Conference Agenda

This study examines the impact of fluid flow and aggregate fragmentation on the dynamic behavior of concrete under uniaxial compression at varying strain rates. Concrete was simulated as a four-phase material consisting of aggregate, mortar, ITZs, and macropores. The concrete mesostructure was obtained from laboratory micro-CT tests. 2D simulations were carried out. A novel, fully coupled DEM/CFD technique, based on a pore-scale thermal-hydro-mechanical model, was employed to predict the effects of strain rate, fluid flow, and aggregate fragmentation on the response of both partially and fully fluid-saturated concrete.

This study investigates the influence of indenter geometry and loading conditions on the determination of viscoelastic parameters of hydrated cement pastes through nanoindentation. Experimental tests with various tip shapes (spherical, Berkovich, cube corner) and load levels were combined with finite element modeling to analyze the stress–strain fields beneath the indenter. The results show that sharp tips generate extremely high stresses and significant plastic strains, violating the assumptions of linear viscoelasticity. In contrast, blunt or spherical tips produce stress levels within the linear regime, allowing reliable evaluation of creep compliance and viscoelastic constants. Analytical models based on the Vandamme approaches were used to interpret the holding segments of indentation curves. The study establishes limits for the applicability of linear creep models at the microscale and provides methodological guidance for obtaining intrinsic viscoelastic properties of cement hydrates for use in multiscale modeling of concrete behavior.

Micromechanical models provide powerful tools for estimating the bulk properties of complex, heterogeneous materials like recycled aggregate concrete (RAC). This is achieved by considering their hierarchical microstructure, from the nanoscale of hydration products to the macroscale of the concrete composite. However, these models inherently rely on various input parameters, each carrying uncertainties that stem from experimental variability, natural fluctuations, or measurement inaccuracies. A rigorous quantification of these uncertainties and an assessment of their propagation through micromechanical models are essential for robust predictions of compressive strength in cementitious materials, with a particular focus on the critical role of the Interfacial Transition Zone (ITZ).
This study leverages a semi-analytical micromechanical multiscale model, which explicitly accounts for the spatially varying properties of ITZ. Unlike conventional approaches that simplify the ITZ as a uniform layer, the adopted model integrates non-uniform characteristics (thickness, porosity distribution) based on experi-mental insights derived from techniques like nanoindentation and SEM-EDX analyses. To quantify the inherent uncertainties associated with RAC, particularly those arising from varying recycled aggregate properties, hy-dration processes, and internal curing phenomena, a variance-based global sensitivity analysis is incorporated.
To overcome the significant computational expense using detailed micromechanical models, a key strategic component of this investigation involves developing an efficient surrogate model which helps to drastically reduce computational effort without significantly compromising accuracy. Furthermore, an assessment of the model uncertainty is integrated, potentially by comparing predictions based on alternative constitutive assump-tions or hydration models, providing a holistic evaluation of prediction capability.

A new 2D framework to simulate concrete at the mesoscopic level of observation is proposed. First, a 3D specimen with randomly generated ellipsoidal aggregates is created. Then a set of parallel cutting planes is defined. For each plane a 2D model with a mesostructure is extracted from the full 3D specimen. Topology of a chosen master plane is iteratively compared with the topology of all remaining planes. A similarity function is derived for all integration points in the FE mesh of the master plane to evaluate differences at the mesostructure between two cuts. Together with out-of-plane weighting function it is used to modify the original constitutive law at an integration point at the master plane in order to mimic 3D mesostructure. Two benchmarks are simulated: tension of a dog-bone specimen and a three-point bending test of a notched beam and the influence of selected parameters is analysed.