Bridges located at moderately or highly aggressive environments can be subjected to different levels of deterioration. Deterioration can be observed as cracking or spalling of concrete and corrosion of reinforcement. Among them corrosion of reinforcement is the main concern in capacity reduction due to loss of steel area and reduction in yield strength. Corrosion may even induce total collapse of bridge. In many cases, reinforced concrete bridges constructed over rivers exposed to reinforcement corrosion at splash zone of pier. Service life of these deteriorated bridges can be reduced significantly due to some loss in load carrying capacity which can also negatively affect the future seismic performance. Focus of these research has been given to investigate reduction of capacity in seismic behavior of bridges subjected to moderately aggressive environmental effects and to propose a condition factor to be used in future bridge analysis. Different deterioration models have been studied and a proper model has been modified to estimate loss in load capacity for a series of standard highway bridges. A condition factor, representing an equivalent deterioration, has been proposed for evaluation and design of bridges at moderately aggressive environment. Use of such condition factor in design can compensate future loss in capacity resulting some overdesign at early years of bridge.