Self-compacting concrete (SCC) offers rational and fast casting process since it merely has to be poured, or pumped, into the formwork without any compaction work needed. But this can be at the cost of high form pressure. However, reported results show that SCC can act thixotropically, i.e., build up a structure at rest, and this can reduce the form pressure considerably. Thus, in order to utilise the favourable possibilities to increase effectiveness without risking form collapses, the need arises for deeper and broader understanding of the mechanisms behind this thixotropic behaviour. Methodologies have been developed for the characterisation and measurement of the structural build-up at rest, both for the fluid (micro mortar) phase and the concrete itself. Hypotheses state that thixotropic mechanisms originate within the colloidal domain and, thus, motivate studies on the fluid phase comprising this domain. The stress-strain methodology is based on the hypothesis stating that the magnitude of the structure is represented by the maximum elastic stress the fresh material can withstand before the structure breaks. An instrumented steel tube is used to simulate various casting heights and rates. Results show that both micro mortar and SCC are thixotropic and this behaviour is influenced by every measure taken influencing the interparticle colloidal forces. The time-dependent structural build-up of SCC is a function of an irreversible structure (slump-loss) and a reversible, thixotropic structure. There is apparently a threshold value of the structural build-up necessary to reach before obtaining any significant form pressure reduction. Housing SCC´s, with W/C = 0.58, show low degree of structural build-up and pressure decrease while civil engineering SCC´s can show the opposite, but this often at the cost of slump-loss. Recommendations are presented and for the nearest future, suggesting a conservatism regarding design of formwork systems when SCC is used. If the behaviour of a SCC is known it should be used to optimise the formwork. If not, calculating with hydrostatic pressure should be done or the knowledge missing should be gained by using this methodology. A third option is given and this is to monitor the form pressure in real time using sensors. / QC 20100812

Abstract : The structural build-up of fresh cement-based materials has a great impact on their structural performance after casting. Accordingly, the mixture design should be tailored to adapt the kinetics of build-up given the application on hand. The rate of structural build-up of cement-based suspensions at rest is a complex phenomenon affected by both physical and chemical structuration processes. The structuration kinetics are strongly dependent on the mixture’s composition, testing parameters, as well as the shear history. Accurate measurements of build-up rely on the efficiency of the applied pre-shear regime to achieve an initial well-dispersed state as well as the applied stress during the liquid-solid transition. Studying the physical and chemical mechanisms of build-up of cement suspensions at rest can enhance the fundamental understanding of this phenomenon. This can, therefore, allow a better control of the rheological and time-dependent properties of cement-based materials. The research focused on the use of dynamic rheology in investigating the kinetics of structural build-up of fresh cement pastes. The research program was conducted in three different phases. The first phase was devoted to evaluating the dispersing efficiency of various disruptive shear techniques. The investigated shearing profiles included rotational, oscillatory, and combination of both. The initial and final states of suspension’s structure, before and after disruption, were determined by applying a small-amplitude oscillatory shear (SAOS). The difference between the viscoelastic values before and after disruption was used to express the degree of dispersion. An efficient technique to disperse concentrated cement suspensions was developed. The second phase aimed to establish a rheometric approach to dissociate and monitor the individual physical and chemical mechanisms of build-up of cement paste. In this regard, the non-destructive dynamic rheometry was used to investigate the evolutions of both storage modulus and phase angle of inert calcium carbonate and cement suspensions. Two independent build-up indices were proposed. The structural build-up of various cement suspensions made with different cement contents, silica fume replacement percentages, and high-range water reducer dosages was evaluated using the proposed indices. These indices were then compared to the well-known thixotropic index (Athix.). Furthermore, the proposed indices were correlated to the decay in lateral pressure determined for various cement pastes cast in a pressure column. The proposed pre-shearing protocol and build-up indices (phases 1 and 2) were then used to investigate the effect of mixture’s parameters on the kinetics of structural build-up in phase 3. The investigated mixture’s parameters included cement content and fineness, alkali sulfate content, and temperature of cement suspension. Zeta potential, calorimetric, spectrometric measurements were performed to explore the corresponding microstructural changes in cement suspensions, such as inter-particle cohesion, rate of Brownian flocculation, and nucleation rate. A model linking the build-up indices and the microstructural characteristics was developed to predict the build-up behaviour of cement-based suspensions The obtained results showed that oscillatory shear may have a greater effect on dispersing concentrated cement suspension than the rotational shear. Furthermore, the increase in induced shear strain was found to enhance the breakdown of suspension’s structure until a critical point, after which thickening effects dominate. An effective dispersing method is then proposed. This consists of applying a rotational shear around the transitional value between the linear and non-linear variations of the apparent viscosity with shear rate, followed by an oscillatory shear at the crossover shear strain and high angular frequency of 100 rad/s. Investigating the evolutions of viscoelastic properties of inert calcite-based and cement suspensions and allowed establishing two independent build-up indices. The first one (the percolation time) can represent the rest time needed to form the elastic network. On the other hand, the second one (rigidification rate) can describe the increase in stress-bearing capacity of formed network due to cement hydration. In addition, results showed that combining the percolation time and the rigidification rate can provide deeper insight into the structuration process of cement suspensions. Furthermore, these indices were found to be well-correlated to the decay in the lateral pressure of cement suspensions. The variations of proposed build-up indices with mixture’s parameters showed that the percolation time is most likely controlled by the frequency of Brownian collisions, distance between dispersed particles, and intensity of cohesion between cement particles. On the other hand, a higher rigidification rate can be secured by increasing the number of contact points per unit volume of paste, nucleation rate of cement hydrates, and intensity of inter-particle cohesion. / Résumé : La structuration des matériaux cimentaires a un grand impact sur leur performance mécanique après le coulage. Par conséquent, la formulation des mélanges devrait être conçue afin d’adapter la cinétique de structuration conformément à l’application considérée. La structuration des suspensions à base des matériaux cimentaires est un phénomène complexe affecté par les deux processus physiques et chimiques. La cinétique de structuration est fortement liée à la formulation des mélanges, des paramètres d’essai et de l’historique du cisaillement. Une meilleure évaluation de ce phénomène est fonction du pré-cisaillement appliqué, afin d’obtenir un état complètement dispersé ainsi que la contrainte appliquée lors de la transition de l’état solide-liquide de la suspension. L'étude des mécanismes physiques et chimiques de la structuration des suspensions à base de ciment peut améliorer la compréhension fondamentale de ce phénomène. Ceci permettra un meilleur contrôle des propriétés rhéologiques en fonction du temps des matériaux cimentaires. Cette recherche porte sur l'utilisation de la rhéologie dynamique pour étudier la cinétique de structuration des pâtes de ciment à l’état frais. Le programme de recherche a été mené en trois phases différentes et complémentaires. La première phase a été consacrée à l'évaluation de l'efficacité de diverses techniques de cisaillement. Les profils de cisaillement étudiés inclus la rotation et l’oscillation et la combinaison des deux modes de cisaillement. Les états initiaux et finaux de la structure de suspension avant et après la dispersion ont été déterminées en appliquant un cisaillement oscillatoire de faible amplitude (SAOS). La différence entre les valeurs viscoélastiques avant et après la dispersion a été utilisé pour exprimer le degré de dispersion. Une technique efficace pour disperser les suspensions concentrées de ciment a été développée. La deuxième phase visait à établir une approche rhéologique afin de dissocier les mécanismes physiques et chimiques individuels de la structuration de pâte de ciment. En effet, la rhéomètrie dynamique non destructif a été utilisé pour étudier à la fois l’évolution du module élastique et de l'angle de phase des suspensions inertes à base de carbonates de calcium et des suspensions de ciment. Deux indices de structurations indépendantes ont été proposés. La structuration de différentes suspensions à base de ciment réalisées avec différentes teneurs en ciment, pourcentages de remplacement de fumée de silice, et dosages de superplastifiant a été évaluée en utilisant les indices proposés. Ces indices ont ensuite été comparés à l'indice de thixotropie bien connu (Athix.). En outre, les indices proposés ont été corrélés à la pression latérale déterminée pour différentes pâtes de ciment coulées dans une colonne sous pression de hauteur 1 m. Le protocole de pré-cisaillement et les indices de structuration proposés (Phases 1 et 2) ont ensuite été utilisés pour étudier l'effet des paramètres de formulation sur la cinétique de structuration dans la Phase 3. Les paramètres étudiés inclus ; la teneur et la finesse du ciment, la teneur en sulfate alcalin, et la température de la suspension de ciment. Le potentiel zêta, les mesures calorimétriques et spectrométriques ont été également réalisées afin d’étudier les changements microstructuraux correspondants dans les suspensions de ciment, tels que la cohésion entre les particules, le taux de floculation Brownien, et le taux de nucléation. Un modèle reliant les indices de structuration et les caractéristiques microstructurales a été développé afin de prédire le comportement de structuration des suspensions des matériaux cimentaires. Les résultats obtenus montrent que le cisaillement oscillatoire est plus efficace sur la dispersion de suspension de ciment concentré que le cisaillement rotationnel. Outre l'augmentation de la contrainte, le cisaillement induit permet d’améliorer la dispersion de la structure de suspension jusqu'à un point critique. Au-delà de cette valeur critique les effets épaississants dominent. Une méthode efficace de dispersion est ensuite proposée. Cette méthode consiste à appliquer un cisaillement rotationnel autour de la valeur de transition entre les variations linéaires et non linéaires de la viscosité apparente avec le taux de cisaillement, suivie d'un cisaillement oscillatoire à la contrainte de cisaillement croisée et la fréquence angulaire élevée de 100 rad/s. Analyser l’évolution des propriétés viscoélastiques des suspensions inertes de carbonates de calcium et de ciment ont permis de développer deux indices pour quantifier la structuration des suspensions de ciment. Le premier indice (le temps de percolation) représente le temps de repos nécessaire pour former le réseau élastique. Le deuxième indice (taux de rigidification) décrit la rigidification de la structure, i.e. sa capacité à supporter des contraintes. D’autre part, les résultats montrent que la combinaison du temps de percolation et du taux de rigidification permet de mieux comprendre le processus de structuration des suspensions de ciment. De ce fait, ces indices se sont avérés être bien corrélés avec la pression latérale de coffrage des suspensions à base de ciment. Les variations des indices de structuration proposés avec les paramètres de mélange ont montré que le temps de percolation est très probablement contrôlé par la fréquence des collisions Browniennes, la distance entre les particules dispersées, et l'intensité de la cohésion entre les particules de ciment. D'autre part, un taux plus élevé de rigidification peut être assuré en augmentant le nombre de points de contact par unité de volume de la pâte, le taux de nucléation d’hydrates de ciment et l'intensité de la cohésion inter-particulaire.

Nowadays, construction industry is rapidly moving towards digitalization and automation that should enable increased rates and efficiency of construction processes, as well as higher possibilities for customization and architectural freedom. Among all technologies under development, digital fabrication with concrete by means of layered extrusion appears to be one of the most promising for purposes of fast mass housing construction. It enables formwork-free production of structures via layer-by-layer concrete printing. Freedom from formwork potentially makes the construction process more cost- and time-saving, but poses multiple challenges to mix design and test methods, especially in terms of concrete rheology. A special focus must be put on the structural build-up of concrete at rest, which is related to its buildability, i.e. capacity of the material to retain the shape of the extruded layers under their own weight and the weight of the subsequently placed layers. This research investigates into the structural build-up of cementitious materials, i.e. evolution of their strength and deformation properties over time at rest, and includes development and refinement of methodology to assess the structural build-up, as well as its modelling and prediction. With respect to methodology, major attention was directed to the constant rotational velocity (CRV) test used for evaluation of the static yield stress development, and rationalization of its application under field conditions. Based on a large amount of experiments performed with two rheometers of different design, characteristic curves and points describing patterns in behaviour of cementitious materials during a CRV test were established. The experimental study also dealt with assessing the effects of alterations in main elements of the CRV test protocol, such as test approach (single- versus multi-batch), pre-shear regime, applied CRV, on the test results. Possible errors in CRV tests were addressed and methods to improve the procedures of testing and data evaluation were suggested. In particular, the single-batch approach was enhanced by implementation of the developed breaking criterion, a concept of zero measurement for non-pre-sheared samples was introduced, and a method for simplified evaluation of elasticity by a single-head rheometer was proposed. General methodological recommendations on the design of a CRV test protocol were formulated. The results are applicable for various cementitious materials and not limited to concretes for layered extrusion. Furthermore, test methods for assessing the structural build-up of printable concretes were studied in terms of their applicability under field conditions, potential for automation, descriptiveness of obtained data and efficiency in predicting the buildability of printed concrete structures. The methods under investigation included CRV test, unconfined uniaxial compression test (UUCT), fast penetration test and newly proposed confined uniaxial compression test (CUCT); all tests were performed on extruded samples of eight printable concrete mixtures with various compositions and rheological properties. The corresponding results were juxtaposed in order to establish correlations between the data and compared to the results of buildability tests, in which hollow cylindrical structures were produced using a laboratory-scale 3D printer. The developed methodology was further used to conduct an in-depth investigation into the influence of aggregates on the structural build-up of ordinary concrete. In the experimental program, the binder composition was kept constant while the aggregate was varied in terms of the volume fraction and the surface area (per unit volume of concrete); the main focus was put on compositions with elevated aggregate content (45−55 % by volume). A mechanism lying behind the effects of the aggregate properties on the structural build-up of concrete was discovered by studying the structure of constitutive paste in concrete. Aggregate-induced inhomogeneity of constitutive paste allowed to introduce a three-component model of fresh concrete. Furthermore, to find models capable of predicting the parameters of structural build-up of concrete, i.e. static yield stress and structuration rate, concrete was viewed as a suspension of aggregate particles in suspending medium. Three approaches were employed to define the correspondent components. Suspending medium was represented by plain cement paste, screened cement paste and fine mortar considered as a part of concrete comprising particles below 0.5 mm; a substantiation for such a definition was provided. Applicability and limitations of the models based on all three approaches were compared. The modelling approach was further extended to printable concretes with nearly identical aggregate compositions, but different properties of paste. Opportunities and challenges in modelling the structural build-up of printable concrete, including the problem of material dependency of the models and the relevancy of fitting coefficients, were discussed. A modified Chateau-Ovarlez-Trung model based on the definition of suspending medium as fine mortar was acknowledged as best suited to describe the structural build-up of both ordinary and printable concrete.

Tato diplomová práce se zabývá aktuálními poznatky, společně s optimalizací oscilačních měření pomocí hybridního reometru, ale hlavně pozorováním změn reologických parametrů v čase past mleté granulované vysokopecní strusky aktivované různými aktivačními roztoky s koncentrací alkálií M: 4 and 7.5 moldm. Výsledky byly pro diskuzi podpořeny výstupy z Vicatovy metody a izotermické kalorimetrie. Nejprve bylo zjištěno, že limit lineární viskoelastické oblasti se zmenšuje v čase. Dále bylo pozorováno, že v time sweep testech vykazovala struska aktivovaná křemičitany nejnižší hodnoty komplexního modulu a struska aktivovaná hydroxidy o koncentraci 4 moldm nejvyšší hodnoty a ostatní pasty vykazovaly hodnoty mezi těmito dvěma extrémy a měly mezi sebou porovnatelné hodnoty. Podobný trend byl pozorován u vývoje kritické meze oscilačního napětí, získaného z amplitude sweep testů (přerušovaně a nepřerušovaně). Oba výše zmíněné parametry, komplexní modul a mez napětí, se vyvíjely v čase. Výsledky všech testů jasně ukázaly, že povrchová chemie hraje roli ve výsledném reologickém chování. Odsud, povaha a koncentrace aktivátoru ovlivňují reologické vlastnosti skrze efekty Na a K iontů, stejně tak skrze koncentraci, která má dopad na elektickou dvojnou vrstvu. Reologické chování může být také ovlivněno dalšími faktory, např. viskozitou aktivačního roztoku a celkovou kinetikou hydratačního procesu.