Shear behaviour of irregular shaped concrete beams with and without steel fibres

Visser, Divan (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: In a rapidly evolving world, the use of concrete for the development of infrastructure is steadily increasing and this trend is set to continue in the future. In order to decrease the environmental impact of the development of infrastructure, it is crucial to reduce the carbon footprint of cement production by reducing the amount of concrete used in structures. Through the application of fabric formwork, geometrically optimised structural members can be cast comprising up to 40% less concrete compared to prismatic members with equivalent strength formed with traditional formwork (Orr et al., 2011). Such optimised members typically have irregular and varying cross-sections along its length, and codified methods that have been applied to these members provided inaccurate results (Orr et al., 2014). This investigation serves as a preliminary study on the shear behaviour of fabric formed beams (FFB) and concentrates only on uniform beams with irregular sections. This investigation strives to augment the knowledge on the shear behaviour of irregular shaped beams inspired by fabric formed beams. Fabric formwork was not used in this investigation but it is suggested that fabric formwork is used in succeeding studies. For this research, a steel fibre reinforced self-compacting concrete (SFR-SCC) mix was developed to cast irregular shaped beams (ISB). Steel fibres are used in the concrete to investigate their potential of replacing vertical reinforcement and self-compacting concrete (SCC) was used to avoid the challenge of external vibration in the complex moulds for proper compaction of the concrete. A total of 12 webbed beams, with four various irregular web shapes and fibres contents of either 0%, 0.6%, or 1%, were cast. Asymmetrical three-point bending tests were performed to investigate the shear behaviour of ISB and to investigate the effect of fibres on the shear behaviour of ISB. Furthermore, nine shear prediction models reported in literature were modified to accommodate the irregular sections of the beams and an analytical shear prediction model for ISB containing steel fibres is proposed. These models were evaluated on the accuracy of their predictions by comparing the predicted and experimental shear strengths. The study showed that the cross-sectional area of the web significantly influences the shear capacity. Larger web cross-sectional areas result in remarkable increases in shear strength. The widely used assumption of calculating the cross-sectional area contributing to shear capacity as the width of the narrowest part of the web (𝑏𝑤) multiplied by the effective depth (𝑑), results in over-conservative strength predictions and an inefficient use of concrete. It was found that the whole cross-sectional area of the web contributes to shear strength. Furthermore, fibres significantly enhance the shear capacity of ISB and an average increase in normalised shear stresses of 49% and 74% was obtained for fibre volumes of 0.6% and 1%, respectively, compared to beams containing no fibres. The comparison of the predicted values with the experimental values indicates that the proposed model predicts the shear capacity of ISB with reasonable accuracy with a mean shear strength ratio and standard deviation value of 0.95 and 0.10, respectively. However, a larger test database must be used to validate the proposed shear prediction model properly.

AFRIKAANSE OPSOMMING: In ‘n vinnig ontwikkelende wêreld is die gebruik van beton baie algemeen om infrastruktuur te ontwikkel en word daar voorspel dat beton al hoe meer gebruik sal word. Om infrastruktuurontwikkeling se impak op die omgewing te verminder, is dit van kardinale belang om die inhoud van beton wat in strukture gebruik word, te verminder. Met die gebruik van gespande materiaal vormwerke, kan die geometrie van elemente geoptimeer word om tot en met 40% minder beton te gebruik in vergelyking met prismatiese elemente van ekwivalente sterkte (Orr et al., 2011). Hierdie geoptimeerde elemente het onreëlmatige deursnitte wat varieër oor die lengte van die elemente. Bestaande metodes wat aangewend is op sulke balke het onakkurate resultate gelewer (Orr et al., 2014). In hierdie studie is die skuifgedrag van balke wat in gespande materiaal vormwerke gegiet word, ondersoek en fokus meer op uniforme balke met onreëlmatige deursnitte. Die doel van die studie is om inligting rakende skuifgedrag van hierdie balke te ondersoek. Gespande materiaal vormwerke is nie gebruik in hierdie studie nie, maar dit word aanbeveel om gespande materiaal vormwerke in toekomstige studies te gebruik. In hierdie navorsing is ‘n eie mengsel van selfkompakterende beton met staalvesels ontwikkel vir die balke met onreëlmatige deursnitte. Staalvesels is bygevoeg tot die beton om die moontlikheid te ondersoek om vertikale wapenstaal met hierdie vesels te vervang en selfkompakterende beton is gebruik om die eksterne vibrasie vir kompaksie in die komplekse bekisting uit te skakel. Daar is twaalf balke gebruik met vier verskillende webvorms met veselinhoud van 0%, 0.6% of 1%. Asimmetriese driepunt buigtoetse is uitgevoer om die skuifgedrag van balke met onreëlmatige deursnitte te ondersoek asook om die effek van vesels op die skuifgedrag te ondersoek. Daar is nege skuifmodele wat uit literatuur gekry is, aangepas om die onreëlmatige deursnitte van die balke in ag te neem. ‘n Analitiese skuifmodel vir balke met onreëlmatige deursnitte word ook voorgestel. Die akkuraatheid van hierdie modele is geëvalueer deur die voorspelde skuifsterktes te vergelyk met die eksperimentele skuifsterktes. Hierdie studie het getoon dat die deursnitarea van die web ‘n groot invloed op die skuifkapasiteit het. Groter webdeursnitareas het ‘n aansienlike verhoging in skuifkapasiteit tot gevolg. ‘n Algemene aanname is om die deursnitarea wat bydrae tot skuifkapasiteit te bereken as die kleinste breedte van die web (𝑏𝑤) vermenigvuldig met die effektiewe diepte (𝑑). Hierdie vereenvoudiging lei tot oorkonserwatiewe skuifkapasiteit voorspellings en dan ook oneffektiewe gebruik van beton. Dit is gevind dat die hele deursnit area van die web ‘n bydrae lewer tot die skuifsterkte. Verder het die vesels ‘n aansienlike bydrae gelewer tot skuifkapasiteit, asook ‘n verhoging van 49% in genormaliseerde skuifspanning vir ‘n veselinhoud van 0.6% en ‘n verhoging van 74% vir ‘n veselinhoud van 1%, in vergelyking met balke wat geen vesels bevat nie. Deur die voorspelde waardes te vergelyk met eksperimentele waardes wys dat die voorgestelde model die skuifkapasiteit van balke met onreëlmatige deursnitte met redelike akkuraatheid voorspel met ‘n gemiddelde skuifsterkte verhouding van 0.95 en ‘n standaardafwyking op die gemiddelde skuifsterkte verhouding van 0.10. Dit word egter aanbeveel om ‘n groter steekproef te ontwikkel om die akkuraatheid van hierdie skuifmodel te bevestig.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107796
This item appears in the following collections: