Concrete behaviors at the time of modeling
Concrete is an exemplary heterogeneous and porous material. Its elementary components, including gravel, sand, cement, water, etc. have complex behaviors under intense loads (very high temperatures, freeze/thaw, etc.) that influence its global properties. Existing digital models are incapable of fully and reliably predicting the consequences of fire or intense cold on concrete structures, and knowledge in this field is based essentially on laboratory tests or field observations made following accidents.
In addition to the purely experimental approach, Frédéric Grondin's PhD has made a significant contribution to the understanding of mechanisms acting inside the concrete microstructure. It is entitled "Multi-scale modeling of the thermo-hydro-mechanical behavior of heterogeneous materials; applications to cementing materials under severe loads", and was directed by Hélène Dumontet from the Modeling, Materials and Structures Laboratory (University of Paris VI) and supervised by Hocine Boussa, Information Technologies and Knowledge Dissemination, CSTB. "By using mathematical models designed for studying mechanical and porous media, I wanted to develop a digital model to predict concrete characteristics from its different components, says Frédéric Grondin. The theoretical results were then compared with experimental results to validate the method."
The model takes account of the different concrete components. "By virtually changing the microstructure of concrete, so-called "homogenization" equations are used to simulate properties of the new material, says Hocine Boussa. And conversely! It is quite possible to change from the macroscopic scale to a microscopic scale to perform a local study of the consequences of a severe load on the microstructure of the material." The evaporation of free water and bound water, shrinkage of the cement paste and expansion of aggregates cause chain constraints in the internal concrete structure. All these physical phenomena related to intense temperature variations can now be studied in the virtual laboratory. All that is necessary is a simple computer. This new approach to the observation of cementing materials opens up the way to a better understanding of the elementary mechanisms that occur within different types of concrete.
Digital concrete in a Symphony
Frédéric Grondin's PhD thesis resulted in the production of a 2D computer program called "Digital concrete". This is an additional module to the Symphonie software developed by the CSTB MOD-EVE (MODeling, Enriched Virtual Environments) Division to simulate the multi-physical behavior of building components and structures subjected to the combined effects of humidity, temperature and mechanics. This model is currently being extended to 3D and implemented in the SALOME open source integration platform.