Abstract: The present invention relates to a process of casting of repairing concrete under water whereby the concrete (mortar) to be cast to a minimum extent contacts water and leaching of the binding agent is prevented. The purpose of the process according to the invention is achieved such that a "plug" of liquid organic material based on a reactive or non-reactive synthetic resin or other organic carrier having a suitable viscosity, good cohesion and poor water solubility and with a specific gravity higher than water, but less than cement-based concrete, is introduced into the bottom of a tight formwork to contact a support cement-based concrete having a good flowability is then introduced below the surface level of the plug, the introduction initially being effected in the very plug.