Process for producing microfiber-reinforced concrete and microfiber-reinforced concrete

Abstract

A method for producing a microfiber-reinforced concrete (1), comprising a cement matrix (10) into which a microfiber additive made of fiber elements (11) is introduced, and wherein the fiber elements (11) comprise a shape memory alloy, the method comprising at least the following steps: - training a fiber shape (12) of the fiber elements (11) at a temperature (T1) above a transition temperature (T2), wherein the fiber shape (12) enables the fiber elements (11) to interlock, - Cooling of the trained fiber elements (11), - plastically deforming the fiber elements (11) from the trained fiber shape into an intermediate shape (13) to form a compacted shape which prevents the fiber elements (11) from becoming entangled, - introducing the fibre elements (11) into the cement matrix (10) to form a fresh concrete (14), - Pouring the fresh concrete (14) and - Heating the fresh concrete (14) to the transition temperature (T2) so that the fiber elements (11) return to the fiber shape (12) by interlocking the fiber elements (11).

Description

The invention relates to a method for producing a microfiber reinforced concrete, comprising a cement matrix into which a microfiber additive made of fiber elements is introduced, wherein the fiber elements comprise a shape memory alloy. The invention is further directed to a microfiber reinforced concrete produced by such a method.PRIOR ARTThe combination of properties of microfiber reinforced concrete with rheological properties of fiber free concrete is a central objective in modern construction. In particular for high-performance concretes such as the ultra-high-strength concrete (UHPC), the use of microfibers is advantageous, in some cases even indispensable. The already very good compressive strengths of the ultra-high strength concrete can be combined with exceptionally high tensile strengths by using micro-steel fibers. This makes the material concrete usable for many very delicate and thus weight-saving construction constructions and makes possible fundamentally novel constructions. It is known that by means of a fiber geometry, for example with end hooks of the fibers, the connection between high-strength steel fiber and the cement matrix is considerably improved. Disadvantageously, however, rheological properties of the concrete are significantly impaired. When using fiber elements with diameters of less than 1 mm with an addition in the range of less than 2 vol %, restrictions on the processability must be determined. This results in so-called gel formations, which describe agglomeration of fiber elements. This results in a clearly uneven distribution of the fiber elements in the cement matrix, so that incomplete compaction of the concrete also results.For example, U.S. Pat. No. 5,858,082 A discloses a method for producing a microfiber-reinforced concrete, having a cement matrix in which a microfiber additive made of fiber elements is introduced, wherein the fiber elements have a shape memory alloy. The shape memory effect of the shape memory alloy is used in such a way that the fiber elements interlock with one another and a prestress builds up in the cement matrix when the fresh concrete is heated to the transition temperature of the shape memory alloy. A microfiber reinforced concrete is thereby produced which has a prestressed reinforcement of microfibers. The training describes the embossing of a certain shape on the fiber elements, which can re-resemble them, i.e. can assume them, when a so-called transition temperature is reached, even if the fiber elements have been temporarily plastically deformed. The transition temperature is usually well below the temperature at which the training of the memory shape of the fibre elements is carried out, for example by means of an embossing process.An improvement in the rheological properties with the fiber elements introduced into the cement matrix cannot be implemented with the previously known application of fiber elements made of shape memory alloy. In particular, it is specified that the fiber elements should have a straight extension in order to process, in particular cast, the concrete in this state of the fiber elements. Here, fiber bundles agglomerate particularly strongly, resulting in marked inhomogeneities in the distribution of the fiber elements in the cement matrix at higher volume fractions of the fiber elements in the cement matrix.DISCLOSURE OF THE INVENTIONThe object of the invention is to improve a method for producing a microfiber-reinforced concrete, wherein the concrete is to have a high proportion of fiber elements, and in addition the microfiber-reinforced concrete is to be well processable, in particular casting of the concrete is to be possible without the fiber elements increasingly accumulating into fiber bundles.This object is achieved on the basis of a method according to the preamble of claim 1 and on the basis of a microfiber-reinforced concrete according to claim 10 having the respectively characterizing features. Advantageous further developments of the invention are specified in the dependent claims.To achieve the object preceding the invention, it is proposed to carry out the method for producing a microfiber-reinforced concrete, having the following steps: training a fiber shape of the fiber elements at a temperature which is above a transition temperature of the shape memory alloy, wherein the fiber shape is to be designed such that interlocking of the fiber elements with one another and / or with the cement matrix becomes possible. Furthermore, the method comprises cooling the trained fiber elements, followed by plastic deformation of the fiber elements from the trained fiber form into an intermediate form, forming a compacted form. The intermediate shape is selected such that this prevents the fiber elements from becoming hooked. The fiber elements are then introduced into the cement matrix to form a fresh concrete, which is subsequently cast without the fiber elements becoming hooked. Finally, the fresh concrete is heated to the transition temperature, so that the fiber elements can be restored into the fiber form with the fiber elements being hooked.As a result, a microfiber reinforced concrete can be produced which can be cast very well and in which the proportion of fiber elements can be significantly increased. The effect of the clearly improved rheological properties, i.e. for example the castability of the microfiber-reinforced fresh concrete, is that during casting the fiber elements are present in the intermediate form with which entanglement of the fiber elements is prevented. As a result, the formation of the gel, i.e. the agglomeration of the fiber elements to form fiber bundles, is also significantly reduced, and a microfiber-reinforced concrete is provided, which has a homogeneous distribution of the fiber elements, even in the case of filigrane casting molds. As a result, this makes it possible to clearly increase the strengths of the concrete, wherein, despite the increased strengths of the concrete, a processability, even for example of filigrane structures, remains possible.The training of the fiber shape of the fiber elements can take place, for example, at a temperature of 300° C. to 400° C. and in particular at a temperature of 340° C. to 360° C. The training of the fiber form is preferably carried out at a temperature of 350° C.It is also advantageous that the fiber elements have a straight extension before the training of the fiber shape. The fiber elements can thus be provided in an inexpensive manner, for example by first providing the fiber elements in wire form and, for example, unwinding them from a coil and cutting them to length.For example, the training of the fiber form is effected by embossing the fiber elements supplied in a straight form into a fiber form with fiber ends bent or bent over. The process of embossing the fiber elements can be carried out in a process-uniform manner with the cutting of the fiber elements to length from an endless wire.The method of training the fiber form is carried out in such a way that the fiber form is formed in particular by end hooks at the fiber ends introduced into the fiber elements. In this case, it may be sufficient, for example, to provide the fiber ends with a bend, bend them over or form them as barbs. The basic shape of the fiber form preferably maintains an elongate extension in order to maintain the effect of the reinforcement.To form the intermediate form from the trained fiber form, the fiber elements are, according to the invention, converted into a compacted form and the compacted form must be designed such that it is no longer possible for the fiber elements to become hooked together and for the fiber elements to become hooked together with the cement matrix. The compacted form can comprise, for example, a kringel form or the intermediate form has a winding form, as a result of which entanglement of the fiber elements is prevented. The winding form or the winding form can be designed in particular such that the ends of the fiber elements in a winding or a kringel, a winding or another compacted form point inward. As a result, the fiber elements can get together in their intermediate form without these becoming hooked together or otherwise forming a form fit with one another and agglomerating. The rheology of the fresh concrete in the cast-capable state is significantly improved as a result of the non-hooking of the fiber elements, since no agglomerations of fiber elements can form. If the fresh concrete with the fiber elements in the intermediate form is subsequently heated again to the transition temperature in the processed, i.e. for example cast state, the fiber elements lose their intermediate form again and re-resemble the trained fiber form which has the desired end hooks, in that the fiber elements re-resemble the trained form. A positive-locking effect of the fiber elements with respect to one another and of the fiber elements with the cement matrix is thus restored, which takes place only after the desired casting mold of the component is produced from the microfiber-reinforced concrete.For example, the transition temperature may have a value of 40° C. to 50° C. The level of the transition temperature depends in particular on the material of the shape memory alloy. Not only the fiber elements but the entire cement matrix are heated to the transition temperature, so that with the return of the fiber elements from the intermediate mold into the trained mold, an improved and accelerated curing of the concrete results at the same time.A particular advantage is achieved if the fibre elements are introduced into the cement matrix with more than 2% by volume. In this case, the fiber elements can be introduced into the cement matrix for example with a length of 5 mm to 50 mm, preferably 10 mm and / or with a diameter of 0.1 mm to 2 mm. The fiber elements do not have to have a circular cross section and can also have a rectangular cross section or even an undefined cross section, for example. The geometric dimensions of the fiber elements depend in particular on the geometry of the component to be produced, which is to be produced with the microfiber reinforced concrete.The invention is further directed to a microfiber reinforced concrete produced by the method of the invention. The fiber elements can have a shape memory alloy comprising a copper- and / or iron-based. It is also advantageous that the fiber elements are hooked together and / or with the cement matrix in the solid concrete, in particular by end hooks introduced into the fiber elements.PREFERRED EMBODIMENT OF THE INVENTIONFurther measures which improve the invention are described in more detail below together with the description of a preferred exemplary embodiment of the invention with reference to the single FIGURE. It shows:FIG. shows a schematic sequence of the steps of the method according to the invention for producing a microfiber reinforced concrete.The FIGURE shows a sequence of a plurality of method steps 101 to 107, which describe a method according to the invention for producing a microfiber-reinforced concrete 1.The method starts with the provision 101 of fibre elements 11 and the heating of the fibre elements 11 to a temperature which is above a transition temperature. The transition temperature of the shape memory alloy fiber elements 11 is the temperature above which the shape memory alloy resembles an originally trained geometric shape by the shape memory alloy component re-forming into this resembled shape.The next step of the method according to the invention is formed by an antraining 102 into a fiber form, wherein the fiber elements 11 are brought into the desired shape, for example, by an embossing process or another plastic deformation process, which is to be reminiscent upon reaching the transition temperature by the shape memory alloy. Subsequently, in a further step, the trained fiber elements 11 are cooled 103 from a temperature T 1 for training the fiber shape, for example, to a room temperature.In a further step 104, the fiber elements are plastically deformed into an intermediate mold 13 which is formed such that the fiber elements are prevented from becoming entangled with one another and also with a cement matrix.This is followed by the introduction 105 of the fiber elements in the intermediate mold 13 into the cement matrix 10 and the casting of the fresh concrete. Once the fresh concrete has been cast, the heating 106 of the fresh concrete to a transition temperature T 2 takes place, so that the microfiber-reinforced concrete is subsequently obtained 107. In method step 107, the fiber elements 11 are again present in the trained form for casting the concrete, so that the strength-enhancing effect of the fiber elements 11 in the cement matrix 10 can be fully utilized despite a simple preceding casting of the fresh concrete.In step 101, the fiber elements 11 are shown which have a rod shape. From step 101 to step 102, heating is effected by adding heat, shown with an arrow, for example to a temperature of 350° C. In step 102, the embossing of the fiber elements 11 is effected into a fiber form 12 in which these have form-fitting end hooks 15 of the fiber elements 11.Following step 102, cooling 103 takes place from the temperature T 1 for antraining of the fiber form 12, for example, back to room temperature.In step 104, a kringle shape of the fiber elements is shown, which corresponds to the intermediate shape 13. The fiber elements 11 in the intermediate mold 13 are introduced into the cement matrix 10 in step 105, whereby the fresh concrete 14 is provided. The heating to the temperature T 2, which describes the transition temperature, then takes place, so that the kringle-shaped intermediate form 13 of the fiber elements 11 from step 105 is formed back into the fiber form 12, which was originally trained in step 102.A substantially larger number of fiber elements 11 per unit volume of the cement matrix 10 can be introduced into the microfiber-reinforced concrete 1, whereby the tensile and / or compressive strength of the microfiber-reinforced concrete 1 is significantly increased. For example, the proportion of the fiber elements 11 in the cement matrix 10 is significantly above 2% by volume, wherein despite the increased fiber proportion the rheological properties of the concrete 1 for processing are promoted, since the rheological step of casting the fresh concrete with fiber elements 11 which are present in the intermediate mould 13 takes place.The embodiment of the invention is not limited to the preferred exemplary embodiment given above. Rather, a number of variants are conceivable which make use of the solution shown even in fundamentally different embodiments. All features and / or advantages arising from the claims, the description or the drawings, including structural details or spatial arrangements, can be essential to the invention both individually and in a wide variety of combinations.List of reference numbers:1 Microfiber reinforced concrete 10 Cement matrix 11 Fiber element 12 Fiber form 13 Intermediate form 14 Fresh concrete 15 End hooks 101 Providing fiber elements and heating the fiber elements to a temperature above a transition temperature 102 Training a fiber form 103 Cooling the trained fiber elements 104 Plastically deforming the fiber elements 105 Inserting the fiber elements into the cement matrix and casting the fresh concrete 106 Heating the fresh concrete to a transition temperature 107 Obtaining a microfiber reinforced concrete T1 Temperature for training a fiber form T2 Transition temperature

Claims

Method for producing a microfiber reinforced concrete (1) comprising a cement matrix (10), in which a microfiber additive is introduced from fiber elements (11), and wherein the fiber elements (11) comprise a shape memory alloy, wherein the method comprises at least the following steps: - training a fiber shape (12) of the fiber elements (11) at a temperature (T1) above a transition temperature (T2), wherein the fiber shape (12) enables a entanglement of the fiber elements (11), - cooling the trained fiber elements (11), - plastically deforming the fiber elements (11) from the trained fiber shape into an intermediate shape (13), forming a compacted shape with which an entanglement of the fiber elements (11) is prevented, introducing the fiber elements (11) into the cement matrix (10) to form a fresh concrete (14), casting the fresh concrete (14) and heating the fresh concrete (14) to the transition temperature (T2), so that the fiber elements (11) are formed back into the fiber mold (12) with the fiber elements (11) being hooked.Method according to Claim 1, characterized in that the training of the fibre form (12) of the fibre elements (11) takes place at a temperature (T1) of 300°C to 400°C, preferably at a temperature (T1) of 340°C to 360°C.Method according to claim 1 or 2, characterised in that the fibre elements (11) have a straight extension before the training of the fibre form (12).Method according to claim 3, characterised in that the training of the fibre form (12) is carried out by means of embossing the fibre elements (12) supplied in a straight form.Method according to claim 4, characterised in that the trained fibre form (12) is formed at least by end hooks (15), which are impressed into the fibre elements (12), at the fibre ends.Method according to one of the preceding claims, characterized in that, in order to form the compacted intermediate mould (13), the fibre elements (11) are brought into a kringle shape, a knaul shape or into a winding shape, as a result of which the fibre elements (11) are prevented from becoming hooked.Method according to one of the preceding claims, characterized in that the transition temperature (T2) is chosen to be from 40°C to 50°C.Method according to one of the preceding claims, characterized in that the fibre elements (11) are introduced into the cement matrix (10) with more than 2% by volume.Method according to one of the preceding claims, characterized in that the fibre elements (11) are introduced into the cement matrix (10) with a length of 5 mm to 50 mm and / or with a diameter of 0.1 mm to 2 mm.Microfiber reinforced concrete (1) produced by a method according to one of claims 1 to 9.Microfiber reinforced concrete (1) according to claim 10, characterized in that the fiber elements (11) comprise a shape memory alloy comprising a copper and / or iron base.Microfiber reinforced concrete (1) according to claim 10 or 11, characterized in that the fiber elements (11) are hooked together and / or with the cement matrix (10) in the solid concrete.

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