Modular, Digital Construction System for 3D Printing Lightweight Reinforced Concrete Spatial Structures

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Spatial structure systems, like lattices, are efficient load-bearing structures that are easy to adapt geometrically and well-suited for column-free, long-spanning constructions, such as hangars and terminals, and in creating free-form geometries. As these structures have to bear both compressive and tensile forces, they are often constructed out of isotropic materials like aluminum and steel. Reinforced concrete is another option, but even with the help of more recent advancements, it’s less common due to high costs and the “heavy” look concrete structures have.

Researchers Norman Hack, Hendrik Lindemann, and Harald Kloft from the Technical University Braunschweig published a paper, titled “Adaptive Modular Spatial Structures for Shotcrete 3D Printing,” about their work developing a “modular, digital construction system” for making lightweight spatial structures out of reinforced concrete.

The abstract reads, “For design and fabrication, a digital workflow is presented, which includes the rationalization of a freeform geometry into adaptive spatial modules made up entirely of planar components. For fast and precise fabrication, these components are 3D printed using a novel 3D concrete printing technology called “Shotcrete 3D Printing”. The ongoing research is demonstrated by an initial real-scale prototype of one exemplary spatial module.”

The goal was to create an “integrative digital construction system for adaptive modular spatial structures,” using reinforced concrete, that considers construction site logistics, economic and ecological aspects, fabrication constraints, material properties, and structural performance.

“In view of that, this paper describes a digital construction system and a digital workflow, in which a freeform geometry is first rationalized into planar panels, secondly developed into spatial modules, and which can finally be fabricated rapidly and without causing construction waste using 3D concrete printing technology,” the researchers explained.

Contemporary examples for spatial structures in reinforced concrete: (a), Hedracrete; (b) Opticut; (c) XTree column in Aix-en-Provence.

Several other research groups have looked into using reinforced concrete for creating geometrically complex and structurally optimized lattice constructions, including:

  • The Hedracrete Pavilion, which looked at spatially complex polyhedral structures developed on 3D static graphics
  • The Opticut project, which explored how to efficiently make topologically optimized structures with robotic abrasive wire cutting
  • XTree, which used four segments of 3D printed concrete to make the formwork and temporary support structure for a truss-like column

The TU Braunschweig team referenced a different project – that of Pier Luigi Nervi, who developed aircraft hangers for the Italian Air Force that were based on a lattice shell system of pre-made, spatial truss girders. He created a system of planar, prefabricated concrete trusses that, when assembled, formed a vault structure shaped like a diamond. Then, the trusses were monolithically joined by casting concrete into the gaps.

Prefabricated Air Force hangars, Pier Luigi Nervi, Orvieto, 1939: (a) interior view of the lattice shell before the planking; (b) prefabricated truss girders during assem-bly; (c) detail of the joint before casting.

This method, which saved Nervi time, weight, and material, inspired the researchers in their efforts to create a spatial structural system with identical components.

Their construction system concept is based on a digital workflow that starts with rationalizing a freeform geometry into spatial modules of planar components. Next, these modules are digitally unfolded so fabrication paths can be generated, and the components are then 3D printed as planar elements.

Because all the elements in the structure had to be planar, the workflow needed to rationalize “a given freeform NURBS surface into spatial modules” of quadrilateral planar components. At the last stage, the planar quadrilaterals are transferred into a spatial lattice structure that’s been adapted to a specific loading condition. Then, the coordinates and tool speed are translated into G-code that can be used with Siemens Sinumeric.

Module generation: (a) initial surface subdivided into double curved quadrilaterals with colours indicating the double curvature; (b) planarized quadrilateral grid with double curved surfaces providing structural depth (c) planarized quadrilateral grid with planar spatial structure, the single colour indicates planarity.

Final geometry: (a) Planar and structurally adapted structure with thicker parts marked in orange and more slender members in blue; (b) top-view of the structure and digitally unrolled modules.

Robotic setup was completed at the university’s Digital Building Fabrication Laboratory: a robotic fabrication facility that uses both additive and subtractive manufacturing methods to produce large-scale structures. The team installed a planar wooden baseplate in the workspace, and then uploaded the fabrication data for each module component, before starting the concrete mixing process.

The researchers used their novel Shotcrete 3D printing (SC3DP) technology, which doesn’t just extrude the material but also sprays it with pressure to build a 3D structure. This method offers superior layer adhesion, can integrate reinforcements, and offers “the potential to spray against vertical surfaces and overhangs.”

Fabrication: (a) Spraying of the elements; (b) cutting the edges in the concrete’s green state; (c) smoothing the mitered surface-edge.

Each of the module’s three components were 3D printed separately. After the first two layers were printed, 8 x 100 cm strips from a carbon fiber reinforcement grid were placed on top, and the final layer was printed and sprayed on to embed it.

After green post-processing and two days of curing, the 60 kg planar elements were detached from the baseplate. The final 2.2 x 1.5 x 1 m prototype, printed in just 25 minutes with 12 minutes of spraying and eight minutes of post-processing, is part of a larger structure that was assembled in just five minutes.

Assembling one module: (a) assembly with three people; (b) assembled element; (c) positioning using a crane.

“Spraying resulted in good compaction, was fast and offered consistent height control. However, the starting and stopping procedures of the spraying process are not yet precisely controlled, requiring to extend the starting and ending point of the contour,” the researchers wrote. “With regards to the reinforcement, it became apparent, that using pre-cut strips of carbon fiber mats is an effective measure to reduce the structural thickness of the elements. However, the manual placement did not prove to be not sufficiently precise, causing collision conflicts during the cutting of the edges during post-processing…Due to the low weight of the components, the assembly was possible with only three persons. As no connection mechanisms were integrated in this first prototype, lashing belts were used to connect the components.”

The researchers concluded that more investigation is required, such as further developing the computational workflow and module fabrication. The next stage will be to “realize larger spatial structures” made with several modules and perform a structural test.

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