## 1 Introduction  
When analysing the behaviour of concrete, it is very important to consider its time-dependent behaviour. Concrete changes the stress and deformation states over time, i.e., it adapts to the external load. Experimental tests of behaviour under long-term loads require significant financial resources and long-term occupation of laboratories, measuring equipment and personnel. On the other hand, numerical models cost significantly less and can be used numerous times with different material characteristics, different loads and other parameters. But, of course, the used model has to be reliable and must produce reliable results. When we speak about the 'time-dependent behaviour of concrete', we often think of creep and shrinkage, which are two main time-dependent physical properties of concrete. Concrete shrinkage is the volumetric change of concrete structures caused by moisture loss due to evaporation. It is a time-dependent deformation, which occurs without the influence of external forces and causes a reduction of concrete volume. On the other hand, creep can be defined as the long-term deformation of concrete under a permanent load. Generally, when a load is applied, pressure changes the shape of the concrete structure, and the strain occurs immediately along the direction of the applied load. But, during this time, deformation increases without the increase in load. We can say that concrete adapts to load or concrete creeps. When the continuous load is removed, the part of the strain decreases immediately and another part over a long period (reversible creeping). In addition, some strain remains permanently on the structure. This physical phenomenon occurs at room temperature and is caused by several reasons: number and position of internal voids, rate of viscous flow of the cement-water paste, and, generally considered as the most important, seepage of water out of the cement gel due to external load and air humidity. The rate and magnitude of creep for most concrete structures are closely related to the drying rate, but it may also occur in massive structures where drying of the concrete is very small. In these structures, most of the creep is caused by external pressure, which forces the flow of the water absorbed from the gel. Creeping in concrete can result in a considerable increase of strains and usually needs to be considered during the design and analysis of reinforced concrete structures. At low-stress levels, the creep of concrete can be described accurately by the linear viscoelasticity model, the so-called linear creep model. It has been experimentally determined that the linear functional relationship (linear creep) is only valid for the small-stress levels of concrete. When the relationship between stress of long-term static stress and concrete uniaxial pressure strength is greater than approximately 0.4, the linear relationship between the short-term and long-term deformations is lost and the progressive nonlinear increase deformation in concrete occurs, which is called non-linear creep [1; 2; 3; 4; 5; 6; 7; 8]. When the stress of concrete with a long static load exceeds 80% of the uniaxial compressive strength of the concrete, creep often leads to the breakdown of concrete in time [9; 10; 11; 12]. Many analytical expressions and empirical formulae can be found in the literature for this physical phenomenon simulation at lower and higher stress levels. Some models of the nonlinear concrete cracking, based on different rheological models and/or experimental tests, can be found in [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. Many authors investigate different rheological influences, especially [PERSON] dedicated much of his work to this problem [1; 2; 3; 4; 5; 6]. None of the offered models provides a complete description of the creep phenomenon. Each of the models has advantages, disadvantages and specifics of the application.