WO2019025699A1

WO2019025699A1 - Method for producing a structural element

Info

Publication number: WO2019025699A1
Application number: PCT/FR2018/051872
Authority: WO
Inventors: Alain SABBAH
Original assignee: Sabbah Alain
Priority date: 2017-08-03
Filing date: 2018-07-20
Publication date: 2019-02-07
Also published as: FR3069870A1; FR3069870B1

Abstract

The invention relates to a method for producing structural elements (10, 20), comprising: - a step of tensioning at least one first pre-tensioner (101) and at least one second pre-tensioner (201) in a prefabricating bench (500), - a step of positioning an interface (300) in the prefabricating bench so that it defines first and second moulding spaces, a step of casting, in the first and second moulding spaces, a material in the fluid state, - a preliminary step of drying said material which, upon hardening, forms first and second elongated bodies (100, 200) which respectively have first and second medium fibres (F100, F200), and - the two elongated bodies being held in abutment against said interface (300, 310, 320), a step of releasing the first and second pre-tensioners so that they exert longitudinal compressive stresses on the first and second elongated bodies respectively along the first and second compression axes (A100, A200), separate from the first and second middle fibres, and the bending stresses on the first and second elongated bodies in opposite directions.

Description

PROCESS FOR MANUFACTURING A STRUCTURE ELEMENT

TECHNICAL FIELD TO WHICH THE INVENTION RELATES The present invention generally relates to the manufacture of infrastructures such as buildings, bridges, dams or any other kind of works.

It relates more particularly to a method of manufacturing a structural element such as a prestressed beam or a prestressed slab.

ARR! TECHNOLOGICAL BACKGROUND

It is common, when building a building, a road structure or any type of infrastructure, to use concrete beams and slabs.

A well-known problem of concrete is that, although it resists compressive forces, it cracks quickly when subjected to tensile forces, even at low intensities.

it is then known to reinforce the concrete by metal reinforcements.

We speak of "reinforced concrete".

If the reinforced concrete presents some advantages, its use becomes counterproductive when the stresses exerted on the beams become important, because of the heavier section of reinforced concrete.

The solution then considered is to use so-called "prestressed concrete", that is to say concrete crossed by a metal frame that compresses the concrete.

The idea is to make sure that the concrete always works in compression and never (or little) in traction. For this, it exerts an initial traction on the metal frame so that once installed in the infrastructure, the concrete beam is compressed by this metal frame.

In this way, when the concrete undergoes traction forces, it decompresses but never works in tension, which prevents the appearance of cracks.

The metal reinforcement used is generally formed of steel cables or bars. When the beam is intended to be installed horizontally on a structure, the cables are generally arranged in the lower part of the section of the beam. In this way, the metal frame tends to bend the beam upward (it is called "counter-arrow"), which allows it to withstand higher loads. One of the methods of manufacturing prestressed concrete beams, called "pre-tension", consists in applying a tension to the metal reinforcement, pouring the concrete around the metal reinforcement and then waiting for the concrete to dry. . The reinforcement is then released, thus putting the concrete in compression by simple adhesion effect. At this time, the beam bends upward, which generates tensile forces in the concrete, especially in the center of the upper face of the beam.

We understand then that it is not possible to stretch the metal reinforcement beyond a certain threshold.

In addition, before decapping (that is to say unmolding) the concrete beam, it is necessary to wait until the concrete has sufficiently hardened so that its mechanical properties are sufficient to withstand these efforts of traction or compression. This waiting time, during which the bench used to manufacture the beam remains unavailable and therefore does not allow to manufacture new beams, is relatively long.

OBJECT OF THE INVENTION

In order to overcome the aforementioned drawbacks of the state of the art, the present invention proposes a method of manufacturing structural elements, comprising:

a step of tensioning at least one first preload tensioner and at least one second preload tensioner in a precast bench,

a step of positioning an interface in the prefabricating bench in such a way that it delimits, completely or partially, first and second molding spaces in which are respectively located each first prestressing tensioner and each second prestressing tensioner,

a casting step, in the first and second molding spaces, of a material in the fluid state,

a step of prior drying of said material which, by hardening, forms first and second elongated bodies which are respectively located in the first and second molding spaces and which respectively have first and second medium fibers, and

the two elongate bodies being held in abutment against said interface by temporary fastening means, a step of releasing the first and second pretensioning tendons so that they exert:

longitudinal compressive stresses on the first and second elongated bodies, respectively along first and second compression axes, distinct from the first and second middle fibers, and

bending stresses on the first and second elongate bodies in opposite directions (to which the said temporary fixing means are opposed).

Due to the tension in the pretensioning tendons, the two elongated bodies are compressed longitudinally and tend to bend in opposite directions. Thanks to the temporary fixing means which keep the two elongated bodies resting against each other over the entire surface of the interfa ce, the bending constraints that oppose compensate each other.

The two elongate bodies then behave as a monolithic assembly within which the bending stresses compensate each other.

This monolithic assembly then only tends to undergo compressive stresses. However, concrete is a material that is very resistant to these compressive stresses, even when it is not completely dry. It is thus possible to release the prefabrication bench after a very short time, so that it can be reused quickly.

Once out of the prefabrication bench, the monolithic assembly must be handled in one piece until the drying is complete, until the concrete has reached its mechanical properties of nominal strength.

Furthermore, thanks to the invention, it is then possible to increase the prestressing force by extending the drying time accordingly outside the prefabrication bench.

Other advantageous and non-limiting features of the manufacturing method according to the invention are the following:

the first and second prestressing tendons are positioned relative to the two elongate bodies in such a way that the first and second compression axes extend parallel to the first and second middle fibers and to said interface;

the first and second pretensioners are positioned by relative to the two elongated bodies such that the first and second compression axes extend in the same plane as the first and second medium fibers;

the first and second biasing tendons are positioned relative to the two elongate bodies such that the first and second compression axes extend between the first and second middle fibers;

the first and second pretensioning tendons are positioned and tensioned in the prefabrication bench so that, before the release step, the product of the intensity of the overall force exerted by the at least one first tensioner prestressing on the prefabrication bench by the distance separating said interface from the first compression axis, and the product of the intensity of the overall force exerted by said at least one second prestressing tensioner on the prefabrication bench by the distance separating said interface of the second compression axis, are substantially equal to 50%, and preferably to 10%;

in the tensioning step, deviators are used to force the first and second pretensioning tendons to extend along a broken line (preferably symmetrically with respect to the interface);

- The positioning step and the casting step are performed concomitantly, said interface being positioned when the first molding space is filled by said material and the second molding space is still empty;

the temporary fastening means comprise stirrups which enclose the two elongated bodies and / or tensors which pass through at least part of the two elongated bodies and which hang thereon;

in the positioning step, said interface is positioned so as to partially delimit the first and second molding spaces by providing at least one passage between the first and second molding spaces, and, at the preliminary drying stage, the material located in each passage forms at least part of the temporary fastening means;

- It is expected, after the release step, a step of simultaneously demolding the structural elements and said interface out of the prefabrication bench, then an additional drying step of the structural elements in a remote storage location of the bench of prefabrication; after the additional drying step, a discharge step is provided during which the temporary fixing means are deactivated;

the temporary fastening means are deactivated by removing the stirrups and / or by releasing the tensors of the elongated bodies;

the temporary fastening means are deactivated by breaking the material located in each passage;

said interface comprises a planar wall and / or a layer of flexible material and / or a layer of retardant or setting inhibitor material adapted to retard or prevent curing of the material;

during the preliminary drying step, the two elongated bodies are held one above the other;

during the preliminary drying step, the two elongated bodies are held next to one another.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT The following description with reference to the accompanying drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be implemented.

In the accompanying drawings:

- Figure 1A is a schematic side view of two beams manufactured according to a first embodiment of the invention;

- Figure 1 B is a sectional view, according to the plane A-A, of Figure 1 A;

- Figure 2A is a schematic top view of two beams manufactured according to a second embodiment of the invention;

- Figure 2B is a sectional view along the plane B-B of Figure 2A; - Figure 3A is a schematic side view of two beams manufactured according to a third embodiment of the invention;

- Figure 3B is a sectional view, according to the plan C ~ C, of Figure 3A;

- Figure 4A is a schematic side view of two beams manufactured according to a fourth embodiment of the invention;

- Figure 4B is a sectional view, according to the plane D-D, of Figure 4A;

- Figure 5 is a schematic side view of the two beams of Figure 1 A, after deactivation of the temporary fixing means;

FIGS. 6A to 6D are diagrams illustrating the bending moments acting in one of the beams of FIG. 1A; FIGS. 7A to 7D are diagrams illustrating the bending moments acting in the two beams of FIG. 1A;

- Figure 8A is a schematic side view of two beams manufactured according to a fifth embodiment of the invention;

FIGS. 8B and 8C are sectional views, along the E-E and F-planes;

F, of Figure 8A.

As a preliminary, it will be noted that the identical or similar elements of the different variants and different embodiments of the invention shown in the different figures will, as far as possible, be referenced by the same reference signs and will not be described at each time.

It should also be noted that the expression "structural element" may designate any element used in a structure to compose and / or stiffen the structure of this structure (this structure can be of any type: building, bridge, canal ...) -

The structural element may have any type of shape, and have various functions. It may for example be a slab, a duct or hourdi. In the remainder of the description, the structural element considered will be a beam.

The beam considered will preferably be profiled and it may have any type of cross section (square, S, T, ....).

As shown for example in Figures 1A and 1B, it will be considered here that each beam 10, 20 has a rectangular section.

Each beam 10, 20 is described as "prestressing" in that it comprises an elongated body 100, 200, preferably made of concrete, as well as preloading tensioners 101, 201 for compressing the body 100, 200. preloading tendons 101, 201 are here formed by metal bars but they could be in other forms (metal cables, carbon fiber cables ...).

The "neutral fiber F100, F200" of the body 100, 200 of each beam 10, 20 will be defined as the geometric line passing through the centers of gravity of the transverse sections of this body, here the cross sections of each beam 10, 20 being rectangular, the geometric centers and the centers of gravity of the cross sections are merged. We will also use the terms "lower" and "upper" with respect to the beam, considering the beam as it is positioned in its prefabrication bench 500. The lower part of an element will then designate the part of this element which is turned from the ground side and the upper part will designate the part of this element which is turned towards the sky.

The prefabrication bench 5 (30 is here a mold that can receive liquid concrete, let the concrete to dry so as to form the beams, and demold the concrete once the latter has hardened.

it is here configured so as to simultaneously mold a pair of two beams 10, 20, for example in a superimposed position one above the other. Of course, it may eventually be possible to simultaneously mold a plurality of pairs of beams (the different pairs of beams being, in the example mentioned above, located side by side).

This prefabrication bench 500 is provided to allow to receive preload tensioners and put them in tension, for example by means of jacks, the time that the concrete dries.

Once manufactured, when a beam is installed in an infrastructure and is held by its ends, it will naturally tend to bend down, because of its own weight and the weight of the loads it will have to support .

This bending may then cause internal tensile stresses at the center of the lower face of the beam. To avoid the appearance of cracks, it is expected to preload the body 100, 200 of the beam by means of pretensioners.

These tensioners are generally located under the neutral fiber of the beam so as to force the latter to bend upward (we speak of "counter-arrow") as long as no load is installed on it, which makes it can then support large loads.

It is understood that at the time of manufacture of the beam (when the beam is not yet loaded), it is risky to stretch excessively the permanent tensioners 120, 220, except to risk seeing the beam too flex and appear cracks in the center of the upper face of the beam.

The invention thus relates to a solution making it possible to avoid bending the beam too intensely at the time of its manufacture, while at the same time making it possible to tending tendons as much as possible and minimizing the manufacturing time of the beam.

This solution consists of a method of manufacturing a pair of beams 10, 20, which comprises the various steps above. This process will be described here with reference to FIGS. 1A and 1B.

During a first step, the prestressing tendons 101, 102 are installed in the prefabrication bench 500.

In a second step, the prestressing tendons 101, 102 are tensioned by means of jacks fixed to the prefabricating bench 500. The prestressing tendons 101, 102 then tend to naturally lengthen, by elastic deformation.

During a third step, which can follow or slightly precede or be concomitant with the second step, a curable material and in the liquid state is poured into the prefabrication bench 500.

This material is preferably concrete. It may be a standard concrete whose compressive strength fc28 is less than 40 MPa. Preferably, it will be rather a concrete with high performance (with a compressive strength fc28 greater than 50 Pa), or a very high performance concrete (with a compressive strength fc28 greater than 70 MPa ), or an ultra high performance concrete (with a compressive strength fc28 greater than 150MPa).

During this third step, the concrete is poured into a single mold, from which will be derived the two beams 10, 20.

In order to make it possible to separate the two beams 10, 20 after hardening of the concrete, it is planned, during this third step, or before it, to position an interface 300 in the mold in such a way that it delimits first and second molding spaces.

These first and second molding spaces have shapes that will correspond, in negative, to the shapes of the beams 10, 20.

In the embodiment for obtaining beams 10, 20 such as those shown in Figures 1A and 1B, the first and second molding spaces are located one above the other.

In this mode, it will then be possible to pour the concrete in two stages:

- firstly in the bottom molding space, which then allows to report the interface 300 on the upper surface of the concrete thus cast,

- then in the molding space at the top.

The interface 300 may be in very different forms.

it may be one or more sheet metal (s), one or more plank (s) of wood, a plastic film or neoprene, a layer of retardant product (adapted to curb concrete hardening at the interface), a layer of setting inhibiting product (adapted to prevent hardening of the concrete at the interface) ... It will preferably extend according to a plane , here corresponding to a plane of symmetry for the first and second molding spaces.

a fourth pre-drying step is then provided during which the concrete is allowed to harden. The duration of this step is much shorter than that required by the concrete to reach its nominal mechanical properties (as defined by the fc28 standard). In this case, the duration of this step is less than 3 days. It is preferably of the order of 24 hours.

By hardening, the concrete forms the elongate bodies 100, 200 of the two beams 10, 20.

During this fourth step, the concrete naturally tends to adhere to the prestressing tendons 101, 201. Then, once they are released, these pretensioning tendons 101, 201 will tend to want to return to their original shapes, which will allow them to compress the elongate bodies 100, 200 of the beams 10, 20.

The prestressing tendons 101 located in the elongated body 100 of the upper beam 10 will compress the latter along an average axis, hereinafter called compression axes A100. The prestressing tendons 201 located in the elongated body 200 of the lower beam 20 will compress the latter along an average axis, hereinafter called compression axes A200.

The relative position of each compression axis A100, A200 with respect to the neutral fiber F100, F200 of the corresponding elongated body 100, 200 is important since it is this which will make it possible to generate a counter-arrow in the beam 10, 20. Au During the aforementioned first step, the positioning of the prestressing tensioners 101, 201 in the prefabrication bench 50 (3 must therefore comply with instructions.

To fully understand this aspect, we can specify the position of prestressing tensioners 101 in the upper beam 10 and the position of the prestressing tendons 201 in the lower beam 20 at the end of the fourth drying step.

As shown in FIGS. 1A and 1B, these tensioners are positioned relative to the elongate bodies 100, 1 10 of the two beams 10, 20 such that the compression axes A100, A200 extend at a distance from the average fibers F100, F200 elongate bodies 100, 200, parallel to these medium fibers F100, F200 and the plane of the interface 300, and in the same yarn as these medium fibers F100, F200.

In this way, the prestressing tendons 1 (31, 201 may exert on the two elongated bodies 100, 200 compression stresses in the same direction and bending stresses tending to bend the two beams 10, 20 in directions opposite (see Figure 5).

In the embodiment shown in FIGS. 1A and 1B, these tensioners are positioned relative to the elongate bodies 100, 1 10 of the two beams 10, 20 such that the compression axes A 100, A 200 extend between the medium fibers F100, F200. In this manner, the upper beam tends to bend upwards and the lower beam tends to bend downward (see Fig. 5).

As shown in FIGS. 1A and 1B, it will be considered here that the two beams 10, 20 are identical and positioned symmetrically with respect to the plane of the interface 300.

So :

- the product (the ₁₀ x d- _i0) of the intensity I ₁₀ of the overall compression force exerted by the prestressing tension 1 (31 of the elongated body 100 of the upper beam 10 of the prefabricating bench 500 by distance d io separating the interface 300 from the compression axis A100 of these pretensioners 101, is substantially equal (to 50%, and even here to 10%)

- the product (i ₂ od ₂ o) of the intensity! ₂ Q of the overall compression force exerted by the prestressing tensioners 201 of the elongated body 200 of the lower beam 20 of the prefabricating bench 500 by the distance d ₂₀ between the interface 300 of the compression axis A200 of these pretensioners 201.

Once the fourth stage of preliminary drying has been completed, it is considered that the concrete is hard enough to withstand the compressive stresses that prestressing tensioners 101, 201 are likely to exert on it.

However, it is considered that it is not hard enough to withstand the tensile stresses that might be exerted on it, because of the bending of the beams. Otherwise formulated, if the tension exerted by the cylinders on the prestressing tensioners 101, 201 was released, the beams would flex until they crack.

The invention then proposes, not to wait for the complete drying of the concrete before releasing this tension, but rather to release this tension while maintaining the elongated bodies 100, 20 (3 in the non-flexed position (bearing against the interface 300), the time for the complete drying of the concrete, for this purpose temporary fixing means are used.

These temporary fastening means can be in various forms, they are preferably implemented in the prefabrication bench 500 before pouring the concrete.

In the embodiment illustrated in Figures 1A and 1B, these temporary fixing means comprise at least one tensor 400 which passes through the two elongate bodies 100, 200 and which is attached thereto.

it is preferably provided at least two tensors 400 respectively positioned near the two ends of the beams 10, 20.

In practice, three tensors 400 are provided here, one of which is situated at half the length of the beams 10, 20. The other two tensors are here positioned at less than one meter from the ends of the beams (here about 70 centimeters from these ends).

These tensors 400 extend orthogonally to the average fibers

F100, F200 beams 10, 20.

they are made here by threaded rods and sheathed, which protrude from either side of the two beams 1 (3, 20 and which are equipped at their ends with nuts respectively bearing on the upper face of the beam 10 from the top and on the underside of the bottom beam.

Alternatively, these tensors could be in other forms (cables ...). Still a variant, instead of tensors, stirrups could be used which would enclose the two elongated bodies.

At the end of the fourth stage of preliminary drying, it is then planned a fifth step of release preloaders 101, 201 so that the latter compress the elongated bodies 100, 200 along the compression axes A100, A200 by simply adhering the concrete on them (we speak of "transfer of the prestressing"").

It is then understood that the preloading tensioners 101, 201 exert on the elongated bodies 100, 200 longitudinal compressive stresses along the compression axes A100, A200.

Because these compression axes are located at a distance from the neutral fibers F100, F200, the prestressing tensioners 101, 201 also exert on the elongate bodies 100, 200 bending stresses directed in opposite directions, which tends to make flex the beams 10, 20 as shown in Figure 5. The tensors 400 then allow to prevent (or at least strongly restrict) this bending, which prevents tensile stresses appear in the concrete and create cracks.

More precisely, because of the distribution of the prestressing tendons

101, 201 on either side of the interface 300, between the two medium fibers F100, F200, the bending stresses that are exerted in one of the beams 10, 20 oppose the bending stresses which exercise in the other beams, so that they cancel each other out.

To understand this phenomenon, we can look at the diagrams shown in Figures 6A to 8D.

In these figures, the beam 10 of the top is represented by a simple segment.

We also consider the ideal case where the two tensors 400 located at the ends of the beams 10, 20 can maintain alone the two beams 10, 20 perfectly straight (the central tensor 400 has no influence in this ideal case) .

FIG. 6A shows the curve C1 of the bending moment exerted by the leftmost tensor 400 on the beam 1 (3 (this bending moment is the moment which tends to rotate the various cross sections of the beam 10 relative to each other, it is relative to the aforementioned bending stresses.) It is observed that it is damaged to the left of the beam 10, that it increases linearly over a length of 70 centimeters (corresponding to the position of the tensor left), then decreases linearly to the right end of the beam.

FIG. 6B shows the curve C2 of the bending moment exerted by the rightmost tensor on the beam 10. It is observed that it is disturbed to the right of the beam 10, that it increases linearly over a length 70 centimeters, then decreases linearly to the left end of the beam.

FIG. 6C shows a curve C3 illustrating the sum of these two bending moments (C3 = C1 + C2),

Finally, in FIG. 6D, the curve C4 of the bending moment generated by the tension exerted by the prestressing tensioners 101 on the beam 10 is represented.

This bending moment is substantially constant over most of the beam 10, and it gradually reduces in "end zones" of the beam 10 (which is due to the fact that the prestressing tendons 101 gradually adhere to the concrete and deliver him progressively efforts).

It has been observed that each end zone extends for a length of about 70 centimeters from the corresponding end of the beam 10. This is the reason why the tensors 400 have been placed at about 70 centimeters from each other. these ends. Thus, the curve C4 corresponds to the inverse of the curve C3, so that the bending moments which are exerted on the beam 10 cancel each other out.

In FIGS. 7A to 7D, the beams 10, 20 are represented by simple segments, and there is shown:

in FIG. 7A, the curves C10, C1 1 of the bending moments exerted by the leftmost tensor on each of the two beams,

in FIG. 7B, the curves C20, C21 of the bending moments exerted by the rightmost tensor on each of the two beams,

in FIG. 7C, the curves C30, C31 corresponding to the sum of the aforementioned bending moments (which can be written as follows: C30 = 010 + C20 and C31 = C1 1 + C21), and

in FIG. 7D, the curves C40, C41 of the bending moments induced by the prestressing tendons 101, 201 on the two beams 10, 20.

In summary, thanks to the tensors 40 (3, the two beams 10, 20 form a monolithic assembly in which only compressive stresses are exerted. These two beams 10, 20 can then be demolded together, with the tensors 400, so as to release the prefabrication bench 500 so that the latter can be quickly reused.

The two beams 10, 20 are then placed in a storage space, so that their drying can be prolonged over a period of several days (until the mechanical properties of the concrete reach values close to the nominal values for which concrete was designed).

Finally, once the concrete is considered sufficiently dry and sufficiently solid, there is provided a discharge step during which the tensors 400 are released.

This step preferably takes place in or near the storage space (before transporting the beams to the building site).

It consists here in unscrewing the nuts located at the ends of the threaded rods of the tensors 400, and in removing these threaded rods from their sheaths.

During this step, the two beams 10, 20 separate naturally and flex in the manner illustrated in Figure 5. The concrete being sufficiently dry, the tensile stresses resulting from the bending of the beams do not create any crack in the concrete.

For its transport, the top beam 10 is transported in its position shown in Figure 5, being supported by its two ends.

For transport, the beam 20 of the bottom is first returned before being transported by being supported by its two ends.

Other embodiments of the preceded manufacture of the beams 10, 20 are conceivable.

Thus, in a second embodiment of the invention illustrated in FIGS. 2A (seen from above) and 2B, the two beams 10, 20 can be manufactured in a juxtaposed position next to each other, and not not in superimposed position one above the other.

In this mode, the interface 3 (30 will preferably be put in place before pouring the concrete, and the concrete will preferably be cast simultaneously in the two molding spaces.

In a third embodiment of the invention illustrated in FIGS. 3A and 3B, the two beams are made substantially in the same way as the beams 10, 20 illustrated in FIGS. 1A and 1B. However, in this mode, no tensor and no stirrup are used to maintain the bodies 1 10, 210 of the two beams bearing against the interface 310, the time of complete drying of the concrete.

On the contrary, the bodies 1 10, 210 of the two beams are held in abutment against the interface 310 by adhesion through the interface 310.

For this, the interface 310 is interrupted and does not completely separate the molding spaces of the two beams. It thus delimits at least one passage 410 between these two molding spaces. H here delimits two passages 410 located at both ends of the beams and which each extend for example 70 centimeters in length.

During the pouring step, concrete is thus placed in these passages 410. During the preliminary drying step, this concrete dries and forms means for temporarily fixing the body 1 10 to the body 210.

At the end of the complete drying of the concrete, these temporary fixing means are deactivated by cutting the concrete located in each passage 410 (by means for example of a diamond saw).

In a fourth embodiment of the invention illustrated in Figures 4A and 4B, the two beams are made substantially in the same way as the beams 10, 20 illustrated in Figures 1A and 1B.

However, in this mode, the prestressing tendons 101, 201 are located, not between the average fibers of the elongate bodies 120, 220 of the two beams, but on either side thereof.

Therefore, when the tension in the pretensioners is released, these tensioners tend to bend the two beams in opposite directions towards each other.

it is then useless to provide, in this mode, a tensor which would pass in the center of the two beams.

In this mode, the interface 320 provided between the two beams then comprises two cleats 321 located near the center of the beams, on either side of this center. It further comprises a layer 322 of retardant product. This product ensures, when the tension is released and the two beams bend very slightly (due to the elasticity of tensors 40 (3), a good separation of the two bodies 120, 220.

In a fifth embodiment of the invention illustrated on the 8A, 8B and 8C, the two beams are manufactured in substantially the same manner as the beams 10, 20 illustrated in FIGS. 1A and 1B.

However, in this mode, each of the prestressing tensioners 101, 201 extends, not in a rectiiigne manner, but in broken line (in a succession of segments inclined relative to each other).

More specifically, the prestressing tendons 101, 201 are arranged so as to extend:

- in a straightforward way and parallel to the interface 330 on their central parts,

- Rectiiigne manner and inclined relative to the interface 330 on their ends.

Two deviators 600 are used for this purpose, which are for example located more than 2 meters apart, and each surrounds the prestressing tendons 101, 201 so as to keep them parallel to the interface 330 on their parts. between the two medium fibers.

These deviators 600 may for example be formed by metal rings.

The ends of the prestressing tendons 101 of the upper beam are however located near the upper face of this beam, while the ends of the prestressing tendons 201 of the bottom beam are located near the underside of this beam. beam.

The prestressing tendons 101 and the prestressing tendons 201 thus extend symmetrically with respect to the interface 330.

Thanks to these deviators 600, the cutting forces exerted by the pretensioners 101, 201 at the ends of the beams are very small. Thus, when they flex, the beams present forms of cocked hat.

In this embodiment, a single tensor 400 situated at the mid-length of the beams is sufficient to hold the two beams in abutment against the interface 330.

Claims

1, Method of manufacturing structural elements (10, 20), comprising:

a step of tensioning at least one first prestressing tensioner (101) and at least one second prestressing tensioner (201) in a prefabricating bench (5 (30),

a step of positioning an interface (300, 310, 320) in the prefabrication bench (500) so that it delimits, completely or partially, first and second molding spaces in which respectively each first prestressing tensioner (101) and each second prestressing tensioner (201),

a step of prior drying of said material which, on hardening, forms first and second elongate bodies (100, 1 10, 120, 200, 210, 220) which are respectively located in the first and second molding spaces and respectively first and second medium fibers (F100, F200), and

the two elongated bodies (100, 1 10, 120, 200, 210, 220) being held in abutment against said interface (300, 310, 320) by temporary fixing means, a step of releasing the first and second tensioners of prestressing (101, 201) so that they exert:

longitudinal compressive stresses on the first and second elongate bodies (100, 1 10, 120, 200, 210, 220) respectively along first and second compression axes (A100, A200), distinct from the first and second middle fibers ( FI 00, F200), and

bending stresses on the first and second elongate bodies (100, 1 10, 120, 200, 210, 220) in opposite directions.

2. Manufacturing method according to the preceding claim, wherein the first and second pretensioners (101, 201) are positioned relative to the two elongated bodies (100, 1 10, 120, 200, 210, 220) in such a way that the first and second compression axes (A100, A200) extend additionally to the first and second middle fibers (FI 00, F200) and to said interface (300, 310, 320).

3. Manufacturing method according to the preceding claim, wherein the first and second pretensioners (101, 201) are positioned relative to the two elongate bodies (100, 1 10, 120, 200, 210, 220) in such a way that the first and second compression axes (A100, A200) extend in the same plane as the first and second middle fibers (F100, F200), preferably between the first and second middle fibers (F100, F200).

4. Manufacturing method according to one of the two preceding claims, wherein the first and second pretensioners are positioned and tensioned in the prefabrication bench so that, before the release step:

the product of the intensity of the overall force exerted by the at least one first prestressing tensioner (101) on the prefabrication bench by the distance separating said interface (300, 310, 320) from the first compression axis (A100 ), and

the product of the intensity of the overall force exerted by said at least one second prestressing tensioner (201) on the prefabrication bench by the distance separating said interface (300, 310, 320) from the second compression axis (A200 )

are substantially equal to 50%, and preferably to 10%.

5, The manufacturing method according to one of the preceding claims, wherein the positioning step and the casting step are performed concomitantly, said interface being positioned when the first molding space is filled by said material and that the second molding space is still empty.

6. Manufacturing method according to one of the preceding claims, wherein, in the tensioning step, diverters (800) are used to force each of said prestressing tendons (101, 201) to extend according to a broken line.

7. Manufacturing method according to one of claims 1 to 6, wherein the temporary fixing means comprise stirrups which enclose the two elongate bodies (100, 120, 200, 220) and / or tensors (400) which pass through at least a part of the two elongated bodies (10 (3, 120, 200, 220) and which hang thereon.

8. Manufacturing process according to one of claims 1 to 6, in which, at the positioning step, said interface (310) is positioned to partially delimit the first and second molding spaces by providing at least one passage (410) between the first and second molding spaces, and prior drying step, the material located in each passage (410) forms at least part of the temporary fixing means.

9. Manufacturing method according to one of the preceding claims, wherein there is provided, after the release step, a step of simultaneously demolding the structural elements (10, 20) and said interface (300, 310, 320 ) outside the prefabrication bench, and then an additional drying step of the structural elements (10, 20) in a remote storage location of the prefabrication bench (500).

10. The manufacturing method according to the preceding claim, wherein, after the additional drying step, there is provided a discharge step during which the temporary fixing means are deactivated.

1 1. Manufacturing method according to claims 7 and 10, wherein the temporary fixing means are deactivated by removing the stirrups and / or releasing the tensors (40 (3) of the elongated bodies (100, 120, 200, 220).

12. The manufacturing method according to claims 8 and 10, wherein the temporary fixing means are deactivated by breaking the material located in each passage (410).

13. Manufacturing method according to one of the preceding claims, wherein said interface (300, 310, 320) comprises a planar wall and / or a layer of flexible material and / or a layer of retardant product or setting inhibitor adapted to slow down or prevent hardening of the material.

14. The manufacturing method according to one of claims 1 to 13, wherein, during the preliminary drying step, the two elongate bodies (100, 1 10, 120, 200, 210, 220) are maintained the one above the other.

15. The manufacturing method according to one of claims 1 to 1 3, wherein, during the preliminary drying step, the two elongate bodies (100, 1 10, 120, 200, 210, 220) are maintained l one next to the other.