Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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STEEL MATERIALS FOR USE WITH PRESTRESSE~ CONCRETE
1 BACKGROUND OF THE INVENTION
The present invention relates to steel materials
for use with concrete that is prestressed by
posttensioning.
Concrete has a relatively low tensile strength.
In order to overcome this disadvantage, prestress~d
concrete has been developed. By means of high strength
steel wires, bars or strands, a concrete member is
precompressed. When the structure receives a load, the
compression is relieved on that portion which would
normally be in tension.
There are two general methods of prestressing,
namely, pretensioning and posttensioning. The present
invention relates to steel materials for use with concrete
of the type that is prestressed by posttensioning.
Structural designs used to prevent direct
contact between steel materials and the surrounding
prestressed concrete are illustrated in Figs 1 and 2.
The design shown in Fig. 1 can be used whether the steel
material is in the form of a wire, bar or strand. A steel
member 1 having a grease coating 2 is sheathed with a PE
(polyethylene) tube 3. When the steel member 1 with the
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1 PE tube 3 is placed within a concrete section 3, the
lubricating effect of the intermediate grease coating 2
reduces the coefficient of friction between the steel
member and concrete to as low as between 0.002 and 0.005
m~lO Because of this low coefficient of friction, the
design in Fig. 1 provides great ease in posttensioning a
long steel cable in concrete. However,- if the steel
material is of short length, the need for preventing
grease leakage from either end of the PE tube presents
treat difficulty in fabricatiny and handling the steel
material. Furthermore, steel members having screws or
heads at both ends are difficult to produce in a
continuous fashion.
The steel member 1 shown in Fig. 2, which is
encapsulated in asphalt 5, has a slightly greater
coefficient ox friction than the structure shown in Fig.
1. This design is extensively used with relatively short
steel materials since it is simplP in construction, is
leak-free, and provides ease in unbonding the steel
material from the concrete, even if the steel member has
screws or heads at end portions.
One problem with the design in Fig. 2 is that
the presence of the asphalt (or, alternatively, a paint)
may adversely affect the working environment due to the
1 inclusion therein of a volatile organic solvent. Moreover,
the floor may he fouled by the splashing of the asphalt
or paint. As another probl'em, great difficulty is involved
in handling the' coated steel material during drying or
positioning ~lithin a framework, and separation of the
asphalt coating can easily occur unless utmost care is
taken in ensuring the deslred coating thickness.
SUMMP~RY OF THE INVENTION
Accordingly t a-primary obj'ect of the present
invention is to provide a steel material for use with
prestressed concrete that is free from the problems
associated with'the prior art techniques. s
These and other objects of the present invention
are'achieved by sheathing a steel material for prestressed
concrete with'a heat-shrinkable synthetic resin tube or a
roamed synthetic resin tube.
BRIEF: DESCRIPTION OF THE DRAWINGS
Figs. 1 and 2 show schematically conventional
designs of steel materials for concrete prestressed by
posttensioning;
Fig 3 lS a schematic presentation of a steel
material of the present invention for use with prestressed
concrete; and
Fig. 4 shows a cross section of a steel strand
sheathed with a resin tube according to the present invention.
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1 Fig. 5 shows measuring apparatus used to determine
the co-efficient of friction as set out in tables 2 and 5
attached to steel material of the present invention for use
with pre-stressed concrete.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, the present invention will be described
in detail with reference to Fiys. 3 and 4, in which -
reference numeral indicates a steel number tl) and
reference numeral 6 (7) a heat-shrinkable synthetic resin
tube (roamed synthe-tic resin tube).
embodiment 1
According to this embodiment, the steel member
is sheathéd with a heat--shrinkabIe synthetic resin tube.
The steel material need not be bonded to the heat-
shrinkable synthetic resin tube with an adhesive material.
If improved rust-preventiny and anti-corrosion effects are
desired, the steel member and the resin tube may be bonded
by an adhesive material. If the steeI member is a bar,
a heat-fusibLe synthetic resin adhesive is coated or placed
on the inner surface of the resin tube or the outer surface
of the steel bar, and, after the resin tube is slipped over
the steel bar, heat is applied to cause the resin tube to
shrink as the resin adhesive melts to provide firm adhesion
between the steel bar and the resin tube. In order for the
resin adhesive to melt while the resin tube only shrinks
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1 when they are heated, the melting point of the resin adhesive
must be lower than that of the resin tube. It has been
found by the present inven-tors that this method is the
simplest and best way to ensure firm bonding between the
steel bar an(l the synthetic resin tube.
The steel material for prestressed concrete
according to the this embodiment is illustrated in Fig. 3,
wherein reference numeral 1 refers to the steeI member and
6 represents the heat-shrinkable synthetic resin tube
coated on the surface of the steel member. In one pre-
ferred example, the steel member 1 is inserted into a pre-
fabricated heat~shrinkable synthetic resin tube, which is
then heated by hot air, steam Gr with an IR (infrared)
heater to shrink and tightly fit it onto the surface o-E the
steel member.
The fall thickness of the heat-shrinkabIe synthetic
resin tube must be at least 300 microns in order to isolate
the steel member 1 and the surrounding concrete layer
sufficiently to provide good slippage between the two
components. The wall thickness to of the synthetic resin
tube after heat shrinking can be approximated by the
following equation:
t = (1/2) (((D + 2tl) - Dl + Do ) 2 - Do3~
where t: wall thickness (mm) after heat shrinking
Do outside diameter (mm) of steel bar
Dl: inside diameter (mm) of the tube before heat
shrinking
l wall thickness (mm) before heat shrlnking.
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l If a steel bar of Do = 17 on is inserted into a
resin tube having an inside diameter of 20 mm and a wall
thickness of 0.3 mm and if the tube is heat-shrunk to
prov'ide intimate'contact with the steeI bar, the tube
around the steeI bar will have a wall thickness as large
as about 0.35 mm. A heat-shrinkable polyolefin tube has a
heat shrinkage of about 35%. Thus, the'inside diameter of
he tube can be sel'ected from the'range'of l.l to 1.5 times
1:he outside'diameter of the steeI bar. This fairly large
:inside diameter of the polyolefin tube permits considerable
ease in inserting the steel bar through the tube. Further-
nore, by properly seIecting the inside diameter and wall
1:hickness of the heat-shrinkable syntheti,c~"resin tube to
be used with'a steel bar having a specific outside diameter 9
the desired wall thickness of the tube will be provided
around the steeI bar after heat shrinkage.
Samples of steel materials for use'with prestressed
concrete that included steel members coated with a heat-
shrinkable synthetic resin tube were fabricated and
subjected to various tests to determine their properties.
The results are shown in Tables 1 to 3,
The method of measuring the frictional coefficient will
be described with reference to Fig. 5.
First, the sample 24 as obtained from the above
procedure was placed in concrete 23 and thereafter the
concrete was solidified. toad cells 21 were provided at
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1 both end portions o~'the sample member of wire 24 which'
were'expose~ from bbth sides of the concret'e 23'and then
tension was applied to the sample member 24 by a jack 22
provided at one end of the sample member 24 as shown
in Fig. 5, At this time, a load applied to one end of
the sample member by using the jack 22 and a load
transmitted through the sample member applied to the other
end of the sample member, i.e., the fixed side of the'
sample member, were simultaneously detec-ted through both
of the load cells 21 by a loadmeasuring detector 25.
Herel if Pi is defined as the'load at the~.applicati~n
side of the tension using the jack and Po is defined as
the load applied to the fixed side of the sample member 24,
the friction between the sample membe'r and the concrete
is obtained by subtracting Po from Pi and the
frictional coefficient at unit length'of the sample
member is obtained from the following equation:
, ;~ = (Pi - Po) /Po.~ = (Pi/Po - l
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1 Table 1
Basic properties of SamPles
Dimensions of Bar having an outside diameter of
steel member: 17 mm and a length of 2,830 mm
. 5 Resin tube: High-density polyethylene tube
that was rendered heat-shrinkable
by cross-linking under exposure to
electron beam
Density: 0.95 g/cm2
Tensile strength: 1.0 kyJmm2
Elongation: 300%
Heat resistance: 350C (1 min.)
Saltwater resistance: OK
Alkali resistance: OK
Acid resistance: ~10% HC1) OK
tl0% H2SO4) OK
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1 Table 2
Unbonding (Frictional) Properties
Load (Kgf) Frictional Frictional
Sample Tensioned Fixed loss (Kgf) coefficient
No._ Side(Po) A (Mel) Remarks
1 19O490 19~110 380 0~00817 Length of
concrete
2 19.540 19.135 405 0.00869 section:
Q=2,435 mm
3 19~530 19.190 340 0.00728
4 19.480 19.105 375 0.008~6
Sample
S l9.S10 19.015 495 0.01059 tempera-
ture:
.6 19O500 19.185 315 0.00674 T=25C
7 19.520 19.065 455 0.00980
Frictional
8 19~500 18.970 530 0.01147 coeffi-
cient:
9 19.510 19.080 430 0.00926 =(Pi 1)
15 10 19.~70 19.110 360 o.oa774
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1 Table 3
Test Conditions Results
1. Continuous JIS Z 2371 No rust or blister
saltwater (5~ aq. NaCl, 35C) formed on the sample
spray test surface
(2,000 hrs)
No rust on the internal
steel bar.
2. Saltwater Immersed in 3% aq~ No rust or blister
immerion test NaCl at 25C formed on the sample
(2,000 hrs) surface.
No rust on the internal
steel bar.
3. Alkali Immersed in 3% aq. Mo rust or blister
resistance NaCl at 25C that formed on the sample
test was adjested to surfact.
(2,000 hrs) pH 11 with KOH
No rust on the internal
steel bar.
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1 Embodiment 2
According to this embodiment, the steel member
is sheathed by a foamed synthetic resin tube 7 in Fig 3.
Various methods may be used to cover the steel member 1
with the resin tube In one method, a synthetic resin
powder containing a blowing agent is applied to provida a
foamed coating on the surface of a preheated steel member
by a fluidized dip coating or electrostatic coating
technique Alternatively, a film oF synthetic resin
containing a blowing agent is formed on the surface of the
steel member 1, which is then passed through a heating
chamber to expand the resin film into a foam. If desired,
a preliminarily formed synthetic resin foam tube 6 may be
slipped over the steel member lo The resln tube 6 may or
may to be bonded to the steel member 1.
In order to isolate the steel material
sufficiently from concrete to facilitate the subsequent
posttensioning, the foamed synthetic resin tube 6 must
have a wall thickness of at least 300 microns.
Furthermore, in order to reduce the frictional resistance
and therefore the slippage between the steel member 1 and
the concrete, the resin tube 6 preferably has a wall
thickness of at least 500 microns.
Steel bars, one example oE a steel member
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1 according to the present invention, were sheathed with a
foamed polyethylene tube. The tube was prepared from a
blowing agent loaded polyethylene powder that was applied
to preheated steel bars using a fluidized dip coating
technique. The properties of these samples were as sown
in Tables 4 and 5:
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1 Tabl _
Basic Properties of Steel Bars
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Bar dimensions: 17 mm~ X 2,830 mm
Polyethylene tube: prepared from medium-density
PE powder (density. 0.925 g/cm3,
m.p. 120C) containing 1.0~
heat-decomposable blowing agent
Wall thickness of 1~3 - 1.5 mm
polyethylene tube:
Occluded cells: Open cells of a size of
0.3 - 0.5 mm distributed
uniformity in a thickness ox
3 - 4 microns
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1 Table 5
Unbondinq_(Fri.ctional) Properties
Load (Kgf) Frictional Frictional
Sample Tensioned Fixed loss (Kgf) coefficient
No. side (Pi) Side(Po) (m~1) Remarks
1 19.510 19.140 370 0.0079 Length of
concrete
2 19.540 19.200 340 0.0073 section:
~=2,435 mm
3 19.500 19.010 490 0~0106
4 19.480 19.040 440 0.0095
Sample
19.510 19O115 395 0.0085 tempera-
. ture:
.6 19.530 19.170 360 0.0077 T=25C
7 19.500 19.040. 455 0.009~
Frictional
S 19.510 18.965 545 0.0118 coeffi-
cient:
9 19.500 19.220 280 0.0060 ~=(Ppi
19.490 19.125 365 0.0078
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1 Table 6
Resin coat
Thickness Surface
Same (microns) features Result
Barax 300 - 500 unscratched No rust ormed
(unbonded) even after 2,000 hrs
Barax 300 - 500 scratched Severe rust formed
(unbonded) around scratches
. after 200 hrs
Foamed 300 - 500 unscrachted No rust formed
polyethylene even after 2,000 hrs
coating
Foamed 300 - 500 scratched Rust formed only
polyethylene at scratches
coating ater 500 hrs
. . .
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1 The present invention is also applicable to a
steel strand composed of a plurality of twisted steel
wires as shown in Fig. 4. The resulting steel strand has
spiral grooves as indicated by A and B in Fig. 4. Not
onlv do these grooves render the posttensioning of the
strand difficult, but they also increase the frictional
resistance on the stressed concrete. In order to avoid
these problems, the grooves are filled with a resin. Such
filling with a resin may be accomplished by extrusion or
other suitable techniques. Subsequently, the thus-treated
steel strand is sheathed with the foamed synthetic resin
-tube as above.
According to the present invention, a steel
material for use with prestressed concrete can be easily
manufacturedO The resulting steel material is easy to
handle during transportation and installation.
