Short Answer:

The anchorage zone in prestressed concrete is the region of concrete around the ends of a prestressed member where the prestressing force is transferred from the tendons to the concrete through anchorage devices. This zone experiences very high stresses due to the sudden application of force and needs special attention during design.

To ensure structural safety, the anchorage zone is provided with extra reinforcement known as bursting or confinement reinforcement, which helps to resist cracking, splitting, or crushing of concrete at the tendon ends.

Detailed Explanation:

Anchorage zone in prestressed concrete

In prestressed concrete construction, tendons are tensioned to introduce compressive forces that improve the structural capacity of concrete members. These tendons are held in place using anchorage systems at both ends. The area of concrete surrounding this anchorage is called the anchorage zone, and it plays a critical role in safely transferring the prestress force into the concrete body.

The anchorage zone is also known as the end block region. It must be carefully designed to resist the complex and concentrated internal stresses that occur due to anchorage.

Functions and Importance of Anchorage Zone

1. Force Transfer Area:
   The anchorage zone is the part where the entire tension from the tendons is transferred to the concrete. This transfer happens through anchorage plates, wedges, or other anchoring devices. As a result, the area faces high local stresses, which are much greater than in other parts of the member.
2. Stress Distribution:
   The force applied at a point needs to spread over a larger area of concrete. As it travels into the beam, the stress gradually becomes more uniform. The initial portion, where this distribution takes place, is the anchorage zone.
3. Bursting and Spalling Forces:
   • Bursting stresses occur in the transverse direction and try to split the concrete outward.
   • Spalling stresses occur near the surface due to the anchorage plate pressing into the concrete.

These need to be controlled to prevent sudden failure.

1. Use of Reinforcement:
   To handle these internal forces, special reinforcements are added in the anchorage zone. These include spiral ties, stirrups, or confinement cages. Their role is to hold the concrete together and avoid cracking or crushing.
2. Length of Anchorage Zone:
   The depth or length of the anchorage zone depends on the type of tendon, magnitude of prestress, and anchorage system used. Proper detailing is important to ensure the load is distributed without damaging the concrete.

Types of Anchorage Zones

• Local Zone: This is the immediate area around the anchorage where the bearing stress is highest.
• General Zone: This surrounds the local zone and helps to spread the stress into the main body of concrete.

Design codes like IS 1343 and ACI 318 provide clear rules for designing and detailing anchorage zones.

Conclusion:

The anchorage zone in prestressed concrete is a critical stress region that allows safe transfer of force from steel tendons to the concrete. Due to high localized stresses, it requires special design with adequate reinforcement to prevent failures such as cracking or splitting. A well-designed anchorage zone ensures the long-term safety and durability of the prestressed structure.