Insights in thixotropic concrete pumping by a Poiseuille flow extension

Thixotropy is a reversible time-dependent phenomenon in fluids, in which an internal structure grows due to flocculation and breaks down under shear action. Numerous fluids are thixotropic, e.g. concretes and cementitious suspensions. Pumping of concrete is an important application. Since current approaches omit thixotropic effects, we aim to develop a simple theoretical model to evaluate or understand the significance of thixotropy on the concrete pumping behaviour. We therefore extended Poiseuille flow for thixotropic concretes and reformulated it in a dimensionless form to gain insights. After a validation, the results and significance are elaborated and concluded. Results showed that for increasing thixotropy and decreasing flow rates, the plug radius, wall shear rate and pumping pressure loss increase. Even though all thixotropy mechanisms may not be covered, a simple model is delivered to interpret or predict the effect of thixotropy on the pumping behaviour of cementitious suspensions. The dimensionless formulations via the Bingham number Bn and related discharge diagrams are sufficiently elegant for computational implementation and very insightful to distinguish a thixotropic flow regime. The model could be extended for more complicated thixotropies, irreversible time-dependent effects or even other pumping related phenomena

CFD implementation of time-dependent behaviour : application for concrete pumping

Sorptivity describes the pore connectivity of cementitious materials. This property is widely used for the assessment of the resistance of concrete to the ingress of aggressive agents. However, anomalous behaviour of cementitious materials during water uptake is usually reported. This is possibly explained by the effect of the hygroscopic nature of cementitious materials on the dynamics of the process. Water affinity of C-S-H might turn it into an imbibant and cause swelling as the material is exposed to water. Development of swelling can cause a variable hydraulic diffusivity of the material with time, and this is consistent with the deviation from the progress of the water uptake with the square root of time in the short term usually reported in the literature for the particular case of cementitious materials. This paper provides experimental results in support of the occurrence of swelling during water uptake in mortar samples. Consequently, the term capillary imbibition instead of capillary absorption seems more appropriate for describing the water uptake by capillarity of cementitious materials, as imbibition is usually connected to swelling. The idea of cementitious materials as rigid materials during water uptake seems incomplete for a complete description of the process

Circulating adrenomedullin estimates survival and reversibility of organ failure in sepsis: the prospective observational multinational Adrenomedullin and Outcome in Sepsis and Septic Shock-1 (AdrenOSS-1) study

Background: Adrenomedullin (ADM) regulates vascular tone and endothelial permeability during sepsis. Levels of circulating biologically active ADM (bio-ADM) show an inverse relationship with blood pressure and a direct relationship with vasopressor requirement. In the present prospective observational multinational Adrenomedullin and Outcome in Sepsis and Septic Shock 1 (, AdrenOSS-1) study, we assessed relationships between circulating bio-ADM during the initial intensive care unit (ICU) stay and short-term outcome in order to eventually design a biomarker-guided randomized controlled trial. Methods: AdrenOSS-1 was a prospective observational multinational study. The primary outcome was 28-day mortality. Secondary outcomes included organ failure as defined by Sequential Organ Failure Assessment (SOFA) score, organ support with focus on vasopressor/inotropic use, and need for renal replacement therapy. AdrenOSS-1 included 583 patients admitted to the ICU with sepsis or septic shock. Results: Circulating bio-ADM levels were measured upon admission and at day 2. Median bio-ADM concentration upon admission was 80.5 pg/ml [IQR 41.5-148.1 pg/ml]. Initial SOFA score was 7 [IQR 5-10], and 28-day mortality was 22%. We found marked associations between bio-ADM upon admission and 28-day mortality (unadjusted standardized HR 2.3 [CI 1.9-2.9]; adjusted HR 1.6 [CI 1.1-2.5]) and between bio-ADM levels and SOFA score (p < 0.0001). Need of vasopressor/inotrope, renal replacement therapy, and positive fluid balance were more prevalent in patients with a bio-ADM > 70 pg/ml upon admission than in those with bio-ADM ≤ 70 pg/ml. In patients with bio-ADM > 70 pg/ml upon admission, decrease in bio-ADM below 70 pg/ml at day 2 was associated with recovery of organ function at day 7 and better 28-day outcome (9.5% mortality). By contrast, persistently elevated bio-ADM at day 2 was associated with prolonged organ dysfunction and high 28-day mortality (38.1% mortality, HR 4.9, 95% CI 2.5-9.8). Conclusions: AdrenOSS-1 shows that early levels and rapid changes in bio-ADM estimate short-term outcome in sepsis and septic shock. These data are the backbone of the design of the biomarker-guided AdrenOSS-2 trial. Trial registration: ClinicalTrials.gov, NCT02393781. Registered on March 19, 2015

Clinical and organizational factors associated with mortality during the peak of first COVID-19 wave: the global UNITE-COVID study

Purpose: To accommodate the unprecedented number of critically ill patients with pneumonia caused by coronavirus disease 2019 (COVID-19) expansion of the capacity of intensive care unit (ICU) to clinical areas not previously used for critical care was necessary. We describe the global burden of COVID-19 admissions and the clinical and organizational characteristics associated with outcomes in critically ill COVID-19 patients. Methods: Multicenter, international, point prevalence study, including adult patients with SARS-CoV-2 infection confirmed by polymerase chain reaction (PCR) and a diagnosis of COVID-19 admitted to ICU between February 15th and May 15th, 2020. Results: 4994 patients from 280 ICUs in 46 countries were included. Included ICUs increased their total capacity from 4931 to 7630 beds, deploying personnel from other areas. Overall, 1986 (39.8%) patients were admitted to surge capacity beds. Invasive ventilation at admission was present in 2325 (46.5%) patients and was required during ICU stay in 85.8% of patients. 60-day mortality was 33.9% (IQR across units: 20%–50%) and ICU mortality 32.7%. Older age, invasive mechanical ventilation, and acute kidney injury (AKI) were associated with increased mortality. These associations were also confirmed specifically in mechanically ventilated patients. Admission to surge capacity beds was not associated with mortality, even after controlling for other factors. Conclusions: ICUs responded to the increase in COVID-19 patients by increasing bed availability and staff, admitting up to 40% of patients in surge capacity beds. Although mortality in this population was high, admission to a surge capacity bed was not associated with increased mortality. Older age, invasive mechanical ventilation, and AKI were identified as the strongest predictors of mortality

Co-infection and ICU-acquired infection in COIVD-19 ICU patients: a secondary analysis of the UNITE-COVID data set

Background: The COVID-19 pandemic presented major challenges for critical care facilities worldwide. Infections which develop alongside or subsequent to viral pneumonitis are a challenge under sporadic and pandemic conditions; however, data have suggested that patterns of these differ between COVID-19 and other viral pneumonitides. This secondary analysis aimed to explore patterns of co-infection and intensive care unit-acquired infections (ICU-AI) and the relationship to use of corticosteroids in a large, international cohort of critically ill COVID-19 patients.Methods: This is a multicenter, international, observational study, including adult patients with PCR-confirmed COVID-19 diagnosis admitted to ICUs at the peak of wave one of COVID-19 (February 15th to May 15th, 2020). Data collected included investigator-assessed co-infection at ICU admission, infection acquired in ICU, infection with multi-drug resistant organisms (MDRO) and antibiotic use. Frequencies were compared by Pearson's Chi-squared and continuous variables by Mann-Whitney U test. Propensity score matching for variables associated with ICU-acquired infection was undertaken using R library MatchIT using the "full" matching method.Results: Data were available from 4994 patients. Bacterial co-infection at admission was detected in 716 patients (14%), whilst 85% of patients received antibiotics at that stage. ICU-AI developed in 2715 (54%). The most common ICU-AI was bacterial pneumonia (44% of infections), whilst 9% of patients developed fungal pneumonia; 25% of infections involved MDRO. Patients developing infections in ICU had greater antimicrobial exposure than those without such infections. Incident density (ICU-AI per 1000 ICU days) was in considerable excess of reports from pre-pandemic surveillance. Corticosteroid use was heterogenous between ICUs. In univariate analysis, 58% of patients receiving corticosteroids and 43% of those not receiving steroids developed ICU-AI. Adjusting for potential confounders in the propensity-matched cohort, 71% of patients receiving corticosteroids developed ICU-AI vs 52% of those not receiving corticosteroids. Duration of corticosteroid therapy was also associated with development of ICU-AI and infection with an MDRO.Conclusions: In patients with severe COVID-19 in the first wave, co-infection at admission to ICU was relatively rare but antibiotic use was in substantial excess to that indication. ICU-AI were common and were significantly associated with use of corticosteroids

Early mobilisation in critically ill COVID-19 patients: a subanalysis of the ESICM-initiated UNITE-COVID observational study

Background Early mobilisation (EM) is an intervention that may improve the outcome of critically ill patients. There is limited data on EM in COVID-19 patients and its use during the first pandemic wave. Methods This is a pre-planned subanalysis of the ESICM UNITE-COVID, an international multicenter observational study involving critically ill COVID-19 patients in the ICU between February 15th and May 15th, 2020. We analysed variables associated with the initiation of EM (within 72 h of ICU admission) and explored the impact of EM on mortality, ICU and hospital length of stay, as well as discharge location. Statistical analyses were done using (generalised) linear mixed-effect models and ANOVAs. Results Mobilisation data from 4190 patients from 280 ICUs in 45 countries were analysed. 1114 (26.6%) of these patients received mobilisation within 72 h after ICU admission; 3076 (73.4%) did not. In our analysis of factors associated with EM, mechanical ventilation at admission (OR 0.29; 95% CI 0.25, 0.35; p = 0.001), higher age (OR 0.99; 95% CI 0.98, 1.00; p ≤ 0.001), pre-existing asthma (OR 0.84; 95% CI 0.73, 0.98; p = 0.028), and pre-existing kidney disease (OR 0.84; 95% CI 0.71, 0.99; p = 0.036) were negatively associated with the initiation of EM. EM was associated with a higher chance of being discharged home (OR 1.31; 95% CI 1.08, 1.58; p = 0.007) but was not associated with length of stay in ICU (adj. difference 0.91 days; 95% CI − 0.47, 1.37, p = 0.34) and hospital (adj. difference 1.4 days; 95% CI − 0.62, 2.35, p = 0.24) or mortality (OR 0.88; 95% CI 0.7, 1.09, p = 0.24) when adjusted for covariates. Conclusions Our findings demonstrate that a quarter of COVID-19 patients received EM. There was no association found between EM in COVID-19 patients' ICU and hospital length of stay or mortality. However, EM in COVID-19 patients was associated with increased odds of being discharged home rather than to a care facility. Trial registration ClinicalTrials.gov: NCT04836065 (retrospectively registered April 8th 2021)

Numerical Reliability Study Based on Rheological Input for Bingham Paste Pumping Using a Finite Volume Approach in OpenFOAM

Rheological quantification is important in many industries, the concrete industry in particular, e.g., pumping, form filling, etc. Instead of performing expensive and time-consuming experiments, numerical simulations are a powerful means in view of rheological assessment. However, due to the unclear numerical reliability and the uncertainty of rheological input data, it is important for the construction industry to assess the numerical outcome. To reduce the numerical domain of cementitious suspensions, we assessed the numerical finite volume simulations of Bingham paste pumping flows in OpenFOAM. We analysed the numerical reliability, first, irrespective of its rheological input by comparison with the literature and theory, and second, dependent on a certain rheological quantification by comparison with pumping experiments. Irrespective of the rheological input, the numerical results were significantly accurate. Dependent on the rheological input, a numerical mismatch, however, existed. Errors below 1% can be expected for proposed numerical rules of thumb: a bi-viscous regularisation, with pressure numbers higher than 5/4. To improve bias due to uncertain rheology, a rheological configuration close to the engineer’s aimed application should be used. However, important phenomena should not be overlooked. Further assessment for lubrication flows, in, e.g., concrete pumping, is still necessary to address concerns of reliability and stability

Active rheology control of cementitious materials : numerical and experimental pumping investigation

Concrete is, worldwide, the most used construction material. This is mainly due to advantages such as its free shape, availability and competitive cost. Innovations in concrete technology allowed for improvement in performance, quality, quantity, practical execution and possible realisations of structures. Although the introduction of self-compacting concrete (SCC) in the 1980s was very promising, SCC has not greatly been adopted by practitioners despite its many advantages. One of the reasons is because, still today, the practical execution, quality and performance of the construction passively relies on the fresh state (or rheological) properties of the concrete. Hence, once the concrete has been mixed and transported on site, it passively relies on the status quo with little room for adaptation. Shortcomings occur due to contradicting demands in rheological performances during different process stages. During transportation, pumping, formwork casting or extrusion the concrete requires to be highly flowable, contrary to directly after casting, 3D printing, shotcreting etc. requiring it to be as little flowable and to set as fast as possible. In order to overcome issues related to the passive character of concrete, to overcome drawbacks associated with the SCC and to provide a next generation innovation in concrete technology allowing for improvement on multiple aspects, the development of Active Rheology and/or Stiffening Control (ARC & ASC) of concrete has been initiated by the SmartCast project. Active rheology control could be understood as the concept of actively triggering smart components in (cementitious) suspensions via an external signal --- which can take various forms such as (electro)magnetic signals --- after which the rheology or fresh state of the suspension is altered in a reversible or non-reversible manner. ASC & ARC may be able to resolve contradicting requirements in rheological performance in cementitious applications. However, due its innovative character, ASC & ARC are a nearly unexplored domain of research in concrete technology. In spite of the vast research on concrete technology, the rheological behaviour of concrete is still not fully understood. This is especially the case for time-dependent rheological behaviour of concrete and more generally cementitious suspensions. Under time-dependent effects, two main aspects are understood affecting the rheological behaviour, i.e. thixotropy which is reversible and hydration which is irreversible. Especially due to their complexity, time-dependent effects are often omitted from concrete rheological studies, even though they can have a significant impact on the execution process and eventual performance. Instead of relying on vast, inconvenient and expensive experiments, numerically modelling the rheological behaviour allows for investigating the performance in certain applications, or may perhaps even provide a framework for mixture design. Rheological input for numerical simulations can be provided by quantification devices, known as rheometers. Since, significant disparities exist among rheometers, the rheological input is an additional uncertainty on top of appropriate constitutive cementitious behaviour modelling. Moreover, deficiencies exist in current modelling techniques, e.g. lacking time-dependent behaviour and more importantly ASC & ARC. In order to provide understanding and insights in ASC & ARC behaviour and potential applications, its modelling cannot lag behind. As part of ASC & ARC development in the SmartCast project, this doctoral dissertation aims at bridging the gap of modelling time-dependent behaviour of cementitious suspensions in view of ASC & ARC, providing a rheological quantification procedure and assessing its adequacy. This research aims to provide fundamental understanding in flow behaviour of cementitious suspensions and how to adequately quantify and model it, independent of time, time-dependent (thixotropic) and with respect to ASC & ARC. The main investigated research hypothesis is whether and how adequately ASC & ARC can be captured by a concept of kinetic temporal evolution of an internal structure, driven by structural build-up and break-down. Neither the full extent of cementitious behaviour was investigated, nor the full extent of potentially possible ASC & ARC. Passive and Active Stiffening Control (PSC & ASC) of cementitious pipe flows were investigated from a fundamental viewpoint, because pumping is an inevitable part of the concrete or cementitious execution process. Paste suspensions form the main baseline for investigating cementitious pipe flows. That is because if paste could adequately be described and modelled, it can easily be superimposed or extrapolated to more complicated flow behaviour involving e.g. shear induced particle migration. Thereby, fundamental and widely applicable knowledge may be obtained in view of ARC. Moreover, such more complicated flow behaviour is still a vast, on-going research domain itself. On the one hand, the adequacy of constitutive behaviour modelling and quantification was assessed. On the other, the numerical adequacy was assessed, especially because numerical simulations rely on rheological input, which uncertainties are directly embedded as a consequence. Assessing the adequacy of the numerical framework is, therefore, a feedback loop between rheological input and numerical outcome. The numerical framework was assessed on three aspects with gradually increased complexity. First the modelling adequacy was assessed independent of time via so-called Bingham behaviour, then including thixotropic behaviour as a predecessor of ASC & ARC and lastly active stiffening control behaviour. Practically, the Bingham behaviour was established by means of limestone powder suspensions, the thixotropic behaviour with additional nano-clay and the ASC & ARC behaviour via cementitious suspensions with magneto-rheological fly ash. To this end, a modular, experimental pumping set-up was designed, referred to as the Small Pumping System (SPS), and a numerical framework was developed based on OpenFOAM, in which time-dependent behaviour as well as behavioural aspects of ASC & ARC were implemented. Furthermore, transparent, easy-to-use rheological quantification procedures were developed to describe cementitious flow behaviour independent of time (by the Bingham model), with thixotropy (by the simplified Roussel model) and with active stiffening control (by a proposed dynamic magneto-structuration model). Based on pumping experiments and rheological quantifications, simulations were performed using a finite volume method. Although the library of constitutive behaviour models is vast and can be fairly complicated, it can be stated that rheological properties can be quantified and depending on the context, it may suffice to describe the flow behaviour of cementitious suspensions from a macroscopic viewpoint. Independent of time a Bingham model characterised by a yield stress and a plastic viscosity suffices. If time-dependent (thixotropic) behaviour is significant, it cannot be omitted from the analysis. It can be included, e.g. by a simplified model proposed by Roussel, for which a transparent, semi-coupled quantification methodology was developed. However, one should pay attention to potential biases involved with rheological quantification, which is part of the difficulty of the state of the art in rheometry. Therefore, it is advised to quantify rheology from a set-up closely related to the engineer's aimed application in order to avoid biases. However, one should be aware not to overlook significant physical phenomena, otherwise the quantification and behaviour description does not suffice as such. If, for instance, shear induced particle migration is significant, which is the case for concrete pumping, microscopic models are inevitable in which the yield stress and the plastic viscosity or even time-dependent parameters are a function of the particle concentration. Non-dimensional numbers may provide a useful indication on the occurring significance of certain effects or on-going phenomena. In particular, three dimensionless parameters are useful, i.e. the pressure number, the non-dimensional discharge and the thixotropy factor. As such, a so-called discharge diagram could be insightful. Fundamental insights were provided based on non-dimensional expressions of Poiseuille flow, its extensions and a derived thixotropic extension based on the simplified Roussel model. The derived, but not further explored, non-dimensional (thixotropic) slippage flow expressions show a huge potential for further investigation. From an experimental viewpoint, investigating the influence of a pre-wetting, pre-lubricating or hydrophobic coating of the pipe surface is of future importance. Based on a sound, five-fold numerical validation, it can be stated that Computational Fluid Dynamics (CFD) can be used to model cementitious suspensions to a significantly accurate extent and from a macroscopic viewpoint only. Although, it should be nuanced that CFD is and will not be able to precisely capture non-Newtonian behaviour, especially with regard to yielding behaviour. A Generalised Newtonian Approach (GNA) will always be a comprise, but if dealt with in an appropriate way, useful results can be provided. At least if considering a proper mesh design, if using proper regularisation and under-relaxation, if using rheological input close to the aimed application thereby omitting uncertainty due to rheometry --- significantly accurate numerical simulations can be achieved, with a theoretical error below 1%. However, in spite of using a sliding pipe rheometer which is closer to pumping than a parallel plate rheometer, an overestimation (from a near perfect match up to a relative error of 100%) was obtained when compared with pumping experiments. This error was mainly attributed to biases in rheological quantification, as well as a slight reduction in pumping pressure loss due to limited intermixed water near the pipe wall surface. Irrespective of significantly accurate theoretical simulations, a numerical simulation is only an approximation of a considered physical problem constrained by its mathematical simplicity and assumptions. Therefore, it is of importance to have sufficient understanding of the problem that needs resolving and the limits of the selected numerical technique. Hence, all significant phenomena should be modelled in order to achieve an adequate numerical simulation. Just as any other technique, it has disadvantages and advantages. One simply needs to bear these in mind and take care of them. Although velocity under-relaxation was able to cope with stability and accuracy issues due to viscoplastic regularisation of yield stress fluids for CFD simulations with a GNA approach, a broader spectrum of simulations still remains unexplored. Especially investigating lubrication pipe flows with respect to regularisation, stability and accuracy is a promising future research path with practical relevance. Other viscoplastic regularisation techniques as well as the augmented Lagrangian technique could be investigated in the future. Apart from the viscoplastic regularisation problem, lubrication flows could also be investigated by means of a slippage boundary condition, implementation of coupled Shear Induced Particle Migration (SIPM), as well as coupled Computational Fluid Dynamics - Discrete Element Method (CFD-DEM) and other techniques. However, calibration and validation of those techniques is not straightforward due to the high model complexity. Valorising the main posed hypothesis, it can be stated that, indeed, it is possible to both qualitatively and quantitatively describe PSC as well as ASC & ARC --- expressed in the form of magneto-rheological behaviour --- by means of a concept of internal structure and its temporal kinetic evolution characterised by a structural build-up and break-down term. Although Passive Stiffening Control (PSC) could not be further verified for more complicated behavioural features due to involved biases in rheology and physical conditions (temperature effects) during the thixotropic pumping tests, predictions by means of a kinetic, temporal approach of an internal structure with a relative overestimation up to 100% were reasonable. Assessment and predictions of ASC were more pronounced, as the numerical simulations provided results both viable and in the same order of magnitude as the magneto-rheological pumping test with ASC. However, this does not prove a hypothesis, it is only evidenced that viable, qualitative results can be captured by the proposed magneto-structuration model. However, the full extent of the magnetic response was not captured by dynamic magneto-structuration only. Therefore, the hypothesis is accepted that the combination of predominant magneto-structuration and subordinate magnetic flow confinement can well capture the magneto-rheological cementitious behaviour. As long as no other evidence is found to falsify or reject the postulated hypothesis on dynamic magneto-structuration, it still stands. The proposed magneto-structuration model does not capture all behavioural features in view of ASC & ARC, but it allows to both qualitatively and quantitatively conceptualise ASC. Hence, it can serve to provide insights in ASC & ARC or to provide a fundamental building block in which more complicated behaviour in general can be incorporated. In perspective of ASC & ARC, promising results were obtained allowing for improved formwork leakage control or 3D printing buildability. Further developments of active triggering controls, e.g. an activating printing nozzle (or pipe) could be explored. The proposed, more general magneto-structuration model could serve as corner stone in development of more complicated and versatile features of ARC, such as micro-vibration improving the flowability, anisotropic magnetisation, magnetic acceleration for improved pumping performance etc. ARC, not only in the form of a magneto-rheological response, nor by only a magnetic fly ash, shows a huge potential for future developments with practical relevance. E.g. making use of magnetite, maghemite or other minerals as well as electrochemically or magnetically activated polymer developments. Thixotropy and passive stiffening control can also be further examined, both numerically as experimentally, given the provided quantification procedure. However, for future thixotropy studies, use of actual cement is advised to avoid exceeding the behavioural context. Future experimental and numerical investigations with respect to pumping re-initiation are relevant for practice.Beton is wereldwijd het meest gebruikte bouwmateriaal. Dit is mede te danken aan voordelen zoals een ongelimiteerde vorm, de beschikbaarheid en voordelige kost. Innovaties in de betontechnologie verbeterden onder andere de eindprestatie, kwaliteit, kwantiteit, praktische uitvoerbaarheid en algemeen mogelijke realisaties. Hoewel de introductie van zelfverdichtend beton in de jaren 80 veelbelovend was, werd het ondanks haar vele voordelen niet veelvuldig toegepast in de praktijk. Één van de voornaamste redenen hiervoor is dat tot op heden de praktische uitvoerbaarheid, kwaliteit en finaliteit passief afhankelijk zijn van de verse toestand of reologische eigenschappen. Eens het beton gemengd en naar de werf getransporteerd is, hangt de prestatie en uitvoerbaarheid passief af van de stand der zaken en is er weinig ruimte voor aanpassingen. Desondanks ontstaan er problemen door de tegenstrijdige eisen in reologie gedurende verschillende uitvoeringsfases. Namelijk, gedurende transport, verpomping, bekisting of extrusie dient het beton zeer vloeibaar te zijn, terwijl het direct na de bekisting, 3D printing, shotcreting etc. zo stijf mogelijk dient te zijn of zo snel mogelijk dient te verharden. Om problemen door het passieve karakter van beton te overkomen, om nadelen geassocieerd met zelfverdichtend beton te verhelpen en om een nieuwe innovatie te voorzien voor de betontechnologie met een verbetering op verschillende aspecten, werd de ontwikkeling van Actieve Reologie- en/of StijfheidsControle (ARC & ASC) gelanceerd in het SmartCast-project. Actieve reologiecontrole kan opgevat worden als een concept van actief aansturen van slimme bestanddelen in (cementachtige) mengsels via een extern signaal --- dat verschillende vormen kan aannemen zoals (electro)magnetische signalen --- waarna de verse toestand of reologie wijzigt op een omkeerbare of onomkeerbare manier. ASC & ARC zou de tegenstrijdige vereisten in reologie van cementachtige toepassingen kunnen verhelpen. Gezien het innovatieve karakter zijn ASC en ARC onbekend terein in betontechnologisch onderzoek. Ondanks veelzijdig onderzoek in de betontechnologie, is reologisch gedrag van beton nog niet volledig verklaard. Dit geldt in het bijzonder voor tijdsafhankelijk reologisch gedrag van beton en meer algemeen van cementachtige suspensies. Concreet worden er twee tijdsafhankelijke effecten opgevat, zijnde thixotropie wat een omkeerbaar verschijnsel is en hydratatie wat een onomkeerbaar verschijnsel is. Mede dankzij de complexiteit in functie van de tijd, wordt tijdsafhankelijkheid meestal niet beschouwd, ondanks het toch een significante impact kan hebben op het uitvoeringsproces en de finaliteit. In plaats van uitvoerige, kostelijke, onpraktische proeven uit te voeren, kunnen numerieke simulaties aangewend worden om reologisch gedrag te onderzoeken in verschillende toepassingen of om zelfs een basis te vormen voor het ontwerp van mengsels. De nodige reologische input voor numerieke simulties kan voorzien worden met behulp van karakterisatietoestellen, gekend als reometers. Aangezien er desondanks veel discrepanties en verschillen bestaan tussen reometers, is de reologische input onderworpen aan bijkomende onzekerheid naast de geschiktheid van constitutieve reologische betrekkingen. Bovendien zijn er ook gebreken in huidige modelleringstechnieken, zoals het ontbreken van tijdsafhankelijk gedrag, maar belangrijker ASC & ARC. Om inzicht in ASC & ARC en potentiële toepassingen te verwerven, kunnen numerieke modellen niet achterblijven. Als onderdeel van ASC & ARC in de ontwikkeling van het SmartCast-project, tracht deze doctoraatsthesis de lacunes te overbruggen met betrekking tot tijdsafhankelijk gedrag gericht op ASC & ARC, karakterisatieprocedures te voorzien en hun bekwaambaarheid te evalueren. Dit onderzoek is gericht op het verwerven van fundamenteel inzicht in het stromingsgedrag van cementachtige materialen en hoe het te karakteriseren en modelleren op een bekwame manier, onafhankelijk van de tijd, tijdsafhankelijk (thixotroop) en gericht op ASC & ARC. De hoofdhypothese die onderzocht werd is of het mogelijk is en hoe accuraat het is om ASC & ARC op te vatten aan de hand van een kinematische tijdsevolutie van een inwendige structuur, gedreven door een structuuropbouwende en -afbrekende term. Noch het volledig spectrum aan cementachtig gedrag werd onderzocht, noch het volledig spectrum aan potentieel mogelijke ASC & ARC. Passieve en Actieve StijfheidsControle (PSC & ASC) van cementachtige stroming in leidingen werd onderzocht vanuit een fundamenteel standpunt, omdat verpompen een onontbeerlijke rol speelt in de uitvoering van beton of cementachtige mengsels. Pastamengsels vormen het uitgangspunt in onderzoek naar stroming in leidingen. Dit komt omdat indien pasta accuraat beschreven en gemodelleerd kan worden het eenvoudig uitbreidbaar is naar meer gecompliceerde stroming met o.a. afschuivingsgeïnduceerde deeltjesmigratie. Daarom kon fundamenteel en wijd toepasbare kennis verworven worden in vooruitzicht van ARC. Bovendien zijn meer gecompliceerde stromingen zelf nog pril in onderzoek. Enerzijds werd de bekwaamheid en accuraatheid van constitutive reologische betrekkingen en hun karakterisatie geëvalueeerd. Anderzijds werd de toepasbaarheid en accuraatheid van numerieke modellen geëvalueerd, specifiek omdat numerieke simulaties afhankelijk zijn van reologische input, welke zelf onzekerheden bevat. De evaluatie van een numeriek werk is daarom een terugkoppelingslus tussen reologische input en numerieke output. De numerieke techniek werd daarom geëvalueerd op basis van drie aspecten met geleidelijk toenemende complexiteit. Eerst werd een analyse uitgevoerd op basis van gedrag onafhankelijk van de tijd via zogenoemd Bingham-gedrag, daarna inclusief thixotroop gedrag als voorloper van ASC & ARC en tot slot voor actieve stijfheidscontrole. Praktisch werd het Bingham-gedrag bekomen door mengsels van kalksteenmeel, het thixotroop gedrag door toevoeging van nano-klei en slim gedrag voor ASC & ARC door cementpastas met magneto-reologische vliegas. In dit opzicht werd een modulair, experimenteel pompcircuit ontworpen, genaamd het "Smal PompSysteem" (SPS) en werd een numeriek model ontwikkel op basis van OpenFOAM, waarin tijdsafhankelijk gedrag alsook verschillende aspecten van ASC & ARC werden geïmplementeerd. Verder werden ook transparante, gebruiksvriendelijke reologische karakterisatieprocedures ontwikkeld om cementachtig gedrag te beschrijven onafhankelijk van de tijd (door het Bingham model), met thixotropie (door het vereenvoudigd model van Roussel) en met actieve stijfheidscontrole (door een vooropgesteld magneto-structuratie model). Op basis van pompproeven en reologische karakterisaties werden numerieke simulaties uitgevoerd met een eindige volume methode. Hoewel de catalogus aan constitutieve gedragsmodellen veelzijdig en behoorlijk gecompliceerd is, kan gesteld worden dat reologische eigenschappen gekwantificeerd kunnen worden, en afhankelijk van de context, volstaat het om

Thixotropic structural build-up of cement-based materials : a state-of-the-art review

Thixotropic structural build-up is an intrinsic property of fresh cementitious materials, playing a significant role in pumpability, stability and formwork filling of concrete, as well as buildability in 3D printing. This paper presents a critical review on the thixotropic structural build-up of cement-based materials from the viewpoints of origins, evaluation methods, influencing factors and applications. Recent research indicates that the origin of structural build-up of cementitious materials is a combining result of colloidal interactions and chemical hydration, with three stages of colloidal network percolation, rigid percolation and rigidification. The evolution of static yield stress and storage modulus can be used to evaluate the structural build-up of fresh cementitious materials, each describing different aspects. Increasing particle volume fraction, decreasing surface coverage and reducing maximum packing fraction exhibit a positive effect on the increase in the structural build-up. Based on the relationships with performance parameters, the thixotropic structural build-up can be used to predict formwork casting, numerical simulations and 3D concrete printing. Furthermore, on-demand stiffening can be obtained by magneto-controlled structural build-up of cementitious materials containing magnetizable particles