>It seems to me that putting pre-stressed concrete into tension via its reinforcements is defeating the purpose of having pre-stressed concrete in the first place.
I think you are correct there. But it looks like the stays were in separate tubes with little allowance around them to separate them from the pre-stressed concrete.
That airspace around the stays then would prevent the concrete from providing any chemical protection against corrosion. And concrete as physical protection against rainwater is often only as good as the paint in the surface of the concrete. It's often way too porous even without cracks.
I think you are correct there. But it looks like the stays were in separate tubes with little allowance around them to separate them from the pre-stressed concrete.
That airspace around the stays then would prevent the concrete from providing any chemical protection against corrosion. And concrete as physical protection against rainwater is often only as good as the paint in the surface of the concrete. It's often way too porous even without cracks.