Steel fibre-reinforced concrete: multi-scale characterisation towards numerical modelling

Zeranka, Stephan (2017-12)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: In recent years, Steel Fibre-Reinforced Concrete (SFRC) applications have increased in quantity and variety, due to its potential to partially or totally replace conventional reinforcement (rebar or welded mesh). Methods for material characterisation, constitutive modelling and design must therefore be improved in order to facilitate the demand for greater structural application of SFRC. A large database of experimental data and empirical models from the analysis of scaled or full-scale shear-critical SFRC structural beams has accumulated. It is therefore concluded from a survey that material-level characterisation and constitutive modelling may be more beneficial in facilitating the demand for greater structural application. Consequently, the primary mechanisms governing the fundamental behaviour of SFRC need to be characterised in order to produce a direct definition of the material’s constitutive model. Far less work has been done on the direct shear response of fibre reinforced concretes. Even fewer investigations attempt to link the Micro-scale (i.e. the transverse pull-out of steel fibres) to the Meso-scale (i.e. at the scale of a single crack) for Mode II fracture. At the time of publication of Soetens & Matthys (2012), only one other study, Lee & Foster (2006) was known that applies this method for investigating the Mode II fracture of SFRC. Analogous to existing numerical tools for reinforced concrete (RC) membrane elements, research towards a constitutive material model and numerical procedure for the analysis of SFRC membrane elements is considered to be essential. A direct and rational approach to model the generic material response also has the potential to allow for tailoring and optimisation in material and structural design. Three scales of observation or analysis are defined in this dissertation, namely the Micro-scale (single fibre level), Meso-scale (single crack level) and Macro-scale (structural level). Here attention is given only to the Micro and Meso-scale. In order to contribute to multi-scale characterisation towards constitutive and numerical modelling, the outcomes of this dissertation in sequence are: Adapt a composite design procedure and develop a SFRC; Classify the composite in terms of standard performance indicators and testing procedures; Design, fabricate and execute experimental tests to characterise the Mode I and Mode II fracture at the Micro and Meso-scale of observation; Develop a material model and verify it numerically: This requires the implementation of an analytical formulation of the material model into a numerical procedure. The material model is calibrated with the experimental data and verified via a Finite Element (FE) representation of the experimental Meso-scale test. Finally, an empirical model is also developed which reconciles the fibre component with the Mode II Meso-scale response. Two useful technologies are utilised to assist in material characterisation, Computed Tomography (CT-scan) and Digital Image Correlation (DIC). The CT-scanning facility provides valuable insight into the fibre distribution and the ability to analyse and quantify the fibre orientation distribution is a powerful tool. The non-contact measurement method (Aramis DIC) proved invaluable in determining the specimen shear displacement and rotation. This dissertation provides insight into experimental design for the fundamental fracture modes of SFRC. A contribution is made to the limited literature available on the link between the single fibre transverse pull-out response and the composite Mode II fracture behaviour. The numerical and empirical models developed simulate the composite response well, given their relative simplicity and limited experimental data. The constitutive model and numerical procedure should aid in material design and provide a foundation for defining material laws.

AFRIKAANSE OPSOMMING: Staalvesel-bewapende beton (SVBB) het die potensiaal om tradisionele bewapening in beton gedeeltelik, of selfs totaal te vervang. Gevolglik het SVBB toepassings die afgelope jare toegeneem, nie net in hoeveelheid nie, maar ook in verskeidenheid. Om die aanvraag vir meer strukturele toepassings van SVBB te fasiliteer, moet metodes vir materiaalkarakterisering, konstitutiewe modellering en ontwerp verbeter word. Die aanvanklike fokus van hierdie verhandeling was die studie van volskaal en/of afgeskaalde bewapende SVBB balke onderworpe aan skuifdominante faling. ʼn Literatuurstudie is uitgevoer met die gevolgtrekking dat ʼn groot databasis van eksperimentele data en empiriese modelle versamel het oor die afgelope 40 tot 50 jaar. Enige nuwe bydrae tot hierdie navorsingsveld vereis die analise van meerdere vlakke van die strukturele probleem, deur middel van materiaalvlak karakterisering en konstitutiewe modellering. Gevolglik moet die hoof meganismes wat die fundamentele gedrag van SVBB oorheers gekarakteriseer word ten einde ʼn direkte definisie van die materiaal se konstitutiewe model te produseer. Minder aandag is gegee aan die direk skuif/Mode II fraktuur gedrag van vesel-bewapende beton (VBB). Nog minder ondersoeke poog om die Mikro-skaal (d.w.s. die dwars uittrek van enkele staalvesels uit ʼn sement matriks) te koppel met die Meso-skaal (d.w.s. die gedrag by ʼn enkele kraak in SVBB) vir Mode II fraktuur. So ver die outeur kon vasstel, is daar net twee vorige studies wat dit al ondersoek het, naamlik Soetens & Matthys (2012) en Lee & Foster (2006). Die modellering van SVBB as ʼn membraanelement, analoog aan bestaande numeriese metodes vir tradisionele bewapende beton, is gevolg. ʼn Direkte en rasionele benadering om die generiese materiaalgedrag te modelleer laat toe vir optimering van materiaal- en struktuur-ontwerp. Drie vlakke van ondersoek is gedefinieer in hierdie tesis, naamlik die Mikro-vlak (gedrag van ʼn enkel vesel), Meso-vlak (gedrag van ʼn enkel kraak in SVBB) en Makro-vlak (struktuur vlak). In hierdie verhandeling is aandag gevestig op die Mikro- en Meso-vlak. Ten einde by te dra tot multivlak karakterisering, en konstitutiewe en numeriese modellering, is die uitkomstes van hierdie tesis soos volg: Ontwikkel ‘n mengontwerp prosedure en ʼn SVBB; Klassifiseer die meng in terme van standaard gehalte-aanwysers en toetsprosedures; Ontwerp, vervaardig en voer eksperimentele toetse uit om die Mode I en Mode II fraktuur te karakteriseer vir beide die Mikro- en Meso-vlak; Skep ʼn materiaalmodel en verifieer dit numeries: Dit vereis die implementering van ʼn analitiese formulering van die materiaalmodel in ʼn numeriese prosedure. Die materiaalmodel is gekalibreer met die eksperimentele data en geverifieer deur middel van ʼn eindige element verteenwoordiging van die eksperimentele Meso-vlak toets. Uiteindelik is ʼn empiriese model ook geskep wat die veselkomponent vir Mode II fraktuur (gekarakteriseer deur die Mikro-vlak toetse) versoen met die Mode II Meso-vlak gedrag. Twee nuttige tegnologieë is gebruik vir materiaal karakterisering, naamlik rekenaar tomografie (Xstraal CT-skandering) en digitale beeldkorrelasie (DIC). Die CT-skandering fasiliteit maak dit moontlik om die vesel-verspreiding in ʼn proefstuk te kwantifiseer in terme van vesel-oriëntasie, aantal vesels in ʼn snit en gemiddelde vesel-verankeringslengte. Die Aramis (DIC) apparaat het dit moontlik gemaak om proefstuk-verplasings en deformasie te meet sonder montering van instrumentasie direk op die proefstuk. Dit het die meet van skuif-deformasie en rotasie aansienlik vergemaklik. Ten slotte gee die verhandeling insig in eksperimentele ontwerp vir die fundamentele fraktuur modes van SVBB. ʼn Bydrae is gemaak tot die beperkte literatuur beskikbaar vir die koppeling tussen enkel-vesel dwars uittrek gedrag (Mikro-vlak) en die Meso-vlak Mode II fraktuur. Die numeriese en empiriese modelle ontwikkel simuleer die materiaal gedrag goed, ten spyte van hul relatiewe eenvoud en beperkte eksperimentele data. Die konstitutiewe model en numeriese prosedure bied ʼn basis vir die definisie van materiaalwette en materiaal-ontwerp.

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