# High-Performance Concrete Nanomodified with Recycled Rice Straw Biochar
## 3 Results
### Evaluation of the Microstructure and Chemical Composition of Rice Straw Biochar
Figure 10: Results of experimental studies of the effect of dosage of rice straw biochar on flexural tensile strength: 1—polynomial of the 3 rd degree; 2—function (1). The graphical dependencies and data on changes in the characteristics of concrete allow finding that the most effective dosage is the addition of rice straw biochar in an amount of 6% by weight of cement. This amount of additive allows for achieving maximum strength characteristics. Thus, the cubic compressive strength value was 55.7 MPa, the axial compressive strength was 41.8 MPa, the tensile strength in bending was 6.6 MPa, and the axial tensile strength was 4.1 MPa. It should be noted that the inclusion of biochar additives of 2% and 4% also contributes to the growth of strength characteristics. However, with the addition of biochar in an amount of 8%, an inverse relationship is observed, the values of the strength characteristics decreased and almost equaled the values of the strength characteristics of the control composition, and with the inclusion of an additive of 10%, and even more intense drop in strength characteristics was observed. Additionally, according to the test results, diagrams of compression \"\(\varepsilon_{\text{b}}-\sigma_{\text{b}}\)\" and tension \"\(\varepsilon_{\text{bt}}-\sigma_{\text{bt}}\)\" were constructed. Graphic dependences of \"stress-strain\" are presented in Figures 12 and 13. Based on the results of the analysis of the obtained deformation diagrams of concretes modified with the addition of rice straw biochar, the following conclusions can be drawn. It has been established that the use of biochar additives affects the deformation diagrams of the resulting concrete as follows: the peak of the diagrams of concretes with the content of biochar additives in the amount of 2%, 4%, and 6% of the mass of cement shifts up and to the left relative to the diagrams of the control concrete sample, and the peak of the diagrams of concretes with the content of additives biochar in the amount of 8% and 10% is shifted up and to the right relative to the diagram of the control sample of concrete. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Characteristics of Concrete**} & \multicolumn{5}{c}{**Change in \% (A) with the Content of Rice Straw Biochar Additive, wt \%**} \\ \cline{2-7}  & **0** & **2** & **4** & **6** & **8** & **10** \\ \hline R\({}_{\text{b,cub}}\), MPa & 0 & +3 & +10 & +19 & \(-2\) & \(-11\) \\ R\({}_{\text{b}}\), MPa & 0 & +6 & +9 & +22 & \(-1\) & \(-9\) \\ R\({}_{\text{btb}}\), MPa & 0 & +5 & +8 & +17 & +3 & \(-13\) \\ R\({}_{\text{btb}}\), MPa & 0 & +7 & +13 & +25 & +1 & \(-9\) \\ \(\varepsilon_{\text{b}\text{b}\text{r}}\), mm/m \(\times 10^{-3}\) & 0 & \(-1\) & \(-5\) & \(-12\) & +2 & +8 \\ \(\varepsilon_{\text{btb}\text{r}}\), mm/m \(\times 10^{-4}\) & 0 & \(-3\) & \(-10\) & \(-24\) & \(-4\) & +8 \\ E\({}_{\text{b}}\) = E\({}_{\text{btb}}\), GPa & 0 & +4 & +6 & +14 & \(-2\) & \(-6\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Changes in the strength and deformation characteristics of test samples of heavy concrete depending on the content of rice straw biochar additive (\(\Delta\)). Figure 12: Stress-strain diagram in compression.