## 5 Conclusions  
In this paper, the fine aggregate in concrete mixes was partially replaced by crushed hazelnut shells to study the influence on the mechanical compressive and tensile strength. Standardized concrete cylinders were tested according to the ASTM standards. A statistical study was performed to correlate the mechanical response with the physical properties. Based on these results, the following conclusions and recommendations could be drawn:  
* The findings indicated a non-linear relationship between the water/cement ratio and compressive strength. For the specimens with a w/c ratio of 0.4, a 9.5% increase in compressive strength at 28 days was observed in one specimen, while the other two specimens showed a decrease of approximately 20%. Similarly, for a w/c ratio of 0.5, the compressive strength at 28 days exhibited a decrease ranging from 2% to 14% compared to the original specimen. This non-linear behavior highlights the complex interplay between the water/cement ratio and compressive strength. * The results suggested that an optimal w/c ratio exists for achieving the highest compressive strength. In this particular study, a w/c ratio of 0.4 resulted in a slight increase in compressive strength in one specimen, indicating that this ratio might be more conducive to optimal strength development. Conversely, a higher w/c ratio of 0.5 resulted in a slight decrease in compressive strength compared to the original specimen, implying that an excessive water content may have a detrimental effect on the strength of the concrete. * For the specimens with a w/c ratio of 0.4, the tensile strength exhibited a general increase as the percentage of crushed hazelnut shell inclusion increased from 2.5% to 5%, reaching a peak value of 6.56 MPa. However, a further increase to 10% resulted in a slight decrease in tensile strength to 6.32 MPa. This suggests that an optimal percentage of crushed hazelnut shell inclusion exists, beyond which the tensile strength starts to decline. * For the specimens with a w/c ratio of 0.5, the inclusion of crushed hazelnut shell resulted in a general decrease in tensile strength across all inclusion percentages. As the percentage of inclusion increased from 2.5% to 10%, the tensile strength decreased from 4.97 MPa to 4.72 MPa. This indicated that the higher w/c ratio negatively influenced the strength enhancement effect of crushed hazelnut shell inclusion. * The use of a 0.4 w/c ratio consistently produced better results for both compressive and bending strength, with fewer and lower reductions in mechanical strength compared to the standard mixture. The correlation analysis showed that the strength results were inversely related to the w/c ratio and air content of the mixture. * Based on the results obtained, concrete mixes with a HS replacement percentage of 2.5% could be used in constructive systems with a compression strength requirement lower than 17 MPa. Figure 7: Average tensile strength results at 28 days for 0.4 and 0.5 water/cement ratios. * Concrete mixtures with a HS replacement percentage of at most 10% of the fine aggregate could be used in structures with tensile bending stress requirements lower than 6 MPa. These results indicate that the use of hazelnut shells as a partial replacement for fine aggregates is a viable option for reducing the environmental impact and improving the mechanical properties of concrete. However, it is important to consider the specific requirements of each application when determining the optimal percentage of HS replacement. To conclude, from the outcomes of the present work, it is possible to identify a few areas of interest for future research. First, in future studies it is advised to increase the number of tests or repetitions performed. The aim of this would be to collect a larger number of results, which could help to reduce the experimental uncertainty. Furthermore, it would be relevant to investigate the compressive and tensile strength of concrete mixes with cure times above 28 days--for example, 90 days.