# Payment Structure Characteristics and Behaviour Analysis with Digital Image Correlation
## 3 Results and Discussion
### The 3D-DIC for Pavement Structure Analyses  
The pavement structure was simulated by a test setup consisting of three layers differing only in the CBGM. For the CBGM, three different cement contents were used (3%, 5%, and 7%) to simulate the change in layer rigidity and its influence on pavement behaviour. Three samples of each kind of CBGM/ pavement structures were tested, and the average results are presented. For every sample, the force and vertical displacement were recorded (Figure 5). Introducing the first loading cycle, characteristic system \"adjustment\" was recorded for each tested sample. Namely, during sample preparation, it was not possible to achieve ideally flat surfaces of each layer, and full contact between the layers was achieved at different values of vertical displacement. This is particularly linked to the condition of the inbuilt UGM layer. System adjustment refers to the closing of the space between the layers and the initial settlement of the UGM layer. However, after the initial stress value was reached, all subsequent steps had a consistent curve slope during loading and unloading, which indicates a uniform stiffness distribution and the absence of further minor adjustments. Furthermore, in all samples, a gradual displacement increase within the same loading cycle was recorded, for example during a certain stress level repetition. This suggests creep occurrence in all samples, meaning slight decreases in stiffness and the occurrence of residual (irreversible) displacements. Within the first loading cycles, no significant changes occurred, so this property can be attributed to the curvature of the stress-strain curve of the individual pavement system layers within the elastic state. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Cement Content** & **f\({}_{\text{c}}\) (MPa)** & **E (MPa)** \\ \hline
3\% & 2.69 & 3.46 \\
5\% & 6.45 & 10.03 \\
7\% & 8.89 & 11.96 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical characteristics of the CBGM. To obtain clearer and more precise information on the pavement system behaviour, from the individual force-displacement curves, the initial state of system \"adjustment\" was removed, and the envelopes of individual cycles are presented in Figure 6. For a 7% cement content CBGM pavement system, a very small total displacement was recorded, while those for a 5% and 3% cement content were relatively similar, but with different displacements levels achieved at different loading cycles. Such a pattern of behaviour points to the conclusion that, in the case of the system with 7% cement content, cracks do not open at all, and therefore, no significant displacements occur. On the other hand, the behaviour of pavement systems with CBGM of 3% and 5 % cement content points to cracks in the lower layers having opened, but at different loading stages, which are then manifested in different ways in the force-displacement record. Figure 5: Force–vertical displacement curves for selected samples of the CBGM with (**a**) 3%, (**b**) 5%, and (**c**) 7% cement content. Figure 6: Force–displacement envelopes for the pavement system with different CBGM layers. Analysing only irreversible displacements within a particular loading cycle for each of the samples, earlier indicators were confirmed, and additional behavioural indicators were revealed. Namely, the sample with 7% cement content achieved the largest residual displacement in the first stage of loading, and the displacement decreased with each following loading cycle, indicating that no cracks had formed. The reduction of the residual displacement by each following loading cycle was exclusively related to the decreasing space for free deformations between the layers and the gradual load bearing through the deformation of the materials themselves. Figure 7 shows the achieved displacement at the end of each loading cycle for each cement content. Based on the field of deformation data for each of the series, some insight can be gained.