Multiscale Modelling of Concrete as a Fully Coupled Porous Medium

Concrete is investigated as both a homogeneous and a composite material model made of cement paste and aggregates. In this meso-level approach, heat and fluid flows and related mechanical effects have different kinetics related to the two basic constituents. Both cement paste and aggregates are treated as saturated-unsaturated porous materials with different porosities, permeabilities and diffusivities, hence where heat and fluid transfer are characterized by different time evolution. Such models have a remarkable importance for understanding specific phenomena in concrete, especially those where aggregate type and content are fundamental. One of this is spalling of heated concrete and hence this model will be used in future to explain and predict the occurrence of this phenomenon. Moreover, since pp fibres are commonly used to prevent spalling, this further constituent is included in the model, starting from basic investigations. Polypropylene (PP) fibres contribute to the reduction of pore pressures in concrete during heating, and hence the reduction of the probability of explosive spalling. Pressure reduction depends upon (a) the molecular structure of PP fibres during heating in relation to the microstructure of the concrete; (b) the mechanical properties of the fibres as functions of temperature; and (c) the type, dosage and dimensional characteristics of the fibres. The experimental basis of this work is the extensive campaign related to heat and mass transfer and cyclic strain behaviour of concrete to temperatures up to 600oC carried out at the Imperial College, London, UK. Temperatures, weight loss and strains are measured in concretes with different aggregates during heating and cooling, with and without compressive load over a period of several days. Heat, mass and mechanical results obtained from the tests are then directly integrated in the 3D fully coupled chemo-thermo-hydro-mechanical numerical model of heated concrete, called NEWCON3D, where aggregates are distinguished from the cement paste. In this model, aggregates and cement paste are treated as multiphase systems where the voids of the skeletons are partly filled with liquid and partly with gas phase. As regards the mechanical field, NEWCON3D incorporates coupled thermal, creep and shrinkage effects, chemo-thermo-mechanical damage and plasticity under low to high temperature levels.