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.

[pt] O fenômeno de fluência em compósitos reforçados com fibras é particularmente importante quando são utilizadas macro fibras sintéticas, que devido ao seu baixo módulo de elasticidade, apresentam comportamento viscoelástico pronunciado mesmo em temperatura ambiente, o que pode levar a alterações no controle de fissuração ao longo do tempo. Testes de arrancamento são comumente usados para prever interações fibra-matriz e neste trabalho foram realizados para cargas de curto e longo prazo em três tipos de macro fibras sintéticas. Diferentes níveis de cargas de longo prazo (20, 30, 40 e 50 por cento da carga máxima de arrancamento em curta duração) e ângulos de orientação das fibras (15 graus celsius, 30 graus celsius e 45 graus celsius) em relação à direção da carga foram considerados para investigar a influência desses parâmetros na interação entre macro fibras sintéticas e matriz. Macro fibras com superfícies onduladas e maior módulo de elasticidade alcançaram maiores tensões de aderência e menores deformações por fluência. Em testes de curto prazo, imagens de microscopia óptica foram obtidas nas fibras arrancadas para correlacionar a degradação superficial das fibras com as curvas de tensão versus deformação. No arrancamento quase estático (curto prazo), foram observadas pequenas reduções na resistência ao arrancamento à medida que o ângulo foi aumentado para todas as fibras, além de uma intensa degradação de suas superfícies devido ao significativo efeito de polia. Em contraste, para os testes de longo prazo, foi observada uma redução da fluência com o aumento do ângulo de inclinação da fibra causada pela redução da fluência da fibra devido ao carregamento não axial e componentes de força adicionais produzidos pelo desvio da força axial. O modelo viscoelástico de Burgers foi aplicado e apresentou boa concordância com as curvas de fluência experimentais, consistindo, portanto, em uma alternativa promissora para modelar o comportamento de longo prazo de fibras individuais. Imagens de microtomografia e microscopia eletrônica de varredura mostraram que uma parte da deformação em tração, sob carga sustentada, pode ser atribuída à fluência da própria fibra, tornando desafiador estimar a fluência deste tipo de compósito, dada a considerável variabilidade de configurações de fibra. / [en] The creep phenomenon in fiber-reinforced composites is particularly important when macro synthetic fibers are used, due to their low modulus of elasticity, exhibit pronounced viscoelastic behavior even at room temperature, which can lead to changes in the cracking control over time. Pullout tests are commonly used to predict fiber–matrix interactions and in this work were conducted for short- and long-term on three types of polymeric macro fibers. Different levels of long-term loads (20, 30, 40 and 50 percent of the maximum short-term pullout load) and fiber orientation angles (15 degrees celsius, 30 degrees celsius, and 45 degrees celsius) with respect to the direction of the load were considered to investigate the influence of these parameters on the interaction between macro synthetic fibers and matrix. Macro fibers with crimped surfaces and higher modulus of elasticity achieved higher bond stresses and lower creep deformations. In short-term tests, optical microscopy images were obtained on the pulled-out fibers to correlate the surface degradation of the fibers with the stress versus strain curves. In quasi-static pullout (short-term), small reductions in pullout strength were observed for all fibers and angles, in addition to an intensive degradation of their surfaces owing to the significant snubbing effect of this type of fiber. In contrast, for the long-term tests, a creep reduction was observed with increasing fiber inclination angle caused by the creep reduction of the fiber due to non-axial loading and additional force components produced by the deviation of the axial force. The Burgers viscoelastic model was applied and showed good agreement with the experimental creep curves, therefore consisting of a promising alternative for modeling the long-term behavior of individual fibers. Microtomography and scanning electron microscopy images showed that a large portion of the strain in tension, under sustained load, can be attributed to the creep of the fiber itself, thus making it challenging to estimate the creep of this type of composite, given the considerable variability of fiber configurations.

The benefits of fibre reinforced concrete (FRC) have been thoroughly investigated. Much of this work has focussed on steel FRC subjected to monotonic loads. Data on the structural behaviour of macro-synthetic FRC or FRC under cyclic loads is scarce. A pilot investigation on the shear behaviour of macro-synthetic FRC and on the behaviour of FRC under reversed cyclic in-plane shear loading was carried out. Five in-plane shear panel tests were performed. The parameters under study were the fibre material type (steel or macrosynthetic) and loading protocol. Additionally, a number of compression, direct tension, and flexural tests were performed to determine the material properties of the concretes for comparison. The material response of 2.0% by volume of macro-synthetic FRC matched closely with 1.0% steel FRC. Finally, building upon an existing steel FRC model, a model for macro-synthetic FRC in tension was proposed and a short verification study was undertaken.

The benefits of fibre reinforced concrete (FRC) have been thoroughly investigated. Much of this work has focussed on steel FRC subjected to monotonic loads. Data on the structural behaviour of macro-synthetic FRC or FRC under cyclic loads is scarce. A pilot investigation on the shear behaviour of macro-synthetic FRC and on the behaviour of FRC under reversed cyclic in-plane shear loading was carried out. Five in-plane shear panel tests were performed. The parameters under study were the fibre material type (steel or macrosynthetic) and loading protocol. Additionally, a number of compression, direct tension, and flexural tests were performed to determine the material properties of the concretes for comparison. The material response of 2.0% by volume of macro-synthetic FRC matched closely with 1.0% steel FRC. Finally, building upon an existing steel FRC model, a model for macro-synthetic FRC in tension was proposed and a short verification study was undertaken.

This thesis presents the results of a research program examining the effects of macro-synthetic fibers on the shear and flexural behaviour of high-strength concrete (HSC) beams subjected to static and blast loads. As part of the study, a series of seventeen fiber-reinforced HSC beams are built and tested under either quasi-static four-point bending or simulated blast loads using a shock-tube. The investigated test parameters include the effects of: macro-synthetic fibers, fiber hybridization, combined use of fibers and stirrups and longitudinal steel ratio and type. The results show that under slowly applied loads, the provision of synthetic fibers improves the shear capacity of the beams by allowing for the development of yield stresses in the longitudinal reinforcement, while the combined use of synthetic fibers and stirrups is found to improve flexural ductility and cracking behaviour. The results also show that the provision of synthetic fibers delays shear failure in beams tested under blast pressures, with improved control of blast-induced displacements and increased damage tolerance in beams designed with combined fibers and stirrups. The study also shows that the use of hybrid fibers was capable of effectively replacing transverse reinforcement under both loading types, allowing for ductile flexural failure. Moreover, the use of synthetic fibers was effective in better controlling crushing and spalling in beams designed with Grade 690 MPa high-strength reinforcement. Furthermore, the results demonstrate that synthetic fibers can possibly be used to relax the stringent detailing required by modern blast codes by increasing the transverse reinforcement hoop spacing without compromising performance. As part of the analytical study, the load-deflection responses (resistance functions) of the beams are predicted using sectional (moment-curvature) analysis, as well as more advanced 2D finite element modelling. Dynamic resistance functions developed using both approaches, and incorporating material strain-rate effects, are then used to conduct non-linear single-degree-of-freedom (SDOF) analyses of the blast-tested beams. In general, the results show that both methods resulted in reasonably accurate predictions of the static and dynamic experimental results.

Fiber reinforcement is being widely used in concrete tunnel linings these days. Using fiber reinforcement can save not only cost, but also labor and time spent on construction. However, many owners hesitate to incorporate fiber reinforcement in tunnel lining due to lack of experience with and knowledge of the behavior of fiber reinforced concrete (FRC) In this study, fiber reinforced concrete was made with various kinds of fibers such as steel fiber, macro-synthetic fiber and hybrid fiber (a blend of macro-synthetic fiber and glass fiber). Many experimental tests were performed to investigate the compressive, flexural and shear behavior of fiber reinforced concrete. In addition to the structural capacity of FRC, the distribution of fiber reinforcement inside the concrete matrix was investigated. Test results of these experimental tests were thoroughly examined to compare and quantify the effects of fiber reinforcement. Next, the test results were used to generate axial force-bending moment interaction diagrams based on current design approaches. In addition, the current design approaches were modified to estimate the accurate and exact value of bending moment. Fiber reinforcement clearly improved the structural performance of tunnel lining. The post-peak flexural and shear strength was significantly influenced by the type and amount of fiber reinforcement. / text

[EN] Several efforts have been made in experimental and theoretical research about shear to understand all the variables that influence the phenomenon. Nowadays, however, due to its complexity, the shear performance of structural concrete elements, especially those without any traditional transversal reinforcement, continue with no clear explanation of the problem. Uncertainty about the problem grows when new variables like fibres are incorporated into the shear study. Research works have demonstrated the effectiveness of steel fibre in improving the mechanical properties of concrete elements. Experimental results reveal that steel fibres have proven effective in improving shear resistance, and they confer some concrete elements more ductility. In adequate amounts, steel fibres can completely or partially substitute traditional shear reinforcements. This is why international codes have included some requirements to take into account the action of fibres on the shear response of concrete elements. However, most recommendations and requirements for steel fibre-reinforced concrete (SFRC) were originally created. New fibres with different materials properties and shapes, such as macrosynthetic fibres, are now available on the market. These fibres, some of which are made of polypropylene, are an alternative in the construction industry given their properties and final cost. Initially, polypropylene fibres were used to control shrinkage cracking. Nevertheless, in the last decade the chemical industry has created larger fibres with better surface shapes, which allows polypropylene fibres to meet the requirements of international codes so they can be used in structural elements. Within this framework, the present PhD thesis aims to contribute to knowledge about fibre reinforced concrete (FRC), especially to study the effectiveness of polypropylene fibres when used as shear reinforcement. For this purpose, a literature review of the material, polypropylene fibre-reinforced concrete (PFRC) and its structural applications is first carried out. This study also discusses the parameters that affect the shear behaviour of traditional concrete and FRC. In order to evaluate the effectiveness of polypropylene fibres in shear, three experimental campaigns are presented. Each campaign represents a different level of study. The first corresponds to the material level, where the shear behaviour of PFRC is evaluated by push-off specimens. The second level involves studying shear in real scale elements. For this purpose, shear critical slender beams were manufactured and tested. The last level corresponds to real application of polypropylene fibres to act as shear reinforcement. In this campaign, deep hollow core slabs, with real sections and supports conditions, were tested. At each level, the shear behaviour of PFRC was evaluated against control reinforced concrete specimens, which were also tested during each campaign. / [ES] Varias investigaciones experimentales y teóricas han sido realizadas para entender el comportamiento a cortante de elementos de hormigón y sus variables. Sin embargo, hoy en día debido a la complejidad del tema, el comportamiento a cortante de elementos de hormigón armado y en especial aquellos que no tienen refuerzo transversal, continúan sin tener una explicación clara. Por otro lado, esta complejidad del cortante aumenta cuando nuevas variables, como las fibras, se incorporan al estudio. Investigaciones han demostrado la efectividad de las fibras de acero para mejorar las propiedades mecánicas de hormigón. Según resultados experimentales, la fibra de acero mejora la resistencia cortante y ductilidad de ciertos elementos. Y en cantidades adecuadas, la fibra puede sustituir total o parcialmente los refuerzos tradicionales de cortante. Es así que varios códigos internacionales han incluido requisitos para tener a las fibras en la respuesta estructural de elementos de hormigón. Sin embargo, estos requerimientos se han creado originalmente para el hormigón reforzado con fibra de acero (Steel fibre-reinforced concrete -SFRC). Nuevas fibras con diferentes materiales y formas, como las fibras macro-sintéticas, han sido introducidas en el mercado. Estas fibras, también llamadas fibras de polipropileno o poliolefina, son una alternativa en la construcción debido a su propiedades y costo final. Inicialmente, las fibras de polipropileno eran usadas únicamente en el hormigón para controlar la fisuración por retracción. Sin embargo, en la última década la industria química ha desarrollado fibras más grandes y con mejores prestaciones de adherencia, que permiten a estas fibras cumplir con requisitos para ser utilizadas estructuralmente. En este contexto, la presente tesis pretende ser una contribución al conocimiento sobre el hormigón reforzado con fibras (Fibre-reinforced concrete - FRC), especialmente en la efectividad de las fibras de polipropileno como refuerzo a cortante. Para esto, primero se realiza un estudio bibliográfico del hormigón reforzado con fibra de polipropileno (PFRC) como material y sus aplicaciones estructurales. Este estudio también tratará sobre los parámetros que afectan el comportamiento a cortante del hormigón tradicional y hormigón reforzado con fibras. Para evaluar la efectividad de las fibras de polipropileno en el cortante, se realizarán tres campañas experimentales. Cada campaña representa un nivel de estudio diferente. El primero es a nivel material en donde se evalúa el comportamiento a cortante a través de especímenes tipo Push-off. El segundo nivel, corresponde al estudio del cortante en elementos a escala real. Para esto se fabrican y ensayan vigas esbeltas críticas a cortante. El último nivel corresponde a una aplicación real de fibras de polipropileno actuando como refuerzo cortante. En esta campaña, se fabrican y ensayan placas alveolares de gran canto con secciones y condiciones de apoyos reales. / [CA] Diverses investigacions experimentals i teòriques han estat realitzades per entendre el comportament a tallant d'elements de formigó i les seues variables. No obstant això, hui en dia a causa de la complexitat del tema, el comportament a tallant d'elements de formigó armat i especialment aquells que no tenen reforç transversal, continuen sense tindre una explicació clara. D'altra banda, aquesta complexitat del tallant augmenta quan noves variables, com les fibres, s'incorporen a l'estudi. Investigacions han demostrat l'efectivitat de les fibres d'acer per a millorar les propietats mecàniques del formigó. Segons resultats experimentals, les fibres d'acer milloren la resistència a tallant i la ductilitat de certs elements. A més, en quantitats adequades, les fibres poden substituir total o parcialment els reforços tradicionals de tallant. És així que diversos codis internacionals han inclòs requisits per a tindre amb compte la resposta estructural de les fibres en els elements de formigó. No obstant això, aquests requeriments s'han creat originalment per al formigó reforçat amb fibres d'acer (Steel fibre-reinforced concrete -SFRC). Noves fibres amb diferents materials i formes, com les fibres macro-sintètiques, han estat introduïdes al mercat. Aquestes fibres, també anomenades fibres de polipropilè o poliolefina, són una alternativa a la construcció a causa de les seues propietats i cost final. Inicialment, les fibres de polipropilè eren usades únicament en el formigó per controlar la fissuració per retracció. No obstant això, en l'última dècada, la industria química ha desenvolupat fibres més grans i amb millors prestacions d'adherència, que permeten a aquestes fibres complir amb requisits per a ser utilitzades estructuralment. En aquest context, la present tesi pretén ser una contribució al coneixement sobre el formigó reforçat amb fibres (Fibre-reinforced concrete - FRC), especialment en l'efectivitat de les fibres de polipropilè com a reforç a tallant. Per això, primer es realitza un estudi bibliogràfic del formigó reforçat amb fibres de polipropilè (PFRC) com a material i les seues plicacions estructurals. Aquest estudi també tractarà sobre els paràmetres que afecten el comportament a tallant del formigó tradicional i del formigó reforçat amb fibres. Per avaluar l'efectivitat de les fibres de polipropilè en el tallant, es realitzaran tres campanyes experimentals. Cada campanya representa un nivell d'estudi diferent. El primer és a nivell material on s'avalua el comportament a tallant a través d'espècimens tipus Push-off. El segon nivell, correspon a l'estudi del tallant en elements a escala real. Per això es fabriquen i assagen bigues esveltes crítiques a tallant. L'últim nivell correspon a una aplicació real de fibres de polipropilè actuant com a reforç a tallant. En aquesta campanya, es fabriquen i assagen plaques alveolars de gran cantell amb seccions i condicions de suports reals. / Ortiz Navas, FR. (2020). Effectiveness of polypropylene fibres as shear reinforcement in structural elements [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/153147