High performance fiber reinforced concrete for the replacement of shear stirrups

Tese de Doutoramento - Programa Doutoral em Engenharia CivilCorrosion of steel reinforcements, especially stirrups, is considered as one of the most common reasons that shorten the service life of the reinforced concrete, RC, structures. In several cases the rehabilitation of corroded RC structures is so expensive and time consuming that a decision for the demolishment of such members is currently taken, by bringing the consequent economic, social and environmental adverse impacts. Hence, in recent years there is an increasing demand for enhancing the durability and sustainability of concrete structures. The main purpose of the present thesis is to introduce a new design framework for constructing highly durable and structurally effective prefabricated concrete beams without stirrups. These elements are produced by means of a tailor-made high performance fiber reinforced concrete (HPFRC), which is capable of suppressing the steel stirrups without occurring shear failure. A hybrid flexural reinforcement system is used for these beams, composed of glass fiber reinforced polymer (GFRP) rebars placed near to the outer surface of the tensile zone and steel reinforcements positioned with higher HPFRC cover to be protected against the corrosion, which is considered another strategy for enhancing the durability, ensuring the required ductility, and attending fire issues in terms of safety at ultimate limit states. This research combines experimental, numerical and analytical approaches to evaluate the possibilities of the proposed strategy for developing pre-fabrication beams of material and structural requisites at competitive costs. The HPFRC is developed in order to have self-compacting requisites and properties at fresh stage suitable for the type of structural application to be developed, taking also into consideration the influence of fiber distribution and orientation. The mechanical properties of this HPFRC at harden stage are deeply characterized, mainly the postcracking behavior, due to the influence of fiber reinforcement in the fracture parameters of this cementitious material. Considering the influence of shear span to effective depth ratio, / a d, on shear behavior of the beams, the response of both HPFRC short span (1 / 2.5 a d ) and slender beams ( / 2.5 a d ) is evaluated under shear loading configuration. During these studies, the effectiveness of fiber dosage and prestress level applied to GFRP and/or steel flexural reinforcements to improve the shear capacity and failure mode of the designed beams is evaluated as well. By considering the obtained experimental results, the predictive performance of some analytical formulations for the shear resistance of fiber reinforced concrete beams is assessed. An analytical and a FEM-based numerical approach capable of capturing the relevant nonlinear phenomena of the intervening materials in this type of RC members are adopted to demonstrate the benefits of fiber reinforcement and prestress level on the load carrying capacity at serviceability limit state conditions and at steel yield initiation. Based on the developed research, it is demonstrated the possibility of combining HPFRC and FRP reinforcement systems for developing innovative construction systems of high structural performance and ductility, immunes to corrosion phenomena, capable of constituting a new generation of durable and cost competitive solutions for a more sustainable built environment.A corrosão das armaduras de aço, especialmente dos estribos, é considerada uma das principais causas da diminuição da vida útil das estruturas de betão armado (RC). Os custos e tempo de reabilitação de estruturas que apresentam elementos com patologias de corrosão são em diversos casos tão elevados que a opção pela demolição destas estruturas é correntemente tomada, com os consequentes impactos económicos, sociais e ambientais adversos. Assim, nos últimos anos tem existido um crescente interesse em encontrar-se estratégias para aumentar a durabilidade e a sustentabilidade das estruturas de betão armado. O principal objetivo da presente tese é desenvolver uma metodologia de dimensionamento, do material à estrutura, que permita a produção de vigas pré-fabricadas de RC de elevada durabilidade e estruturalmente eficazes sem recorrer à utilização de estribos. Estes elementos são produzidos pelo desenvolvimento de um betão de elevado desempenho reforçado com fibras (HPFRC), com capacidade de dispensar a utilização de estribos de aço sem que ocorra rotura por corte. Nas vigas utilizou-se um sistema híbrido de reforço à flexão composto por varões de fibra de vidro (GFRP) posicionados próximo da superfície exterior da zona tracionada, e varões de aço localizados de forma a terem um maior recobrimento de HPFRC para que sejam protegidos da ocorrência de fenómenos de corrosão, o que constitui outra forma para melhorar a sua durabilidade e mitigar algumas questões relacionadas com a resistência ao fogo em termos de segurança em estados limite último. É realizada investigação experimental, numérica e analítica para se avaliar a possibilidade de utilização da metodologia proposta no desenvolvimento de vigas pré-esforçadas com requisitos materiais e estruturais a custos competitivos. O HPFRC é desenvolvido de forma a apresentar requisitos de auto-compactabilidade, com propriedades em estado fresco apropriadas para este tipo de aplicação, e considerando a influência da distribuição e orientação das fibras. As propriedades mecânicas deste HPFRC em estado endurecido são profundamente caracterizadas, principalmente as relativas ao comportamento pós-fissurado, dada a influência do reforço das fibras nos parâmetros de fratura deste material cimentício. Considerando a influência da relação entre o vão de corte e a altura efetiva, / a d, no comportamento ao corte de vigas, são ensaiadas e estudadas vigas com 1 / 2.5 a d e com / 2.5 a d sob carregamentos de corte. Durante estes estudos também se avalia a eficiência da dosagem de fibras no HPFRC e do nível de pré-esforço aplicado aos varões de GFRP e/ou cabo de aço de reforço à flexão no melhoramento da capacidade de resistência ao corte e no modo de rotura dos elementos concebidos. Considerando os resultados experimentais obtidos, é avaliada a capacidade de previsão de algumas formulações analíticas para previsão da resistência ao corte de vigas de betão reforçado com fibras. É adotada uma abordagem analítica e numérica (esta última baseada no método dos elementos finitos), capaz de captar os fenómenos mais relevantes que governam o comportamento material e estrutural deste tipo de elementos, para demonstrar os benefícios dos mecanismos de reforço das fibras e da pré-tensão nas armaduras de flexão na capacidade de carga e ductilidade em estados limites de serviço e últimos. Os conhecimentos resultantes dos estudos realizados foram aplicados na demonstração de ser possível combinar HPFRC e reforços em FRP no desenvolvimento de inovadores sistemas construtivos de elevado desempenho estrutural e ductilidade, imunes a fenómenos de corrosão, e capazes de constituírem uma nova geração de soluções duráveis a custo competitivo para o ambiente construído

Steel fiber reinforced self-compacting concrete : from material to mechanical behaviour

Report: 12-DEC/E-19This study is a part of the research project entitled “DURCOST – Innovation in reinforcing systems for sustainable prefabricated structures of higher durability and enhanced structural performance” supported by the FCT, PTDC/ECM/105700/2008

Interfacial bond behaviour of GFRP bar in self-compacting fiber reinforced concrete

In an ongoing research project, discrete steel fibers are being used in a self-compacting concrete (SFRSCC) to replace completely steel stirrups for pre-fabricated beams reinforced longitudinally with pre-stressed glass fiber reinforced polymer (GFRP) and steel bars. To take the advantages of the non-corrodible character and high tensile strength of GFRP bars, the minimum SFRSCC cover needs to be determined in order to assure the adequate bond performance between these bars and the surrounding SFRSCC. Since bond of the longitudinal bars has a relevant impact on the cracking behavior of RC elements (crack opening and crack spacing), an extensive experimental program composed of pullout bending tests was carried out where the influence of the following parameters was assessed in terms of bond behavior: GFRP bar diameter, surface characteristics of the GFRP bars, bond length, SFRSCC cover thickness. The local bond law was derived from inverse analysis and it was used to define the slip mode of the constitutive law adopted for interface finite elements. These interface finite elements were used to assess the crack opening and crack spacing on SFRSCC beams flexurally reinforced with GFRP bars. This paper resumes the experimental program, describes the strategy to derive the local bond law and presents and discusses the numerical simulations

Study of the fracture behaviour of fiber reinforced concrete under direct shear loading

Report 12-DEC/E-18The behavior of Fiber Reinforced Concrete (FRC) under Mode II of fracture is rarely reported due to the lack of information on standard testing procedures. Between the few numbers of researches performed to characterize the shear behavior of concrete, no one have dedicated to the effects of fiber orientation at different places of a concrete element subjected to second mode of fracture. In addition the efforts made on mode II loading effects are rather limited in plain concrete in general and in fiber reinforced concrete in particular. In the present study the double shear specimen (DSS) has been used to characterize the shear behavior of Fiber Reinforced Self Compacting Concrete (FRSCC). Considering the effects of fiber orientation on Mode II of fracture, some beams 150 × 150mm cross section and 60mm length has been discretized in to 8 or 12 specimens included different shear sections. Using the designed specimen, fracture energy and toughness, stress intensity factor and shear stress-slip relationship is calculated along the beams. The results show that none of the specimens are subjected to pure Mode II due to the considerable ratio of the stress intensity factors in first mode of fracture to that of the second mode " . " Unlike the crack propagation in mode I of fracture, the shear crack propagation is accompanied by compression failure after the maximum load. The fiber bridging not only restrains the crack opening but also leads to higher interlocking. Furthermore, the dowel action of the fibers, leads to a lower slip and higher shear transfer. The suitable fracture energy and consequently high toughness is detected not only at the middle of the beam but even in the case of the specimen located at the farther distances from the center of the beam. It is indicated that using the proper Self Compacting Concrete (SCC) mix can be helpful to have the uniform shear behavior along the beam. Additionally increasing the shear plane appears to have an increasing trend in ductility, rate of fracture and toughness of the specimens. Furthermore, the orientation and distribution of fibers in concrete is investigated by the help of image analyzing. It is observed that fibers dispersed homogeneously in all concrete series. Although the density of the fibers detected on the shear plane of all the specimens is almost the same, the ductility of the specimens is different. It shows that the ductility of the specimens can be affected by the area of the shear plane

High performance fiber reinforced concrete for the shear reinforcement: experimental and numerical research

High performance fiber reinforced concrete (HPFRC) is developing rapidly to a modern structural material with unique rheological and mechanical characteristics. Despite applying several methodologies to achieve self15 compacting requirements, some doubts still remain regarding the most convenient strategy for developing a HPFRC. In the present study, an innovative mix design method is proposed for the development of high17 performance concrete reinforced with a relatively high dosage of steel fibers. The material properties of the developed concrete are assessed, and the concrete structural behavior is characterized under compressive, flexural and shear loading. This study better clarifies the significant contribution of fibers for shear resistance of concrete elements. This paper further discusses a FEM-based simulation, aiming to address the possibility of calibrating the constitutive model parameters related to fracture modes I and II.The research in this paper is part of the project "DURCOST - Innovation in reinforcing systems for sustainable pre-fabricated structures of higher durability and enhanced structural performance" with reference number of PTDC/ECM/105700/2008, supported by FCT. The authors also thank the collaboration of the following companies: Casais to manufacture the moulds, Ibermetais for supplying the steel fibres, Secil/Unibetao for providing the Cement, BASF for supplying the superplasticizers. The first author acknowledges the research grant in the ambit of this project

Assessment of the sustainability of fibre-reinforced concrete by considering both environmental and mechanical properties

The environmental consequences of human activities, e.g., the depletion of non-renewable fuel resources, consumption of natural raw materials, and release of huge amounts of CO2 into the atmosphere, resulted in new challenges in materials engineering. Based on these challenges, building materials must fulfil not only mechanical performance criteria, but also produce the least environmental impact accompanied by their production. In the present study, the possibility of employing scrap tire recycled steel fibres (RSF) as a substitution to industrial steel fibres (ISF) for developing more sustainable fibre-reinforced concretes was explored by adopting a life-cycle approach, integrated both environmental and mechanical properties. Four different fibre-reinforced self-compacting concretes–FRSCCs–were tailored by means of replacing the ISFs partially/totally (i.e., 0%, 50%, 67%, 100% by mass of) with the recycled ones. The effect of applying various dosages of RSFs on mechanical behavior of FRSCC–namely compressive, flexural, and splitting tensile responses–were evaluated experimentally. The environmental impacts associated with the production of each FRSCC were also assessed through life-cycle analysis. The potentiality of the RSFs to be used as concrete reinforcement with a comparable mechanical performance to that of ISF-reinforced concrete and lower environmental footprint was evaluated through a consolidated environmental and mechanical index (EM). In this study, using RSFs instead of industrial fibres for developing FRSCC has provided up to 37% higher EM index. The results confirmed the promising prospects for the application of RSFs in developing more eco-efficient and sustainable reinforced concrete.FOATIDE, reference POCI-01-0145-FEDER-028112, co-financed by the European Regional Development Fund (ERDF), through the Operational Programme for Competitiveness and Internationalization (COMPETE 2020), under Portugal 2020, and by the Fundação para a Ciência e a Tecnologia—FCT (National Agency for Science and Technology). The first author also acknowledges the Scientific Employment funding, No. CEECIND/01627/2017, provided by FCT. The financial support provided by FCT under the project UIDB/04033/2020 is kindly acknowledged by the last author

Shear capacity of HPFRC beams flexurally reinforced with steel and prestressed GFRP bars

This paper presents the relevant results from an experimental program to assess the shear capacity of high performance fiber reinforced concrete (HPFRC) beams flexurally reinforced with a hybrid system of passive steel and prestressed GFRP longitudinal bars. Three series of two beams with different level of prestressing were tested. The effect of prestressing level on the shear capacity of the beams was the main investigated parameter. The results showed an enhancement of the load carrying capacity, ductility and energy absorption with the increase of the prestress level. Based on the obtained results, the predictive performance of the analytical formulations of CEB-FIP Model Code 2010 and RILEM TC 162-TDF for the shear capacity of FRC beams was assessed. Both formulations seem appropriate for design purposes, but the CEB-FIP formulation predicts more conservative shear capacity. The experimental results demonstrated that the prestressing level has an effect on the shear capacity much higher than the one recommended by the codes

Shear resistance of SFRSCC short-span beams without transversal reinforcements

Corrosion of steel reinforcements, especially stirrups, is considered as one of the most common reasons that shorten the service life of the reinforced concrete structures. This study aims to replace the stirrups of the beams by means of a tailor made steel fiber reinforced self-compacting concrete (SFRSCC). A hybrid flexural reinforcement system was used for all these beams, composed of glass fiber reinforced polymer (GFRP) rebars placed near to the outer surface of the tensile zone and steel reinforcements positioned with higher SFRSCC cover to be protected against the corrosion, which is considered another strategy for enhancing the durability and attending fire issues in terms of safety at ultimate limit states. The effectiveness of varying the prestressing force applied to GFRP bars to improve the shear capacity and failure mode of the designed elements is evaluated. By considering the obtained experimental results, the predictive performance of some analytical formulations for the shear resistance of fiber reinforced concrete beams was assessed. All formulations demonstrate acceptable accuracy for design purposes, but the one proposed by CEB-FIP Model Code 2010 predicts more conservative shear resistance.European Regional Development Fund (FEDER) - “Inotec”, with reference number 23024Portuguese Foundation for Science and Technology (FCT) - “SlabSys-HFRC”, with reference PTDC/ECM/120394/201

Effect of fiber dosage and prestress level on shear behavior of hybrid GFRP-steel reinforced concrete I-shape beams without stirrups

Corrosion of steel reinforcements embedded in concrete elements is generally known as one of the most common reasons that shorten the service life of the structures. The present study aims to contribute in overcoming this problem by replacing steel stirrups as shear reinforcement of concrete beams using a steel fiber reinforced self-compacting concrete (SFRSCC). In the present research the potential of SFRSCC for improving the shear resistance of the beams without stirrups is explored. In order to further reduce the risk of corrosion in this type of beams, a hybrid system of flexural reinforcement composed of a steel strand and GFRP rebars is applied and properly arranged in order to assure a relatively thick concrete cover for the steel reinforcement. The GFRP bars are placed with the minimum cover thickness for providing the maximum internal arm and, consequently, mobilizing efficiently their relatively high tensile strength. The effectiveness of applying different dosages of steel fibers and varying the prestress force to improve the shear behavior of the designed beam are evaluated. By considering the obtained experimental results, the predictive performance of a constitutive model (plastic-damage multidirectional fixed smeared crack model) implemented in a FEM-based computer program, as well as the one from three analytical formulations for estimating shear resistance of the developed beams were assessed. The FEM-based simulations have provided a good prediction of the deformational response and cracking behavior of the tested beams. All the analytical formulations demonstrated acceptable accuracy for design purposes, but the one proposed by CEB-FIP Modal Code 2010 predicts more conservative shear resistance.The first and second authors, respectively, acknowledge the research grant in the ambit of the project “UrbanCrete”, with reference number of 30367, supported by the European Regional Development Fund (FEDER), and “SlabSys-HFRC”, with reference PTCD/ECM/120394/2010, supported by the Portuguese Foundation for Science and Technology (FCT). The authors also thank the collaboration of the following companies: Tensacciaci in the name of Eng. F. Pimenta for the assistance on the application of prestress reinforcements, Sireg and Schoeck for providing the GFRP rebars, Casais to manufacture the moulds, Exporplas for supplying the polypropylene fibers, Secil/Unibetão for providing the Cement, BASF for supplying the superplasticizer and CiviTest for collaborating in producing the specimens

Analytical bond model for GFRP bars to steel fiber reinforced self-compacting concrete

The objective of this study is to present a computational algorithm to analytically evaluate the bond behavior between GFRP bar and steel fiber reinforced self-compacting concrete (SFRSCC). The type of information to be derived is appropriate to study the flexural behavior of SFRSCC beams reinforced with GFRP bars in terms of serviceability limit states requirements; in fact the bond between bars and surrounding concrete influences significantly the crack width and crack spacing. The proposed bond model was established by calibrating the parameters of a multilinear bond-slip constitutive law using the experimental results of pullout bending tests carried out by the authors, taking into account the experimental pullout force versus slip at loaded and free ends. According to the comparison between theoretical and experimental pullout force-slip, an acceptable accuracy of the model was observed. Additionally, by considering the proposed bond-slip relationship, a parametric study was carried out to evaluate the influence of the involved bond-slip law’s parameters on the maximum force transferred to the surrounding concrete. Finally, the development length of two GFRP bars utilized in the experiments (deformed and smooth bars) was determined by means of the proposed model, and it was compared with the values recommended by codes.Fundação para a Ciência e a Tecnologia (FCT