End blocks

The full prestressing force enters a post-tensioned member through a small anchor plate at the end. Immediately behind the plate the stress is highly concentrated; a little further in, roughly one member depth away, it has spread out to the linear distribution of ordinary beam theory. The region in between is the end block, or anchorage zone, and it behaves quite differently from the rest of the member.

As the compression spreads out from the anchor, the lines of force bulge outwards and then turn parallel again. That bulging creates transverse tension, the bursting force, across the line of the load, a short distance behind the anchor. Near the loaded face, at the corners and between multiple anchorages, there is also spalling tension. If either is not reinforced, the end block can split along the tendon line, sometimes violently during stressing.

The bursting force depends on how concentrated the load is: the smaller the anchor plate compared with the end block, the larger the force. A widely used empirical relation, found in standard textbooks and reflected in codes, gives the bursting force as the prestressing force times a factor that falls from roughly 0.3 for a very small plate to almost nothing as the plate approaches the full end face. Reinforcement, usually closed links or a spiral around each anchorage, is placed across the bursting zone to carry this force at a limited steel stress.

Anchor manufacturers supply local zone details, spirals and minimum edge distances from tested systems. The designer checks the general zone, including the interaction of several anchors, and the bearing stress under the plates, which depends on the concrete strength at the time of stressing.