Fatigue behaviour of steel fibre reinforced concrete

Fataar, Humaira (2022-04)

Thesis (PhD)--Stellenbosch University, 2022.

Thesis

ENGLISH ABSTRACT: Concrete is a heterogeneous material that is known to have a weak tensile capacity. The construction industry has successfully utilised steel reinforcement in concrete to overcome its brittle tensile behaviour, however, steel reinforcement is generally not sufficient to resist the formation and propagation of tensile cracks. As a result, discrete fibres are added to the concrete mixing stage to produce fibre reinforced concrete (FRC). The use of steel fibres has been known to improve the post-cracking behaviour of steel fibre reinforced concrete (SFRC), and is one of the most readily available fibres in industry. Due to concrete’s popularity as a construction material, many of its applications may experience flexural fatigue loading at some point during its lifespan. A significant amount of research has been conducted on the fatigue behaviour of plain concrete and FRC in the past century. However, the research focused primarily on the uncracked behaviour, with few researchers considering the cracked behaviour. In FRC, the fibres are activated only at crack initiation and therefore, this work aimed to investigate the fatigue life and failure mechanisms of pre-cracked SFRC subjected to fatigue loading. Experiments were conducted at a single fibre and macroscopic level, using hooked-end steel fibres. The pre-cracks ranged from 0.6 mm to 2.5 mm, at fatigue load levels of 50%, 70% and 85% of the maximum static load. Various methods were used to attempt to predict the fatigue life and failure mechanisms. A single fibre pull-out model was developed to categorise the various fibre pull-out phases and the level of deformation associated with each phase. When subjected to a static pre-slip of the fibre, followed by fatigue loading, the failure mechanisms were both fibre pull-out and fibre rupture. The fatigue capacity and failure mechanisms of the single fibre specimens vary depending on the combination of pre-slip and load level. The pull-out failures are generally unable to resist many load cycles due to a diminished fibre anchorage. The rupture failures tend to occur after a significant number of cycles have already passed, since the fibres rupture due to fatigue failure. The macroscopic behaviour subjected to fatigue loading shows fibre rupture to be the dominant failure mechanism, which differs from the static behaviour, where fibre pull-out occurs. The fatigue resistance decreases with an increase in pre-crack and load level. The single fibre pull-out model is used to classify the fibres into the different phases along the height of the crack for the various pre-cracks. A fatigue life prediction approach developed from the macroscopic fatigue results in the form of a modified S-N curve, was used to predict the fatigue behaviour. Experimental results from similar work performed was compared with the modified S-N curve and the results show that for deflection softening behaviour of SFRC, the model overestimates the fatigue capacity. Therefore, post-cracking behaviour influences the fatigue capacity of SFRC. The framework for an analytical model was developed to predict the fatigue failure mechanisms of pre-cracked SFRC. Fibre pull-out is likely to occur when large pre-cracks are present, whereas fibre rupture is more likely at low pre-cracks.

AFRIKAANSE OPSOMMING: Beton is ‘n heterogene materiaal met ‘n swak trek kapasiteit. Die konstruksiebedryf het staalbewapening suksesvol gebruik om die bros trekgedrag van beton te oorkom. Die staalbewapening is egter nie genoeg om die vorming en verspreiding van trekkrake te voorkom nie. Gevolglik, word diskrete vesels in die mengfase by die beton gevoeg om vesel versterkte beton (VVB) te produseer. Dit is bekend dat die gebruik van staalvesels die gekraakte staal vesel versterkte beton (SVVB) verbeter, en dit is een van die mees vrylik beskikbare vesels in die industrie. As gevolg van die gewildheid van beton as ‘n konstruksiemateriaal, is daar verskeie toepassings waar beton sikliese belading in buiging ondervind. Baie navorsing is gedoen oor die sikliese gedrag van gewone beton en VVB in die afgelope eeu. Die navorsing fokus egter hoofsaaklik op die ongekraakte gedrag, met min navorsers wat onlangs die gekraakte gedrag oorweeg het. In VVB, word die vesels slegs geaktiveer by die aanvang van die kraak. Daarom is hierdie werk daarop gemik om die sikliese lewensduur en falingsmeganismes van gekraakte SVVB te ondersoek wat aan sikliese belading blootgestel word. Eksperimente word uitgevoer op ‘n enkele vesel en makroskopiese vlak, met behulp van gehakte staalvesels. Die vooraf-krake wissel van 0.6 mm tot 2.5 mm, met belading van 50%, 70% en 85% van die maksimum statiese las. Daarna is verskillende metodes geïmplementeer om die sikliese gedrag en falingsmeganismes te voorspel. ‘n Enkeleveseluittrekmodel is ontwikkel om die verskillende fases van vesel uittrek te kategoriseer, tesame met die vlak van vervorming wat met elke fase geassosieer word. Met ‘n statiese voorafglip van die vesel gevolg deur sikliese belading, is die falingsmeganismes beide veseluitrekking en veselbreek. Die sikliese kapasiteit en falingsmeganismes van die enkelveselmonsters wissel afhangende van die kombinasie van voorafglip en beladingsvlak. As gevolg van die verminderde veselverankering, kan die uittrekfalings oor die algemeen nie baie belading siklusse weerstaan nie. Die breekfalings kom gewoonlik voor nadat ‘n beduidende aantal siklusse reeds verby is, aangesien die vesel faal as gevolg van sikliesefaling. Die makroskopiese gedrag onder sikliesebelading het getoon dat veselbreek die dominante falingsmeganisme is, en verskil van die statiese gedrag, waar veseluittrek plaasgevind het. Die sikliese weerstand neem af met ‘n toename in voorafkraak en beladingsvlak. Die enkelvesel uittrekmodel word gebruik om die vesels in verskillende fases oor die hoogte van die kraak te plaas. ‘n Sikliese lewensduur voorspellingsbenadering wat ontwikkel is uit die makroskopiese sikliese resultate in die vorm van ‘n aangepaste S-N kurwe word gebruik om die sikliese gedrag te voorspel. Eksperimentele resultate van soortgelyke werk wat voorheen uitgevoer was, is vergelyk met die aangepaste S-N kurwe en die resultate toon aan dat die model die sikliese kapasiteit oorskat vir defleksie versagtende-gedrag van SVVB. Daarom, beïnvloed die nakraak gedrag die sikliese kapasiteit van SVVB. Verder is die raamwerk vir ‘n analitiese model ontwikkel om die sikliese falingsmeganismes van vooraf-gekraakte SVVB te voorspel. Uittrekking van vesels sal waarskynlik voorkom as daar groot voorafkrake is, terwyl veselbreek meer waarskynlik is by lae voorafkrake.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/124767
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