EP0095413B1 - Prestressing ducts and methods for their manufacture - Google Patents

Abstract

To manufacture a bent prestress conduit (5) adapted to receive without damage cables (7) subjected to extremely high pulling forces, one of the faces of a metal strip (1) is coated intimately with a metal phosphate layer (4) itself covered by a soap and this metal strip is wound in helical form so as to obtain a continuous tube whose inner face is formed by the thus coated face of the metal strip.

Description

The invention relates to metal sheaths intended to be embedded in concrete structures and to receive the prestressing cables of these structures, cables which are put in tension with a bearing on said structures at the ends of said sheaths.

It also relates to the processes for manufacturing these sheaths.

It relates more particularly to the case where the axes of said sheaths are not straight and where the tensile forces exerted on the cables are high, which develops high application pressures between the cables and the sheaths in the bent areas of these past.

To facilitate the sliding of the cables along the sheaths, that is to say reduce their mutual friction, and thus increase performance (in particular by reducing the tensile forces at the ends of the cables as well as the section of these cables), it has already been proposed to bring on the inner face of the sheaths and / or on the outer face of the cables a "suitable lubricant product, constituted in particular by a graphite or molybdenum disulfide powder.

Such a method is generally satisfactory.

However, when the tensile forces exerted on the prestressing cables, and therefore the local pressures for applying these cables to the sheaths, are very high, destruction and expulsion of the intermediate lubricating layer is sometimes observed.

This is in particular the case when the walls of the sheaths are corrugated and above all when the cables consist of strands or bundles of wires twisted or wound in a helix.

In this case, in fact, the contact zones between the cables and the sheaths are reduced to very small surfaces, or even almost pointwise, and the local application pressures which result from these surface reductions are increased by the same amount.

Thus, in certain recent embodiments, it is not uncommon for the value of said pressure to reach or even exceed 1000 bars.

The Applicant has observed that, in order to solve a problem having no relation to that set out above, namely to facilitate overall deformations of wires or similar continuous elements by drawing or hot drawing through appropriate dies, it has already been proposed to line these wires or tubes beforehand by chemically applying to their external surfaces a coating composed of a layer of metallic phosphate, itself lined with gelled soap: this coating is so intimately attached to the surfaces of the elements to be deformed that it is not torn off by the dies and therefore ensures effective guidance of the deformation.

The preliminary phosphating and soaping treatments in question are hardly applicable to interior surfaces of tubes or sheaths.

However, it turns out that, in order to manufacture prestressing sheaths which lend themselves easily to bending, it is particularly advantageous to proceed by helically winding a "strip or continuous metal strip.

However, the faces of such a strip lend themselves perfectly to the above treatments.

The invention is essentially characterized in that the said phosphating and soaping treatments, known per se for the external surfaces of wire to be drawn, are applied to at least one of the two faces of such a strip, after which coils this strip in a helix so as to form a sheath whose face treated above constitutes the inner face.

Experience shows that the locally and cold sliding contacts established between the prestressing cables and the inner surfaces of the sheaths thus produced are ensured with excellent lubrication, even for the highest values of the tensile forces exerted on these cables.

The invention includes, apart from this main arrangement, certain other arrangements which are preferably used at the same time and which will be more explicitly discussed below.

• La figure 1 de ce dessin, montre en coupe axiale une portion coudée d'une gaine de précontrainte établie conformément à l'invention et contenant un câble tendu.
• La figure 2 est une coupe transversale partielle de ladite gaine et dudit câble selon II-II, figure 1.

In what follows, a preferred embodiment of the invention will be described with reference to the accompanying drawing, of course in a non-limiting manner.

• Figure 1 of this drawing shows in axial section a bent portion of a prestressing sheath established in accordance with the invention and containing a tensioned cable.
• FIG. 2 is a partial cross section of said sheath and said cable according to II-II, FIG. 1.

A mild steel strip 1 is used, deformed according to longitudinal corrugations, that is to say having a corrugated cross section with ridges 2 and hollow 3.

At least the face, of the strip 1, intended to become the inner face of the sheath, is subjected to a chemical treatment known in itself leading to the intimate bonding, on this face, of a phosphate layer externally coated with a veil of gelled soap.

• décapage à l'acide sulfurique (ou chlorhydrique) dilué, durant 10 secondes environ,
• deux rinçages successifs à l'eau durant environ 3 secondes chacun, séparés par un brossage mécanique,
• application par vaporisation, durant 70 secondes environ, d'un bain de phosphatation composé de phosphate mixte de zinc et de fer. d'oxydants (chlorates, nitrites, ...) et d'additifs appropriés,

This treatment comprises the following operations, all of which are carried out in a tunnel oven heated to a temperature of the order of 70 to 80 ° C. inside which said strip is scrolled at constant speed:

• pickling with dilute sulfuric (or hydrochloric) acid, for about 10 seconds,
• two successive rinses with water for approximately 3 seconds each, separated by mechanical brushing,
• application by spraying, for approximately 70 seconds, of a phosphating bath composed of mixed zinc and iron phosphate. oxidants (chlorates, nitrites, ...) and suitable additives,

new water rinse,

application by spraying, for 5 to 6 seconds, of a soap consisting of an aqueous solution of sodium stearate and sodium palmitate,

and finally drying.

For information only, it is indicated that the above phosphating bath could be an aqueous solution at 6% by volume of the liquid distributed by the French Company of Industrial Products (CFPI) under the name Thermogranodine 63, the soap then being itself constituted by an aqueous solution of the powder distributed by said company CFPI under the name Prolub.

• une couche cristalline présentant une épaisseur de l'ordre de 0,5 micron composée de cristaux de phosphate qui présentent la forme générale de feuilles de palmier et sont imbriqués dans les cristaux superficiels du métal constitutif du feuillard,
• puis une couche superficielle de savon dont l'épaisseur peut être de l'ordre de 100 microns : la plus grande partie de cette couche de savon peut être arrachée par frottement, mais non sa base, laquelle est constituée par un voile très mince, d'épaisseur environ 1/100 de micron, se présentant sous la forme d'un velours dont les poils sont plantés sur la couche cristalline sous- jacente.

The above treatment has the effect of successively reporting, on the treated face of the strip 1:

• a crystalline layer having a thickness of the order of 0.5 micron composed of phosphate crystals which have the general shape of palm leaves and are embedded in the surface crystals of the metal constituting the strip,
• then a surface layer of soap whose thickness can be of the order of 100 microns: most of this layer of soap can be torn off by friction, but not its base, which is constituted by a very thin veil, d '' thickness about 1/100 of a micron, in the form of a velvet whose hairs are planted on the underlying crystalline layer.

There is shown diagrammatically at 4 in FIG. 2 the coating with a “lubricating property” thus applied very intimately to the treated face of the strip 1 and covering this face.

This coating 4 is recognizable by its matt state, its light gray color and its creamy feel, reminiscent of that of Marseille soap.

This strip is then wound around a helix so as to form a tubular sheath 5 of which said treated face constitutes the internal face.

This winding is carried out with lateral overlapping of the contiguous turns of the propeller so that the ridges 2 form external helical ribs and the hollows 3, helical grooves each connecting two of these ribs.

Then one staples one over the other the edges of the overlapping turns by folding them back on each other like a crimping (at 10).

The sheath 5 thus obtained is easy to bend according to the needs of the prestress.

We see, in Figure 1, the concrete structure 6 in which was embedded such a curved sheath 5, as well as one of the prestressing cables 7 put in place in this sheath 5, cable subjected to a tensile force F exerted between its two ends.

This cable 7 is preferably constituted, as illustrated, by a plurality of wires or strands 8 wound helically around another wire 9 constituting a central core.

Theoretically, the surface of application of such a cable 7 against the sheath 5 is reduced to a succession of points or punctual zones M (FIG. 2) which, for each strand, are separated longitudinally from each other by a distance equal to the not of the strand winding propeller, not generally of the order of 20 cm.

In addition, the fact that the contact between each cable and the sheath can only be established at the level of the bottoms of the helical grooves of the sheath corresponding to the hollows 3 above further reduces the possible application surface area between cables and sheath.

This double circumstance, combined with the very high values given to the force F, leads to the development of local pressures P, themselves extremely high, at the contact points M.

Despite these enormous pressures, the pulling of the cables results in the establishment of perfectly "lubricated" sliding contacts between these cables and the inner surface of the sheath, that is to say causing no tearing of the layer. intermediate phosphate at the level of the small contact zones M.

Of course, after this pulling, the volume inside the sheath between the cables and the inner surface of this sheath is filled with an injected grout or a protective grease (not shown) in the manner usual.

Following this, and whatever the embodiment adopted, there is finally a prestressing sheath whose constitution and advantages, in particular the possibility of receiving, with perfectly lubricated sliding contacts, cables subjected to extremely high tensile forces, result sufficiently from the above.

As is obvious, and as already follows from the foregoing, the invention is in no way limited to those of its modes of application and embodiments which have been more especially envisaged; on the contrary, it embraces all its variants.

Claims (3)

1. A duct for prestressing formed by a helically wound metal strip (1), characterized in that the inner surface of this duct is intimately coated with a metal phosphate layer, itself covered with a soap.

2. The duct according to claim 1, characterized in that it comprises a succession of helical ribs and grooves corresponding respectively to the peaks (2) and hollows (3) of longitudinal undulations of the starting metal strip (1).

3. A method for manufacturing a duct for prestressing according to anyone of claims 1 and 2, according to which a metal strip (1) is helically wound with lateral overlapping of the adjacent turns of this helix and preferably mutual crimping of the overlapping edges, characterized in that one of the faces of said metal strip is previously subjected to one of the successions of phosphatation and soaping treatments known per se in the wire-drawing technique, and in that the helical winding of said metal strip is then carried out so that its thus treated face forms the inner face of the duct.

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