Be it a house or a skyscraper, motorway bridge or railway tunnel: Concrete is one of our most important building materials without which most modern structures would be unimaginable. Its many uses, its stability, resilience, and durability as well as its ready availability make concrete so indispensable. At the same time, it is also a cause of considerable emissions: The manufacturing of cement, which forms the basis of concrete, causes yearly CO2 emissions that are three to four times higher than those produced by the entirety of global air traffic. The reason for this: the raw material, which mostly comes from limestone, is burned in a furnace at temperatures ranging from 1,400 to 1,450 degrees Celsius often using fossil fuels to power the furnace. During the subsequent cooling process the carbon dioxide stored within the limestone is released- after the initial burning this is the second largest source of CO2. With time the results will become more environmentally friendly, however, the cement industry still accounts for around 6 per cent of worldwide CO2 emissions.
Manufacturers of concrete components are, therefore, interested in solutions which help to save on materials and that allow the components to be produced more cost effectively and in a more environmentally friendly way. One especially promising process is so-called concrete grading. This involves manufacturing the building components with different material properties in accordance with their static and structural load profiles: where a component bears a high load, such as on the outer surface of for example, a thick, high strength concrete mix is used. Conversely, in reas of the component, that bear lower loads, such as inside the component, a more porous mixture is used. This helps to save on materials and reduces the weight of the component. “We are familiar with this principle from its numerous examples found in nature. The constitution of our bones would be one such example”, says Mark Wörner from the University of Stuttgart’s Institute of System Dynamics (ISYS). “The outside of the bone which is impacted upon by high forces, is hard and solid- the inside on the other hand, is elastic and porous. That makes it strong and yet highly durable at the same time”.
Wörner and his colleagues from ISYS are part of an interdisciplinary research project at the University of Stuttgart: Together with scientists from the Institute of Light Construction, Drafting and Constructing (ILDC), as well as from the Institute of Construction Materials (ICM), they are working on a way to produce graded concrete components for use in the Construction Industry. During their research, the dry-mix procedure emerged as the most suitable: Two different concrete mixes (one solid, the other one porous) are mixed in the mixing nozzle and then sprayed into the required shape. The material properties change with the mixing ratio. This can be used to produce more solid and durable concrete or lighter, more porous concrete depending on what is required.
Automation for Reproducible Results
“For industrial use it is especially important that we achieve exactly reproducible results”, Wörner explains. This is a big challenge with a coarse process such as concrete spraying: Every uncontrolled movement of the spraying nozzle, every fluctuation of the distance between the nozzle and the target surface can change the properties of the component, not to mention inaccuracies in the mix ratio. Wörner thinks (that) “this cannot be efficiently accomplished manually”. “Therefore, we are trying to automate the entire production process, as much as is possible”.
The scientists developed an automated application system, which allows them to precisely manage and change the different variables of the spraying process. The spraying nozzle guidance is managed through a SLE (Steel-Linear-Unit) heavy-duty multi-axis portal made by the linear technology specialists Winkel. This allows the nozzle to move three-dimensionally: The traverse ranges are 3,000 millimetres on the X-axis, 2,200 millimetres on the Y-axis, and 800 millimetres on the Z-axis. Wörner and his team also mounted a hexapod on the Z-axis, which enables the nozzle to move independently from the portal. Consequently, the researchers are able to manage and vary the concrete spraying process in detail, in order to obtain optimal results.
Our contact with Winkel started at the Motek trade fair, a couple of years ago at the Motek trade fair, Wörner remembers: “We were looking for a solution that also works reliably under extreme operating conditions”. When you dry-spray concrete there is a large build-up of cement dust. Moreover, part of the sprayed material also splashes back, the so-called splash back, from the target surface and lands on the apparatus. This can prove especially problematic with linear technology and can lead to disruptions. “The robust heavy-duty Winkel portal was best suited to our needs as it can effortlessly deal with even the most trying circumstances”, adds Wörner. We were delighted that Winkel agreed to help the University of Stuttgart’s research into this innovative manufacturing process with both their portal and their know-how.
Robust yet Dynamic
The experts at Winkel developed a solution in line with the specifications set down by the ISYS and that was perfectly tailored to the demands posed by the research project. The building and commissioning of the apparatus was carried out in close cooperation. “Besides the tough operating conditions, the dynamics were also an important criteria”, says Athansios Loupas, sales manager at Winkel. This is due to the fact that the nozzle guidance has to be able to move quickly and change direction during the spraying process. “With a top speed (velocity) of up to one metre per second and capable of acceleration of up to 2 metres per second squared our portal more than fulfils the necessary criteria”, Loupas explains.
With the help of the multi-axis portal the research team was able to achieve a fully automated production process: The two concrete mixes are placed ready for use in their respective silos. The process controller then determines a graded layout based on key statistical figures. Therefore, the density that each part of the component should be manufactured to, is determined by the mix ratio. The spray nozzle applies the concrete layer by layer in order to form the correct shape. The movement of the portal and the spray nozzle is also automatically controlled. “With this we have created a reproducible process from which we can gather and document exact parameters”, says Wörner. “Therefore, we can ascertain what effect a change in the production process has on the end result and thereby optimize the entire process”.
The multi-axis portal has been in use at the University for 15 months. The scientists think that the project is showing a lot of promise. “At the moment we are still in the research phase, however, we hope to develop the technology to the stage where it is ready for series production within the coming years” says Wörner. He also adds “This project would not have been possible without Winkel’s support.” Should the grading process be able to be used in the industry, one day, then it has the potential to revolutionize the manufacturing of concrete building components: “ It allows us to save up to 60% of the mass relative to the previous mass and also 35%, in terms of CO2 emissions”, explains Wörner. This would be a meaningful step forward from both, a scientific and an ecological standpoint.