As opposed to conventional concrete, the binding component of polymer concrete is not cement, but polyester resin. A precise relation and mix of three components is required to ensure the material stiffness. For this project, a total of 120 kg of Epument 140/5 A1 polymer concrete was used. The three necessary components were delivered by the RAMPF Machine Systems GMBH & CO KG company in Wangen bei Göppingen, Germany and were mixed using the ratio shown in the Table 1 below.

Therefore, due to the exact mixture, no different material consistencies are possible, as this is the case with cement-based concrete. This proves to be a disadvantage when applying and pouring the mixture. Another disadvantage is the delicate handling due to resin-based components and the relatively high price as compared with cement-based concrete.

However, the application of polymer concrete offers also large advantages. The cured material is not brittle, has also high tension (not only compression) abilities, and proves to have a high adherence to wood materials. Esthetically, it can be produced (depending on the additives) with different grain sizes, so that the surface quality can be very smooth or coarse, depending on the formwork surface. Polymer concrete does not need to be compacted by vibration.

There is a large palette of applications in the industry for polymer concrete. Polymer concrete is an industrially manufactured material with precise and controllable material qualities such as the grain sizes and stiffness qualities. Because all components (there is no water component) are industrially produced, they can be controlled to a high degree. An important current application of the material is the fabrication of machinery foundations. This is because the guaranteed stiffness is high, the material is not as brittle as normal concrete, and can withstand vibrations and tension forces to a certain degree. See Table 2 below for a comparison of traditional cement-bound concrete and polymer concrete (Institut für Betonbau, 1998; Schober, 2008).

In recent years, polymer concrete testing and small-scale applications were conducted at the Hochschule Trier (Prof. W. Becker, Prof. R. Thum) in Germany. The DigDesFab15 project builds upon the knowledge gained through this preliminary research in Germany; however, this type of construction and structural typology has been tested with this research pavilion for the first time at a larger scale. The intention was to create a fully self-supporting structure with structural stiff nodes on a rectangular grid (without the need of using a triangulated grid). This is innovative, because under normal conditions, it is very difficult to achieve structural bending stiff nodes in wood joints; therefore, in many cases, triangulated (truss) systems are used. This problem naturally reduces the availability of cost effective design options for architects and structural engineers.

The design process of the research pavilion followed a strict systematic approach. Twenty-four students were organized in four groups of six students each. These groups were given a design task and were tutored toward developing initial design proposals.

The project design was developed from two points of interest. On one hand, the architectural brief asked for an examination to create a light-weight structure, and, on the other hand, a structurally feasible construction technique and application of the polymer concrete in combination with wood had to be developed. It was not required by the architectural brief to design a water-proof shelter for rain protection. The architecture students developed the required Rhino Grasshopper and Karamba analysis setup and definition files with the help of experts from Bollinger + Grohmann Engineers.

The structural system has been developed during the design tasks from the so-called Zollinger system (Menges et al., 2016), where small modules (Tamke et al., 2010) are prefabricated and joined to form a greater structure (Sack, 2013). A parametric model in the Rhinoceros and Grasshopper software was created and tested virtually through various subdivision densities and node sizes (Figure 2). The overall structure was evaluated with the structural analysis tool Karamba (plugin for Rhino Grasshopper), the different heights of the lamellas in the pavilion arise from the structural analysis of stresses and deflections under vertical and horizontal loading.

The Grasshopper definition loads a NURBS surface created in RhinoVault. The definition applies a rectangular subdivision (Figure 3) and creates the corresponding four legs of each node as sheets (or lamellas) of different structural heights. The Karamba plugin is used to apply gravity (vertical) and mesh (horizontal) loads to the structure. The mesh view component and corresponding material utilization results in Karamba is linked to differentiate the height of the lamellas parametrically. The rectangular mesh subdivision is optimized while keeping utilization below 60% of the maximum material capacity.

Laminated spruce timber boards (3-layered, 19 mm thick) were used for construction. All the pieces from the 3D model were flattened and distributed on standard size boards (1.25 m × 2.00 m) parametrically, using the Rhino plugin Grasshopper. The pieces were cut by a 3 axe CNC mill (Figure 5), and the edges were sanded to avoid injuries when exhibiting the final piece. The geometrically unique pieces (cut two times for double layering) were labeled in the digital model and the students sorted and labeled and grouped the adjacent pieces (node by node) immediately and manually after production. Each node was screwed together with metal screws as a temporary fixture (Figure 6).

The three-dimensional geometry was predetermined by the shape of the assembled pieces in each node (Figure 4). Neighboring pieces would only fit together on the angles predetermined by the geometry of the milled pieces; therefore, no measurements had to be transferred from the 3D model to the physical model at this state. This means, the four pieces for each node and the bottom sealing plate would fit together only in one way. For helping the assembly process and for purposes of geometry control, marker slits were milled in each piece, indicating where the neighboring piece would need to be attached precisely. The screws only served as a temporary fixation, until the polymer concrete was filled in the center of node for final fixation (Figure 7). Each node was sealed on the bottom with a custom-cut plate out of a thinner (8 mm) plywood material, labeled with a unique code. The project builds up on a rectangular grid, each node has four legs around a central node cavity (reciprocal system) where the polymer concrete was poured (Figure 8). The polymer concrete (curing time about 8 h) adhered with the wood and, therefore, created a structurally stiff node in wood, in a large range of geometrical variations. Normally, it is very difficult to achieve stiff structural nodes in wood structures; therefore, this applied composite method might offer interesting potential in future.

All 122 nodes were cast separately and bolted on-site with screws. An assembly strategy was developed to easily connect the nodes with each other through using milled geometrical references. In the final structure, the wood sheets of the four node legs overlap, so the final material thickness is 2 mm × 19 mm = 38 mm (Figure 11). The overlapping pieces fits at certain positions, this creates a reference when assembling the 1:1 structure. The geometry has been developed with the assistance of the software plugin Rhino Vault to develop a purely compressive load freeform shell. In this case, no supporting formwork (with the exception of the scaffolding for the purposes as described below) was needed for construction, final assembly, and de-construction. The assembly pieces were added starting from the lowest row and the structure reached final structural stability after the arches were completed. During the construction, scaffolding was needed for the assemblers to reach and mount the higher levels of the structure. Equally, the scaffolding helped to stabilize the not completely closed arches of the structure against horizontal loads (wind) during construction.