Creep behaviour of cracked steel fibre reinforced self-compacting concrete laminar structures

Tese de Doutoramento - Civil EngineeringFibre reinforced self-compacting concrete (FRSCC) is a novel type of concrete containing discrete elements, which enhance the concrete’s post-cracking properties. For structural applications, commonly, it contains short discrete steel fibres that are randomly distributed and oriented. Fibre reinforcement can be consider as an alternative to conventional steel bars in order to improve the structural efficiency as well as the in-situ working conditions. Although, in the last decades, fibres were mainly applied in nonstructural elements or even to control early thermal cracking, and plastic or drying shrinkage, they can be used toreduce or even replace the ordinary steel reinforcements in structural concrete elements. In the laminar structures with lower safety requirements such as grade slabs, nowadays, fibres can already be considered as an alternative in order to replace the ordinary bars completely. One of the most important benefits of steel fibre reinforced concrete, SFRC, is the durability improvement, as a consequence of its improved post-cracking properties. For a cracked section, under a sustained load, the time-dependent crack widening has been attributed to two mechanisms: fibre pull-out process and time-dependent fibre creep. Creep is a visco-elastic phenomenon, which mainly occurs in the viscous hydrated cement paste. This may be a concern, since steel fibre reinforced self-compacting concrete (SFRSCC) has a high binder content, in part to attain its self-compactibility. Thus SFRSCC may exhibit a relatively high deformability due to long-term loads. The present work aims to increase knowledge within this research area. The main purpose was to achieve as much as possible a consistent comprehension of the behaviour of this material under monotonic and long-term (in the cracked state) loading conditions. Therefore, in the first phase, an experimental campaign was executed in order to understand how fibres were distributed and oriented in SFRSCC laminar structures, and, furthermore, how these parameters influence the overall composite behaviour at a macrolevel. Then, the micro-mechanical aspects of fibre reinforcements were analysed by performing a series of monotonic and long-term fibre pull-out tests. Finally, based on the fibre’s micro-mechanical properties, an integral approach was used to predict the flexural behaviour of SFRSCC laminar structures under monotonic and long-term loading conditions.Betão auto-compactável reforçado com fibras (BACRF) é um material relativamente recente contendo elementos de reforço discretos, os quais contribuem para o melhoramento das propriedades pós-fissuradas do material. Para aplicações estruturais, usualmente, são utilizadas fibras de aço distribuídas e orientadas aleatoriamente. A aplicação das fibras discretas pode ser encarada como uma alternativa viável ao uso de armadura convencional, fomentando a eficiência estrutural e melhorando as condições de trabalho in-situ. No entanto, nas últimas décadas, as fibras têm sido principalmente utilizadas com um propósito nãoestrutural, para o controlo da fissuração devida ao calor de hidratação, retração plástica e por secagem;as fibras podem ser uma alternativa plausível para a redução ou eliminação da armadura convencional em estruturas de betão armado. Em estruturas laminares com menores exigências do ponto de vista estrutural, como por exemplo lajes / pavimentos térreos, o uso de fibras, atualmente, já é encarado como uma alternativa viável à armadura convencional. Uma das maiores vantagens do betão reforçado com fibras aço, BRFA, é a sua durabilidade, que advém do seu comportamento melhorado no regime pós-fissurado. Para uma secção fissurada submetida a uma carga constante, a evolução da abertura da fissura com o tempo é atribuída a dois mecanismos, nomeadamente, o arrancamento da fibra e a fluência da mesma ao longo do tempo. A fluência é um fenómeno visco-elástico que ocorre principalmente na pasta de cimento hidratada. No caso de betões auto-compactáveis, tal facto pode ser uma preocupação dado que este tipo de betões têm uma quantidade elevada de ligante, em parte para cumprir requisitos de auto-compactibilidade. Por esta razão, os betões auto-compactáveis poderão exibir uma elevada deformabilidade ao comportamento diferido quando sujeitos a cargas constantes. Com este trabalho pretende-se aumentar o conhecimento do comportamento diferido de BACRF no regime pós-fissurado. Numa primeira fase estudou-se a distribuição e orientação das fibras em estruturas laminares de BACRF, e como esses parâmetros influenciam o comportamento mecânico do compósito. Posteriormente, o comportamento micro-mecânico das fibras foi estudado através se uma série de ensaios de arrancamento monotónicos e diferidos no tempo. Finalmente, com base nos ensaios de arrancamento, é proposta uma abordagem integrada para prever o comportamento de estruturas laminares de BACRF em flexão sob ação de cargas monotónicas e diferidas no tempo

Creep behaviour of cracked steel fibre reinforced self-compacting concrete: from micro-mechanics to composite behaviour

This work presents the results of an extensive experimental program that aims to study the long-term behaviour of cracked steel fibre reinforced self-compacting concrete (SFRSCC). At the micro-level, a series of single fibre pull-out creep tests were performed, and the fibre slip was monitored as a function of time. The influence of fibre orientation angle (0, 30 and 60 degrees), as well as pre-imposed fibre slip levels 0.3 and 0.5 mm on the creep behaviour was investigated. At the composite level, prismatic specimens were extracted from a SFRSCC panel, having in consideration the concrete flow direction. The specimens were pre-cracked up to 0.3 and 0.5 mm and subjected to a sustained flexural loading. In this stage, the influence of the pre-crack level and fibre orientation on the long-term crack opening was appraised. Furthermore, instantaneous fibre pull-out tests and bending tests were performed on undamaged specimens to quantify the effects of the creep phenomenon. It was found that a higher damage level in the specimens accelerated the creep rate. Moreover, the creep performance of the SFRSCC was influenced by the fibre orientation. Finally, the assembled creep curves did not differ considerably from the instantaneous behaviours for the adopted pre-damaged levels.T his work is supported by the FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the project SlabSys - HFRC - PTDC/ECM/120394/2010. Radmix and Maccaferri (fibres), SECIL (cement), SIKA and BASF (superplasticizers), Omya Comital (limestone filler), and Pegop (Fly ash). Civitest Company for the development of the Steel Fibre Reinforced Self - Compacting Concrete used in this wor

Time-dependent fibre pull-out behaviour in self-compacting concrete

In the present study, the effectiveness of a fibre as an element for transferring stresses across cracks under a sustained load was assessed. Single fibre pull-out creep tests were performed, in which fibre slip was monitored as a function of the time. The influence of the fibre orientation angle (0, 30 and 60 degrees), as well as pre-imposed fibre slip levels, spr, 0.3 and 0.5 mm on the creep response was investigated. Additionally, instantaneous fibre pull-out tests were carried out on undamaged-bond specimens in order to quantify the effects of the pull-out creep behaviour. The damage introduced by the pre-slip levels in the bond of the fibre/matrix interface influenced the long-term fibre pull-out behaviour and, consequently, accelerated the creep rate. However, the assembled pull-out creep behaviour did not differ considerably from the instantaneous pull-out behaviour for the adopted pre-imposed fibre slip levels.This work is supported by the FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the project 18 SlabSys-HFRC-PTDC/ECM/120394/2010. The authors would like to acknowledge the materials supplied by Radmix and Maccaferri (fibres), SECIL (cement), SIKA and BASF (superplasticizers), Omya Comital (limestone filler), and Pegop (Fly ash).info:eu-repo/semantics/publishedVersio

Determinação da resistência à tração de camadas de UHPFRC com base em ensaio não-destrutivo

[ES] No presente artigo apresenta-se um procedimento para estimar a resistência à tração de camadas de materiais cimentícios de ultra-elevado desempenho reforçados com fibras de aço (Ultra-High Performance Fibre-Reinforced cement Composites – UHPFRC na sigla em Inglês), recorrendo a um modelo mecânico simples proposto inicialmente por Naaman [1]. Este procedimento baseia-se na aplicação de um método de ensaio não-destrutivo (NDT), desenvolvido na FEUP, que permite avaliar a dosagem e um coeficiente de orientação das fibras; e em ensaios laboratoriais de tração para estimar a tensão de aderência fibra-matriz equivalente, assumindo um comportamento rigido-plástico. Foi desenvolvido um programa experimental que permitiu estabelecer a relação entre o coeficiente de orientação determinado pelo método NDT e os parâmetros de orientação determinados a partir de análise de imagem. Finalmente, seguindo o procedimento proposto, foi estimada a resistência à tração de diversos provetes, com uma gama variada de dosagens e orientação das fibras, e comparada com os resultados experimentais. A boa correlação encontrada demonstra o potencial do método NDT proposto para a implementação de procedimentos de controle de qualidade em aplicações com UHPFRC.Este trabalho foi financiado por: Projeto POCI-01-0145-FEDER-007457 - CONSTRUCT - Instituto de I&D em Estruturas e Construções - financiado pelo Fundo Europeu de Desenvolvimento Regional (FEDER), através do COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI) e por fundos nacionais através da FCT.Nunes, S.; Pimentel, M.; Abrishambaf, A. (2018). Determinação da resistência à tração de camadas de UHPFRC com base em ensaio não-destrutivo. En HAC 2018. V Congreso Iberoamericano de hormigón autocompactable y hormigones especiales. Editorial Universitat Politècnica de València. 331-340. https://doi.org/10.4995/HAC2018.2018.5506OCS33134

Tensile stress–crack width law for steel fibre reinforced self-compacting concrete obtained from indirect (splitting) tensile tests

In this work, the fracture mode I parameters of steel fibre reinforced self-compacting concrete (SFRSCC) were derived from the numerical simulation of indirect splitting tensile tests. The combined experimental and numerical research allowed a comparison between the stress-crack width (σ - w) relationship acquired straightforwardly from direct tensile tests, and the σ - w response derived from inverse analysis of the splitting tensile tests results. For this purpose a comprehensive nonlinear 3D finite element (FE) modeling strategy was developed. A comparison between the experimental results obtained from splitting tensile tests and the corresponding FE simulations confirmed the good accuracy of the proposed strategy to derive the σ – w for these composites. It is concluded that the post-cracking tensile laws obtained from inverse analysis provided a close relationship with the ones obtained from the experimental uniaxial tensile tests.The studies reported in this paper are part of the research project LEGOUSE (QREN, project nº 5387). This project is co-supported by FEDER through COMPETE program (“Programa Operacional Factores de Competitividade”). The materials were supplied by Radmix and Maccaferri (fibres), SECIL (cement), SIKA and BASF (superplasticizers), Omya Comital (limestone filler), and Pegop (Fly ash).The studies reported in this paper are part of the research project LEGOUSE (QREN, project nº 5387). This project is co-supported by FEDER through COMPETE program (“Programa Operacional Factores de Competitividade”). The materials were supplied by Radmix and Maccaferri (fibres), SECIL (cement), SIKA and BASF (superplasticizers), Omya Comital (limestone filler), and Pegop (Fly ash)

The influence of fibre orientation on the post-cracking tensile behaviour of steel fibre reinforced self-compacting concrete

Adding fibres to concrete provides several advantages, especially in terms of controlling the crack opening width and propagation after the cracking onset. However, distribution and orientation of the fibres toward the active crack plane are significantly important in order to maximize its benefits. Therefore, in this study, the effect of the fibre distribution and orientation on the post-cracking tensile behaviour of the steel fibre reinforced self-compacting concrete (SFRSCC) specimens is investigated. For this purpose, several cores were extracted from distinct locations of a panel and were subjected to indirect (splitting) and direct tensile tests. The local stress-crack opening relationship (σ-w) was obtained by modelling the splitting tensile test under the finite element framework and by performing an Inverse Analysis (IA) procedure. Afterwards the σ-w law obtained from IA is then compared with the one ascertained directly from the uniaxial tensile tests. Finally, the fibre distribution/orientation parameters were determined adopting an image analysis technique.Fundação para a Ciência e a Tecnologia (FCT

Mechanical performance of fibre reinforced concrete : the role of fibre distribution and orientation

Adding fibres to concrete provides several advantages, especially in terms of controlling the crack opening width after the cracking initiation of the composite. However, distribution and orientation of the fibres toward the crack plane are significantly important in order to provide the maximum benefit for controlling crack width. Therefore, in this study, the effect of the fibre distribution and orientation on the tensile behaviour of the steel fibre reinforced self-compacting concrete (SFRSCC) specimens is investigated. For this purpose, cores that are extracted from distinct locations of a panel will be subjected to indirect (splitting) and direct tensile tests. By modeling the splitting tensile test under the finite element framework and by performing an Inverse Analysis (IA), the achieved stress-crack opening relationship (σ-w) is compared with the one obtained directly from the experimental curve obtained in the direct tensile tests.Fundação para a Ciência e a Tecnologia (FCT

A two-phase material approach to model steel fibre reinforced self-compacting concrete in panels

This work presents an experimental and numerical approach to ascertain the mechanical behaviour of steel fibre reinforced self-compacting concrete in laminar structures. Four-point flexural tests were performed on prismatic specimens extracted from a SFRSCC panel; the specimens’ behaviour was then modelled under the FEM framework. SFRSCC is assumed as a two-phase material, i.e. plain concrete and discrete steel fibres. The nonlinear material behaviour of the plain matrix was simulated using 3D smeared crack model, while the fibre reinforcement mechanisms were modelled using micro-mechanical behaviour laws determined from experimental fibre pull-out tests. The good performance of the developed numerical strategy was demonstrated.FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the project SlabSys-HFRC-PTDC/ECM/120394/201

Time-dependent flexural behaviour of cracked steel fibre reinforced self-compacting concrete panels

In the present work are described and discussed the results of an extensive experimental program that aims to study the long-term behaviour of cracked steel fibre reinforced self-compacting concrete, SFRSCC, applied in laminar structures. In a first stage, the influence of the initial crack opening level (wcr = 0.3 and 0.5 mm), applied stress level, fibre orientation/dispersion and distance from the casting point, on the flexural creep behaviour of SFRSCC was investigated. Moreover, in order to evaluate the effects of the creep phenomenon on the residual flexural strength, a series of monotonic tests were also executed. It was found that wcr = 0.5 mm series showed a higher creep coefficient comparing to the series with a lower initial crack opening. Furthermore, the creep performance of the SFRSCC was influenced by the orientation of the extracted prismatic specimens regarding the direction of the concrete flow within the cast panel.This work is supported by the FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the project SlabSys-HFRC-PTDC/ECM/120394/2010. The authors would like to acknowledge the materials supplied by Radmix and Maccaferri (fibres), SECIL (cement), SIKA and BASF (superplasticizers), Omya Comital (limestone filler), and Pegop (Fly ash)

Assessment of fibre orientation and distribution in steel fibre reinforced self-compacting concrete panels

The benefits of adding fibres to concrete lie, mostly, in improving the post-cracking behaviour, since its ability to transfer stresses across cracked sections is substantially increased. The post-cracking strength is dependent not only on the fibre geometry, mechanical performance and fibre/matrix interface properties, but also on the fibre orientation and distribution. Previous works have shown that in self-compacting concrete matrices, there is a preferential fibre alignment according to the concrete’s flow in the fresh state. Having in mind that fibres are more efficient if they are oriented according the principal tensile stresses, a preferential fibre alignment on a certain direction could either enhance or diminish the material and the structural performance of this composite. In this paper, it is investigated the influence of the fibre orientation and distribution on the post-cracking behaviour of the steel fibre reinforced self-compacting concrete (SFRSCC). To perform this evaluation, SFRSCC panels were casted from their centre point. Two self-compacting mixtures were prepared using the same base mix proportions. For each SFRSCC panel cylindrical specimens were extracted and the post-cracking behaviour was assessed from a crack width controlled splitting tensile test