Post Tensioning System

Mono strand – unbonded tendon systems

          The single strand system is normally adopted for unbonded tendon which the strand is greased and PE coated for corrosion protection as well as reducing friction.The strand is gripped in single strand anchorage type. To provide for a corrosion protection system the anchorage is encapsulated by a special plastic cap after the stressing and the strand end has been cut.

Stressing anchorage data

Strand Dia. 0.5” (12.7 mm.) 0.6” (15.2 mm.)
No. of strand 1 1
Type of anchorage SF105 SF106
Ultimate tensile force, KN 186 265
Stressing force at 0.8 U.T.S.,KN 149 212
Jack Type V16 V24

          The same anchorage for stressing anchorage can be used as intermediate stressing anchorage and dead end anchorage. For dead end anchorage, the wedge is pre-seated in the anchorage by applying nominal force to the strand using the stressing Jack.
          Alternatively a loop dead end anchorage can be used in which case the strand is formed in loop shape around the loop bearing plate and stressing anchorages are used at the stressing ends of the loop tendon.
          A nominal reinforcing bar of 2 DB 12 grade 40 is normally provided behind the stressing anchorage to act as brustling reinforcement in the anchorage zone.

Dead end anchorage

1. Loop dead end
2. Loop plate
3. Burstling reinforcement

Multistrand flat duct – bonded tendon systems

          multistrand flat duct systems is normally adopted for bonded tendon. The strands are individually gripped in one flat anchor head unit and tramsmits their prestressing forces by means of flat type anchor plate casting unit. The strands are stressed individually by means of a mono strand jack. The strands are contained in one flat duct which is made of corrugated galvanized or non-galvanized metal. To ensure corrosion protection and to give adequate bond strength, the tendons are filled with suitable cement grout mix after complete stressing of the strands.

Stressing anchorage data

Strand Dia 0.5” (12.7mm)   0.6” (15.2mm)  
Tendon Type 305 405 505 206 306 406
No. of strand 3 4 5 2 3 4
Anchor Head Type SF305 SF405 SF505 SF206 SF306 SF406
Anchor Plate Casting Type SF305/206 SF405/306 SF505/406 SF305/206 SF405/306 SF505/406
Ultimate tensile force per tendon, KN 558 744 930 558 837 1,116
Stressing force at 0.8 U.T.S.,KN 446 595 744 446 670 892
Flat duct inside
dimension /mm. x mm.		 63.5 x 20
or 50 x 20		 63.5 x 20 76.5 x 20 63.5 x 20
or 50 x 20		 63.5 x 20 76.5 x 20
Jack Type V16 V16 V16 V24 V24 V24

          The same anchorage for stressing anchorage can be used as dead end anchorage in which case the wedges are preseated in the anchor head unit by applying nominal force to the strand using the stressing jack.
           Alternatively a bulb dead end anchorage can be used with simpler solution and more cost effective. In this case each strand is formed into a bulb shape by means of special bulb forming jack.
           Burstling reinforcement should be provided at the anchorage zone for both stressing anchorage and dead end anchorage type.

Dead end anchorage

1. Bulb dead end
2. P.C. strand
3. Grout vent with mastic seal tape
4. Corrugated metal flat duct
5. Burstling reinforcement

multi strand round duct

          The systems are commonly used in bridge construction as well as other civil Engineering structures such as Silo, Water tank etc. The systems are normally adopted for bonded tendons. The tendons consist of a bundle of strand with a nominal diameter of 0.5″ (12.7 mm.) or 0.6″ (15.2 mm.). The number of strands per tendon can be from 4 strands up to 31 strands of diameter 0.5″ or 22 strands of diameter 0.6″. The strands in the tendons are contained in one round duct which is made of corrugated galvanized or non-galvanized metal.  
          The strands are individually gripped in one anchor head unit and transmits their prestressing force by means of anchor plate casting unit type M. For each anchor size a special spiral reinforcement is provided at the anchor plate casting to give adequate splitting reinforcement for burstling stresses developed at the anchorage zone. The strands in tendon are stressed simultaneously by means of a multi strand stressing jack from capacity 1,100 KN up to 5,000 KN. In case of small size tendon unit such as type M405 the strands can also be stressed individually by means of mono jack.  
         To ensure corrosion protection and to give adequate bond strength, the tendons are filled with suitable cement grout mix after complete stressing of the strands.

Anchorage components

1. Wedges 2. Anchor head type M 3. Anchor plate casting type M
4. Grout vent 5. Corrugated metal round duct 6. Anchorage spiral reinforcement

Stressing anchorage data

Number of strands dia. 0.5”(12.7mm) 4 7 12 19 31
Tendon Type 405 705 1205 1905 3105
Ultimate tensile force per tendon, kN 747 1302 2232 3534 5766
Stressing force at 0.8 u.t.s.,kN 595 1042 1786 2827 4613
Duct ID, mm 45 55 70 85 105
Jack Type V16 M110 M200 M300 M500
Number of strands dia. 0.6”(15.2mm) 4 7 12 19 22
Tendon Type 406 706 1206 1906 2206
Ultimate tensile force per tendon, kN 1116 1953 3348 5301 6138
Stressing force at 0.8 u.t.s., kN 893 1562 2678 4241 4910
Duct ID, mm 50 65 80 100 110
Jack Type M110 M200 M300 M500 M500

          The same anchorage can be used as dead end anchorage in which case the strands are gripped by means of wedges and are secured by springs and covered with cap fastened against anchor plate casting to prevent intrusion of cement slurry into the anchorage.
        In case of prestressing continuity is required, the tendons are coupled by means of anchorage coupling connected to the stressed anchorage at the stressing joint. The anchorage couplings are available for each type of anchor size.

Anchorage coupling

  Strand Dia. 0.5″ Strand Dia. 0.6″
Number Of Strands 7 12 19 31 4 7 12 19 22
Coupling Type 705 1205 1905 3105 406 706 1206 1906 2206
Length h, mm. 420 565 655 775 420 565 655 775 815
Diameter d, mm. 156 206 247 307 156 206 247 307 337

Technical Features of External Prestressing

External prestressing is characterized by the following features :

– The tension elements resp. prestressing tendons are placed on the outside of the physical cross-section ( mostly in concrete ) of the structure
– The forces exerted by the prestressing tendons are only transferred to the structure at the anchorages and at deviators
– No bond is present between the cable and the structure, unless at anchorages and deviators bond is intentionally created
– The application of external prestressing is indeed not bound to the use with concrete, but it can be combined with any construction material as composite materials, steel, timber, steel and concrete combined and other modern plastic materials. This can considerably widen the scope of the post-tensioning applications.
– Due to the absence of bond, it is also possible to restress, destress and exchange any external prestressing cable, provided that the structural detailing allows for these actions.

Advantages :

– Facilitating of concrete placing due to the absence of tendons in the webs, and ease for placing tendons.
– Improvement of conditions for tendon installation which can take place independently from the concrete works.
– Reduction of friction losses, because the unintentional angular changes, known as wobble, are practically eliminated. Furthermore, in case of the use of a polyethylene sheathing the friction coefficient is drastically reduced compared to standard internal and bonded prestressing using corrugated sheat metal ducts.
– Generally the webs can be made thinner, resulting in an overall lighter structure
– As an overall result, better concrete quality can be obtained leading to a more durable structure and facilitating execution.

Typical Applications for External Prestressing

Typical applications where external tendons are feasible, practical and economical, are:
– Repair work and strengthening of all kinds of structures
– Underslung structures – Precast segmental construction simple and continuous spans – Incremental launching procedure, in particular centric prestressing

The PBL multi strand anchorage systems can be used for external prestressing tendon

– The tendon is formed from plain 0.5” or 0.6” dia. strands according to ASTM 416 or BS standard
– The duct is from high density polyethylene and continuous from one anchorage to the other. The Tendon sheathing passes freely through intermediate diaphragms, respectively deviators, whereby a metal or HDPE sleeve provides the required reservation opening.
– A standard PBL multi strand anchorage assembly consisting of anchor head, wedges, anchor plate casting safely transfers the prestressing force to the end cross-beam
– The tendon is filled with cement grout after it has been tensioned. As no double pipe is provided at the anchor points through the end-diaphragms a certain bond between tendon and structure is created which hinders the replaceability. Depending on requirements, the anchor head may be protected by a cap, or alternatively the anchorage recess is filled with non-shrinkage concrete.