This section would define “fracture toughness” as a material’s ability to resist fracture in the presence of a flaw. It would explore the mechanisms by which fracture toughness is measured and modeled in cementitious materials, discussing key parameters such as crack tip opening displacement and stress intensity factors. The relationship between microstructure (such as porosity and aggregate interlock) and the fracture toughness of concrete would be detailed, emphasizing how these microscopic features influence macroscopic properties.