## 1 Introduction  
Concrete members of centrifugally cast annular cross sections are useful in a variety of scenarios, including industrial and hydraulic engineering and energy and transportation engineering design practice [1]. Reinforced concrete annular cross section piers and columns are produced by centrifugation. These elements are reinforced with a longitudinal reinforcement evenly distributed over the entire cross sectional perimeter. The resistance of such elements to mechanical impact is the same in all directions, which makes centrifugal annular piers and columns economical, especially under horizontal loads [2]. Steel spirals are used for transverse reinforcements. Durability, stability, high elasticity, compressive and tensile strength are characteristics of centrifuged, reinforced concrete structures [3]. The strength of bending and eccentric compressive reinforced concrete elements of the annular cross section is more dependent on the strength of the concrete than on the element of the square or I-beam section [2, 4, 5, 6, 7, 8]. The amount of longitudinal reinforcement in the tensile or compressive zone depends on the position of the neutral axis. Longitudinal high-strength steel reinforcements Are more effective than the conventional ones, with a yield strength of 400 MPa, which prevent the concrete from deforming in the falling part of the stress-strain diagram [9]. In elements of other cross sections, the compressive and tensile reinforcements are most often located in the furthest layers, i.e., those most distant from the neutral axis. This explains why the correct determination of the real strength of centrifuged concrete in both prestressed and non-prestressed tensioned elements of the annular cross sections continues to be a problem. The strength of the concrete and its other physical and mechanical characteristics in reinforced concrete products may vary considerably from those of the control tests (tests of prisms, cylinders or annular cross-section concrete). The real compressive strength of centrifuged concrete is in the cylinder strength. The manufacturing and testing process of tubular samples of the annular cross section is complex, and it is difficult to determine this value even for control samples, let alone for the real concrete structures. Indeed, it is common knowledge that concrete strength in real structures can differ significantly from the strength of concrete present in control samples. Centrifugation is one-way of compacting concrete in order to create a product that is fundamentally different in terms of structure and texture from that produced by means of vibration. Centrifuged concrete has a high tensile strength; the process of centrifugation able to increase the compressive strength of concrete by a factor of two or more. However, the process of centrifugation leads to an anisotropy of the concrete strength and deformability through the wall thickness [10; 11; 12]. With a thin ring of centrifuged wall, components of the concrete mixture are distributed over the wall thickness, depending on the weight of the particle. For example, coarse aggregates collect closer to the outer surface of the element, finer particles are located between the coarse aggregates. Towards the inner surface the amount of coarse aggregate reduces in concentration and the sand is evenly distributed in the cement stone. The inner layer of the surface consists of cement, fine aggregate particles and a sludge [13; 14]. Centrifugation of the concrete allows water filtration in the mixture from one layer to another, so that capillaries extending from the outer to the inner surface form in the centrifuged concrete. These join together to form macrocapillaries in the middle of the element wall, which reduces the strength of concrete. The intensity and amount of microcapillaries depend on the initial water and cement ratio in the concrete mixture.