||It has long been recognized that the cohesion of composite materials, in low confinement, is strongly affected by the properties of the interfacial transition zone (ITZ) between inclusions and matrix. While the effect of the ITZ on the elasticity properties of composites has been studied by many authors in the context of linear homogenization methods, the upscaling of the cohesion strength of highly filled composite materials has not been addressed. This is the focus of the non-linear homogenization procedure developed in this paper, which is based on the separation of the heterogeneous material system in phases of constant strength properties, a non-linear elastic representation of the limit stress state in each phase, and the definition of appropriate effective strain quantities that capture the morphological features of the microstructure. Applied to a three phase composite model composed of rigid inclusion, interface zone and matrix, the model provides a quantitative means of studying the effect of the interface cohesion and the interface volume fraction on the composite cohesion. In particular, we identify a critical interface-to-matrix cohesion ratio, below which the composite cohesion is smaller than the one of the matrix. Furthermore, the model lends itself readily to the study of the degradation of the interfacial properties in composite materials. This is shown for non-degraded and chemically softened cement-based materials, for which we provide conclusive evidence (1) that the interface strength properties of mortar are far more affected by chemical degradation than the one of the cement paste matrix; and (2) that chemical degradation does affect the mechanical strength performance of the cement paste not only through a change of volume proportions (i.e. increase of porosity), but as well through a pure chemical softening of the solids cohesion.