The elastic modulus of cement paste is determined by its porosity. The water to cement ratio (w/c), air content, admixture dosage and degree of cement hydration control the porosity of cement paste.
The modulus of elasticity of concrete can be represented based on the following simple equation:
Ec = Eag + Ep(1 – g)
Ec – Modulus of elasticity of concrete
Ea – Modulus of elasticity of aggregate
Ep – Modulus of elasticity of cement paste
g – Volume fraction of the aggregate
1 – g – Volume fraction of the cement paste
Ep is low due to the poor density of the transition zone.