Anchorage in Prestressed Concrete

Anchorage as the name signifies is a component of post tensioning system which is used to anchor the tendons into the concrete while terminating or joining two tendons. Main function of anchorage is to transfer the stressing force to the concrete once the stressing process is completed.

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It is one of the most important 5 components of post tensioning systems. Anchorage is an inevitable part of prestressing system. Efficiency of anchorages affects the service life of a prestressed structure.

Components of Anchorages

Important components of Anchorage are Anchor plate or anchor head, Removable grouting cap, Iron Block/force transfer unit, Bursting reinforcement, Deviation cone and duct coupler. The prestressing force is applied to the strands and locked in place by the wedges in the anchor head which is supported on the force transfer unit cast into the concrete.

The force transfer unit ensures the transmission of the prestressing force into the concrete. The force transfer unit and the deviation cone ensure the correct deviation of the strands from the anchor head to the duct.

Types of Prestressed Concrete Anchorages

Post-tensioning tendons anchorages are classified into following types

1. Live anchorages,
2. Dead anchorages,
3. Couplers,
4. Annulars,
5. External and electrically insulated.

Live Anchorages

In prestressing, the side of tendon which is stressed is referred to live end. In both side stressing systems, two ends of tendons are live ends. Live anchorages are those which are used at the stressing/live end of the the tendon.

Electrically insulated post-tensioning

Some other types of live anchorages includes Electrically insulated anchorages and external types. Electrically insulated anchorages are used were protection of pre-stressing cables from corrosive agents is required. The cable is enveloped completely in a protective sheathing from outside agents from the starting of anchoring blocks. A dielectric material made of high-density polyethylene is used for covering the tendons.

External type are used with external cables where full replaceability as well as protection of tendon is required. System can be demounted and substituted through the presence of an inner steel
cone that separates strands and inner protective injection from the surrounding elements in the anchorage zone.

External tendons can reduce congestion in concrete and better corrosion resistance since it makes inspection and replacement possible. Friction losses are minimum compared to internal tendons as friction occurs at the deviators & anchorage points.

Adjustable post-tensioning

Adjustable anchorages are used where force adjust and re-stressing are required, depending on the structure behaviour and construction needs. It also gives the possibility to monitor loads,
especially in the period following the installation of cables, when either the strands relaxation or the viscous effects of concrete can affect the evaluation of forces.

To allow the load check and adjustment, these anchorages are composed of a threaded anchorage with a nut, all protected with a cap filled with grease. The load adjustment operation is carried out by screwing and unscrewing the thread with a special annular jack, especially designed for these applications. This system is fully replaceable.

Bonded Flat System

Bonded Flat System Anchorage is a flat system used mainly in slabs and for transverse stressing in bridge decks. It can also be used in transfer beams, containment structures and other civil applications and for both 13mm and 15mm strands. The system connects bare strands which run through a steel or plastic flat oval duct. The strands are stressed individually using a monostrand Jack.

Dead End Anchorages

Dead end anchorages are used at non stressing end of a tendon. They are used where the end of a prestressing cable is buried in concrete or is inaccessible during the stressing operation of the tendon. The dead end anchorage configurations is same as the standard anchorage as it uses the same tube unit to guide the strands.

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Different types of dead end anchorages are

1. Compression Fitted Dead End Anchorage
2. Plate Dead End Anchorage
3. Loop Dead End Anchorage
4. Basket dead end anchorages

Anchorage is a permanent anchorage device used in prestressed concrete. It is an anchorage tool used in post-tensioning structure or member to maintain the tension of prestressed reinforcement and transfer it to the concrete interior. It is also called pre-stressed anchorage.

Anchorage classification by mounting position

A permanent anchorage device for concrete.Anchors can be divided into two categories: (a) Tensioned end anchors: anchors that are installed at the end of prestressed reinforcement and can be tensioned, are also called prestressed anchors. The so-called anchors are anchors in post-tensioned structures or members that maintain the tension of the prestressed reinforcement and transfer it to the concrete; (b) Fixed end anchorage: anchorage installed at the end of prestressed reinforcement, usually embedded in concrete and not tensioned.The standard of anchorage for prestressed reinforcement is: the National standard of the People's Republic of China (GB/T -)

Common specification

At present, the anchorage specifications commonly used in China are: (a) M15-N anchorage. M stands for anchorage (the first letter of Chinese pinyin for anchorage); 15 represents the steel strand with a specification of 15.24mm (China's generally used steel strand strength of MPa class 15.24mm); -N is the number of strands to be threaded. (b) M13-N anchorage. M stands for anchorage (the first letter of Chinese pinyin for anchorage); 13 Represents the 12.78mm steel strand (the 12.78mm steel strand strength of MPa that is commonly used in foreign countries); -N is the number of strands to be threaded.

Prestressed anchor products include: group anchor, P anchor, connector, ring anchor, Frasier anchor and other specifications. And each specification is divided into 15 series and 13 series. Anchors of 15 series are widely used nowadays.

Common system classification

(1) Round cylinder anchorage. The specifications and models are: YM15-N or YM13-N(Y, the first letter of pinyin for round anchor, which means round anchor); The anchoring tool has good anchoring performance and self-anchoring performance. In general, through-center jacks are used. 

(2) Rectangular flat anchorage. The specifications and models are: BM15-N or BM13-N (B, the first letter of pinyin for flat anchor, which means flat anchor); The flat anchor is mainly used for transverse prestressed bridge deck, hollow slab and low-height box girder to make the stress distribution more even and reasonable and further reduce the thickness of the structure. 

(3) Holding and wrapping anchorage. (Fixed end anchor) Specification and model: AM15P-N or AM13P-N; It is applicable to the case where the design stress of the end cloth is large or the end space is limited. It uses an extruder to compress the squeeze sleeve on the steel strand. It is embedded in the concrete and arranged as required. 

 Application

Highway bridge, railway bridge, city overpass, urban light rail, high-rise building, water conservancy and hydropower DAMS, port terminals, and anchorage of rock mass slope protection, foundation reinforcement, the tunnel vault crown and prestressed steel grids, subway, large government building projects, warehouse, workshop, tower building, heavy lifting, synovial intermittent the pusher, large container and ship propulsion, tunnels, Bridges and buildings, sleeper, replace bearing reinforcement, reinforcement engineering, antimagnetic and anticorrosive engineering anchorage (fibers), carbon fiber reinforcement, construction of pre-tensioned beam field, external prestressed engineering, stay cables, suspension cables, etc.

The Ministry of Communications stipulates that the prestressed anchor must be used in beams and plates with a span of more than 15 meters.This fully reflects the importance and wide application of prestressed anchorage products in bridge construction.

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