Anchorage zones and bursting reinforcement in PT slabs

What happens at the anchorage

At each stressing and dead-end anchorage, the full tendon force enters the concrete over a small bearing plate. As the force spreads out, tensile stresses form across the line of the tendon (bursting) and at the loaded face (spalling). Without proper reinforcement these can crack the slab edge, especially when stressing early at low concrete strength.

Local zone and general zone

• Local zone: the concrete immediately around the anchorage device, usually covered by the tested anchorage system with its own confinement spiral or hairpins.
• General zone: the larger region where the force spreads into the full slab or beam section. ACI 318 sets out the design of the general zone, with PTI guidance for slabs; strut-and-tie or simplified methods are used to find the bursting force and the reinforcement to carry it.

Detailing points

• Bursting bars or hairpins at each anchorage, positioned at the depth where bursting tension peaks.
• Back-up bars along slab edges between anchorages, and minimum edge distance and spacing to the system data.
• Extra care at closely spaced or grouped anchorages, at re-entrant corners and at construction joints and pour strips.
• Concrete strength at transfer confirmed by cube tests before stressing.
• Stressing pockets sealed and anchorages protected for the exposure class.

Anchorage zone reinforcement is shown on the PT shop drawings and checked as part of every PT design review and full design package.