Abstract:
Robust models are required in geomechanics to make reliable predictions of engineering applications close to collapse both in the field of cementitious materials and soils.
The lecture is aimed at giving some highlights on possible strategies to the numerical modelling of concrete materials, in particular proving the soundness of FEM numerical models for the correct simulation of its mechanical behaviour, when close to failure.
The suggested approach is an elasto-plastic-damaged formulation in function of two invariants of the deviatoric stress tensor and in line with non-associated plasticity, i.e. the hardening, non-associated model by Menétrey and Willam [1], enriched with the potential function proposed by Grassl et al. [2] with a reformulation of what proposed in [3] for the damage effects; in this case a non-local integral type regularization technique is included [4] to avoid mesh-dependency in the results. In addition to this, to overcome the limitations in the iterative return-mapping scheme of the elasto-plastic model for the cement paste under tensile regime, due to the presence of singularities, or apex points in the adopted non-smooth yield surface, an improved return-mapping procedure is numerically implemented, able to catch locally the optimal return point on the active yield surface [5]. When dealing with concrete, ITZ can be modelled or not, and so characterized mechanically or not, depending if it is expected to have an important role in the failure mechanism; this may happen when maybe the collapse is interface-driven. In case ITZ is mechanically characterized, a possible cohesive formulation is illustrated, accounting for its lower stiffness and different degree of compactness than the surrounding cement paste.
On the other hand, the predictive simulation of damage triggering and evolution in concrete under generic 3D stress states requires the definition of the continuum at a meso-scale level, i.e. as a heterogeneous material, where coarse aggregates are explicitly modeled together with the cement paste, while the finer aggregates are included in the latter component, which is treated as homogeneous. At this purpose, special procedures to conduct meso-scale FE analyses on ordinary concrete made with calcareous aggregates, as well as sustainable concrete made with recycled aggregates from Electric-Arc Furnace (EAF) steel slag, is proposed, based on 3D X-ray computed tomography (X-ray CT) for the digitalization of the outer geometry of the aggregates and for the definition of their orientation in the matrix.
References:[1] P. Menétrey, K.J. Willam, Triaxial failure criterion for concrete and its generalization. ACI Structural Journal, 92(3), 311-318, (1995).
[2] P. Grassl, L. Karin, G. Kent, Concrete in compression: a plasticity theory with a novel hardening law, International Journals of Solids and Structures, 39(20), 5205-5223, (2002).
[3] J. Mazars, F. Hamon, S. Grange, A new 3D damage model for concrete under monotonic, cyclic and dynamic loadings, Materials and Structures, 48(11), 3779-3793, (2015).
[4] G. Mazzucco, G. Xotta, V.A. Salomoni, C.E. Majorana, Integral-type regularization of non-associated softening plasticity for quasi brittle materials', submitted for publication in International Journal, (2019).
[5] G. Mazzucco, B. Pomaro, V.A. Salomoni, C.E. Majorana, Apex control within an elasto-plastic constitutive model for confined concretes. Mathematics and Computers in Simulation, 162, 221-232, (2019).
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