Mechanical and structural characterisation of extrusion moulded SHCC

Visser, Christo Riaan (2007-12)

Thesis (MScIng)--University of Stellenbosch, 2007.

Thesis

ENGLISH ABSTRACT: SHCC (Strain-Hardening Cement-based Composite) is a type of HPFRCC (High Performance Fibre Reinforced Cement-based Composite) that was designed and engineered to overcome the weaknesses of ordinary concrete. It shows a high ductility as it can resist the full tensile load at a strain of more than 3%. This superior response is achieved with multiple cracking under tensile loading which has a pseudo strain-hardening phenomenon as a result. The purpose of the research project reported in this thesis document was to design and build a new piston-driven extruder for the production of SHCC as well as R/SHCC (reinforced SHCC) elements and to investigate and characterise the structural and mechanical behaviour of extrusion moulded SHCC. A new piston-driven extruder, specifically for academic purposes, was designed based on the principles of fluid flow mechanics. Although fluid flow is not an ideal model to represent the flow of viscous material through an extruder, it was deemed sufficient for this specific study. A new extruder with the capacity to extrude SHCC and R/SHCC was built. Provision was made that this extruder can be fitted with extruder dies and transition zones of varying shapes and sizes. A comparative study between unreinforced as well as reinforced cast SHCC and extruded SHCC as well as a suitable R/C (Reinforced Concrete) was conducted. Three-point bending tests, representative of the envisioned structural application, were performed on specimens of each of the composites. The unreinforced cast SHCC and especially the unreinforced extruded SHCC have a comparative level of performance to the cast R/C. These specimens displayed a similar cracking pattern of multiple cracks, although less pronounced in the extruded SHCC. The extruded SHCC has superior first cracking and ultimate strength in comparison to cast SHCC, but with accompanying lower ductility. The reinforced SHCC specimens failed in a combination of flexure and shear. The extruded R/SHCC specimens formed multiple diagonal cracks before failure, while the cast R/SHCC specimens formed only a few diagonal cracks, before delaminating along the reinforcement. The higher shear capacity and thus the ability to form multiple diagonal cracks of the extruded R/SHCC can be ascribed to the better fibre orientation of the specimens in the longitudinal direction, while the cast specimens have a random orientation of fibres. R/SHCC and especially extruded R/SHCC could be a far superior structural material to R/C. Mechanical characterisation of extruded SHCC was done with the use of uni-axial tensile and compressive tests. The results of these tests were compared with the results of uni-axial tensile tests previously performed on cast SHCC as well as uni-axial compressive tests that were performed on cast SHCC in this research study. The extruded SHCC displayed superior tensile performance in terms of first cracking and ultimate strength in comparison to cast SHCC, but with accompanying lower ductility. In terms of compressive performance the extruded SHCC has a higher ultimate strength, but with a lower ductility than cast SHCC. The extruded SHCC also has a much higher E-modulus than cast SHCC. This can partly be attributed to the difference between the water/binder ratios of the cast and extruded SHCC, but can mainly be ascribed to the lower porosity as a result of high extrusion forces involved in the manufacturing of extruded SHCC. A simple bending model for SHCC has also been introduced. This model is based on the mechanical characteristics of SHCC. The model somewhat underestimates the resistance moment of the extruded and cast SHCC, but this underestimation is more pronounced in the case of the cast SHCC. Various reasons for the underestimation is discussed, but it is postulated that the main reason for the difference in experimentally determined and the calculated resistance moment of the cast SHCC is the possible variation in ingredient properties and specimen preparation and testing, since the characterisation of the cast SHCC was done over a long period of time and by different researchers. The bending model is however deemed sufficient for the design purposes of SHCC.

AFRIKAANSE OPSOMMING: SHCC (“Strain-Hardening Cement-based Composite”) is ‘n tipe HPFRCC (“High Performance Fibre Reinforced Cement-based Composite”) wat ontwerp is om die swakhede van gewone beton te oorkom. Hierdie materiaal het ‘n hoë duktiliteit en kan die volle trekkrag weerstaan met ‘n vervorming van meer as 3%. Hierdie uitstaande gedrag word gekenmerk deur meerdere krake wat vorm gedurende ‘n trek belasting wat vervormingsverharding tot gevolg het. Die doel van die navorsingsprojek wat weergegee word in hierdie tesis dokument was om ‘n nuwe suier-aangedrewe ekstrueerder vir die produksie van SHCC sowel as R/SHCC (bewapende SHCC) te ontwerp en te bou en om die strukturele en meganiese gedrag van ge-ekstrueerde SHCC te ondersoek en te karakteriseer. ‘n Nuwe suier-aangedrewe ekstrueerder, spesifiek for akademiese doeleindes, is ontwerp gebaseer op die beginsels van vloeistof vloeimeganika. Alhoewel vloeistof vloeimeganika nie ‘n ideale model is vir die voorstelling van die vloei van ‘n viskose materiaal deur ‘n ekstrueerder nie, word dit beskou as aanvaarbaar vir die doeleindes van hierdie spesifieke studie. ‘n Nuwe ekstrueerder met die kapasiteit om SHCC en R/SHCC te ekstrueer is gebou. Voorsiening is ook gemaak dat ekstrueerder vorms (“dies”) en oorgangsones van verskillende vorms en groottes aan die ekstrueerder geheg kan word. ‘n Vergelykende studie tussen onbewapende sowel as bewapende gegote en ge-ekstrueerde SHCC, sowel as ‘n gepasde R/C (“Reinforced Concrete”) is uitgevoer. Drie-punt buigtoetse, verteenwoordigend van die voorgestelde strukturele toepassings vir SHCC, is uitgevoer op proefstukke van elk van die bogenoemde materiale. Die meganiese gedrag van die onbewapende gegote SHCC en spesifiek die onbewapende geekstrueerde SHCC is vergelykbaar met die meganiese gedrag van gegote R/C. Hierdie proefstukke het ooreenstemmende kraakpatrone van veelvuldige krake getoon, alhoewel dit minder prominent was in die geval van ge-ekstrueerde SHCC. Die ge-ekstrueerde SHCC het hoër eerste kraak- en maksimum sterktes in vergelyking met gegote SHCC, maar met gepaardgaande laer duktiliteit. Die bewapende SHCC proefstukke het in ‘n kombinasie van buig en skuif gefaal. Die geekstrueerde R/SHCC proefstukke het meerdere diagonale krake gevorm voor faling, terwyl die gegote R/SHCC proefstukke slegs ‘n paar diagonale krake gevorm het, voordat dit langs die bewapening gedelamineer het. Die hoër skuifkapasiteit van ge-ekstrueerde SHCC en dus die vermoë om meerdere diagonale krake te vorm, kan toegeskryf word aan die longitudinale orientasie van vesels van die proefstukke, terwyl gegote proefstukke se vesels meer lukraak georienteerd is. R/SHCC en spesifiek ge-ekstrueerde R/SHCC kan‘n superieure strukturele materiaal in vergelyking met R/C wees. Die meganiese karakterisering van ge-ekstrueerde SHCC is gedoen met die gebruik van direkte trek- en druktoetse. The resultate van die hierdie toetse is vergelyk met die resultate van direkte trektoetse uit ‘n vorige studie op gegote SHCC,, sowel as met die uitslae van direkte druktoetse wat op gegote SHCC in hierdie navorsingstudie gedoen is. Die ge-ekstrueerde SHCC het superieure trekgedrag in terme van eerste kraak en maksimum sterktes in vergelyking met gegote SHCC getoon, maar met gepaardgaande laer duktiliteit. In terme van drukgedrag het die ge-ekstrueerde SHCC ‘n hoër maksimum druksterkte, maar met ‘n laer duktiliteit in vergelyking met die gegote SHCC. Die ge-ekstrueerde SHCC het ook ‘n veel hoër Emodulus as gegote SHCC. Dit is gedeeltelik as gevolg van die verskil in die water/binder verhouding van die gegote en ge-ekstrueerde SHCC, maar kan grootliks toegeskryf word aan die laer porositeit van ge-ekstrueerde SHCC as gevolg van die hoë ekstrusie kragte. ‘n Eenvoudige buigmodel vir SHCC word ook voorgestel. Hierdie model is geabseer op die meganiese gedrag van SHCC. Die model onderskat die weerstandsmoment van ge-ekstrueerde SHCC sowel as gegote SHCC, maar hierdie onderskatting is meer prominent in die geval van gegote SHCC. Verskeie redes vir hierdie onderskatting word genoem, maar dit word beweer dat in die geval van gegote SHCC dit grootliks as gevolg van moontlike variasies in die materiaal eienskappe en proefstukke se voorbereiding en toetsing is, aangesien die karakterisering van die gegote SHCC oor ‘n lang tydperk en deur verskillende navorsers gedoen is. Die buigmodel word nogtans as voldoende beskou vir die ontwerpdoeleinde van SHCC.

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