This thesis presents a study on crack growth and the development of fracture zones in plain concrete and similar materials, conducted by Per-Erik Petersson at Lund University. The research focuses on the behavior of concrete under tensile stress, the formation of micro-cracks, and the resulting stress transfer capabilities of the material. The study introduces the Fictitious Crack Model, which simulates the fracture zone as a crack that can transfer stress, with the stress transfer capability depending on the width of the crack. The model is based on the material's deformation properties, represented by a σ-ε curve for the material outside the fracture zone and a σ-w curve for the fracture zone itself. The research also explores the applicability of linear elastic fracture mechanics to concrete, finding that it is too dependent on specimen dimensions to be useful unless the dimensions are on the order of meters. The study evaluates the usefulness of the J-integral, crack opening displacement approach, and R-curve analysis for cementitious materials, finding them to be limited in applicability. The thesis presents methods for determining the σ-ε and σ-w curves, which are essential for the Fictitious Crack Model. These curves can be approximated if the tensile strength, Young's modulus, and fracture energy are known. The fracture energy (GF) is determined using a stable three-point bend test on a notched beam, and the results indicate that GF is suitable as a material property for concrete. The study also discusses the determination of the σ-w curve from stable stress-deformation curves, using a new type of stiff testing machine. The results show that the shapes of the σ-w curves are similar for different concrete qualities, allowing for good approximations of the curves using simple tests. The characteristic length (ℓch) is defined as GF*E/ft², and it is found that for most concrete qualities, ℓch is between 200-400 mm, indicating the material's brittleness. The research concludes that the Fictitious Crack Model provides a realistic material model, a functional calculation model, and methods for determining the necessary material parameters for analyzing the fracture process of concrete and similar materials. This work serves as a foundation for further studies on the fracture process of concrete and related materials.This thesis presents a study on crack growth and the development of fracture zones in plain concrete and similar materials, conducted by Per-Erik Petersson at Lund University. The research focuses on the behavior of concrete under tensile stress, the formation of micro-cracks, and the resulting stress transfer capabilities of the material. The study introduces the Fictitious Crack Model, which simulates the fracture zone as a crack that can transfer stress, with the stress transfer capability depending on the width of the crack. The model is based on the material's deformation properties, represented by a σ-ε curve for the material outside the fracture zone and a σ-w curve for the fracture zone itself. The research also explores the applicability of linear elastic fracture mechanics to concrete, finding that it is too dependent on specimen dimensions to be useful unless the dimensions are on the order of meters. The study evaluates the usefulness of the J-integral, crack opening displacement approach, and R-curve analysis for cementitious materials, finding them to be limited in applicability. The thesis presents methods for determining the σ-ε and σ-w curves, which are essential for the Fictitious Crack Model. These curves can be approximated if the tensile strength, Young's modulus, and fracture energy are known. The fracture energy (GF) is determined using a stable three-point bend test on a notched beam, and the results indicate that GF is suitable as a material property for concrete. The study also discusses the determination of the σ-w curve from stable stress-deformation curves, using a new type of stiff testing machine. The results show that the shapes of the σ-w curves are similar for different concrete qualities, allowing for good approximations of the curves using simple tests. The characteristic length (ℓch) is defined as GF*E/ft², and it is found that for most concrete qualities, ℓch is between 200-400 mm, indicating the material's brittleness. The research concludes that the Fictitious Crack Model provides a realistic material model, a functional calculation model, and methods for determining the necessary material parameters for analyzing the fracture process of concrete and similar materials. This work serves as a foundation for further studies on the fracture process of concrete and related materials.