The defective or cracked portion of the concrete or plaster has to be removed. On doing this care should be taken that the cracks are entirely removed from the surface and a well-defined cavity is formed without affecting the good concrete or plaster. The size of the cut depends upon the size, depth and extent of repair. Before doing this, all the structural load carrying members like columns, beams and slabs, must be supported, propped or isolated so that the load on these members is reduced or temporarily transferred to other good structural members. Care should also be taken during repairs that the weakened members do not lose their strength further due to chipping and other removal procedures.
Rust from the steel must be completely removed by tapping lightly, wire brushing and cleaning. The entire surface should be cleaned from dust and dirt by sand blasting or any other effective method and washed thoroughly. The excess water is removed from the cavity and all the cracks and joints are to be properly inspected and sealed with mortar and cured for 7 days.
The reinforcing bars are to be given a protective coating and the concrete surface is given a coating with a thin layer of cement grout before placing the patching material.
The cavities are then filled with good quality mortar or concrete as required. Repair material should be filled in layers of about 10 mm thick. For columns, beams and slabs, guniting is resorted to with either ordinary cement concrete or polymer modified cement concrete. The surface is properly cured for 7 days.
The surface is then allowed to dry and finishing coats of mortar are applied in two coats. Curing is done for the surface also for 7 days.
Cracks can also be repaired by the use of Ferro cement. This is done by peeling off the affected plastering and replastering the surface after placing a sheet of steel wire mesh of required size inside the plaster in position.
Plugging and sealing of cracks can be done using cement grout or various brands of epoxy compounds available in the market.
It is important that during repair the safety of the inhabitants and other people in and around be given due consideration.