Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
2001-03-16
2004-04-20
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S724000
Reexamination Certificate
active
06723162
ABSTRACT:
This invention relates to a new fiber concrete allowing to make structure components and having better properties than those of the prior art components, in particular with respect to the tensile stress behaviour (bending and direct tensile stress). The fibres being used are organic fibres.
A structural analysis of concrete has shown that their mechanical properties are closely linked to the presence of structural defects. Many types of structural defects can be observed in these concrete mixes when they are subject to mechanical loads. They differentiate from each other with their size.
At the lowest scale, the so-called microporosity defect of the concrete is observed, that means so-called capillary pores, derived from intergranular spaces initially present in the fresh paste. Their size lies in the range from 50 nanometers to a few micrometers.
At an intermediary scale, microcrack defects are observed. These are microcracks having openings in the range between 1 and 100 micrometers. These are non coalescent, i.e. they do not form a continuous path through the structure. These are essentially due to the heterogeneous character of concrete, the granulates having mechanical and physical properties different from those of the binder/cement. They occur upon mechanical loading. This defect type is mainly responsible for the poor mechanical tensile stress properties of concrete and its breakable character.
At the upper scale, macrocrack defects are observed. The crack opening varies from 100 &mgr;m to 1 mm. These cracks are coalescent.
Millimetric size major defects may also be observed which are due to a wrong concrete preparation (occluded air, filling defects).
Solutions have been proposed either to reduce the presence of these various defects or to attenuate their effects.
Thus, it has been possible to partially control microporosity by reducing the water/cement weight ratio and using fluidizing agents. The use of fine fillers, in particular with a pozzolanic reaction, has also made it possible to reduce the micropore size.
As far as microcracking is concerned, it has been strongly reduced:
by improving the concrete homogeneity, for example, by reducing the granulate size to 800 &mgr;m,
by improving the material compactness (granular optimization and optional pressing before and during setting),
thermal treatments after setting.
As to microcracking, it has been controlled by the use of metal fibres.
WO-A-95/01316 can be mentioned as prior art document. It relates to controlling the size ratio between the metal fibres and the granular elements (sand, granulates). This improved fibre concrete comprises cement, granular elements, fine elements with pozzolanic reaction and metal fibres. The granular elements must have a maximum grain size D of 800 micrometers at the most, the fibres must have an individual length I in the range between 4 mm and 20 mm and the ratio R between the average length L of the fibres and D should at least equal 10.
The resulting concrete shows a flexural ductile behaviour or pseudo cold working.
Concrete or mortar formulations comprising organic fibres have also been suggested for various purposes, optionally conjugated with metal fibres, as disclosed, for example, in the publication “Fibre reinforced cementitious composites” by A. BENTUR, S. MINDESS (Elsevier Applied Science, 1990).
The state of the art shows that the man skilled in the art who aims at formulating a fibre concrete faces multiple possible choices of materials and proportions, as well as regards the concrete cement matrix than the fibres, so that the problem still remains that a concrete is to be formulated having improved properties compared to existing concrete mixes and the cost of which is not to be redhibitory for its efficient use in the building industry and public works.
An answer to the aimed properties is to be found at the level of the use of organic fibres instead of metal fibres: increase in ductility, in particular tensile stress, reduction of corrosive effects, weight reduction of fibre concrete structures. A less important attenuation of the radioelectric signals can also be mentioned.
An interesting effect provided by the presence of polymer type reinforcing fibres is an improved fire behaviour of the fibre concrete mixes.
Another further solution is to be found at the level of the elimination of the above-mentioned defects, more particularly microcracks, because it has been observed that the implementations described in the prior art are mainly designed to avoid the development of macrocracks and not of microcracks: the microcracks are not stabilized and develop under stress.
The object of the present invention is a concrete mix comprising reinforcing organic fibres and having improved properties compared to the prior art concrete mixes, more particularly in tensile stress (flexion and direct tensile stress).
Another aim of the present invention is to provide a concrete mix the cold working of which is improved beyond the first damage by controlling the macrocrack propagation. The invention aims thus at increasing the field of use of concrete beyond the first damage by providing a ductile behaviour to concrete.
FIG. 1
of the accompanying drawings is a typical direct tensile stress curve of a concrete mix with a ductile nature according to the prior art.
In the case of a break being not of the breakable type (breakable means here that break is sudden, not progressive), both the engineer designing a structure and the engineer who calculates it or must check its safety, needs to have access to the behaviour law of the material or to a feature that shows it. The material ductility only corresponds to the non elastic strain occurring, in direct tensile stress, before the constraint peak A.
In order to illustrate the advantage of ductility, one can merely imagine the behaviour of a tie-rod (a strut for example built-in at its upper end), subjected to an increasing tensile load (weights being added to the lower end). As soon as this load has reached the peak value, a break occurs and is complete (in the direct tensile stress test, in particular, the post-peak portion can only be seen if the test is carried out upon stress).
The ductility of a non elastic material is characterized by the whole stress-strain curve in a simple tensile stress, but considered only up to the peak. It can also be defined as being the ratio of the breaking stress &egr;
A
to the elastic stress &egr;
él
=&egr;
B
·(&sgr;
A
/&sgr;
B
) corresponding to the breaking stress (provided &sgr;
A
is higher than (&sgr;
B
); this ratio is equal to that of the elastic modulus (OB slope) divided by the secant modulus at break (peak stress divided by peak strain or OA slope).
The ductility may be described by means of a ductility coefficient &dgr;:
δ
=
ε
A
·
σ
B
ε
B
·
σ
A
where &egr;
A
=peak strain, and
ε
e1
=
ε
B
·
σ
A
σ
B
with &egr;
el
=strain that would be obtained under peak stress by elastically extrapolating the resulting strain under the running stress.
This definition is perfectly in line with the physical behaviour observed on a test specimen (multicracking): upon the first cracking, the so-called first crack peak B (which is only a local or partial maximum), is locally reached followed by an opening that can be read on
FIG. 1
between first peak B and the point C where the curve goes beyond the value of this peak; at this time, the first crack is stabilized because stress again increases in the whole stressed volume until the occurrence of a second crack, etc. This behaviour is strong, as it can only be more stable in higher size volumes.
Another aim of the present invention is to increase the stress level where the first concrete damage occurs (i.e. microcracks) and thereby to extend the field of use for the concrete (elastic linear behaviour).
Still another aim of the invention is to improve, by a synergistic effect between the cement matrix and the organic fibres, the behaviour of the concrete both wit
Boivin Sandra
Cheyrezy Marcel
Dugat Jerome
Froin Laurent
Orange Gilles
Bouygues
Marcantoni Paul
Young & Thompson
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