Research Summary Report of A04

Böhler, David¹; doctoral researcher, d.boehler@ibmb.tu-bs.de

Rudolph, Jennifer¹; doctoral researcher, j.rudolph@ibmb.tu-bs.de

Freund, Niklas¹; doctoral researcher, n.freund@ibmb.tu-bs.de

Lowke, Dirk²; project leader, lowke@tum.de

¹TU Braunschweig, Institute of Building Materials and Concrete Construction and Fire Safety (iBMB)

²TU Munich, Department of Materials Engineering

Main Goal

Project A04 investigates cooperative Additive Manufacturing (AM) processes based on Shotcrete 3D Printing (SC3DP). The aim of this project is a fundamental understanding of the SC3DP technology to manufacture sustainable, multi-objective optimised, reinforced concrete components with geometrically precise surface quality and improved building physics by functional integration.

Current state of research

3D concrete printing (3DCP) has the potential to significantly reduce the total amount of material used and therefore the CO₂ emissions of the manufactured buildings (Flatt and Wangler, 2022)[1]. However, the resulting savings will only be beneficial if 3D printed concrete structures are as durable as conventionally cast ones. Therefore, research on the durability of 3D printed concrete is increasingly becoming a key topic of current research. In general, concrete is considered durable when it can withstand normal environmental conditions and its intended use for a defined period of time. However, pollutants from the environment (e.g. water, chlorides or CO₂) can enter the concrete through the pore systems and cracks. If the pollutants penetrate to the steel reinforcement, it can cause corrosion of the reinforcement and, in the worst case, failure of the entire structure. The way in which the pollutants penetrate the concrete depends largely on the micro- and meso-structure of the concrete matrix, i.e. the distribution of aggregate, binder and pores. Conventionally cast concrete is considered to be a homogeneous building material (see Figure 1 (left): This can be identified by the uniform distribution of aggregate (white dots) and cement paste (grey)) and therefore show a homogeneous penetration front of the pollutants. In contrast, 3D printing processes, can form an inhomogeneous micro and meso structure due due to the layered application and the inherent process sequences such as pumping, extruding or spraying[2]^,[3],[4],[5]. Accordingly, this can result in an inhomogeneous pollutant penetration front, see Figure 3. For the SC3DP process in particular, it has already been shown by Freund et al. (2023)² and Böhler et al. (2023)³ that the concrete components accumulate locally differently depending on the selected material and process parameters, see Figure 1 (right). This can be seen by the cement paste-rich layer in the interface zone (grey band) and the aggregate-rich bulk zone (white dots).

In a recent study, durability specimens were produced both by the SC3DP method and conventionally cast in moulds. The SC3DP specimens were prepared with an accelerator dosage of 0 % (SC3DP) and 3 % (SC3DP_3). All samples were then tested for resistance to carbonation, freeze-thaw and chloride migration. A comparison of durability properties is shown in Figure 2, where low penetration depth is associated with high durability or high resistance to penetration by pollutants. The SC3DP samples with 0 % accelerator dosage (grey: SC3DP) show a higher or equivalent resistance in the chloride migration and freeze-thaw tests compared to the conventionally cast samples (black: cast), but the carbonation resistance is less pronounced. With an increased accelerator dosage of 3 % (white: SC3DP_3), the chloride migration resistance is in the range of the cast samples, but freeze-thaw and carbonation resistance are less pronounced. Thus, for the SC3DP printed samples, an increase in accelerator dosage leads to a decrease in durability. In addition, it has been shown that the inhomogeneous microstructure leads to an inhomogeneous penetration front of the pollutants, especially in the SC3DP_3 samples, see Figure 3.

To sum up, it has been shown that SC3DP printed concretes without accelerator dosage have comparable or higher resistance to freeze-thaw or chloride migration than cast concretes, while carbonation resistance is lower. If a higher accelerator dosage is selected, this leads to a decrease in durability and to inhomogeneous durability across the cross-section. In the future, the influence of the variation of material and process parameters, as well as the post-processing of the surfaces on the durability behaviour, will be investigated in more detail.

[1] Flatt, R.J., Wangler, T., On sustainability and digital fabrication with concrete, CCR 158 (2022) 106837. https://doi.org/10.3929/ethz-b-000548718

[2] Freund, N.; David, M.; Böhler, D.; Mai, I.; Ullmann, S.; Dröder, K.; Lowke, D. (2023): Shotcrete 3D Printing ‐ Interaction of nozzle geometry, homogeneity and hardened concrete properties. In: ce papers 6 (6), S. 746–754. https://doi.org/10.1002/cepa.2818

[3] Böhler, D.; Freund, N.; Mai, I.; Lowke, D. (2023): Shotcrete 3D Printing ‐ Effect of material‐process interaction on the global and local material density. In: ce papers 6 (6), S. 770–776. https://doi.org/10.1002/cepa.2822

[4] Mohan, M.K., et al., Extrusion-based concrete 3D printing from a material perspective: A state-of-the-art review, CCC 115 (2021) 103855. https://doi.org/10.1016/j.cemconcomp.2020.103855

[5] van der Putten, J., Mechanical properties and durability of 3D printed cementitious materials. Dissertation, 2021. http://hdl.handle.net/1854/LU-8717381