*Sustainability of Concrete with Synthetic and Recycled Aggregates*

large amounts of energy during strength resistance of the construction. Therefore, CFST of circular cross-section are increasingly used in construction practice.

The high strength and deformability of the concrete core ensure its main advantages, especially for short centrally loaded circular cross-section concrete-filled tubular elements. Due to the complicated nature of CFST load resistance, regulations of the Europe, Australia, Brazil, India, Canada, China, the USA, Japan, and a number of other countries recommend using empirical formulas to calculate their bearing capacity.

Despite the large number of the experiments serving as a base for these formulas they do not always allow to obtain valid results [6, 7]. They have significant limitations in the field of application. They were obtained either from the results of specific laboratory sample testing, or due to statistical processing of the relevant data. First, these formulas are valid only for normal concrete. They give unreliable results for the columns from other types of concrete (for example, fine-grained ones). Secondly, these methods, as a rule, do not allow the calculations of eccentrically compressed concrete filled steel tube elements, which have any differences from a "classical" design, for example, the presence of a high-strength rod [8, 9] and (or) spiral reinforcement [10–12], the application of various types of concrete [13], the effect of preliminary lateral reduction in a concrete core [14], etc.

According to the results of researches carried out by many scientists, the most reliable calculations of the strength of CFST columns can be performed based on the recommendations of the EN 1992-1-1 standard. Moreover, a simplified method is often used in the calculations. But it is based on empirical formulas and is very limited in scope. It is proposed to consider the general case of calculation as well. For its implementation, the following assumptions are made:


Design of column structural stability should take into account second-order effects including residual stresses, yielding of structural steel and of reinforcement, local instability, cracking of concrete, creep and shrinkage of concrete, geometrical imperfections.

However, there are no specific methods for practical implementation of such a calculation.

The purpose of this monograph is to propose the method of deformation calculation of the bearing capacity of compressed CFST under short-term load action based on the phenomenological approach.
