Browsing by Author "Kruger, Pienaar Jacques"
Now showing 1 - 1 of 1
Results Per Page
Sort Options
- ItemRheo-mechanics modelling of 3D concrete printing constructability(Stellenbosch : Stellenbosch University, 2019-12) Kruger, Pienaar Jacques; Van Zijl, P.A.G Gideon; Zeranka, Stephan; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Three industrial revolutions have occurred in the last 250 years that resulted in increased productivity for most economic sectors. This generally improved the standard of living for all individuals. Recent research has however found that the productivity of the USA’s construction sector regressed at 0.6 % per year over the last 55 years. This indicates that the construction industry has not yet experienced the benefits of industrialisation. The current industrial revolution, commonly referred to as Industry 4.0, presents technology such as 3D printing. Applying this technology in the construction sector yields 3D printing of concrete (3DPC), or digital construction, that holds tremendous potential for enhanced productivity. Early estimates indicate possible construction time savings of 50 % and waste savings of 30 %. Additionally, the realisation of geometrically-complex elements is possible without the need for formwork. Although a promising technology, it remains in the early stages of commercialisation and presents many challenges before mass adoption thereof. No material characterisation test currently exists that specifically appertains to 3DPC. Fresh state mechanical tests are mostly performed; however, they provide insignificant information on the appropriateness of a material for 3DPC. This process requires a material to be easily transported via pumping, but then also to possess sufficient strength after extrusion to support the weight of subsequent filament layers. The latter is commonly referred to as buildability in 3DPC terminology. Furthermore, limited constructability design guidelines are currently available; consequently, elements are typically printed at randomly chosen speeds and filament layer heights with the hope of a successful outcome i.e. the element does not collapse whilst being printed. The main aim of this research is thus to develop practicable analytical models based on rheological material properties that collectively contribute towards constructability design guidelines for 3DPC. This research initially presents the design and manufacture of an industrial-grade gantry type 3D concrete printer with a build volume of roughly 1 m3. The development of a high-performance, 3D printable, thixotropic concrete via the Fuller Thompson theory is explained. Progression and mastering of basic 3DPC technology is demonstrated by means of pictures from initial 3D prints conducted at Stellenbosch University to the latest X-project. Thereafter, a bi-linear thixotropy model that specifically appertains to 3DPC is developed. A current thixotropy model that accounts for structuration (Athix) is extended to account for re-flocculation after agitation (Rthix), which is a physical process. The material characterisation is solely conducted by the use of a rheometer. In addition to presenting insight into a material’s thixotropy behaviour, the bi-linear static yield shear stress evolution curve depicts the strength gain of a material after it has been extruded from the nozzle. The following chapter presents the development of a filament shape retention model that is based on the bi-linear thixotropy model. The model is simple and practicable, while numerical validation via finite element analysis depicts the conservatism of the model. Thereafter, an analytical buildability model is developed which is also based on the bi-linear thixotropy model. The model only accounts for material failure in the form of plastic yielding. In addition, the model accounts for various filament aspect ratios, which influence the apparent compressive strength of a material. The model is verified by experimental testing, yielding an 8.33 % under prediction of the total number of filament layers before failure. Finally, the three models are combined to yield a constructability design model for 3DPC. The model predicts the optimum print parameter combination, i.e. filament layer height and print speed, that successfully yields the entire specified print object in the least amount of time. In addition, a statistical design model is presented by incorporating material partial factors to reduce the model’s probability of failure to 10 %. In summary, this research contributes a statistically safe and time-optimised constructability design model for the 3DPC industry to facilitate commercialisation of the technology.