Grouting inside tendon ducts after pre-stressing operation

protects tendon from possible corrosion .

helps in maintaining pre-stress force reducing losses.

protects structure from sudden failure, if local damage takes place for tendon.

Thanks Ishwar Sonar .. for your answer..

Cementitious grout provides an alkaline environment that passivates the

surface of the steel which inhibits the corrosion process.

Thanks Chitaranjan Pany ... for your answer..

In addition to what have been said above, using grout can change the structural behaviour of the prestress forces. It can change the forces form two forces at the beam ends to uniformly distributed forces along the whole length of the beam. This can only occur if the ends constrained are released!

All arguments mentined are correct. However, the main task of grouting the pipes with mortar is to produce a composite effect. As a result, later local damage or failure cases have no effect on the overall function of prestressing.