## 1 Introduction  
Shrinkage, a time-dependent volumetric change, is a complex material response affecting concrete in an adverse manner. Several factors impact concrete drying such as relative humidity, exposed drying surface, cement characteristics, shape and even the size of the structure. It is important to distinguish between plastic shrinkage, autogenous shrinkage and drying shrinkage. The first occurs while the concrete is still wet and may generate significant cracking during the setting phase. Plastic shrinkage is particularly a concern when using high strength concrete; the reinforcements are ineffective during the setting phase. The second, usually quantified using sealed specimens, is a result of various chemical reactions and self-desiccation leading to water loss due to the hydration reaction. The third is caused by the loss of water during the drying process. Concrete shrinkage strain is the sum of both the autogenous and drying shrinkage strains. It is an increasing function of time with a decreasing rate. For the sake of clarity, shrinkage strain is used when talking about the total shrinkage strain in the remainder of this paper. Despite decades of research, shrinkage remains an important property to be controlled. To be more precise, unrestrained shrinkage, where the material is free to shrink, does not generate stresses and is hence not a major concern for structural engineering. In reality, the concrete structural elements are rarely, if ever, free to shrink; they can be restrained by their support, by the adjacent members or even by any bonded reinforcement. Consequently, shrinkage introduces a heterogeneous stress field leading to damage development when exceeding the concrete tensile strength. This threatens the integrity of the concrete structures and leads to their premature failure. It could be translated by an increase of permeability for structures such as dams, tanks and containment vessels or a decrease of performance for structures such as tunnels and bridges. It should be noted that 'free' shrinkage estimations are not enough for evaluating cracking tendencies. In other words and although shrinkage data are a major actor, it does not determine how the concrete will behave during service [1]. For this reason, concrete damage and cracking tendencies are normally evaluated under different restrained conditions via a series of cracking tests such as slab, bar and ring tests. The latter is wide-spread due to its simplicity and versatility and it is also adopted in the present study. A great deal of researchers have investigated cracking in circular ring tests under different restrained conditions. Their work covered everything starting with the effect of the used mixture [2; 3; 4; 5; 6; 7], the ring size/geometry [8; 9; 10], the drying conditions [5; 8; 11] and even the restraining steel ring wall thickness [12] on the concrete cracking tendencies. Consequently, the circular ring test became a standard testing method recommended by the AASHTO and ASTM in 2005 [13; 14]. Despite the simplicity and adaptability of the circular ring test, the authors of [8; 9; 15] highlighted key challenges summarized by the long time (up to one month) needed to develop a visible crack and the high dependency on the restraining steel stiffness. Solving the latter problems as an objective in mind, the authors of [16] proposed a variation of the ring test by adopting elliptical rings instead. In later works [17; 18], the study was extended to a wider range of specimens covering also thin and thick ring geometries. Using elliptical rings yielded a shorter period to generate visible cracks. In addition, the measuring instruments' cost is reduced since the cracks' positions are concentrated in known positions unlike circular rings (where the cracks have an equal opportunity to be initiated all over the circumference).