05-PRESTRESSING



 PRESTRESSING

1.INTRODUCTION

This part includes the quality control requirements of the materials and workmanship for prestressing in bridge construction. The materials required for prestressing and the prestressing operations shall conform to IRC: 112 and MoRTH Specifications.

  1. Prestressing operation shall be carried out only under the direction of an experienced and competent supervisor and all personnel operating the stressing equipment shall have been properly trained in its use.
  2. In addition to the normal precautions against accident, special care shall be taken when working with or near tendons which have been tensioned or are in the process of being tensioned.
  3. The system of prestressing used shall be a system indicated in Drawing or approved by the Engineer. Such system shall be used strictly in accordance with the recommendations of the system manufacturer.
  4. Under no circumstances shall the equipment or fittings designed for use with one system of prestressing be used in conjunction with equipment and fittings designed for use with another system.
  5. Prestressing components shall be stored in clean dry conditions. They shall be clean and free form rust and loose mill scale at the time of fixing in position and subsequent concreting. Slight rusting of the steel, which can be removed by moderate rubbing, is acceptable, but the surface shall not show signs of pitting.
  6. Prestressing accessories like jacks, anchorages, wedges, block plates, etc. shall be procured from authorised manufacturers only.

2. MATERIALS

 2.1. Wires and strands

  1. Prestressing steel shall be in the form of plain or indented wires, stress-relieved multi-ply strands or high tensile steel bars and conform to the following BIS subject to the stipulations in IRC: 112 (Refer figure 3400-1):

Plain cold drawn stress-relieved wire                                      -           IS: 1785 (Part-I)

Indented hard-drawn stress relieved wires                           -           IS: 6003

Stress relieved multi-ply strands of normal relaxation       -           IS: 6006

Stress relieved multi-ply strands of low relaxation              -           IS: 14268

High tensile steel bars                                                                   -         IS: 2090

  1. The requirements for prestressing wires are given in table 3400-1 (Ref: Table 18.3 of IRC: 112).
  2. The requirements for prestressing strands are given in table 3400-2 (Ref: Table 18.4 of IRC: 112).
  3. All wires or strands to be stressed at the same time shall be taken from the same parcel. The coil numbers of the steel used for each tendon shall be recorded.
  4. Welding of tendons shall not be permitted.

 Table: Requirements of hard drawn stress relieved wires

Type

Diameter

(mm)

Minimum 

tensile

strength (MPa)

      Minimum

elongation at

fracture (%)

Plain wires

4

1715

3

5

1570

4

7

1470

4

8

1375

4

Indented wires

4

 1715

3

5

1570

4

 

Table: Requirements of hard drawn stress relieved strands

Class

Designation

Nominal

area

(mm2 )

Normal relaxation

Low relaxation

Breaking

load (kN)

 0.2%  proof

load (kN)

Breaking

load (kN)

0.2% proof

load (kN)

 

I

11.1 mm

7ply

70.0

124.54

105.86

120.1

108.0

12.7 mm

7ply

92.0

166.18

139.9

160.1

144.1

15.2 mm

7ply

139.0

226.86

192.83

240.2

216.2

 

II

11.1 mm

7ply

74.2

137.89

117.21

137.9

124.1

12.7 mm

7ply

98.8

183.71

156.11

183.7

165.3

15.2 mm

7ply

140.0

261.44

222.23

260.7

234.6

 

                     

                                                             

 

 

 

 

                                   

Fig : Prestressing wires and strands.

  1. Cutting and trimming of wires and strands shall be carried out using a high-speed abrasive cutting wheel or friction saw at not less than one diameter from the anchorage or flame cutters at not less than 75 mm from anchorage or jacks. Care shall be taken to ensure that the flame does not come in contact with other stressed steel when a flame cutter is used.
  2. Strands shall be sufficiently projected from the anchorage to allow jacking to take place at either end of the tendon.
  3. The cable or individual strands comprising the cable shall not be kinked or bent. No strand that has become unravelled shall be used in the works.

2.2. Sheaths and cores

  1. The sheathing shall conform to the requirements specified in IRC: 112 as given in table (Refer figure 3400-2).

Table: Requirements of sheathing 

Strands

Nos./ Diameter

(mm)

Internal dia. of  duct

(mm)

Thickness of  sheathing

(mm)

Metallic

HDPE

Metallic

HDPE

7/13

50

50

0.3

2.0

12/13

75

75

0.4

2.5

19/13

85

85

0.4

2.5

27/13

100

100

0.5

3.0

12/15

85

85

0.4

2.5

19/15

100

100

0.5

3.0

27/15

125

130

0.5

4.0

 Fig.: Metallic and HDPE sheaths for prestressing

               

                                            

 
  1. Sheaths shall be accurately located both vertically and horizontally as described in the Drawing.
  2. All sheaths shall be maintained in their correct position during the placing of the concrete. Unless otherwise specified in the Drawing, sheaths shall be placed in correct position and profile by providing suitable ladders and spacers at intervals of approximately 1.0 m. The method of support shall be approved by the Engineer.
  3. Where sheaths are used, the number of joints shall be kept to a minimum and sleeve connectors shall be used for jointing. Each joint shall be adequately water tight and shall withstand a pressure of 1.1 times the grouting pressure and maximum grouting head.
  4. Joints in sheathing shall be neatly fitted without internal projection or reduction of diameter. Joints in adjacent sheaths shall be staggered by at least 300 mm.
  5. Within 24 hours of the concrete being placed, the Contractor shall satisfy the Engineer that the tendons are free to move, if they are in ducts or that the ducts are free from obstruction.
  6. The number and position of grout vents for entry and exit points shall be agreed with the Engineer before the ducts are formed so that the grouting for the entire length can be checked.
  7. Vents shall be provided at low points in the tendon profile to allow the disposal of water that may have collected as a result of rain or curing. The vents shall be sealed before grouting operations begin.

2.3 Anchorages

  1. Anchorages, end blocks and plates shall be positioned and maintained in position during concreting so that the centre line of the duct passes axially through the anchorage assembly (Refer figure 3400-3).
  2. All bearing surfaces shall be clean before concreting and tensioning.
  3. No damaged anchorages shall be used. Steel parts shall be protected from corrosion at all times. Threaded parts shall be protected by greased wrappings and tapped holes shall be protected by suitable plugs till used. Anchorage components shall be kept free from mortar and loose rust and any other deleterious coating.
  4. Where swaging/button-heading is envisaged, the Contractor shall furnish details of his methodology and obtain approval of the Engineer, prior to taking up the work.

                              

                                                            

Fig.: Components of a typical prestressing anchorage.

2.4. Mechanical couplers

Couplers or other similar fixtures in conjunction with the prestressing strands or bars, shall have an ultimate tensile strength of not less than the strengths of the individual strands or bars being jointed and shall meet the requirements of individual anchorages as specified in IRC: 112.

3. WORKMANSHIP

3.1. Jacking equipment

  1. All jacking equipment used for stressing operations shall be of the type applicable to the system adopted.
  2. Jack and pumps shall be calibrated at an independent facility, in the presence of the Engineer, before beginning stressing operations. Calibration of the equipment shall take place at six-month intervals for equipment permanently present at site.
  3. Whenever new equipment is brought to the site, or equipment is removed and returned, or serviced, recalibration of the equipment as described will be required.
  4. All gauges, load cells, dynamometers and other devices used for measurement shall have a reading accuracy of within ±2%.

3.2. Tensioning

  1. The Contractor shall submit details of the proposed stressing loads and stressing sequence to the Engineer for approval.
  2. Tensioning shall be carried out only in the presence of the Engineer or his representative unless permission has been granted otherwise.
  3. The Contractor shall ensure that personnel carrying out the stressing are provided with particulars of the required tendon loads, order of stressing and extensions.
  4. Immediately before tensioning, the Contractor shall prove that all tendons are free to move between jacking points.
  5. Unless otherwise permitted in the Drawing, concrete shall not be stressed until two test cubes taken from it has attained the specified transfer strength.
  6. The friction factors assumed for the calculation of tendon extension shall be verified by onsite measurement of the force-extension relationship of a typical sample of installed tendons.
  7. When the required force, including overloads of short duration, has been applied to the satisfaction of the Engineer, the tendons shall be anchored. The jack pressure shall then be relieved in such a way as to avoid jerks to the anchorage or tendons.
  8. If the pull-in of tendons at the completion of anchoring is greater than that acceptable to the Engineer, the tendons shall be de-tensioned and re-tensioned again.
  9. If it is necessary to crop the tendons to enable the ducts to be grouted, this shall be delayed as long as practicable up to the time of grouting. In all other cases, unless otherwise agreed with the Engineer, the tendons shall not be cropped less than three days after grouting.
  10. The Contractor shall keep full records of all tensioning operations, including the measured extensions, pressure gauge or load cell readings and the amount of pull-in at each anchorage. Copies of these records, on suitable formats, shall be supplied to the Engineer within 24 hours of each tensioning operation.

3.3. Grouting

                                      

                                     

 

 

 

 

 

a) Ideal location of vents in cable                    b) Cross section of well grouted cab

  1. Grouting shall be done only with the written approval of the Engineer.
  2. All ducts shall be thoroughly cleaned by means of compressed air and all anchorages shall be sealed before grouting.
  3. Ducts shall be grouted at the earliest after the tendons in them have been stressed and the Engineer’s written permission to commence has been obtained. Grout shall be injected in one continuous operation and allowed to flow from the vents until the consistency is equivalent to that being injected. The maximum time between mixing and injection shall not exceed 30 min.
  4. The ducts shall be completely filled with grout (Refer figure 3400-4).
  5. Vents shall be sealed consecutively in the direction of flow and the injection tube sealed under pressure until the grout has set. The filled ducts shall be protected to the satisfaction of the Engineer to ensure that they are not subject to shock or vibration for one day.
  6. Two days after grouting, the level of grout in the injection and vent tubes shall be inspected and made good if necessary.
  7. The Contractor shall keep full records of grouting including the date each duct was grouted, the proportions of the grout and admixtures used, the pressure, details of interruptions and topping up required. Copies of these records shall be supplied to the Engineer within three days of grouting.

4. Grout mixer

  1. It is essential that the grout is maintained in a homogenous state and of uniform consistency so that there is no separation of cement during the entire grouting process. It is necessary that the grout be continuously mixed in a colloidal mixer with a minimum speed of 1000 rpm and travel of discharge not exceeding 15 m/sec.
  2. The equipment shall be capable of maintaining pressure on completely grouted ducts and shall be fitted with a nozzle which can be locked off without loss of pressure in the duct.
  3. The pressure gauges shall be calibrated before they are first used in the works and thereafter as required by the Engineer. All equipment shall be thoroughly washed with clean water at least once every three hours during grouting operations and at the end of use for each day.
  4. The Contractor shall ensure that standby grouting equipment is available in the event of a breakdown.

5. Properties of grout

  1. Only clean potable water shall be used.
  2. The same type of cement as used in construction of pre-stressed elements shall be used for making the grout.
  3. It is not recommended to use sand for grouting of pre-stressing tendons.
  4. Approved admixtures conforming to IS: 9103 may be used if tests have shown that their use increase fluidity, reduce bleeding, entrains air or expand the grout. Admixtures containing chloride, nitrates, sulphates or any other products which are likely to damage the steel shall not be used.
  5. When an expanding agent is used, the total unrestrained expansion shall not exceed 10%. Aluminium powder as an expanding agent is not recommended.
  6. Admixtures shall be used only with the written permission of the Engineer and shall be used strictly in accordance with the manufacturer’s instructions.
  7. Water-cement ratio shall be as low as possible consistent with the required workability. Under no circumstances shall the water to cement ratio exceed 0.45.
  8. The temperature of the grout after accounting for the ambient temperature of the structure shall not exceed 25°C.
  9. The compressive strength of 100 mm cube of the grout shall not be less than 17 MPa at 7 days.
  10. Initial setting time of grout shall be more than 3 hours and less than 12 hours. The final setting time shall not be less than 24 hours.
  11. The bleeding shall not exceed 0.3% of volume of the initial volume of grout after three hours kept at rest.
  12. The volume change of grout kept at rest for 24 hours and tested as per ASTM C1090 shall be within the range of -0.5% and +0.5% of the original volume.
  13. Fluidity is tested as per ASTM C939 standard using standard flow cone.

6. Post tensioning                            

  1. Prestressing tendons shall be accurately located and maintained in position, both vertically and horizontally as per drawings.
  2. The method of supporting and fixing shall be such that profile of cables is not disturbed during vibrations, by pressure of wet concrete, by workmen or by construction traffic.
  3. Sheathing in which the permanent tendon will not be in place during concreting shall have a temporary tendon inserted or shall be stiffened by some other method to the approval of the Engineer. The temporary tendon shall be pulled out by a special threading machine or other contrivance, before threading the permanent tendon.
  4. Where possible, tendons shall be placed prior to concreting. Tendons shall be handled with care to avoid damage or contamination, to either the tendon or the sheathing. Any tendons which are damaged or contaminated shall be cleaned or replaced.
  5. Tensioning force shall be applied in gradual and steady steps, in such a manner that the applied tensions and elongations can be measured at all times. The sequence of stressing, applied tensions and elongations shall be in accordance with the approved Drawing or as directed by the Engineer.
  6. It shall be ensured that in no case, the load is applied to the concrete before it attains the strength specified in the Drawing or as stipulated by the pre-stressing system supplier, whichever is more.
  7. After prestressing steel has been anchored, the force exerted by the tensioning equipment shall be decreased gradually and steadily so as to avoid shock to the pre-stressing steel or anchorage.
  8. The tensioning force applied to any tendon shall be determined by direct reading of the pressure gauges or dynamometers and by comparison of the measured elongation with the calculated elongation.
  9. A complete record of prestressing operations along with elongation and jack pressure data shall be maintained in the required format.

4 TOLERANCES

   4.1 Permissible tolerances for structural unit

The dimensional tolerances for precast girders shall be as under:

Length                                                           : ±10 mm.

Flange width and thickness                    : ±5 mm.

Depth                                                             : ±5 mm.

Web thickness                                            : ±5 mm.

Minimum surface unevenness               : 1.5 mm on a 3.0 m template.

    4.2 Tolerances for prestressing strands

Permissible tolerances for positional deviation of prestressing strands shall be as under:

Variation from the specified horizontal profile                 : 5 mm.

Variation from the specified vertical profile                      : 5 mm.

Variation from the specified position in member             : 3 mm.

5. SAMPLING AND TESTING

  1. The materials shall be tested in accordance with the required specifications and shall meet the prescribed criteria and requirements.
  2. Prestressing steel, sheaths and accessories like jacks, anchorages, wedges, block plates, etc. shall be procured from authorised manufacturers only.
  3. Anchorages shall conform to “Recommendations for acceptance and application of pre-stressing systems” published by FIB.
  4. The prestressing accessories shall be subjected to an acceptance test prior to their actual use on the work. Test certificates from a laboratory fully equipped to carry out the tests shall be furnished to the Engineer. Such test certificate shall not be more than 12 months old at the time of making the proposal for adoption of a particular system for the project.

RELATED CODES:

1.IRC: 112-Code of practice for concrete road bridges.

2.IS: 1785-Specification for plain hard-drawn steel wire for prestressed concrete.

   (Part -1)-Cold drawn stress-relieved wire.

   (Part -2)-As drawn wire.

3.IS: 2090-Specification for high tensile steel bars used in prestressed concrete.

4.IS: 6003-Indented wire for prestressed concrete - specification.

5.IS: 6006-Specification for uncoated stress relieved strand for prestressed concrete.

6.IS: 9103-Concrete admixtures - specification.

7.IS: 14268-Uncoated stress relieved low relaxation seven ply strands for prestressed concrete - specification.

8.ASTM C1090-Standard test method for measuring changes in height of cylindrical specimens from hydraulic-cement grout.

9.ASTM C939-Standard test method for flow of grout for preplaced aggregate concrete (flow cone method).

10.Technical report bulletin 7:- "Corrugated plastic ducts for internal bonded post tensioning", published by FIB.

11.Technical report:-"Recommendations for the acceptance of post-tensioning systems", published by FIB.

12.Technical report:-"Quality assurance and quality control for post-tensioned concrete structures", published by FIB.

13.MoRTH-Specification for Road and Bridge works (5th revision).

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