Precast Prestress Camber

[GoDucks]

I was asked today about why a precast prestressed girder's camber goes up between initial release and 3 months and 5 years.....
I have always just accepted that it does. I am sure that I was proven it in college, but the reason has left me.

Can anyone please help with the reasoning? thanks

 

[civilperson]

Creep in the stressed concrete and eccentricity of the centroid of the stressing steel,

 

[yakpol]

The negative moment due to post-tensioning is singificantly larger than positive moment due to girder selfweight, therefore, initial deflections during release are upward and getting larger with time due to creep. To reduce creep deformations some special provisions require preloading of girders if they spend too much time on the casting yard.