February 2024

Where the concrete comes from

Trip to the heart of precast concrete production: Euromecc Planetary Mixers.

Introduction
We often focus on large prestige works such as bridges, viaducts and tunnels: infrastructures that make national and international travel possible. All these works are united by the presence of very high quality concretes that require an efficient, effective and replicable production process to be manufactured. This need, which is common to all producers of precast elements, whether blocks, beams, tunnel segments or railway sleepers, translates into the adoption of mixers which are usually of the planetary type.

The operating principle.
Concrete static mixers, whether they are planetary or twin shaft, are equipment used to homogenise the concrete components according to precise recipes and timings. Differences between the two models are related to the mixing technology:

• Twin shaft mixers uses two horizontal axles where there are different arms and paddles; thanks to their rotation it creates a turbulence which agitates and mixes the aggregate with water, cement and chemicals.
• Planetary mixers, on the other hand, are equipped with a gearbox that transfers to the vertical arms, and therefore to the mixing paddles, the number of which depends on the size of the mixer, a dual effect of rotation and revolution, which enables better results to be obtained with lower tank filling coefficients.

Euromecc planetary mixers.
Euromecc has been designing and manufacturing high-efficiency planetary mixers for over 15 years, during which time it has succeeded in developing and improving a product that today represents a reference for the industry. The initial study started from the users, loyal customers who decided to share their experience and indicate the main criticalities in the use of this equipment, among them the difficulty of cleaning the tank and its accessibility for maintenance activities. These two aspects, although simple and intuitive, laid the foundations for an innovative project that would additionally combine high-performance and technologically efficient solutions.
To begin with, it was decided to adopt a feeding system that left the tank free, without obstructing its top, by incorporating the transmission components outside of it using a slewing ring; this made it possible to maintain a flat tank roof, and to obtain a regular geometry for the mixing chamber. Furthermore, the chosen technology made it possible to keep the centre of the tank free and make it available to accommodate the inlet of water and cement, this being the optimal point for the production cycle.
In fact, the planetary mixer has a tank with axial symmetry and the entry of cement, water and additives at this axis can only be an advantage compared to an asymmetrical load, in terms of the speed of diffusion throughout the tank and consequent mixing speed. This effect finds increasing benefits as the tank diameter increases, peaking for the 3.5 m3 output mixer which has a tank diameter of almost 4 metres.
The other aspect was then taken into consideration, namely the accessibility of the tank. In order to meet this need, it was decided to adopt a sector construction of the tank, which would allow the dismantling of an entire 90° sector to gain wide access during more invasive maintenance operations. In addition, large inspection hatches with gas spring-assisted opening were included for daily cleaning and easier operations.
The new doors come with a flat surface on the inside and have sealing profiles that perfectly seal against dust and, where present, the high-pressure water sprays from the washing system.
The experience accumulated over the years has also made it possible to optimise the profile and the material used for themixing arms so as to have a perfect balance between rigidity and compliance. The impact of a shovel is infrequent, but its occurrence must be taken into account to prevent it from causing major damage. In such cases it is the arm that behaves as sacrificial element in order to isolate and protect the transmission.
In addition, the arms are covered with 90 shore polyurethane sleeves, which increases wear resistance and limits the formation of concrete deposits.
The last analysis phase focused on the quality of the mixing. A kinematic study was conducted in order to optimise both the rapid coverage by the paddles of the entire bottom of the tank as well as to maximise the period between two consecutive passes of the same paddle at the same point in the tank.

Configuration flexibility.
In terms of use, Euromecc mixers are extremely versatile and able to meet the needs of any construction site, starting with the discharge configuration: since the tank and its liners are made symmetrically, there is the advantage of being able to orient and choose the number of discharge doors, up to a maximum of four per mixer.
In addition, the discharge conveyors can also be modified to customise the height according to the destination mean, whether it is a flying bucket, a crane skip or a concrete truck mixer, and reduced secondary discharge
outlets have also been designed for the evacuation of washing water only
Having mentioned the liners, it must be emphasised that these are also customisable: starting from a standard Hardox® 500 wear covering, it is possible to choose to use alternative materials such as Ni-hard cast iron, the more valuable Chrome alloy, plates with reported-welding coating, etc.
Starting from a standard Hardox® 500 12-15 or 20 mm thick, you can choose to use alternative materials such as hard metal welded plates; furthermore, for particularly fine materials and/or to extend the life of the tank bottom, 90 shore polyurethane mixing paddles are available.
As far as customisation is concerned, it is worth noting that planetary mixers can be equipped with numerous accessories: from high-speed agitators used to ‘break up’ any lumps in the mix, to humidity probes to constantly monitor the water/cement ratio and determine the consistency of the mix, temperature probes, passing through high-pressure washing systems equipped with retractable nozzle heads, and key-transfer safety systems to replace magnetic limit switches on the inspection doors.

Conclusion.
In more than fifteen years since the first model was produced, Euromecc has succeeded in perfecting its planetary concrete mixers to a very high level of quality, which guarantees its users phenomenal results in terms of mix quality and cycle times. With hundreds of models installed and their use in a variety of sectors and major projects such as the construction of the Storstrøm Bridge in Denmark and the doubling of the ME-CT railway line, Euromecc planetary mixers have consolidated their position in their category, ready to reach many more milestones.

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