EP0922817A1

EP0922817A1 - Use of fibers in concrete compositions for the production of structural elements in prestressed concrete and relevant reinforced elements

Info

Publication number: EP0922817A1
Application number: EP98119688A
Authority: EP
Inventors: Claudio Failla
Original assignee: Larco Astori SpA
Current assignee: Larco Astori SpA
Priority date: 1997-10-21
Filing date: 1998-10-19
Publication date: 1999-06-16
Also published as: ITPC970031A0; ITPC970031A1; IT1296234B1

Abstract

The invention, relates to the use of steel fibres, carbon fibres, poli-vinil-alcool fibres etc., for the production of concrete suitable for the manufacture of pre-stressed reinforced concrete structural elements, as well as fibre-reinforced pre-stressed concrete structural elements (C.A.P.). In particular the invention relates to the use of fibre-reinforced concrete with the pre-stressing technology, in order to obtain carrying structures without the use of conventional not-stressed reinforcement bars, obtained without the need of using particular procedures during their manufacturing, without particular treatments or seasoning of the mixture, but only applying the usual producing systems and seasoning of the concrete.

Description

The invention, relates to the use of steel fibres, carbon fibres, poli-vinil-alcool fibres etc., for the production of concrete suitable for the manufacture of pre-stressed reinforced concrete jobs, as well as fibre-reinforced pre-stressed concrete structural elements (C.A.P.).
In particular the invention relates to the use of fibre-reinforced concrete with the pre-stressing technology, in order to obtain carrying structures without the use of conventional not-stressed reinforcement bars, obtained without the need of using particular procedures during their manufacturing, without particular treatments or seasoning of the mixture, but only applying the usual producing systems and seasoning of the concrete.
It is known that, while calculating the structures, the resistance to the traction of the concrete is never taken into consideration, because of the weak tensile strength of this material.
For this reason the concretes are reinforced with a passive reinforcement made of metal bars apt to absorb the traction forces in the concrete itself.
It is also known that adding fibres of different kind to the concrete mixture causes a variation in the force/deformation ratio, providing, also after a breaking, a resistance of the concrete mixture, which is not obtained in the fiberless concrete.
In particular it is known that with the use of fibres, even when the tensile strength does not vary in a relevant way, the amount of the energy that is dissipated during the breaking of fibre-reinforced concretes is higher than in the case of normal concrete, with a reduction of the peculiar fragility of this material.
This characteristic of the fibre reinforced concrete allows to safely absorb the forces that locally overtake the breaking value of the concrete, transferring, thanks to the flexibility of the section, the forces to the adjacent fibres.
Despite this, it is not possible to provide great structures using only fibre-reinforced concrete without passive reinforcements, since the traction will overtake the limits of this material.
Some studies have been published, relating to the use of fibres instead of the main reinforcement, but limited to "not-structural" elements like pipes, airport runways, coverings or floorings etc, which are not subjected to tensile stresses. The fibres in these elements are used, for example, instead of steel net to absorb small traction stresses.
It is also known the use of fibres mixed to a cement mortar to obtain, underground, columns of consolidated soil, by the technique known as "jet grouting".
So the technique of mixing fibres to the cement is already known, even if only for "normal" elements, like the ones casted "in situ".
U.S. Patent 5.635.263, for example, describes the use of carbon-fibre sheets applied, by means of resins, to concrete structures in order to reinforce them.
U.S. Patent 5.503.670 describes a fibre reinforced concrete, wherein the use of metal fibres is provided, instead of a conventional reinforcement.
This patent, which represents the most advanced state of the art, suggests solutions suitable to be applied only to not-structural elements, of the type described above.
Moreover this patent relates to a particular kind of concrete, which requires aggregates of very fine dimensions, less than 0.8 mm (see, e.g., lines 5, 6 of the abstract and figgs. 1A and 1B), the use of particular mixing apparatuses as well as a heat curing phase (see col. 10, lines 16-19).
Even if this patent gives no teaching about the use of fibres instead of cross reinforcement in C.A.P., it provides us several information about the main resistance features of this kind of concrete.
The present invention relates to the use of fibre reinforced concrete for the production of structural C.A.P. elements, in particular elements having only the pre-compression reinforcement, while the crossed reinforcement or passive reinforcement is substituted by fibres.
In short, the invention provides to combine the pre-compression technology with the use of the fibre reinforced concrete, in order to avoid the need of the passive reinforcement.
This allows several advantages, because since the passive reinforcements as well as a convenient covering of the same (which must be not less than 2 cm in thickness, according to the Italian Laws) are not necessary, it is possible to obtain thinner structures.
In particular these advantages are greater in the case of structural elements of complex shape and/or variable section.
As an example we can think about the covering tiles made of concrete, or the sheds.
The actual structures of this kind can have different length, for example 20 mt, a width from 1 to 3 metres and a thickness can be 5 to 15-20 cm, and can have a "L", a "U" or, better, a "TT" shape and/or asymmetrical sections in different shapes.
As they must only carry their own weight and the snow weight, to support these loads it would be enough a thickness of few centimetres of the structure. But since these structures are reinforced by providing ribs and a passive reinforcement, their thickness, to ensure the required covering of the reinforcement, must be at least 5-6 cm.
One could provide thinner and consequently lighter structures by removing the passive reinforcement, with advantages during the manufacturing and other economical advantages.
Furthermore the manufacturing is quicker, and a simplification of the calculation is achieved since the fibre-reinforced concrete, with convenient characteristic in granularity and percentage of fibres, can be compared for the calculation, to a substantially homogeneous material.
It is also possible to use non-metallic fibres, like carbonium or PVA, which are not sensible to the atmospheric agents.
Then it is also possible, thanks to these homogeneity characteristics, to greatly reduce the cracking or to get a uniform distributed micro-cracking.
The invention will now better described with reference to the enclosed tables from A to E, which show the results of the tests carried out on reinforced concrete samples with different kind of fibres, which can be used to obtain C.A.P. structural elements according to the invention.
Several kind of mixtures have been tested, in order to get a concrete that, added with fibres, may ensure the characteristics of resistance considered as necessary for the production of C.A.P. structural elements without passive reinforcement.

The following kind of fibres have been used for the preparation of the mixture:

Kind of Reinforcement	Diameter	Length (mm.)	Amount
RK 10 Carbon	0,008	25	15 to 40 Kg/m³
New Cem 3 Carbon	0,009	25	15 to 40 Kg/m³
PVA RF 1500	0,42	30	10 to 30 Kg/m³
PVA RF 4000	0,66	30	10 to 30 Kg/m³
Dramix Steel 40/,50	0,5	40	30 to 80 Kg/m³
Dramix Steel 60/,80	0,8	60	30 to 80 Kg/m³

A reference concrete mixture without fibres with the following composition has been prepared:

Base mixture

525 R1 cement: 350 Kg/mc
Italeux Naftalensulphonate additive: 5,5l/mc
H₂O: 166l/mc
Sieved sand 0-3: 120 Kg
Mixed sand 0÷12: 985 Kg
Fine gravel 8÷15: 793 Kg

Two mixtures made of fired clay, cement and water with the following aggregates have also been prepared:

Mixture N. 2

• Light aggregates TC 0-6: 460 Kg
• Light aggregates TC 6-12: 460 Kg
• Sieved sand 0-3: 400 Kg

Mixture N. 3

• Light aggregates TC 0-6: 420 Kg
• Light aggregates TC 6-12: 420 Kg
• Sieved sand 0-3: 560 Kg

The following mixtures have also been prepared, using the above Base Mixture, added with the following fibres:

• Mixture 4: 12,5 Kg of Kuralon fibres RF 4000
+ 10 litres of H2O
• Mixture 5: 12,5 Kg of Kuralon fibres RF 4000
+ 4 litres of super fluidificant
• Mixture 6: 25 Kg of Kuralon fibres RF 4000
+ 15 litres of superfluidificant
• Mixture 7: 12,5 Kg of Kuralon fibres RF 1500
+ 4 litres of superfluidificant
• Mixture 8: 25 Kg of Kuralon fibres RF 1500
+ 4 litres of superfluidificant
• Mixture 9: 40 Kg of Dramix fibres 40/.5
+ 4 litres of superfluidificant
- 20 litres of H2O
• Mixture 10: 60 Kg of Dramix fibres 40/.5
+ 4 litres of superfluidificant
• Mixture 11: 40 Kg of Dramix fibres 60/.8
• Mixture 12: 17,5 Kg of carbon fibres New Cem 3
•: + 4 litres of superfluidificant
• Mixture 13: 17,5 Kg of carbon fibres RK 10
+2 litres of superfluidificant
• Mixture 14: 35 Kg/mc of carbon fibres RK10
+ 10+31 litres of superfluidificant

The mixtures have been prepared using a laboratory concrete mixer having a capacity of 0,03 mc.
The compounds were weighted every time in the laboratory and, once calculated the humidity of the aggregates and dried them, the amount of added water was so balanced.
For every mixture the following tests were carried out:

room temperature
mixture temperature
initial slump
slump after 30'

For each mixture, the following tests were carried out:

4 tests of 2 blocks (size 100x100x100 mm)
4 tests of 3 prisms (size 40x40x160mm)
4 cylinders (size 300x150mm)

Half of the tests (abbreviations M.V.) were seasoned in a thermostatic tank at the temperature of 70° after 2 hours of pre-setting.
The tests were kept 14 hours in the tank before being brought at room temperature and subsequently picked up within 24 hours.
The remaining half of the tests (abbreviation M.N.) were seasoned at a room humidity and temperature and picked up within 24 hours. All the tests were kept until the execution of the test, at room humidity and temperature.
The tests undertook the following tests:

2 blocks: M.V. compression for 24 hours
2 blocks: M.N. compression for 24 hours
2 prisms: M.V. flexion for 24 hours
2 prisms: M.N. flexion for 24 hours
1 cylinder: M.V. indirect traction for 24 hours
1 cylinder: M.N. indirect traction for 24 hours

The tests were repeated at the end of 28 days.
The results of the tests are shown in tables A and B.
Table A shows all the results, specifying for every test the water/cement ratio and the kind and percentage of the fibres.
The values of the resistance are expressed in N/cmq.
Table B resumes the flexual stregth and compression strength values, as well as the values of the energy absorbed during the break.
By a comparison between the values of energy absorbed by the reference concrete (0,42 J) and by the concrete added with steel fibres (11, 86 J - reference 10) it is clear that the fibre-reinforced concrete can absorb, during breaking, an energy at least 20 times more than the normal concrete, and this confirms our hypothesis. The results of the average of the measurement held out on the tests for every kind of mixture are shown in tables C and D and underline the minimum and maximum values.
From the structural point of view, the pre-compressed reinforcement of the elements according to the invention allows a reduction of the longitudinal flexural stresses within the limits supported by the reinforced concrete which will absorb the traction stresses generated by the transversal flexion and shearing. The effects of this last action will be reduced by the presence of pre-compression.
The presence of the fibres improves only marginally the value of the tensile strength of the mixture, but it certainly confers to the concrete a very ductile behaviour in case of breaking, so to allow to safely take advantage even from the tensile strength of the concrete.
The presence of a passive reinforcement, where it is necessary, is limited to the edges of the structure where the pre-compression effects do not completely spread. Despite the usual structures made in reinforced concrete or reinforced pre-compressed concrete, where thickness of the structures themselves depends on the presence of the passive reinforcements which must be inserted inside the mixture and covered by a layer of concrete of a suitable thickness, in the case of the invention, the suppression of the passive reinforcements makes unnecessary such a thickness, allowing the manufacturing of lighter structures.
The structures manufactured with fibre-reinforced concrete without any passive reinforcement will provide high duration since there is almost no cracking and, whenever it may locally be, the dimension of the cracks is limited by the presence of the fibres which avoid their spreading.
In the conventional C.A.P. structures, the reinforcement steel bars which are nearer to the surface are the main cause of the structural ruin because of the atmospherical agents which cause their oxidation and consequently the improvement of their volume, that causes the cracking of the concrete.
The possibility of suppressing such a reinforcement avoids the possibility of such a cause of the structural elements granting long durability.
It is possible getting such results, as seen before, by combining the pre-cpmpression technology with known techniques like the addiion of different kind of fibres (steel, carbon, glass, plastic materials, etc. that have a lenght and diameter depending on the mixture) in concrete the characteristics of which, like specific weight, dimension of the aggregates, resistance, depends on the kind of its use.
The usual ways of manufacturing and seasoning the concrete are enuogh to get such a result and particular manufacturing, treatments nor seasoning of the concrete are not necessary.
This invention can be applied to all the structures and a particular advantage is obtained in the structures which must bear flexural or shearing stresses and for which it is possible to produce sections in variable concrete mixture, so as to follow - with the design - the internal tensions in order to reduce the quantity of the material employed.
This invention is in particular useful for the pre-compressed prefabricated elements with adherent and post-stressed cables such as beams, fiat roofing-tiles, variable section roofing-tiles, curtain walls, etc.
In particular the invention is useful for the manufacturing of covering prefabricated elements with variable section from the head to the middle zone for the discharge of water, and having closed ends.
Some rising hooks and only a passive reinforcement are provided, only in the zone in which there are the effects of the pre-compression.

Claims

Structural use of concretes of any density, class of resistance, dimension of the aggregates, workability, etc. obtained without any particular production, treatment nor seasoning of the mixture, characterised by the fact of being reinforced by fibres (steel, carbon, plastic materials, etc.) combined with the pre-compression thecnology, in order to obtain the suppression or the reduction of the passive usual reinforcement in all or some parts of the structure.
Concrete for the producrion of structural elements in pre-stressed reinforced concrete (C.A.P.), characterised in that it is made of a mixture comprising a suitable amount of cement, water, aggregates and reinforcing fibres.
Concrete for the producrion of structural elements in pre-stressed reinforced concrete (C.A.P.) according to claim 2, characterised in that it consists of a mixture comprising, for 1 m³ of concrete:

350 to 500/ m³ of cement;

100 to 250 lt of water

1800 to 2000 kg/ m³ of aggregates having dimensions up to 20 mm.;

12,5 to 60 Kg./ m³ of reinforcing fibers;

fluidificants from 1 to 3% of the weight of the cement.
Reinforced pre-compressed structural concrete elements, characterised by the fact of being made of concrete according to any of the claims 1 to 3, wherein it is provided the use of reinforcement fibres instead of part of the whole the passive reinforcement.
Structural elements in accordance with claim 3, in which said fibres are steel fibres in an amount from 30 to 80 Kg./ m³.
Structural elements in accordance with claim 3, in which said fibres are carbon fibres in an amount from 15 to 40 Kg./ m³.
Structural elements in accordance with claim 3, in which said fibres are PVA fibres in an amount from 10 to 30 Kg./ m³.
Structural elements in accordance with claim 4, in which said steel fibres have a diameter from 0,4 to 0,8 mm and a lenght from 20 to 60 cm.
Structural elements in accordance with claim 5, in which said carbon fibres have a diameter from 0,08 to 0,09 mm and a lenght from 20 to 30 cm.
Structural elements in accordance with claim 6, in which said PVA fibres have a diameter from 0,40 to 0,70 mm and a lenght from 25 to 35 cm.
Prefabricated pre-compressed elements such as beams, flat roofing-tiles, variable section roofing-tiles, curtain walls, made of fibre-reinforced concrete in accordance with any of the preceding claims, characterised by the fact of providing the suppression or the reduction of the usual passive reinforcement in the whole or in part of the structure.
Variable section covering prefabricated elements from the share-cropping top to the head in order to allow the discharge of waters and to follow with the design the internal section of the tensions, characterised by the fact of being made with fibre reinforced concrete in accordance with any of the preceding claims combined with the precompression technique, and in which in the head zones are fitted the rising hooks and the passive reinforcement for the distribution of the precompression, said passive reinforcement being limited to the head zone.