The Modulus of Elasticity reduces with time because of the effect of creep. Creep is the phenomenon that occurs in hardened concrete over time, where for constant load or stress, deformation increases.
Keeping that in mind, let us look at the definition of modulus of elasticity of concrete. It is the slope of the initial tangent modulus of the stress-strain curve of concrete.
The above graph shows the stress-strain curve of the concrete at an early stage.
Due to creep, for a given load/stress, deformation increases, thereby strain increases. This has been explained in the following graph.
As a result, the slope of the curve reduces and becomes like this.
As the curve descends, the slope of the initial tangent modulus of the stress-strain curve of concrete reduces.
As a result, the modulus of elasticity also reduces.