Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
coNcKErE AND ITS PRESTRESSING PROCESS AND CONTADNER
MANUF~CTUK~ WrrH THIS CONCRETE
DESCRIPTION
_ ELD OF THE INVENTION
In general te~ms the invention relates to high performance ccncretes,
i.e. having to withstand high mechanical stresses, such as in the case
of prestressed concretes. It more particularly relates to high dura-
bility concretes more particularly suitable for producing drums or
enclosures having to contain solid or liquid, dangerous industrial
waste and in particular highly toxic chemical or radioactive waste.
It also applies to any civil engineering structure.
PRIOR ART
Conventional concretes are generally constituted by a mixture of
cement, natural or artificial mineral aggregates such as gravel, sand,
possible additives and a hydraulic binder such as water.
The conventional proces~ for the production of a cast concrete prcduct
is as foll~s:
1. The mixing takes place with the aid of a mixer in order to obtain
an intimate mixture of the different components.
2. The thus obtained concrete is transferred into the form, whose
shape corresponds to the product to be obtained.
3. The concrete is po~Led into the form, possibly accompanied by a
vibrating action during filling.
4. The moulded product is demculded by removing the fonm.
5. The concrete is hardened during a ~cure~, which can last f~cm one
day to one month, as a function of the type of concrete used and the
product produced.
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The quality of the product produced with this concrete is significantly
dependent on the concrete curing conditions. Thus, microcracks can
e~ppear and deteriorate the final mechanical prcperties of the product.
1'hese microcracks then lead to the start of a decay phenomenon with
respect to the structure of the product and which can advance with the
appearance of macrocracks carried along by the mechanical stresses
reaching the strength limits of the concrete.
It is known to reinforce concretes by the introduction of fibres into
the mixture in order to improve the mechanical strength and signifi-
cantly reduce the appearance of microcracks prior to the appearance ofvisible cracks. However, it d oe s not make it possible to delay the
appearance of macrocracks in the structure of the product, when ccm-
pression or tension is applied to the latter.
As is described in French patent d~lication, published under No. 2 640
410, such a concrete is used for manufacturing radioactive waste stor-
age containers. The container comprises a drum having an opening for
the introduction of the waste and a tight sealing cover. The container
is entirely melde from cancrete reinforced by metal fibres.
It is cansidered necessary to imprcve the performance characteristics
of such cancretes in order to manufacture containers, whose strength
and durability characteristics are better than in the prior art. It
has therefore been proposed to supply a prestressed concrete, which
can have superior mechanical characteristics to the presently produced
prestressed cancretes. The main process for prestressing a concrete
block at present consists of embedding in the block one or more sheaths
within which are located metal cables, which are tensioned once the
hardening of the cancrete has been obtained, so as to compressively
prestress the concrete block.
The object of the invention is to supply a concrete which can meet the
existing require~ents with respect to the production of radioactive
waste containers, whilst avoiding the use of the concrete prestressing
type described in the preceding paragraph.
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SUMMARY OF THE INVENTION
To this end, a first object of the invention is a process for prestres-
sing a cancrete produced fram a given mixture consisting of intr~ducing
in a randcm manner into the mixture fibres, whose shapes or sizes can
evolve in time under the action of an energy transfer, pouring the
S concrete into a fonm, de~oulding the cast concrete, hardening the con-crete during a curing period and then carrying out a heat treatment
in such a way that the dimensions or shape of the fibres evolve, so as
to impose mechanical campressive stresses within the concrete.
Thus, at all points of the concrete product obtained it is possible to
impose a cpressive stress, which can have the same effects as the
campressive stresses imposed by the metal cables under tension used
conventionally in prestressed concrete blocks.
A second main object of the invention is cansequently a fibre-containing
concrete canstituted by an agglamerate of sand, gravel and ce~ent
cambined by means of a hydraulic binder and artificial fibres e~bedded
in the agglcmerate at various, randcm positions.
According to the inventian, the fibres used are of the type whose
shapes or dimensions can evolve in time under ~le actian of an energy
transfer in order to create a compressive stress in the cancrete.
It is possible to use two fibre types, namely shape memory metal alloy
fibres and thermoretractable fibres.
m e preferred shape of the said fibres is a very flat lamella, whose
length is between 1 and 10 centimetres.
The third main object of t'ne invention is a radioactive waste storage
container incorporating a drum provided with a waste introduction
cpening and a cover for the tight sealing of said cpening. According
to the inventian, the container is entirely made from cancrete of the
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type described in the preceding paragraphs.
DETAILED DESCRIPTION OF A PREFERRED EME~ODIMENT
The invention and its technical characteristics will be better under-
stood frcm reading the following description, illustrated by the single
S drawing showing a container according to the invention.
In order to prcduce the ooncrete according to the invention, use is
made of the base materials necessary for the prGduction of a conven-
tional concrete. These materials are Gonstituted by mineral aggregates,
i.e. natural or artificial gravel, sand and cement. To this mixture of
solid products in powder fa~m are added loose fibres, i.e. said fibres
can assume any randam position. Once this mixture has been made, the
hydraulic binder, preferably oonstituted by water, is added. This is
followed by mixing in a machine, so as to obtain an intimate mixture
of the different components.
The second phase of the process aocording to the invention obviously
Gonsists of pouring the fresh ooncrete into the foIm, whose internal
shape Gorresponds to the external shape of the praduct to be obtained.
In order to facilitate this operation, the form or mould is generally
plaoed an an operating vibrating table. The vibratians make it pos-
sible to obtain a very oompact concrete and give a product having avery low porosity. A good surfa oe appearance and a high mechanical
strength can also be obtained by using the said vibrating table.
Internal vibrating means can be used far large parts. It should be
noted that the po~ition of the fibres is of a randam nature.
Certain products require the use of an internal reinforcement, such as
a metal reinforcement, with a view to reinforcing the mechanical prop-
erties of the product obtained. The reinforcement is then intraduced
into the mould prior to the pouring of the concrete.
Following a first drying period, the concrete is demoul~ed, i.e. the
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fram is dismantled and removed fram the fluid or cast cancrete. This
is followed by a drying phase with respect to the product obtained for
the purpose of hardening the concrete. This phase is a cure, which
can last fram one day to one month. The duration of the cure varies
as a function of the cement type used and the type of praduce manufac-
tured. During this cancrete cure, there is a cement hydratian
reaction, which is known as concrete setting.
According to the invention, this is followed by a heat treatment per-
mitting an energy transfer by varying the temperature of the fibres
so that their shapes or sizes evolve. Thus, under the action of a
slight temperature rise or fall, such a fibre type assumes a retracted
shape or reassumes an initial shape.
A first category of fibres used for this purpose is constituted by
so-called shape memory metal fibres. Thus, when a metal or an ordinary
alloy is subject to a mechanical stress in excess of its yield point,
it underg oe s a plastic defonmation which persist~ after the stress has
been discontinued. Its shape and size are then v1rtually subject to
no further evolution if the alloy or metal undergoes subsequently a
heat treatment of any type. However, shape memory alloys do not have
this behaviour, because within a certain temperature range, a sa~ple
of such a material can undergo an apparently plastic deformation of
several percent and integrally reassume its initial shape by simply
heating at a later time. This shape memory phenamen~n is associated
with a reversible structural transformation of the ~martensitic thermo-
elastic~ type occurring between a first temperature Tl, at which the
sample was fonTed, and a second temperature T2 higher or lower than
Tl, to which the sa~ple must be heated or cooled fa,r it to reassume
its shape.
In the process according to the invention use is made of fibres whose
initial shape was that which it was wished to give them to enable the
concrete to be compressed, i.e. prestressed. This shape is preferably
a retracte~, curved, more or less curled up sh~pe compared with an
intermedLate shape, which will tend to be linear or completely straight.
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hmong the materials used, reference can be made to alloys of the NiTi
type (nickel, titanium and dopants) and the CuZnAl type (brass,
aluminium and dopants). For such alloy types, the temperature varia-
tion can be a rise between 50 and 70C, i.e. for a concrete prcduct
at 20C, a heat treatment at a temperature between 70 and 90C can make
it possible to return to the initial shape of the metal fibres cast in
the concrete. The same result can be obtained by cooling to a temper-
ature between -10 and -30C.
The duration of the application of the heat treatment is a function of
the shape of the product produced with the concrete. In general tenms
the entire product, i.e. even the product core, must reach the phase
transfonnation temperature.
If the product produced is transportable, the heat treatment can be
applied in the oven. However, if the product is moulded in situ, i.e.
is not transportable as a result of its weight and dimensions, the heat
treatment application process can take place by means of high fre-
quencies or microwaves using heating sheaths or jackets.
Tension tests by bending at three points were carried out on 4 x 4 x
16 om testpieces. The concrete used had the following constitution:
oenent CLC 45 890 g
Bayeux sand 2660 g
water 450 g
fluidifier 8 g
The incorporation of the shape memory fibres takes place after mixing
the four components. The heat treatment takes place for 24 hours at
80% in a tight pack to avoid dIying out and then for 24 hours at
ambient temperature fo,r cooling prior to the mechanical tests. The
tests were carried out in the aOGEMA, m e Hague, concrete laboratory
in a 1.5 litre mixer.
The following table summarizes the values obtained:
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AGE 14 days 28 days
FIBRES (kg/m ) O 50 90 0 50 105
No heat treatment 6.17 6.89 7.65 6.987.68 7.21
With heat treatment 7.37 7.89 7.43 7.758.00 7.84
For fibre-free testpieces, the heat treatment impr~ves the bending
strength of the cancrete or mortar (improvement of the cement setting
process) by approximately 1 MPa.
For fibre-oontaining testpieces, the tests performed with fibre con-
cretes with 50 and 100 kg of fibres per m3 of f m ished concrete shcws
that the results are more conclusive (excluding experimental err~rs)
with the fibres than without them and with the heat treatment than
without it.
A second category of fibre used consists of thermoretractable fibres.
Such a fibre is subject to a reduction in size with a slight temper-
ature rise or fall. If the shape is not rect;linear and preferably
well curved, the thermoretractable fibres, an retracting, impart a
compressive stress to the concrete in which they are embedded. The
shape of the fibres used is preferably that of flat lamellas, whose
length can be between 1 and 10 centimetres. The thickness can be less
than one tenth of a millimetre.
Any randam product can be prcduct with the aid of such a concrete.
However, a special application of the invention is the prcduction of
radioactive waste storage containers. Thus, with reference to the
single drawing, the container according to the invention essentially
comprises a drum 10,, whose upper cpening is sealed by a cover 12.
The sealing is tight in order to make it possible to store low or
medium activity radioactive waste coated with a filling material. In
the example shown in the drawing, the container is parallelepipedic,
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has a flat bottom and a side wall with a square section. The upper end
of the latter defines the opening permitting the closure with the aid
of the co~er 12.
According to the invention, the complete container, i.e. the drum 10
and the cover 12 is made fram a concrete reinforced by fibres 20 either
of the shape memory, or of the thermoretractable type.
me drum 10, like the cover 12, is produced by moulding. In particular,
the upper end of the side wall of the drum 10 has a stepped shape
successively defining, passing from the outside to the inside of the
drum 10, a planar end faoe 13 and a planar support or bearing face 7
parallel to the first planar fa oe 13, but set back with respect thereto.
me bearing face 7 is connected to the upper surface 13 by an inclined,
intem al, peripheral edge 6 forming a Z in cross-section with its two
first surfa oes 7 and 13. me diameter of the inclined edge 6 increases
on approaching the bearing surfaoe 7, 80 that said inclined edge 6
forms an angle of at least 10- with the axis of the drum 10.
The cover 12 also has a peripheral stepped zone with, starting fram
its upper surface, an inclined, outer, peripheral edge 11 and a vert-
ical edge 15 set back from the inclined edge 11. These edges 11 and
15 are connected by a second planar bearing face 5 parallel to the
upper and lower faces of the cover 12. The diameter of the inclined
edge 11 increases on appr~aching the second bearing surface 5, so that
said inclined edge 6 fo,rms an angle of at least 10- with the axis of
the cover 12.
When the co~er 12 is placed an the drum 10, the lower part of the oover12 defined by the edge 15 is fitted into the opening fonmed at the top
of the drum 10, until the horizontal bearing surface 5 of the cover 12
bears on the horizantal bearing surface 7 of the drum 10. As can be
seen in the drawing, the inclined edges 6 and 11, which have the same
height, then face ane another and define between them a do~etail-shaped
annular space 14, whose width is substantially canstant from the upper
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face 13 of the ccntainer 10 to the bearing surface 7. This annular
space 14 constitutes a keying slot.
In order to tightly flx the said cover 12 to the drum 10, into the
aforementioned annular space 14 is cast a keying joint produced frcm
the same material as the remainder of the container, i.e. concrete
reinforced by shape memory or thenmoretractable fibres 20.
This fo~mation of the keying joint in the upwardly c~en annular space
14 ensures the tight sealing of the container without it being neces-
sary to use a fonm. Moreover, the shape of the key mg joint avoids
any risk of the cover flying off when the joint is prcduced.
Advantageously, the cover 12 is centrally provided with a large opening
8 in which is formed a keying slot 9. Therefore the container 10 can
be filled follcwing the sealing of its cover 12. When the waste has
been intrcduced by the c~ening 8, the filling material is introduced
until the opening 8 is completely sealed. When the f~lling material
i8 constituted by thermoretractable or shape memory fibre-reinforced
concrete identical to that forming the actual container, a hamogeneous
assembly is formed, where the risks of cracks and breaks are
eliminated.
Finally, to permit the handling of the container, various means can be
provided. In exemplified manner reference can be made to rods or rings
4 sealed anto the upper, planar end face 13 of the drum 10. A handling
groove or slot 3 can be fonmed by moulding on the outer peripheral sur-
face of the drum lO in the vicinity of said upper end face 13.
This descriptian of a container only represents an exemplified embcdi-
ment of objects which can be manufactured with the aid of the concrete
according to the invention. Thus, it is possible to prestress any
randam object made in general terms from an ordinary cancrete by simply
introducing such fibres into the mixture prior to the praduction of the
concrete. A heat treatment subsequently applied after the object has
been dried makes it possible to obtain within the concrete ccmpressive
stresses equivalent to the prestresses imposed by canventional methods
used far prestressed cancrete.
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