The present invention relates to a method to repair a crack on an external wall of a concrete structure. More specifically, the present invention relates to a method to repair a crack of a concrete structure, which is able to use a crack repair agent, which does not cause exfoliation in case of rust prevention of an exposed rebar and painting by water-based paint, to make a waterproof function last longer, and to allow a clean exterior to be maintained for a long time by painting an external wall with water-based paint. The present invention is able to prevent rust on an exposed rebar with an excellent rust prevention materials and undercoating materials with superior penetration, to strengthen a surface of an existing external wall painting, and to convert minute bits of rust not removed from the exposed rebar into an oxidized steel film, thereby preventing corrosion of the rebar and neutralization of concrete by moisture penetration. The present invention is able to use an acrylic repair mortar as a sheath repair agent, to prevent painting from spoiling by alkaline materials coming from cement, and to prevent delamination of the repair mortar, thereby making an effect of sheath repair last for a long time. The present invention applies an acrylic film repair agent twice, which is able to provide a good extension rate and to deal with a change in an external environment, to deal with seasonal changes in temperatures, and to have no deformation of properties, thereby keeping an extension rate of the film repair agent and maintaining a subsequent painting using water-based paint on an external wall of an apartment building for a long time without discoloration. The film repair agent applied is friendly with water-based paint to show no stain of a crack film agent on an external side of water-based paint as time goes by, thereby being capable of maintaining a water-proof effect of the apartment for a long time and keeping a clean exterior. The present invention is able to use different colors in accordance with the number of times using the crack film repair agent, thereby providing convenience in construction and supervision. According to the present invention, the method comprises: a step of pre-treatment of a part with a crack; a step of applying an agent for rust prevention for an exposed rebar and for undercoating; a step of restoring a concrete sheath; and a step of repairing a crack film.
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