The exceptional mechanical characteristics that Glass Fibre Reinforced Cement (GRC) shows are obtained by merging the properties of the glass fibres and the brittle matrix. Cement mortar provides a notable compressive strength and correspondingly the presence of glass fibres enhances the material toughness. Moreover, the fracture energy is also increased due to the presence of the glass fibres that add a multiple cracking damage pattern and, hence, a large damaged surface. In order to provide available resources that may ease and widen the structural design of GRC, the assessment and verification of its constitutive relations is of high significance, given that such relations may reproduce the fracture behaviour. The softening function of GRC under flexural tensile fracture tests in an in-plane disposition has been obtained by combining numerical simulations with an inverse analysis. The application of trilinear softening functions reproduces with notable accuracy the fracture behaviour of GRC. The constitutive relations found can be used for future modelling and structural design and, therefore have widened the feasible applications and reliability of GRC in the construction industry.