Numerical modeling in timber engineering – moisture transport and quasi-brittle failure

With the rising popularity of timber structures and the increasing complexity of timber engineering projects, the need for numerical simulation tools specific to this building material is gaining rapidly in importance. in particular, moisture transport processes and the quasi-brittle failure behavior, both difficult to describe, present major challenges and are of great relevance in practical construction. For these reasons, this article presents numerical modeling concepts for predicting moisture gradients, estimating effective stiffness and strength, and numerically identifying potential cracking mechanisms in wooden components. These concepts are validated through experimental test programs, and the associated challenges are addressed. selected results ultimately demonstrate the capabilities and relevance of such methods for timber engineering

Mehrskalenmodellierung der Mikrostruktur in Zement und Beton : von der Hydratation zu Poroelastizität, Kriechen und Festigkeit

Zusammenfassung in deutscher SpracheDie makroskopischen mechanischen Materialeigenschaften von Beton werden durch seine hierarchisch organisierte, heterogene Mikrostruktur bestimmt. Auf einem Beobachtungsmaßstab von einigen Milli- bis Zentimetern kann man visuell zwischen Sand- und Gesteinszuschlagskörnern und der sie umgebenden Zementsteinmatrix unterscheiden. Betrachtet man die Matrix auf einem kleineren Beobachtungsmaßstab, offenbart sich ein überraschend komplexes Materialsystem, bestehend aus unhydrierten Zementpartikeln, Poren, Hohlräumen und Hydraten, die aus der chemischen Reaktion von Zement mit Wasser entstehen. Besonders die Modellierung des jungen Betons ist herausfordernd, da, aufgrund des ständigen Ausfallens weiterer Hydrate, die Mikrostruktur einer kontinuierlichen Transformation unterzogen ist. Das Erfassen dieser komplexen, sich entwickelnden Mikrostruktur mithilfe eines mathematischen Modellierungskonzepts stellt das erste Ziel dieser Arbeit dar. Die Volumina der Materialbestandteile sollen als Funktion der volumetrischen Zusammensetzung und des Aushärtegrades ermittelt werden, wobei auf erst kürzlich entdeckte Phänomene, wie die zunehmende Verdichtung der Calciumsilicathydrate (C-S-H) oder die durch Wasser in der zugänglichen Oberflächenporosität der Zuschlagskörner ermöglichte -innere Nachbehandlung-, eingegangen werden soll. Die Bestimmung der Hierarchie der Materialbestandteile, deren Morphologie, deren mechanischer Eigenschaften und deren Interaktion sind weitere Themen, die in dieser Arbeit behandelt werden. Die Mehrskalen-Mikrostrukturmodelle werden mit publizierten Messoder Modellergebnissen aus unterschiedlichen Disziplinen der Betonwissenschaften gespeist. Die Festigkeitskennwerte und das Verdichtungsverhalten von C-S-H, beispielsweise, stammen von Nanoindentationtests bzw. von Messungen der Kernspinresonanzrelaxation. Dadurch verbleibt die Anzahl der Modellparameter auf einem absoluten Minimum und alle eingeführten Materialkonstanten sind direkt physikalisch interpretierbar. Methoden der Kontinuumsmikromechanik ermöglichen den Skalenübergang zwischen Mikrostruktur und makroskopischen mechanischen Materialverhalten. Durch einen Bottom-up Ansatz werden die homogenisierten mechanischen Eigenschaften auf der Makroskala basierend auf physikalischen Gesetzen auf der Mikrostruktur bestimmt; ein Top-down Ansatz quantifiziert Materialkonstanten auf der Mikrostruktur, die bis heute experimentell nicht zugänglich sind. Dabei wird auf die drei zentralen mechanischen Eigenschaften der zementgebundenen Materialien eingegangen: poroelastisches Verhalten und Kriechverhalten sowie auf die einaxiale Druckfestigkeit. Es wird gezeigt, dass die Steifigkeitshomogenisierung ausgehend von den Nanometer großen C-S-H Festkörpern bis hinauf zum makroskopischen elastischen Materialverhalten möglich ist, wenn man berücksichtigt, dass die beengten Platzverhältnisse in den wassergefüllten Porenräumen die Dichte und die Morphologie des C-S-H steuert. Die alters- und zusammensetzungsabhängige Kriechaktivität von Zementstein, Mörtel und Beton - die aus mehreren tausenden von Kriechtests bestimmt wurde - kann auf eine einzige universelle Kriechfunktion in den Hydraten zurückgeführt werden. Außerdem wird gezeigt, dass Hydrate in umweltfreundlichen -grünen- Zementsteinen und Mörteln, hergestellt mit Hochofenschlacke oder Flugasche als Zementersatz, wesentlich fester sind als in gewöhnlichen Portlandzementsteinen.Concrete is a microheterogeneous material. Therefore, mechanical properties of concrete are related to the hierarchically organized microstructure of the material. At an observation scale of millimeters to centimeters, one can visually distinguish sand grains, gravel aggregates, and the surrounding cement paste matrix. Resolving the cement paste matrix at smaller scales of observation reveals a surprisingly complex material microstructure. It consists of cement grains, pores, and hydrates; whereby the latter represent products of the chemical reaction between cement and water. Material modeling is particularly challenging at early material ages, because the microstructure of cement paste undergoes a continuous transformation due to the progressive consumption of cement and water, and the corresponding precipitation of hydrates. Describing the evolving microstructures within a mathematical framework is the first objective of this work. We aim at quantifying the volumes occupied by the material constituents, as functions of the initial volumetric composition and of the maturity of the material. Thereby, we account for recently quantified phenomena like the progressive densification of calciumsilicate- hydrates (C-S-H) and the -internal curing- capacity provided by water residing in the open surface porosity of aggregates. Additional important challenges tackled in this thesis are: identification of the morphology of the individual material constituents and of their arrangement within the hierarchically organized microstructure, quantification of their mechanical properties, and modeling their interactions. Corresponding multiscale models are fed with measured or modeled input data, taken from several fields of cement science reported in the open literature. The mass density and the elastic stiffness of solid C-S-H nanoparticles are taken from small angle scattering experiments and from atomistic modeling, respectively. Strength properties and the densification behavior of C-S-H gel are taken from limit state analysis of nanoindentation tests and from nuclear magnetic resonance relaxometry tests, respectively. This way, the number of model parameters is kept at an absolute minimum and all involved quantities are physically meaningful. Methods of continuum micromechanics are used as vehicles for scale transitions, i.e. for establishing links between microstructure and microstructural properties, on the one hand, and macroscopic mechanical properties of cementitious materials, on the other hand. Bottomup homogenization is used to upscale physical laws introduced at material microscales and top-down identification is used to quantify constants of material constituents, which are nowadays not accessible by direct material testing. Thereby, the present thesis addresses all three major mechanical properties of cementitious materials: their elastic stiffness, their creep properties, and their uniaxial compressive strength. As for poroelasticity, it is shown that stiffness homogenization starting at nanoscopic solid C-S-H particles all the way up to the macroscopic elastic behavior of cement paste is possible, if one considers that space confinements in the water-filled pore spaces govern (i) the shape of precipitating solid C-S-H particles and (ii) the overall density of the evolving C-S-H gel. As for creep, it is shown that the maturity- and composition-dependent creep properties of cement pastes, mortars, and concretes - as quantified in several thousands of macroscopic creep experiments - can be traced back to one universal creep function of microscopic hydrates. As for strength, it is shown that hydrates of environmentally friendly -green- cement pastes and mortars, produced with slag or fly ash as cement replacement materials, are considerably stronger than the hydrates in ordinary Portland cement pastes.18

Validated hydration model for slag-blended cement based on calorimetry measurements

Modeling of the complex interlinked chemical reactions involved in the hydration process of slag-blended cement is rather challenging, in particular since accurate prediction of the hydration kinetics of clinker and slag hydration, respectively, is still out of reach. To overcome this challenge, we propose a hybrid modeling approach based on calorimetry measurements combined with state-of-the-art hydration models. The model features intrinsic C-(A)-S-H nanoparticles related to clinker and slag hydration, respectively, with precipitation space-dependent densities of the corresponding gel phases. Successful validation against published experimental data, obtained on 54 different mixes in 7 different laboratories, proves the model's applicability and corroborates that C-(A)-S-H gel densifies progressively during hydration, that portlandite consumption upon slag reaction is caused by the precipitation of calcium-aluminate hydrates and that the ultimate heat of slag depends significantly on its chemical composition.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Micromechanical multiscale modeling of ITZ-driven failure of recycled concrete: Effects of composition and maturity on the material strength

Recycled concrete, i.e. concrete which contains aggregates that are obtained from crushing waste concrete, typically exhibits a smaller strength than conventional concretes. We herein decipher the origin and quantify the extent of the strength reduction by means of multiscale micromechanics-based modeling. Therefore, the microstructure of recycled concrete is represented across four observation scales, spanning from the micrometer-sized scale of cement hydration products to the centimeter-sized scale of concrete. Recycled aggregates are divided into three classes with distinct morphological features: plain aggregates which are clean of old cement paste, mortar aggregates, and aggregates covered by old cement paste. Macroscopic loading is concentrated via interfacial transition zones (ITZs)-which occur mutually between aggregates, old, and new cement paste-to the micrometer-sized hydrates resolved at the smallest observation scale. Hydrate failure within the most unfavorably loaded ITZ is considered to trigger concrete failure. Modeling results show that failure in either of the ITZs might be critical, and that the failure mode is governed by the mutual stiffness contrast between aggregates, old, and new paste, which depend, in turn, on the concrete composition and on the material's maturity. The model predicts that the strength difference between recycled concrete and conventional concrete is less pronounced (i) at an early age compared to mature ages, (ii) when the old cement paste content is small, and (iii) when recycling a high-quality parent concrete.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Effect of Solution-to-Binder Ratio and Alkalinity on Setting and Early-Age Properties of Alkali-Activated Slag-Fly Ash Binders

The growing use of blends of low- and high-calcium solid precursors in combination with different alkaline activators requires simple, efficient, and accurate experimental means to characterize their behavior, particularly during the liquid-to-solid transition (setting) at early material ages. This research investigates slag-fly ash systems mixed at different solution-to-binder (s/b) ratios with sodium silicate/sodium hydroxide-based activator solutions of varying concentrations. Therefore, continuous non-destructive tests—namely ultrasonic pulse velocity (UPV) measurements and isothermal calorimetry tests—are combined with classical slump flow, Vicat, and uniaxial compressive strength tests. The experimental results highlight that high alkali and silica contents and a low s/b ratio benefit the early-age hydration, lead to a faster setting, and improve the early-age strength. The loss of workability, determined from the time when the slump flow becomes negligible, correlates well with ultrasonic P-wave velocity evolutions. This is, however, not the case for Vicat or calorimetry tests

Monitoring early age elastic and viscoelastic properties of alkali-activated slag mortar by means of repeated minute-long loadings

This study investigates the development of elastic and creep properties of sodium hydroxide-activated blast furnace slag mortar, utilizing three different molarities of the activator solution and two solution-to-binder ratio and a reference OPC-based mixture, since the earliest age. The experimental phase involves a series of hourly-repeated 5-min long creep tests on the aging material. This approach enables continuous monitoring, facilitating the characterization of early-age elastic stiffness and creep properties. Linear regression is employed to calculate the tangent unloading (elastic) modulus and Poisson's ratio. To model short-term creep behaviour, a power-law creep function is utilized. The integration of these findings with calorimetry-derived evolutions of cumulative heat release establishes linear correlation between (compressive) strength and heat release, along with a power function relationship between unloading (elastic) modulus and heat release. An optimal alkali dosage (Na2O content) appears to be vital for long-term strength development. Additionally, the creep parameters, namely amplitude (A) and kinetic (K), demonstrate a gradual decrease, although they maintain values higher than those of the corresponding OPC mixture, as the heat release progresses

Hydrate failure in ITZ governs concrete strength: A micro-to-macro validated engineering mechanics model

Ever since the early days of Féret (1892) and Abrams (1919), concrete research has targeted at relating concrete composition to uniaxial compressive strength. While these activities were mainly characterized by empirical fitting functions, we here take a more fundamental approach based on continuum micromechanics. The loading applied at the concrete level, is first concentrated (“downscaled”) to maximum stresses related to cement paste volumes which are directly adjacent to the aggregates, i.e. to the interfacial transition zones (ITZ). These maximum stresses are further “downscaled” to the micron-sized hydrates, in terms of higher-order stress averages. The latter enter a Drucker-Prager failure criterion with material constants derived from nanoindentation tests. The model is successfully validated across the hydrate-to-concrete scales. Strength magnitude is governed by ITZ stress concentrations, and the water-to-cement ratio is its dominant mixture design parameter.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

DataSheet1_Development of early age autogenous and thermal strains of alkali-activated slag-fly ash pastes.docx

Replacing ordinary Portland cement-based materials with alkali-activated industrial wastes is often limited because of significant volume changes occurring in these materials at early age. This experimental study aims to quantify the extent of the volume changes and explore the underlying mechanisms of pastes composed of slag and fly ash (ratio 50:50) which are activated by sodium hydroxide and sodium silicate. Eight compositions were tested, with silica modulus (Ms) varying between 1.04 and 1.58 and with solution-to-binder ratios (S/B) varying between 0.47 and 0.70. Specimen length changes in sealed conditions are monitored by applying repeated thermal variations in an adapted AutoShrink device and are accompanied by isothermal calorimetry, uniaxial compressive strength, and internal relative humidity (IRH) tests. This way, the temporal evolutions of autogenous strains, the coefficient of thermal expansion (CTE), the heat release, the apparent activation energy (Ea), the IRH and the strength are determined and compared to each other. Both the measured autogenous shrinkage and CTEs are rather large; they amount to 4,000–5,000 μm/m and roughly 40 μm/m/°C, respectively, at material ages of 2 weeks. An increase in S/B leads to a decrease in autogenous shrinkage and an increase in CTE. An increase in the Ms causes a decrease in both the autogenous shrinkage and the CTE. Most strikingly, autogenous shrinkage evolves linearly with the cumulative heat released by the binders. The IRH remains continuously above 94% during the first 2 weeks. The apparent activation energy amounts to roughly 74 kJ/mol and is virtually unaffected by S/B and Ms.</p