PRE-STRESSED CONCRETE (PSC)

PRE-STRESSED CONCRETE (PSC)

⇒ Definition of PSC: 

The PSC is one in which there have been introduced internal stresses of such magnitude and distribution that the stresses resulting from external loading can be counter balance up-to desired degree.

⇒ Necessity of very high strength steel & concrete for PSC

1. Steel: 

Due to various losses a large stress value is lost only due to creep & shrinkage strain. Strain=0.0008, ∴ Loss=0.0008 x 2 x 10^5 (Es)=160 N/mm2.

∴ The total loss is about=200-300 N/mm2 and the initial pre-stress value should be in the range of 1000 N/mm2 to 2000 N/mm2

2. Concrete:

High strength of concrete is required:

• To bear high stress due to high stress in steel.
• To bear high bond stress in case of Pre-Tensioned.
• To keep the loss due to creep & shrinkage to a minimum value.

⇒ Types of PSC

1. Pre-Tensioned:

• Tensioning: Steel Reinforcement provided is tensioned by applying pull at ends.
• Casting of concrete: Now concrete is casted and is allowed to gain sufficient strength for 28 Days.
• Transfer of stress: Now steel Reinforcement is cut, pre-stressing force is transferred by bond action between steel 7 concrete. Steel is in direct contact with concrete in this case. Repetitive type of member are casted by this method.
• Eg: Railway Sleeper.

2. Post-Tensioned:

• Casting of concrete: Concrete is casted 1st by leaving a duct of required shape at required location in concrete.
• Tensioning: Now steel reinforcement is provided in duct and then tensioned either from one end or from both end.
• Anchoring the reinforcement: The reinforcement is anchored at ends by any suitable method after tensioning process.The pre-stressed force is transferred by anchoring at ends.

⇒ Advantages of PSC over Reinforced Cement Concrete

• It can take very load over large span.
• Concrete area is fully utilized so we need a smaller section of PSC member.
• There will be saving on super structure and supporting structure like column or foundation.

⇒ Assumptions:

• Stress in the reinforcement does not change along the length of reinforcement.
• Prestressing force is considered constant in the beam at all cross-section.
• Any gain or loss of stress due external loading is to be ignored.
• Hook's law is valid.
• Strain diagram is linear.