## 3 Results and Discussion
Another reason may be that the aggregate used in their manufacture has lower mechanical resistance (Figure 2) than the aggregate used in GC2/8 concrete. The aggregate with the 2 \(\div\) 8 mm fraction has a lower ACV coefficient by about 23% and 63%, compared to the 8 \(\div\) 16 mm and 16 \(\div\) 31.5 mm aggregates, respectively. In order to explain the differences found in the properties of concrete, the properties of gravel aggregate examined were analysed. The ACV was considered to be the most variable, and its values depend on the thickness of coarse aggregate grains, and there is a very strong correlation between them (Figure 2), with statistically significant differences (\(p=5.6\times 10^{-5}<p=0.05\)). It should be noted that the increase in the ACV value of the aggregate corresponds to its lower mechanical resistance (lower crushing strength resistance). For this reason, the potential for relationships between the properties of the concrete and the corresponding ACV values of the aggregate and how correlated they are was analysed. The strongest correlation was found between the splitting tensile strength f\({}_{\rm cm,sp}\) of concretes and ACV values of aggregates (Figure 3). The correlation coefficient \(r=0.91\) indicates a very strong correlation. The correlation shows that the increase in D\({}_{\rm max}\) of the aggregate and, at the same time, the  
Figure 2: Correlation between aggregate crushing value and aggregate fraction. decrease in the mechanical resistance of the aggregate (increase in ACV) corresponds to the decrease in the splitting tensile strength. There is a strong correlation between the compressive strength and ACV values (\(r\) = 0.84) (Figure 4). The increase in D\({}_{\text{max}}\) of the aggregate has corresponded to a slight increase in the compressive strength of the concrete. However, it is surprising that in this case, despite the fact that the mechanical resistance of the aggregate significantly decreases, the compressive strength of the concrete increases slightly. A very weak correlation was found between aggregate crushing value and elastic modulus of concretes (Figure 5). These parameters do not differ significantly statistically. Analysing the influence of D\({}_{\text{max}}\) of aggregate on the values of strength properties of the tested concretes, an opposite relation to the commonly accepted one was found, a slight increase in compression strength (within the same concrete strength class) corresponds to a clear decrease in splitting tensile strength. The relations described above indicate that the influence of D\({}_{\text{max}}\) of the aggregate on the values of strength properties is greater and more significant for splitting tensile strength. It is widely recognised that the most significant impact on the mechanical properties of concrete is exerted by the interfacial transition zone (ITZ), where the first micro-cracks form under load. As the load increases, micro-cracks propagate, the increasing scratches start to merge and expand until the concrete is destroyed. In the splitting test, aggregate grains are very often split (cracked), and also in the case of gravel, especially the weaker ones. Therefore, apart from the area of ITZ with increased porosity [5], which  
Figure 4: Correlation between aggregate crushing value and compressive strength of concrete. Figure 3: Correlation between aggregate crushing value and splitting tensile strength of concrete. plays the most important role in the destruction, the share of aggregate grains with lower mechanical resistance (higher ACV) will also be significant. [PERSON] et al. demonstrated that the reduction of D\({}_{\text{max}}\) of the aggregate causes the formation of ITZ, with lower porosity around the grains [40]. This explains the results obtained in the tests. The growth of D\({}_{\text{max}}\) of the aggregate from 8 mm to 31.5 mm resulted in the formation of a more porous ITZ around the aggregate grains in GC2/31.5 and GC2/16 in comparison to ITZ in GC2/8.