# Strengthening Polymer Concrete with Carbon and Basalt Fibres
## 3 Results and Discussion
### Analysis of the Fracture Surface of the Tested Samples  
In order to investigate the reasons for the change in strength characteristics with the increasing percentage of the dispersed reinforcement and to study the peculiarities of fracture of carbon fibre concrete, we analysed the microstructure of the fracture surface of the experimental samples. The obtained data allowed us to establish that the reinforcement in the range from 0 to 1.0% leads to a uniform distribution of carbon fibres (Figure 9b), while increasing the concentration up to 1.5% and above, clusters of fibres of different density are observed. These clusters disturb the homogeneity of the fibre concrete structure, which negatively influences its force structure, promotes the appearance of stress concentration zones and cracks, which ultimately reduces the strength properties of the material. The analysis of the contact zone between the carbon fibres and the cement matrix showed that deposits of newly formed cement hydration products are formed on the surface of the fibres, which helps to improve the adhesion between the surface of the fibres and the matrix, as well as their anchoring in the sample material (Figure 9a). These results are in line with the findings of other researchers [28], confirming the significance of these phenomena in the context of strengthening the strength characteristics of the composite material. The concentration of reinforcing fibres has a significant influence on the mechanical properties of polymer concrete. This applies to both the carbon fibre reinforcement and the basalt fibre reinforcement. A uniform distribution of fibres at low concentrations contributes to a better strength behaviour of the material, as it ensures a uniform distribution of loads (Figure 10a). However, the increase in defect areas due to fibre clustering at higher concentrations reduces the strength of the composite (Figure 10b). The formation of such clusters disrupts the continuity of the matrix, resulting in insufficient bonding and sites for the initiation of microcracks. Under mechanical load, these microcracks can coalesce and propagate, especially if stress concentrations from agglomerates are present. The development of microcracks in the course of time significantly reduces the durability and reliability of the material. Continuous loading exacerbates this problem, leading to premature failure. It is known that the failure mechanism of carbon fibre concrete with an arbitrary fibre orientation depends on their physical and mechanical properties and sizes. This process can be accompanied by the rupture of a certain number of fibres with the subsequent pulling out of the rest or the complete pulling out of all fibres from the concrete composition. During the investigation of the fracture surface of the carbon fibre concrete, we found that failure occurred with partial rupture of some fibres followed by the pulling out of the rest. Some fibres showed signs of rupture while others were pulled out of the concrete matrix. This fact was confirmed by the observed signs of rupture and craters in the concrete body (Figure 9b). Figure 9: The fracture surface of carbon fibre-reinforced polymer concrete: (**a**) formations on the surface of carbon fibre; (**b**) broken fibres and voids of torn out fibres. The adhesion between the polymer matrix and the reinforcing fibres is of fundamental importance for the material performance. A good adhesion ensures an effective stress transfer from the matrix to the fibres, allowing the composite material to withstand higher stresses without failure. Based on the obtained test results and microstructure photographs, one can state that when a crack appears in the polymer concrete matrix, the fibres continue to function and resist its further development. This leads to an increase in the plastic properties of such concrete. In particular, a significant increase in relative elongation is observed for fibre-reinforced samples, which indicates an increased deformation capacity of the material.