## 4 Conclusions  
The following key conclusions can be drawn from this study:  
1. The influence of D\({}_{\text{max}}\) of the aggregate on the values of strength properties is greater and more significant for splitting tensile strength. Analysing the obtained results, it was found that, with the increase of D\({}_{\text{max}}\), the splitting tensile strength of \(f_{\text{ctm,sp}}\) deteriorated-for concrete GC2/31.5 is 34.5% lower than the strength of concrete GC2/8. An increase in aggregate D\({}_{\text{max}}\) with a simultaneous decrease in the mechanical resistance of the aggregate (increase in ACV) significantly reduces the tensile strength.
2. Comparing the obtained results of \(f_{\text{ctm,sp}}\) with standard values, it was found that GC2/16 and GC2/31.5 concretes are characterized by 5% and 16% lower values respectively, i.e., one and two  
Figure 6: Three-dimensional surface plot: (**a**) the splitting tensile strength (MPa) against the compressive strength (MPa) and the aggregate crushing value (ACV), (**b**) the splitting tensile strength (MPa) against the elastic modulus (GPa) and the aggregate crushing value (ACV).  
classes of concrete strength lower. This may indicate that an increase in \(\rm D_{max}\) may make the element more susceptible to earlier scratching.
3. The influence of the maximum size of the aggregate grain on the modulus of elasticity was negligible. The changes in the size of the coarse aggregate did not affect the modulus of elasticity of concrete-the \(\rm E_{cm}\) for GC2/8 was only 1% higher than the GC2/31.5 concrete module, which was the lowest of all.
4. With the growth of \(\rm D_{max}\) of the aggregate, the brittleness of the tested concretes increases.
5. The opposite to commonly accepted relationships have been observed--despite a decrease in the mechanical resistance of the aggregate, the compressive strength of the concrete has increased slightly, and a slight increase in the compressive strength corresponds to a clear decrease in the splitting tensile strength.  
To summarise, the issue of the influence of the maximum size of aggregate grain on the properties of concrete requires further extensive research. As can be seen, the few test results presented are not consistent, and thick aggregates of different fractions are used for structural concretes. Undoubtedly, it is also necessary to analyse what effect on the changes caused by the maximum grain size will be exerted by the use of crushed aggregate, instead of pebbles, with irregular grains and rough texture. Mechanical adhesion of the grout to the aggregate will be improved in this case, but how this will affect the properties of the concrete needs to be explained by means of experiments.  
Conceptualization, J.G.; methodology, J.G.; validation, J.G.; formal analysis, All; investigation, J.G.; data curation, J.G.; writing--original draft preparation, All; writing--review and editing. All; visualization, M.S.; translation, M.S.; supervision, J.G.; project administration, All; funding acquisition, J.G. All authors have read and agreed to the published version of the manuscript. This work was financially supported by the Ministry of Science and Higher Education, within the statutory research number FN14/ILT/2019. The authors declare no conflict of interest.