## 4 Results
The influence of the porous aggregate on the physical and mechanical properties of compacted concrete in which porous material replaced part of the dense fractions was established by control tests using specimens for axial compression, axial tension, and bending tension. The replacement of the concrete components was conducted at a constant cement consumption rate without changing the total volume of the aggregate mixture. Figure 4: Dependence of the bulk density of a slag pumice mixture depending on the volumetric content of various types of fractions. Figure 5: Dependence of the bulk density of a vibro-compacted mixture of slag pumice on the volumetric content of various types of fractions. In the studied compositions, up to 50% of the volume of dense aggregate was replaced with the same volume of the porous one. To identify the most effective grain compositions of the combined aggregates, the combinations shown in Table 9 were investigated. The test results of the concretes made with the indicated grain compositions are presented in Table 9 and Figure 7.  
CCQ characterizes the possibility of reducing the weight of buildings while maintaining or even improving the basic properties of materials. The structural quality factor is defined as the ratio of the compressive strength to the relative density MPa \(\times\) m\({}^{3}\)/kg:  
\[\text{CCQ}=\frac{R_{b,cub}}{\rho_{CM}} \tag{9}\]  
where \(R_{b,cub}\) is compressive strength, MPa;  
\(\rho_{cm}\) is the density of the concrete mixture, kg/m\({}^{3}\). SC--designation of the concrete control composition (without slag pumice); S1-S5--designation of the compositions (with different contents of slag pumice). Analysis of the obtained results and their comparison with the results obtained by other authors [2; 8; 20; 21; 33; 34] indicates that the inclusion of slag pumice in the composition of concrete instead of as part of the dense aggregates leads to an increase in the CCQ, that is, a decrease in the compressive strength of the concrete is compensated by an even more significant decrease in the density of the material, which means a decrease in mass. The maximum increase of CCQ on the combined aggregate is observed when part of the coarse and small dense aggregate is replaced with slag pumice at 40% of the total volume of these aggregates in the concrete mixture. Further, this research was conducted to study the strength and deformation characteristics of LFRC on a combined aggregate. Based on preliminary studies, the optimal composition of concrete on a combined aggregate (S4) was determined, which was subsequently used to manufacture LFRC prototypes on a combined aggregate. The dependence of the change in the strength and deformation characteristics of lightweight fiber concretes on a combined aggregate was studied by changing the volume content of the fiber in the composition of the concrete mixture and the technological methods of its distribution. As a result of testing the control samples (S4), the strength characteristics of the original concrete were established: \(R_{h\_cub}\) = 59.1 MPa, \(R_{b}\) = 23.6 MPa; \(R_{ht}\) = 5.9 MPa; \(R_{htb}\) = 4.1 MPa; \(\varepsilon_{hR}\) = 1.87 mm/m \(\times\) 10\({}^{-3}\); \(\varepsilon_{htbR}\) = 1.24 mm/m \(\times\) 10\({}^{-4}\); \(E_{b}\) = \(E_{htb}\) = 35.3 MPa \(\times\) 10\({}^{3}\). The determination of the strength characteristics of LFRC on a combined aggregate was conducted using regression dependencies, the type, and the values of the coefficients that were determined by the mathematical planning methods of the experiment. As functions, the strength and deformation characteristics of LFRC on a combined aggregate vary depending on the volumetric content of the fiber in the composition of the concrete mixture and its distribution method during the preparation of the concrete fiber-reinforced concrete mixture are taken.