DK1293314T3 - A method and device for compressing a mixture - Google Patents

Description

The invention relates to a method for compacting and molding a mixture, preferably a concrete mix, in a mold receiving the mixture. The invention further relates to an arrangement for compacting and molding a mixture with regard to the problem of the uniformity of compacting the mixture.

The compaction of a mixture is common in many fields of technology. It may concern, for example, bulk goods such as sand or cereal grains, the volume of which is to be reduced for the purpose of storage. Compaction is of outstanding importance in the prefabrication of structural units from a concrete mix. What is important here is a high and uniform density of the mixture, a property imperative for high compressive strength and good material quality. For this reason, a great number of methods and arrangements for compacting a concrete mix have been developed and become known in prior art. The essential idea with all these methods is to impress upon the mold an oscillating or rotating movement that leads to vibrations acting on the concrete mix. In the end, this reduces frictional and adhesive forces in the mix and induces therein a flow process that, in interaction with the force of gravity, causes trapped air to escape to the top and permits a more compact arrangement of the constituents of the mixture; thus, for example, in a mixture of constituent particles of varied size, larger interstices between large-volume particles are filled up with particles of smaller volume.

For a good survey of methods and arrangements for compacting concrete mixes by means of vibrating or jolting the mold (the two verbs being used as synonyms herein), reference is made to the book „AuBenrQttler - Grundlagen und praktische Anwen-dungen der Rutteltechnik" by S. Warnbach and W. Schneider, published by „verlag moderne Industrie", Landsberg 1992. In the arrangements described there, harmonic vibrations are generated and transmitted to the mixture via the mold. The vibrations are generated by means of one or several rotating mass unbalances mechanically coupled to the mold.

For a homogeneous and high compaction of the concrete mix, it is decisive that for one thing, a sufficiently high vibrational energy - though not as high as to cause damage to occur to the arrangement or the mixture to separate - is transferred into all areas, and for another, this happens uniformly all over the contact surface between the mold and the concrete mix. The bigger the mold is, the greater is the effort needed to satisfy these requirements, and the greater is the noise level connected with generating the vibrations and jolting the mold.

Another drawback generally inherent in the design of larger arrangements of this kind is the fact that the acceleration amplitude — the second differentiation of the vibrational function with respect to time, and a measure of the energy input to the mixture — is not distributed uniformly throughout the bottom of the mold, which inevitably leads to irregularities in compaction. These become greater as the mold bottom surface, under which the vibrators are affixed, is enlarged, because the mold acts increasingly less like a rigid body, which entails problems since differences in the acceleration amplitudes, in case of low amplitudes, lead to locally insufficient compaction, but in case of high amplitudes lead to damages to the compacted mixture or to separation phenomena. With higher frequencies and larger mold dimensions, therefore, the flexural properties of the mold bottom become ever more relevant. The flexural vibrations induced are modes of forced vibration in the shape of standing waves, i.e., waves featuring local nodes and antinodes, which constitutes a non-uniform distribution of the acceleration amplitude and thus rather aggravates the disadvantage of inhomogeneous energy input. However, theoretical investigations as described in „Betonwerk + Fertigteil-Technik", issue 8, 1999, pages 52 to 59, come to the very obvious conclusion that uniform compaction can be achieved even with flexible forms, provided that the acceleration amplitudes are distributed statistically evenly with regard to space and time. There is no evidence, though, that this fact has been put into practice in prior art. DE 34 27 780 A 1 discloses a method for compacting and molding a mixture in accordance with the generic part of Claim 1. A different way is adopted with arrangements working in a range of low frequencies of up to 15 Hz, which are outside the range of human acoustic perception and generate vibrations in the horizontal plane. In DE 43 41 387 A 1, for example, an arrangement is proposed in which the mold is excited to perform circular jolting movements in the horizontal plane. The magazine „Hochbau" in its 3rd issue of 1996, pages 18 to 21, also proposes an arrangement using circular jolting movements. While these arrangements have lower sound pressure levels than the other arrangements, they have other disadvantages, though. In addition to high manufacturing costs due to the complexity of their design, they also suffer from rather high material load and wear: To achieve sufficient compaction, a high transmissible vibrational energy must be available, for which purpose waves with a high energy density have to be generated in the concrete mass. The energy density is proportional to the square of the frequency for one thing, and to the square of the amplitude for another. With low frequencies, therefore, the mold must be set vibrating with a sufficiently high amplitude, which leads to the disadvantages mentioned above. Another disadvantage with exciting low-frequency horizontal vibrations is the fact that this procedure can, with proper quality, compact only easily workable concrete mixes, i.e., mixtures of soft to flowable consistency. In addition, the concrete height to be processed is limited because, with greater concrete heights, the low-frequency horizontal vibration is not transmitted uniformly, resulting in non- uniform compaction.

Whereas, then, with smaller molds of rigid design one can achieve good results as regards a uniform energy input into the concrete mix, the fabrication of larger structural units in molds having extended mold bottoms requires either non-uniform compaction or long manufacturing times and higher manufacturing costs to be put up with.

Proceeding from the prior art as described, the invention, therefore, is based on the problem of developing a method by which, in the fabrication of large-area precast concrete units, a uniform input of vibrational energy into the concrete mix plus short manufacturing times can be achieved. The invention is further based on the problem of developing an arrangement for manufacturing large-area precast concrete units, with which a uniform input of vibrational energy is achieved in connection with low-cost operation and short manufacturing time.

According to the invention, the problem is solved by a method of the kind described at the beginning, in which the movement of a progressive transverse wave is impressed upon the bottom of the mold, by which process the mixture is compacted. To generate the wave, it is expedient that forces causing an elastic deformation of the bottom surface are, periodically and time-shifted by one phase each, passed into regions of the mold bottom that are spaced at specified distances from each other. The phase is a function only of the mutual spacing of the regions and of the propagation velocity of a wave in the material of the bottom of the mold.

As a progressive and transverse wave is impressed onto the bottom of the mold, i.e., as the material particles of the bottom are deflected normal to the propagation direction of the wave, the same vibrational energy is passed into the concrete mix in all regions of the bottom. Whereas in a conventional jolting method the acceleration amplitude varies with location and, with a constant frequency, is constant with regard to time, the acceleration amplitude in the invented method varies with time and is independent of the location. At a time average, therefore, the bottom receives an evenly distributed acceleration amplitude, as required for uniform compaction. In addition, as the vibrations generated are vertical, manufacturing times are short.

For selecting the wavelength and the forces, various parameters play a part: The type of the mixture, the design of the mold and its fundamental oscillations, and the load distribution in the vibratory mold vary the vibrational properties and therefore make it necessary to adapt the wavelengths to the respective circumstances. Accordingly, the exciting frequencies may be between 5 and 300 Hz. With the mix commonly used for the fabrication of large-area precast concrete units, it is expedient, however, to select accelerations and phase so that the frequencies of the wave are in a range of 30 to 150 Hz and accelerations are in a range of three to ten times the gravitational acceleration. This will yield the best compaction results.

According to the invention, the problem regarding arrangements of the kind described at the beginning is solved with an arrangement comprising a mold for receiving the mixture in which at least the mold bottom is designed to be flexible, and, arranged under the mold bottom and parallel to an axis, one or several rows of at least two means, coupled together, for force transmission into the mold bottom for the purpose of its deformation, with the said means being driven so as to transmit force periodically and each means being driven with the same cycle, and with force transmission taking place successively by adjacent means, whereby the movement of a progressive transverse wave is impressed upon the mold bottom.

By the arrangement of means for force transmission along an axis, the possible propagation direction of the wave is predetermined, and in case of molds with large-area bottoms, a parallel arrangement of the axes ensures that the wave has essentially only one propagation direction. The means are coupled to each other and driven periodically. By transmitting forces into the mold bottom in regions below which the means are arranged, the mold bottom is subjected to targeted elastic deformation and forced vibration. As all means transmit force with the same cycle while this force transmission takes place successively, i.e. first with the first means, then with the second, then with the third means on an axis, etc., the movement of a progressive transverse wave is impressed upon the mold bottom. The amplitude of the wave is displaced in the course of time, so that all regions of the mixture are successively compacted with the same acceleration amplitude.

Further, it is expedient for the times at which force is transmitted in succession to be selected in such a way that there is a relationship between the times of maximum force transmission of every two means in such a way that the amount of the quotient of the difference between the times of maximum force transmission and the difference between the mutual spacing of the means along the axis and the greatest integral multiple of the wavelength that does not exceed the said spacing, is constant and, at different times, is equal for all pairs of means. Thereby, the phase of the wave is determined explicitly. If the wave movement of the mold bottom is to be independent of the vibrational properties of the face contact material, the distance between two adjacent means along the axis must be distinctly smaller than the wavelength. As the mutual distance between any two means along the axis may be greater than the wavelength, though, this distance is considered modulo the wavelength, i.e., the greatest integral multiple of the wavelength An s - with λ being the wavelength and n a natural number including zero - that does not exceed the distance is subtracted from the distance, with only the residue being considered. In this way of looking at things, the same point in time corresponds to the same locus. The wave, then, satisfies a simple, continuous wave equation, the solution of which is a function of the form

where å denotes the amplitude, Ω the angular frequency, t the time, c the propagation velocity of the wave and x the locus, which here is a unidimensional coordinate, since the means are arranged along an axis. The time of maximum force transmission will generally be at a zero crossing of the derivative. Due to the design of the mold and the means, the point in time may shift, but will be repeated periodically. Here, it serves as a reference; any other point in time may be employed just as well, e.g., the time of the smallest, negative-going acceleration.

In either case, there results a phase relationship of the following kind between two phases φ1, φ2 and two times H, t2, if the cycle is the same with all means:

As a phase φ is the product of the angular frequency Ω with the quotient of locus x relative to a point of reference and the propagation velocity c of the wave, it follows that the constant quotient of the time difference (ti — ti) and the distance (x2 — xi — ηλ) between two means, from which the greatest integral multiple of the wavelength λ, ηλ that does not exceed the mutual distance of the means along the axis has been subtracted, is equal to the reciprocal of the propagation velocity of the wave. Here, the distance of two means explicitly relates only to the distance projected onto an axis along the axis direction; it does not relate to directions across the said axis direction.

In a favorable embodiment of the invention, the mold bottom is provided with stiffening elements spaced at specified distances across the axis. While the parallel arrangement of the rows and the selection of the phases already essentially predetermine the form of a plane wave, the stiffening elements enhance this effect and force it onto the wave.

To ensure its being supported in a plane, the mold preferably rests on elastic elements, the vibrational properties of which have to be selected in such a way that the frequencies of the fundamental oscillations essentially do not superpose on the vibrations induced by the wave. Preferably eligible elastic elements are springs, especially rubber or steel springs or pneumatic suspensions, but elastic layers such as, e.g., insulating mats are preferred as well.

In order that the forces to be transmitted are generated in a row, a favorable embodiment that is easy to implement provides mass unbalances arranged on a drive shaft along the propagation direction of the wave. Due to the rotation of the wave and an arrangement according to the above explanations, the force imparted by the wave is transferred by the mass unbalances to the mold bottom successively in a row, thereby elastically deforming the mold bottom. A transmission of the force can be expediently achieved if, e.g., the drive shaft consists of segments that are elastically coupled to each other, with a mass unbalance assigned to each segment and each segment provided, as means for force transmission, with one or several coupling elements connected with the mold bottom for the purpose of imparting the forces to the same. These coupling elements may be identical, for example, with the stiffening elements provided across the axis. The segments, due to their elastic coupling, are deflected out of the drive shaft’s axis of rotation in accordance with the mass unbalance movement, this deflection passing forces into coupling elements and the mold bottom and deforming the latter. Because of the phase-shifted arrangement of the mass unbalances, a wave is formed the wavelength of which is a function of the phase shift and the spacing of the segments. The embodiment may provide for changing the frequency of the wave during operation, whereby the propagation velocity of the wave will change accordingly. As in this arrangement the force to be transmitted depends on the speed of rotation, the amplitude also depends on that speed, i.e., on the frequency, which makes it possible to change the energy input. Furthermore, it is possible, in case of several rows, to have pairs of drive shafts rotate synchronously in opposite senses. In this way, the horizontal forces occurring can easily be eliminated and need not, or only to a slight degree, be received by other attenuating devices.

In another favorable embodiment of the invention, the means provided for force transmission are eccentric cams arranged on a drive shaft along the propagation direction of the wave. Compared with the arrangement of elastically coupled mass unbalance segments, this arrangement has the advantage that the path amplitude is independent of the frequency and the mass, which permits flexible fabrication of different structural units of different masses with the same vibrational energy input. The geometric form of the eccentric cams determines the maximal amplitude attainable at a frequency; the frequency can be varied via the speed of rotation. In a favorable simple configuration, the eccentric cams, during rotation, are in contact with the mold bottom at least from time to time, but indirect, perma nent connections are feasible, too, whether force-closed or form-closed, the former causing less noise.

In another favorable, particularly low-noise embodiment, the means provided for generating the forces and for force transmission are piezo actuators, hydraulic or pneumatic elements, which are connected to a control device for the successive application of force upon the mold bottom.

Below, the invention will be explained with reference to an exemplary embodiment. In the accompanying drawings,

Fig.1 is the total view of an arrangement according to the invention, and Fig.2 is the view of a drive shaft provided with mass unbalances.

Fig.1 shows the fundamental design of an arrangement with which the movement of a wave can be impressed upon a mold bottom 1. The mold bottom 1 rests on elastic elements 2, which here are configured as a continuous layer. This layer is connected with a base 3, which supports the entire arrangement. The mold bottom 1 is provided with stiffening elements 4. Mass unbalances 5 arranged on a drive shaft 6 serve to generate the forces necessary for deformation. These forces are passed into the mold bottom 1 through the stiffening elements 4, which are also connected with the drive shaft 6, whereby the wave adopts the form of a plane wave.

Fig.2 shows the design of such a drive shaft 6 provided with four mass unbalances 5. The mass unbalances are spaced at equal distances and arranged in such a way that their mass centers are at different locations outside the axis of rotation, but with such regularity that the rotation of the drive shaft 6 will cause the mass centers of the mass unbalances 5 to reach their nearest point to the mold bottom 1 in succession at equal time intervals. Flere, the drive shaft 6 consists of separate segments that are elastically coupled to each other via coupling elements 7, with each segment being assigned one mass unbalance. Because of the elastic connection of the segments, these are deflected out of the axis of rotation of the drive shaft 6 in accordance with the mass unbalance movement, whereby forces are passed into the coupling elements 4 and the mold bottom 1, which thereby is deformed. Because of the phase-shifted arrangement of the mass unbalances 5, a wave is formed the wavelength of which depends on the phase shift and the spacing of the segments.

Claims (12)

1. A method for compressing and forming a mixture, preferably a concrete mix into a mold, taking up the mixture, characterized in that the bottom of the mold characterized by the movement of a continuous transverse wave, since the areas of the bottom of the mold, situated in the predetermined distances relative to one another, cyclically and in each case time-shifted with a phase begins forces to the elastic deformation of the bottom surface, wherein the mixture is compressed.

2. The method of claim 1, characterized in that the wave frequency selected from the range 30 to 150 Hz.

3. A process according to one of claims 1 or 2, characterized in that the generated accelerations in the range of three to ten times the acceleration of gravity.

4. A device for compressing and forming a mixture, preferably a concrete mixture, comprising - a form in which at least the form bottom (1) is formed flexible, for receiving the mixture, - one or more rows which are arranged parallel to an axis Having the form bottom (1), of in each case at least two interconnected agents for power transmission to the form bottom (1) for the elastic deformation thereof, wherein - the means are driven intermittently transmission, and all the means are driven with the same period, and characterized in that that - the power transmission takes place in time progressive using adjacent means by which the form bottom (1) is characterized by the movement of a continuous transverse wave.

5. Device according to claim 4, characterized in that, between the times of maximum power transfer for each two agents is such a relationship that the sum of the quotient of the difference between the times of maximum power transfer and the difference between the agents' spaced apart relative to the axis with the largest integer multiple of the wavelength, which does not exceed the funds spaced apart along the axis, is constant and at different times are the same for all pairs of means by which the form bottom (1) is characterized by the movement of a continuous transverse wave whose propagation velocity is kvotientens reciprocal.

6. Device according to claim 4 or 5, characterized in that the form bottom (1) at predetermined distances transverse to the axis are arranged elements (4) which stiffens the form bottom (1), whereby the wave form of a plane wave.

7. Device according to claim 4 to 6, characterized in that the mold is mounted on the elastic elements (2), preferably of rubber springs, air springs, steel springs or elastic layer.

8. Device according to one of claims 4 to 7, characterized in that for the generation of the forces to be transferred, is provided imbalances (5) arranged in a row on a drive shaft (6) along the wave propagation direction.

9. Device according to claim 8, characterized in that the drive shaft (6) consists of individual segments which are connected elastically to each other, that an imbalance (5) is assigned to each segment, and that in each segment as a means for power transmission is provided one or more coupling elements which are connected to form the bottom (1), the opening of the forces in the form bottom (1).

10. Device according to one of claims 4 to 7, characterized in that as means for power transmission are provided cam motions, which is arranged on a drive shaft (6) along the wave propagation direction.

11. Device according to claim 10, characterized in that the excenterne at least periodically affects the form bottom (1).

12. Device according to one of claims 4 to 7, characterized in that for the generation of forces, and as means for power transmission is provided piezoaktuato acids, hydraulic or pneumatikelementer, and these are connected to a control device for time-sequential application of force of the the form bottom (1).