# Jointing Principles in AMC--Part 1: Design and Preparation of Dry Joints
## 2 State of the Art and Research Outline  
Dry joints performed with CNC-milling were successfully used by the construction company Max Bogl in the serial production of precast high-strength segmental post-tensioned concrete towers for wind turbines [14]. Based on this experience, the arch of a tied-arch bridge [16] and a prototype for a segmental ultra-high performance concrete railway bridge [17] were built. The segments were produced conventionally in all cases, and the joints were milled. The CNC-milled joints were executed as smooth dry joints, and the flatness of the joint end faces was limited to 0.1 mm/m. These high accuracy requirements were determined based on tests because normative regulations are still missing.  
The joints between the segments could be weak points and hence decisive for the structural design. Investigations of the load-bearing capacity of dry joints, along with segmental components with dry joints made of normal concrete and produced conventionally, were carried out, e.g., by [18; 19; 20; 21]. An overview of the tests carried out on normal concrete can be found in [22]. Based on these investigations, a calculation approach for determining the load-bearing capacity of normal strength dry joints was proposed [22]. Furthermore, a new computation method for the parametric evaluation of joint geometries by machine learning techniques was addressed [23].  
Recent investigations dealt with the load-bearing capacity of dry joints and segment components with dry joints made of ultra-high performance fibre-reinforced concrete (UHPFRC) [24; 25; 26]. The investigations show that the load-bearing capacity of UHPFRC dry joints differs from normal strength dry joints. In former studies, new joint geometries were proposed and evaluated [27; 28]. Nonetheless, the match-cast process produced all the test specimens of these investigations.  
For this study, as part of a tandem research project funded by the German Research Foundation under TRR 277, subproject C05 investigated the jointing principles in Additive Manufacturing in Construction (AMC). The subproject C05 contains four consecutively structured working packages WP1-WP4 (Figure 2). The WP1 aims to design and select joints profiles for 3D-printed elements. A set-up database (joint catalogue) is used, containing the main imaginable joint profile types and geometries taken from timber, steel and bionic connections. It also contains evaluation criteria, weighting factors, and an algorithm to evaluate the gathered joint profiles regarding the applicability for 3D-printed concrete elements. The algorithm then, proceeds to select four joint profiles to be further analysed by packages WP2, WP3 and WP4. In WP2 and WP3, the goals are to understand the load-bearing behaviour under compression and shear by numerical simulation and experimental testing, while in WP4, the results of the previous packages are used to design, simulate and test a 3D-printed, and via dry joints connected component.  
Considering the process described above, this paper shows the results of WP1, i.e., which joint profile types are associated, evaluated by various criteria, and selected by the algorithm. Furthermore, this contribution deals with the preliminary investigations for the subtractive post-processing required for manufacturing the specimens for experimental investigation in WP2-WP4.  
Figure 2: Outline of C05 subproject in TRR277.