METHOD STATEMENT FOR PRESTRESSING OF I GIRDERS

 

METHOD STATEMENT FOR PRESTRESSING OF
I GIRDERS

1. SCOPE

The purpose of pre-stressing is as per design requirement for Post Tensioning of Concrete I Girders at Major Bridge.

This work shall consist of:

1.    Profiling

2.    Cutting and threading of tendons

3.    Stressing of tendons

4.    Grouting of tendons

5.    End Cap Concreting

2. Reference:

      MoRT&H Specifications for Road and Bridge works  (Fourth              Revision).

          IRC : 18-2000: Design Criteria for Pre-stressed Concrete            Road Bridges,  

           IRC: SP 47-1998: Guidelines on quality systems for road                bridges (Plain, reinforced, pre-stressed and composite                  concrete).

            IS: 2502-1963: Code of practice for bending and fixing of            bars for concrete reinforcement.

          IS: 1786-1985: Specification for high strength deformed                steel bars and wires for concrete reinforcement.

           IS: 8112-1989:43 grade ordinary Portland cement- Specification.

           IS: 12269-1987: Specification for 53 grade ordinary Portland               cement.

           IS: 516-1959: Methods of tests for strength of concrete.

           IS: 383-1970: Specification for coarse and fine aggregate            from natural sources for concrete.

3.  MACHINERY & EQUIPMENT:

B) MACHINERY & EQUIPMENT:

Sl. No	Description	Capacity	Nos. Required	Remarks
1	Multi strand Jack-MG -1800		2
2	Chain pulley block	2 T	2
3	Grout Pump with pressure devices with agitator		1
4	Concrete Batching Plant	90/30m³	1
5	Transit Mixers	6.00m³	2
6	Gantry	40TON	2
7	Tool box		1	As per
8	Diesel Generator	125 KVA	1	requirement As per
requirement

 

4. PROCEDURE:

The following sequence of activities has been described as under.

a)    Profiling

b)    Installation of tendons

c)    Stressing of tendons

d)    Grouting of tendons

e)    End Cap Concrete

a) Profiling:

Profiling (layout of prestressing tendon)shall be carried out according to the ordinates given in the reference drawings.

Cable supports/Chairs shall be welded to the vertical stirrups and sheathing will be tied with the help of binding wire so as to avoid displacement of cable during concreting. However, care should be taken in providing smooth profile and there should be no sudden kinks.

Precautions during profiling:

The following precautions should be taken during profiling.

·  Between the points given the cable must retain a regular curvature without undulations or sharp changes in the direction, which will cause entries in the friction loss at the time of stressing.

·     The cable axis must coincide with anchorage axis for a length.

·      The cable must be placed in the concrete section in such a way so as to allow easy concreting and sufficient cover.

Steps in profiling:

~ Preparing cone boxes and fixing guides to it.

~ Profiling and fixing of sheathing.

b) Installation of tendon

Checking of Identification Tag details.

    The packing of HTS strands shall be removed just prior to use.

   The strand shall be de coiled on an elevated platform along its length and the required length of catch strand shall be marked, cut and recorded in the format correlating the Cable No. to Coil No. The coils may be mounted on a rotating table to facilitate unpacking.

· Cut-strands shall be inspected for defects like opening of wires, if any.

· The cut strands shall be threaded inside the duct for full length.

· Strand shall be cleaned up before the threading operation.

· In case any Delay in stressing operation after inserting the strands, the ends of strands protruding beyond the face of girders should be wrapped with plastic sheets to protect from water/dust.

b) Stressing of Tendons

  Jack Efficiency test shall be carried out at site before commencement of stressing activity.

  Pressure gauges third party calibrated certificate shall be made available at site.

  Pressure gauges shall be calibrated once in 3 months or once for every 200 cables are stressed.

 Pressure gauges shall be calibrated either against a master proving ring or preferably by a dead weight tester at regular intervals.

Calibration certificate required for Master proving ring.

 The stressing operation shall be undertaken by the pre-stressing technicians.

  Before commencing the stressing operation, Tube Unit & strands projecting outside the anchorages shall be cleaned.

  The Bearing plates, Wedges shall be cleaned and visually checked for any damages before using.

  Bearing plates shall be assembled on both sides and the threaded strands shall be secured with compact wedges by tamping with pipe.

  Multi strand jacks will be fixed at the both ends.

  The stressing pumps shall be connected to the jack through the hose pipes.

  The stressing sequence shall be observed as indicated on the post-tensioning drawings.

  When reaching the design stressing load, measure the elongation.

  Stressing shall proceed on the indicated elongation.

              In case elongation reached earlier and corresponding jack pressure is                        not   reached, further jacking 5% extra elongation or desired jack pressure which earlier, shall be done.

 In case jack pressure is reached and elongation is not reached, stressing shall be continued up to a maximum jack pressure of 1.05 times the calculated pressure or desired elongation whichever obtained earlier.

  In case of variation more than the prescribed limits, the matter shall be referred to the design office.

  Initial slackness in pre-stressing cables shall be removed by applying tension of 1/3 the maximum pressure.

  The pressure shall then be increased gradually by intervals of 50kg/Sq.cm and elongation shall be recorded.

  When conformity of the design elongation with the measured one are ascertained, the tendon force shall be anchored.

  On reaching the Full load, the wedges shall be locked of hydraulically at 250 to 280 kg/Sq.cm lock off Pressure.

  Upon successfully completed stressing of the tendon, the jacking unit shall be removed and installed to the next tendon.

~ After removal of jack, a mark shall be done on strands at a distance of 100 mm to observe 24 hrs for any further wedge slip. If any slip is observed, the observation shall be submitted to Designer for their review & approval.

  The strands protruding from the anchor head shall be cut off at a distance of not less than 50 mm from the anchor-head surface by means of an abrasive disc cutter.

  The 1st stage stressing shall be done after 3 days or on achieving 30N/mm2 whichever is later.

~ After checking the slip for 24 hours the Girder shall be shifted from casting bed to stacking bed.

  The 2nd stage Stressing shall be done after 21 days or on achieving 45Mpa whichever is later.

~ After checking the 24 hours slip the strands protruding from the anchor heads shall be cut and grouting shall be carried out.

~ A stressing record shall be prepared for each cable to be stressed on which all steps and occurrences of the procedure are recorded. The stressing record sheet shall be filled up by the pre-stressing technician in charge of the operation.

c) Grouting of tendons

·  The tendon shall now be prepared for grouting. Between stressing and grouting of a tendon with in 10days after tensioning, but not before 24 hours after tensioning.

·  Grout vents shall be provided at anchorages for each cable.

·  Grout caps shall be fixed on both sides of anchor heads and tightly secured.

·   Ducts shall be flushed with water for cleaning as well as wetting the duct walls. The entire water shall be drained through lowest drain pipe or by blowing compressed air through the duct.

·  As soon as flushing is completed the grout operation shall be started.

·  The proportions of grout materials shall be based on field trials conducted before commencement of grouting.

·  Water shall be added in mixer first, followed by cement and admixture.

·  The additive admixture is mixed with cement in proportion as recommended by additive manufacture.

·     The grout mix shall have water cement ratio not exceeding 0.40. Cebex 100 in right proportion shall be added to the grout mix to avoid the shrinkage of the grout mix.

·      The compressive strength of 100mm cubes of the grout shall not be less than 17 Mpa at 7 days.

·      The temperature of the Grout shall not Exceed 25 degrees Celsius.

·      Mixing time of grout shall be 2 to 3 minutes so as to obtain uniform and thoroughly blended grout. The grout shall be continuously agitated with agitator till it is injected. Water shall not be added once the grout is mixed thoroughly. The locking pressure 5 kg/sq.cm is to be maintained for approximate one minute. Consistency of grout shall be ensured.Once the desired pressure of injection reaches, the valve at the entry point shall be closed.

 ·   Records for grouts shall be maintained.

d) End Cap Concrete

    · The rebar shall be tied as per the approved drawings.

  · The side formwork shall be placed and secured properly.

  · Before placing the end shutter, Nitobond EP shall be applied on the

old concrete to achieve proper bonding between old & new concrete.

    After the concreting is done, it shall be cured to achieve the strength.

  ·The formwork shall be removed and now the girder is ready for erection

after being stressed.

6. HEALTH, SAFETY & ENVIRONMENT:

· Safety Officers shall be engaged for monitoring to avoid unsafe work conditions & unsafe workers acts.

· Green helmets shall be provided for safety stewards.

· All the persons involved shall be given Pep talk before start of the Pre-stressing activity.

· All the staff and work men shall be provided with necessary personal protective equipment (Safety helmets, safety hand gloves, safety shoes, safety belts, safety goggles, retro-reflective jackets etc) always at site.

· Drinking water facility shall be provided at site.

· First aid box  shall be available at site location.

·  General Lighting shall be provided in work place.

·  Only certified plant & machineries shall be put in operation.

·  The operators shall have relevant experience and valid license to operate machinerie.

· A warning sign, “Stressing Operations in Progress” shall be put up at locations. 

·   Strand bundle protruding from the bearing plate must be encased in a plastic or sheet metal pipe to prevent injury by the sharp ends of the strands.

· During the process of strand cutting, only abrasive disc cutters with guard shall be used, and goggles shall be worn during the process. 

· Only intact and tight pressure hoses used shall be intact and tight pressure. 

·   No stranger shall be allowed in the immediate vicinity of the stressing equipment during stressing to avoid injury in event the tendon or concrete should fail.

·   The pump and jack shall be securely tied to a solid object when working at an elevated level.

·    No stressing equipment or accessories shall be used that are not complete as supplied from the manufacturer or post-tensioning supplier.

 ·  Stressing shall take place on a proper working platform, as required by the regulation agency. Hands shall be kept out of jack while stressing operation is taking place and while retracting.

  · All persons shall stand clear of equipment, hoses or electric cords while stressing is taking place; always use the remote switch and not the toggle switch located on the pump unit.

· If a tendon shall fail or a blowout occur, or the jack grippers fail; the jack, pump, hoses and electrical cords shall be thrown off the structure. If this occurs anyone standing close the hoses or cords shall be dragged off the structure with the equipment.