Methodology of Post tensioning or Prestressing in details

Prestressing process / Post tensioning method is primarily divided  into the following parts

  • Cable  layout and profiling of the sheathing pipe
  • Fixing of Trumpet or Guide unit  
  • Insertion & cutting of  cable 
  • Concreting of the element
  • Assembling of various parts of prestressing jack 
  • Calculation for Stressing 
  • Process of Prestressing
  • End sealing

Prestressing Jack

Cable  layout and profiling of the sheathing pipe

  • Cables should be placed in right position as per given drawing profile. 
  • Ordinates shall be plotted with help of total station and sheathing duct should be aligned and hold in in that position with help of tendon support or reinforcement bars and binding wire as necessary to fix its firmly so that not capable of dislocation during any construction period. 
  • Sheathing ducts are generally of two types namely Metallic sheathing duct which is made by rolling CRCA Steel Strips with a spiral corrugation and HDPE sheahing duct made of High  Density Poly Propylene (HDPE) or Poly-Propylene.
  • These corrugations take better bonding with the Concrete of outside and the Cement Grout of inside.
  • As corrugated HDPE are more corrosion resistive than the previous one so that advantageous to use in modern days construction.  
Sheathing Duct Profiling
  • Cables are to be profiled with tolerance limit of  ± 3 mm
  • Couplers proved at each end are usually used for connection of sheathing ducts and check the duct for ant type of opening and damages to seal it properly using the adhesive tape or any suitable sealant.

Fixing of Trumpet or Guide unit  

Trumpet or Guide unit is a tube alike unit with square base at front position made of graded cast iron. It is generally provided at the end of sheathing duct profile to connect them and also to provide a flat surface to bearing plate . it is generally embedded in concrete which ensures gradual distribution of prestressing forces to the concrete to avoid the damage of concrete due to sudden impact of loading. It  comes with a injection port to facilitate the grouting of duct.

 

Insertion & cutting of  cable 

The H.T. Strands are only capable for inducing the stressing forces in the elements. Placement or insertion of strands can be happen  before or after concreting of the element but it is preferable to insert it before concreting because it becomes more problematic to place it as use of the De-Coiler and the Strand Pushing Machine is made compulsory.

Strands are to be cut equally by an abrasive cutter in chosen length. Cutting length of cables are generally include 750 mm free length beyond the anchorage system to facilitate the prestressing process. Precaution should be made during pulling of strands in to the sheathing ducts ( strands end shall be smooth and ground well)

 

Concreting of the element

Grade of concrete is varying project to project and determined by design of PC members.

Precast end block of concrete should be atleast 28 days old and shall have attained a minimum cube strength of 350 Kg/Sqcm before very first cable is stressed.

Precaution should be taken that duct not to get damages due to use of vibrators during casting as cement slurry may clog the duct after admission in duct through opening or damages. Clogged strands are making unequal elongations and meanwhile blocking grout to ingress in to duct smoothly.  

 

Assembling of various parts of prestressing jack 

Freyssinet prestressed concrete  anchorage system is of various types namely 12T13, 6T13, 7T13  etc.  In our case we are going to discuss about 12T13 strands system.(Assumption is made for calculation purpose and preference over the system adoption depends on the design of PC system). 12T13 strands meaning 12 nos of strands are there and nominal diameter of each strands is 13 mm.

Bearing plate is placed over square base of guide with inserting strands in to the conical holes.  Live wedges are to be placed on the holes through the strands and push wedges in to the bearing plate holes to its maximum extent with help of hollow pipe. 

Bearing Plate with strands

Wedges/Grips– This is a conical shaped components made of of Alloy Carbon Steel. These grips are slatted in three parts which are bounded together with help of a special wire clip.

Live and Master wedges– Wedges used for bearing plate holes are called live wedges and used for pulling plate inside the jack is known master wedges. )

Bearing collar is placed over bearing plate and fix it with help of screws and insert lock off plate over collar inserting strands in to holes of plate. It has to be ensured that there is no point of detachment between the lock off plate and the bearing plate. 

 

Bearing collar

Presstressing jack is need to be hanged on a beam structure made of any steel structural element ( ISMB, Channel section , Tube section depending upon the jacks) with help of chain pulley system for mounting of jack along the tendons.

Jack is to be mounted towards lock off plate by hammering chain pulley block until the proper impregnation of lock off plate into the jack back. After proper settlement put pulling plate in front of jack and push master wedges in to holes of pulling plate over the strand with help of pipes. Waxing over the grips is to be done before pushing into the holes.

Prestressing Jack hanging in Chain Pulley
Pulling plate inside

Prestressing Jacks are being operated using ELECTRICALLY OPERATED HIGH PRESSURE PUMP (E.O.H.P). Prestressing jack is needed to be connected in correct manner with pressure pump and ensuring all the connections are done,  jack is then needed to hold tightly with help of supporting chain before stressing process gets started.

 

Calculation for stressing 

A real work example of cable presstressing a bridge girder cable presstressing is considered to explain each every steps with proper imagination.

Before starting stressing process some assumptions made for calculation purpose are following

Tendon to be stressed is denoted – T1

Each prestressing cable consist of 12nos of high tensile steel stands of 12.7 mm Dia.

HDPE sheathing pipe of 75 mm internal dia is used for encasement of cable.

Anchorage prestressing system- 12T13

Jack is having piston area of 479.00 Sqcm

T1 tendon having each strands properties as below

Theoretical elongation- 117 mm

Theoretical Elastic Modulus- 190.56 N/mm2

Theoretical area – 96.2 sqmm

Actual area – 98.33 sqmm

Actual Elastic Modulus-  193.87 N/mm2

Calculation of Elongation (Modified)

Modified elongation = MOE(Theoretical)/ MOE(Actual) x  Area (Theoretical)/Area(Actual)  x Elongation (Theoretical)

                                   = 190.56/193.87 x 96.2/98.33 x117 mm      

                                   =  112.50 mm

Tolerance limit for modified elongation is ±5 % i.e 106.88 mm to 118.13 mm (It may be varied depending upon design)

Design theoretical Jack gauge pressure  = 350.9 Kg/Cm2

Efficiency of Jack as per manufacturer = 99.5%

Modified gauge pressure= 350.90/0.995 = 352.663 Kg/Cm2

Tolerance limit for modified gauge pressure is ±5 % i.e 335.03 Kg/Cm2 to 370.30 Kg/Cm2 (It may be varied depending upon design) 

 

Process of Prestressing

  •  Stressing should be done at the both ends of cable simultaneously. Stressing should be started applying to the cables by E.O.H.P only after ensuring all the connections are done properly. 
Electrically Operated High Pressure Pump (E.O.H.P)
  • Stressing will be done as per sequence of cables given in drawing. Pressure should be applied in range of 50 Kg/Sqcm each time and pressure gauge is used to measure that which needs to calibrated before in use. No one should stand infront or along the stressing line during entire stressing process. 
  • Elongation in terms of ingress of front ram with respect to prestressing jack edge is needed to measure for each and every 50 Kg/Sqcm pressure applied and take note it down in table prepared earlier for both ends. 
 
 
Measurement taken
  •  For very first 50 Kg/Sqcm pressure is generally acted as seating pressure so that elongation for that initial pressure is calculated from next three consecutive readings (Average of three readings).
  •  Now application of stress is continued in interval of 50 Kg/Sqcm until the cumulative elongation is within the allowable limit (106.88 mm to 118.13 mm ) and final gauge pressure should also be within tolerance limit (335.03 Kg/Cm2 to 370.30 Kg/Cm2).
  • Note the final pressure and elongation. Locking pressure should be applied to lock plate which embed the live wedges to the bearing plate for anchorage of strands to hold it in position. Generally 80% of final stress is taken as locking pressure which need to be applied simultaneously to lock plate in pre specified numbers. 
  • After locking pressure applied to lock plate elongation in terms of ram protrude is measured again to find out slip of strands while anchoring which always need to be within design specified allowable limit.
  • After all process comes to end,  it needs to remove prestressing jack safely from its current position and taken out others accessories carefully. 
  • Anchored cables are need to observed after 24 hrs of presstressing process to ensure there is no further slip. 

End sealing

After stressing and locking process of cable is happened satisfactory, strands should be cut at a length of approximately 40 mm from bearing plate front face and end sealing should be done in a proper way with help of grouting cap or cement mortar as per requirement. After end sealing grouting of cable may be done as per codal specification. (Which may be covered in another blog)

Conclusion 

This process may vary project to project or case to case but this overall process is nevertheless similar to the each other. Special attention shall be paid to safety during entire process to eliminate unwanted accidents.

Pressure should be applied in correct manner not exceeding the limit and should always be carried out in intense observation of experienced person. During stressing no one should allowed to stand infront or behind the jack.

If any unwanted sound is there during the stressing process, stop the application of  pressure immediately and check chain block for any looseness and continue the process only after ensuring all is in correct way.

FAQS

 
1. What are the steps involved in prestressing?
Ans-
  • Cable  layout and profiling of the sheathing pipe
  • Fixing of Trumpet or Guide unit  
  • Insertion & cutting of  cable 
  • Concreting of the element
  • Assembling of various parts of prestressing jack 
  • Calculation for Stressing 
  • Process of Prestressing
  • End sealing
 2. What are the Sheathing ducts in post tensioning?
 
Ans- Sheathing ducts are generally of two types namely Metallic sheathing duct which is made by rolling CRCA Steel Strips with a spiral corrugation and HDPE sheahing duct made of High  Density Poly Propylene (HDPE) or Poly-Propylene.
 
3.What is Trumpet or Guide unit?

Ans- Trumpet or Guide unit is a tube alike unit with square base at front position made of graded cast iron. 
 
4. What is meaning of 12T13 Freyssinet system in prestressing?
 
Ans-12T13 strands meaning 12 nos of strands are there and nominal diameter of each strands is 13 mm.
 
5. What is Bearing Plate?
 
Ans-Bearing plate is placed over square base of guide with inserting strands in to the conical holes.  
 
6. What is wedges? What are the types of Wedges used in prestressing? 
 
Ans- Wedges/Grips- This is a conical shaped components made of of Alloy Carbon Steel.
Two types of wedges are generally used for prestressing process, these are Live and Master wedges
 
7. What is Bearing Collar in post tensioning?
 
Ans-Bearing collar is placed over bearing plate and fix it with help of screws and insert lock off plate over collar inserting strands in to holes of plate. It has to be ensured that there is no point of detachment between the lock off plate and the bearing plate. 

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