# Investigating the Poisson Ratio of 3D Printed Concrete
## 5 Conclusions  
This research set out to perform a preliminary investigation into the Poisson ratio of extrusion-based 3D printed concrete. Three-dimensionally printed specimens were prepared from an arbitrary printed element in two orthotropic directions: OI, denoting interlayers perpendicular to the testing direction, and OIII, denoting interlayers parallel to the testing direction. The specimens were equipped with target markers to accurately record lateral and longitudinal strains through digital image correlation technology. A reference case was also adopted and comprised conventionally cast specimens that were core-drilled from cast concrete cubes. Testing guidelines provided by ASTM C469-02 were followed to determine the Poisson ratios. Additionally, micro-computed tomography scans were performed on all specimens to investigate their microporosity, specifically the void topography. The main conclusions derived from the results of this study are:  
1. Three-dimensionally printed concrete exhibits anisotropic behaviour. The Poisson ratios for the OI and OIII directions were 0.21 and 0.24, respectively;
2. Three-dimensionally printed concrete exhibits larger Poisson ratios than its cast counterpart. The same concrete mix that was 3D printed was also cast and tested, which yielded a Poisson ratio of 0.18. Although only five specimens were tested per configuration, a one-way ANOVA test confirmed that the means of the three groups are statistically different;
3. The -CT scanning results confirm the widespread presence of oblate voids in the 3D printed specimens of this study. For the OI direction, these voids are oblate with respect to the testing direction, whereas in the OIII direction, the voids are prolate. These varying void orientations are the cause of the observed anisotropy in printed concrete;
4. Results from the compactness and sphericity investigations indicate that the printing process has a larger influence on porosity topography than the pumping and extrusion process. Hence, proper selection of printing parameters is paramount to reducing anisotropy;
* A spatial variation in porosity and void topography was observed over the height of the 3D printed element. Although not explicitly tested in this research, it is postulated that the Poisson ratio might vary spatially over the height of a printed object.  
This research forms a basis on which further, more detailed, investigations into the anisotropic behaviour of 3D printed concrete can be performed. The findings presented herein are especially valuable for modelling and designing 3D printed concrete structures, from the material to the structural level. Suggested focus points for future studies include (i) larger sampling, (ii) investigating potential spatiotemporal influence on Poisson's ratio and (iii) comparing anisotropy between various 3D concrete printing methods.  
Conceptualisation, J.K. and J.-P.v.d.W.; methodology, J.K. and J.-P.v.d.W.; software, J.-P.v.d.W.; validation, J.K. and J.-P.v.d.W.; formal analysis, J.-P.v.d.W.; investigation, J.-P.v.d.W.; resources, J.K.; data curation, J.-P.v.d.W.; writing--original draft preparation, J.K. and J.-P.v.d.W.; writing--review and editing, J.K.; visualisation, J.-P.v.d.W.; supervision, J.K.; project administration, J.K.; funding acquisition, J.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors thank PPC Cement and Chryso SA for donating research material toward this study.  
The authors declare no conflict of interest.