Effects of grout-strand interface modelling on the degradation of external grouted post-tensioning tendons

In recent years, relevant brittle fractures of external grouted post-tensioning tendons in bridges have been reported due to corrosion damage, compromising the structural safety and stability of the bridges. A previous detailed finite element (FE) modelling approach for grouted tendons has been developed by the authors and compared with experimental results presented in the literature, ensuring an accurate reproduction of experimental results by accounting for steel plasticity and large deformations. This modelling approach considered a bonded contact to model the strand-grout interface. That is, an immediate re-anchoring of the strands in the grout is assumed in case of failure and, consequently the influence of the bond stress-slip behaviour is not considered in the modelling approach. This paper presents an alternative FE modelling strategy where the differences with the previous one are: i) the modelling of the strands as beams instead of solid bodies, ii) the presence of the sheathing duct, and iii) a non-linear model to reproduce the strand-grout bond stress-slip behaviour. The objective is to investigate the influence of different models to define the grout-strand interface: a bonded model or a bond stress-slip model, while validating the author’s previous FE approach. These models are also compared with the experimental results from the literature. Normal stresses along the strands and in the grout are studied, and degradation curves are derived, that is, the effective tensile force and natural frequencies versus damage (defined as the percentage of broken strands). These degradation curves serve as a key performance indicator of the structural performance of the tendon for structural health monitoring systems, anticipating to severe damage and potentially dangerous scenarios.