The floors of the Trocadéro Commercial Centre car-park, which was built in 1999, were made using pre-stressed concrete pre-slabs. Cracks rapidly appeared in the joints between and inside the slabs. Surveys carried out by the client revealed little reinforcement of the floor pre-slabs, it was envisaged therefore to install a waterproof polyurethane membrane as the most economically viable solution. However, the conformity of this solution is only guaranteed if the cracks covered do not open up more than 2 millimetres. As a result of the tests carried out by OSMOS, the client obtained precise information on the instantaneous progression of the opening of these cracks and confirmed the installation of a waterproofing membrane.