Experimental and numerical study of size effect on long-term drying behavior of concrete: influence of drying depth

This study aims at rationalizing the analysis of drying shrinkage tests and taking better advantage of the measurements, by studying the influence of the specimen size. Three self-consolidating concrete and one vibrated concrete mixes were studied during 3 years. Drying started 24 h after casting. The tests were carried on three sizes of cylinders: Φ78, Φ113 and Φ163 mm, under the same experimental conditions (20 ± 1 °C, 50 ± 5 % relative humidity), according to RILEM recommendations. The results show that the classification of concrete depends not only on the experimental conditions, but also on the duration of the shrinkage measurement. A scientific approach based on a simple mathematical model was proposed to analyze shrinkage data. The ultimate drying shrinkage did not depend on specimen size. The existing empirical models of shrinkage and drying used in construction codes take into account the size effect only through a geometrical parameter called notional size of cross-section, assuming that concrete is an homogeneous and non-aging material. With the time variable change by the ratio of the square root of the time and the notional size of cross-section, a master curve can be found for the theoretical curves of different sizes. However, experimentally, upshifts were observed between the drying shrinkage curves for different specimens sizes, thus phenomena that occur at early age are not taken into account by current models. The drying depth notion was introduced to explain the part of drying behavior responsible for the observed difference between drying curves. The influence of this concrete layer on the long-term behavior was confirmed by a two-step modeling of the mass-loss evolution. The first one included coupled drying-hydration model. The same classification was observed numerically and experimentally, with higher drying kinetics corresponding to smaller specimens, but the experimental shift could not be reproduced. The drying depth notion was incorporated in the second model; an increase in permeability was introduced in the outer layer of the concrete specimens. The size effect on the long-term mass-loss was reproduced numerically.