Concrete stiffness increases in time with its elastic modulus, as you can find in all the ACI recommendations, Eurocodes and others. This is related to the increasing strength during curing days. After a brief period of time (a few months from casting) this tendency becomes negligible. After that, shrinkage and creep effects reduces with time the overall stiffness; due to the theory of linear viscoelaticity of concrete with aging, it is possible to explain the increasing strains attributable to sustained loads with a useful expedient: the so-called effective modulus of concrete Ec,eff = Ec/(1+f(t,t0)) where f(t,t0) is the creep coefficient. For subsequent sustained loads applied in different times to the concrete structure it is possible to extend the theory through the so-called AAEM (age-adjusted effective modulus) method, for which a reduced effective modulus which considers the concrete aging is defined.

Summarizing: time-dependent phenomena as creep and shrinkage in concrete have an effect equivalent to a decreasing modulus of elasticity which is the interpretation of the increasing strain due to sustained loads (self weight and other permanent loads) even though cracking is not considered. In high-rise buildings this effect is significant because it concerns with increasing axial shortening of columns and cores. In a similar way it is important for bridges, especially prestressed concrete bridges. For more info you can read my paper and the related bibliography:

Simplified Procedure for Evaluating the Effects of Creep and Shrinkage on Prestressed Concrete Girder Bridges and the Application of European and North American Prediction Models

http://ascelibrary.org/doi/abs/10.1061/(ASCE)BE.1943-5592.0000483

There you can find info also on the codes useful for the estimation of these phenomena.

I hope this can be useful to give a correct answer to your question

Stiffness increases in concrete with time as concrete is being settled, after significant time due to aging of concrete there may be significant decay in the stiffness of concrete. Stiffness can be termed as strength or load resistivity

At the beginning, the stiffness increases with the time.

After long time period and due to cracks and drying shrinkage ...etc, the stiffness decreases.

Take a view in the paper of Korea congress

"Nonlinear seismic assessment of eight-storey reinforced concrete building according to Eurocode EN 1998-3"

V. Lekidis1, S. Anagnostopoulos 2, D. Baros3, Ch. Karakostas 4,

Th. Salonikios5, T. Makarios 6

1), 4), 5), 6), Earthquake Planning & Protection Organization, ITSAK, Thessaloniki, Greece

1) [email protected]

2), 3) Department of Civil Engineering, University of Patras, Greece

The basic principle of stiffness increase in concrete can be attributed to unhydrated cement particles. As concrete ages, with the permeation of moisture some of these particles could hydrate and further increase stiffness.

Stiffness only decreases as mentioned earlier through crack formation, concrete deterioration (ASR/Delayed ettringite formation/reinforcement corrosion etc)  and in some cases creep.

Stiffness is basically related with section properties and elastic modulus. After long time period, section damage(crack, deterioration)  will greater impact than elastic modulus increase.

Stiffness of concrete is directly in function with properties of concrete, and in first phase is increase , but during the long time of using or under load applied will be the decrease , but depend of many factors: cracks; environmental conditions, ect

Can somebody give an estimation in terms of time spans (years) in which the increase in stiffnes stops, and at which point the stiffness starts decaying?

Thanks R.V. Silva!

Many national codes, in my case Chile, don't go to such extends.