Experimental and computational mechanics for the constitutive modelling of extrusion-based 3D concrete printing

Van den Heever, Marchant (2021-12)

Thesis (PhD)--Stellenbosch University, 2021.

Thesis

ENGLISH ABSTRACT: The structural use of 3D concrete printing (3DCP) has globally gained traction owing to the host of benefits permitted by an industrialised manufacturing style approach to construction. However, due to the novelty of this technology, no standardised hardened-state mechanical characterisation or design specifications have been established. Moreover, limited research has been conducted on the non-linear hardened-state finite element (FE) analysis of 3DCP components. Therefore, this research proposes mechanical testing protocols, computational modelling strategies and advanced microstructural characterisation procedures to evaluate the fundamental failure mechanisms in 3D printed concrete (3DPC) to provide critical insights into the design and analysis of 3DCP components or structures. Novel hardened-state material characterisation procedures are proposed, which elucidate the anisotropic strength and deformation attributes of 3DCP components and provide the necessary material and model parameters for FE simulation. An analogy between the well-established computational modelling strategies for masonry structures and 3DCP is drawn. Two anisotropic non-linear simulation strategies are adapted and introduced for application in the 3DCP design space. Both techniques are novel within the context of 3DCP and provide accurate results of the experimentally attained capacity and cracking patterns of the members. The constitutive models of the proposed simulation strategies are studied and found to overestimate the shear capacity of interfacial regions due to the assumption of a Mohr-Coulomb failure criterion. To further advance the current knowledge basis, the microstructural morphology is comprehensively characterised via X-ray computed tomography and deemed a potential contributor to the reduced strength and stiffness portrayed by 3DCP specimens. Subsequently, it is demonstrated how the microstructure and orientation of the axes of the composite material influence the direction of crack propagation, validating that porosity content and pore geometric attributes have distinct but complementary effects on the mechanical capacity of 3DPC. It is then revealed why anisotropy is so prevalent in 3DCP and why the shear strength of interfacial regions is overestimated. Thereafter, it is divulged how to improve predictions of the non-linear shear constitutive behaviour computationally through a novel modified Mohr-Griffith yield criterion. Solid theoretical descriptions relate the microstructural morphology to damage mechanisms and the resulting anisotropic shear strength in mould-cast and 3DCP elements. Employing the enriched understanding of the mechanical performance of 3DPC's, potential remedies to alleviate the anisotropic response in 3DCP components are proposed. In essence, this research demonstrates how synergies between the experimental and computational mechanics and advanced microstructural characterisation techniques permit improved constitutive modelling for 3DCP. Finally, it is recommended how the knowledge gained from this dissertation can be utilised to take an incremental step towards the detailed design and analysis of 3DCP structures.

AFRIKAANSE OPSOMMING: Die strukturele gebruik van 3D-gedrukte beton (3DGB) het wêreldwyd belangstelling verwerf as gevolg van die vele voordele wat 'n geïndustrialiseerde vervaardigingsstylbenadering tot konstruksie bied. As gevolg van die nuutheid van hierdie tegnologie, is daar egter geen gestandaardiseerde meganiese karakterisering of ontwerpspesifikasies vasgestel nie. Daar is boonop geringe navorsing gedoen oor die nie-lineêre ontleding deur eindige element (EE)-analise van 3DGB komponente. Dus, stel hierdie navorsing meganiese toetsprotokolle, modelleringstrategieë en gevorderde mikrostruktuur karakteriseringsprosedures voor om die fundamentele falingsmeganismes in 3DGB te evalueer en om kritiese insigte te bied vir die ontwerp en analise van 3DGB komponente of -strukture. Nuwe prosedures vir die karakterisering van 3DGD komponente in die verharde toestand word voorgestel om die anisotropiese sterkte- en vervormingseienskappe van 3DGB toe te lig en om die nodige materiaal- en modelparameters vir EE-simulasie te bepaal. 'n Analogie tussen die gevestigde rekenaarmodelleringstrategieë vir messelwerk en 3DGB word getrek. Twee anisotropiese nie-lineêre simulasiestrategieë word aangepas en voorgestel vir toepassing in die 3DGB ontwerpruimte. Albei tegnieke is nuut binne die konteks van 3DGB en bied akkurate resultate van beide die komponente se kapasiteit en eksperimentele kraakpatrone. Die konstituerende modelle van die simulasiestrategieë is bestudeer en daar is bevind dat die aanname van 'n Mohr-Coulomb-falingskriterium die skuifweerstand van 3DGB se tussenlae oorskat. Om die huidige kennisbasis verder te bevorder, word die mikrostruktuur morfologie volledig gekarakteriseer deur middel van X-straaltomografie. Hiermee word daar bevestig dat die mikrostruktuur morfologie bydra tot die verminderde sterkte en styfheid tipies bevind in 3DGB. Daarbenewens word aangetoon hoe die mikrostruktuur en oriëntasie van die samestelling se material-asse die groeirigting van krake beïnvloed, wat bevestig dat die porositeit en porie-geometriese eienskappe unieke, maar aanvullende effekte op die meganiese gedrag van 3DGB het. Daarna word onthul waarom anisotropie so algemeen in 3DGB voorkom en waarom die skuifsterkte van 3DGB tussenlae oorskat word. Daarvolgens word voorgestel hoe om die nie-lineêre skuifkonstituerende gedrag berekeningsmatig beter te voorspel deur 'n nuwe, gewysigde Mohr-Griffith falingskriterium. Vaste teoretiese beskrywings verbind die mikrostrukturele morfologie aan die falingsmeganismes en gevolglike anisotropiese skuifsterkte in gegote en 3DGB elemente. Gebaseer op 'n fundamentele, dieper begrip van die meganiese optrede van 3DGB, word potensiële metodes om die anisotropiese gedrag in 3DGB komponente te verminder, voorgestel. In essensie demonstreer hierdie navorsing hoe sinergieë tussen die eksperimentele- en berekeningsmeganika en gevorderde mikrostruktuur karakteriseringstegnieke verbeterde konstituerende modellering vir 3DGB moontlik maak. Ten slotte, word daar aangetoon hoe die kennis wat uit hierdie proefskrif verwerf is, gebruik kan word om 'n inkrementele stap te neem na die gedetailleerde ontwerp en analise van 3DGB strukture.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/123688
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