Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
2000-06-28
2002-11-12
Wood, Elizabeth D. (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S640000
Reexamination Certificate
active
06478867
ABSTRACT:
The present invention relates to the field of concretes, more particularly to fibre-reinforced concretes. The main subject of the invention is an improved concrete, especially making it possible to manufacture elements of civil engineering structures, intended for constructing buildings and highway structures, and having properties superior to those of elements in the prior art. In particular, the present invention aims to obtain, for structural concretes, mechanical behaviour which is tough and ductile at the same time.
Structural analysis of concretes has shown that their mechanical properties are intimately linked to the presence of structural defects. Several types of defects distinguishable by their size may be observed in these concretes when they are subjected to mechanical loads.
On a smaller scale, the defect called microporosity is observed in concrete. This consists of pores, called capillaries, emanating from the intergranular spaces initially present in the fresh paste. Their size varies between 50 nm and a few &mgr;m.
On the next scale up, microcracking defects are observed. These are microcracks having openings ranging from 1 &mgr;m to a few hundreds of Am. They are non-coalescent, that is to say that they do not form a continuous path through the structure. They are mainly due to the heterogeneous character of concrete, the aggregate having mechanical and physical properties different from those of the binder/cement. These microcracks appear during mechanical loading. This type of defect is a major reason for the poor mechanical properties of concrete in tension and for its brittle character.
On the final scale, macrocracking defects are observed. The opening of these cracks varies from a few hundreds of &mgr;m to a few mm. These cracks are coalescent.
Major defects several millimetres in size may also be observed, these being due to poor preparation of the concrete (entrained air, faults in filling).
Solutions have been suggested either for decreasing the presence of these various defects or for reducing their effects on the mechanical properties of the concrete.
In order to improve the mechanical properties of concretes, it has been proposed to replace the sand of the cementitious matrix by other, higher-performance, constituents, but the cost of the concrete rises to an unacceptable level for it to be conceivably used widely in civil engineering because of the economic constraints which burden this field.
It has also been proposed to incorporate high-hardness aggregate into the concrete composition, but the amounts involved in order to achieve the desired performance also increase the manufacturing cost of the concrete excessively, given the high cost of such aggregate.
It has also been proposed to improve, sometimes spectacularly, certain mechanical properties of concrete by incorporating into it a high content of reinforcing fibres, namely, typically, a content of 10 to 15% by volume, but this content not only has a very significant effect on the manufacturing cost of the concrete but also makes its mixing, homogenization and possibly its casting too difficult or too critical to be applicable in civil engineering, especially under the working conditions of a construction site.
Also, it has been possible to partially control the microporosity by decreasing the water-to-cement weight ratio and by using plasticizers. The use of fine fillers, especially pozzolanic-reaction fillers, has also allowed the size of the micropores to be reduced.
However, the organization of the aggregate skeleton by the usual methods does not make it possible to obtain concrete with a satisfactory rheology under acceptable civil engineering operating conditions (poorly dispersed fibres, microstructural defects, etc.).
Microcracking itself has been greatly reduced by:
improving the homogeneity of the concrete, for example by limiting the size of the aggregate to 800 &mgr;m;
improving the compactness of the material (aggregate optimization and optional pressing before and during the setting phase);
carrying out heat treatments after setting.
With regard to macrocracking, this may be controlled by the use of metal fibres, but with the operating difficulties mentioned above.
By way of an illustrative document of the prior art, mention may be made of Patent Application WO-A-95/01316 which relates to a metal-fibre-reinforced concrete in which the fibre content is controlled and the fibre dimensions are chosen in defined proportions with respect to those of the aggregate particles.
This fibre-reinforced concrete comprises cement, aggregate particles, fine pozzolanic-reaction particles and metal fibres. The aggregate particles must have a maximum size D of at most 800 &mgr;m, the fibres must have an individual length 1 of between 4 and 20 mm and the ratio R of the average length L of the fibres to D must be at least equal to 10, the fibre content being such that the fibres occupy a volume of from 1 to 4% of the volume of the concrete after it has set.
The concrete obtained exhibits ductile behaviour or undergoes pseudo-work-hardening.
There is still a need to remove the aforementioned defects or to greatly reduce their effects, especially microcracks, as it may be seen that the work described in the prior art serves mainly to prevent the development of macrocracks and not of microcracks; microcracks are then only partially stabilized and develop under load.
The object of the present invention is to provide a concrete containing metal reinforcing fibres and having improved properties compared with similar concretes of the prior art.
Improved properties should be understood to mean both mechanical properties that are superior to those of known fibre-reinforced concretes and properties that are at least equal to those of known fibre-reinforced concretes, but these properties being achievable on an industrial scale in a constant and reproducible manner.
Another object of the present invention is to increase the stress level at which the first damage (i.e. microcracks) appears in the concrete and thus to increase the range of use of the concrete, namely the linear elastic behaviour of the concrete.
Yet another object of the present invention is to improve the work hardening of the concrete beyond the first damage by controlling the propagation of macrocracks. The purpose of the invention is thus to increase the range of use of the concrete beyond the first damage by improving the ductile behaviour of the concrete.
Another object of the invention is also to improve, by a synergy effect between the cementitious matrix and the fibres, the behaviour of the concrete both with respect to the appearance of microcracks and to the propagation of macrocracks.
“Cementitious matrix” should be understood to mean the hardened cementitious composition apart from the metal fibres.
Yet another object of the present invention, which is particularly important for obtaining concrete structures which, because of their size or the work site conditions, could not undergo a heat treatment, is to obtain, under improved conditions over the prior art and especially at temperatures close to ambient temperature (20° C.), a concrete having mechanical properties (in the sense mentioned above) at least equal to those which can only be obtained at the cost of a heat treatment in the case of the best known fibre-reinforced concretes.
In addition, the subject of the present invention is the cementitious matrix, which allows the concrete of the invention to be produced, and the premixes which comprise all or some of the constituents necessary for preparing this matrix or the concrete.
In its general form, the invention relates to a concrete consisting of a hardened cementitious matrix in which metal fibres are dispersed, obtained by mixing, with water, a composition which comprises, apart from the fibres:
(a) cement;
(b) aggregate particles having a maximum particle size Dmax of at most 2 mm, preferably at most 1 mm;
(c) pozzolanic-reaction particles having an elementary particle size of at most 1 &mgr;m, preferably at most 0.5 &
Cheyrezy Marcel
Clavaud Bernard
Dugat Jerome
Frouin Laurent
Orange Gilles
Bouygues Travaux Publics
Foley & Lardner
Wood Elizabeth D.
LandOfFree
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