The bond properties at joint interfaces in concrete structures were evaluated through tension softening diagrams. For the joint surfaces between old and new concrete, the conditions to obtain good bond properties were clarified. The size effect on the flexural bond strength was well predicted through numerical analysis with tension softening diagrams. For the interfaces between repair materials and substrate concrete, the properties of repair materials and the conditions of interfaces were characterized by the shape of the tension softening diagrams and fracture energy. The shrinking and cracking behavior of repair materials was also investigated through experiments and numerical analysis.