The objectives of this communication is to characterize the soft impact response behaviour of a reinforced concrete mock-up (OECD/NEA IRIS Phase 3 mock-up, part of a benchmark), and to evaluate the ability of some mechanical models to represent the energy dissipation during and after the impact. Results from the benchmark show insufficient dissipation in numerical models. Using time-frequency analysis, the dissipative behaviour of the mock-up under impact is evaluated. The stiffness and dissipation associated with the first mode are shown to be amplitude-dependent. This non-linear effect seems to be of a different nature than damage or plastic deformation. A simple rheological model with friction exhibits a similar behaviour.