Cementitious Materials and Structures Group at TU Wien
Experimental and Theoretical Multiscale Analysis
Civil Engineering infrastructure and the materials making up these structures are typically hierarchically organized, i.e. characteristic heterogeneities manifest themselves at different, frequently size-separated scales of observation. Physico-chemical processes taking place at small scales frequently trigger the apparent behavior at larger scales. This is the motivation for bottom-up multiscale analysis of materials and structures. Reliable multiscale modeling includes strict quantitative testing of the predictive capabilities of the developed material models. To this end, it is essential to combine theoretical with experimental approaches.
Based on a combined experimental-theoretical approach, research work focuses on the design and the carrying out of macroscopic mechanical experiments involving materials such as cement-based composites and geomaterials, as well as on using the related measurements for testing hypotheses used to develop multiscale models predicting macroscopic mechanical properties of microheterogeneous materials. The strong interplay of theory and experiments increases the theoretical understanding of microstructural processes, and it allows for the design of improved laboratory tests for studying macroscopic consequences of microscopic processes. Our research approach aims at introducing the smallest possible number of material constants at the microscale, i.e. to formulate simple physical laws on microstructures of materials. These laws are then upscaled in order to obtain their macroscopic counterparts. Identification of introduced material constants at the microscale and confronting model predictions of macroscopic behavior with measurements from experiments involves two independent sets of test data, stemming, e.g. from different testing techniques. Once material models were shown to perform reliably, they are involved in the numerical analysis of civil engineering structures such as, e.g., tunnels. This allows for studying how microstructural processes within hetero-geneous materials govern the behavior of infrastructure.
Drying of porous media results in their shrinkage and may cause cracking provided that shrinkage deformations are hindered by kinematic constraints. In more detail, microcrack propagation requires that cracks are partially saturated, because liquid saturated cracks progressively close under the action of the drying-induced underpressure of the liquid. Once air enters the cracks, they can be expected to dry out quickly such that consideration of either liquid-saturated or gas-saturated cracks is sufficient for many engineering mechanics applications. The cracking risk depends on the kinematic constraints hindering the free shrinkage and on the poromechanical properties of the material, in particular the pore-size distribution.
The macroscopic early-age strength evolution of cement pastes and mortars can be explained by the composition and maturity-dependent stress concen-tration from the macroscopic material scale down to stress peaks in microscopic hydration products. Reaching the load carrying capacity of hydrates is associated with the overall strength of cementitious material. The macro-micro stress concentration is particularly amplified by the porosity of the material, while unhydrated clinker grains can be interpreted as micro-reinforcements. The latter positive effect is significant for sub-stoichiometric mixes at large maturities.
Strength models are essential when dealing with safety and durability analyses of civil engineering infrastructure such as, e.g., tunnels. In this context, micromechanics-based strength models provide the possibility to study the influence of material composition on the structural behavior. Related sensitivity analyses provide insight into how microstructural properties trigger the performance of large civil engineering infra-structure. Identification of load carrying mechanisms at fine scales, in turn, contributes to an improved understanding of hetero-geneous materials and, hence, supports future optimization activities.
1. B. Pichler and L. Dormieux “Cracking risk of partially saturated porous media – Part I: Micromechanical model, Part II: Application to drying shrinkage” IJ Num Anal Meth Geomech, 34(2), 135-186 (2010).
2. B. Pichler, S. Cariou, and L. Dormieux “Damage evolution in an underground gallery induced by drying” IJ Multiscale Comp Eng, 7(2), 65-89 (2009).
3. S. Ullah, B. Pichler, S. Scheiner, and C. Hellmich “Influence of shotcrete composition on load level estimation in NATM tunnel shells: micromechanics-based sensitivity analyses” Comp Mod Eng Sci, 57(3), 279-314 (2010).
4. B. Pichler and C. Hellmich “Upscaling quasi-brittle strength of cement paste and mortar” Cem Con Res, 41(5), 467-476 (2011)