For the reinforced concrete (RC) bridge, neutralization of concrete causes the deterioration of concrete material and weakens the protection of reinforcing steel against corrosion. The more the neutralization reveals, the poorer the properties of the materials of the bridge will be apt to get into. As time goes on, the degression of the structural performance, particularly in a polluted environment, will be more significant accordingly. If the degression can be predicted with an acceptable precision, the durability assessment of the existing RC bridge can be carried out and give a great help to the bridge management which is quite important for the developed countries. This paper collected the inspected and detected data of twenty-two existing RC bridges in western Taiwan to conduct the necessary regression analyses in predicting the neutralized effect of the bridges. In addition to the strength deterioration of material, the effective cross sections of the corroded reinforcement and of the neutralized concrete can be also acquired by the proposed approach. As a result, the performance degression curve of the whole structure can also be determined quantitatively via structural analysis. The results obtained can not only tell systematically the life cycle of the existing RC bridges but also give the crucial occasion to the necessary repairing or retrofitting for the neutralized bridges losing qualified performance. This study might benefit the optimal maintenance and the minimization of life cycle cost (LCC) for the bridges.