A Comparison Between Explicit and Implicit Modelling of Transient Creep Strain in Concrete Uniaxial Constitutive Relationships

Transient creep strain has to be included within the constitutive relationships for concrete at high temperatures. However, the necessity of taking into account this term explicitly is not clearly defined. In the Eurocode 2 uniaxial concrete material model, transient creep is included implicitly. This paper aims to highlight the capabilities and limitations of concrete uniaxial models at elevated temperatures for thermo-mechanical behaviour modelling, depending on the implicit or explicit consideration of transient creep strain in the model.

The characteristics inherent to the two types of models are described and compared. It appears that one of the major limitations of implicit models concerns the unloading stiffness because implicit models treat transient creep as reversible. Based on numerical analysis performed on loaded concrete columns subjected to natural fire, it is shown that the stress-temperature paths experienced by structural concrete are varied and complicated and that concrete material models cannot handle properly these complex situations of unsteady temperatures and stresses without explicit consideration of transient creep.

The paper proposes a new formulation of the Eurocode 2 concrete material model that contains an explicit term for transient creep. The new model is implemented in the software SAFIR and validated against experimental data of the mechanical strain developed by concrete cylinders under different unsteady temperatures and loads. It is shown that the actual material behaviour is better matched with the new explicit model than with the current implicit Eurocode 2 model. Finally, a comparison is given between experimental and calculated results on an axially restrained concrete column subjected to heating and cooling.