Many slightly reinforced concrete structures of operating hydraulic structures need strengthening. Traditional methods of strengthening (by reinforced concrete, metal structures, etc.) have significant disadvantages. A method of strengthening by external reinforcement systems based on carbon fiber which is effective in cases where there is an access to the tension zone of the reinforced structures begins to spread. The authors propose to strengthen slightly reinforced concrete structures of the operated hydraulic structures with pre-stressed basalt composite reinforcement placed in the pre-drilled holes in the concrete of the reinforced structures. To substantiate the proposed technical solutions there was carried out a set of experimental studies of characteristic slightly reinforced concrete structures of hydraulic facilities (including those with inter-block construction joints) reinforced with pre-stressed basalt composite reinforcement. The results of experimental studies have shown the effectiveness of strengthening low-reinforced concrete structures of operating hydraulic facilities with inter block construction joints by means of pre-stressed: longitudinal basalt composite reinforcement and transverse reinforcement.