Methodology of Post-Tensioning work

Methodology of Post-tensioning work

Post-tensioning  work  is  divided  into  three  major  activities.  The  general methodology is given below.

  1. Profiling.
  2. Stressing.
  3. Grouting.

1) Profiling:

  • Read the  drawings  and  identify  type  of  tendons,  layout,  locations,  and  number of strands per tendon, sequence of laying, number of tendons per pour, etc.
  • Calculate the materials requirement such as Tube Units, Bearing Plates, Wedges, Sheathing and Prestressing Steel (HTS).
  • Calculate the cut length of Strand inclusive of Jack Grip length & profile for each tendon and prepare Strand cutting schedule as per drawing.
  • HT Strand must be cut by using cut-off wheel/abrasion blade.H T Strand shall be uncoiled using coil dispenser and it should not contact soil or dust while cutting.
  • Mark the Tendons as per Tendon layout drawing, after the completion of formwork& reinforcement.Fix  the  fabricated  tendon  support  bars  (bar  chairs)  rigidly  at  every  1m  (max.) interval according to the tendon layout and profile drawings.
  • Lay the sheathing over the tendon support bars as per the tendon layout drawing and jointhe ducts using couplers and seal the joint using self-adhesive tape.
  • Thread therequired number of Strands into Sheathing according to the  Tendon type (strand schedule) and tie thesheathing with Bar Chairs using Binding wires rigidly.
  • Profiling of Tendons, both horizontally and vertically as per drawingsand arresting it rigidly to avoid any disturbances in profile during concreting.
  • Fix the Stressing anchorages and make Dead-end anchorages position as per the layout drawing. Fix the Grout vents at end portions and Air vents at high profile locations and seal it properly.
  • Tie Bursting and Spiral reinforcements at the anchorages zones as per the details given in drawing. Final checking of all Tendons for profiling, supporting and sealing after the completion of reinforcement and formwork works and before concreting.
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2) Stressing:

  • Stressing shall  be  carried  out  only  after  the  concrete  has  attainedthe  required Compressive Strength and age of concrete as specified in the drawing.
  • Check the  Stressing  equipments  and  hoses  before  stressing  to  ensure  proper/smooth functioning.
  • H T Strand/Wire/Bars shall be completely cleaned (oil, dust & etc.) for gripping of Jacks. Stressing can be carried out stage by stage if required, as per drawing.
  • Calculate the Elongation or Modified Elongation for each tendon as per the physical values in TC.
  • Calculate the Jack Pressure for the Design Load including Jack & Anchorage losses.
  • Stressing shall be carried out only as per the specified sequence. Load shall be applied to the tendons by using calibrated hydraulic pumps and jacks and monitored through calibrated Pressure gauges.
  • Fix the Bearing plate to the tube unit and lock it by wedges and mark a  reference  point  on  Strand  to  measure
  • Stressing can  be performed in single / multi strokes depend on Jack piston length.Fix the Hydraulic Jack (Mono/Multi) at stressing end (Single/Double) and  ensure proper seating of jack with bearing plate and also connect the Jack & Pump by hydraulic hosepipes.
  • Apply the  calculated  pressure  (load)  gradually  in  incremental  basis  on  each  strand  or  tendon  by  using  hydraulic  Jack  and
  • Apply jack pressure at both ends simultaneously for double-end stressing.While applying load check for required elongation & jack pressure during  stressing and also note the Jack pressure & Elongation reading at every interval / stage in stressing format.Compute the total Jack Pressure applied and the total elongation arrived  during stressing.
  • If elongation  reached,  but  pressure  not  reached  apply  5%  additional      If  pressure  reached,  but  elongation  not  reached  apply  5% additional pressure. After applying the additional pressure / elongation, if the Stressing result    is within +/-5%, release the jack pressure slowly and then proceed for next stressing. If  the  stressing  result  /  value  is  more  than  +/-5%  inform  the  Design    Engineer  for  further  instruction.  Same  method  shall  be  followed  for  all other Strands / Tendons.12 hours after the completion of stressing of all tendons, cut the strands  leaving 25mm from the face of wedges by using angle grinder.
  • After cutting  fill  the  recess  pockets  (Stressing  pocket)  in  layers  using    approved materials.
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3) Grouting:

  • After the  setting  /  curing  time  of  recess  pockets,  flush  the  tendons    (sheathing) completely by water and then by compressed air. Potable water and OPC Cement should be used for preparation of grout. Suitable  admixture  (Non-Aluminium  /  Non-Chloride  based)  may  be added to the grout as per specification.Fix the valves at all grout and air vents and also fix the pressure gauge.Mix Water followed by Cement of ratio 0.40 in the grouting machine.The grout shall be mixed in mixture for 2-3 minutes and until pumping of
  • Grout should be colloidal mixture and free from any lumps and it should passed through sieve.Pump the grout into the tendon through grout vent pipe (inlet) and release  the air inside the tendon through air vents.Plug all air vent pipes and also the outlet vent pipe after consistent grout    is passed.
  • After plugging  all  the  vents  pipes  tightly,  built  the  pressure  of  5kg/cm2 and maintain for one minute. Then close the inlet valve also. The same method shall be followed for all other tendons.
  • Grouting should be performed continuously to avoid grout setting. Any breakage  occurred  during  grouting,  flush  the  tendon  with  water  and compressed air and do fresh grouting.
  • Clean the vicinity by water after the grouting is completed.
  • Cut all vent pipes projecting outside the surface after the grout is set.