Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
. CA 0222~166 1997-12-18
A CABLE SHEATH HAVING A MULTILAYER STRUCTURE, A METHOD OF
MANUFACTURING SUCH A SHEATH, AND A MACHINE FOR
IMPLEMENTING THE METHOD
The present invention relates to a cable sheath, in
particular a prestress sheath or a stay sheath, to a
method of manufacturing such a sheath, and to a -~hinQ
for implementing said method.
Prestress sheaths designed to be embedded in
concrete constructions, and to receive the prestress
cables of said constructions, must satisfy several
requirements:
they must offer certain watertightness
characteristics;
they must be very resistant to abrasion and to
~?ch~nical stress;
they must have at least a minimum thickness of wall
to avoid being punctured on tensioning the cables;
they must offer low friction on tensioning the
cables; and
they must enable the prestress forces to be
transferred to the structure via the set cement by being
provided with relief on their outside surfaces, the
cement slurry being injected after tensioning.
It is known that prestress sheaths can be made of
metal, in particular steel. A simple t~hn;que consists
in helically winding a metal strip, and in fast~n;ng
together the adjacent edges of the strip after W;n~ing.
That t~hn;que offers the advantage of making a sheath
that can be provided with a spiral rib over its external
surface enabling sheath couplings to be fixed merely by
being screwed onto the ends of the sheaths. However,
such metal sheaths suffer from the drawback of rusting
easily, which can give rise to local damage and thus to
loss of watertightness and/or of strength. To overcome
that drawback, it is possible to cover the metal strip
with a protective layer. Unfortunately, that increases
the manufacturing cost considerably.
CA 0222~l66 l997-l2-l8
Another solution for ove~ ng that drawback is to
make sheaths of plastic. Unfortunately, because of the
~;n; ~m thickness required for the wall of a sheath
(about 2 millimeters) and because of the shape memory of
the suitable plastics materials, it is n~c~ary, in
order to use the above-mentioned method of manufacturing
metal sheaths, to heat the entire strip of plastic, to
wind it, to weld together the adjacent edges, and to cool
the sheath rapidly, all of this taking a long time, and
involving a high cost in energy. That method of
manufacturing is therefore too costly and cannot be
performed in situ, i.e. on the building site. It is
therefore necessary to manufacture such plastics sheaths,
e.g. by extrusion or molding, in complicated and costly
=~h~n~.~ and to transport them to the building site from
the place of manufacture, which is also very costly, in
particular since they are very voluminous. In addition,
if the sheaths have to be transported, it is impossible
to make long straight lengths of sheath.
Furthermore, such plastics sheaths generally include
bands serving as projecting relief over their outside
surfaces. Such bands generally do not enable sheath
couplings to be screwed on. To overcome that drawback,
threaded portions are generally provided at the ends of
the sheaths, which portions are generally formed by
molding. In addition to increasing the manufacturing
costs, such threaded end portions prevent the sheaths
from being cut to any desired length on site.
Alternatively, special sleeves in the form of half shells
are provided which are assembled after the sheaths have
been laid. That also increases manufacturing costs.
~ ch;ne~ exist for manufacturing by extrusion
plastics sheaths provided with spiral threads over their
outside surfaces. Unfortunately, such r-ch;~ are very
complicated and very costly and they cannot be used in
situ. The above-mentioned drawbacks are therefore not
~1; ;n~ted.
CA 0222~166 1997-12-18
Another drawback with plastics sheaths is that it is
difficult to find a plastics material that satisfies all
of the above-mentioned requirements simultaneously.
An ob;ect of the present invention is to provide a
cable sheath satisfying the above-mentioned requirements,
while ~li ;n~ting the above-mentioned drawbacks.
An ob~ect of the present invention is thus to
provide an improved cable sheath which is watertight and
strong, which has at least a i n i thickness of wall,
which offers low friction on the inside, and which can
transfer the prestress forces to the concrete. A
particular object of the present invention is to provide
an improved cable sheath which has a structure such that
each of the above-mentioned requirements is satisfied as
well as possible.
A further object of the present invention is to
provide such an improved cable sheath that is simple,
quick, and cheap to manufacture.
A further object of the present invention is to
provide such an i~ uved cable sheath that can be
manufactured in situ to any desired length.
A further object of the present invention is to
provide such an improved cable sheath that is essentially
made of plastics material.
The present invention thus provides a cable sheath
comprising a multilayer structure made up of at least two
superposed continuous layers of plastics material, each
layer being constituted by a strip disposed helically and
whose side edges are tol~ch; ng and sealed together.
This multilayer structure offers numerous
advantages. It is thus easy to vary the thickness of the
wall of the sheath by changing the number of layers, each
of which may be very thin, which eliminates, to a large
extent, the difficulties of wi n~; ng due to the shape
memory of plastics materials. In addition, it is
possible to incorporate layers of different materials,
each layer then having particular properties.
CA 0222~166 1997-12-18
The multilayer structure may include more than two
layers of plastics material. In particular, the
invention provides a cable sheath that is made entirely
of plastic.
Preferably, said at least two layers of plastics
material are welded together at at least one point,
advantageously along a line, e.g. by heat sealing. This
characteristic makes it possible to perform irreversible
bo~;ng between the layers, and guarantees repeatability
and uniformity for the sheath.
Preferably, the cable sheath is provided with at
least one continuous helical thread over its outside
surface. The presence of the continuous helical or
spiral thread makes it possible for sheath couplings to
be fixed merely by being screwed on. Furthermore, the
fact that the thread extends continuously over the entire
length of the sheath makes it possible for the sheath to
be cut to any desired length. In addition, depen~ing on
the pitch of said spiral thread, the rigidity of the
sheath may be modified.
According to an aspect of the invention, an insert
such as a metal wire may be disposed in the multilayer
structure of the sheath, in particular inside at least
one longitll~ln~l rib of at least one strip. The insert
may act to reinforce the rigidity of the sheath, or
optionally as a heater insert.
A particularly suitable use of the sheath of the
invention is as a prestress sheath.
However, the sheath of the invention may be used as
a stay sheath. In which case, the advantageous presence
of a continuous spiral thread over the outside surface of
the sheath offers aerodynamic advantages with respect to
wind, and to rainwater which can run off more easily by
being ~h~nn~led, and the multilayer structure makes it
possible to make an outer layer having any desired
practical or esthetic characteristic.
CA 0222~166 1997-12-18
The present invention further provides a method of
manufacturing a cable sheath, the method including the
following steps:
supplying at least two strips of plastics material;
assembling them helically and in superposed ~n~
by means of assembly apparatus such that the respective
edges of each of said strips are tollch; ng SO as to form a
sleeve having a multilayer structure made up of at least
two layers of plastic.
Preferably, the method involves welding together
said at least two superposed layers of plastic at at
least one point. Advantageously, the welding is
performed by heat sealing along a continuous helical
line, which makes the sheath more watertight.
Preferably, prior to the assembly step, the method
involves forming at least one longit~ n~l rib in each
strip.
The invention further provides a ~hi n~ for
implementing the method, said ?Ch; n~ including at least
two drums for receiving rolls of strip so as to supply
said at least two strips, and assembly apparatus for
assembling the strips helically such that their
respective side edges are tollch; ng.
In a first variant of the invention, the assembly
apparatus includes a stationary cylindrical former which
may be provided with at least one continuous spiral
thread over its outside surface, on which thread a
respective longitll~in~l rib of each strip is wound, the
rib being formed previously by means of a correspon~;ng
rib-forming module, the w;n~;ng being advantageously
performed by means of drive and/or presser wheels. This
variant makes it possible to make sheaths of
substantially circular cross-section.
In a second variant of the invention, the assembly
apparatus includes two rotary cylindrical formers around
which the strips are helically wound, the set formed by
said two rotary formers itself being rotary. The w; n~; ng
CA 0222~166 1997-12-18
is also advantageously performed by means of drive and/or
presser wheels. This variant makes it possible to make
sheaths of oblong cross-section.
Other characteristics and advantages appear from the
following detailed description given by way of non-
limiting example and with reference to the ~ nying
drawings, in which:
Figure 1 is a diagrammatic longitll~i n~l section view
through the wall of a sheath having a multilayer
structure of the invention;
Figure 2 shows a portion of the assembly apparatus
of the invention, and diagrammatically shows how the
sheath having a multilayer structure of the invention is
assembled;
Figure 3 is a diagrammatic view of a ~ hine for
implementing the method of manu~acturing a sheath having
a multilayer structure of the invention, said machine
incorporating a first variant of the assembly apparatus;
and
Figure 4 is a view similar to Figure 3, the -~h i n~
incorporating a q~on~ variant of the assembly apparatus.
As shown in Figure 1, the sheath 1 of the invention
comprises a multilayer structure that may, for example,
comprise three superposed layers 10a, 10b, 10c.
Naturally, the number of layers 10, of which there are at
least two, is not limited and the embodiment of the
sheath comprising three layers as shown in the figures
merely represents an advantageous embodiment. The three-
layer structure is advantageous when the sheath is used
as a prestress sheath because, to obtain the required
~ n; lm thickness of walls, i.e. about 2 millimeters
prior to tensioning the cables, it is then possible to
use layers of material of st~ ~d thickness of about 0.7
m;ll; ?ters. Naturally, if the cable sheath of the
invention is not to be used as a prestress sheath, the
thickness of the wall may be smaller or larger, and the
number of layers may therefore be different.
CA 0222~166 1997-12-18
According to an aspect of the invention, the sheath
having a multilayer structure includes at least one layer
of plastics material. Preferably, said sheath includes
at least two superposed layers of plastics material,
thereby enabling them to be intimately bonded together as
explained below in the description of the method of
manufacturing the sheath.
The detailed description of the invention given with
reference to the drawings conc~ns more particularly a
sheath having a multilayer structure comprising three
layers lOa, lOb, lOc of plastics material, but clearly
the sheath may include a layer made of another material,
e.g. such as glass fiber, or metal.
A particularly advantageous characteristic of the
sheath having a multilayer structure of the present
invention lies in the fact that it is possible to choose
a particular material having particular intrinsic
characteristics for each layer so that each layer of the
multilayer structure performs a particular function. For
example, the inner layer lOa of the sheath may be made of
a material having low friction during tensioning of the
prestress cables, e.g. Teflon or graphite-cont~; n ~ ng
high-density polyethylene (HDPE). The second layer lOb,
i.e. the layer that is superposed directly on said inner
layer lOa, may, for example, be made of a material having
relatively high rigidity. The outer layer lOc may be
made of a material having good resistance to ultra-violet
radiation. This is advantageous particularly when the
sheaths are stored on building sites situated in hot
countries. Softening and distortion of the sheath caused
by solar radiation can thus be limited. This multilayer
structure also makes it possible for the cable sheath of
the invention to be adapted to uses other than as a
prestress sheath. Such a sheath may be used as a stay
sheath. In which case, the characteristics required for
the inner layer lOa and the outer layer lOc of the sheath
are not necessarily the same as those of a prestress
CA 0222~166 1997-12-18
sheath. In a stay sheath, the outer layer lOc may, for
example, be made of a colored material to satisfy
esthetic requirements, said colored material also having
good resistance to ultra-violet radiation so as to
guarantee that the color does not fade.
According to the invention, each layer 10 of the
sheath is continuous, i.e. each layer of the sheath forms
a closed sleeve 10, the multi-layer structure therefore
being constituted by a set of superposed individual
sleeves lOa, lOb, lOc. As shown in the figures, each
layer 10 of the sheath is made from a strip or sheet of
material 11 which is assembled such that its side edges
12, 13 are ad;acent and touching so as to form said
sleeve. According to a characteristic of the invention,
said side edges 12a, 13a; 12b, 13b; 12c, 13c of each
strip lla, llb, llc of plastics material, which are
adjacent and tollch;ng after they are assembled, are
welded together by heat se~l;ng to form a homogeneous and
watertight plastics sheath layer. Preferably, the heat-
sealing zones 17 in which the side edges of each strip of
plastics material are welded together are offset from one
layer to another axially along the sheath 1. This offers
a further guarantee that the multilayer structure of the
sheath is properly watertight, since the zones 17 of the
layers of the sheath where the watertightness is
relatively weak are not situated facing one another.
According to an example of application of the
invention, the sheath 1 is provided with at least one
continuous helical or spiral thread 15 over its outside
surface. The spiral thread 15 acts in particular to
satisfy the above-mentioned requirement of transferring
forces to the concrete, after the cement slurry has set.
Preferably, the continuous outer spiral thread 15 of the
sheath is formed by one or more longitll~lnAl ribs 14a,
14b, 14c provided on each strip lla, llb, llc making up
said multilayer structure. Said longitudinal ribs 14a,
14b, 14c of the various strips are preferably equidistant
CA 0222~166 1997-12-18
so that they are superposed on one another on assembling
said sheath. Thus, in the example shown in Figure 1,
each of the strips lla, llb, llc respectively
constituting the layers lOa, lOb, lOc of the sheath 1 is
provided with three longitudinal ribs of identical shape
which are superposed on one another on assembling said
strips spirally, as described below with reference to the
method. In this embodiment, the sheath 1 is thus
provided with three continuous helical threads 15 on its
outside surface. This characteristic is particularly
advantageous when the sheath is of circular cross-
section, because it enables sheath couplings to be fixed
merely by being screwed onto said spiral threads 15, at
the ends of the sheath 1.
The width of each of the various strips lla, llb,
llc forming the multilayer structure and the number of
and/or the spacing between the longitl~;n~l ribs 14a,
14b, 14c formed in each of said strips may be varied,
thereby advantageously enabling the rigidity of said
sheath 1 to be modulated.
According to another advantageous aspect of the
invention, an insert 19 is disposed in the multilayer
structure of the sheath. The insert 19, which may, for
example, be a metal wire, may serve to reinforce the
rigidity of said sheath, but it may also act as a heater
insert by passing an electric current. In which case,
said insert 19 acts to heat the sheath when the
temperature is e~L~ ly low, thereby enabling the cement
slurry to be in;ected even in cold weather. This insert
or metal wire is advantageously disposed inside one or
more of said longitll~i n~l ribs 14a, 14b, 14c of one or
more of said strips lla, llb, llc. In which case, it
extends over the entire length of the sheath in spiral
nne~ .
As mentioned above, the sheath of the invention may
have a circular cross-section, but it may have a
different cross-section, in particular an oblong cross-
CA 0222~166 1997-12-18
section as shown in Figure 4. In certain applications,
this provides better distribution of the prestress
forces, in particular in thin structures.
The invention also ~on~ns a method of
manufacturing a sheath having a multi-layer structure as
described above. The method essentially involves
spirally assembling said strips in superposed ~ nn~ by
means of assembly apparatus 20. Assembling is performed
such that the respective side edges of each strip touch
so as to form continuous layers of sheath. As mentioned
above, at least one of said strips lla, llb, llc is made
of a plastics material, and, according to a preferred
aspect of the method of manufacturing, at least two of
said strips lla, llb, llc are made of plastics material
so as to form at least two superposed plastics layers in
the multilayer structure of the sheath.
Each strip lla, llb, llc, is paid out from a roll or
a drum 50a, 50b, 50c, and it advantageously penetrates
firstly into a rib-forming first module 40 in which at
least one longit~ n~l rib 14a, 14b, 14c is formed in
said strip lla, llb, llc. The modules 40 are organized
to form identical ribs 14a, 14b, 14c in the various
strips making up the multilayer structure, and they
advantageously include heater means enabling the material
to be deformed to form said ribs lla, llb, llc in the
respective strips. Said strips lla, llb, llc making up
the multilayer structure are then assembled, preferably
simultaneously as shown in Figures 3 and 4, by means of
assembly apparatus 20. Assembling is preferably
performed by spirally wi n~i ng said strips so that the
their longitll~i n~l ribs lla, llb, llc are mutually
superposed. Said strips thus form the various layers
lOa, lOb, lOc of the multilayer structure of the sheath.
In a first variant of the invention, said assembly
apparatus 20 includes a stationary cylindrical former 21
which, in this example, is provided with at least one
continuous helical or spiral thread 22 over its outside
CA 0222~166 1997-12-18
11
surface. In this case, the assembly apparatus 20 is
organized to assemble a sheath having a multilayer
structure that has a substantially circular cross-
section. As shown in Figure 2, the former 21 is provided
with a continuous spiral thread 22 for each longit~ n~ 1
rib 14a, 14b, 14c of the various strips llAa, llb, llc.
Thus, in the example shown, each of the strips is
provided with three longitll~;n~l ribs and the former is
provided with three threads. Thus, each of said
longitl~;n~l ribs 14a, 14b, 14c of each of said strips is
engaged on a respective spiral thread 22 of the former
21, during assembly. Assembling is advantageously
performed by helically w; n~; ng said strips about said
former 21. As shown in Figure 3, the win~ing is
preferably performed by means of drive wheels 25 which
are advantageously disposed sloping so as to follow said
continuous spiral threads 22 of the former 21, and which
are rotated by suitable drive apparatus 26. The wheels
25 act in particular to guide the strips so that their
longitll~; n~l ribs 14a, 14b, 14c engage on said spiral
threads 22 of the former 21. Each strip lla, llb, llc is
thus wound around said former 21 to form a respective
layer lOa, lOb, lOC of the multilayer structure of the
sheath 1, the w; n~; ng being performed such that the side
edges 12a, 13a; 12b, 13b; 12c, 13c of each strip lla,
llb, llc are adjacent to each other and tol~h;ng after
the strip is wound around said former 21. This can be
seen in particular in Figure 2 which also shows how the
three strips lla, llb, llc are wound such that they are
mutually superposed with their respective side edges
offset axially. Preferably, after the longitll~;n~l ribs
14a, 14b, 14c have been formed by means of the respective
forming modules 40, and prior to w;n~;ng by means of said
assembly apparatus 20, the method advantageously makes
provision to dispose a heater module 30 for each strip of
plastics material serving in particular to heat its side
edges 12a, 13a; 12b, 13b; 12c, 13c. In this way, during
CA 0222~166 1997-12-18
winding around the former 21, the heated adjacent edges
of each strip of plastics material are welded together by
heat sealing by means of said drive wheels 25 which, in
this case, also act as presser wheels. This
characteristic makes it possible to obtain a watertight
and homogeneous layer of plastic with each strip of
plastics material.
Preferably, each of said heater modules 30 also
serves to heat "fusing" zones 18 locally in each strip so
that, during wl n~; ng, the respective heated zones 18 of
two strips which are superposed during said w; n~; ng are
pressed together by means of said presser wheels 25 so
that said superposed layers formed by said two strips are
intimately bonded in said fusing zones 18. Irreversible
bonds are thus obt~; ne~ between the various layers of the
multilayer structure, thereby guarant~i ng that the
sheath 1 is properly uniform.
In certain applications in which the sheath is not
embedded in concrete, the spiral ribs no longer serve to
transfer forces to the concrete. They may therefore be
omitted. It should however be noted that the ribs
contribute to imparting good rigidity to the sheath.
Figure 4 shows a second variant of the above-
described assembly apparatus 20. In this variant,
instead of having one stationary former, the assembly
apparatus 20 has two rotary cylindrical formers 23a, 23b,
each of which may be provided with one or more continuous
spiral threads (not shown) over its outside surface. The
set made up of said two rotary formers 23a and 23b is
itself mounted to rotate about an axis. Thus, the strips
are wound around said two formers 23a, 23b to form a
multilayer structure having an oblong cross-section.
Once the various strips have been assembled together
to form the multilayer structure, said structure
advantageously passes through a cooling module 60 to set
the shape of said sheath 1. Since the heater modules 30
act only locally, i.e. on the side edges and in
CA 0222~166 1997-12-18
particular fusing zones of each strip, cooling is not
essential, and in any event does not need to be very
considerable.
Then, the multilayer structure preferably passes
through a cutting module 70, in which said sheath is cut
to any desired length.
The above-described method thus offers the following
very important advantages:
the sheath obt~ne~ comprises a multilayer
structure, which offers the above-mentioned advantages,
namely that, depen~;ng on the desired characteristics,
the number of layers and/or the materials of the strips
constituting the layers can be varied very simply;
the ~h~ne for implementing the method is simple
and ~ ct, and can therefore be used on site, i.e. on a
building site, to manufacture the sheath having a
multilayer structure of the invention;
the sheath obt~1ne~ by implementing this method may
be provided with one or more continuous spiral threads
over its outside surface, thereby enabling sheath
couplings to be made merely by screwing on suitable
sleeves, regardless of the desired length of the sheath;
the cutting module makes it possible to cut the
sheath obt~lne~ to any desired length; the fact that the
method can be implemented on the building site optionally
makes it possible to make very long sheaths, which is
impossible when the sheaths have to be transported from
the factory to the building site;
the costs in terms of energy of said method are low
because heating is applied at localized points only; and
merely by replacing the stationary former of the
first variant of the ~QA hly apparatus with a former
having a slightly larger outside diameter, the ~ch1 n~
for implementing the method of manufacturing the sheath
of the invention also makes it possible to make said
coupling sleeves on site, which sleeves are screwed onto
the ends of said sheaths to couple them together.
