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Concrete Casting News from the Hill and Griffith Company

Lightweight Precast Concrete Roof with Optimized Load-Bearing Design

Posted by Hill and Griffith Company on Mar 14, 2019 3:56:33 PM

3D Sand printing for formwork manufacturing and showing oil based precast concrete form release agent application

Researchers at ETH Zurich have fabricated an 80 m2 light­weight concrete slab at the DFAB House, making it the world's first full-scale architectural project to use 3D sand printing for its formwork. Just 20 mm thick at its thinnest point, decoratively ribbed and not even half as heavy as a conventional concrete ceiling: with "Smart Slab", the name says it all. The slab combines the structural strength of concrete with the design freedom of 3D printing.

Developed by the research group of Benjamin Dillenburger, Assistant Professor for Digital Building Technologies at ETH Zurich, Smart Slab is one of the core elements of the residen­tial unit DFAB House at Empa's and Eawag's research and in­novation platform NEST in Dubendorf. The 80 m2, 15 t ceiling consists of eleven concrete segments and connects the lower floor with the two-story timber volume above.

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The Smart Slab segments being placed piece by piece on the 12 cm wide mesh mould wall.

(Thanks to CPI - Concrete Plant International May 2018)

Only as much concrete as needed

3D concrete printing is currently experiencing a boom in ar­chitecture, and entire houses have already been printed layer by layer. However, for the Smart Slab project, the researchers did not produce the building components themselves with 3D printing but rather the formwork - i.e. the mould. To achieve this, they used a large-scale 3D sand printer, which means the resulting moulds consist of a kind of artificial sand­stone. One of the advantages over the layered concrete print­ing process is that high performant fibre-reinforced concrete can be used and the structure can be fabricated in the preci­sion of millimeters.

Formwork production is the most labour-intensive step in con­crete construction, particularly for non-standardized compo­nents. Since concrete is relatively cheap and readily abundant, the temptation is for the construction industry to produce the same solid ceilings over and over again, but the disadvantage is excessive material consumption and implicitly, a big carbon footprint. Digital fabrication methods can make a key contri­bution here: components can be optimized, enabling the necessary stability with far less material. The geometric com­plexity of a component does not matter in 3D printing, nor does it cause any additional costs - the printer simply prints what it is told to.

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The 3D sand printer used for the fabrication of the form work. The printer has a build volume of 8 cubic meters and a reso­lution of a fraction of a millimeter.

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Post-processing of the 30 printed form work parts. Unconsol­idated sand particles are being removed from the print bed.

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The formwork parts are assembled seamlessly and prepared for concreting.

Computational design coordinates parameters

Dillenburger's research group developed a new software to fabricate the formwork elements, which is able to record and coordinate all parameters relevant to produc­tion. In addition to basic data such as room dimensions, the researchers also en­tered a scan of the curved wall, accurate down to the last millimeter, which acts as the main support for the concrete ceiling. With the software, one could adapt the geometry of the slab so that at each point it was applied only as thick as structurally necessary to support the force flow. "We didn't draw the slab; we programmed it," says Mania Aghaei Meibodi, Smart Slab project lead and senior researcher in Dil­lenburger's group. "It would not have been possible to coordinate all these aspects with analogue planning, particularly with such precision."

If you look at the ceiling from below, you see an organic ornamental structure with different hierarchies. The main ribs carry the loads, while the smaller filigree ribs are mainly used for architectural expression and acoustics. Statics and ornamentation go hand-in-hand. The lighting and sprinkler systems are also integrated into the slab structure. Their size and position were similarly coordinated with the planning software. In this way, the building technology disappears elegantly into the slab to occupy very little space. This saves only a few centimeters in the DFAB House proj­ect, but in high-rises this may mean a few extra floors could be fitted into the same height.

Fabrication at the push of a button

After planning on the computer is completed, the fabrication data can then be ex­ported to the machines at the push of a button. This is where several industry part­ners came into play for Smart Slab: one produced the high-resolution, 3D-printed sand formworks, which were divided into pallet-sized sections for printing and trans­port reasons, while another fabricated the timber formwork by means of CNC laser cutting. The latter gives shape to the upper part of the Smart Slab and leaves hollow areas that reduce material and weight and at the same time create space for electrical cables.

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An oil-based release agent facilitates the removal of the formwork once the concrete hardens.

The two types of formwork for the concreting were then brought together by a third company, which first sprayed the fibre-reinforced concrete onto the sand formwork to produce the finely ribbed surface of the lower concrete shell and then casted the remaining concrete into the timber formwork.

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Glass-fibre reinforced concrete being sprayed on the 3D printed formwork in several consecutive layers.

Strong thanks to prestressing

After a two-week hardening process, the eleven individual concrete segments were ready for transport to the NEST. Thanks to the precise planning and prefabrication, the installation time at the construction site was reduced to a minimum: a crane hoisted the concrete elements onto the load-bearing wall, where the prestressing took place. Workers pulled steel cables lengthwise and crosswise through the concrete sup­port and into the channels already inserted in the formwork. Tensioning the cables massively increases the system's load capacity.

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The hierarchical grid of structural ribs of the Smart Slab.

"It was spectacular to see on the construction site how seam­lessly our elements fitted with each other and with the existing components of the DFAB House," says Dillenburger. "We owe this in part to the outstanding interdisciplinary collaboration with our partners. The meticulous work that we had invested into planning completely paid off."

Smart Slab partners

ETH Zurich research groups: Chair for Digital Build­ing Technologies, Benjamin Dillenburger (lead); Chair for Building Materials, Robert Flatt; Chair of Structural Design, Joseph Schwartz

Industry partners: Burgin Creations; Frutiger AG; voxeljet AG; Georg Ackermann GmbH; Stahlton AG; Christenguss AG; Fischer Rista AG; Rudolf Glauser AG; Gorn International AG

See here a video about "Smart Slab."

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