A thixotropy model for fresh fluid concretes: Theory, validation and applications

Rheological requirements for printable concretes

Additive manufacturing and digital fabrication bring new horizons to concrete and cement-based material construction. 3D printing inspired construction techniques that have recently been developed at laboratory scale for cement-based materials. This study aims to investigate the role of the structural build-up properties of cement-based materials in such a layer by layer construction technique. As construction progresses, the cement-based materials become harder with time. The mechanical strength of the cement-based materials must be sufficient to sustain the weight of the layers subsequently deposited. It follows that the comparison of the mechanical strength, which evolves with time (i.e. structural build-up), with the loading due to layers subsequently deposited, can be expected to provide the optimal rate of layer by layer construction. A theoretical framework has been developed to propose a method of optimization of the building rate, which is experimentally validated in a layer-wise built column.

https://link.springer.com/content/pdf/10.1617%2Fs11527-015-0571-0.pdf

Structural built-up of cement-based materials used for 3D-printing extrusion techniques

The objective of this study is to link the molecular structure of polycarboxylate-ether-type superplasticizers with the performance of cementitious systems in order to develop new products with enhanced properties, e.g. improved water reduction with a wide range of cements or a reduced retardation of cement hydration. Different experimental superplasticizers have been synthesized varying length and density of the polyether chains as well as the molecular weight of the polymer. The influence of these polymers on the properties of cement pastes and mortars was determined using various characterization methods like mortar flow, rheological and calorimetric measurements, adsorption measurements and mortar compressive strength. Characteristic connections between molecular structure of the polycarboxylate-type water reducers, adsorption behaviour, workability and retarding effect have been determined allowing the synthesis of new superplasticizers with improved performance.

Effects of the molecular architecture of comb-shaped superplasticizers on their performance in cementitious systems

Digital fabrication has been termed the “third industrial revolution” in recent years, and promises to revolutionize the construction industry with the potential of freeform architecture, less material waste, reduced construction costs, and increased worker safety Digital fabrication techniques and cementitious materials have only intersected in a significant way within recent years In this letter, we review the methods of digital fabrication with concrete, including 3D printing, under the encompassing term “digital concrete”, identifying major challenges for concrete technology within this field We additionally provide an analysis of layered extrusion, the most popular digital fabrication technique in concrete technology, identifying the importance of hydration control in its implementation

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Digital Concrete: Opportunities and Challenges

Optimizing fracture spacing and sequencing in horizontal well fracturing

Optimizing Fracture Spacing and Sequencing in Horizontal-Well Fracturing

From ordinary rhelogy concrete to self compacting concrete: A transition between frictional and hydrodynamic interactions

Rheological behavior of cement pastes from MRI velocimetry

Various thixotropy models have been proposed in literature but in general their predictions have not been compared to the flow characteristics of a given material under a wide range of conditions. Here we compare the predictions of a model not only under different flow conditions but for the first time with local and macroscopic flow properties. In this aim we used a very simple model which appeared capable to predict qualitatively the “viscosity bifurcation” of thixotropic pastes [J. Rheol. 46 (2002) 573]. We first study in dimensionless terms the detailed theoretical predictions of this model under homogeneous and heterogeneous, controlled stress or controlled velocity conditions. The qualitative predictions of the model are in perfect agreement with experimental observations. After having fitted to data the three basic parameters, this model appears capable to predict quantitatively rather well the steady state and transient (decrease of rotation velocity) velocity profiles measured by NMR for a bentonite suspension in a coaxial cylinder geometry. Then the predictions of this model with the parameters thus determined are compared with (macroscopic) flow properties, i.e. rotation velocity versus time, under controlled stress tests. The model surprisingly appears capable to predict characteristics of transient flows even over times much shorter than in the NMR rheometrical tests, but the (linear) restructuration term appears incapable to predict the rapid viscosity increase for the fluid at rest or evolving towards stoppage. This work at the same time shows the global consistency of our simple thixotropy model with regards to flow characteristics under various conditions and the difficulty to predict detailed flow characteristics under strongly different conditions or times of observations.

Nicolas Roussel

Papers

Optimizing fracture spacing and sequencing in horizontal well fracturing

Optimizing Fracture Spacing and Sequencing in Horizontal-Well Fracturing

From ordinary rhelogy concrete to self compacting concrete: A transition between frictional and hydrodynamic interactions

Rheological behavior of cement pastes from MRI velocimetry

Thixotropy modelling at local and macroscopic scales