Question

Which one of the following statements is not correct with respect to PSC beams?

• A. Loss due to shrinkage is proportional to water-cement ratio used in concrete.
• B. Loss due to creep is proportional to the age of the member at the time of loading.
• C. Loss due to elastic deformation is directly proportional to the modulus of elasticity of concrete.
• D. Loss due to friction occurs in post-tensioned concrete members.

Answer 1

The correct option is C Loss due to elastic deformation is directly proportional to the modulus of elasticity of concrete.

The shrinkage of concrete in prestressed members is due to the gradual loss of moisture which results in changes in volume. The drying shrinkage depends on the aggregate type and quantity, relative humidity, water/cement ratio in the mix, and the time of exposure.
Loss due to elastic deformation of concrete$=\frac{E_s}{E_c}\times f_c$
$f_c=$ Prestress in concrete at the level of steel. In the case of post-tensioned members. the tendons are housed in ducts preformed in concrete. On tensioning the curved tendons, loss of stress occurs in the post-tensioned members due to friction between the tendons and the surrounding concrete ducts.
The sustained prestress in the concrete of a prestressed member results in creep of concrete which effectively reduces the stress in high-tensile steel. The magnitude of the creep coefficient, $,\varphi ,$ varies depending upon the humidity, concrete quality, duration of applied loading and the age of the concrete when loaded.