This paper discusses the effect of defects in a steel-concrete interface under horizontal top-cast steel bars on the development of corrosion in reinforced concrete beams under sustained loading exposed to a chloride environment. The work was based on four beams exposed to climate accelerated natural corrosion for 29 months. The corrosion-induced cracking patterns were drawn after 8 and 24 months of exposure to the chloride environment, and cracking maps with crack widths were recorded at 29 months. The experimental results show that corrosion-induced cracks always developed much more quickly along the top-cast steel bars due to the existence of top-casting-induced defects. Such defects formed at the steel-concrete interface under horizontal top-cast bars due to bleeding, segregation and settlement of fresh concrete, and they were favorable to both initiation and propagation of corrosion.