CS257185B1 - Method for compacting concrete vibration mixtures and apparatus for carrying out the method - Google Patents

Abstract

A method of compacting concrete mixtures, in which a form is placed on a vibrating threshold, into which a concrete mixture is placed, which is compacted by vibration with a circular frequency of the excitation force <wv satisfying the condition 0.9Ω<ων< < Ι,ΙΩ , where Ω satisfies equation (5), where Ω - optimal frequency <or - reduced frequency 1¾ - mass of the concrete mixture mr - reduced mass of the system - damping. The device for carrying out the method consists of a vibrating threshold, to which a vibration exciter is firmly connected, connected by means of a belt transmission with a drive, whereby the vibrating threshold is connected by means of springs to the first adjusting plate and to the second adjusting plate, which are height-adjustable on screws, which are fixed at one end in the base frame.

Description

The invention relates to a method of compacting concrete mixtures and a device intended for carrying out this method.

Modern concrete production cannot do without compaction machines, among which vibrators are the first. The basis of every vibrator is a vibration exciter - a vibrator, characterized by two parameters - excitation force and operating frequency. So far, it is known that

That the vast majority of vibrators produced are rotary vibrators, most often with an operating frequency of 25, 50, 100, 150 and 200 Hz. This wide range poses the question to the technology designer, which of the above frequencies is suitable. The aspects from which this question can be assessed are various, for example, the aspect of the service life of rolling bearings, the geometric dimensions of the vibrator and the like, one of the last aspects is the aspect of the effect of vibration on the compacted concrete mixture, for which the most common criterion is the acceleration of vibration, which is determined based on the diagram shown in Fig. 1: A layer of concrete mixture with a mass lying on a vibrating surface with a mass m _v , to which a vibrator with a centrifugal force F is attached, is vibrated with an intensity ^S o

I /v* a -- ^m b ^{+ m} v where

I - vibration intensity

F - vibrator excitation force

OX m^ - weight of concrete mix m _v - weight of vibrating table with vibration exciter

From this principle, it is clear that the concrete mix is still considered a passive system that participates in the vibration process only through its weight.

However, numerous practical experiences over time have forced a number of additions to the above procedure, for example, it was found that the intensity of vibration decreases with the height of the concrete mixture layer, which led to the compilation of various tables, the so-called attenuation coefficients, which took into account the influence of the layer height. On the other hand, cases were recorded where the intensity of vibration did not decrease with the height of the concrete mixture layer, but instead increased. Similar discrepancies were also found, for example, in the area of the power consumption of electric motors driving vibrators.

So while in one case the motors had to be compensated for poor power factor with capacitor banks, in other cases the same motors were overloaded to the point of unusability.

This eventually led to the general design of a vibrator with variable unbalances, which partially allow the size of the electric motor load to be adjusted. These and other negative phenomena causing serious problems in the design of vibrating machines were solved and even explained by granting the concrete mixture a certain elasticity according to the scheme shown in Fig. 2. The mathematical treatment of this case then showed that the acceleration acting on the concrete mixture does not depend only on the size of the vibrator excitation force and the size of the masses, as follows from formula (1), but that it also depends on the frequency. Thus, it was specifically deduced that the acceleration of the concrete mixture will be

4.2 2 ⁺ 9Í <^v <ω;

ω ² ) ² + «, ² ω ² (2) where = stiffness of the compacted concrete mixture <

— + —) ^m b ^m v

ÍC· = damping coefficient of concrete mixture (— + — ) ^m b ^m vq - circular frequency of the excitation force m, - mass of concrete mixture ^b 111 nv - reduced mass of the system — = — + — ^vm b ^m v

If we illustrate this relationship graphically (Fig. 3), we see that the intensity of vibration of the concrete mixture is significantly dependent on the frequency and it is obvious that from a technological point of view it will be appropriate to use a frequency at which the concrete mixture will vibrate as intensely as possible.

Similarly, it can be deduced that the intensity of a vibrating surface depends on the frequency, according to the relationship:

(Cj2 - coj) ² ₊

-)ω,

J ² (3) where a _v - acceleration of the action surface of the vibrating table

F - excitation force of the vibrator o

- weight of concrete mix m _v - weight of vibrating parts of the vibrating table

- natural reduced circular frequency of the vibration system CJ _r = (stiffness of the layer of compacted concrete mixture)/m ) r

- circular frequency of the excitation force

9C - proportional coefficient of damped concrete mix

If we express this relationship graphically and compare it with relationship (2), we find that there is a frequency range in which the vibration intensity of the concrete mixture increases, while the vibration intensity of the surface through which the vibration is introduced into the concrete mixture is attenuated, which is desirable in terms of the vibrations transmitted to the operator and in terms of noise (see Fig. 4). Finally, it can also be shown that the concrete mixture will load the vibrator with a power of (m _b +m _v ) (cs ² - to ² ) ² + Χ ² ω ² (4) where

P - power consumption

F - excitation force of the vibrator om^ - mass of the concrete mixture m _v - mass of the vibrating parts of the vibrating table.

Gj> _r “ natural reduced ring frequency

- proportional damping coefficient of the concrete mixture

If we plot (2), (3), and (4) in a common picture (see Fig. 5), we find that optimal vibration of the concrete mixture occurs when the frequency according to the relation /5) /--------—j is used for compaction

o: <3»a, q / ,8 /V / ^ω Γ m + 2ií ² a ⁶ (2 + ^) rm r 1/ ^year in

m. m. o

2(1 + ^)ω* -(2+ (5) where the symbols used have the same meaning as in formula (3), or the frequency is very close to it. As can be seen from Fig. (5) at this frequency the mixture will vibrate with great intensity, while the vibration of the mold will be damped and the motor driving the vibrator will be optimally loaded.

In all other cases, there will be unnecessary intense vibration of the vibrating device, weak vibration of the concrete mixture and underutilization or overloading of the vibrator drive;

It follows from the above that the vibration frequency is the dominant parameter of the compaction process, which clearly documents the measurement of the dependence of individual parameters of the vibration process carried out during the tests of the experimental vibrating table - see Fig. 6. The figure also shows that the time required for compaction is significantly dependent on the vibration frequency. Failure to respect the importance of frequency selection leads to significant energy losses, increased cement consumption and increased health hazards for operating personnel due to excessive noise.

As can be seen from Fig. 6, a random choice of vibration frequency not only does not take advantage of the described damping effect, but also lengthens the vibration time and thus the noisy interval. However, lengthening the vibration time also results in a lengthening of the production cycle and thus an impact on the economy of the entire production.

All these shortcomings are eliminated by the method of compacting concrete mixtures, the essence of which is that a form is placed on the vibration threshold, into which the concrete mixture is placed, which is compacted by vibration with a circular frequency of the excitation force fJ _in the satisfying condition 0.9.f]<^CO _v ·ζ 1, where JI satisfies the equation

(5) where the optimal frequency

Ct>. - reduced frequency

- weight of concrete mix m _r - reducible weight of the system

36. - damping

A device intended for carrying out this method, the essence of which consists in that it consists of a vibrating threshold to which a vibration exciter connected by means of a belt transmission with a drive is firmly attached, the vibrating threshold being connected by means of springs to a first plate and a second plate, which are fixed at one end in a base frame.

The advantages of the method and device are that it saves electricity and cement, reduces vibrations that are harmful to the operator, reduces noise and, last but not least, increases the quality of the final product.

The principle of the invention is illustrated in the attached drawings, where Fig. 1 is a diagram of the simultaneous assessment of vibration and concrete mix, Fig. 2 is a diagram of the proposed method of assessing vibration of concrete mix, Fig. 3 is the course of the acceleration of the surface of the concrete mix according to the new assessment, Fig. 4 is the course of the acceleration of the surface of the concrete mix and the acceleration of the vibrating machine, Fig. 5 is the course of the main vibration parameters when applying the new assessment method, Fig. 6 is the course of the main vibration parameters determined by measurements on a vibrating table, Fig. 7 is an example of a practical arrangement of the device from the drive side and Fig. 8 is a side view of the device.

In Fig. 1 and 2, m is the mass of the concrete mix and m is the mass of the vibrating table with b in the vibration exciter. The individual lines in Fig. 5 and 6 relate to the following facts: solid line acceleration of the action surface, dashed line acceleration of the BS surface, dashed line power supplied to the BS, broken lines squares time required for compaction, broken lines triangles vibrator power input, broken lines X acceleration of the BS surface and broken lines circles acceleration of the mold bottom.

The device consists of a vibrating threshold 1, in which a vibration exciter 2 is mounted, driven by an electric motor 2 using a belt transmission 4_, the belt transmission £ being selected so that the circular frequency of the vibration exciter 2 satisfies the condition given by formula (5). The vibrating frame 2 is connected by means of springs j5 to the first adjusting plate £ and the second adjusting plate 2, which are height-adjustable on screws 2. The electric motor 2 ^is arranged on the first adjusting plate 2, which ensures that the length of the belt transmission 2 does not change when the height of the bench is adjusted.

The description of the device's function is as follows: after switching on the electric motor 2, the vibration exciter 2 is driven into rotation by means of a belt transmission 4, which develops a centrifugal force that oscillates the vibration threshold 2r, which is made possible by the threshold being mounted on springs 2.

The method of compacting concrete mixtures according to the invention can then be implemented by placing a mold on the vibration threshold, into which the concrete mixture is placed, which is compacted by vibration with a frequency satisfying the condition 0.9-ΩΛ ^ω ν < l,líl, whereXl satisfies equation (5)

where n - optimal frequency

- reduced frequency m, - mass of concrete mix b

- reduced system weight

- damping

The solution according to the invention has wide application in the construction industry. This device is used, for example, to process concrete mix in the production of ceiling panels for residential and civic facilities, in the production of railway sleepers, engineering prefabricated products and other building components used in the construction industry.

Claims (2)

A method for compacting concrete mixes by vibration, wherein a mold in which a concrete mix is introduced is deposited on a vibration threshold, characterized in that the concrete mix is compacted by a vibration with a circular excitation force CO satisfying the condition 0.9H-CJ1, ljQ, where n satisfies the equation i -------- » ₊ 2? ύ ^{2 * * *} ω ⁶ (2 ₊ rm _r rm _v m. «m. o 2 (1 + - (2 + r (5)) where XX - optimal frequency Ct) _r ”reduced frequency m, - concrete mixture weight bm _r - reduced system weight - damping) Device for carrying out the method according to claim 1, characterized in that it consists of a vibration threshold (1) to which a vibration exciter (2) is fixedly connected to the drive (3) by means of an arm transmission (4), the vibration threshold (1) is connected by means of springs (5) to the first adjusting plate (6) and the second adjusting plate (7), which are height-adjustable on screws (8) fixed at one end in the base frame (9) »