Prestressed Concrete Quiz 2_Prestress Losses

What is prestress losses?

• Prestress losses are the increase in prestressing force in concrete.
• Prestress losses refer to the initial tension applied to concrete before curing.
• Difference between initial prestress Po and

  the effective prestress P' that remains in a member.

• Prestress losses are the reduction in prestressing force in concrete due to factors like creep, shrinkage, and relaxation.

Which of the following is NOT a type of prestress loss?

• Friction

• Elastic deformation of concrete

• Relaxation of stress in steel

• Temperature changes

What is the difference between immediate losses and time-dependent losses?

• There is no difference; they are the same

• Immediate losses are always larger than time-dependent losses

• Immediate losses are due to initial conditions, while time-dependent losses occur due to ages and sustained loads

• Immediate losses occur over time, while time-dependent losses happen instantly

The short-term losses are caused by ​ ​ (a)   , ​ (b)   , (c)   ; while time-dependent losses are due to ​ (d)   , ​ (e)   .

• elastic shortening
• friction
• anchorage slip
• creep
• steel relaxation

Which factor does NOT affect concrete creep?

• Age of the concrete

• Applied stress level

• Color of the concrete

• Type of aggregate used

What is the primary cause of prestress loss during the transfer of prestress?

• Temperature fluctuations

• Air pressure changes

• Elastic shortening of concrete

• Water content in concrete

Match the losses and cause:

• Loss of bond at the anchorage point

• Creep

• Deformation over time under constant load

• Shrinkage

• Reduction in length due to stress

• Friction

• Decrease in volume as moisture evaporates

• Anchorage Slip

• Resistance to motion between surfaces

• Elastic Shortening of Concrete

Which losses only happened in post-tensioned? (Think: Why?)

• Anchorage Slip

• Relaxation Steel

• Friction loss

• Elastic Shortening

Answer explanation

Think Why? Answer:

In Post tensioned, the loss due to friction between tendon and the duct or between the tendons themselves.

Anchorage loss can be accounted for at the site by over-extending the tendon during prestressing operation by the amount of draw-in before anchoring.

(see Figure from Midas)

Which of the following is a time-dependent loss?

• Elastic shortening

• Anchorage slip

• Friction loss

• Concrete creep

What is the primary reason for relaxation of steel in prestressed concrete?

• Increase in load

• Change in humidity

• Reduction in stress over time at constant strain

• Increase in temperature