Constitutive equation is a mathematical formula to present the mechanical properties of materials. Generally, the deformation of materials at elevated temperatures can be described by four strain components in the following constitutive equation

ε = εth(T) + ε σ(σ,T) + εcr(σ,T,t) + εtr (σ,T)

where εth = thermal strain; ε σ = instantaneous stress-related strain; εcr = creep strain; εtr = transient strain which only applies to first-time heated concrete; σ = stress; T = temperature; and t = time.