WHAT IS PRE-STRESSED CONCRETE - TYPES AND ADVANTAGES

Pre-stressed Concrete

Pre-stressed concrete is a combination of high strength concrete and steel strands.Prestressing is generally a way to overcome concrete weakness in tension.Generally the concrete undergoes compression on top flange and tension at bottom flange.In prestressing the tendons are stretched along the axis and cement is poured, later when the tendons and released the compression is generated at the botttom which tries to counter-balance the compression due to loading at the top part of the beam.The upward force along the length of the beam counteracts the service loads applied to the member.

Pre-stressed concrete was invented and patented in 1886 by Henry Jackson, an engineer from San Francisco USA.Pre-stressed concrete makes a strong structural materials that is used in the building of roof slabs, bridges girders and railroad ties.The walnut Lane Memorial Bridge built in 1951 in Philadelphia, Pennsylvania become the first structure in USA made of this type.

Pre-stressed concrete can be created using two different methods, they are as fallows:

1. Pre-tension 
2. Post-tension

1. Pretensioning 

The pretensioning method involves in stretching high tensile steel stands between abutments located at both ends of the concrete casting bed.After the stands are taught, concrete is poured into beds, where it surrounded and adheres to the stands.Once the concrete is dry it will have bonded to the steel.After the concrete has reached the desired strength the strands are released, resulting in the concrete developing a slight arch that makes it more resistant to heavy loads.

2. Post Tensioning

In post tensioning the steel is prestressed only after the beam is cast, cured and attain strength to take the prestress.Within the sheathing, the concrete is cast.For the passage of steel, ducts are formed in the concrete.

Once the casted concrete hardens completely, the tendons are tensioned.One end of the tendon is anchored and the other end is tensioned In same cases, the tensioning can be performed from either side and anchored subsequently.

Once the prestressing is complete, there is space between the tendons and the duct.This leads to:

1. Bounded Construction
2. Unbounded Construction

1. Bounded Construction

In bounded construction the space between the duct and the tendon is filled with cement grout.The grouting process helps the steel to resist corrosion to a large extent.The ultimate strength is increased as this method increases the resistance to live loads acting.The grout is a mixture of cement and water without admixtures.In this grout sand is not used.

2. Unbounded Construction

In unbounded construction no grout is used to fill the space between the duct and tendon.Here steel is galvanized to protect from corrosion.A waterproofing material is used for galvanizing.

Advantages of Pre-stressed Concrete

• It has high compressive strength.
• It is a best for construction of sleek and slender structures.
• It is more durable than reinforced concrete.
• The prestressing system works for a span greater than 35m.
• Prestresseing helps to reduce the dead load of the concrete structure.
• Prestressed concrete are free from cracks.
• Prestressing enhance shear strength and fatigue resistance of concrete.

Disadvantages of Pre-stressed Concrete

• It has higher material cost.
• Prestressing is an additional cost.
• High skilled labor is required.
• Advanced technical knowledge and strict supervision is very important.