In this post the different losses in prestressed concrete is explained. Briefly explained each loss of both pre and post tensioned concrete.
When a prestressed concrete member is first tensioned, the applied prestress is maximum.
But this value does not remain constant with time.
As the structure ages, the effective prestress gradually reduces.
This reduction happens due to material behavior and construction-related effects.
These reductions are known as losses in prestressing.
Understanding these losses is essential for safe and realistic prestressed concrete design.
Definition: Losses in prestressing refer to the reduction in effective prestress in concrete from the time of transfer onward.
Nature of Losses: These losses occur due to time-dependent properties of concrete and steel, as well as mechanical effects.
Design Importance: Prestressed members are always designed considering these losses so that the required prestress remains effective during service life.
Losses depend on whether the member is pre-tensioned or post-tensioned.
Elastic Deformation of Concrete: Shortening of concrete due to prestress causes loss of stress in steel.
Relaxation of Stress in Steel: Reduction of steel stress under constant strain over time.
Shrinkage of Concrete: Volume reduction of concrete independent of applied load.
Creep of Concrete: Increase in deformation with time under sustained load.
Elastic Deformation of Concrete: No loss if all tendons are stressed simultaneously; loss occurs if stressed successively.
Relaxation of Stress in Steel: Similar to pre-tensioned members.
Shrinkage of Concrete: Reduction in concrete volume over time.
Creep of Concrete: Time-dependent increase in strain.
Friction Loss: Loss due to friction between tendon and duct during stressing.
Anchorage Slip: Loss caused by slight movement of tendons at anchorage.
Elastic Deformation of Concrete:
Prestressing causes concrete to shorten elastically, reducing stress in steel.
Loss is given by:
Loss = m × f₍c₎
where m is the modular ratio and f₍c₎ is the concrete stress at steel level.
Loss Due to Shrinkage of Concrete:
Shrinkage is a volume change that occurs without loading.
It depends on humidity, cement type, aggregates, and curing method.
Loss Due to Creep of Concrete:
Creep is continuous deformation under sustained load.
Its magnitude may be several times the initial elastic deformation.
Relaxation of Stress in Steel:
Relaxation is the reduction in steel stress under constant strain and temperature.
It typically ranges from 2% to 8% of the initial stress.
Friction Loss in Post-Tensioned Members:
Occurs due to wobble effect in straight tendons and curvature in curved tendons.
This loss affects stress transmission along the tendon length.
Loss Due to Anchorage Slip:
Occurs when wedges grip the tendons and slip slightly before fully locking.
This causes a reduction in tendon strain near the anchorage.
To understand the fundamentals of prestressed concrete, refer to:
What is Prestressed Concrete?
Losses in Prestressing: Reduction of effective prestress over time.
Time-Dependent Behavior: Caused mainly by creep, shrinkage, and relaxation.
Construction Effects: Friction and anchorage slip affect post-tensioned members.
Design Consideration: All losses must be accounted for to ensure required prestress during service.
Engineering Insight: Prestressing is not about applied force alone, but about how much force remains effective.
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