It's very Christmassy

Prof Dr Sandra Gelbrich and Enrico Rudolph from the Institute of Lightweight Structures at Chemnitz University of Technology are working on 3D printing. They are currently printing candle arches, just in time for Christmas. But they are not using plastic as usual: together with their team and the Steinbeis Innovation Centre, they have developed a special concrete that can be printed.

Why are you making candle arches out of concrete?

Prof Dr Sandra Gelbrich: We've been working with additive manufacturing processes for concrete for about five years and that's when we came up with the idea of making something regional.

Enrico Rudolph: In keeping with the Advent season, we thought about what else can be made from concrete. With 3D printing in particular, you can produce relatively delicate structures by concrete standards. That's why we came up with the idea of realising a candle arch.

Sandra Gelbrich: We wanted to combine tradition and Chemnitz as a centre of science - so our idea was to implement a Christmas motif as a reference.

Did you create several motifs for the candle arches?

Enrico Rudolph: We came up with several motifs: In addition to the motif with the angel, shooting star, church and Christmas tree, there was also a classic motif with miners and a coat of arms, as well as one with a reindeer, sleigh and Father Christmas. The advantage of 3D printing with concrete is that we are very flexible. We can go from a computer-based drawing to the finished print programme and implement it relatively quickly and easily. We don't need any moulds and can simply discharge the concrete from the nozzle to create our final structure. Concrete production usually requires a mould into which the concrete is poured or sprayed, which is not the case with concrete 3D printing. The huge advantage here is that we can react variably, are flexible and also have freedom of design.

Sandra Gelbrich: We wanted to show that this 3D printing technology is very suitable for precast concrete parts and has a lot of potential. Now it's up to artists to take up the ideas and come up with new structures. Of course, 3D printing with concrete would also be very interesting in the area of furniture or gardening and landscaping.

In the future, Prof Dr Sandra Gelbrich and her team want to take development one step further: They want to print load-bearing components. "In basic research, we are at the point where we can combine different processes to produce façade parts in this way, for example," says Prof Dr Sandra Gelbrich. For example, they want to combine concrete casting with 3D printing. This has a major advantage: "You can save material and still generate the appropriate strength properties. We want to use the technology to produce reproducibly and sustainably. " And they do this with a special, fibre-reinforced concrete:

What does "fibre-reinforced" mean?

Sandra Gelbrich: The concrete recipe is a fine concrete with a relatively fine maximum grain size. This fine concrete formula is additionally reinforced with an alkali-resistant short glass fibre. This can be thought of as a kind of small needles that are incorporated into the concrete mixing process.

What is special about the formula?

Sandra Gelbrich: We have developed a fibre-reinforced, high-strength fine concrete mix with special properties. We have to pump the concrete over a relatively long hose length. This means that it has to be pumpable on the one hand, but on the other hand it has to have a certain stability so that we can print several layers on top of each other. The hardening process is set in such a way that we can manupulate the precast concrete element after just a few hours. How long does it take for it to harden? Sandra Gelbrich: Conventional concrete takes 28 days to fully harden. We demould after 12 hours. By then, the concrete arch is so strong that it can be transported.

Two robots are responsible for 3D printing the candle arch. One of them is able to convey the concrete and apply the motif. The other then takes over the finishing work: It can mill, i.e. ensure a smooth surface, or create grooves and embed fastenings or optical fibres.

How do the robots know what to do?

Enrico Rudolph: We have a CAD programme (editor's note: programme in which a computer takes over the technical drawing), as we know it from engineering, and can use it to draw the candle arch as 3D geometry. We then transfer this to another programme that breaks down the 3D geometry into layers. Our candle arch, for example, is three centimetres thick, with each of the layers being one centimetre high. I use the programme to transfer the outer paths of the layers into the corresponding robot movements. We have developed our own software for this purpose, with which new design variants can be transferred and realised quickly and easily. Within half an hour, I have a new design ready and can transfer it to the printing robot. With just a few mouse clicks, the CAD models can be changed very quickly and the candle arch can be made five centimetres thicker, for example.

Sandra Gelbrich: Programming was one of our main areas of research and we have developed this over the last five years: this input option, the parameterisation, but also the material and the technology. Now we are working with the companies on product development.

What is innovative about the way you printed the candle arch?

Sandra Gelbrich: The primary aim was to make 3D printing technology "fit" for precast concrete construction. The innovative aspect is to develop the entire value chain - from the design to the finished part. The first step was to parameterise moulds, so that we could quickly arrive at geometries and path planning. A further challenge was to ensure that the pumping of the fibre-reinforced fine concrete worked evenly, that there were no material dropouts, that the concrete did not solidify in the hose, but could be applied in layers in variable shapes and stiffened in a targeted manner after discharge - in principle, the entire chain of material and technology development. No classic concrete is used, but a highly complex seven-material system whose components are precisely harmonised. Another innovative feature is the complete technology, which is reproducible and automated and will soon be transferred to series production together with project partners.

The candle arches are already illuminated, will they also be given a colour?

Sandra Gelbrich: We are now working with a white concrete and it could be made in almost any colour. So neon pink is not possible (laughs), but various other concrete colours can be incorporated. We already have companies interested in printing in two colours. But for us, the candle arch is a reference to show what is possible overall. Now we need to find companies that want to develop the technology. The more you show what is possible, the more creativity will be awakened.

Where would you like to set up the candle arches?

Sandra Gelbrich: There are already several candle arches in employees' gardens and one in our institute building. Our Dean of the Faculty of Mechanical Engineering, Professor Lampke, has already received one and set it up at the institute. If we had lectures on site, I would also like to put one in our lecture theatre buildings to show the students what is possible with additive manufacturing processes. Perhaps we could attract more students to study in Chemnitz this way.

What are your hopes for the Capital of Culture year 2025?

Sandra Gelbrich: That we can show more of what we are developing at the university. I would like to see some kind of competition to show what we are doing: There is an idea and the idea is looking for realisers. That we work together with artists and designers and show what we can develop and realise in Chemnitz. It is always very important to us that we don't develop "for the drawer", but that we work together to put developments into practice. We are very strong in Saxony. We have a large number of small and medium-sized companies with whom we can do this together. And we should also publicise this more.