FR2707625A1 - Concrete mixture and concretes obtained - Google Patents

Abstract

Concrete mixture consisting essentially of cement, granular components, fine components with pozzolanic reaction, metal fibres, optionally adjuvants, to be mixed with water, characterised in that the preponderant granular components have a maximum particle size D not exceeding 800 microns, in that the preponderant metal fibres have an individual length 1 within the range 4 mm - 20 mm and in that the ratio R of the mean length L of the fibres to the said maximum size D of the granular components is at least 10. Figure 1 is a diagram illustrating the concrete of the invention (Fig.1B) in comparison with a conventional concrete (Fig.1A). Application to the construction of structures which are prestressed or otherwise.

Description

 The invention relates to a mixture for concrete, to be kneaded with water, to obtain an improved concrete, for general use, comprising fibers but not reinforced in the traditional sense of the term, used for the production of prestressed or non-prestressed structures.

 Traditional concretes have a granular skeleton formed of the following three phases: cement which constitutes the binding phase with a grain size of between 1 micron and 100 microns; - sand with a grain size between 1 and 4 mm: - aggregates or gravel with a size between 5 and 20 mm or between 5 and 25 mm.

 Traditional metal fiber concretes comprise steel fibers whose length is between 30 and 60 mm. The maximum length of the fibers that can be used is limited on the one hand by the kneading possibilities without excessive degradation and on the other hand by the requirements of pouring the concrete (placement and vibration).

 The smooth metal fibers are anchored in the concrete by adhesion. To ensure a good behavior of the smooth fiber, it is important that the form factor equal to the quotient of the length by the diameter of the fiber be between 50 and 100. This optimal form factor can be reduced when the anchoring of the fiber is improved by a modification of its geometry: undulations, hooks at the ends, crenellations, etc ...

 The fiber dosages used in traditional fiber concretes range from 30 kg / m3 to 150 kg / m3; they are generally between 40 kg / m3 and 80 kg / m 3, which corresponds to a volume percentage of between 0.5% and 1%.

The length L of the fibers is generally between 30 mm and 60 mm, whereas the diameter D of the largest aggregates is generally between 20 and 25 mm, so that the ratio R =
L / D is between 1.2 and 3.0.

 In traditional concrete, the interface between the aggregates and the hardened cement paste constitutes a weak zone of resistance because of its greater porosity (transition halo). This interface is also the seat of local constraints due to the anisotropy of behavior between the granulate and the paste. In the case of aggregate traction exerted on the concrete, the aggregates may remain connected to each other only if there are tensile-resistant fasteners with a development length of at least approximately ten times the size. of the largest granulate.

 Since the ratio R is at most 3.0, the fibers can not effectively bind the aggregates to each other.

 This is confirmed by the fact that the addition of metal fibers in traditional concrete only slightly improves the tensile strength of the concrete. This improvement is a few percent for usual fiber dosages of 0.5 to 1% by volume.

 The metal fibers used in concretes that do not have traditional reinforcements do not make it possible to avoid the cracking of the concrete, they only allow to distribute it, that is to say that one has a large number of micro - cracks sewn by the fibers instead of fewer but larger cracks.

As a result, the use of metal fiber concretes without traditional passive reinforcement is limited to the following two areas
- paving concrete
- projected concretes.

 So-called structural concretes subjected to significant mechanical forces always include traditional passive frames.

 An object of the invention is to obtain a conventional passive-free metal fiber concrete having a tensile strength of at least about 30 MPa to about 60 MPa.

 Another object of the invention is to obtain a metal fiber concrete having a fracturing energy of at least from about 10 000 to about 40 000 J / cm 2.

 An object of the invention is also to obtain a metal fiber concrete having a compressive strength of at least about 150 to about 250 MPa.

 The concrete of the invention is obtained by mixing with water a mixture consisting essentially of cement, granular elements, fine pozzolanic reaction elements, metal fibers, optionally adjuvants, characterized in that the predominant granular elements have a maximum grain size D at most equal to 800 microns, in that the predominant metal fibers have an individual length 1 in the range 4 mm - 20 mm and in that the ratio R between the average length L fibers and said maximum size D granular elements is at least 10.

 By the term "predominant granular elements" is meant those granular elements which represent at least 90%, preferably at least 95%, or more preferably at least 98%, of the total mass of the granular elements.

 The expression "predominant metal fibers" denotes metal fibers which represent at least 90%, preferably at least 95%, or even more preferably at least 98%, of the total mass of the metal fibers.

 Ideally, the predominant granular elements constitute all the granular elements and the predominant metal fibers constitute the totality of the metallic fibers.

 In particularly advantageous embodiments: D is at most equal to 600 microns, or better still to 400 microns, (sizes of 800, 600 and 400 microns respectively corresponding to sieve equivalents of 30, 29 and 27 of the AFNOR series NFX 11-501 - 1 is in the range 8 - 16 mm, or better in the range 10 - 14 mu - the predominant metal fibers have a diameter in the range 80 - 500 microns, or better 100 - 200 microns.

In a typical example, the aggregates of the concrete mix have a diameter of at most 400 microns and the metal fibers have a length greater than 12 mm, which gives a ratio of
R = 30.

 The behavior of the 12 mm long fiber in the reactive powder concrete matrix is similar to the behavior of a traditional smooth reinforcement with a length of 30 x 12 = 360 mm.

 The mechanical operation of the reinvention concrete is therefore identical, to a scale effect, to the mechanical operation of the concrete. traditional armed with traditional frames of length 360 mm.

 Whereas traditional fiber-free concrete concretes can not be used as structural concretes, ie for the manufacture of beams, columns and slabs, the concrete of the "micro-reinforced" invention constitutes on the contrary, a new material that can be used for such applications.

 In a preferred embodiment, the concrete mix comprises, per 100 parts by weight of cement, 60 to 150 (or better 80 to 130) parts by weight of fine sand having an average particle size of 150 to 400 microns, 10 to 40 ( or preferably 20 to 30 parts by weight of amorphous silica having a grain size of less than 0.5 microns, 10 to 80 (or better still 15 to 40) parts by weight of metal fibers having an average length of 10 to 14 mm and optional adjuvants, and is kneaded with 10 to 24, preferably 13 to 20, parts by weight of water.

Example
About 16 parts by weight of water are mixed with a mixture consisting essentially of about 100 parts by weight of cement.
Portland, about 110 parts by weight of fine sand of particle size less than 400 microns, about 25 parts by weight of amorphous silica with a particle size less than 0.5 microns, about 1.8 parts by weight of superplastliiant (dry extract) and about 20 parts by weight of 13 mm long steel fibers having a diameter of 175 microns.

The table below gives comparative examples of the characteristics of the concrete of the invention and other concretes.

<tb><SEP> RESISTANCE <SEP> A <SEP><SEP> ENERGY <SEP> RESISTANCE <SEP> A <SEP> LA
<tb><SEP> TRACTION <SEP> BY <SEP> FRACTURING <SEP> COMPRESSION
<tb><SEP> FLEXION <SEP> 3 <SEP> POINTS
<tb> Traditional <SEP> concrete
<tb> B25 <SEP> to <SEP> B <SEP> 40 <SEP> 2MPa <SEP> to <SEP> 3 <SEP> MPa <SEP> 90 <SEP> to <SEP> 120 <SEP> J / m2 <SEP> 25 <SEP> to <SEP> 40 <SEP> MPa
<tb> Concrete <SEP> to <SEP> High <SEP> and
<tb> very <SEP> high
<tb> performance <SEP> 4MPa <SEP> to <SEP> 5 <SEP> MPa <SEP> 120 <SEP> to <SEP> 150 <SEP> J / m2 <SEP> 50 <SEP> to <SEP> 100 <SEP> MPa
<tb><SEP> matrix of <SEP> concrete
<tb> to <SEP> reactive <SEP> powders <SEP> 16 <SEP> MPa <SEP> to <SEP> 20 <SEP> MPa <SEP> 80 <SEP> to <SEP> 100 <SEP> J / m2 <SEP> 180 <SEP> to <SEP> 290 <SEP> MPa
<tb> Concrete <SEP> according to
<tb> Invention <SEP> 30 <SEP> MPa <SEP> to <SEP> 60 <SEP> MPa <SEP> 10000 <SEP> to <SEP> 40000 <SEP> J / <SEP> m2 <SEP> 150 <SEP> to <SEP> 250 <SEP> MPa
<tb><SEP> I
<Tb>

 The concrete of the invention is prepared by mixing the solid constituents and water, in a manner known per se.

 Preferably, the concrete obtained is subjected to baking at a temperature between ambient and 100 ° C, in particular baking between 60 and 100 ° C, preferably at a temperature of the order of 90 ° C.

 The duration of the cooking is preferably between sixteen hours and four days with an optimal duration of the order of two days, at least one day after the beginning of the setting of the mixture.

 The cooking is carried out in a dry or wet atmosphere or in cycles alternating the two environments, for example twenty four hours of cooking in a humid environment, followed by twenty four hours of cooking in a dry atmosphere.

 This cooking is carried out on concretes at least one day old, preferably at least about seven days old.

 The invention is not limited to the use of a particular super plasticizer, but polyacrylate super plasticizer is preferred over melamine and naphthalene superplasticizers. Preferably 0.5 to 3.0, or more preferably 1.2 to 2.5 parts by weight of superplasticizer (dry extract) is used.

 The silica used is preferably silica fume from the zirconium industry rather than silica fume from the silicon industry.

 Figure 1 shows schematically the mode of operation of the metal fibers in the concrete of the invention, compared to that of traditional passive frames: - FIG. 1A is a diagram of a fiber of length L = 60 mm surrounded by aggregates of maximum diameter D = 25 mm, the ratio R = L / D max = 2.4 and FIG. 1B is a diagram (on a different scale) of a metal fiber surrounded by aggregates in a concrete according to the invention with L = 12 mm and DmaX = 0.4 mu.

 FIG. 2 represents the results obtained during three-point bending tests carried out on notched specimens 4x 4 x 16 cm.

 The fracturing energies GF are proportional to the areas delimited under the behavior curves.

 It is calculated that the fracturing energy of the concrete of the invention is 270 times greater than that of the traditional reference mortar.

Claims (18)

1. Concrete mix consisting essentially of cement, granular elements, pozzolanic reaction fine elements, metal fibers, optionally adjuvants, to be mixed with water, characterized in that the predominant granular elements have a maximum grain size D at most equal to 800 microns, in that the predominant metal fibers have an individual length l in the range 4 mm - 20 mm and in that the ratio R between the average length L of the fibers and the said maximum size D of the granular elements is at least 10. 2. Concrete mix according to claim 1, characterized in that said maximum thickness D is at most equal to 600 microns. 3. Concrete mix according to claim 2, characterized in that said maximum thickness D is at most equal to 400 microns. 4. Concrete mix according to one of the preceding claims characterized in that said individual length is included in the 8-16mm range, 5. Concrete mix according to the preceding claim characterized in that i is in the range 10 - 14 mm. 6. Concrete mix according to one of the preceding claims characterized in that the predominant metal fibers have a diameter of between 80 and 500 microns. 7. Concrete mix according to the preceding claim characterized in that the metal fibers have a mean diameter of between 100 and 200 microns. 8. Concrete mix according to one of the preceding claims characterized in that the maximum thickness of the predominant granular elements is at most equal to 500 microns and in that the predominant metal fibers have a length greater than 10 mm. 9. Concrete mix according to one of the preceding claims characterized in that said ratio R is at least 20. 10. Mixture according to one of the preceding claims characterized in that it comprises, per 100 parts by weight of cement, 60 to 150 parts by weight of fine sand having a mean grain size of between 150 and 400 microns, 10 to 40 parts by weight of amorphous silica having a mean grain size of less than 0.5 microns and 10 to 80 parts by weight of metal fibers having an average length of 10 to 14 mm, and optional adjuvants. 11. Concrete mix according to claim 10 characterized in that it comprises 0.5 to 3.2 parts by weight of plasticizer. 12. Concrete mix according to claim 10 or 1 1 characterized in that it is kneaded with 10 to 24 parts by weight of water. 13. Concrete mix according to claim 11, characterized in that it comprises, for 100 parts by weight of cement, 80 to 130 parts by weight of fine sand having an average particle size in the range 150 - 400 microns 20 to 30 parts by weight of amorphous silica having an average grain size of less than 0.5 microns and 15 to 40 parts by weight of metal fibers having an average length of 10 to 14 mm, and optional adjuvants. 14. Concrete mix according to the preceding claim, characterized in that it comprises 1.2 to 2.5 parts by weight of plasticizer. 15. Concrete mix according to one of claims 13 or 14, characterized in that it is kneaded with 13 to 20 parts by weight of water. 16. Concrete mix according to one of the preceding claims, characterized in that the cement of the mixture is a Portland cement. 17. Concrete obtained by kneading a mixture according to one of the preceding claims. 18. Concrete according to the preceding claim, characterized in that it was baked at a temperature between 60 and 100 C for a period of time between sixteen hours and four days, at least one day after the start of setting.