During the 1980s and 1990s, external grouted post-tensioning tendons were widely used in concrete box-girder bridges. However, recent brittle failures related to corrosion have been reported. In these tendons, re-anchoring of broken strands makes early damage detection difficult as tensile force loss is not proportional to steel loss. Vibration-based non-destructive testing techniques may be useful for detecting tendon anomalies, and estimating their tensile force and other key performance indicators sensitive to damage. Tensile force estimation from vibration measurements of taut grouted external tendons involves updating their analytical dynamic models, which requires continuously solving an optimisation problem. Moreover, tendon evaluation under corrosion conditions often relies on detailed finite element (FE) models. Both cases highlight the need for simplified and efficient models for in-line assessing their risk of collapse within a continuous monitoring system. Thus, a simplified mechanical model of an external grouted tendon is proposed to simulate tendon degradation. The model consists of a set of series and parallel springs representing the grout, healthy strands, and corroded strands. Damage is modelled by decreasing the axial stiffness of the corroded spring, enabling efficient estimation of the evolution of the tendon tensional state without an FE model. This paper validates the proposed mechanical model with experimental tests from the literature. Then, a parametric study previously performed by the authors with a detailed FE model is reproduced here with the proposed mechanical model to obtain the necessary number of broken strands that will lead to tendon failure in different corrosion situations. The advantages of the proposed simplified approach have been demonstrated.