Delayed ettringite formation (DEF) is a concrete pathology which may occur in structural parts submitted to high temperature during their casting, such as some massive elements cast in-situ. DEF consequences can lead to structural disorders (expansions, cracking, deterioration of mechanical properties), which modify and jeopardize structures serviceability and sometimes, safety. This communication deals with a concrete lock for which a large prestressed beam supporting the lower gate is affected by DEF for several years. First, the structure and its pathology are described. The global strategy of the study, mixing together documents analysis, laboratory tests on concrete samples and use of finite-element models is then presented. These numerical models have multiple purposes: assessing the early-age thermal history of the beam in order to confirm the origin of DEF; assessing the structural behaviour and its modification due to DEF, based on chemo-mechanical coupling models; mechanical interaction between the beam and the other parts of the lock. This modelling is performed to assess structural state at present time, as well as to predict its evolution in the future. Finally, conclusions drawn from the study are used to help structure owner in their planning of his maintenance policy.