ~ the formats for anchorages used in conjunction with post-tensioned prestressed concrete works depends mainly onwhether the tendons are to be stressed individually or as a group,but most systems use a form of split cone wedges or jaws acting against a form of bearing or pressure plate.

Comparison with Reinforced Concrete ~ when comparing prestressed concrete with conventional reinforced concrete themain advantages and disadvantages can be enumerated but in the final analysis each structure and/or component must be decided onits own merit.

Main advantages :-

1. Makes full use of the inherent compressive strength of concrete.

2. Makes full use of the special alloy steels used to form the prestressing tendons.

3. Eliminates tension cracks thus reducing the risk of corrosion ofsteel components.

4. Reduces shear stresses.

5. For any given span and loading condition a component with asmaller cross section can be used thus giving a reduction inweight.

6. Individual precast concrete units can be joined together to forma composite member.

Main Disadvantages :-

1. High degree of control over materials, design and quality of workmanship is required.

2. Special alloy steels are dearer than most traditional steels used in reinforced concrete.

3. Extra cost of special equipment required to carry out the prestressing activities.

4. Cost of extra safety requirements needed whilst stressing tendons.


As a general comparison between the two structural options under consideration it is usually found that :-

1. Up to 6000 span traditional reinforced concrete is the mosteconomic method.

2. Spans between 6000 and 9000 the two cost options arecomparable.

3. Over 9000 span prestressed concrete is more economical than reinforced concrete.It should be noted that generally columns and walls do not need prestressing but in tall columns and high retaining walls where the bending stresses are high, prestressing techniques can sometimes be economically applied.