This thesis describes the development of an in-situ longterm testing device for shotcrete in subsurface construction especially tunnelling. The novel concept integrates the specimen as part of the shotcrete lining, which is therefore exposed to the same influences. The testing device is able to load the specimen in freely selectable stress-levels in a range of 0.3 to 25 MPa. The necessary load for the testing is created by a hydraulic unit and a hydraulic cylinder with a operating pressure of 700 bar. The constant load is ensured by a closed loop pressure control. The layout and the functionality of the four main components - casing, hydraulic installations, closed loop control, data acquisition and recording - are described. The assembly and the instrumentation of the unit and the specimen are demonstrated by an application on a construction site. Furthermore a listing of some reference installations and their measuring results and graphical interpretation are part of this thesis.