Conference Agenda

The interfacial transition zone (ITZ) in cement-based composites is a thin region surrounding aggregates, typically 20–50 μm thick, characterized by higher porosity and lower stiffness than the bulk paste. Its properties are governed by mix composition, aggregate characteristics, and curing conditions, making it a
critical factor influencing stiffness, crack initiation, and durability. This study quantitatively evaluates the mechanical influence of ITZ characteristics using image-based modeling. High-resolution X-ray micro-computed tomography (micro-CT) was employed to characterize ITZ geometry and property gradients near aggregate boundaries. These experimentally derived characteristics were implemented into finite element (FE) models, representing the ITZ as a zone with distance-dependent material properties. Parametric simulations examined the effects of ITZ on damage propagation. A bio-inspired modification was further investigated by coating aggregates with a bacteria-induced calcium carbonate layer through microbially induced calcium carbonate precipitation (MICP). The biomineralized coating produced a denser and more homogeneous ITZ, improving bond strength and reducing microcracking. Digital simulations confirmed enhanced load transfer and delayed crack initiation in MICP-modified specimens. The integration of microstructural imaging, biomineralization, and numerical modeling provides an effective approach to design durable and sustainable cementitious materials.

The incorporation of renewable materials such as mineral additions and recycled aggregates in mortars and concretes formulations reduces greenhouse gas emissions and promotes low-carbon construction. These substitutions significantly affect the hydration mechanism and modify the physical, mechanical and durability properties of mortars and concretes. In the present work, experimental and micromechanical investigations are conducted on recycled mortars incorporating ground granulated furnace slag (GGBFS) and recycled sand. Various mortar mixtures are designed involving different replacement ratios of Portland cement and natural sand by GGBFS and recycled sand . Microstructural and macroscopic properties are therefore investigated thanks to experimental setups such as nano and micro indentation, thermogravimetric test analysis and mechanical testing (compressive strength and elastic modulus). On the other hand, a modeling approach is proposed based on a simplified representation of the representative volume element (REV) of low carbon mortars and fed with data collected on the microstructure. Slag blended cement hydration model including kinetic aspects is therefore developed using Avrami and Knudsen kinetic laws [1] for respectively clinker and slag hydration mechanisms. The influence of parameters such as the replacement ratio of Portland cement by slag, the water-to-binder ratio, the portlandite consumption is captured using stoichiometric relationships between the reactants and the products. Volume fractions of hydration products in slag blended cement pastes at different curing ages are also estimated and discussed in this work. Finally, the overall elastic properties of REVs of recycled mortars are computed so as the compressive strengths at different curing ages using the downscaling principle and the well-known Drucker-Prager failure criterion [2]. Compared to the experimental results, the established model is found to predict accurately the macroscopic elastic modulus and the compressive strengths of mortars containing slag and recycled sand at different curing ages.

Concrete carbonation reduces the pH of the cementitious matrix, which can potentially lead to reinforcement corrosion. In urban tunnels, where CO₂ concentrations are elevated, accurately assessing the progress of carbonation is essential for planning maintenance and repair actions. This study presents a predictive model for concrete carbonation in such environments, based on established formulations and calibrated through a dedicated laboratory experimental campaign. The model effectively estimates carbonation depth and porosity
evolution, providing a reliable tool for service-life assessment of concrete in urban tunnel conditions.

Creep and shrinkage are essential characteristics of the material behaviour of concrete. To study the influence of real environmental conditions on the time-dependent material behaviour, back in 2017, a longterm testing campaign on large-scale specimens was initiated by the Research Unit Structural Concrete at TU Wien, and the measurements are still ongoing. The long-term testing campaign consists of 12 large-scale specimens, which are stored outside and they are exposed to real environmental conditions. The measurements of more than eight years indicate the influence of environmental conditions on the time-dependent behaviour. Furthermore, a pronounced influence of the production date (the season of the year in which the concrete elements are cast) is observed. A theory capable of implementing the influence of the changing environmental boundary conditions is needed to capture this influence in the model. The extended micro-prestress solidification theory (XMPS) provides an appropriate framework for capturing this influence. Therefore, the paper presents a fourstep approach to model the influence of environmental boundary conditions on the temperature, moisture and hydration field. To clarify the question posed in the title, the measurements of more than eight years are compared with the prediction of the model using the measured environmental boundary conditions. The comparison shows, that the measured behaviour of the concrete specimen is replicable with the model if the environmental boundary conditions are appropriately implemented. Furthermore, the influence of the coupling between the hydration field, temperature field and moisture field is studied, and the comparison with the measured data shows that the full coupling between these fields leads to the best model prediction.

Extending French nuclear power plants to 60 years requires verifying the long-term integrity of concrete containment buildings under Severe Accident (SA) conditions, namely, exposure to 150°C and 90% RH. An autoclave with an internal hydraulic jack was used to reproduce SA conditions, axial delayed strains were measured, and the in-situ Young’s modulus of concrete cylinders was determined. In parallel, a simplified thermo-hydro-mechanical (THM) model is developed assuming the gas phase is only composed of water vapor. The TH part includes temperature-dependent sorption and the change of porosity, density, and permeability. The mechanical model adopts Burger’s rheology to predict basic creep, drying creep, shrinkage, transient ther-mal deformation (TTD), and Young’s modulus degradation. A 2D homogeneous axisymmetric finite-element model is used to simulate the creep test. Experiments and simulations show rapid strain changes within the first 24 h, followed by a steady state, indicating that most strains accumulate early during SA exposure.

Cement-based materials such as concrete are among the most common in civil engineering applications. The complexity of its response can mainly be attributed to its heterogenous nature. Sound knowledge of features such as the wall effect, and their relation to the internal structure, remain critical to the accurate representation of concrete behaviour in computational models. The presence of the wall effect on the material meso-structure has previously been demonstrated both experimentally and numerically, though only limited work addressed its effect on the material properties. The current contribution aims at performing such investigations by means of the Lattice Discrete Particle Method (LDPM). It is a powerful discrete mechanical model that effectively can simulate the heterogenous nature of concrete on the meso-scale. Recently, the framework was enhanced by a two-phase formulation, thereby allowing to introduce meaningful variability in elastic material properties linked to the heterogeneous mesostructure. This paper demonstrates the capabilities of the model in reproducing the wall effect in concrete.