To repair concrete it was formerly necessary to use the costly, time-consuming method of chipping, drilling, roughening, acid washing or scarifying the old concrete base prior to applying a slurry coat of cement to bond the new topping. Even with all the precautions that could be taken, such repairs were oftentimes far from satisfactory. The picture has changed radically. With bonding materials now available it is possible to bond new toppings to old, and plaster directly to concrete or any structurally sound surface. In many cases the bonding materials prove to be stronger than the materials being bonded. The materials being used today for bonding cementitious materials include synthetic latex emulsions, epoxy resins, and aqueous resinous emulsions of the vinyl type. The synthetic latex emulsions are used as an admixture in cementitious materials, giving these normally brittle materials the qualities of resilience, elasticity, durability, and good adhesion. Epoxy adhesives consist of liquid polymer, liquid epoxy resin, amine catalyst, and inert filler. By variation of the type and amount of each ingredient, innumerable variations in adhesive properties may be obtained. The aqueous resinous emulsions of the vinyl type will bond new plaster base or finish coats to concrete, or to any structurally sound surface. Why are modern bonding materials so strong? How do they work? The answers to these questions should be a matter of interest. First, a mechanical bond is achieved by the penetration of the bonding material into the pores of the old surface, forming a mechanical interlock. Secondly, as the moisture evaporates form the bonding material adhesion takes place. Thirdly, a chemical reaction takes place as the new material is applied. The bonding agent penetrates voids and forms a bond within the crystalline structure of the new concrete.