Fracture toughness and fracture surfaces morphology of metakaolinite-modified concrete

• Fracture toughness of concrete with metakaolinite additive was studied.
• Metakaolinite increases fracture toughness of concrete.
• Developed model gives possibility for fracture toughness prediction.
• Fractal analysis of concretes with metakaolinite additive is highlighted.
• Fracture toughness correlates with fractal dimension and water/binder ratio.

The authors’ main aim was an in-depth study of the issue of fracture toughness of metakaolinite modified concretes, the problem not investigated to a great extent so far. The results of tests on the fracture toughness of metakaolinite modified concretes have proved that it is higher compared with that of non-modified concretes. Fracture toughness increases with the higher addition of metakaolinite. The possibility of relating the morphology of fracture surfaces forming during cracking of concretes modified with the addition of metakaolinite to the fracture toughness, as expressed by the critical stress intensity factor KIcS, is explored in the paper using statistical apparatus. The mechanical properties and fractal dimension examinations were carried out according to the central composition plan, while assuming two variables: the metakaolinite addition fraction of the binder mass, varying in the range from 2 to 15%, and the water/binder ratio, varying in the range from 0.35 to 0.53. The fractal dimension Dc was calculated by the chord method for profile lines isolated from the fracture surfaces, and then the fractal dimension was related to the fracture toughness as expressed by the critical stress intensity factor, KIcS.