# Investigating the Poisson Ratio of 3D Printed Concrete
## 2 Materials and Methods
### DIC, Target Marker Design, and Assembly  
ASTM C469-02 [36] specifies that either an unbonded extensometer or strain gauges may be used to determine the transverse or lateral strain for Poisson ratio determination. Due to the laboratory not possessing an unbonded extensometer setup, and considering the large number of specimens to test combined with the small surface area for gauge mounting, the authors decided to employ 2D digital image correlation (DIC) technology. This technology, which yields high resolution in concrete applications if correctly set up [38], enables the synchronization of lateral and vertical strain capturing during testing that results in highly accurate Poisson ratios. In this research, a Basler acA2040-55 \(\upmu\)m camera with resolution 2048 \(\times\) 1536 pixels was coupled with an Edmund Optics 35 mm/F1.65 lens for image acquisition. The Basler Pylon camera software (V7.2.1) suite facilitated the capturing and data storage process. By utilising the Basler Lens Selector tool, it was determined that the camera must be located at a working distance of 585 mm in order to achieve a field-of-view with dimensions 112 mm wide and 83 mm high, i.e., slightly higher than the test specimen. This configuration yielded a pixel size (resolution) of roughly 50 \(\upmu\)m. An acquisition rate of 1 Hz was chosen.  
GOM Correlate software (2021) was used for the analysis of the data [39]. Since only one camera was used, only a 2D analysis could be performed. However, since the test specimens were cylindrical in a 3D space, in-plane aligned target markers were required to provide a 2D reference point at any point of surface contact on the test specimen. These  
Figure 4: Core-drilled specimens of 20 mm height and 30 mm diameter for \(\upmu\)-CT scanning.  
target markers were identifiable by the software and were used to compute the relative in-plane strain difference over the monochrome image series input. The target markers contained nine differentiable point components that each represented an ellipsoid with major and minor axis lengths of 10 \(\pm\) 2 pixels, based on the field-of-view dimensions. A summary of the DIC setup information is given in Table 2.  
The target markers were fixed to the test specimen by using tiny 2 mm thick Perspex blocks, epoxy-glued to the specimen, with a 1 mm hole in the centre through which the target marker's pin fit. This facilitated planeness, i.e., aligned all target markers depth-wise, achieving a consistent distance from the camera. An image of a test specimen with two fixed lateral Perspex blocks is shown in Figure 5.  
Target markers were also used for longitudinal strain capturing. A steel platen was placed below and above the test specimen for the uniaxial compression test. Since a target marker could not be placed directly between the specimen's surface and each platen, the authors decided to fix the target markers on the inside of the steel platens, next to the test specimen. Such positioning would only capture the test specimen's deformation and exclude that of the steel platens. The final test specimen DIC assembly comprised six in-plane target markers. Four target markers were used to capture longitudinal strain, and two markers placed laterally at the specimen's mid-height, where maximum lateral strain would occur. A DIC recorded image of an assembled test specimen is shown in Figure 6.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Resolution** & **Working Distance** & **Field-of-View** & **Pixel Size** & **Ellipse Major Axis** & **Ellipse Minor Axis** \\
**Pixels** & **mm** & **mm** & **\(\mu\)m** & **\(\mu\)m** & **\(\mu\)m** \\ \hline
2084 \(\times\) 1536 & 585 & 112 \(\times\) 83 & \(\approx\)50 & 656 & 547 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of the DIC setup information.  
Figure 5: Test specimen with two lateral Perspex blocks attached for target marker positioning.