Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
1999-08-23
2001-05-22
Green, Anthony (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S643000, C428S399000, C428S400000, C428S603000
Reexamination Certificate
active
06235108
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a straight steel fiber for reinforcement of high-performance concrete or mortar.
BACKGROUND OF THE INVENTION.
It is known in the art to reinforce high-performance concretes by means of steel fibers.
BE-A3-1005815 (N.V. BEKAERT S.A.) teaches that for conventional concretes with a compressive strength ranging from 30 MPa to 50 MPa, it makes no sense to increase the tensile strength of a steel fiber above 1300 MPa since an increase in tensile strength does not add any increase in flexural strength to the reinforced concrete. BE 1005815 further teaches, however, that for concretes with an increased compressive strength, the tensile strength of the steel fibers should increase proportionally.
WO-A1-95/01316 (BOUYGUES) adapts the average length of metal fibers to the maximum size of granular elements which are present in high-performance concrete so that metal fibers act as conventional rebars in high-performance concrete. The volume percentage of metal fibers in high-performance concrete is relatively high and ranges
DE-A1-33 47 675 (LAMPRECHT Gerd) relates to an artificial stone of cement or gypsum reinforced by means of thin fibers made of a high-alloyed steel. The high-alloyed steel fibers are provided with roughnesses on their surface in order to increase the adhesion in the cement and the gypsum. The fibers have a diameter ranging from 0.05 mm to 0.15 mm and the depth of the roughnesses is limited to 30% of the diameter of the fibers.
SUMMARY OF THE INVENTION
It is an object of the present invention to further optimize the geometry and the tensile strength of steel fibers to high-performance concrete.
It is also an object of the present invention to reduce mixing problems when reinforcing high-performance concrete with high volume percentages of steel fibers.
It is another object of the present invention to improve the anchorage of steel fibers in the reinforcement of high-performance concrete.
According to one aspect of the present invention, there is provided a straight steel fiber for reinforcement of high-performance concrete or mortar. The steel fiber has a length ranging from 3 mm to 30 mm, a thickness ranging from 0.08 mm to 0.30 mm and a tensile strength greater than 2000 MPa, e.g. greater than 2500 MPa, or greater than 3000 MPa. The steel fiber is provided with anchorages the dimension of which in a direction perpendicular to the longitudinal axis of the steel fiber is maximum 50%, e.g. maximum 25%, e.g. maximum 15%, of the thickness.
The terms ‘high-performance concrete or mortar’ refer to concrete or mortar the compression strength of which is higher than 75 MPa (1 MPa=1 Mega-Pascal=1 Newton/mm
2
), e.g. higher than 200 MPa. The compression strength is the strength as measured by ASTM-Standard N° C39-80 on a cube of concrete of 150 mm edge, where the cube is pressed between two parallel surfaces until rupture.
The term ‘thickness’ of a steel fiber refers to the smallest cross-sectional dimension of a straight steel fiber without the anchorages.
The term ‘anchorage’ refers to any deviation from a straight steel fiber with a uniform transversal cross-section where the deviation helps to improve the anchorage or staying of the steel fiber in the concrete.
Within the context of the present invention, the terms ‘straight steel fiber’ excludes normal bendings but does not exclude small bendings, i.e. bendings with a high radius of curvature, in the steel fiber which are a result of the steel wire having been wound on a spool before the final drawing and/or cutting. Steel fibers with only such small bendings which are the result of the previous winding of the steel wire, are still considered as ‘straight steel fibers’.
The advantage of the present invention may be explained as follows. Concretes have a so-called interfacial zone between the cement paste and aggregates added to the concrete. This interfacial zone can be studied by means of a scanning electronic microscope (SEM). It has been observed that due to an increased presence of water in the neighbourhood of the aggregates, cement hydration is accelerated in the interfacial zone, resulting in the presence of calcium hydroxide intermixed with calcium-silica-hydrates and ettringite in the interfacial zone. The consequence is an interfacial zone with a relatively high degree of porosity. This interfacial zone forms the weakest link of the concrete and determines to a large extent its strength which tends to be smaller than the strength of its cement paste. The thickness of the interfacial zone ranges from about 50 &mgr;m (micrometer) to about 100 &mgr;m around the aggregates. A similar interfacial zone has been observed around steel fibers added to the concrete.
In comparison with conventional concretes, high-performance concretes are characterized by:
(a) a relatively low water/cement ratio (smaller than 0.45);
(b) the addition of superplasticizers which much increase the workability of concrete in spite of the low water/cement ratio;
(c) the addition of mineral additives such as silica fumes, fly ashes, blast furnace slag, pulverized fuel, micro-fillers and/or pozzolans and/or the addition of chemical additives such as water glass and tensides.
The additives mentioned under (c) result in an increased bond between aggregates and cement and result in an interfacial zone the thickness of which is substantially decreased, if not disappeared. Indeed silica fumes, for example, transform the calcium hydroxides of the interfacial zone into calcium-silica-hydrates.
In order to have an effective anchorage or staying in conventional concretes, steel fibers must have anchorages with dimensions that are a few times the thickness of the interfacial zone, i.e. a few times 50 &mgr;m-100 &mgr;m. Anchorages with smaller dimensions will not work to the same degree, since they would not bridge adequately the interfacial zone. In contradiction with the interfacial zone of conventional concrete, the interfacial zone of high-performance concretes is either not so weak or not so thick or even not existent. The result is that steel fibers provided with anchorages of a small dimension work effectively.
A supplementary advantage of the smaller dimensions of the anchorage is that the mixing problem of steel fibers in the concrete is reduced since there are no substantial bendings any more.
Another advantage is that, due to the improved anchorage, the volume of steel fibers needed for a required performance of the concrete, may be reduced, which also reduces considerably the degree of mixing problems. This is very important since the volume percentage of steel fibers in high-performance concrete is substantially higher (normally 1.0% to 4.0%) than in conventional concretes (normally 0.40% to 1.0%), and the higher this volume percentage the greater the risk for mixing problems.
Within the context of the present invention the anchorages are not limited to a particular form or way of manufacturing. The anchorages may take the form of bendings or waves on condition that their dimension in a direction perpendicular to the longitudinal axis of the steel fiber is limited in size. The anchorages may also take the form of micro-roughenings, e.g. obtained by means of a controlled oxidation or by means of a controlled etching operation.
In a first preferable embodiment of the invention the anchorages are indentations which are distributed along the length of a straight steel fiber. The depth of these indentations ranges from 5% to 25% of the thickness of the steel fiber without indentations. For example, the depth of these indentations ranges from 0.01 mm to 0.05 mm. The indentations may be provided at regular distances along the length of the steel fiber.
In a second preferable embodiment of the invention the steel fiber is provided with flattenings at both ends of the steel fiber. The thickness of the flattened ends may range from 50% to 85% of the thickness of the non-flattened steel fiber. Such a steel fiber has preferably an elongation at fracture which is greater than 4%.
In order to prov
Foley & Lardner
Green Anthony
N.V. Bekaert S.A.
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