## 2 Materials and Methods
### Methods  
When conducting scientific research, relying on the existing regulatory and technical base containing testing and research methods was crucial. As a concrete sample, concrete of the Russian class B30 was chosen, corresponding to SC in the range from 40 to 50 MPa. The workability grade was P1 (cone draft 1-4 cm, determined according to GOST 10181 \"Concrete mixtures. Methods of testing\"). A metal cone for determining the flowability of concrete was \"filled with a concrete mixture through a funnel in three layers of the same height on a smooth sheet\" [59]. The concrete mix was rodded 25 times, tightly pressing the cone to the sheet. Next, the loading funnel was removed, the excess mixture was cut off, and the surface was smoothed. The time for filling and removing the cone was no more than 3 min. The rise of the cone lasted 5-7 s. The draft of the cone was determined twice. The error was no more than 0.5 cm. The total test time did not exceed 10 min [60]. The process of determining the workability of a concrete mixture is shown in Figure 4.  
Figure 4: Testing the concrete mix for workability: (a) filling and bayoneting the mix in a cone; (b) slump measurement. \begin{table}
\begin{tabular}{c c} \hline \hline
**Name of Indicator** & **Actual Value** \\ \hline Diameter (\(\mu\)m) & \(21\pm 1.1\) \\ \hline Titer (Tex, g/km) & \(30\pm 1.3\) \\ \hline Length of fiber (mm) & \(30\pm 2\) \\ \hline Density (kg/m\({}^{3}\)) & 1200 \\ \hline Porosity (\%) & 32–35 \\ \hline Cellulose content (\%) & 32–43 \\ \hline Lignin content (\%) & 41–45 \\ \hline Crystallinity (\%) & 27–33 \\ \hline Tensile strength (MPa) & 175 \(\pm\) 8.2 \\ \hline Ultimate strain (\%) & 3.6 \(\pm\) 0.2 \\ \hline Elasticity modulus (GPa) & 22.0 \(\pm\) 0.2 \\ \hline Relative fiber strength (cN/Tex) & 15.3 \(\pm\) 0.6 \\ \hline Angle of orientation (\({}^{\circ}\)) & 30–49 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristics of coconut fiber. The calculations of the concrete composition parameter were based on GOST 27006 \"Concretes. Rules for mix proposing\". The selection of the concrete composition was carried out according to the following steps:  
* Selection and characterization of raw materials for concrete;
* Calculation of the initial basic composition;
* Calculation of initial additional compositions of concrete;
* Production of experimental batches from the initial and additional compositions:
* Taking samples for testing the concrete mixture and making control samples;
* Concrete testing to determine standardized quality parameters;
* Processing of the results obtained with the establishment of dependencies reflecting the influence of the parameters of the concrete composition on the standardized quality indicators;
* Designation of the nominal composition of concrete, ensuring the receipt of a concrete mixture and concrete of the required quality [61]. The parameters of the concrete composition obtained in this way, the method of preparing the concrete mixture, the curing of the samples, and subsequent tests were carried out based on verified methods, confirmed by a large amount of experimental and regulatory data. The parameters of the composition are presented in Table 4.  
The concrete components were mixed using a BL-10 laboratory concrete mixer (ZZBO, Zlatoust, Russia). The components were loaded in this order: fine aggregate, then Portland cement, then water. Coconut fiber was introduced into a homogenized mixture of binder solution and fine aggregate. The resulting mixture was stirred for 3 min, and then the coarse aggregate was loaded. The resulting concrete mixture was mixed until homogeneous. The mixture was vibrated on an SMZh-539-220A unit (IMash, Armavir, Russia).