Views: 0 Author: Site Editor Publish Time: 2025-11-20 Origin: Site
Anchorage in prestressed concrete is a critical structural component responsible for securely holding the prestressing tendons in place and transferring prestressing forces effectively to the concrete member. These anchorages must be robust, corrosion-resistant, and precision-manufactured to ensure the longevity and safety of prestressed concrete structures. The following comprehensive overview of anchorage in prestressed concrete covers types, components, materials, installation, standards, specifications, and performance considerations.
Fundamentals of Anchorage in Prestressed Concrete
Prestressed concrete utilizes steel tendons that are tensioned to introduce compressive stresses into the concrete before external loads are applied. The anchorage system is where these tendons are anchored after tensioning, providing a "dead end" for the force and ensuring it transfers into the concrete. Without efficient anchorage, tendons could slip or fail, leading to structural insufficiency.
Securely hold tendons under high tensile forces, often reaching thousands of kilonewtons.
Transfer the prestressing force into the concrete to maintain compressive stress.
Prevent tendon slippage, cracking, and structural splitting.
Protect tendons from corrosion to extend service life.
Dead-end anchorages firmly fix one end of the prestressing tendon in concrete. This type is mainly used in pre-tensioned concrete where tendons are tensioned before concrete placement. The anchor resists the tendon force permanently embedded in the concrete block, often with anchor plates and wedges holding the strands.
Used primarily in post-tensioned concrete, where tendons are placed inside ducts cast into concrete, tensioned afterward, and then anchored to maintain tension. This type includes:
Anchor heads equipped with wedges to grip strands.
Bearing plates distributing force onto concrete.
Hydraulic jacks apply the tension before locking the anchor.
Grouting follows to bond tendons and prevent corrosion in bonded systems.
Bonded Anchorage: Tendons are grouted inside ducts, creating a bond with concrete for uniform force distribution. Grouting prevents corrosion and provides structural integrity.
Unbonded Anchorage: Tendons are free within ducts and only anchored at ends, allowing some movement and flexibility in stress distribution, useful for certain specialized applications.
Anchor Plate / Bearing Plate: A steel plate that spreads the load from the tendon over a broader concrete surface area, minimizing concrete crushing.
Wedges: Tapered steel pieces that grip the tendon strands securely by friction inside the anchor head.
Anchor Head: The housing for wedges and anchor plates; transmits force to the concrete.
Ducts: Steel or plastic tubes housing unbonded tendons in post-tensioned members.
Grouting Materials: Cementitious or chemical grout injected to fill ducts in bonded systems, protecting against corrosion and bonding tendons to concrete.
Threaded Bars and Nuts: In some systems, prestressing bars are tensioned and locked against bearing plates using nuts.
Single-Strand Anchorage: Designed for individual steel strands, mostly used in precast or smaller structural members to provide precise force transfer and tensioning.
Multi-Strand Anchorage: Utilized for multiple parallel strands, typical in large-scale construction projects like bridges and high-capacity slabs. These anchor assemblies consist of larger anchor plates and multiple wedges to secure several strands at once.
Freyssinet System: Uses concrete cylinders with grooves and corrugations to grip wires within a reinforced cylinder, enabling simultaneous tensioning of multiple wires.
Gifford Udall System: British-origin single-wire system with tube and plate anchorages using double-acting jacks.
Lee McCall System: Employs threaded steel bars tensioned and secured by nuts on bearing plates for certain applications.
In countries like China, standard anchorage models are designated with codes such as:
M15-N or M13-N: M indicates anchorage; 15 or 13 denotes strand diameter (e.g., 15.24 mm); and N indicates the number of strands.
Variants like BM15-N denote flat anchorage used in lateral prestressing for uniform stress and reduced structure thickness.
Extruded sleeve or wrapped anchors address conditions with high end stresses or space restrictions in components.
Steel components in anchorage systems undergo hot-dip galvanization or are coated to resist corrosion.
Precision manufacturing ensures tight tolerances and uniform load transfer.
Corrosion protection extends the lifecycle of prestressing tendons and anchorage hardware, essential in chloride-rich or aggressive environments.
Anchorage must accommodate ultimate prestressing loads, often expressed in kilonewtons (kN), depending on tendon size, number of strands, and prestress level.
The anchorage zone in concrete undergoes reinforcement to cope with stress concentration and avoid splitting cracks.
Design optimizations can reduce anchorage size and weight significantly, enhancing economic efficiency in bridge design and other applications.
Tendons are threaded through ducts or placed in position.
Wedges and bearing plates are positioned correctly.
Hydraulic jacks tension tendons to design force.
Wedges lock tendons in place against bearing plates.
For bonded systems, ducts are grouted post-tensioning.
Proper seating of wedges and fixing of plates is essential for grip and load transfer.
Verify anchorage torque and no tendon slippage.
Monitor grout acceptance and curing.
Test anchorage device load capacities conforming to ASTM, ISO, or regional standards.
Surveillance for corrosion resistance adherence.
Feature | Description | Examples/Notes |
Anchorage Types | Dead-end, Tensioned (post-tensioning), Bonded, Unbonded | Pre-tensioned uses dead-end; post-tensioned uses tensioned anchorage |
Components | Anchor head, wedges, bearing plate, ducts, grout | Critical for force transfer & corrosion protection |
Strand Configuration | Single-strand or multi-strand anchorage | Varies with structural load and design |
Material & Protection | Hot-dip galvanized or coated steel | Extends lifespan in corrosive environments |
Load Capacity | Thousands of kN, per tendon or tendon groups | Must meet engineering safety factors |
Installation & Testing | Hydraulic tensioning, wedge seating, grout injection | Ensures structural safety and durability |
Standards | ASTM, ISO, GB/T (China) | Guarantees performance and compatibility |
Anchorage in prestressed concrete is a sophisticated system designed to achieve secure force transfer from prestressing tendons to the structural concrete. Whether in single or multiple strands, bonded or unbonded configurations, anchorages must satisfy stringent standards for load capacity, corrosion resistance, and durability. Precision manufacturing, proper installation, use of corrosion-resistant materials such as hot-dip galvanized steel, and adherence to international standards ensure the long-term performance and safety of prestressed concrete structures.
Modern systems like the Freyssinet, Gifford Udall, and Lee McCall demonstrate the variety and adaptability of anchorage designs tailored for specific engineering needs. Standardized model designations, such as M15-N in China, facilitate quality control and specification in construction projects.
Overall, prestressed concrete anchorage devices are engineered to support complex loads, environmental challenges, and structural demands, playing a pivotal role in the success of contemporary civil engineering projects from bridges to buildings.
