> I would assume that if the cables themselves were applying compressive force to the concrete, then their use as stays releases some of this force since the st

> I would assume that if the cables themselves were applying compressive force to the concrete, then their use as stays releases some of this force since the stays are in tension. Since concrete itself performs poorly in tension, it's going to provide a negligible counter to reinforce the cables even if it is still compressed, since the compression forces are provided by the cables. 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. Not quite. High resistance steel is used in prestress applications, which provides a considerable headroom to determine what cables are used (number of strands per cable group and number of cable groups per cable) and how to design a structure (target prestress values, dynamic/vibration properties, load variation/fatigue design, etc). Therefore, just because a designer selects a specific prestress cable that doesn't mean the cable will be designed to be stressed closed o yield or even fatigue limits. The designer can pretty much design a structure that uses prestress cables loaded at a fraction of their design limits but subjected to higher loads at construction to be able to unload the initial prestress load as compression. This strategy is often used in bridges, as is designing a bridge to e intentionally heavier so that variable loads are a fraction of the dead load to increase fatigue life.