Nanofiber Concrete: Multi-Level Reinforcement
Concrete is the most commonly used building material worldwide. One of its main disadvantages is the fragility of fracture and low crack resistance. The use of dispersed reinforcement of concrete composites is a promising direction in solving this type of problem. Dispersed fibers, evenly distributed over the entire volume of the material, create a spatial frame and contribute to the inhibition of developing cracks under the action of destructive forces. In order to increase the fracture toughness of concrete, dispersed fiber reinforcement is increasingly used in practice. The beginning of crack nucleation occurs at the nanoscale in the cement matrix. Thus, the use of nano-reinforcement with dispersed nanofibers can have a positive effect on the crack resistance of the cement composite. It is proposed to consider carbon nanotubes as such nanofibers. The presence of carbon nanofibers changes the microstructure and nanostructure of cement modified with carbon nanotubes. The result of the processes occurring in capillaries and cracks are deformations in the intergranular matrix, the free flow of which is prevented by rigid clinker grains and nanocarbon tubes, which creates a certain stress intensity at the tips of the separation cracks. The working hypothesis is confirmed that the required fracture toughness of structural concrete is provided by multi-level reinforcement: at the level of the crystalline aggregate of cement stone - carbon nanotubes, and at the level of fine-grained concrete - various macro-sized fibers (steel, polymer). Reinforcement of a crystalline joint with carbon nanotubes leads to an increase in the fracture toughness of the matrix (cement stone) by 20 %, compressive strength by 12 %, and tensile strength in bending by 20 %. When reinforcing at the level of fine-grained concrete, we obtain a composite - nanofibre-reinforced concrete with fracture toughness.