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.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:85519 |
| Date | 24 May 2023 |
| Creators | Ivanova, Irina |
| Contributors | Mechtcherine, Viktor, Khayat, Kamal H., Lowke, Dirk, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | urn:nbn:de:bsz:14-qucosa-234594, qucosa:30907, info:eu-repo/grantAgreement/Deutsche Forschungsgemeinschaft/SPP 2005 Priority Program “Opus Fluidum Futurum – Rheology of reactive, multiscale, multiphase construction materials”/387152958/ |
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