## 1 Introduction
According to [PERSON] [7], creep is defined as a time-dependent deformation higher than the initial deformation in concrete exposed to permanent load, while ACI Committee 209 [8] defines creep as a time-dependent increase in deformation in hardened concrete exposed to long-term loads. With the model proposed by [PERSON] [9], the accuracy of estimation of deformation due to shrinkage and creep of concrete was increased compared to the ACI 209 model [8]. Of course, all the improvements usually reduce the simplicity of the model's mathematical definition and increase the number of input parameters. The user needs to decide the level of accuracy he needs, depending on the sensitivity of the problem being analysed. Typically, models are defined for a uniaxial stress state and then mapped to a multiaxial stress state. In these mappings and defining the functional dependence of uniaxial and multiaxial stress states, experimental studies of concrete creep have shown that the corresponding Poisson's creep coefficient at exposure to multiaxial compressive stress is not constant over time [10; 11; 12]. It depends on the multiaxial character of the stress state and is therefore not isotropic, and its initial value is lower than in the elastic region of the material. According to the literature [13], resulting from a twelve-year study of concrete creep exposed to uniaxial and biaxial stresses, the variation of the Poisson coefficient is negligible. It ranges from 0.32 to 0.27 for autogenous creep, while for total creep it remains at boundaries between 0.30 and 0.35, which justifies the mathematical formulations of creep and contraction defined with a constant Poisson coefficient. Concrete creep has the characteristics of viscous deformations and occurs in both compression and tension zones. Although the tensile zone creep is less pronounced than in the compression zone, due to the lower tensile strength compared to the compression strength, creep in the tension is interesting in many practical situations [14], for example in assessing the possibility of opening cracks due to changes in temperature [15]. The studies reported in the literature [16] analyse the influence of the water-cement ratio on the creep development when the concrete is subjected to dominantly tensile stress. The development of creep deformation in reinforced concrete structures over time can lead to redistribution of stresses in the structure itself, whose neglecting, in combination with other adverse effects, could significantly impair the load-bearing capacity and stability of the observed structures. Concrete shrinkage represents the total time-dependent decrease of volume caused by the change in humidity of the concrete mix. When deformations of concrete are prevented, shrinkage leads to the occurrence of additional stresses in the concrete mainly transferred to the reinforcement, which contributes to the reduction of shrinkage deformation [10; 11; 16; 17; 18]. Shrinkage also reduces the development of continuous time-dependent displacement [19], thereby reducing the creep effect. The main goal of this research is to develop and validate an efficient numerical algorithm/model and identify its parameters by fitting experimental data. The final objective is to create software for simulating full spatial concrete structures under long-term load. This software combines two models. The first is according to Ref. [8], which provides the prediction of the creep coefficient at any time, and the second is a 3D model for describing reinforced concrete structures that provide a stress-strain state at any time under a long time loading.