The article deals with the problem of determining the bended reinforced concrete beams deformability indices, which are subjected to a combined long-term load and aggressive environment influence during operation. It is noted that due to the concrete creep, the corrosion damages accumulation and cracks formation, the reinforced concrete structures deformability increases in time. In current methods of beams deflections calculating, a stepwise description of stiffness along the span is implemented, which in some cases can lead to significant errors and, as a consequence, to the limiting state. Therefore, during calculations, it is important to take into account the proper stiffness parameters. In this regard, on the basis of predetermined deformations method, a new methodology deformability indices calculation has been developed, in which the hardness in the span is distributed along the parabola. For its validation, special experimental studies of samples of reinforced concrete beams that were under a static load in sulfate and chloride-containing aggressive environments for a long time were carried out. After comparing the corresponding experimental and calculated samples deflections values, it was concluded that the developed method has sufficient accuracy and, thus, can be used for practical calculations of bended concrete elements deformability under long-term and environmental influences.