Nonlinear creep of concrete: the driving role of viscoelasticity and cracking-induced damage

Creep of concrete refers to the progressive increase of deformation of a representative material volume subjected to a sustained stress. At low ratios between applied stress and material strength, an increase of the stress relates to an approximately linear increase of the deformation. Nonlinearities occur once the stress exceeds some 40% of the strength. In this work, the origins of such nonlinearities are explored, explicitly separating the contributions from viscoelastic processes and damage due to cracking. A multilevel uniaxial compressive creep test on a mature concrete [1] is re-analyzed. Additionally to documenting the prescribed loading and to measuring the strains, the acoustic emission technique was used to monitor the creation of microcracks. These data were used as input to formulate an analytical model as follows. The reported strength was related to the hydration degree of the material via a validated multiscale model. The obtained hydration degree, together with the mix design, served as input for quantifying linear, nonaging, basic creep properties of uncracked concrete by means of another well-validated multiscale model for creep of cementitious materials under saturated conditions. The obtained creep function was then extended to account for (i) different levels of time-invariant relative humidity (constant moisture during creep testing), by means of a creep reduction factor, (ii) nonlinear viscoelastic processes, by means of the affinity concept [2], and (iii) diffuse microcracking, by means of a micromechanics-motivated damage factor which relates the creation of microcracks to a proportional increase in the compliance of concrete. In [5], it was shown that cracks created during both quasi-static load application and sustained loading have an important influence on the deformation behavior of concrete. Herein, further experiments are analyzed. This confirms that nonlinear viscoelastic phenomena govern the creep behavior at medium stress levels, while cracking-induced damage dominates the behavior at high stress levels.