Dear Will,

Pretension  technique is usually used with precast concrete because it saves the cost of mechanical anchorage at cables ends, besides that, the  required stressing equipment can easily be provided in precast factories. However, in some projects it is cheaper to prepare a cast yard in the site to produce the precast element due to rare site location, transportation problems and shortage in precast factories. It is difficult to provide this cast yard with required pretension equipment, hence, contractors have to use post tension technique. So, I think your question is related to precast bridge girders or may be precast piles.    

I think there are 2 main differences between post tension precast and CIP concrete and both are related to shrinkage as follows:

1- For CIP concrete, contractor have to stress the element as fast as possible to speed up the construction and reduce form-work cost, because of this early stressing, the losses due to shrinkage and elastic shortening  will be significant. On other hand, precast elements could be stored till most of shrinkage occurs and elastic modulus increases without affecting the construction schedule. that reduces the losses significantly.

2- Precast concrete usually cured using steam unlike the CIP concrete, that gives the precast concrete early strength and speed up shrinkage, not mentioning that quality control of precast is much better that CIP.

Based on that, I think post tension precast concrete is structurally more effective than CIP post tension concrete.

I hope this helps you

best regards

2-         

Sir Ahmed, thank you for your answer. So, there is nothing related to creep loss? Only due to shrinkage loss and elastic shortening ?

Creep  (unlike shrinkage) requires long time and sustained compression stresses, that means creep will not initiated until the element stressed , hence, storing the precast element will not reduce the creep effect. 

On other hand, creep effect depends on the age of concrete (hence, strength)  when the sustained load applied. from this point of view, precast concrete is better than CIP because steam curing gives much earlier strength.