## 3 Results and Discussion
### Tensile Flexural Strength and Compressive Strength  
The results of the determination of the flexural and compressive strengths for the comparative mixture containing standardized aggregate according to CSN EN 196-1 [42] and the experimental mixtures Z2, Z3 based on recycled rubber are shown in Figures 3-5. The strengths were determined after 3, 7, 14, 28 and 90 days.  
Figure 3 presents the values of tensile flexural strength and compressive strengths depending on the age of the comparative test specimens. The trend of increasing compressive strength has a linear character with a reliability value of R = 0.96, which means that it is a very narrow dependence. This also applies to flexural strength, where the reliability value R = 0.97. The determination of the flexural strength and compressive strength of the test specimens based on natural aggregates was performed only to verify the strength characteristics of the binder (cement CEM III/A 32.5 N). This type of cement was used as the binder for mixtures Z2 and Z3.  
Figure 4 presents the results of the flexural strength of mixtures Z2 and Z3. Both mixtures show the same trend of increasing flexural strength in the specified time intervals with the value of the reliability coefficient of R = 0.98 for Z2 mixture and of R = 0.97 for Z3  
Figure 2: Grain size curve of standardized aggregate and rubber granulate fr. 0/1 mm, fr. 1/3 mm.  
mixture as the comparative mixture based on natural aggregate. This trend of increasing flexural strength of mixtures Z2 and Z3 (with recycled rubber) correlates with the trend of increasing flexural strength of the comparative mixture (see Table 1) containing natural aggregate (standardised sand as filler).  
Figure 5 shows the compressive strength results of mixtures Z2 and Z3. Both mixtures show the same trend of increasing compressive strength as the test specimens of the comparative mixture based on natural aggregate. The value of the reliability coefficient for Z2 is R = 0.99 and for mixture Z3, R = 0.99. This trend of increasing compressive strength of mixtures Z2 and Z3 (with recycled rubber) correlates with the trend of increasing compressive strength of comparative mixture (see Table 1) containing natural aggregate (standardised sand as filler).  
Figure 4: Tensile flexural strength of mixtures Z2 and Z3.  
Figure 3: Tensile flexural strength and Compressive strength of comparative mixture.  
Based on the results of the strength characteristics of the mixture based on natural aggregate and the mixture based on recycled rubber, it is evident that the recycled rubber used as the filler caused a decrease in both compressive and flexural strengths. The decrease in tensile flexural and compressive strength values is caused by the high elasticity of rubber compared to the comparative sample with aggregate [49], which has no elastic properties at all. This causes lower tensile flexural and compressive strengths. The cohesion of rubber particles with cementing compound is significantly lower than the cohesion of cementing compound with aggregate. For clarity of the graphs, the standard deviations of the values of tensile flexural strength and compressive strength are given separately in Table 2.