PSC WORKS DEATILS PPT AT A GLANCE PRESENTATION

PLS CHECK

1. GUIDES TO GOOD PRACTICE
IN
PRESTRESSED CONCRETE
WORKS

2. 1.Making of prestressing cables
Check whether the strand is of approved quality
and obtained from approved manufacturer.
Ensure availability of Manufacturer’s Test
Certificate along with important test results.
Ensure proper storage of strands as per
recommendations of the manufacturer.
Check whether the strands are free of rust, pits
etc.
The prestressing strands should be cut by
suitable cutting machine for required length of
the particular cable as per drawing.

3. Extra length required for each strand from the face
of guide:
 At stressing end for fixing jack 750 mm
 At dead end 200 mm
Each strand should be identified with suitable
number/colour code with paint at both the ends of
the strand.
The relative position of wires in a cable should be
accurately maintained by suitable means.
For this purpose, it is preferable to use specially
designed spacers available in the market at
adequate spacing.

4. •After making of cables it should be placed in
position as early as possible.
Don’ts
•Never allow use of gas flames for cutting
prestressing strands.
•Do not allow cables to be kept in exposed
condition for a longer period.
•The strands as supplied are self-straightening
when uncoiled. Never allow application of heat to
facilitate straightening or bending of prestressing
steel.

5. 2.Positioning of cable and its profiling
Prestressing cables should be located carefully and
accurately in the exact positions shown in the
drawing. The permissible tolerance in locating
prestressing cables is 5 mm.
The sheathings (duct formers) should be supported
at adequate intervals and fixed in position in such a
manner that they do not get disturbed or displaced
by heavy prolonged vibration or by pressure or
upward thrust of wet concrete.
During cable profiling, reinforcement works and
concreting care should be taken to ensure that the
sheathing is not damaged. Any welding work and
use of needle vibrators shall be done with extreme
care by well-experienced staff only.

6. •All unintended holes or openings in the sheathings
must be repaired prior to placement of concrete. The
joints of couplers and sheathings should be made
water proof by using suitable adhesive tapes.
•Where cables are threaded after concreting, it is
preferable to protect the duct during concreting by
inserting temporary PVC pipes inside the sheathing.
•The guide should be firmly secured in true position and
alignment before concreting. The sheathing should be
inserted into the guide mouth and the joint should be
made water proof.
•The anchorage zone reinforcement should be carefully
placed as per drawing and also as recommended by the
manufacturer for the prestressing system. The spiral or
mesh reinforcement should be tack welded with the
guide cone.

7. 3.Precautions during concreting
•During concreting and at least up to 4 hrs. after
concreting ensure that all the cables are moved in
both the directions at an interval of not more than
30 minutes by light hammering or pulling so that
any ingress of mortar/concrete inside the duct
looses bond with the cables and thereby prevents
jamming of cables. If required, compressed air
may be pumped to clear leaked mortar plug.
•The anchorage zone is normally very congested
with reinforcement. Concreting in this area should
be done very carefully. It is good practice to
provide inspection windows in the formwork to
ensure dense concreting.

8. 4.Checks before starting of prestressing
operation
•From test cube results ensure that the concrete has
attained specified strength for transfer of prestress.
•Check that all components of anchorage system viz.
bearing plates, grips, circlips etc. are of approved
quality.

•Check that the jack is equipped with the proper set of
hoses, pump and bearing ring for the type of
anchorage to be used.
•Check that all spare parts for prestressing jack and
pump is readily available.
•Check that the pressure gauge of prestressing pump
is calibrated.

9. •The duct should be thoroughly cleaned by air and
water jets.
•Ensure free movement of cable inside duct by
hammering or pulling by jack, if required.
•The length of prestressing strand from initial
griping to final griping points should be
thoroughly cleaned with petrol.
•The holes of bearing plates and outer surface of
grips should also be thoroughly cleaned with
petrol.
•A thin coat of Graphite Grease should be applied
on the outer surface of grips to reduce slip.
•Check that the bearing plate of the anchorage is
properly located on the guide and is butting
uniformly.

10. •After fixing of jack check that the alignment of
cable and the centre are collinear.
•Ensure that the strands of the cable having same
identification mark are positioned at same location
of the bearing plates on both ends of the cable.
•Check that the elongation of cables mentioned in
the drawing has been modified based on actual
values of ‘A’ and ‘E’ specified in the manufacturer’s
test certificate.
•Ensure proper communication system between two
ends of the girder.
•The sequence and stage of prestressing of cables
should be clearly understood and must be executed
as mentioned in the drawing.

11. 5.Good practices during prestressing
operation
•Simultaneous stressing from both ends of the strand
should be done.
•It is preferable that all the strands of a cable are
stressed simultaneously using multi-strand (multi-
pull) jacks.
•A nominal initial pressure of 50kg/cm
2
shall be
applied on both ends to wipe off the initial slackness
in the strand as well as to make the jack self
supporting. With the help of a standard steel scale
accurately measure a suitable fixed distance (say
300 mm) from a point on the face of the concrete
plate to a point on the strand and mark with paint. It
is desirable that an additional mark be made on each
strand just behind the barrel grip assembly to
ensure that there is no slippage of the strand from
the grips.

12. •Gradually increase the jacking pressure on both
ends simultaneously and as the pump dial gauge
shows pressure of 100kg/cm
2
stop pumping and
measure the distance between the above two
points. The difference in measurement is the
extension in the strand. The extension and
corresponding gauge pressure shall be recorded.
•It may so happen that the gauge pressure on one
end has reached 100/cm
2
but the other end has
not. Under such circumstances continue pumping
on the end where the pressure lagging till it
reaches 100kg/cm
2
.
•The amount of extension in the strand at both ends
at each stage should be more or less the same. If
not, pumping on the end having less extension
should be started first for the next stage and
pumping on the other end should be held up till the
extension on both the ends are equal.

13. •In a similar manner as described in the above
paragraphs, the jacking pressure shall be increased
at an increment of 50kg/cm
2
i.e. to 150kg/cm
2
,
200kg/cm
2
, 250kg/cm
2
and so on till the measured
extension plus the extension at initial pressure of
50kg/cm
2
(to be determined as given below)
equals the desired extension for the cable as
mentioned in the drawing duly modified as per
actual A and E values mentioned in manufacturer’s
test certificate.
•However, it must be remembered that at no point
of time the gauge pressure shall increase beyond
1.05 times the required gauge pressure for
achieving the desired extension given in the
drawing.
•For determining the amount of extension at initial
pressure of 50kg/cm
2
, graphical method as
described in IRS CBC (shown below) shall be
followed.

14. Gauge Reading
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x
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T
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A
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E
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A
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A
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D

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Initial Effective Elongation

15. •After achieving the desired extension (or after
applying maximum permissible pressure in case of
not getting desired extension) at both ends,
blocking of grips shall be done simultaneously at
both ends. During this period of blocking, maintain
the tensioning pressure by closing the valve on the
tensioning circuit. The blocking pressure shall be
limited to the maximum level as per
recommendations of the manufacturer of
prestressing system.
•Thereafter, the jacking pressure shall be released as
slowly as possible to minimize slip.
•Instantaneous slip shall be recorded by again
measuring the distance between the two points
immediately after release of jack pressure. The
difference in measurement before and after release
of jack pressure is the instantaneous slip of cables.
The same measurement shall be taken at an interval

16. of 24 hrs. up to 72 hrs. The difference between the
instantaneous slip and 72 hrs. slip shall not be more
than 1 mm. If it happens to be more, the strand is to be
destressed from both ends and re-stressing shall be
done using new grips. The total slip loss considered in
the design is normally 6mm.
•The net extension achieved is the criteria and not the
applied jack pressure. Thus if there is any shortfall
extension in the previous strand, try to compensate in
the next cable (if there is no provision for emergency
cables) to be stressed by extra tensioning, keeping in
view that maximum allowable gauge pressure as
mentioned above is not exceeded. However, extension
in any particular strand shall not exceed more than
1.05 times the designed extension given in the
drawing.
•If the difference between the extension and
corresponding gauge pressure is more than 5%, the
cause should be investigated and the matter should be
brought to the notice of higher authorities.

17. •The entire prestressing operation shall be done under
the control of an experienced Engineer; and from
Railway’s side the supervision should be done by
person not below the rank of AEN.
•All records of prestressing operation shall be jointly
signed by the AEN and contractor’s representative in at
least two copies. One copy shall be retained by the
contractor and the other copy shall be with Railways.
Don’ts
Never extend the strands beyond 1.05 times the design
extension given in the drawing.
Never allow the gauge pressure to exceed beyond 1.05
times the gauge pressure required for achieving the
design extension given in the drawing.
Never stand behind the jack while stressing of strands
is undergoing.

18. 6.Good practices during Grouting of cables.
•Grouting should be done as early as possible after
prestressing of cables but not later than one week.
•Before grouting, mix proportion shall be got
approved by competent authority. The water cement
ratio shall be as low as possible, consistent with
workability and shall not exceed 0.45. The
compressive strength of 100 mm cubes of the grout
shall be not less than 170 kg/cm
2
at 7 days. Tests
shall be conducted for each job periodically.
•It is essential that the grout is maintained in a
homogeneous state and uniform consistency so that
there is no separation of cement. For this purpose it
is necessary that the grout be continuously agitated
by a suitable mixer with a minimum speed of 100
RPM.

19. •Use of hand pumps for grouting is not recommended.
Use of compressed air operated equipment for
injection of grout is prohibited as it is likely that
there will be some air entrapped in grout.
•The grouting equipment should contain a screen
having a mesh size of 100 m. Prior to introduction of
grout into the pump, the grout should be passed
through such screen.
•Ducts shall be flushed with water for cleaning as well
as for wetting the surfaces of the duct walls. Water
used for flushing should be of same quality as used
for grouting. It may, however, contain 1% slaked
lime or quick lime. All water should be drained
through the lowest drain pipe or by blowing
compressed air through the ducts.
•The connection between the nozzle of the grout
injection pipe and the duct should be such that air

20. can not be sucked in. Connections and details of
fixing inlets to the sheathing or anchorage shall be
followed as recommended by the supplier of the
system of prestressing.
•All outlet pipes including vent opening shall be kept
open prior to commencement of injection of grout.
•A stand by direct feed high-pressure water pump
shall be available at site for emergency. In case of
any problem in grouting the ducts, such pump shall
be immediately connected to the duct and grout
shall be flushed by using high pressure flushing.
Storage of adequate quantity of clean potable water
on each accessibility shall be ensured.
•Before grouting a particular cable it must be
ensured that all the nearby cable ducts are
completely filled with water.

21. •If grouting is to be discontinued for a longer period
(say more than 8 hrs.), then all the ungrouted ducts
of the girder including where prestressing has not
been done shall be thoroughly flushed with clean
water and water shall be drained with compressed
air.
•Once mixed, the grout shall be kept in a continuous
movement.
•Injection of grout shall be continuous and shall not
be interrupted.
•Grouting should be initially commenced with a low
pressure of injection (upto 0.3 N/mm
2
); increasing it
until the grout comes out at the other end. The grout
shall be allowed to flow freely through the other end
until the consistency of grout at this end is same as
that of the grout at injection end. When grout of
same consistency flows from the other end, it should

22. be closed and building up of pressure be allowed. Full
injection pressure at about 0.5 N/mm
2
shall be
maintained for at least one minute before closing the
injection pipe. Where there are more than one
outlets, the closer of outlets shall be one after the
other staring from the lowest point to the highest
point of the cable profile.
•If there is a built up of pressure in excess of 1
N/mm
2
without flow of grout coming at the other
end, the grout operation should be discontinued and
the entire duct shall be flashed with high water
pressure.
•Use of short length flexible tubes at both inlet and
outlet points facilitate closing by U-shape bending
followed by binding with binding wire. This can also
be easily cut off after the grout is hardened.
•Grout not used within 30 minutes of mixing shall be
rejected.

23. •Grouting should preferably be done in the early
morning or late evening hours when the ambient
temperature is not likely to exceed 40
o
C.
•Consider use of sand for grouting only in case where
the internal diameter of the ducts exceed 150mm.
•Protective glasses must be used during grouting
operation especially when near to or disconnecting a
grouting hose.
•Once mixed no water shall be added to the grout to
increase its fluidity.
7.Protection of cable ends.
•End faces where anchorages are located are
vulnerable points for entry of water. They have to be
necessarily protected with effective cover.

24. •All the strands of the cable are required to be cut at
a distance of about 100 mm from the face of guide
cone with the help of cutting machine. The recess
and the protruded strands after cutting shall be
sealed with epoxy mortar/concrete of suitable mix.
A minimum cover of 75 mm shall be maintained all
around. However, in case grouting is done after
mounting, a metallic pipe of suitable length shall be
threaded and fixed with the hole provided in the
guide cone/bearing plate for grouting of duct before
sealing with epoxy mortar. The pipe shall protrude
outside the mounting.