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INTRODUCTION:
Prestressed Concrete Circular Spun Piles are
an International phenomenon and are in use
all over the World particularly in Japan and
Germany for quite sometime. In recent times
spun piles have been taken to use
extensively in neighbouring countries like
Malaysia, Indonesia, Taiwan and Korea.
TECHNOLOGY:
HBL in the process of product
diversification from spun poles, have
developed a design for spun piles,
manufactured a prototype and got the same
tested at Indian Institute of Technology,
Madras. The test results are as under:
|
Indian
Test |
Japanese Test |
%
increase |
|
Axial Load |
Bending Moment |
|
242.0 |
15.3 |
|
|
Axial Load |
Bending Moment |
|
172.0 |
6.6 |
|
|
Axial Load |
Bending Moment |
|
40.1 |
131.8 |
|
From the
above test results, it would be apparent
that the product is comparable to
International Standards. HBL has the
technical know-how and machinery for
commercial manufacture of piles.
SPLICE JOINTING:
The joint welding procedure for piles longer
than 12 metres is a well-known method
adopted in a number of countries. The method
involves circumferentially welding together
the plates (mild steel, annular discs)
firmly embedded in concrete at ends of
piles. The welded joint has strength equal
to that of the concrete portion of the pile.
The joint details are given in sketch below.
UNIFORM STRENGTH AND HIGH PRODUCT
RELIABILITY:
HBL PC-PILES are manufactured by
centrifugal compaction process which gives
uniform and assured strength with high
reliability. Concrete of grade M-50 is used
in the manufacture of piles.
DURABILITY:
Even if there be a tensile load on the pile
during transportation, handling or pile
driving work, the prestress prevents
occurrence of any cracking in the pile.
However, if crack appears due to sudden
excessive tensile load of temporary nature
such cracks will disappear as soon as the
load is removed. The possibility of
corrosion of the prestressing steel wires
is, therefore, remote and life of spun piles
is longer.
COST EFFECTIVE:
In the conventional piles, driven or
cast-in-situ, a minimum of 2% reinforcing
steel has to be provided as per relevant
code whereas the prestressing steel in spun
piles is 0.50% to 0.60% leading to a saving
in cost of pile.
PILE DRIVING:
Spun piles do not require any special
driving equipment. The conventional hammers
can be used for pile driving.
ADVANTAGES OF HBL SPUN PILES:
-
Manufactured in longer length up to 12
m.
- Low
unit weight, hence lower cost.
- High
strength concrete of 500 kg/sqcm is
used.
-
Spinning results in denser concrete,
reduces water cement ratio and hence
increases concrete strength further.
- High
flexural strength.
-
Absence of torsional cracking.
- High
resistance to corrosion especially from
sea water.
-
Consistent quality under factory
conditions.
- High
rate of production ensures early
completion of projects.
-
Eliminate casting yard at site – a
distinct advantage in urban
re-development projects.
- High
early strength enables the pile to be
driven 10 days after manufacture.
MANUFACTURING PROCESS The process
consists of several activities starting from
raw materials to testing and despatch.
The source of raw materials for concrete,
cement, natural stone aggregate and sand are
identified and materials tested for
suitability. Similarly, H.T. wires
conforming to stipulations are procured from
renowned manufacturer and test reports
verified.
H.T. wires are cut to required size of poles
with a margin of about 300 mm and button
heading done at ends of pretensioning. Cages
are formed with M.S. Wires for helical
reinforcement.
The cage is placed in the bottom half of the
mould and the calculated quantity of
concrete is placed. The top half is than
bolted and prestressing of H.T. wires done
by a hydraulic jacking system. The mould is
then placed on spinning machine and spun at
predetermined speeds and duration.
The spun mould is placed in a sump for steam
curing for six hours at 65 degrees Celsius.
The demoulded piles are placed in a water
sump for normal curing. The top & bottom
open ends are sealed with cement mortar.
The poles are inspected, marked and stacked
for testing and despatch.
Stagewise inspections and testing of
material, concrete and poles are carried
out. Bent test is carried out on random
selected poles as per provisions of IS
codes.
TECHNICAL SPECIFICATION FOR
PRESTRESSED SPUN CONCRETE PILES
1.
SCOPE:
This specification covers design,
manufacture, testing and supply of
prestressed concrete circular SPUN piles.
2. MATERIALS:
2.1 High strength ordinary Portland cement
conforming to IS:8112 or ordinary Portland
cement conforming to IS:269, which shall
have the following additional requirements,
shall be used.
a) Initial Setting Time : Not less than 90
minutes.
b) Final Setting Time : Not more than 600
minutes.
2.2 As far
as possible cement shall be obtained from
the same source to minimize variation in
quality.
3. AGGREGATES:
3.1 Coarse and fine aggregates used for
manufacture of piles shall be from natural
source conforming to IS: 383-1970. The
nominal maximum size of aggregates shall in
no case exceed 20mm provided further
that the size of aggregates shall be atleast
5mm less than the spacing between the
prestressing wires.
3.2 Each size of aggregates shall be stocked
in different storage bins or stock piles and
shall be mixed only after the quantity
required for each size has been
separately weighed.
3.3 Water:
Water shall be free from chlorides,
sulphates, other salts and organic matter.
Portable water shall generally be suitable.
3.4 Admixtures:
Admixtures, if used, shall not contain
calcium chloride or other chlorides and
salts which are likely to promote corrosion
of pre-stressing steel. Use of any
admixture shall be got approved by the
Engineer-in-charge.
3.5 Reinforcement:
3.5.1 Reinforcement bars and high tensile
prestressing wires used for the manufacture
or prestressed concrete piles shall conform
to the following Indian Standards.
a) IS: 1785 (Part-I) – Specification for
plain hard drawn steel wire for prestressed
concrete: Part-I cold drawn stress relieved
wire (second revision).
b) IS:1785 (Part-II) – Specification for
plain hard drawn steel wire for prestressed
concrete: Part-II as drawn wire (first
reivision).
c) IS: 2090:1983 – Specification for high
tensile steel bars used in prestressed
concrete (first revision).
d) IS: 6003:1983 – Specification for
indented wire prestressed concrete (first
revision).
e) IS: 6006:1983 – Specification for
uncoated stress relieved strand for
prestressed concrete (first revision).
3.5.2 The surface of all high tensile and
reinforcement bars shall be free from loose
scale, oil, grease, clay or other material
that may have detrimental effect on the
bond between the bars and the
concrete.
4. CONCRETE:
The concrete mix shall be designed to
meet the requirement stipulated in IS:456
(code of practice for plain and reinforced
concrete) or IS:1343 (code of practice
for prestressed concrete).
a) Minimum works test cube strength at 28
days shall not be less than 50N/Sqmm. In
occasional cases where the minimum works
test cube crushing strength falls below
50N/Sqmm but not below 47N/Sqmm may be
accepted subject to increased frequency of
tests.
b) The concrete strength at transfer shall
be atleast half the 28 days strength adopted
in the design or strength adopted for
transfer or prestress.
5. MANUFACTURE OF PILES:
5.1 Moulds:
Moulds shall be of steel and of rigid
construction to prevent distortion and so
arranged as to provide smooth surfaces. The
moulds shall not allow any leakage of
cement slurry during spinning. The holes
in the end plates for prestressing wires
shall be accurately drilled to ensure
inter-changeability. The end plates shall be
designed to withstand the forces arising
out of the change in direction of
prestressing wires during tensioning.
5.2 Tensioning of Wires:
5.2.1 The H.T. wires shall be placed axially
at regular spacing along the circumference.
The spacing shall be as per IS:13158.
5.2.2 Helical steel shall be 3.2mm dia M.S.
wires and the pitch shall be 150mm.
5.2.3 The clear cover shall be 20mm.
5.2.4 Pre-tensioning shall be by automatic
machines. Force shall be applied on the
entire group of H.T. wires to ensure all
wires are equally stressed.
5.2.5 The anchoring of the prestressed wires
shall be such that during manufacture and
until the wires are released, no slipping
occurs. The force at the time of initial
stretching, in addition to imparting
of designed prestress, shall also be
sufficient to overcome the friction on
account of any change in the inclination of
wires and slippage that might occur
during the anchoring process which shall be
suitable
5.2.6 The tensioning of prestressing steel
shall be carried out in a manner that will
induce a smooth and even rate of increase of
stress in the wires.
5.2.7 The force induced in the prestressing
wires shall be determined by means of
pressure gauge attached to the tensioning
equipment and cross checked by an
elongation scale determined in
advance. The accuracy of the devices for
measuring of the tensioning force shall be
within plus or minus 5%.
5.3 Mixing and consolidation of
Concrete:
5.3.1 Provision shall be made to measure the
quantities of cement, fine and coarse
aggregates by weight only. The accuracy of
the measuring equipment shall be plus
or minus 3%. All the measuring equipment
shall be maintained in clean, serviceable
condition and accuracy checked at regular
intervals. Modern high speed mixers,
preferably pan type, shall be used.
5.3.2 The freshly cast piles shall be
protected during the first stage of
hardening from harmful effects of sunshine,
dry winds, cold rains.
5.4 Curing:
5.4.1 The curing shall be done with
saturated steam at 65 degree celsius to
ensure that concrete strength for release of
prestress as adopted in design is reached in
6/8 hours. Thereafter the H.T. wires
shall be cut, poles demoulded and cured in
water in sumps or by sprinklers for atleast
7 days.
5.4.2 During manufacture, periodical tests
on concrete cubes shall be carried out till
the concrete achieves the required strength
at transfer. Thereafter, the test on
concrete shall be carried out a s detailed
in IS:13148/IS:456. The Manufacturer shall,
if so required by the purchaser, furnish
results of compressive test conducted
in accordance with IS:456 on concrete cubes
made from the concrete used for the piles.
If the purchaser so desires, the
manufacturer shall supply cubes for
test purposes and such cubes shall be tested
in accordance with IS:456.
5.5. Detensioning of Wires:
5.5.1 The anchoring system shall provide a
device for gradual detensioning of the
wires. No back pulling of the wires shall be
permitted in the gradual detensioning
device for the purpose of release of
any wedge or other parts of the detensioning
device. Flame cutting of the wires before
release of the full tension shall be
strictly prohibited.
5.5.2 The transfer of prestress shall not be
effected until the concrete in the piles has
attained the specified strength as
established of cube tests.
6. TESTS:
6.1 During manufacture, tests on concrete
shall be carried out.
7. MARKING:
The piles shall be clearly and indelibly
marked with the following particulars either
during or after manufacture but before
testing at a position so as to easily read
after erection in position:
a) Month and year of manufacture.
b) Transverse strength of pile in Kg.
c) Serial number of the pile.
d) Position of centre of gravity of the
pile with the word C.G.
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Brochure for Details.
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