# Payment Structure Characteristics and Behaviour Analysis with Digital Image Correlation
## 2 Materials and Methods
### Spatial Digital Image Correlation (3D-DIC System)  
Digital image correlation is an optical system for measuring displacement and determining the surface deformations of the observed object, regardless of the type of material. Generally, it is a system composed of equipment for measuring and collecting data and a software package for their analysis and processing. Within this research, the Aramis 3D optical deformation analysis system, developed by Gesellschaft fur optische Messtechnik (GOM) was used. The basic units of the system are a sensor with two cameras (for 3D measurements), a stand for cameras with LED lighting, a camera controller and power supply, and a computer with a software package (Figure 3). Using Aramis, it is possible to monitor the experiment with high temporal and spatial resolution, thus replacing many common measuring devices (extensometer, LVDT, strain gauge measuring tapes, etc. ), and it is easy to integrate with existing equipment using analogue or digital inputs and outputs, which significantly speeds up the instrumentation and testing process. After processing the measurement results, the program package provides the possibility of a graphical display of the desired data, which enables a better understanding of the behaviour of the measured object. The main advantage of this system compared to classic testing methods is that it is contactless. This prevents the contact between an extensometer and the specimen to be compromised during loading due to the roughness of the surface and slight movements, which makes it particularly suitable for pavement material testing. The digital cameras used in this research support a resolution of 2448 \(\times\) 2050 pixels and are equipped with a CCD chip. The lenses have a focal length of 12 mm, and they are set at an angle relation of 25\({}^{\circ}\). To ensure a full view of the specimen volume during all loading stages, the cameras were mounted at 280 mm from the cylinder face and were set to form 86 mm between each other. For this camera alignment, 0.005 mm\({}^{2}\) on the specimen surface represents a pixel on the image. With the given hardware components, a strain accuracy of up to 0.01% was established, which was at all times controlled by the unit software monitoring of the mean points' intersection deviation, which was kept lower than 0.1 pixels [43]. The testing preparation included volume determination, system calibration, and specimen treatment. The calibration of the cameras was performed using a certified, coded calibration panel. Specim treatment was performed by applying a white-colour spray stochastic pattern, since the specimen surface lacks contrast in its natural state. Figure 3: The 3D-DIC testing setup. The standardised method EN 13286-43 [40] defines testing the CBGM modulus of elasticity. The guidance in this standard suggests that the modulus can be determined from compressive strength tests, direct tensile tests, or indirect tensile tests. Although these static test methods are usually used to characterise the CBGM, there are numerous difficulties and problems in the very procedure of testing and measuring the modulus of elasticity. The biggest challenge is the correct measurement of the displacement given the rough, uneven, and, at times, loose surface of the samples (Figure 4). As pointed out in [44], test results can be inconsistent, varying greatly among samples within the same mixture, making this method difficult and time-consuming. Therefore, in this research, the 3D-DIC method was applied for the non-contact measurement of sample displacements and deformations with the calculation of the modulus of elasticity according to the specified norm. For the pavement structure test, the contact surface between the tire and pavement was adopted as an equivalent rectangular surface, while its actual shape depends on the axle load and inflation pressure (tire air pressure). The tire pressure of heavy goods vehicles is usually taken to be around 700 kPa and ranges from 500 to 1000 kPa.