## 3 Research Results and Analysis
In addition, according to the data given in Table 3, ES-models (4) and (5) were calculated, reflecting the action of the hardening accelerator and fiber amount on the compressive strength of the investigated repair concrete and fiber-reinforced concrete at the ages of 2 and 28 days. For the ES-model (4), the experimental error is 0.301 MPa, and for the ES-model (5), it is 0.642 MPa:  
\[f_{\text{c.cub2}}\ (\text{MPa})=53.80+4.78x_{1}+0.42{x_{1}}^{2}+2.55x_{2} \pm 0{x_{2}}^{2}-0.25x_{1}x_{2}, \tag{4}\]  
\[f_{\text{c.cube}}\ (\text{MPa})=85.02-4.68x_{1}+2.02{x_{1}}^{2}+4.97x_{2}-3.23{x_{ 2}}^{2}+1.23x_{1}x_{2}, \tag{5}\]  
The ES-models (4) and (5) were used to draw the diagrams shown in Figure 6.  
The examination of the diagram in Figure 6a allows us to enclose that the amount of the hardening accelerator most significantly effects the early compressive strength of  
Figure 5: The combined effect of varied composition factors on the flexural strength of concretes and fiber-reinforced concretes at the age of 2 days (**a**) and 28 days (**b**). concrete and fiber-reinforced concrete for rigid pavement repair. With the addition of Sika Rapid 3 admixture in the amount of 2.4% by cement mass, the \(f_{ck.cube2}\) value increases by more than 9 MPa. Steel fiber also provides positive effects on the early compressive strength, despite the fibers' effect on the \(W/C\) mixture. With the maximum amount of hardening accelerator, regardless of the fiber amount, the concrete has a compressive strength at the age of 2 days of at least 55 MPa (grade C32/40). It allows us to begin the operation of the reconstructed highway section. At the same time, fiber-reinforced concretes with fiber amount in a composition of 50 kg/m\({}^{3}\) and more have a strength of 55 MPa and more already with an amount of hardening accelerator from 1.4%. Using the maximum amount of dispersed reinforcement and Sika Rapid 3 admixture, the early strength (2 days) of fiber-reinforced concrete is at least 60 MPa (grade C 35/45). At the design age, concretes and fiber-reinforced concretes containing the hardening accelerator have lower compressive strength than concretes of analogous compositions without the addition of Sika Rapid 3 (Figure 6b). Using an average amount of accelerator (1.2% of the cement mass), the compressive strength of the investigated concretes decrease on average by 6 MPa. When using the maximum amount (2.4% of the cement mass),they decrease by 9 MPa. This effect is known in concrete technology [41], but the main purpose of using the hardening accelerator is to achieve a high-early strength of repair concrete. At the design age, the positive effect of dispersed steel fiber reinforcement on compressive strength is more palpable than at the age of 2 days. So, when using fiber in the amount of 50 kg/m\({}^{3}\), the concrete compressive strength increases by an average of 8 MPa, and when using the maximum amount of fiber, that is, 100 kg/m\({}^{3}\), the strength increases by an average of 10 MPa. In general, the study of the modified fiber concrete's strength showed that by using a rational amount of the hardening accelerator Sika Rapid 3 and steel anchor fiber (70-90 kg/m\({}^{3}\)), the received repair concrete materials are characterized by high compressive and flexural strengths at an early age and at the age of 28 days. Frost resistance is an important durability indicator of cement-concrete roads and air-field pavements. During the experiment, frost resistance was measured by the accelerated method according to DSTU B V.2.7-49-96 [42] (the third method, freezing and thawing in salt water). The frost resistance grade is taken according to the number of cycles of  
Figure 6: Change in the concrete and fiber-reinforced concrete compressive strengths with usage of varied factors: at the age of 2 days (**a**) and 28 days (**b**). alternating freezing-thawing, after which the strength loss of the main samples in comparison with the strength of the control samples was no more than 5%, and the mass loss was no more than 3%.