Variable Density Concrete

The bones in your body are sophisticated structural elements. The classic bone shape – slim in the centre and wider towards the joints – has evolved to cope efficiently with the forces acting upon it. But look inside and you find further refinement: a variably porous structure allows bone to be denser where strength and stiffness is required, or lighter where it is not.

“We are aiming to do something similar with our graded concrete,” says Lucio Blandini, head of the Institute for Lightweight Structures and Conceptual Design (ILEK) at the University of Stuttgart and partner at engineering consultant Werner Sobek. “A floor slab, for example, does not usually need to be completely solid throughout.

So we have developed methods for creating ‘pores’ or cavities in a controlled way, so material can be concentrated where it is most needed.” Two very different ways of achieving this have been used to create M212, a prototype developed by ILEK and Werner Sobek, and now on display at the Vienna Technical Museum. It is a 250mm-thick slab, the left half of which has been lightened by using carefully distributed hollow concrete spheres. The other half has been made by pouring concrete around specially developed soluble formwork.

Both sides of M212 contain much less concrete than a standard slab. The left side has 217 hollow spheres, each 15cm in diameter, reducing its weight by 30%. The side with the soluble internal formwork is almost 50% lighter than a standard slab. The ability to vary the concentrations of cavities allows concrete to be more efficiently used, says Blandini. “So you might make the slab solid where it connects to a supporting wall and maximum stiffness is required, but increase the cavities towards the floor centre where weight is less helpful.”

Using less concrete reduces resource consumption, lowers the carbon footprint of elements and, by making them lighter, can help reduce the size and weight of a building’s entire structure. The spheres were made by putting a small amount of fibre-reinforced grout inside a spherical silicone mould and then rotating it in a centrifuge. “This forces the grout to the inner surface of the mould, coating it to a depth of around 1-3mm,” explains Daria Kovaleva, project lead at ILEK.

“Remove the moulds, and you are left with lightweight hollow spheres that can be distributed throughout an element. In M212, we’ve used the reinforcement grid to hold the spheres in place, and to stop them rising to the surface of the wet concrete. The advantage over polystyrene void formers is that the element is fully mineral, so is easier to recycle.” The other side of M212 is equally ingenious. The soluble internal formwork is created with a specially developed 3D printer, which uses sand, water and an organic binder based on cornstarch.

“The forms are then set with the help of infrared lamps,” says Kovaleva. “In this state they are solid enough to resist the pressure of concrete poured around them.” Once the concrete has cured, the element can be hosed down or immersed in water to dissolve the formwork, which can then be recycled. This leaves a pattern of cavities in the cured concrete, determined by the CAD file.

The complex geometries that result are not always compatible with the use of steel rebar. For M212, the concrete is reinforced with basalt fibre rebar instead. “The pattern created by the process can be attractive,” adds Kovaleva. “So as well as its material efficiency, architects are showing interest in its aesthetic possibilities.” Clever stuff, but can technologies like these really make it out of the lab and onto a construction site? “We’re very focused on making this happen,” says Blandini. In fact, some 20,000 spheres are currently in production, and will be integrated into a project under construction at the university campus.

The research building for the Cluster of Excellence on Integrative Computational Design and Construction for Architecture will include a foundation slab with 30cm spheres and ground-floor slab with 15cm spheres, covering a total area of 1,200m2. “It’s been a challenge, getting this technology peer-reviewed and managed through building regulations, but we recognise the importance of refining not just the technology but the whole process of applying it on site.”

“As we move from research to application, we are talking to both architects and precast producers,” adds Kovaleva. “Building designers are interested in solving the problem of sustainably producing something that is structurally complex. Manufacturers are attracted by the potential to reduce the weight not only of structural elements but also facade units, where cavities in concrete can be used to increase insulation.” Variably dense material could clearly have many applications. And as so often with good ideas, nature thought of it first.

Interview by Tony Whitehead

Photos Patrick Johannsen