The bond properties of concrete joints in different conditions were investigated through tension softening diagrams in addition to conventional flexural bond strength. Conditions to obtain good bond properties were clarified. The size effect on the flexural bond strength was discussed. The fracture energy (area under the tension softening diagram) was a more sensitive index than the conventional flexural bond strength for the evaluation of concrete joints. The wash-out method combined with a paper sheet containing retarder was efficient to obtain a rough joint surface and good bond properties. The size effect on the strength was well predicted through numerical analysis with tension softening diagrams. The effect of size on the shape of the determined tension softening diagram was not remarkable.