Reinforced Concrete:
Reinforced concrete is a composite material in which a framework of bars, often of steel, are set into the concrete, in order to increase its tensile strength. Whilst concrete has a very high compressive strength, its tensile strength is very poor, and this is an effective method of increasing tensile strength.
Prestressed Concrete:
Prestressed concrete describes concrete that has been treated by tensioning cables inside of it, often of steel, to best resist wear the concrete might endure as part of its function. There are two main types of prestressing concrete: Pre-tensioning and Post-tensioning.
Pre-tensioning:
This is when the steel cables are put in tension and in the desired shape, and the concrete is subsequently poured into the mould in which the cables have been tensioned. Once the concrete has dried, the steel cables are released from tension and this creates compression within certain parts of the concrete where a greater compression strength was deemed necessary.
Whilst pre-tensioning is effective, due to the exposure of the steel cables to the wet concrete, the cables tend to corrode faster than in post-tensioning. In addition, pre-tensioning cannot be done on-site and it is difficult to pre-tension concrete if it has a particularly unique shape. This means it is not used as frequently as post-tensioning in construction.
Post-tensioning:
This is when ducts are set within the concrete, in the desired shape. Steel cables are then inserted into these ducts and are tensioned with the help of anchor points which are found at either end of the ducts.
This method of prestressing concrete does not expose the steel cables to the moisture in the concrete, both due to the insulation provided by the ducts but also that the cables are inserted after the concrete has dried. Post-tensioning can be done on-site and allows for the strengthening of unique shapes. This means that it is the most used method of prestressing in construction.
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