Debonding of external CFRP plates from RC structures caused by cyclic loading effects

Badenhorst, Adriaan Jakobus (2012-03)

Thesis (MScEng)--Stellenbosch University, 2012.

Thesis

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.

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