Krejčí Engineering

Ilja KREJCI began working at the Brno University of Technology in 1968. His most important finding was the discovery of the absorption frequency of concrete.

The history of concrete compaction machines currently manufactured by Krejci Engineering began in the middle of the 20th century. At that time, there was a major need to build a large number of flats, schools and transport infrastructure in the Czechoslovak Republic. Prefabricated technology would play an important role in meeting these requirements. It was necessary to produce components for housing, ceiling beams in new schools and large pre-stressed panels as basic components for bridges within a new transport infrastructure.

The first experiments were carried out in such a way that the large forms were equipped with vibromotors. However, that solution was found to be very energy consuming as each individual vibromotor was not synchronised with the others the vibration was of low efficiency. The concrete compacted in some areas, but not in others. Where the intensity of the vibration was too big it reduced the product compactness. In addition the vibromotors were overloaded if they came into resonance and could not perform satisfactorily. Often, during a single production shift, it was necessary to replace several vibromotors on one mould.

A number of attempts to create technologically sophisticated solutions were carried out at that time. The most successful solutions, that promoted the practice and were patented, were based on vibration tubes.

The Vibration Tube, which transmits vibrations at certain points along its length, contains the common rotor, on which is located a number of unbalances that produce vibrations as it rotates. This solution was proposed by Ing. Ilja KREJCI and it became the basis for a wide range of machines.

The key advantage of using a vibrating tube, to create vibration, lies in the fact that all of the unbalances are located on one common axis. Therefore the exposure to vibration is synchronised. Therefore an appropriate way of attachment of the vibration tube to the mould is achieved through very effective transmission of vibrations into a mould. It also avoids the resonance phenomena. The longest vibration tubes produced are up 12 m long.

The technical solution described above results in the reduction of installed power at the same compaction efficiency. Compaction of large beams with a length of 10 meters or more needs the use of a vibrating power engine of about 5 to 8 kW when the vibration tubes are used as vibration exciters.

When using vibromotors it is often necessary to install numerous machines, each with a power of 1 - 2 kW. In this situation the total power can be higher than 20 kW.

Use of vibration tubes therefore brought energy savings, reduced the time needed for concrete compaction and decreased noise levels.

Vibration technology has been made possible on the basis of several patents by Ing. Ilja KREJCI. Using this technology the company GEOtest, Uhřínov, produced vibration tubes and supplied them extensively throughout Czechoslovakia. Some archive photos are shown below in Archive Pictures.

Archive Pictures
Construction of a bridge using prefabricated beams	The mould for bridge beam 30 m long, with a final beam weight of 150 tons	A mould for bridge beams

Concrete compaction depends on the height of its layer, with a specific frequency at which concrete absorbs vibration energy in a very effective manner. Therefore compaction is obtained very quickly as all vibration energy is absorbed by the concrete and transformed into an increase in its compactness.

As the vibration is transferred to the concrete, which dampens the vibration machine, then the operating noise significantly decreases. On the basis of this knowledge Krejci Engineering compaction machines have been further improved and machines were created that allowed the production of large concrete products with a quality comparable, in some cases, to metallic materials.

The comparison of concrete, produced by traditional methods 30 years ago, and vibration compacted concrete, made using Krejci Engineering methodology can be seen in the figure below.

The comparison of traditional concrete compaction and Krejci Engineering technology after 30 years

The beam was compacted using optimised vibration and the rest of the construction was made from standard concrete. The influence of water and frost is obvious