FR2673223A1 - CONCRETE AND PRECONTROLLING METHOD, CONTAINER MADE WITH THE CONCRETE. - Google Patents

Abstract

The concrete according to the invention has superior durability and mechanical resistance characteristics equivalent to prestressed concrete. It comprises fibers (20) embedded in traditional concrete, these fibers being either shape memory metal alloy fibers or heat shrinkable fibers. Subsequent heat treatment allows these fibers to take a shape allowing compressive stresses to be imposed throughout the volume of the poured concrete. Application to the manufacture of radioactive waste containers.

Description

i

  CONCRETE AND METHOD FOR PRETREATMENT,

CONTAINER MANUFACTURED WITH THIS CONCRETE

DESCRIPTION

FIELD OF THE INVENTION

  The invention generally relates to high performance concretes, that is to say to undergo significant mechanical stresses such as prestressed concrete It particularly concerns high durability concretes particularly suitable for producing drums or enclosures to contain hazardous industrial waste, solid or liquid, and in particular radioactive waste or highly toxic chemical waste. It also applies to all

civil engineering.

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

like water.

  The traditional manufacturing process

  of a cast concrete product is as follows.

  1) The mixing is carried out using a kneader to obtain an intimate mixture of the various components. 2) The concrete thus obtained is transferred into the formwork whose shape corresponds to the product

to obtain.

  30) The concrete is poured into the formwork, possibly with the contribution of a vibration during

filling.

  ) The molded product is demolded by ablation

formwork.

  Concrete is cured during a "cure" that can last from one day to one month, depending on the type of concrete used and the product manufactured. The quality of the product manufactured with this

  concrete depends a lot on concrete cure conditions.

  Indeed, microcracks can appear and

  alter the final mechanical properties of the product.

  These microcracks then lead to the beginning of a

  phenomenon of "ruin" of the product structure.

  This phenomenon of ruin can be prolonged by the appearance of macrofissures driven by mechanical stresses reaching the limits of

concrete resistance.

  It is known to strengthen concretes by the introduction of fibers in 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 allow to delay the appearance of macrocracks in The product structure When efforts in

  compression or traction are applied to the latter.

  As described in French Patent Application No. 2,640,410, such a concrete is used to make radioactive waste storage containers. This container comprises a barrel provided with an opening for introduction.

  waste and a sealing cover.

  The container is made entirely of reinforced concrete

by metal fibers.

  However, there is a need to improve the performance of this type of concrete to manufacture containers whose characteristics of strength and durability would be more efficient than the previous ones. It is therefore proposed to provide prestressed concrete which may have mechanical characteristics. superior to prestressed concretes currently manufactured Gold, The main method of prestressing a concrete block is currently to embed in the b Loc one or more ducts inside which there are metal cables which are energized a concrete hardening is obtained, so that

  prestress in compression The concrete block.

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

SUMMARY OF THE INVENTION

  For this purpose, a first object of the invention is a method of prestressing a concrete made from a specific mixture, consisting in: introducing into the mixture fibers whose shapes or dimensions may change in time under the action of a transfer of energy; pour concrete into a formwork; unmolded Poured concrete; harden Concrete during a cure period; heat treatment so that the dimensions or the shape of the fibers evolve, so as to impose mechanical stresses of

compression in the concrete.

  It is thus possible at each location of the concrete product obtained to impose a compressive stress which may have the same effects as the compressive stresses imposed by the tensile metal cables used in the concrete blocks.

traditionally prestressed.

  A second main object of the invention is therefore a fiber concrete consisting of: an agglomerate of sand, gravel and cement joined by means of a hydraulic binder; artificial fibers embedded in the agglomerate. According to the invention, the fibers used are of the type of those whose shapes or dimensions can evolve in time under the action of a transfer of energy, to create a constraint

compression in concrete.

  Two kinds of fibers can be used: metal alloy fibers with shape memory;

heat-shrinkable fibers.

  The preferred form of these fibers is a very flat lamella whose length is included

between one and ten centimeters.

  The third main object of the invention is a storage container for radioactive waste, comprising a drum provided with a waste introduction opening and a sealing cover of this opening. According to the invention, the container is made completely with concrete, as it comes

  to be summarized in the preceding paragraphs.

DETAILED DESCRIPTION OF A MODE OF

CARRYING OUT THE INVENTION

  The invention and its technical characteristics will be better understood when reading

  the following description, illustrated by the single figure

  representing a container according to the invention.

  In order to produce the concrete according to the invention, the basic materials required for the preparation of a traditional concrete are used. These materials consist of mineral aggregates, that is to say natural or artificial gravels, sand and After mixing, the hydraulic binder, preferably water, is added. The mixture is kneaded in a machine so as to obtain an intimate mixture.

of these different components.

  The second phase of the process according to the invention is of course to pour the fresh concrete into the form whose inner shape corresponds to the external shape of the product to obtain To facilitate this operation, the formwork or the mold is generally placed on a vibrating table in operation Vibrations make it possible to produce very compact concrete and obtain a product with very low porosity. A good surface appearance and good mechanical resistance can be achieved.

  also obtained using this vibrating table.

  Internal vibration means can be used

  for Large parts.

  Some products require the use of an internal reinforcement, such as a metal reinforcement, in order to reinforce the mechanical properties of the product obtained The reinforcement is then introduced into the mold before pouring the concrete. After a first drying period, the concrete is demolded, that is to say that the formwork

  is dismantled and removed from poured concrete.

  This is followed by 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

  cement used and the type of product manufactured.

  During this cure of the concrete, a hydration reaction of the cement occurs, also called

concrete".

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

initial form.

  A first category of fibers used for this purpose consists of metal fibers, called "shape memory" Indeed, when a metal or an ordinary alloy is subjected to a mechanical stress greater than its elastic limit, it undergoes a plastic deformation which persists after the cessation of the stress Its shape and its dimension do not evolve practically any more if one submits again the alloy or the metal to a heat treatment of any kind On the other hand, the alloys with memory of shape They do not have this behavior because, in a certain temperature range, a sample of such a material can undergo a seemingly plastic deformation of several percent and recover completely its initial shape by simple subsequent heating. This phenomenon called shape memory is associated with a reversible structural transformation of the "thermo-elastic martensitic" type, occurring between a first temperature T 1, to which the sample had been formed, and a second temperature T 2 greater or less than T 1, to which the sample must be heated or cooled for

that he regains his form.

  In the process according to the invention, fibers whose initial shape is the one which one wishes to see them take so that the concrete is compressed, that is to say prestressed This form is preferably a retracted form, curved, more or less curled up, compared to an intermediate form that would be rather linear or

completely straight.

  Among the materials used, mention may be made of alloys of the Ni Ti type (nickel, titanium and dopants) and Cu Zn AL (brass, aluminum and dopants). For this type of alloy, the variation in temperature may be an elevation which can be between 50 and 70 ° C. That is, for a concrete product at 20 ° C., a heat treatment at a temperature of between 70 and 90 ° C. can make it possible to obtain the return to the initial form fibers

  cast metal in concrete.

  The same result can be obtained also by proceeding, not to a heating, but to a cooling at a temperature between

-10 and -30 C.

  The duration of the application of heat treatment depends on the shape of the product made with the concrete In general, it is necessary that the entire product, that is to say, even the core of the product, reaches the temperature of transformation

phase.

  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 the spot, and therefore not transportable by its weight and its dimensions, the process of application of the heat treatment can be done by means of high frequencies, of microwaves, by using coats and heating sheaths. A second category of fibers used is heat-shrinkable fibers. This type of fiber undergoes, with a slight rise or fall in temperature, a reduction in its dimensions. If the shape is not rectilinear, and preferably curved, the heat-shrinkable fibers, by retracting, print a compression constraint

  to the concrete in which they were drowned.

  The shape of the fibers used is preferably that of flat slats whose length may be from one to ten centimeters.

  may be less than one-tenth of a millimeter.

  However, the invention provides a special application for the production of radioactive waste storage container. Indeed, with reference to the single figure, the container, object of the invention, mainly comprises a barrel 10 whose upper opening is closed by a cover 12 The shutter is sealed to be able to store radioactive waste of low or medium activity, embedded in a filling material In the example represented in this figure, the container is of parallelepipedal shape,

  flat bottom and side wall of square section.

  The upper end of the latter delimits the opening allowing the closure using the

cover 12.

  According to the invention, the entire container, that is to say the barrel 10 and the lid 12, is made of a fiber-reinforced concrete

  Either with shape memory or heat-shrinkable.

  The barrel 10, like the lid 12, are made by molding. In particular, the upper end of the side 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, but set back from, the first planar face 13 The support surface 7 is connected to the upper surface 13 by an inner peripheral edge inclined 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 an angle with the axis of the barrel 10

at least 10.

  The cover 12 also has a stepped peripheral zone comprising, starting from its upper surface, an inclined outer peripheral edge 11 and a vertical edge 15 set back from the inclined edge 11. These edges 11 and 15 are connected by a second surface. support plane 5 parallel to the upper and lower faces of the cover 12 The diameter of the inclined edge 11 increases while approaching the second bearing surface 5, so that this inclined edge 6 is made with

  the axis of the cover 12 an angle of at least 10.

  When the lid 12 is placed on the barrel 10, the lower portion of the lid 12 delimited by the edge 15 engages in the opening formed at the top of the barrel 10, until the horizontal bearing surface 5 of the cover 12 comes to rest on the horizontal bearing surface 7 of the barrel 10 As seen in this figure, the inclined edges 6 and 11, which have the same height, are then opposite one of the another 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 support surface 7.

  This annular space 14 constitutes a keyway groove. In order to seal this cover 12 on the barrel 10, a keying joint made of the same material as the remainder of the container, that is to say reinforced concrete, is cast into the annular space 14 above. 20 heat-shrinkable fibers

or shape memory.

  This embodiment of the keyway seal in the annular space 14 open up ensures the sealing of the container, without the need to use a formwork Moreover, the shape of the keyway seal avoids any risk of sending the cover, when this seal

is realised.

  Advantageously, the cover 12 has at its center an opening 8 of large dimension in Which is formed a keying groove 9. In this way, the filling of the container 10 can

  be carried out after sealing the lid 12.

  When the waste has been introduced through the opening 8, the filler material is introduced in turn until the opening 8 is completely closed using as filling material the fiber-reinforced concrete 20 with shape memory or thermo-retractab identical to that which forms the container itself, one realizes a homogeneous set in which the risks of

  cracks and breaks are removed.

  Finally, to allow the handling of

  container, different means can be provided.

  Among these means, there may be mentioned, for example, rods or rings 4 sealed on the flat top end face 13 of the barrel 10. A groove or a handling groove 3 may be formed by molding on the outer peripheral surface of the barrel. 10, close to the upper end face

13.

  This description of a container is

  An example of embodiment of objects that can be manufactured using the concrete according to the invention It is understood that it is possible to preload any object made generally with ordinary concrete by simply introducing this type. of fibers in the mixture before the concrete is made A heat treatment applied later after the drying of the object, makes it possible to obtain inside the concrete compressive stresses equivalent to the prestresses imposed by the traditional methods used for the concrete. prestressed.

Claims (5)

  1 Method for prestressing a concrete made from a specific mixture, characterized in that it consists in: introducing into the mixture fibers (20) of the type whose dimensions or shape may change over time under the action of a transfer of energy; listen Concrete in formwork; unmolded Poured concrete; harden the concrete during a cure period; heat treatment so that the dimensions or the shape of the fibers (20) evolve so as to impose mechanical stresses of compression in the concrete.   2 Fiber concrete, consisting of: aggregate of sand, gravel and cement combined by means of a hydraulic binder; artificial fibers (20) embedded in the aggtomerate, characterized in that the fibers (20) are of the type whose dimensions or shape can change over time under the action of a transfer of energy, to create compressive stress in concrete. Concrete according to Claim 2, characterized in that the fibers (20) are in one   a L Metal bonding with shape memory.   4 Concrete according to claim 3, characterized in that the metal alloy with memory   form is an alloy of titanium and nickel.   Concrete according to claim 3, characterized in that the shape memory alloy   is an alloy of Brass and aluminum.   Concrete according to claim 2, characterized in that the fibers (20) are heat-shrinkable. 7 Concrete according to any of the   Claims 2 to 6, characterized in that the fibers   (20) are very flat slats whose length   is between one and ten centimeters.   8 Storage container for radioactive waste, comprising a barrel (10) provided with an opening (8) for introducing waste, and a lid (12) for sealing the opening (8), characterized in that it is made with a   beton according to any one of claims 2 at 7.