UNCOILING OF H.T.STRANDS :
This operation is to be carried out on an elevated platform to prevent soiling of strands. The coil should be bound by pipes & couplers to ensure no sudden release of internal pressure in the coil. The strand shall be uncoiled layer by layer to prevent flying off of strands from the coil. Atmost caution shall be exercised while decoiling operations are in progress and no personnel/Labour shall be allowed to move in the area around the coil.
CUTTING OF STRANDS :
The uncoiled strands shall be laid on the elevated platform and cut to size as per the execution drawings. The cutting of strands shall be done only by abrasive cutter and no gas cutting shall be allowed. The length of the cable as given in the execution drawings shall be verified as to whether it includes the gripping length of 750mm on either side of the girder in case of both end stressing of cables is being done. For one end stressing 750mm extra on the stressing end & 250mm for dead end of the girder should be provided.
MAKING AND MARKING CABLES OUT OF CUT STRANDS :
The strands shall not be moved from the elevated platform till they are attached with tags bearing the specific cable nos. The cables shall be formed by tying the strands with binding wire at a distance centre to centre as necessary for the entire length of the cable.
FIXING OF GUIDE CONES :
The guide cone shall be fixed with the grout hole on the topside of the cone. The cable arrangement as per the drawing shall be maintained while fixing the cones. The sheathing shall be properly fixed into the mouth of the guide cone by properly turning the sheathing into the grooves meant for the purpose.
PROFILING OF SHEATHING :
The sheathing shall be profiled with the help of jigs. The jigs are to be prepared of 16mm dia reinforcement bars and shall be fixed @ 1000 mm c/c by tying firmly with the reinforcement. It is preferable to weld the supporting bars with reinforcement cage of the girder after the profiling of the cables is checked and approved.
THREADING OF STRANDS :
The cables prepared on the elevated platform shall be then brought to site carefully. The cable end tied with binding wire and then wrapped (at the tip) with PVC tapes shall be introduced into the duct from one end of the guide cone. Care shall be taken while threading the cables not to puncture the sheathing at any location.
CONCRETING OF GIRDER :
The girder may be cast with either pre threaded ducts or post threaded ducts.
a) Pre threaded:
The girder shall be concreted after the cables are introduced and strand ends are enclosed in gunny bags as protection. The cables shall be moved back and forth until concreting is completed to ensure free movement of strands. This will prevent any slurry from setting inside the sheathing and blocking the ducts. If there is a doubt of any blockage, the duct shall be flushed with water and subsequently with compressed air. Concrete cubes have to be taken (at least two sets per girder-set comprising of 6 cubes for the same batch) and marked to identify girder no. / Date of casting etc. During pouring of concrete care should be taken not to pour concrete directly on the tube units or the brusting reinforcements and careful observation should be made to ensure no pocket formation anywhere. Shutter vibrator and needle vibrator have to be operated properly.
b) Post threaded:
Provide a H.D.P.E. pipe of suitable diameter through the duct. During concreting this pipe is push back & forth to ensure no blockage in the duct. In this method threading is done just before pre stressing activity is taken up.
Before the stressing operations are undertaken following things have to be checked:
1. Check the correct functioning of jacks, Power pack pumps and leads.
2. Re-establish the modulus of elasticity of H. T. strands from the particular lot to be used from an approved laboratory. Re calculate and modify the elongation of strands, corresponding to the modulers of elasticity and section area for the particular lot of strand to be used for stressing.
3. Move the cables inside the duct to ensure pressure in the duct.
4. Clean the strands thoroughly to ensure no rusting on the strands in the gripping area at the permanent wedge location and master grip area to ensure this a length of strands protruding out from the face of the tube unit and length of 300mm inside the duct should be cleaned with wire brush and petrol to remove all the dirts, grease, oil etc. from the surface of the strands.
5. Wedges and bearing plates are inspected visually and cleaned thoroughly with Nylon wire brushing and petrol to remove all dirts, grease etc. from surface of the female parts of the bearing plates and serretions of wedges.
6. Wire brushing is not recommended for cleaning of wedges. We should use Nylon bristled brush.
THREADING OF BEARING PLATES AND JACKS:
On the attainment of strength of concrete as determined by cube crushing strength as specified by the consultant steps are taken to take up stressing.
Ø A tripod / stand or scaffolding is erected on each end of the girder to hang the prestressing jacks with the help of 3 to 5 Ton. chain pulley block in position. Height of scaffolding / tripod should be such to provide sufficient driving of the jack to thread the strands into the jack difficulties. Specific bearing plates duly cleaned are threaded with the strands.
Ø Cleaned Grips are threaded into the groove of the bearing plates.
Ø It is to be observed that strands in the area of gripping are cleaned.
STRESSING :
The power pack is supplied with 3 phase Power supply and it is connected to the jack. Now at both ends of the girder stressing is done simultaneously. The pressure is increased up to 50 kg/cm2 and halted. At this stage initial slackness is removed. Subsequently elongation at an interval of every 50 kg/cm2 is noted down. Zero correction is done and total elongation is calculated. Slip after 24 hours is also recorded by making a mark on the cable at 100 mm from the face of girder. The cables are cut from a distance of 50 mm from the wedges and grout cap is bolted.
SPECIAL NOTE DURING TRANSFER OF PRESTRESSING FORCE:
The transfer of the prestressing force shall be carried out gradually. Since sudden application of stress may result in severe eccentricities of Prestressing force in the concrete. This will also ensure transfer of prestress to the concrete is uniform along the length of the tension line.
CABLE AND STRESSING PREPARATION SHEET :
Before a cable is stressed, Cable and Stressing preparation sheet is updated. This sheet involves girder no., cable no., length of cable etc. Theoretical "E" value (assumed Young's modulus during design) and theoretical "A" value (assumed area of the cable during design) are entered into the sheet. Actual "E" and actual "A" value can be obtained from the strand manufacture's test certificate. Theoretical elongation & theoretical jacking force (obtained from the drawings) are updated. Jack efficiency and Jack ram area can be obtained from the Jack test certificates. Modified elongation and modified jacking force are computed as per the formulae given in the sheet. Finally jacking pressure is computed as per the modified elongation.
STRESSING READINGS SHEET:
While a cable is stressed the stress readings are entered into this sheet. From this total elongation is computed. The clients should authorize this sheet.The total tension imparted to each tendon shall conform to the design requirements and modified values based on the calculation. Initial slack in Prestressing tendons may be removed by applying small initial tension.
If the calculated elongation is reached before the specified gauge pressure, continue tensioning till attaining the specified gauge pressure provided the elongation does not exceed 1.05 times the calculated elongation. If 1.05 times the calculated elongation is reached before the specified gauge pressure is attained. Stop stressing and note the readings as per the charts.
If the calculated elongation has not reached at the specified gauge pressure, continue tensioning by intervals of 5kg/sqcm of gauge pressure until the calculated elongation is reached restricted to the upper limit of gauge pressure 1.05 times the specified gauge pressure.
LOCK OFF PRESSURE :
Lock of pressure should be in the order of 2/3rd of the stressing pressure. In case wedge set is found to be more than 6mm the lock off pressure may be increased by another maximum 10kg/cm2. In no case lock off pressure is more than 280kg/cm2. in case by increase of lock off pressure wedge sets did not improve. One has to take more care regarding cleaning of strands, wedges etc. before taking up the pre-stressing work.
TESTING OF JACKS AND PUMPS:
1. Interconnect the corresponding lines of both EOHP power pack and start one power pack to read corresponding gauge pressure on the others. If the gauge pressure are found to be same then go for the Jack test.
2. Put the jack face to face and fit the jack in line. Thread the strands of suitable length to grip on to the strands at the end of the jack. Give the initial movement to jack ram upto 50mm for both the jack and fix master grips on both jacks. The system is ready for efficiency test.
One Jack is first made active and increase the jack pressure @ intervals of 50Kg/sqcm and note the readings on the other jack pressure provide upto 85%of the U.T.S. of the strands work out the efficiency. Now the other jack is made active and former one is made passive and repeat the test and work out the efficiency. Average of these readings will give the efficiency of the jacks.
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