Constitutive models to simulate failure of structures made by cement based materials

Tese de Doutoramento - Plano de tese no âmbito do Programa Doutoral em Engenharia CivilNonlinear Finite Element Analysis (NFEA) has been widely adopted as an effective and reliable method to analyze reinforced concrete (RC) structures subjected to various loading scenarios. Amongst many key factors that affect the reliability of a NFEA tool used for analysing RC structures, the selected constitutive model still remains the foremost challenging task due to the complexity of concrete behaviour associated to the cracking in tension and crushing in compression. The present work proposes a new constitutive model for cement based materials, allowing the possibility of simulating the complex functioning of concrete under both tension and compression. The model proposes a unified approach combining a multidirectional fixed smeared crack model to simulate the crack initiation and propagation with a plastic-damage model to account for the inelastic compressive behaviour of concrete between cracks. The smeared cracking model considers the possibility of forming several cracks in the same integration point, whose orientations, conditioned by an adopted criterion, are however preserved constant during the cracking process. The crack initiation is governed by the Rankine failure criterion, whereas the crack propagation (crack opening process) is simulated by a trilinear (or a quadrilinear) softening diagram. Two approaches are available to simulate the fracture mode II: one based on the concept of shear retention factor, and the other one on a shear softening diagram that requires some information about the fracture mode II propagation. The plasticity model is defined by four entities: yield function (yield surface); flow rule; evolution law for the hardening variable; and condition for defining loading–unloading process. Evolution of the yield surface during the plastic flow is governed by a single hardening parameter for compression. The plasticity part is responsible for simulating irreversible strains and volumetric strain in compression, whereas the strain softening and stiffness degradation of the material under compression are simulated by a strain based isotropic damage model. In this damage approach the state of damage in concrete under compression is equally distributed in all directions, and can be represented by a scalar damage parameter. Calculation of the scalar damage parameter is an explicit operation as this parameter is driven by the plastic hardening parameter. Two versions of the model are developed, one dedicated to concrete structures subjected to plane stress fields, and the second for being applied to concrete structures submitted to three dimensional stress states. Both versions of the model are implemented into FEMIX 4.0 computer program. To appraise the performance of the model and to evidence the interaction between cracking and plasticity-damage parts of the model, some numerical tests at material level are executed, and the obtained results are discussed. The model appraisal at the structural level is also considered. The set of experimental tests simulated in this thesis covers a wide range of specimens regarding geometry, concrete type, loading configurations, and reinforcement conditions, in order to demonstrate the robustness of the developed model. These structures are of particular interest for the assessment of the reliability of the model, since in these examples the failure mechanism involved simultaneous occurrence of cracking and inelastic deformation in compression. The predictive performance of the model in terms of load carrying capacity, ductility, crack pattern, plastic zones, and failure modes is obtained by comparing the results of the numerical simulations and the available experimental data.O método dos elementos finitos (MEF) tem-se revelado eficaz na análise não linear de estruturas de betão armado submetidas a diferentes tipos de carregamentos. De entre os muitos fatores que podem afetar a fiabilidade de uma ferramenta capaz de efetuar uma análise não linear usando o MEF, o modelo constitutivo selecionado ainda continua a ser o desafio mais importante, nomeadamente devido à complexidade do comportamento do betão associado à fendilhação quando sujeito a tração e ao esmagamento em compressão. O presente trabalho propõe um novo modelo constitutivo, capaz de simular o comportamento complexo de materiais de matriz cimentícia quando sujeitos a esforços de tração e de compressão. O modelo propõe uma abordagem unificada, combinando um modelo de múltiplas fendas fixas distribuídas que permite simular o início de fendilhação e a sua propagação com um modelo de dano e plasticidade para simular o comportamento inelástico do betão entre fendas. O modelo de fendilhação permite a formação de várias fendas por ponto de integração, cuja orientação é condicionada por um determinado critério e preservada constante durante o processo de fendilhação. A abertura de fenda é condicionada pelo critério de Rankine, sendo o seu desenvolvimento simulado por intermédio de um diagrama de amolecimento trilinear ou quadrilinear. Duas abordagens estão disponíveis para simular o modo II de fratura: uma baseada no conceito de fator de retenção ao corte, e o outro utilizando um diagrama de amolecimento definido com base nos parâmetros do modo II de fractura. O modelo de plasticidade é definido: pela função de cedência (superfície de cedência); lei de escoamento plástico; lei de endurecimento; condição para a definição do processo de carga e descarga. A evolução da superfície de cedência durante o escoamento plástico é governada por um único parâmetro de endurecimento. A parte da plasticidade é responsável por simular as deformações irreversíveis e a deformação volumétrica em compressão, enquanto o amolecimento e a degradação da rigidez do material sob compressão são simulados por um modelo de dano isotrópico. Nesta abordagem, o estado de dano no betão sob compressão é igualmente distribuído em todas as direções, e pode ser representado por um escalar denominado parâmetro de dano. O modelo proposto é desenvolvido numa primeira fase para estados planos de tensão e posteriormente generalizado para estados de tensão tridimensionais. Estas duas versões do modelo foram integradas no código computacional denominado FEMIX 4.0. De forma a evidenciar as partes do modelo que têm em conta a simulação da propagação da fendilhação, do dano e da plasticidade, bem como da sua interação, são executados alguns testes numéricos focados no comportamento do material, sendo os seus resultados discutidos. Os ensaios experimentais escolhidos para avaliar a robustez do modelo a nível estrutural abrangem uma ampla gama de elementos no que respeita a geometria, tipo de betão, configurações de carga e de reforço. Estas estruturas são de particular interesse para a avaliação da fiabilidade do modelo, uma vez que nestes exemplos ocorrem simultaneamente fendilhação e deformação plástica em compressão. O desempenho do modelo em termos de previsão da capacidade de carga, da ductilidade, do padrão de fendilhação, das zonas plásticas e dos modos de rutura é obtido comparando os resultados das simulações numéricas com os dos ensaios experimentais disponíveis.The research reported in the present thesis is carried out at the Department of Civil Engineering of University of Minho. The financial supports provided by the research projects “PREPAM” with reference number of PTDC/ECM/114511/2009, and “SlabSys- HFRC”, with reference PTCD/ECM/120394/2010, both supported by the Portuguese Foundation for Science and Technology (FCT), are gratefully acknowledged

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

Behaviour of GFRP-steel reinforced I shape beams with steel fibers as shear reinforcement

This paper evaluates the possibility of developing prefabricated beams without stirrups by using fiber reinforcement for increasing the concrete shear capacity, and a hybrid flexural reinforcement system composed of glass fiber reinforced polymer (GFPR) and steel rebars. A high compressive strength and high post-cracking tensile capacity steel fiber reinforced self-compacting concrete (SFRSCC) was developed, aiming at supressing the need of steel stirrups in this type of beams while providing sufficient ductility for structural applications. The experimental results were analysed in terms of failure mode, deformational and cracking behaviour, as well as load carrying capacity. A constitutive model, capable of simulating three types of material nonlinearities simultaneously in an integration point (IP), was used and its predictive performance was assessed by simulating the experimental tests. The numerical approach was then used to assess the potentialities of this material system and structural concept when applied to relatively large span beams.The authors wish to acknowledge the funding provided by project FOATIDE, reference POCI-01- 0145-FEDER-028112

Proposal of a test set up for simultaneous application of axial restraint and vertical loads to slab-like specimens: sizing principles and application

Cracking control in reinforced concrete (RC) is a key factor to ensure proper service life behaviour. However, current design recommendations are unable to provide straightforward methodologies for crack width prediction in RC structures subjected to the combined effects of applied loads and restrained deformations, which is a common situation in civil engineering. This is motivated by the lack of knowledge about the complex interactions that take place between self-imposed deformations, viscoelasticity and the effects of applied loads in the process of crack development. A major challenge in studying these combined effects is the validation of numerical simulations with real scale experimental data. For that purpose, an experimental system for testing real scale RC slabs subjected to the above-mentioned conditions was developed. This system is capable of inducing a prescribed axial restraint to the slab, in correspondence to a high restraint degree that induces cracking in view of expectable shrinkage. At the same time, the setup enables the application of vertical loads. The experimental results obtained in this work allowed for the validation of the test setup, as well as the suitability of the slab geometry and reinforcement.This work was financially supported by: Project POCI-01-0145-FEDER-007457 (CONSTRUCT - Institute of R&D in Structures and Construction) and by project POCI-01- 0145-FEDER-007633 (ISISE), funded by FEDER funds through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI), and by national funds through FCT - Fundação para a Ciência e a Tecnologia. FCT and FEDER (COMPETE2020) are also acknowledged for the funding of the research project IntegraCrete PTDC/ECMEST/1056/2014 (POCI-01-0145-FEDER-016841). The financial support of COST Action TU1404 through its several networking instruments is also gratefully acknowledged.info:eu-repo/semantics/publishedVersio

Application of plastic-damage multidirectional fixed smeared crack model in analysis of RC structures

This paper describes a plasticity-damage multidirectional fixed smeared cracking (PDSC) model to simulate the failure process of concrete and reinforced concrete (RC) structures subjected to different loading paths. The model proposes a unified approach combining a multidirectional fixed smeared crack model to simulate the crack initiation and propagation with a plastic-damage model to account for the inelastic compressive behaviour of concrete between cracks. The smeared crack model considers the possibility of forming several cracks in the same integration point during the cracking process. The plasticity part accounts for the development of irreversible strains and volumetric strain in compression, whereas the strain softening and stiffness degradation of the material under compression are controlled by an isotropic strain base damage model. The theoretical aspects about coupling the fracture, plasticity, and damage components of the model, as well as the model appraisal at both material and structural levels, have been detailed in a former publication. This study briefly summarizes the model formulations, and is mainly dedicated to further explore the potentialities of the proposed constitutive model for the analysis of concrete and RC structures. The model is employed to simulate experimental tests that are governed by nonlinear phenomenon due to simultaneous occurrence of cracking and inelastic deformation in compression. The numerical simulations have predicted with good accuracy the load carrying capacity, ductility, crack pattern, plastic (compressive) zone, and failure modes of all types of structures analysed. The influence of the model parameters that simulate the nonlinear behaviour of concrete under tension and compression is analysed through a parametric study.Portuguese Foundation for Science and Technology in the scope of the SlabSys-HFRC research project, with reference PTDC/ECM/120394/201

Early age measurement of the coefficient of thermal expansion of concrete: a new test setup based on internal heating/cooling

Um dos fatoresque influencia a durabilidade das estruturas de betão armado é o aparecimento de fendas nas primeiras idades devido às tensões internas que se geram por restrição de deformações térmicas durante a cura. A sensibilidade da deformação térmica do betão, representada pelo coeficiente de dilatação térmica (CDT), é uma propriedade que se altera durante as primeiras idades,devidoàs modificações micro-estruturais que ocorrem durante a hidratação do cimento. O conhecimento da evolução do CDTnas primeiras idades é fundamental para uma correta previsão das extensões e tensões internas que se geram logo após a presa do betão. Neste trabalho é apresentado um método integrado para determinar a evolução do CDT de provetes de betão desdeidades anteriores à finalizaçãoda presa, ultrapassando deste modo as principais limitações dos métodos atualmente utilizados para determinação deste coeficienteem materiais cimentícios. O método proposto consiste na exposição de um provete de betão a ciclos de variações térmicas de ±2.5°C,tendo-se desenvolvido um molde de ensaio que permite a leitura da resposta do provete durante as primeiras idades. Asvariações térmicassão impostasatravés da imersão do molde de ensaio em água, a temperatura controlada, e impondo a circulação dessa água no interior do provete através de um tubo em espiral embutido, permitindo desta forma reduzir o gradiente de temperatura entre a superfície e o núcleo e, consequentemente, o tempo de duração dos ciclos térmicos.No presente trabalho são testadas diferentes composições de betão para verificar a aplicabilidade do método proposto para determinação do CDT do betão nas primeiras idades, tendo-se obtido resultados que estão em acordo com o atual estado da arteOne of thefactors influencing the durability of reinforced concrete structures is the occurrence of cracking during the early ages, caused by the internal stresses generated by restrained thermal deformations during cement hydration. Thermal deformation sensitivity, represented by its coefficient of thermal expansion (CTE), is a property that changes during early ages due to, essentially, micro-structural changes that occur during cement hydration.Knowing the evolution of the CTE sinceearly ages is essential for a correct prediction of the internal stresses that occur right after setting of the concrete. In this work, an integrated methodfor determination of concreteCTE evolution duringthe early ages is proposed, in which the main limitationsof the current methods are circumvented. The proposed method consists in exposing a concrete specimen to thermalvariations of ±2.5°C,and a test rig that enables measuringthe specimen’sresponse to those thermal variations since early ages. The thermal variationis imposed by immersing the concrete specimen in a water bath, with controlled temperature, and imposing water circulation inside the specimen through an embedded spiral tube. This allows reducingthe difference between the surface and the core temperatures,andso minimizingthe duration of thermal cycles. In the present work several concrete compositions are tested in order to verify the applicability of the proposed method for determination of concrete CTE in early ages,beingobserved that the results are in accordance with the current state of the artPOCI-01-0145-FEDER-007457 (CONSTRUCT -Institute of R&D in Structures and Construction) e POCI-01-0145-FEDER-007633 (ISISE), financiados por fundos FEDER através do COMPETE2020 -Programa Operacional Competitividade e Internacionalização (POCI), e por fundos nacionais através da FCT –Fundação para a Ciência e Tecnologia. Agradece-se ainda à FCT e FEDER (COMPETE2020) o financiamento do projeto IntegraCrete PTDC/ECM-EST/1056/2014 (POCI-01-0145-FEDER-016841

Numerical Model‐Software for Predicting Rock Formation Failure‐Time Using Fracture Mechanics

Real‐time integrated drilling is an important practice for the upstream petroleum industry. Traditional pre‐drill models, tend to offset the data gathered from the field since information obtained prior to spudding and drilling of new wells often become obsolete due to the changes in geology and geomechanics of reservoir‐rocks or formations. Estimating the complicated non‐linear failure‐time of a rock formation is a difficult but important task that helps to mitigate the effects of rock failure when drilling and producing wells from the subsurface. In this study, parameters that have the strongest impact on rock failure were used to develop a numerical and computational model for evaluating wellbore instability in terms of collapse, fracture, rock strength and failure‐time. This approach presents drilling and well engineers with a better understanding of the fracture mechanics and rock strength failureprediction procedure required to reduce stability problems by forecasting the rock/formation failuretime. The computational technique built into the software, uses the stress distribution around a rock formation as well as the rock’s responses to induced stress as a means of analyzing the failure time of the rock. The results from simulation show that the applied stress has the most significant influence on the failure‐time of the rock. The software also shows that the failure‐time varied over several orders of magnitude for varying stress‐loads. Thus, this will help drilling engineers avoid wellbore failure by adjusting the stress concentration properly through altering the mud pressure and well orientation with respect to in‐situ stresses. As observed from the simulation results for the failure time analysis, the trend shows that the time dependent strength failure is not just a function of the applied stress. Because, at applied stress of 6000–6050 psi there was time dependent failure whereas, at higher applied stress of 6350–6400 psi there was no time dependent strength failure

Three dimensional plastic-damage multidirectional fixed smeared crack approach for modelling concrete structures

A constitutive model to simulate the behaviour up to the failure of concrete structures subjected to multiaxial loading is presented. The proposed model is based on the combination of 3D multidirectional fixed smeared crack model to simulate the crack initiation and propagation, and a plastic-damage model to account for inelastic compressive behaviour of the intact concrete between cracks. The adopted smeared crack model considers the possibility of having more than one crack of different orientation in the same integration point and in different evolution of the cracking process. The plasticity part accounts for the development of irreversible strains and volumetric strain in compression, whereas the strain softening and stiffness degradation of the material under compression are controlled by an isotropic strain based damage model. The constitutive model was included in the 3D solid finite element of the FEMIX computer code, and the model appraisal is performed by simulating experimental tests with structural reinforced concrete (RC) elements. The numerical simulations have predicted with good accuracy the load carrying capacity, deformation, crack pattern, and plastic (compressive) zones of all analyzed tests. A parametric study is also performed to appraise the sensitivity of the numerical simulations to the values adopted for the model parameters.The authors wish to acknowledge the FCT financial support provided by the Portuguese Foundation for Science and Technology in the scope of the SlabSys-HFRC research project, with reference PTDC/ECM/120394/2010. The second author wish to acknowledge the grant SFRH/BSAB/114302/2016 provided by FCT.info:eu-repo/semantics/publishedVersio

A life-cycle approach to integrate environmental and mechanical properties of blended cements containing seashell powder

The adverse consequences of producing ordinary Portland cement (OPC) on the environment have introduced cement production as the fourth largest source of anthropogenic carbon emissions after petroleum, coal, and natural gas. Managing and reducing the environmental concerns regarding the impacts of cement production on the environment, namely the depletion of non-renewable fuel resources, consumption of natural raw materials, and releasing huge amounts of CO2 into the atmosphere should be, therefore, one of the key priorities of the cement industry. Application of locally available minerals and wastes that can be blended with OPC as a substitute could considerably reduce the environmental impact. The present study evaluates the potentiality of waste seashell to be used as an additive in the production of blended cement through a modified life cycle approach integrating environmental and mechanical performances. In this regard, 34 cements consisting of different blends of OPC, seashell powder (within the range of 4–30% by OPC mass), and natural pozzolan (up to 30% by OPC mass) were tested to identify the optimal dosage of OPC substitution. Environmental impacts of the cements were assessed through life-cycle analysis. The possibility of mitigating the carbon dioxide emissions in the production of cements, with similar mechanical performance compared to that of OPC, was evaluated by considering both the mechanical and environmental results. The outcome of this study introduced more environment-friendly and sustainable options for future cements.The first three authors wish to acknowledge the funding provided by project 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 I.P. (National Agency for Science and Technology). The first author also acknowledges the Scientific Employment funding, No. CEECIND/01627/2017, provided by FCT I.P. The financial support provided by FCT I.P. under the project UIDB/04033/2020 is kindly acknowledged by the last author