The goals of concrete compaction are to give the concrete element the required rigidity and its visible surfaces the aesthetic effect desired, either directly or through subsequent mechanical working, for example
- Bush hammering,
- Sand blasting and
The mechanical vibrations to which the mixture is subjected lead to compaction of the concrete. The compaction process itself consists of three individual processes with differing degrees of contribution. There are
- the odering process,
- the transport process and
- the separation process.
A model demonstrates the processes which occur in the concrete mixture during the vibration process. In the model there should only be one fraction, according to size and shape with the same grain size, balls. Then there is either the loosest or the most compact form of storage with either 48 % or 26 % pore volume respectively. The ordering process leads to a change in the pore volume. Noise protection walls with rough no-fines structures and masonry of expanded clay are made with a concrete with almost exactly this grain distribution.
Lower pore volume and greater solidity require the well-known grain distribution in the mixture. Fractions with ever smaller grain sizes must be transported into the hollow spaces through a rough grain mesh, which hardly changes its position after the mould has been filled (Fig. 3).
The fractions transported must replace air and excess water which is then precipitated via the surface of the mixture. The mechanical vibrations the mixture is subjected to must lead to movement of the particles in the mixture, in other words they must accelerate the particles and thus overcome the retention force between the particles or at least temporarily increase the distance between them; the individual processes run their course.
As a source of mechanical vibration, which does not propagate like acoustic vibration, but rather through grain to grain contact, it is easiest to think of the concrete internal vibrator whose surface area periodically makes contact with the fractions lying closest to it.
In the industrial manufacture of pre-cast concrete elements, it is not and should not be possible to produce using internal vibrators for reasons of concrete technology, production technology, and manufacturing costs. In other words, the mechanical vibrations required must originate at the shuttering surface of the mould. In order to achieve this, external vibrators are installed on the skeleton of the concrete moulds.
These skeletons today are almost always made of steel profiles. The rotor cams are unbalanced and deform the steel profile and thus the skin of the shuttering in contact with the concrete, i.e. what is so easy to understand for the internal vibrator, becomes a thin line between destruction and usefulness for the external vibrator.
Half the deformation effect is therefore half the displacement of the mechanical vibration generated. The product of this and the square of the vibration frequency is the acceleration amplitude (b = a x f²).
At 8 g, a level of acceleration which in practice should be the average generated at the concrete skin, the peak/peak displacement should be 1,6 mm using a 50 Hz vibration frequency - with electric vibrators this is possible directly using the normal 50 Hz net, i.e. without a converter, at 100 Hz vibration frequency it only needs to be 0,4 mm. These two figures show that the use of so-called high frequency vibrators on moulds is done largely for reasons related to the machines, and not for reasons related to the production of concrete.
There are usually several resonance frequency ranges for each mould resulting from the characteristics of the steel profile, the material of the shuttering, the characteristics of the rubber dampers, and their spacing. There are also frequency ranges in which it is exceedingly difficult to generate vibrations in the mould. If one has a vibrator with fixed vibration frequencies, i.e. dependant on the normal power net, with a selection of one of five frequencies between 50 Hz and 200 Hz, then it is entirely possible to land in a frequency range in which the compaction result is unsatisfactory.
By means of electronic frequency converters it is both technically and economically possible today to generate any vibration frequency. In other words to vibrate any mould at the frequency or in the frequency range in which it will react most favourably (Fig. 7).
The compaction of the concrete is of great importance for the success of the product, i.e. that the pre-cast concrete element achieves the characteristics which the designer intended and that it can be produced at the calculated price.
For this reason vibrators used in compaction equipment today should consist entirely of units which are supplied by frequency converters with variable output frequency. In this way the compaction process can be optimised and freed from any uncertainty which may result from having to choose between one of five fixed frequencies.
If one selects an electronic frequency converter from among the innumerable suppliers which is willing to adapt the software for the equipment to the specific requirement of operating a vibrating station and supply equipment that is up to the physical conditions prevailing in pre-cast concrete plants, then the concrete compaction process and the results of it can be truly revolutionised. In the following only compaction equipment will be described which owes its efficient operation to the opportunity of optimising the vibration frequency and other adaptations required for use in a pre-cast concrete plant.
Example: Manufacture of element floors
The problem was to equip a fixed production line for element floors with external vibrators. However, since on one side power rails for an automatic concrete spreader were to be installed, it was only possible to mount the external vibrators along one side, instead of along both sides as usual.
First a 12 metres segment of the line was measured to see if it was even possible to install them on only one side. For the test operation a frequency of 77 Hz was chosen, which should provide adequate displacement and acceleration amplitude for successful production. The equipment in use is depicted in figures 11 and 12.
The production hall in Berlin-Köpenick used five production lines. The vibrators on the production lines are powered sequentially from a central power supply and have a central control unit. The equipment cabinet for the vibrators of each production line is equipped with a rotary potentiometer.
This can be used to adjust the frequency determined in the trials for each of the seemingly identical steel structures which is necessary due to the differing weld tensions so that the optimum vibration frequency can be found for each production line. The workers simply switch on the vibrators, which are divided into four groups, for each line via a radio control and the vibrators are supplied with the optimised fixed frequency.
Manufacture of rod-shaped and large-scale pre-cast concrete elements:
The vibrators are connected to shuttering with greatly differing types of structures, but are connected to a power supply which is centrally installed in the production hall. There is shuttering in which the shuttering consists of glued wooden panels which are screwed to the steel skeleton and ones in which the shuttering is steel plates which are welded onto the steel skeleton, in addition to a combination of these two forms. This circumstance means that each type of the shuttering achieves its optimum compaction at another frequency.
In the compaction facility, the worker makes the connection between the central power supply and the shuttering vibrators at the beginning of the concrete filling and compaction process by pressing a button. At the same time as this switching action, the start and maximum frequencies, i.e. the vibrator frequency range, is determined at which the shuttering can be optimally vibrated.
Via a radio remote control the worker switches the required vibrator groups on and off and during operation adjusts the vibration intensity to the amount of concrete as it is being filled into the mould. A very visible large-digit display informs the worker of the intensity currently set. The intensity is displayed in percent, i.e. the maximum and minimum values for the various shuttering types are not recognisable so they cannot confuse him. For the worker the start frequency is always 0 %, the maximum is always 100 %.
Example: Hollow floor production, untensioned
Above and beyond the advantages mentioned in the examples, the use of electronic frequency converters makes it possible to prevent the problem of undesired self-synchronisation of the vibrators. When several vibrators are installed on the steel profile of the shuttering, it sometimes happens that the rotors fall into sync, with the result that their cams come into phase in such a way that the compaction effect is neutralised. This state is easy to spot.
Periodically no compaction takes place. It can be heard, it is recognisable by the rising and falling noise level.
Vibrators which are used in the sheet steel moulds used in the manufacture of masonry in floor production are installed with vertical rotors very close to each other. The phenomenon of undesired synchronisation would prevent any production whatsoever. In order to avoid this, the externally mounted electric motors which power the vibrators are supplied with varying frequencies. A frequency difference of approximately 7 Hz prevents self-synchronisation.
This method has proven to be effective over many years and may be transferred to the manufacture of large-format pre-cast concrete elements.
The vibrators for such moulds are supplied by two frequency converters. They are then alternately connected so that two neighbouring vibrators are not connected to the same converter. In production plants for the manufacture of untensioned hollow floors of a well-known manufacturer in Henningsdorf near Berlin and in Nersingen near Ulm, the vibrators on the production palettes are powered with a frequency difference of 7 Hz. The vibrators used on the core tubes use a third frequency which is approximately 30 Hz lower. Since any form of self-synchronisation is thus impossible, the compaction takes place within the fixed vibration time very intensively and with great exactness in repeated manufacture.
When it is possible, i.e. when the dimensions and weight of the vibrator units allow it, the displacement of the mechanical vibrations should not undertake the deformation of the mould profile. The correct selection and installation of vibrators should move the entire vibration unit without deformation.
Example: Vibration table for compaction of high-quality concrete elements
Vibration tables with edge lengths of up to ca. 4 metres can be made so rigid that two vibrators installed along the same parallel axis - it could be two vibrators which are forced into synchronisation - can move the upper portion of the table, which is vibration insulated, exclusively vertically linearly.
Displacement and acceleration amplitude have a sinus shaped course, good compaction effects rely on this. Today such vibrators are supplied by a frequency converter; the intensity of the vibration can then be adjusted to the amount of concrete that has been filled into the mould.
If pre-cast elements are to be made whose visible surfaces are to be mechanically worked after hardening, e.g. by sandblasting, then the concrete structure under the surface must be completely closed. Only then can the sandblasting lead to the desired structuring effect.
In addition to the compaction vibrators installed under the table, there are two vibrators with vertical rotors installed diagonally on two of the four corners of the table. Their vibration frequencies to each other and to the compaction vibrators are different.
The horizontal direction of the mechanical vibrations attracts the fine grained fraction of the mixture to the edge of the mould. This guarantees that the sandblasting does not make any faults in the surface visible.
Example: Concrete pipe compaction
Today pipes are produced in which a plastic pipe is coated with concrete. The plastic pipe stretched over the core of the pipe making machine does not allow the use of vibrators. External vibrating may not and cannot deform the exterior mould, especially for pipes with a small diameter. This would lead to fatigue breaks after only a short time in operation.
Two tandem groups consisting of 2 single vibrators whose rotors are forced into synchronisation using a cam shaft are mounted on the circumference of the exterior mould and set off from each other by 90 degrees. In order to prevent undesired synchronisation of the vibrators, each tandem group is supplied by a subordinate converter which operate with a frequency difference of 7 Hz to each other. The torsion vibrations generated overlap each other; the result is good compaction without the transport which separates the fine grained fraction from the large grained fraction.
For the last two examples the displacement, as has already been stated, does not proceed from the deformation of the shuttering skin, but from the relation of the cam masses of the vibrator rotors shafts to the vibrating mass of the system. This means the choice of vibrating frequency can be largely determined by the concrete manufacturing aspects of the system.
The possibility to "quietly" compact concrete
The noise which is caused by the vibrating of moulds and shuttering is radiated by the surfaces of the shuttering skin and the profiles of the skeleton, which may measure many square meters in total. For vibrating systems in which the displacement does not arise from the deformation of the steel profile and shuttering skin, as in the last two examples, the proper acceleration amplitude for compaction can be formed by the product of a relatively large displacement at low vibration frequency.
Electronic frequency converters make it possible to "gently" start up rotors with extremely large cam masses, compared with normal cam masses, and to operate them at any desired rotational frequency. If the rotor shafts of several vibrators are forced into synchronisation, great masses can be subjected to mechanical vibrations with large displacement.
Lately ceiling and wall elements have been "quietly" compacted in the vibrating stations of a number of palette circulation systems. The same can be said of a number of tipping tables whose palette-like upper portions "floats" on carefully chosen and selected rubber shock absorbers on the heavy parallel supports for the tipping direction. The noise level during compaction is normally 80, max. 85 dB (A).
The linear horizontal displacement is generated in practice by oscillating vibrators which practically deliver tractive force and pressure when mounted on their base plates. This method is being used in the field since several years now and has proven that 'quiet' compaction with vibrators is possible.