Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: This study set out to determine the debonding of externally applied Carbon Fibre Reinforced
Polymer (CFRP) plates from RC structures under cyclic loading. Triplet shear tests and finite
element (FE) analyses were done on the epoxy to determine the bond stress between the
CFRP plate and a reinforced concrete specimen. From these tests and analyses the
average shear strength of the bond between the epoxy and concrete substrate was
determined and the shear strength of the epoxy specified by the supplier could be confirmed.
A case study of a statically loaded beam was performed to verify the bond strength.
Finally a reinforced concrete (RC) T-section was designed and pre-cracked to simulate a
damaged beam in practice. These sections were then externally reinforced by bonding
CFRP plates onto the face of the web. The sections were subjected to static and cyclic
loading at different force amplitudes. Along with the experimental tests, FE models were
developed and analysed which had the same geometrical and material properties as the
experimental specimens. Due to time constraint a FE mesh objectivity study was not done,
but the chosen element size is believed to be sufficiently small to replicate the experimental
tests objectively.
The FE analyses and the experimental tests yielded results that were close to each other on
both the global scale and in terms of localised behaviour, thus it was decided that the
computational approach could be used for the final design of a model of the debonding of
CFRP plates bonded onto RC beams under cyclic loading because the data can be
analysed more easily and a large variation of tests can be done.
For the T-section 3 tests were conducted; a pull-off (static) test where the bonded CFRP
plate was pulled from a specimen to get the ultimate failure envelope of the test specimens.
The static test was followed by cyclic tests with force amplitude of 85% and 65% of the
ultimate pull-off strength. Different measurements were taken to get the global and local
displacement behaviour of the section. The global displacement was measured by means of
a linear variable displacement transducer (LVDT, displacement meter) clamped onto the
CFRP plate that pushed on the top of the concrete and the local displacement was
measured with the help of the Aramis system. The displacement was then compared to the
same displacements of nodes and elements in the FE models. The result was a
confirmation that the results from the FE models were sufficient to design a model for cyclic
debonding of CFRP plates from RC structures. From the FE models the relative displacement between the CFRP plate and concrete was
obtained in the vicinity of a crack. This relative displacement was then normalised by the
respective stress range of the different tests, from which the normalised relative
displacement was plotted against the number of cycles to get an equation limiting the
number of cycles for a specific stress range.
From the results, it appears that for cyclic load levels up to 65% of the peak static resistance,
a threshold number of load cycles are required for delamination initiation. Subsequently, a
near constant delamination rate is reached. The delamination rate is significantly lower for
lower cyclic load levels. Finally, an unstable delamination stage is reached at a level of
about 65 μm for all the analyses, after which CFRP pull-off is imminent.
Service life design of CFRP reinforcement of RC beams should take into consideration the
delamination initiation threshold, the subsequent delamination rate and finally the initiation of
unstable delamination. / AFRIKAANSE OPSOMMING: Die projek is uitgevoer om die delaminasie van ekstern aangewende Koolstof Vesel
Versterkte Polimeer (KVVP) stroke op gewapende beton strukture te bepaal onder sikliese
belasting. Triplet skuif toetse is gedoen op die gebruikte epoksie om die verband-sterkte te
bepaaltussen die KVVP stroke en die beton proefstuk. Die skuif toetse is ook met behulp
van die eindige element (EE) metode geanaliseer. Die resultaat van die toetse en analises
het gewys dat die verband sterkte tussen die KVVP stroke en beton gelyk is aan die skuif
sterkte van die epoksie wat verskaf is. `n Gevalle studie van `n monotonies belaste balk is
gedoen om die verband-sterkte te verifieër.
`n Gewapende beton T-snit is ontwerp en voor-af gekraak om `n beskadigde balk in die
praktyk voor te stel. Die beskadigde proefstukke is vervolgens ekstern versterk met KVVP
stroke wat aan die web van die T-snit vas geplak is. Die versterkte T-snitte is getoets onder
statiese en sikliese belasting. Die sikliese toetse is ook onder verskillende
spanningsamplitudes getoets. Om die eksperimentele toetse te verifieër is EE modelle
gebou en geanaliseer wat dieselfde geometriese en materiaal eienskappe as die
eksperimentele proefstukke gehad het, maar as gevolg van `n tydsbeperking is `n
sensitiwiteit studie oor die element grootte nie gedoen nie. Die element grootte is klein
genoeg gekies en word beskou as voldoende om die gedrag objektief te simuleer.
Die EE analises en eksperimentele resultate was na genoeg aan mekaar op beide globale
en lokale vlak. Dus is `n analitiese benadering tot die toetse vervolgens gebruik vir die
ontwerp van `n model vir delaminasie van KVVP stroke van gewapende beton strukture
onder sikliese belasting. Die EE metode stel die analis in staat om `n verskeidenheid van
toetse relatief vinnig uit te voer en om die data van die toetse vinniger te interpreteer as deur
fisiese eksperimentele toetse.
Drie eksperimente is uitgevoer op die T-snitte, `n aftrek-toets (staties) waar die KVVP strook
van `n proefstuk afgetrek is om die falingsomhullende diagram te kry en dan ook twee
sikliese toetse teen 85% en 65% van die krag amplitude van die falingskrag.
Verplasingsmeters is gebruik om die globale verplasing te kry, deur dit vas te klamp op die
KVVP strook en dan die verplasing te meet relatief tot die bokant van die beton. Die lokale
veplasing is met behulp van die Aramis sisteem verkry. Die eksperimentele verplasings is
dan vergelyk met verplasings van die ooreenstemmende nodes en elemente in die EE
modelle. Deur die vergelyking van die resultate is dit bevestig dat die eindige element modelle voldoende is om die model vir sikliese delaminasie van KVVP stroke van
gewapende beton strukture te gebruik vir die ontwerp.
Uit die EE modelle is die relatiewe verplasing tussen die KVVP strook en die beton gekry in
die omgewing van `n kraak. Die relatiewe verplasing is genormaliseer deur elkeen se
spanningsamplitude. Die genormaliseerde relatiewe verplasing is dan teenoor die aantal
siklusse geteken waarvan `n vergelyking vir die maksimum verplasing afgelei is om die
aantal siklusse vir `n gegewe spanning amplitude te beperk.
Uit die resultate blyk dit dat vir sikliese laste tot en met 65% van die piek statiese weerstand
`n aantal siklusse moontlik is voordat delaminasie begin waarna `n konstante delaminasie
tempo bereik word. Die delaminasie tempo is stadiger vir sikliese laste teen `n laer
belastings amplitude. Laastens word `n onstabiele delaminasie fase bereik by `n vlak van
ongeveer 65 μm, na die oorgang delamineer die KVVP strook binne enkele siklusse.
Die beginpunt van delaminasie, die delaminasie tempo en laastens die begin van onstabiele
delaminasie moet in gedagte gehou word by die ontwerp diens leeftyd van KVVP versterkte
gewapende beton balke.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/20308 |
| Date | 03 1900 |
| Creators | Badenhorst, Adriaan Jakobus |
| Contributors | Van Zijl, G. P. A. G., 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 | 84 p. : ill. |
| Rights | Stellenbosch University |
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