Q:

do prestressing tendons located in the compression zone contribute to compression reinforcement?

when doing the normal RC design of an RC beam, it is determined that compression steel reinforcement is required, does a prestressed tendon in the compression zone contribute to the area of compression steel? 
Possibly it could contribute, but with a reduced contribution because it already has an axial load within it.

A:

The short answer would be "yes", but there is a lot of things to be aware:

First of all, you should check that the building code you are following allows you to do so.
You should check adherence between steel and concrete. Since the ratio between strength and lateral surface is a lot higher for tendons than for passive reinforcing steel, adherence conditions are a lot worse. Furthermore, beware that some kinds of tendons (unbonded?) don't have any adherence at all and can't be counted as reinforcing.
Most building codes require that longitudinal bars of reinforcing in compression zones to be inside the cage - with some layout of other bars - to avoid buckling. Placing of tendons not always complies with such a requirement.
To evaluate the contribution of tendons, please remember that deformation of steel and concrete must be compatible, and that ultimate deformation of high tensile steel is way larger than that of concrete in compression. Then, in your calculations you can't just replace tendons by the rebar of the same ultimate resistance.

And about your last sentence, contribution of steel is not reduced because of axial load, but it's just the other way: in the compression zone, the load introduced to steel by bending is in opposite direction of the load introduced by prestressing. However, prestressing do reduce the contribution of concrete, because both prestressing and bending cause compression to concrete in the compression zone.
And just as an end note: prestressing in the compression zone should be avoided if possible, but you probably already know that.
Disclaimer: I'm not used to write about reinforced concrete in English. If my vocabulary is too weird to be properly understood, please comment and I'll try to fix it.

