Monitoring Durability of Limestone Cement Paste Stored at Conditions Promoting Thaumasite Formation

The durability of Portland-limestone cement with high limestone content was monitored at conditions promoting thaumasite formation. Pore structure and deterioration characteristics were assessed with X-ray micro-computed tomography and correlated with material‘s strength. Changes in crystalline and amorphous phases of the cement paste were investigated with X-ray powder diffraction and solid state nuclear magnetic resonance spectroscopy. Rapid deterioration was observed, evolving as a front causing concentric crack patterns followed by detachment of the part of specimen in contact with the corrosive solution. This ultimately led to loss of structural integrity after 4 months of exposure. During sulfate attack, thaumasite, ettringite and gypsum formed at the expense of portlandite, calcite and monocarboaluminate hydrate. Furthermore, polymerization of silicate chains in C-S-H and deterioration of C-S-H also occurre

Distribution of hydration products in the microstructure of cement pastes

This case study focuses on the quantification of the amorphous hydrate distribution in the microstructure of hardened cement paste. Microtomographic scans of the hardenend cement paste were thresholded based on histogram image analysis combined with microstructural composition obtained from CEMHYD3D hydration model, to separate unhydrated cement grains, crystalline and amorphous hydrates, and capillary pores. The observed spatial distribution of the amorphous hydrate exhibited a strong spatial gradient as the amorphous gel tended to concentrate around dissolving cement grains rather than precipitate uniformly in the available space. A comparative numerical study was carried out to highlight the effect of the spatially (non)uniform hydrate distribution on the compressive strength of the hardened cement paste

Complex analysis of a prestressed concrete girder with GFRP reinforcement subjected to freeze/ thaw cycles

experimentální centru

Analysis of the Design Process and Product Development

Bakalářská práce se zabývá vlivem procesů v konkrétní společnosti na konstrukční návrh produktu z pohledu kvality a kreativity. Cílem bakalářské práce je zhodnotit vliv daných procesů na kvalitu a kreativitu návrhu světlometu. V hlavní části práce jsou představeny principy tvorby konstrukčního návrhu se zaměřením na inovace a kreativitu. Dále pak představení fází vývoje a vývojových týmů ve společnosti. V praktické části práce jsou popsány změny na nejdůležitějších dílech světlometu v souvislosti se změnou vedení projektových týmů. V závěru práce je předložen návrh na efektivnější předávání informací v týmech.The bachelor's thesis deals with the influence of processes in a particular company on the design of a product in terms of a quality and creativity. The aim of the bachelor thesis is to evaluate the influence of the stated processes on the quality and creativity of the headlight design. The main part of the thesis presents the principles of creating a design with focusing on an innovation and creativity. Furthermore, the introduction of development phases and development teams in the company. The practical part of the work describes the changes in the most important parts of the headlight in connection with the change in the management of project teams. In the final part of the work, a proposal for more efficient team transfer of information is presented.345 - Katedra mechanické technologievelmi dobřeOdklad je požadován z důvodu nutné lhůty na uplatnění ochrany duševního vlastnictví na technické řešení navržené v kvalifikační práci. A ochranu před zneužitím informací z práce na aktuálně probíhajícím projektu ve společnosti

Mikromechanische Analyse von Kompositmaterialien aus Mischzementen

Zusammenfassung in tschechischer SpracheIn order to reduce CO2 emissions associated with the production of building materials, the cement and concrete industry has developed new binders by blending ordinary Portland cement with supplementary cementitious materials such as the industrial waste products blast furnace slag and y ash, and/or with finely ground inert materials such as limestone and quartz. This is setting the scene for the present thesis which is motivated by the fact that the new binders exhibit different hardening characteristics compared to their predecessors that were produced with ordinary Portland cement alone. In order to improve the predictability of properties of blended cementitious materials, a combined experimental-computational approach is used. In the experimental part, a comprehensive test database is elaborated by combining state-of-the-art microstructural characterization techniques and mechanical testing. Microstructural characterization combines methods including thermogravimetric analysis, X-ray diffraction with Rietveld analysis, and scanning electron microscopy, in order to determine the microstructural phase assemblages of the initial raw products as well as 1, 3, 7, 28, and 91 days after mixing with water. This allows for resolving phase volume evolutions point wisely. Mechanical testing, in turn, includes characterization of stiffness and strength. The early-age evolution of static unloading modulus is determined with a test protocol including cyclic loading-unloading tests which are hourly repeated from 24 hours after production up to material ages of 8 days. Dynamic stiffness is determined based on measurements of ultrasonic pulse velocities of longitudinal and shear waves, evaluated on the basis of elastic wave propagation in isotropic media. The uniaxial compressive strength evolution is determined both on pastes and mortars, crushed 1, 3, 7, 28, and 91 days after production. The tensile splitting strength, in turn, is determined 1, 3, 28, and 91 days after production. All the measured data is then stored in a newly established database \CemBase" along with additional data collected from the literature. At the end of this thesis \CemBase" contained information on 399 entries out of which approx. 20 % were measured during the experimental campaign and approx. 80 % were collected from available literature. The computational part focuses on multiscale strength homogenization in the frameworks of two complementary modeling methods: multiscale Finite Elementbased homogenization and continuum micromechanics. Finite Element-based homogenization uses nonlinear fracture mechanics with an isotropic damage model to establish a link between nanoscopic calcium-silicate-hydrates and the uniaxial compressive strength of cement pastes, emphasizing the nonuniform distribution of C-S-H around clinker grains. The model focuses on the special role of C-S-H as the main material phase contributing to the macroscopic mechanical properties as well as introduces the C-S-H/space ratio as the main microstructural descriptor. In addition, the model identifies key factors in uencing the compressive strength of cement pastes. Continuum micromechanics, in turn, is used for elasto-brittle modeling of both compressive and tensile strength. The method accounts for key features of cementitious microstructures in terms of their hierarchical organization, quasi-homogeneous material phases, their volume fractions, characteristic shapes, and mutual interaction. The compressive strength model considers that the macroscopic strength is reached once stress peaks in micronsized needle-shaped hydrates reach their strength. The latter is described based on a Mohr-Coulomb failure criterion including the angle of internal friction and the cohesion of low-density calcium-silicate-hydrates, as quantified by limit state analysis of nanoindentation tests. The model accounts for stress concentrations in the immediate vicinity of sand grains, and it uses strain energy-related stress averages for the scale transition from cement paste down to needle-shaped hydrates. It is found that microfillers effectively reinforce the hydrate foam, and that hydration of supplementary cementitious materials has a strengthening effect which is not only related to the increase of hydrate volume and the corresponding decrease of capillary porosity, but also to an increase of the cohesion of low-density calcium-silicate-hydrates. The tensile strength model, in turn, is based on upscaling elasticity and fracture energy from nanoscopic calcium-silicate-hydrates { as quantified by molecular dynamics simulations { all the way up to the uniaxial tensile strength of mortars and concretes. The model considers that cementitious materials suffer from pre-existing cracks which are only somewhat smaller than the maximum aggregate size, and that the tensile strength of the material is reached once these cracks start to propagate. Summarizing, the three developed models define a new state-of-the-art regarding multiscale homogenization of strength properties of cementitious materials.20

Simulation of heat transport in extruded concrete structure

The extrusion-based additive manufacturing process is rapidly establishing itself as a plausible construction alternative to the traditional design paradigm employed for building concrete structures. The structure can be newly produced by successively stacking extruded filaments of fluid concrete, which rapidly harden without the need for traditional formwork. This feature frees the constraints from traditional concrete design and is capable of delivering shape-optimized structures into common practice. However, the material requirements for successful production often pose a significant challenge from a technological perspective. The interconnection between neighboring extruded filaments depends on their mutual adhesion. Any interruption in the production process, resulting in a prolonged deposition of the successive layer, results in the formation of a “cold joint”. This planar inter-layer region with increased porosity effectively compromises the long-term durability of the extruded structure. The present paper sets the framework for an initial assessment of cold joint formation. The cementitious binder used in concrete generates notable heat during hardening, which accelerates humidity transport throughout the extruded structure. Herein, we address the temperature fluctuation within the concrete structure at two scales. By reflecting the microstructural development during cement hydration at the microscale, the accompanying heat release serves as an internal heat source and fuels the temperature fluctuation at the mesoscale of the extruded concrete structure. A fully-developed multiscale model should provide indicators for optimal material design for this technological process

A collection of three-dimensional datasets of hydrating cement paste

This dataset contains a collection of digitized three-dimensional hardened cement paste microstructures obtained from X-ray micro-computed tomography. Four sets of ordinary Portland cement-based pastes were produced and X-ray screened, varying in the initial water-to-cement ratio (wcr=0.35 and 0.50) and fineness of cement used (391 and 273 m2/kg Blaine). Individual paste samples from each set were screened after 1, 2, 3, 4, 7, 14, and 28 days of elapsed hydration at 20˚C in saturated conditions. Each digitized paste specimen captures a realistic spatial configuration of the principal microstructural phases (anhydrous cement, hydration products, and large capillary porosity). The dataset may be further used for assessing changes in the mix design on the resultant spatial configuration of the paste microstructure or aid the development of microstructure-inspired micromechanical models based on realistic material configuration.ISSN:2352-340

Surface area and size distribution of cement particles in hydrating paste as indicators for the conceptualization of a cement paste representative volume element

The conceptualization of a representative volume element (RVE) of hardened cement paste for numerical homogenization of mechanical problems rests on identifying the largest discernible microstructural feature, i.e. unreacted cement grains. While the particle size distribution (PSD) of anhydrous cement is a well-controlled production parameter, the size evolution of a representative cement grain throughout hydration remained unresolved. This study analyzes digitized 3D cement paste microstructures obtained from X-ray micro-computed tomography, coupled with CEMHYD3D hydration model, and segmented by image-processing tools, to obtain the full PSD and specific surface area evolutions of unreacted grains throughout hydration. Results provided indicate a representative grain size in the range of 30−40μm regardless of hydration elapsed, implying a cement paste RVE should amount to 150−200μm to realistically represent cement grains. The PSD shape remained self-similar and two distinctive hydration regimes were identified, differing in dissolution rate and specific surface area decrease, correlating with calcium sulfate reactivity peak. Both measures provide easily accessible microstructural features that may be used for constructing artificial RVEs of hardened cement paste in micromechanical models and related simulations, resting on experimental data.ISSN:0958-9465ISSN:1873-393

Modelling of the Flocculated Polydisperse Microstructure of Fresh Cement Paste

Granular materials present in nature are commonly polydisperse, featuring an inconsistent grain size distribution. This characteristic directly affects particle packing, and consequently influences attributes such as maximal packing fraction, bulk density, rheology, and derived properties. For cement, this has direct consequences as polydispersity, among other factors, influences considerably the rheology and concurrent microstructural build-up in the fresh paste. Previous studies on polydispersity in granular systems considered only mechanical contact forces among particles, but omitted non-contact colloidal interactions, which dominate the microstructure in cement paste. Therein, the flocculation of particles, caused by these interactions, creates a percolated network, which is strongly affected by the polydispersity, but is not fully understood yet. In this paper, we show how flocculation affects the microstructural build up and the resulting rheological and mechanical properties of fresh cement paste. By accounting for flocculation in our numerical modelling, we can produce statistically representative microstructures, which maintain physically consistent rheological properties. We observed that the mechanical properties of percolated particle networks due to flocculation depend on their packing fraction and size distribution. A higher packing ratio offers a higher shear resistance of the fresh paste. Accounting for polydispersity in flocculated build-up does influence the shear resistance, as opposed to a granular system with only contact forces, where the shear strength is independent of grain size span. Our model provides insight into the micro-mechanical origin of the rheology of fresh cement paste, contributing to a better balance between workability and buildability in early stage, and to a better understanding reignited for the design of new concrete mixtures for digital fabrication

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