# Influence of Variotropy on the Change in Concrete Strength under the Impact of Wet-Dry Cycles
## 1 Introduction  
### Background  
Currently, reinforced concrete structures form the basis of modern construction. Buildings and structures of various levels of responsibility are still being built from a concrete composite reinforced with metal rods, which makes such a material and such structures very susceptible to various types of aggressive influences, especially considering cyclic influences. One of the most dangerous types of cyclic impacts, especially inherent in several regions of the world, is the alternating effect of wetting and drying on concrete and reinforced concrete structures. In view of the fact that concrete in reinforced concrete structures has a protective function and provides a bearing capacity in the long term, protecting concrete from such cyclic exposure is an important task for process engineers and materials scientists [1; 2; 3; 4]. However, it should be noted that to develop corrective measuresto improve the concrete of reinforced concrete structures operating in aggressive conditions of alternating wetting-drying, these processes should be studied in detail, identifying the fundamental processes and dependencies that occur under the destructive influence of moistening and drying cycles. These processes include corrosion of steel [5; 6], diffusion of chlorides and the content of chlorides on the surface of concrete [7; 8], fatigue damage to concrete caused by water [9]. Studies of various types of concrete exposed to aggressive environments during the moistening-drying cycles are presented in [10; 11; 12]. The study [10] evaluated the mechanical characteristics of concretes under conditions of combined exposure to sulfates and moistening and drying cycles. According to the results, it was found that during the cycling of moistening and drying, the compressive strength increases up to 60 cycles and then gradually decreases. As for the sulfate attack, here the dependence of the change in strength is directly proportional: with an increase in the concentration of the sodium sulfate solution, the rate of loss of compressive strength also increases. Studies [11; 12] have a similar topic to [10], but the difference lies in the fact that concretes reinforced with steel fibers were studied. In general, the change in strength characteristics depends on the impact of the \"moisture-drying\" cycles and the degree of aggressiveness of the environment is of a similar nature. In [13], the authors studied \"the effect of nano-TiO\({}_{2}\) additives on the corrosion characteristics of concrete\". Following the results of experimental studies, it was found that the addition of a nanomodifying additive of titanium oxide increased the sulfate resistance of the composite during its alternating moistening and drying. Thus, for samples of the control composition, the maximum value of strength loss was 40%, and for modified ones, 36%. In [14; 15], the authors studied the effect of fly ash additives on the durability characteristics of concrete. So, for example, in [14], for samples with 40% fly ash content, the coefficient of corrosion resistance and relative dynamic modulus of elasticity are higher than for samples of the control composition. The authors explain the higher corrosion resistance of samples with fly ash by a change in the nature of hydration processes. In [15], the authors note some negative aspects that arise when fly ash is introduced into the composition of concrete. As for the sulfate resistance of concrete, it directly depends on the percentage of ash. According to the results of the study, the best characteristics of the durability of concrete can be provided by replacing the binder with ash in an amount of 20%; however, with a replacement of 30%, a significant deterioration in these characteristics is observed. In addition to filler additives in the form of waste, chemical additives, for example, hydrophobic ones, have also been studied [16]. In [17; 18], the authors studied the mechanism of concrete destruction under the conditions of \"dry-wet\" cycles during a sulfate attack.