## 1 Introduction
For such analysis, one (for 2D-DIC) or two (for 3D-DIC) digital cameras are used instead of conventional surface-attached measuring devices (e.g., linear variable displacement transformers or LVDTs, foil strain gauges, dial indicators, etc.). During the mechanical test, a specimen can be continuously monitored by recording a video or capturing images. Afterwards, the acquired images are processed by DIC software that evaluates the desired QoIs by analysing both point displacements and deviations from the initial state. The 2D-DIC technique is effective when the object is planar to the camera sensor and not moving with respect to the camera [34], whereas the 3D-DIC system (also known as stereo-DIC) tolerates relative movements and allows one to complete the calculations properly even with convex/concave-shaped specimens [35]. Cost-effectiveness is one of the reasons that researchers increasingly more often prefer non-contact measuring techniques over conventional systems. Since 2D-DIC is a monocular system and requires a single camera (with a stand, positioning mechanism and lighting kit), the expenses are significantly lower than for the 3D-DIC setup that requires at least double the amount of investment. Furthermore, computer software for 2D-DIC is normally less expensive (or even free of charge) and easier to operate. As 3D-DIC is beyond the scope of this research work, it will not be discussed in further detail. Various scientists have validated optical-based research techniques by using scientific-grade digital cameras. Such selection of equipment is also advised by the International Digital Image Correlation Society [34]. However, there have been successful studies that prove relatively inexpensive digital single-lens reflex (DSLR) cameras to be appropriate as well [36; 37]. In practical use, the DIC technique can significantly ease surveys of CCC structures such as bridges, tunnel linings, containment vessels of nuclear reactors and others, that can be described as inaccessible to humans, hazardous environments or excessively large in the surface area [38; 39]. Another advantage of DIC is the ability to analyse QOIs at any position and direction within the field of view (FOV). Therefore, unlike with surface-attached measuring devices, with image correlation software, various changes can be evaluated that could not be predicted before the test (e.g., strain and crack propagation at unsuspected locations or amplitude) [34; 40]. Furthermore, in some cases, deformations can exceed the limitations of the attachable extensometer gauge lengths and fastening accessories, leading to faulty data, an absence of data or even ending the test [37]. In general, the versatility of the DIC method allows it to be applied for both direct and indirect mechanical tensile testing. In this research work, 2D-DIC was employed to monitor fibre-reinforced Portland cement composite tensile creep deformation. Compact Tension (CT) specimens were monitored under sustained uniaxially applied tensile load over 50 days. Similar specimen geometry is suggested by ISO 7539 [41], ASTM E647 [42] and ASTM E399 [43] standards. Such specimen geometry allows to investigate both elastic and plastic deformations as well as fracture mechanics. An entry/mid-level DSLR camera was used to capture the images, whereas the analysis was done using the GOM Correlate free software. In addition, the acquired data were compared with conventional surface-attached strain gauges measurements. Such long-term DIC experiments have not been previously done.