## 3 Results and Discussion  
Generally, when assessing the quality of the aggregate used in concrete, the requirements of the PN-B-06712 [34] standard can be referred to. It should be noted that the requirements contained in this standard have been developed by the Polish Committee for Standardization on the basis of several dozen years of testing experience and are still used by bridge or road construction. According to this standard, the basic property classifying the quality of aggregate is its crushing strength, which is measured by the so-called aggregate crushing value (ACV). On the basis of its value, classification is made into the so-called aggregate brand, on which, in turn, the limit requirements relating to the remaining properties of the aggregate, such as the content of irregular grains or content of mineral dusts, depend. Taking into account the average ACV values of the gravel aggregate used for the tested concretes (Table 4), it is classified to the highest brand 30 (ACV value \(<\) 12%), according to the PN-B-06712 [34] standard. The other results of the properties, which can directly affect both the strength properties of concrete and the adhesion of cement paste to aggregate grains, should be considered as very good, the content of irregular grains is significantly lower than the 20% limit and the content of mineral dusts is lower than the 1.5% limit (Table 2). Figure 1: The test stand for testing the static modulus of elasticity. Table 5 shows the physical properties of gravel concrete, while, in Table 6, the mechanical properties are shown. The obtained results were compared to the values quoted on the basis of the EN 1992 standard [33] (Table 6). When assessing the air content in the tested mixtures, slight differences were found, which did not exceed 0.3%, and it was considered that they do not affect the mechanical properties of the tested concrete. Similarly, for concrete mixture consistency, the differences did not exceed 1.0 cm (Table 5). The concrete strength classes determined from the results of the compressive strength tests are the same, and correspond to the same class C40/50 (Table 6). The difference between extreme \(\mathrm{f_{cm}}\) values is 2.1 MPa, and no statistically significant differences in compression strength results were also confirmed by ANOVA test results (\(p=0.22>p=0.05\)). Different relations were found between splitting tensile strengths--the greater the maximum size of aggregate grain (\(\mathrm{D_{max}}\)) the lower the value of \(\mathrm{f_{cm,sp}}\) of concrete. The difference between the extreme values of \(\mathrm{f_{cm,sp}}\) is 1.73 MPa and the GC2/31.5 concrete splitting tensile strength is 34.5% lower than the GC2/8 concrete strength. The significant differences in splitting tensile strength values were confirmed by ANOVA test results. The value of \(p=5.6\times 10^{-9}\) is significantly lower than \(p=0.05\), which clearly and strongly confirms the statistically significant differences between the concrete tensile strength results. The least significant difference (LSD) test showed that each pair of averages differs significantly from each other (differences are statistically significant). The obtained values of \(\mathrm{f_{cm,sp}}\) were compared with the respective values of \(\mathrm{f_{cm,sp}}\) according to the EN 1992 standard [33] corresponding to specific classes of concrete. It was found that the concretes GC2/16 and GC2/31.5 were characterized by lower values by 5% and 16% respectively, i.e., by one and two classes of concrete strength lower. From a practical point of view, this means that increasing the \(\mathrm{D_{max}}\) of the aggregate can cause higher susceptibility to earlier scratches of the element. The modulus of elasticity of concretes was practically not susceptible to changes caused by different sizes of coarse aggregate. The difference between the extreme values was 0.8 GPa, with the highest \(\mathrm{E_{cm}}\) value for GC2/8 being only 1% higher than the lowest modulus of elasticity for the GC2/31.5 concrete.