# Strengthening Polymer Concrete with Carbon and Basalt Fibres
## 3 Results and Discussion
### Analysis of the Fracture Surface of the Tested Samples
The fibres resist not only the formation of cracks, but also their propagation, even when a crack has already been initiated in the main matrix. They act as bridges across the cracks. This phenomenon allows the material between the cracks to contribute to the overall load-bearing capacity. This bridging action delays the propagation of cracks and distributes loads more uniformly, increasing both the bending and compressive strengths. Our studies demonstrated an increase in the bending and compressive strengths. According to the data, presented by other authors, an increase in the hardness of fibre-modified concrete was also observed, although this increase was not significant [29]. Therefore, an increase in all the above-mentioned strength characteristics also entails an increase in the rigidity of the material. These findings emphasize an important role of fibres in improving the mechanical properties of polymer concrete and indicate the potential for their use in creating more reliable composite materials. The study of fracture mechanisms of carbon fibre concrete with a volumetrically random fibre orientation is key to understanding its strength properties. The analysis of the fracture microstructure can provide valuable data on the interaction of carbon fibres with the concrete matrix and reveal the peculiarities of the fracture processes. The research shows that reinforcement with both carbon and basalt fibres improves the strength properties of concrete. However, the cost of these fibres varies significantly. Basalt fibre practically does not increase the cost of concrete since its price is relatively low and the required amount is only 1-2%. While carbon fibre, whose cost is almost 10 times higher than that of basalt fibre, significantly increases the costs of the production process. The economic feasibility for using carbon fibre is justified in cases when it is necessary to ensure maximum bending strength of the material. At the same time, the use of basalt fibre is more appropriate in terms of cost and efficiency for a wide range of construction applications. In view of this, the choice of the type of reinforcing fibre should be based on a balance between the required performance characteristics and financial considerations. Figure 10: The fracture surface of polymer concrete reinforced with basalt fibres: (**a**) uniform distribution of fibres during the 1.5% reinforcement; (**b**) non-uniform distribution with clotted fibres during the 5% reinforcement. Mechanical Tests (Flexure, Compression) of the Samples with Additions of Fibres of Different Lengths  
The uneven distribution of fibres throughout the volume of polymer concrete significantly affects its properties. In addition, the strength is largely influenced by the bonding force between the fibres and the polymer concrete matrix. Figures 11-14 show the data on the effect of the length of reinforcing carbon and basalt fibres on the strength of the polymer concrete specimens under study (bending and compressive strengths). Figure 11: Bending strength for the samples made of polymer concrete reinforced with basalt fibres of different lengths. Figure 12: Compressive strength for the samples of polymer concrete reinforced with basalt fibres of different lengths. Analysis of Figures 13 and 14 shows that for carbon fibre reinforcement the optimum fibre length is 10-15 mm. Further increase in fibre length leads to a decrease in both bending and compressive strengths. This is due to the fact that with longer lengths it is more difficult to ensure a uniform distribution of fibres throughout the specimen. A longer length promotes the formation of fibre clusters. Also with longer lengths, the total number of fibres in the specimen decreases for the same percentage of reinforcement. A shorter fibre length increases the total number of fibres in the sample. However, the fibres have a shorter length and a smaller bonding surface to the matrix. Therefore, such fibres are more easily torn out of the matrix and are less able to withstand the load.