Why does the modulus of elasticity of concrete reduce with time?
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The modulus of elasticity of concrete can be improved. As the porosity decreases, the modulus of elasticity of cement paste increases and consequently, the elastic modulus of concrete increases. The porosity of cement paste is controlled by water to cement ratio (w/c), air content, admixture dosage and degree of cement hydration.
Modulus of elasticity of concrete is defined as the ratio of the applied stress to the corresponding strain. Not only does it demonstrate the ability of concrete to withstand deformation due to applied stress but also its stiffness. In other words, it reflects the ability of concrete to deflect elasticity.
The higher the value of the modulus, the stiffer the material is. Thus comparing a high performance concrete will be higher, thereby making it a stiffer type of concrete.
The development of the modulus of elasticity of the uncured concrete S i.e. the decrease in the decrease in the value of Ecu between the ages of 28 and 90 days, can be explained by a lack of water necessary for the cement to fully hydrate.
The elastic modulus of concrete gets reduced with age due to creep. Creep is the process of gradual development of deflections, deformations, and loss of strength with time due to.prolonged action of a constant load.
The modified modulus of elasticity becomes :
E’ = E/Θ
where Θ : creep coefficient
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.