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Tensile creep of cracked macro synthetic fibre reinforced concreteBabafemi, Adewumi John 03 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Macro synthetic fibres are known to significantly improve the toughness and energy absorption
capacity of conventional concrete in the short term. However, since macro synthetic fibre are flexible
and have relatively low modulus of elastic compared to steel fibres, it is uncertain if the improved
toughness and energy absorption could be sustained over a long time, particularly under sustained
tensile loadings.
The main goal of this study is to investigate the time-dependent crack mouth opening response of
macro synthetic fibre reinforced concrete (FRC) under sustained uniaxial tensile loadings, and to
simulate the flexural creep behaviour. For the purpose of simulating the in-service time-dependent
condition, all specimens were pre-cracked.
Experimental investigations were carried out at three levels (macro, single fibre and structural) to
investigate the time-dependent behaviour and the mechanisms causing it. At the macro level,
compressive strength, uniaxial tensile strength and uniaxial tensile creep test at 30 % to 70 % stress
levels of the average residual tensile strength were performed. To understand the mechanism causing
the time-dependent response, fibre tensile test, single fibre pullout rate test, time-dependent fibre
pullout test and fibre creep test were done. Flexural test and flexural creep test were done to simulate
the structural level performance.
The results of this investigation have shown significant drop in stress and increase in crack width
of uniaxial tensile specimens after the first crack. The post cracking response has shown significant
toughness and energy absorption capacity. Under sustained load at different stress levels, significant
crack opening has been recorded for a period of 8 month even at a low stress level of 30 %. Creep
fracture of specimens occurred at 60 % and 70 % indicating that these stress levels are not sustainable
for cracked macro synthetic FRC.
The single fibre level investigations have revealed two mechanisms responsible for the time-dependent crack widening of cracked macro synthetic FRC under sustained loading: time-dependent fibre pullout and fibre creep. In all cases of investigation, fibre failure was by complete pullout
without rupture.
Flexural creep results have shown that the crack opening increases over time. After 8 months of
investigation, the total crack opening was 0.2 mm and 0.5 mm at 30 % and 50 % stress levels
respectively.
Since the crack opening of tensile creep and flexural creep specimens cannot be compared due to
differences in geometry, specimen size, load transfer mechanisms and stress distribution in the
cracked plane, a finite element analysis (FEA) was conducted. Material model parameters obtained
from the uniaxial tensile test and viscoelastic parameters from curve fitting to experimental uniaxial
creep results have been implemented to successfully predict the time-dependent crack opening of
specimens subjected to sustained flexural loading. Analyses results correspond well with experimental
result at both 30 % and 50 % stress levels. / AFRIKAANSE OPSOMMING: Makro sintetiese vesels is bekend daarvoor dat dit die taaiheid en energie absorpsie van
konvensionele beton beduidend verbeter in die kort termyn. Aangesien makro sintetiese vesels
buigsaam is met 'n relatiewe lae styfheidsmodulus in vergeleke met staalvesels, is dit onseker of die
verhoogde kapasiteit vir energie absorpsie en taaiheid volgehou kan word oor die langer termyn,
veral in gevalle waar dit aan volgehoue trekkragte blootgestel is.
Die hoofdoel van die studie is om die tydafhanklike-kraakvergrotingsgedrag van makro sintetiese
veselversterkte beton (VVB) wat blootgestel is aan volgehoue trekkragte te ondersoek asook die
simulasie van die kruipgedrag in buig. Ten einde die werklike toetstande te simuleer is al die
proefstukke doelbewus gekraak in 'n beheerde manier voor die aanvang van die toetse.
Die eksperimentele ondersoek is uitgevoer op drie vlakke (makro, enkelvesel en strukturele) om
die tydafhanklike gedrag sowel as die meganismes verantwoordelik vir hierdie gedrag te ondersoek.
Op die makro-vlak is druktoetse gedoen saam met eenassige trek- en eenassige kruiptoetse met
belastings tussen 30 % en 70 % van die gemiddelde residuele treksterkte. Om die meganisme wat die
tydafhanklike gedrag veroorsaak te verstaan is veseltoetse, enkel vesel uittrektoetse, enkel vesel
uittrek kruiptoetse asook kruiptoetse op vesels gedoen. Buigtoetse en buig kruiptoetse is ook gedoen
om die gedrag op die strukturele vlak te ondersoek.
Die resultate van hierdie ondersoek wys dat daar 'n beduidende val in spanning is en dat daar
gepaardgaande kraak opening in die eenassige trek proefstukke plaasgevind het na die vorming van 'n
kraak. Die na-kraak gedrag wys beduidende taaiheid en energie absorpsie kapasiteit. Gedurende die
volgehoue trekbelasting by verskillende spanningsvlakke is beduidende kraakvergroting opgemerk,
selfs by 30 % belasting na 8 maande. Kruipfaling het plaasgevind by proefstukke met belastings van
60 % en 70 % wat daarop wys dat hierdie spanningsvlakke nie geskik is vir gekraakte makro sintetiese VVB nie. Op die enkel veselvlak is twee meganismes geïdentifiseer wat verantwoordelik is vir die
kraakvergroting oor tyd vir gekraakte makro sintetiese VVB met volgehoue trekbelasting:
tydafhanklike vesel uittrek en vesel-kruip. In alle gevalle in hierdie ondersoek was die
falingsmeganisme vesels wat uittrek.
Buig kruiptoets resultate wys dat die krake vergroot oor tyd. Na 8 maande van ondersoek was die
kraakwydtes 0.2 mm en 0.5 mm by 30 % en 50 % spanningsvlakke onderskeidelik.
Aangesien die kraak opening van eenassige trek kruiptoetse en die buig kruiptoetse nie direk met
mekaar vergelyk kan word nie weens die verskille in geometrie, proefstuk grootte en
spanningsverdeling in die kraakvlak, is 'n eindige element analises (EEA) gedoen. Materiaal
eienskappe is bepaal deur gebruik te maak van die eenassige kruip trektoets se resultate en viskoelastiese
parameters is bepaal deur middel van kurwepassing van die resultate. Dit was gebruik om
suksesvol die buig kruip kraak opening gedrag te simuleer. Die analises se resultate vergelyk goed
met die eksperimentele data by beide 30 % en 50 % spanningsvlakke.
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Construction in in-situ cast flat slabs using steel fibre reinforced concreteJarrat, Robert 12 1900 (has links)
Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Fibre reinforced concrete (FRC) transforms concrete from a characteristically brittle material to one with a post-crack tensile residual capacity. Its application in industry has varied over the past of which the tensile properties have generally been used in the form of crack mitigation. More recently, the introduction of steel fibres has broadened this scope to structural applications in which the resisting tensile stresses that develop within a steel FRC (SFRC) element can be rather significant. This thesis reviews the existing practices and design models associated with SFRC and the suitability of its implementation as the sole form of reinforcement in in-situ cast flat slab systems. As a material SFRC is dependent on a number of factors which include the fibre type and volume, fibre distributions, element size, as well as the support and applied load conditions. Thus, its performance can be considered rather variable in comparison to conventional concrete should the incorrect practices be implemented. In order to adequately define the material characteristics, it is necessary to use test procedures that accurately reflect on the intended structural application. As a result a number of test procedures have been developed. In addition to this, the post-crack material performance is associated with a non-linear behaviour. This attribute makes the design of structural SFRC elements rather difficult. In an attempt to simplify this, existing design models define stress-strain or stress-crack width relations in which assumptions are made regarding the cross-sectional stress distribution at specified load states. This thesis takes on two parts in defining the suitability of SFRC as the sole form of reinforcement in flat slab systems. The first is a theoretical investigation regarding the micro and macro scale material performance of SFRC, the practices that exist in defining the material properties and its application in structural systems (particularly suspended slab systems), and a breakdown of the existing design models applicable to strain softening deflection hardening SFRC materials. The second part is an experimental program in which the fresh state and hardened state material properties of specified SFRC mix designs defined
through flow and beam testing respectively. These properties are then implemented in the
design and construction of full scale flexural and punching shear test slabs in an attempt to
verify the theory applied.
The investigation reveals that the use of SFRC significantly improves the ductility of
concrete systems in the post-crack state through fibre crack bridging. This ductility can result
in deflection hardening of flat slab systems in which the redistribution of stresses increases
the load carrying capacity once cracking has taken place. However, the performance of large
scale test specimens is significantly influenced by the construction practices implemented in
which the material variability increases as a result of non-uniform fibre distributions. The
results indicate that the load prediction models applied have potential to adequately predict
the ultimate failure loads of SFRC flat slab systems but however cannot account for possible
non-uniform fibre distributions which could result in premature failure of the system. / AFRIKAANSE OPSOMMING: Vesel versterkte beton (VVB) verander beton van die kenmerkende uiters bros material na ‘n material met ‘n residuele post-kraak trekkapasiteit. Die toepassing daarvan in die bedryf het
in die verlede gewissel en die trek eienskappe is oor die algemeen gebruik vir kraak
vermindering. Meer onlangs het die bekenstelling van staal vesel hierdie omvang verbreed
na die strukturele toepassings waar trekspannings wat ‘n VVB element kan weerstaan
noemenswaardig kan wees. Hierdie tesis ondersoek bestaande praktyke en ontwerpmodelle
met die oog op staalvesel versterkte beton (SVVB) en die geskiktheid van die
implementering daarvan as die enigste vorm van bekisting in in-situ gegiete plat blad stelsels.
As ‘n materiaal, is SVVB afhanklik van ‘n aantal faktore wat die tipe vesel en volume, vesel
verspreiding, element grootte, sowel as die randvoorwaardes tipe aangewende las insluit. As
gevolg hiervan, kan die gedrag van SVVB, wat korrek geïmplimenteer word, as redelik
varieerbaar beskou word wanneer dit met konvensionele beton vergelyk word. Ten einde die
materiaaleienskappe voldoende te definieer, is dit noodsaaklik dat prosedures wat die
strukturele toepassing akuraat voorstel, getoets word en daarom is ‘n aantal toets prosedures
ontwikkel. Verder het die post-kraak materiaalgedrag ‘n nie-lineêre verband wat
struktuurontwerp met SVVB redelik moeilik maak. Om dit te vereenvoudig, definieer
bestaande ontwerpmodelle spanning-vervorming of spanning-kraakwydte verhoudings
waarin aannames gemaak word ten opsigte van die spanningsverdeling oor ‘n snit, gegewe
sekere lastoestande.
Hierdie studie bestaan uit twee dele wat die geskiktheid van SVVB as die enigste vorm van
bikisting in plat blad stelsels definieer. Die eerste deel bestaan uit ‘n teoretiese ondersoek
wat handel oor die mikro- en makro-skaal materiaalgedrag van SVVB, die praktyke wat
bestaan om die materiaaleienskappe en toepassing in strukturele sisteme (spesifiek opgelegde
blad stelsels) te definieer, en ‘n uiteensetting van die bestaande ontwerpmodelle wat van toepassing is vir defleksie as gevolg van vervormingsversagting wat SVVB material verhard.
Die tweede deel bestaan uit ‘n eksperimentele program waarin die materiaaleienskappe van
gespesifiseerde SVVB meng-ontwerpe in die vars toestand en in die verharde toestand
gedefinieer word deur middel van vloei- en balktoetse onderskeidelik. Hierdie eienskappe
word dan toegepas vir die ontwerp en konstruksie van volskaalse buig- en ponsskuif
toetsblaaie ten einde die modelle en teorie wat toegepas is, te bevestig.
Die ondersoek toon dat die gebruik van SVVB die duktiliteit van beton sisteme
noemenswaardig verbeter in die post-kraak toestand deur kraak oorbrugging. Hierdie
duktiliteit kan defleksie verharding van plat blad stelsels veroorsaak waarin die herverdeling
van spannings, nadat kraking plaasgevind het, die lasdraende kapasiteit verhoog. Die gedrag
van die grootskaalse toetsmonsters word egter noemenswaardig beïnvloed deur die
konstruksiemetodes wat geïmplementeer word waarin die materialveranderlikheid toeneem as
‘n gevolg van nie-uniforme vesel verdelings. Die resultate dui daarop dat die modelle wat
toegepas is om die laste te voorspel, die potensiaal het om die grens falingslas van SVVB plat
blad stelsel voldoende te voorspel, maar neem nie moontlike nie-uniforme veselverdelings
wat kan lei tot vroeë faling van die stelsel in ag nie.
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Development of Design Procedures for Fiber Reinforced Concrete (FRC) & Ultra-High-Performance Concrete (UHPC) Based on Experimental EvaluationsJanuary 2018 (has links)
abstract: A comprehensive study was performed on non-proprietary ultra-high-performance concrete (UHPC) material and several design methods were suggested based on numerous experimental results. Several sets of compression tests, direct tensile tests, and flexural tests were performed on UHPC to provide a better understanding of the mechanisms involved in the mechanical behavior of the fiber reinforced material. In addition to compressive tests, flexural tests, based on ASTM C1609 and EN 14651, were performed. The effect of the strain rate on the UHPC material was also investigated through the high-speed tensile tests at different strain rates. Alongside the usual measurement tools such as linear variable differential transformers (LVDT) and clip gages, digital image correlation (DIC) method was also used to capture the full-range deformations in the samples and localized crack propagations. Analytical approaches were suggested, based on the experimental results of the current research and other research groups, to provide design solutions for different applications and design approaches for UHPC and hybrid reinforced concrete (HRC) sections. The suggested methods can be used both in the ultimate limit state (ULS) and the serviceability limit state (SLS) design methods. Closed form relationships, based on the non-linear design of reinforced concrete, were used in the calculation of the load-deflection response of UHPC. The procedures were used in obtaining material properties from the flexural data using procedures that are based on back-calculation of material properties from the experimental results. Model simulations were compared with other results available in the literature. Performance of flexural reinforced UHPC concrete beam sections tested under different types of loading was addressed using a combination of fibers and rebars. The same analytical approach was suggested for the fiber reinforced concrete (FRC) sections strengthened (rehabilitated) by fiber reinforced polymers (FRP) and textile reinforced concrete (TRC). The objective is to validate the proper design procedures for flexural members as well as connection elements. The proposed solutions can be used to reduce total reinforcement by means of increasing the ductility of the FRC, HRC, and UHPC members in order to meet the required flexural reinforcement, which in some cases leads to total elimination of rebars. / Dissertation/Thesis / Masters Thesis Civil, Environmental and Sustainable Engineering 2018
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Behavior of synthetic fiber-reinforced concrete circular columns under cyclic flexure and constant axial load / Comportement des poteaux circulaires en béton renforcé avec fibres synthétiques soumis à charge axiale constante et flexion cycliqueOsorio Gomez, Laura Isabel January 2008 (has links)
La ductilité et la capacité à dissiper de l'énergie sont deux qualités très importantes pour les éléments structuraux des structures situées dans les régions sismiques comme l'est du Canada. Soulignons que Montréal occupe la deuxieme place en ce qui a trait au risque sismique au Canada. De plus, la réduction des coûts de maintenance des infrastructures est un sujet d'intérêt pour les propriétaries alors que ces derniers doivent en tout temps garantir la sécurité des usagers. Or, le béton renforcé avec des fibres synthétiques semble être un matériau qui remplit ces caractéristiques. Pourtant, son utilisation est actuellement limitée aux éléments non structuraux ou structuraux mais non principaux. Afin de généraliser l'utilisation du béton fibre dans le domaine structural, il faut continuer à produire et à analyser des données expérimentales qui permettront de valider et d'améliorer les prescriptions de design et les modèles analytiques actuels pour la conception des éléments en béton armé avec des fibres dans les zones sismiques. Dans ce contexte, six poteaux circulaires à grande-échelle ont été testés sous une charge axiale constante (25% de Agf'c) et en flexion cyclique. Trois poteaux ont été confectionnés en béton normal (BN) et les trois autres en béton renforcé avec des fibres synthétiques (BRFS). La résistance à la compression du béton spécifiée à 28 jours pour les spécimens était de 30 MPa. Le volume de fibres synthétiques en polypropylène-polyéthylène utilisé a été de 1%. Les trois poteaux en BN étaient renforcés par une armature transversale constituée d'une spirale ayant un pas de 42, 75 et 100 mm respectivement. Ces trois spécimens ont été comparés avec des spécimens similaires en BRFS. Les résultats montrent que la présence des fibres synthétiques dans la matrice de béton améliore le comportement ductile et la capacité a dissiper de l'énergie des spécimens. Il a été observé que cette amélioration n'est pas directement proportionnelle à la quantité d'armature transversale. Toutefois, l'utilisation du béton fibre semble rendre possible une réduction de l'armature transversale tout en conservant un aussi bon sinon un meilleur comportement.
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