Tunnels are a critical component of transportation infrastructure, but they are vulnerable to severe environmental conditions, accidental blast events as well as some structural damages coming from design or construction errors. Under these circumstances, they require repair and maintenance activities for which tunnels must be closed to traffic and the functionality of the transportation network is significantly impacted with increase of the costs of transport. This paper provides an innovative retrofitting technique of the tunnels concrete lining. This technique involves the application of a thin layer of high-performance mortar on the damaged area of the existing lining. The main advantage of the analysed technique is that it doesn’t need to apply directly loads in the foundation; moreover, it is possible to preserve a partial road circulation during the application of the strengthening with a consequent reduction of the economic losses. Since there are few experimental and theoretical studies on existing structures with on-side reinforcements, in this study a preliminary experimental investigation on eighteen 1:2 scale concrete lining elements was carried out under monotonic compressive load. The aim of this preliminary experimental study is to investigate the performance equivalence between control lining elements (concrete elements without structural damages) and damaged concrete lining elements (thinner with respect to control elements due to an undesirable damage) one-sided strengthened. The experimental program includes studies on different thicknesses of strengthening and on different kinds of high-performance mortar. Results showed the reliability of the analysed repair technique.
