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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96679 |
| Date | 03 1900 |
| Creators | Babafemi, Adewumi John |
| Contributors | Boshoff, William Peter, Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xxvii, 174 pages : illustrations |
| Rights | Stellenbosch University |
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