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.
