Slight and moderate amounts of deterioration caused by alkali silica reaction (ASR)/delayed ettringite formation (DEF)-induced expansion in reinforced concrete specimens do not markedly or detrimentally affect its structural load-carrying capacity. However, effects of severe (late-stage) ASR/DEF deterioration on the load-carrying capacity of structures is not known. A large-scale reinforced concrete C-beam specimen with heavy ASR/DEF deterioration is load tested in the laboratory. A comparison of the force-deformation results of the severely deteriorated specimen with the undamaged control specimen, and slightly and moderately ASR/DEF-deteriorated specimens, shows that there is no reduction in the load-carrying capacity. The mode of failure of the specimen is a brittle joint-shear failure similar to the other C-beam specimens. However, a reduction in the ductility of the heavily deteriorated specimen is observed. Postfailure examination shows a significant degree of corrosion in the longitudinal and transverse reinforcement. Significant bulging and complete debonding of the lapped transverse U-bars in the specimen knee-joint caused considerable loss in confinement of the softened concrete, resulting in an embrittled performance and sudden early loss of load in the heavily damaged specimen. It is not known how further corrosion in conjunction with severe ASR/DEF deterioration could affect the future behavior of the structure. © 2018 American Society of Civil Engineers.