## 3 Results and Discussion
Simultaneously, with the growth of D\({}_{\text{max}}\), the mechanical resistance of the aggregate also deteriorated, and the lowest ACV value was determined for the aggregate in the GC2/31.5 concrete. The overlapping of these two factors caused that, despite the same class of all three concretes and similar values of their compressive strength, there was a clear decrease in the splitting tensile strength as the maximum aggregate grain size increased. Analogous relations to those obtained in this study were obtained by [PERSON] et al. [41], but instead of natural aggregate they used steel balls with diameters of 9, 12, 19, 25 and 32 mm, i.e., with very similar D\({}_{\text{max}}\) to the gravel used in the concretes studied. Low strength concrete (LSC, 25 MPa) and high strength concrete (HSC, 47 MPa) were tested. They also found a significant decrease in tensile strength (to about 15%) as the aggregate size increased, while the compression strength increased only slightly. The observed decrease in tensile strength along with the increase in grain size in both LSC and HSC was justified by the decrease in bond strength. This is due to the increased volume of the aggregate in relation to the total volume of the composite, which makes the significant difference between the elastic modules of the two phases more pronounced, thus creating an increased stress concentration and more micro-cracks near the aggregate. The negative influence of the smooth texture of the aggregate surface on the binding force due to the increased aggregate surface was also emphasized. The smooth surface texture and high modulus of elasticity of the aggregate result in a greater reduction of tensile strength in composites with a lower w/c ratio (_w/c_ = 0.42). According to the authors [41], the interfacial bond was considered decisive in the reduction of tensile strength and played a minor role in the influence on the values of the compressive strength. It was found that the tensile strength decreases as the size of the aggregate increases. The rate of tensile strength reduction with increasing size of a single aggregate becomes higher in HSC [41]. In turn, the results of this study and [PERSON] et al. [41] differ significantly from the results of the research on mortars and concretes presented by [PERSON] [10]. The tests carried out in the study by [PERSON] [10] showed that when aggregates from 2 mm to 8 mm were used in concrete production, the tensile strength value increased from 2.35 MPa to 2.7 MPa, respectively. This shows a 15% increase in this value. However, when aggregates with a larger diameter of 8 mm to 32 mm were used, the tensile strength of the concrete remained constant at 2.86 MPa. It was found that there is a systematic difference between the strength of mortar (aggregate size up to 4 mm) and concrete (from  
Figure 5: Correlation between aggregate crushing value and elastic modulus E of concrete. 8 mm upwards). As noted, whether this conclusion is valid for all types of concrete and aggregate sizes should be confirmed by experiments and numerical simulations, which have not been conducted so far [10]. Figure 6a shows a mathematical/experimental model of the splitting tensile strength of concretes which depends on two other concrete characteristics: x\({}_{1}\)--compressive strength and x\({}_{2}\)--aggregate crushing value. The interaction of compressive strength and aggregate crushing value, as described in the three-dimensional graph, indicates that within the tested ranges a higher compressive strength with a lower aggregate crushing value would significantly increase splitting tensile strength. The statistical analysis carried out in the STATISTICA 12 program (StatSoft, Inc., Tulsa, USA) showed that the whole presented regression model is statistically significant. The dependent variable-splitting tensile strength-is explained in 80% using the above model, although the correlation matrix before the analysis indicated that one of the independent variables is statistically insignificant-compressive strength.