Abstract Numerical simulations with a simple lattice model in which the properties of the individual components in the concrete can be varried are shown. Fracture in laboratory scale specimens is simulated. It is shown that the fracture mechanism is influenced by the strength of aggregates, matrix and interfacial zone. Varying the bond strength leads to a change in the amount of microcracking outside the main crack, but has only a slight influence on the overall specimen strength. Keywords: Lattice Model, Interfacial Properties, Concrete Fracture.