Abstract: For use in strengthening of natural or artificial structures, e.g. buildings, monuments, ruins, there is drawn under vacuum into the structure a liquid of low viscosity and comprising a monomer or monomers containing some polymer which may be polymerized in situ. The polymer may be obtained by polymerization of part of the monomer or monomers. The liquid resin is cured at ambient temperature. Acrylates and methacrylates are preferred.