Monitoring and simulating humidity profiles in concrete elements during drying

Integrated approaches towards the measurement and simulation of the internal humidity of cement based materials, in which the same team accomplishes all the tasks are scarce in the literature. Furthermore, the measurement of humidity in cement-based materials is a subject that requires experience and significant care to obtain reliable data. In view of this reasoning, the research here reported pertains to an integrated approach that focuses in two main topics: (a) several issues regarding the experimental measurement of internal humidity in cement based materials through sleeved humidity probes, with test series devoted to each issue (namely the type of sensor, the existence of Gore Tex fabric to protect the sensor and the influence of the permanence time of probes within measuring sleeves); (b) an experimental program for humidity measurement in concrete specimens, followed by the simulation through the formulation forwarded in the MC1990/2010. The successful achievement of the measurements and simulations reported herein corroborate the validity of the adopted strategies and assumptionsFEDER funds through the Competitivity Factors Operational Programme - COMPETE and by national funds through FCT within the scope of the projects POCI-01-0145-FEDER-007633 and POCI-01-0145-FEDER-016841 (IntegraCrete - PTDC/ECM-EST/1056/2014). The PhD grant of the second author is also gratefully acknowledged (SFRH/BD/80682/2011

Bender-extender elements for characterization of cement paste at early ages

The embedment of bender-extender elements in cement-based materials for assessment of the early development of mechanical properties is a relatively unexplored field. This technique provides the opportunity of embedding piezoelectric elements (emitter and receiver) into the tested material at the fresh state, generate waves and assess the velocity of propagation. It has the interesting feature of allowing distinct frequencies of wave to be explored at the several stages of stiffening of the testing material, thus maximising signal intensity and facilitating the identification of velocities. This paper presents an exploratory application of bender- extender elements to cement paste specimens, in parallel with other established experimental techniques, such as the Vicat needle, ultrasound pulse velocity measurements (with external probes), measurement of E-modulus through cyclic compressive testing and continuous assessment of the E-modulus of the cement paste through EMM-ARM. The results are evaluated and discussed in an integrated manner and conclusions are drawn in regard to the potential of using bender-extender elements in cement-based materialsFEDER funds through the Competitivity Factors Operational Programme - COMPETE and by national funds through FCT – Foundation for Science and Technology within the scope of the projects POCI-01-0145-FEDER-007633 and POCI-01-0145-FEDER-016

Continuous characterization of stiffness of cement-based materials: experimental analysis and micro-mechanics modelling

Philosophy Doctorate Thesis Civil EngineeringThe structural performance and durability of reinforced concrete structures are strongly influenced by the material properties of concrete. Concrete’s characteristics endure strong evolution since casting, passing from a solid suspension to a structural material. Therefore, it is extremely important to understand and predict the structural behaviour of concrete since the beginning of the hardening process for a good structural design, particularly in regard to the development of self-induced stresses (due to heat of hydration and shrinkage). Apart from these issues related to structural design, relevant urges are brought about by the necessity of shortening construction schedules, both due to pressures by society, as well as due to economic and sustainability concerns. In view of these motivations, there are enough reasons to justify the importance of having experimental methods that allow continuous monitoring of the evolution of mechanical properties of concrete since very early ages, both in laboratory environment and “in-situ”. In such concern, several methods experimental have been proposed throughout the years, particularly in regard to the evaluation of the E-modulus of concrete. However, the most widespread methods still present limitations/complexities which make them inadequate for the wider intents mentioned above. Thus a new experimental method called EMM-ARM (Elasticity Modulus Measurement through Ambient Response Method) was proposed in 2009, which is based on the modal identification of a composite beam (acrylic and concrete) during the curing period of concrete, allowing the continuous measurement of concrete E-modulus since casting. Despite the good results obtained during the first implementation prior to this thesis, the EMM-ARM is still lacked extensive validation and presented several laminations that needed to be overcome. Following the encouraging results obtained in the first application of EMM-ARM, the work reported in this thesis intended to achieve an improved robust tool based on EMM-ARM to provide early information of the cementitious materials stiffness, readily available for application on behalf of both scientists and practitioners. In pursuit of that goal, relevant changes were introduced in EMM-ARM, particularly in concern to the geometry and materials involved in the EMM-ARM mould, as well as to the modal identification of technique. These changes allowed overcoming the identified constraints and to significantly improve the usability and robustness of the method. This thesis also presents a systematic study of the application of EMM-ARM compared to competing methods that mechanical characterization of cementitious materials at early ages with mutual validation objectives. This systematic study allowed proving that the results of EMM-ARM are metrologically robust and also to clearly identify the strengths and limitations of EMM-ARM. After the optimization and validation of EMM-ARM the method was applied in different conditions such as: (i) different isothermal curing temperatures in the range 10-40ºC; (ii) the implementation in a construction site; and (iii) non-isothermal conditions. This research also permitted demonstrating that EMM-ARM can be used to characterize a wide range of materials that undergoes chemical hardening such as structural epoxy adhesives. In addition, a new version of EMM-ARM for monitoring the concrete viscoelasticity during the fresh state was suggested. The thesis ends with a foray into the microstructural simulation of the stiffness evolution of cementitious materials by taking advantage of the unprecedented quantitative experimental information obtained with EMM-ARM. The stiffness evolution of cement pastes, simulated by μic/AMIE, developed at EPFL (École polytechnique fédérale de Lausanne) was validated through comparison with EMM-ARM results.A performance estrutural e a durabilidade de estruturas de betão armado são fortemente influenciadas pelas propriedades do betão. As características do betão sofrem uma grande evolução desde a betonagem, passando de uma suspensão de sólidos para um material estrutural. Desta forma torna-se extremamente importante compreender e prever o comportamento estrutural do betão desde o inicio do processo de endurecimento para se conseguir efetuar um correto dimensionamento estrutural, especialmente no que diz respeito ao desenvolvimento de tensões autoinduzidas (devido ao calor de hidratação e à retração). Adicionalmente a estas questões relacionadas com o dimensionamento estrutural, o desempenho estrutural do betão é também relevante do ponto de vista da redução dos períodos de construção devido a pressões da sociedade assim como devido a questões económicas e de sustentabilidade. Tendo em conta estas motivações, há razões suficientes para justificar a importância a existência de métodos experimentais que permitam a monitorização continua da evolução das propriedades mecânicas do betão desde as primeiras idades, tanto para a aplicação em laboratório assim como “in-situ”. Nesse sentido vários métodos experimentais têm vindo a ser propostos ao longo dos anos, particularmente no que diz respeito à avaliação do módulo de elasticidade de betão. No entanto, os métodos mais disseminados ainda apresentam limitações e/ou complexidades que os tornam inadequados para os propósitos mais amplos acima mencionados. Desta forma um novo método experimental foi proposto em 2009 chamado EMM-ARM (Elasticity Modulus Measurement through Ambient Response Method) que é baseado na identificação modal de uma viga composta (acrílico e betão) durante o processo de cura, permitindo a monitorização continua do modulo de elasticidade do betão desde a betonagem. Apesar dos bons resultados obtidos durante a primeira aplicação do método antes desta tese, o EMM-ARM ainda requer uma extensa validação e apresenta algumas limitações que necessitam de ser eliminadas. Na sequência dos resultados encorajadores obtidos na primeira aplicação do EMM-ARM, o trabalho reportado nesta tese pretende alcançar uma ferramenta melhorada e robusta baseada no EMM-ARM para fornecer informação antecipada sobre a rigidez materiais cimentícios e fornece-la em tempo real ao utilizador. Na busca deste objetivo foram introduzidas alterações relevantes no EMM-ARM, particularmente no que diz respeito à geometria e materiais envolvidos no molde, assim como na técnica de identificação modal. Estas adaptações permitiram superar as limitações identificadas e melhorar significativamente a usabilidade e a robustez do método. Esta tese apresenta também um estudo sistemático da aplicação do EMMARM comparado aos métodos concorrentes capazes de caracterizar as propriedades mecânicas dos materiais cimentícios nas primeiras idades com o objetivo de fazer a validação mútua dos métodos. Este estudo sistemático permitiu provar que os resultados obtidos pelo EMM-ARM são metrologicamente robustos e ainda identificar claramente os pontos fortes e limitações do método. Após a otimização e validação do EMM-ARM, o método foi aplicado sob diferentes condições, tais como: (i) diferentes temperaturas isotérmicas de cura na gama entre 10-40ºC; (ii) a implementação num estaleiro de obra; e (iii) condições não-isotérmicas. Este trabalho permitiu também demonstrar que o EMM-ARM pode ser utilizado para caracterizar uma vasta gama de materiais que sofre endurecimento químico, tais como adesivos epoxídicos. Adicionalmente, foi ainda sugerida uma nova versão do EMM-ARM para monitorizar a viscoelasticidade do betão durante o estado fresco. A tese termina com uma incursão na simulação microestrutural da evolução da rigidez de materiais cimentícios, tirando partido da informação experimental quantitativa sem precedentes obtida com o EMM-ARM. A evolução rigidez de pastas de cimento, simulada pelo modelo μic/AMIE, desenvolvido na EPFL (École polytechnique fédérale de Lausanne) foi validada através de comparação com os resultados obtidos pelo EMM-ARM.Fundação para a Ciência e Tecnologia (FCT

Effects of curing temperature on pull-out behavior and stiffness evolution of epoxy adhesives for NSM-FRP applications

The efficiency of the FRP-concrete strengthening system is strongly influenced by the mechanical properties of the epoxy adhesive, which depend on the curing temperature.In the present work, the influence of temperature on the curing process of the epoxy was examined. Two different temperatures were studied: 20 and 30 °C. The elastic modulus of the adhesive was continuously monitored by using a variant of the classical resonant frequency methods, called EMM-ARM (Elasticity Modulus Monitoring through Ambient Response Method). A simultaneous study of direct pull-out tests with concrete specimens strengthened with NSM carbon FRP laminate strips was carried out at the same two different temperatures to compare the evolution of bond performance with the E-modulus of epoxy since early ages. The results showed that increasing the curing temperature significantly accelerated both the curing process of the epoxy adhesive and the evolution of bond performance. Moreover, the EMM-ARM technique revealed its ability in clearly identifying the hardening kinetics of epoxy adhesives, allowing measurements since very early ages and in different environmental conditions

Quality control and monitoring of NSM CFRP systems: E-modulus evolution of epoxy adhesive and its relation to the pull-out force

The present paper describes the application of an innovative technique (termed EMM-ARM: Elasticity Modulus Monitoring through Ambient Response Method) for continuous monitoring of the stiffening process of an epoxy adhesive used in near-surface mounted (NSM) fibre reinforced polymer (FRP) reinforcements. A simultaneous study of direct pull-out tests with concrete specimens strengthened with NSM carbon FRP laminate strips was carried out to compare the evolution of bond performance with the E-modulus of epoxy since early ages. A relationship between the evolution of epoxy E-modulus and the maximum pull-out force is assessed, highlighting the potential of applying EMM-ARM for quality control and decision-making assistance of NSM systems.This work is supported by FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the projects CutInDur PTDC/ECM/112396/2009 and VisCoDyn EXPL/ECM-EST/1323/2013. The authors also like to thank all the companies that have been involved supporting and contributing for the development of this study, mainly: S&P Clever Reinforcement Iberica Lda., Artecanter - Industria de Transformacao de Granitos, Lda., Vialam - Industrias Metalurgicas e Metalomecanicas, Lda. The first and second authors also acknowledge the grants SFRH/BD/80338/2011 and SFRH/BD/80682/2011, respectively, provided by FCT

Influence of temperature on the curing of an epoxy adhesive and its influence on bond behaviour of NSM-CFRP systems

In NSM-CFRP installations, the mechanical behaviour of the strengthening system is strongly influenced by the epoxy adhesive, particularly at early ages. In the present work, the influence of temperature on the curing process of the epoxy was investigated. Three distinct temperatures were studied: 20, 30 and 40 °C. The elastic modulus of the adhesive was monitored through EMM-ARM (Elasticity Modulus Monitoring through Ambient Response Method). Direct pull-out tests with concrete specimens strengthened with NSM CFRP strips were carried out at the same three distinct temperatures to compare the evolution of bond performance with the E-modulus of epoxy since early ages. The results showed that increasing the curing temperature significantly accelerated both the curing process of the epoxy adhesive and the evolution of bond performance. The EMM-ARM technique has revealed its ability in clearly identifying the hardening kinetics of epoxy adhesives, allowing also thermal activation analysis. Finally, existing models for predicting temperature-dependent mechanical properties were extended to also describe the bond behaviour of NSM-CFRP applications.This work is supported by FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the projects FRPreDur FCOMP-01-0124-FEDER-028865 (FCT no. PTDC/ECM-EST/2424/2012) and VisCoDyn FCOMP-01-0124-FEDER-041751 (FCT no. EXPL/ECM- EST/1323/2013). The authors also like to thank all the companies that have been involved supporting and contributing for the development of this study, mainly: S&P Clever Reinforcement Ibérica Lda., Artecanter - Indústria de Transformação de Granitos, Lda., Vialam – Indústrias Metalúrgicas e Metalomecânicas, Lda. The second and third authors also acknowledge the grants SFRH/BD/80338/2011 and SFRH/BD/80682/2011, respectively, provided by FCT

Monitoring the early stiffness development in epoxy adhesives for structural strengthening

The present work aimed to assess the early-age evolution of E-modulus of epoxy adhesives used for Fibre-Reinforced Polymer (FRP) strengthening applications. The study involved adapting an existing technique devised for continuous monitoring of concrete stiffness since casting, called EMM-ARM (Elasticity Modulus Monitoring through Ambient Response Method) for evaluation of epoxy stiffness. Furthermore, monotonic tensile tests according to ISO standards and cyclic tensile tests were carried out at several ages. A comparison between the obtained results was performed in order to better understand the performance of the several techniques in the assessment of stiffness of epoxy resins. When compared to the other methodologies, the method for calculation of E-modulus recommended by ISO standard led to lower values, since in the considered strain interval, the adhesive had a non-linear stress-strain relationship. The EMM-ARM technique revealed its capability in clearly identifying the hardening kinetics of epoxy adhesives, measuring the material stiffness growth during the entire curing period. At very early ages the values of Young's modulus obtained with quasi-static tests were lower than the values collected by EMM-ARM, due to the fact that epoxy resin exhibited a significant visco-elastic behaviour.FEDER funds through the Operational Program for Competitiveness Factors – COMPETE and National Funds through FCT – Foundation for Science and Technology under the projects CutInDur PTDC/ECM/112396/2009 and VisCoDyn EXPL/ECM-EST/1323/2013. The authors also like to thank all the companies that have been involved supporting and contributing for the development of this study, mainly, S&P Clever Reinforcement Ibérica Lda. and Vialam – Indústrias Metalúrgicas e Metalomecânicas, Lda

Reinforcement design for the combined effect of restrained shrinkage and applied loads in slabs: a design challenge

The quantification of the necessary reinforcement for crack width control in highly restrained RC slabs still remains a subject of discussion in both scientific and practitioner communities, particularly when the simultaneous effects of applied loads and restrained shrinkage deformations are considered. Indeed, different authors/designers follow distinct approaches to deal with the problem. This is however a very important matter, because in slabs, the quantity of reinforcement is frequently determined by Service Limit States (SLS) of cracking. Therefore, the use of different design criteria for SLS can bring different performance levels, and also different global costs (e.g. reinforcement can be overdesigned, or under designed and then repairs may be in order). In such context, this paper presents and analyses the results of a design challenge launched by the research teams at UMinho and UPorto to a set of design offices. The design challenge consists in the sizing of the necessary reinforcement to satisfy adequate cracking performance in a highly restrained slab. All information about geometry, materials, loads and boundary conditions are provided in the design challenge sheet provided to participants. A total of 7 teams have provided answers to this design challenge. Results are treated anonymously in regard to participating teams. A discussion is held with basis on common and differentiating points, and finally an analysis of the authors using non-linear finite element analysis is made, targeting to better assist interpretation of the expectable behaviour of reinforcement solutionsPortuguese Foundation for Science and Technology (FCT) to the Research Projects PTDC/ECM/099250/2008, EXPL/ECM-EST/1323/2013 and PTDC/ECM-EST/1056/2014 (POCI-01-0145-FEDER-016841), as well to the Research Units ISISE (POCI-01-0145-FEDER-007633) and CONSTRUCT (POCI-01-0145-FEDER-007457) is gratefully acknowledged. A word of acknowledgment is also given to the participants in the design challenge, with fundamental importance for the discussion held herein. The authors would also like to acknowledge the benefits of cross-linking research through the network of COST Actio

3D Thermo-hygro-mechanical approach for simulation of the cracking behaviour of a RC slab under the combined effects of applied loads and restrained shrinkage

The design of reinforced concrete (RC) structures that meet safety, functionality and aesthetic requirements during their lifespan, without unforeseen maintenance costs, depends on adequate design practices that allow engineers to properly control and predict crack widths on concrete. Even though there is a wide body of design codes and recommendations providing methodologies for reinforcement design on elements subjected to applied loads or imposed deformations, they do not provide unambiguous rules for RC structures under the combined effect of these actions, which is a typical situation in RC slabs applied in buildings. This is motivated by the lack of knowledge about the complex interactions that take place between self-imposed deformations, viscoelasticity and the effect of applied loads. This work intends to contribute for deepening the knowledge on this subject by performing a 3D thermo-hygro-mechanical analysis on a highly restrained slab in service load conditions, in which the temperature and moisture fields of the slab are determined in order to take into account the non-uniform distribution of stresses (in space and time) due to hydration and drying shrinkage. This analysis shows that the real restraint forces applied to the slab are in fact just a mere fraction of the those that would be expected in a hypothetical tie subjected to total restraint due to the loss of rigidity caused by crack development induced by a combination of flexural and self-induced stressesPortuguese Foundation for Science and Technology (FCT) to the Research Project IntegraCrete (PTDC/ECM-EST/1056/2014 -POCI-01-0145-FEDER-016841) as well to the Research Units ISISE (POCI-01-0145-FEDER-007633) and CONSTRUCT (POCI-01-0145-FEDER-0074

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