EP0501879B1 - Concrete and method of pre-stressing; container made of such concrete - Google Patents

Description

FIELD OF THE INVENTION

The invention generally relates to high-performance concretes, that is to say which must undergo significant mechanical stresses such as prestressed concretes. It relates in particular to high durability concretes which are particularly suitable for making drums or enclosures which must contain hazardous industrial waste, solid or liquid, and in particular radioactive waste or very toxic chemical waste. It also applies to all civil engineering works.

PRIOR ART

Traditional concretes generally consist of a mixture of cement, natural or artificial mineral aggregates such as gravel, sand, any additives and a hydraulic binder such as water.

• 1°) Le mélange est effectué à l'aide d'un malaxeur pour obtenir un mélange intime des différents composants.
• 2°) Le béton ainsi obtenu est transféré dans le coffrage dont la forme correspond au produit à obtenir.
• 3°) Le béton est coulé dans le coffrage, avec éventuellement l'apport d'une vibration au cours du remplissage.
• 4°) Le produit moulé est démoulé par ablation du coffrage.
• 5°) Le béton est durci au cours d'une "cure" qui peut durer de un jour à un mois, suivant le type de béton utilisé et du produit fabriqué.

The traditional process for manufacturing a poured concrete product is as follows.

• 1 °) The mixing is carried out using a mixer to obtain an intimate mixture of the various components.
• 2) The concrete thus obtained is transferred into the formwork, the shape of which corresponds to the product to be obtained.
• 3 °) The concrete is poured into the formwork, possibly with the addition of a vibration during filling.
• 4 °) The molded product is removed from the mold by removing the formwork.
• 5 °) The concrete is hardened during a "cure" which can last from one day to a month, according to the type of concrete used and the product manufactured.

The quality of the product produced with this concrete depends very much on the conditions of curing of the concrete. Indeed, microcracks can appear and alter the final mechanical properties of the product. These microcracks then lead to the start of a "ruin" phenomenon in the structure of the product. This ruin phenomenon can be prolonged by the appearance of macrocracks caused by mechanical stresses reaching the concrete resistance limits.

It is known to strengthen concrete by the introduction of fibers into the mixture to improve the mechanical strength and greatly reduce the appearance of microcracks before the appearance of visible cracks. On the other hand, it does not make it possible to delay the appearance of macrocracks in the structure of the product when compressive or tensile forces are applied to the latter.

As described in the French patent application, published under number 2 640 410, such concrete is used to manufacture storage containers for radioactive waste. This container comprises a barrel provided with an opening for the introduction of waste and a sealed closure cover. The container is entirely made of concrete reinforced with metallic fibers.

However, there is a need to improve the performance of this type of concrete to manufacture containers whose characteristics of solidity and durability would be more efficient than the previous ones. It is therefore proposed to supply a prestressed concrete which may have mechanical characteristics superior to prestressed concretes currently manufactured. However, the main method of prestressing a concrete block currently consists of drowning in the block one or more sheaths inside of which there are metal cables which are tensioned once the concrete has hardened. , so as to prestress the concrete block in compression.

The object of the invention is to provide a concrete which can meet the current requirements imposed for the production of radioactive waste containers, while avoiding using the type of concrete prestressing described in the previous paragraph.

SUMMARY OF THE INVENTION

• introduire de façon aléatoire dans le mélange des fibres dont les formes ou les dimensions peuvent évoluer dans le temps sous l'action d'un transfert d'énergie ;
• couler le béton dans un coffrage ;
• démouler le béton coulé ;
• durcir le béton durant une période de cure ;
• procéder à un traitement thermique pour que les dimensions ou la forme des fibres évoluent, de manière à imposer des contraintes mécaniques de compression dans le béton.

To this end, a first object of the invention is a method of prestressing a concrete made from a determined mixture, consisting of:

• randomly introducing fibers into the mixture, the shapes or dimensions of which can change over time under the action of an energy transfer;
• pour the concrete in a formwork;
• unmold the poured concrete;
• harden concrete during a curing period;
• carry out a heat treatment so that the dimensions or the shape of the fibers change, so as to impose mechanical compressive stresses in the concrete.

It is thus possible at each location of the concrete product obtained to impose a compressive stress which can have the same effects as the compressive stresses imposed by the metallic cables in tension used in traditionally prestressed concrete blocks.

• d'un agglomérat de sable, de gravier et de ciment réunis au moyen d'un liant hydraulique ;
• de fibres artificielles noyées dans l'agglomérat dans des positions diverses et aléatoires.

A second main object of the invention is therefore a fiber concrete consisting of:

• an agglomerate of sand, gravel and cement united by means of a hydraulic binder;
• artificial fibers embedded in the agglomerate in various and random positions.

According to the invention, the fibers used are of the type of those whose shapes or dimensions can change over time under the action of an energy transfer, to create a compressive stress in the concrete.

• des fibres en alliage métallique à mémoire de forme ;
• des fibres thermorétractables.

Two kinds of fibers can be used:

• shape memory metal alloy fibers;
• heat-shrinkable fibers.

The preferred shape of these fibers is a very flat strip whose length is between one and ten centimeters.

The third main object of the invention is a storage container for radioactive waste, comprising a barrel provided with an opening for introducing the waste and with a sealed closure cover for this opening. According to the invention, the container is made entirely of concrete, as just summarized in the previous paragraphs.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The invention and its technical characteristics will be better understood on reading the following description, illustrated by the single figure representing a container according to the invention.

To make the concrete according to the invention, the basic materials necessary for making a traditional concrete are used. These materials are made up of mineral aggregates, that is to say natural or artificial gravels, sand and cement. To this mixture of solid powder products are added the loose fibers, that is to say that these can take any position. Once this mixing has been carried out, the hydraulic binder, preferably water, is then added. The whole is kneaded in a machine so as to obtain an intimate mixture of these various components.

The second phase of the process according to the invention naturally consists in pouring the fresh concrete into the formwork, the internal shape of which corresponds to the external shape of the product to be obtained. To facilitate this operation, the formwork or mold is generally placed on a vibrating table in operation. The vibrations make it possible to produce a very compact concrete and to obtain a product having a very low porosity. A beautiful surface appearance and good mechanical resistance can also be obtained by using this vibrating table. Internal vibration means can be used for large parts. It is noted that the position of the fibers is arbitrary.

Certain products require the use of an internal reinforcement, such as metal reinforcement, in order to reinforce the mechanical properties of the product obtained. The reinforcement is then introduced into the mold before the concrete is poured.

After a first drying period, the concrete is released, that is to say that the formwork is dismantled and removed from the poured concrete.

We then proceed to a drying phase of the product obtained to harden the concrete. This phase is a cure that can last from one day to a month.

The duration of the cure varies according to the type of cement used and the type of product manufactured. During this concrete cure, a hydration reaction of the cement takes place, also called "setting of the concrete".

According to the invention, a heat treatment is then carried out allowing energy transfer by varying the temperature of the fibers, so that their shapes or dimensions change. Indeed, under the action of a slight increase, or decrease, in temperature, such a type of fiber takes a retracted form or regains an initial form.

A first category of fibers used for this purpose is constituted by metallic fibers, called "shape memory". Indeed, when a metal or an ordinary alloy is subjected to a mechanical stress higher than its elastic limit, it undergoes a plastic deformation which persists after the cessation of the stress. Its shape and size practically no longer evolve if the alloy or metal is subjected to any heat treatment again. On the other hand, shape memory alloys do not have this behavior, because in a certain range of temperatures, a sample of such a material can undergo an apparently plastic deformation of several percent and fully recover its initial shape by simple subsequent heating. This phenomenon called shape memory is associated with a reversible structural transformation of the "thermoelastic martensitic" type, occurring between a first temperature T₁, at which the sample was formed, and a second temperature T₂ higher or lower than T₁, to which the sample must be heated or cooled in order for it to regain its shape.

In the method according to the invention, fibers are used whose initial shape was that which one wishes to see them set so that the concrete is compressed, that is to say prestressed. This shape is preferably a retracted, curved, more or less curled shape, compared to an intermediate shape which would be rather linear or completely straight.

Among the materials used, mention may be made of alloys of the NiTi type (nickel, titanium and dopants) and CuZnAl (brass, aluminum and dopants). For this type of alloy, the temperature variation can be an elevation which can be between 50 and 70 ° C. That is to say, for a concrete product at 20 ° C, heat treatment at a temperature between 70 and 90 ° C can achieve the return to the initial shape of the metallic fibers cast in concrete.

The same result can also be obtained by not heating, but cooling to a temperature between -10 and -30 ° C.

The duration of the application of the heat treatment depends on the shape of the product produced with the concrete. In general, the whole product, that is to say, even the core of the product, must reach your phase transformation temperature.

If the manufactured product is transportable, the heat treatment can be applied in an oven. On the other hand, if the product is molded on site, therefore not transportable due to its weight and dimensions, the process of applying the heat treatment can be done by means of high frequencies, microwaves, using coats and heating sheaths.

Three-point bending tensile tests were carried out on 4 x 4 x 16 cm test pieces. The concrete used had your following constitution: - CLC 45 cement 890 g - Bayeux sand 2,660 g - water 450 g - thinner 8 g

• pendant 24 heures à 80 % dans un emballage étanche pour éviter la dessication ;
• puis 24 heures à température ambiante pour refroidissement avant essais mécaniques.

The incorporation of shape memory fibers is carried out after the mixture of the four components. The heat treatment was carried out as follows:

• for 24 hours at 80% in a sealed package to avoid drying out;
• then 24 hours at room temperature for cooling before mechanical tests.

The tests were carried out at the COGEMA la Hague concrete laboratory in a 1.5 liter mixer.

The table below summarizes the values obtained:

For fiber-free specimens, the heat treatment improves the flexural strength of the reference concrete or mortar (improvement of the cement "setting" process) by around 1 MPa.

For specimens with fibers, the tests carried out with fiber concrete at a dosage of 50 kg and 100 kg of fibers per m³ of finished concrete shows that the results are more conclusive (apart from experimental errors) with fibers than without fibers and with heat treatment than without heat treatment.

A second category of fibers used is heat-shrinkable fibers. This type of fiber undergoes, with a slight rise or decrease in temperature, a reduction in its dimensions. Yes the shape is not straight, and preferably well bent, the heat-shrinkable fibers, by shrinking, impart a compressive stress to the concrete in which they have been embedded.

The shape of the fibers used is preferably that of flat strips, the length of which can be from one to ten centimeters. The thickness can be less than a tenth of a millimeter.

Any product can be made using such concrete. The invention however provides a special application for the production of a radioactive waste storage container. Indeed, with reference to the single figure, the container, object of the invention, mainly comprises a barrel 10, the upper opening of which is closed by a cover 12. The closure is sealed so as to be able to store radioactive waste of low or medium activity, coated in a filling material. In the example shown in this figure, the container has a rectangular shape, a flat bottom and a side wall of square section. The upper end of the latter delimits the opening allowing closing using the cover 12.

According to the invention, the entire container, that is to say the barrel 10 and the cover 12, is made of a concrete reinforced with fibers 20 either in shape memory or heat shrinkable.

The barrel 10, like the cover 12, are produced by molding. In particular, the upper end of the lateral wall of the barrel 10 has a stepped shape defining successively, going from the outside towards the inside of the barrel 10, a flat end face 13 and a flat bearing face 7 parallel to the first planar face 13, but located back from the latter. The bearing surface 7 is connected to the upper surface 13 by an inclined inner peripheral edge 6 forming a Z in section with its first two surfaces 7 and 13. The diameter of the inclined edge 6 increases as it approaches the bearing surface 7, so that this inclined edge 6 makes with the axis of the barrel 10 an angle of at least 10 °.

The cover 12 also has a peripheral peripheral region comprising, starting from its upper surface, an inclined external peripheral edge 11 and a vertical edge 15 set back with respect to the inclined edge 11. These edges 11 and 15 are connected by a second flat bearing surface 5 parallel to the upper and lower faces of the cover 12. The diameter of the inclined edge 11 increases as it approaches the second bearing surface 5, so that this inclined edge 6 forms with the axis of the cover 12 an angle of at least 10 °.

When the cover 12 is placed on the barrel 10, the lower part of the cover 12 delimited by the edge 15 fits into the opening formed at the top of the barrel 10, until the horizontal support surface 5 of the cover 12 comes to bear on the horizontal support surface 7 of the barrel 10. As can be seen in this figure, the inclined edges 6 and 11, which have the same height, are then opposite one of the 'other and delimit between them an annular space 14 of dovetail shape whose width is substantially constant from the upper face 13 of the container 10 to the bearing surface 7. This annular space 14 constitutes a keying groove.

In order to tightly fix this cover 12 on the barrel 10, a keying joint made in the same material as the rest of the container, that is to say concrete reinforced with 20 heat-shrinkable or shape memory fibers.

This embodiment of the keying joint in the annular space 14 open upwards makes it possible to ensure the sealed closure of the container, without the need to use a formwork. In addition, the shape of the keying joint makes it possible to avoid any risk of the cover coming off when this joint is made.

Advantageously, the cover 12 has at its center an opening 8 of large dimension in which a keying groove 9 is formed. In this way, the filling of the container 10 can be carried out after sealing of its cover 12. When the waste has been introduced through the opening 8, the filling material is introduced in turn until the opening 8 is completely closed. By using as filling material the concrete reinforced with shape memory or heat shrinkable fibers 20 identical to that which forms the container proper, a homogeneous assembly is thus produced in which the risks of cracking and breaking are eliminated.

Finally, to allow handling of the container, various means can be provided. Among these means, there may be mentioned, by way of example, rods or rings 4 sealed on the flat face. upper end 13 of the barrel 10. A groove or a handling groove 3 can be formed by molding on the outer peripheral surface of the barrel 10, near the upper end face 13.

This description of a container is only an exemption from making objects which can be produced using the concrete according to the invention. It is understood that it is thus possible to prestress any object produced in general with ordinary concrete by simply introducing this type of fiber into the mixture before making the concrete. A heat treatment applied subsequently after the object has dried, makes it possible to obtain, inside the concrete, compressive stresses equivalent to the prestresses imposed by the traditional methods used for prestressed concrete.

Claims (8)

1. Process for prestressing a concrete produced from a given mixture consisting of introducing in a random manner into the mixture fibres (20), whose shapes or sizes can evolve in time under the action of an energy transfer, pouring the concrete into a form, demoulding the cast concrete, hardening the concrete during a curing period and then carrying out a heat treatment in such a way that the dimensions or shape of the fibres (20) evolve, so as to impose mechanical compressive stresses within the concrete.
2. Fibre concrete constituted by an agglomerate of sand, gravel and cement joined by means of a hydraulic binder, artificial fibres (20) embedded in the agglomerate at various, random positions, characterized in that the fibres (20) are of the type whose dimensions or shape can evolve in time under the action of an energy transfer in order to create a compressive stress in the concrete.
3. Concrete according to claim 2, characterized in that the fibres (20) are made from a shape memory metal alloy.
4. Concrete according to claim 3, characterized in that the shape memory metal alloy is an alloy of titanium and nickel.
5. Concrete according to claim 3, characterized in that the shape memory alloy is an alloy of brass and aluminium.
6. Concrete according to claim 2, characterized in that the fibres (20) are thermoretractable.
7. Concrete according to any one of the claims 2 to 6, characterized in that the fibres (20) are very flat lamellas, whose length is between 1 and 10 centimetres.
8. Radioactive waste storage container, incorporating a drum (10) provided with a waste introduction opening (8) and a cover (12) for tightly sealing the opening (8), characterized in that it is made from a concrete according to any one of the claims 2 to 7.

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