Surveys to ascertain the conditions and possible deterioration of the materials of cooling towers made of reinforced concrete have been carried out for many Italian geo-thermal power plants. Physically and geometrically non-linear FE models have been developed for the structural analyses of the towers under static loads and analysed with DIANA v.7.1. The towers build in the 40s-70s, are now approaching their service life and deterioration is visible in the reduction of the nominal thickness and also decreased material performance. Using material non-linearity and modelling the real conditions of the structures, all the ultimate resources have been taken into account, allowing determining the safety coefficient of the towers. The FE models have been used also to design the most effective technique for the structural rehabilitation. For the largest tower a fibre reinforced tie has been designed. The structural models have been used also to guide the design in the demolition-reconstruction phases.