During the reconstruction, or upon expiration of the service life, as well as after external impact, reinforced concrete structures require examination and verification calculations. The most modern and accurate calculations are based on diagram methods. Existing diagrams of concrete deformation are focused on designing new structures and are not adapted to the concretes that have changed their strength and deformation properties over time or because of external influences. The main parameter describing the deformability of concrete is the secant modulus ratio. A technique for determining the secant modulus ratio is proposed in this arctical. The technique is based on using the exponential concrete deformation model. The obtained secant modulus ratio can be used for the widely adopted description of the concrete deformation diagram. The technique is illustrated on a specific example. The proposed technique significantly expands the application range of diagram methods and extend them to concrete after long-term operation or external impact.