73 research outputs found

    Characterization of concrete materials using non-destructive wave-propagation testing techniques

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    Non-destructive testing (NDT) of concrete members has been widely used for characterisation of material and assessment of functional structures without impairing their functions and performances. This thesis focuses on addressing critical challenges related to the practical implementation of NDT techniques based on wave-propagation approaches for characterisation of concrete members used in civil infrastructures. Specially, this research aims to achieve three interdependent objectives related to developing NDT techniques with piezoceramic-based transducers: monitoring of very early-age concrete hydration process, detection, and monitoring of cracking in concrete members of different complexity under loading. The concept of piezoceramic-based Smart Aggregate (SA) transducers is central to this research. Embedded SA transducers with an active sensing method have shown great potential for characterisation of construction materials such as concrete and concrete-steel composites. Based on the developed SA based approaches, an active sensing approach with appropriate arrangement of SAs in and on concrete members, and analysis of the received signal using the power spectral density, total received power and damage indexes is developed and applied in this thesis. To confirm its applicability for characterisation of very early-age concrete, a systematic investigation is performed into concrete specimens with different values of water-to-cement ratio due to slightly different initial water amounts, and different separation distances between the embedded SAs. For the detection and monitoring of cracking in concrete members under loading the mounted SA based approach is proposed and applied. It is shown that NDT systems, based on this approach, provide detection and monitoring of cracking in a variety of concrete members under loading, including relatively simple concrete beams and reinforced concrete beams under bending, and reinforced concrete slab as a part of a complex composite member under cyclic loading. Comparisons are provided between the proposed system and conventional load cell and strain gauge systems with each tested member

    Experimental Study on Stress Monitoring of Sand-Filled Steel Tube during Impact Using Piezoceramic Smart Aggregates

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    The filling of thin-walled steel tubes with quartz sand can help to prevent the premature buckling of the steel tube at a low cost. During an impact, the internal stress of the quartz sand-filled steel tube column is subjected to not only axial force but also lateral confining force, resulting in complicated internal stress. A suitable sensor for monitoring the internal stress of such a structure under an impact is important for structural health monitoring. In this paper, piezoceramic Smart Aggregates (SAs) are embedded into a quartz Sand-Filled Steel Tube Column (SFSTC) to monitor the internal structural stress during impacts. The piezoceramic smart aggregates are first calibrated by an impact hammer. Tests are conducted to study the feasibility of monitoring the internal stress of a structure. The results reflect that the calibration value of the piezoceramic smart aggregate sensitivity test is in good agreement with the theoretical value, and the output voltage value of the piezoceramic smart aggregate has a good linear relationship with external forces. Impact tests are conducted on the sand-filled steel tube with embedded piezoceramic smart aggregates. By analyzing the output signal of the piezoceramic smart aggregates, the internal stress state of the structure can be obtained. Experimental results demonstrated that, under the action of impact loads, the piezoceramic smart aggregates monitor the compressive stress at different locations in the steel tube, which verifies the feasibility of using piezoceramic smart aggregate to monitor the internal stress of a structure

    Advanced microwave embedded sensors for infrastructure health monitoring

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    Microwave sensor systems have been widely investigated for many applications due to their ability to provide non-destructive, noncontact, one-sided and wireless testing. Among these applications infrastructure health monitoring of bridges, building, and dams using microwave sensors, which are mounted on or embedded in composite structures of infrastructure has been attracting an increasing interest. One of the current needs of infrastructure health monitoring includes the detection and monitoring of disbonding and gaps in concrete-based structures, which are also required for simultaneous characterization of concrete. A recently proposed microwave sensor technique exploiting a relatively simple waveguide sensor embedded in a concrete-metal structure such as a concrete-filled steel tube exhibited great potential. However, it suffers from a few drawbacks that need to be solved. This thesis aims to develop and investigate advanced microwave embedded sensors to solve main problems in the current microwave sensory technique including characterization of concrete in concrete-based structures at different stage of its life, size of the interface under inspection, detection and monitoring of a small gap between concrete and dielectric material surfaces and sensitivity to gaps. To achieve this aim the following five research contributions have been made: The first contribution is the methodology for the determination of the complex dielectric permittivity of concrete using both measurement data and simulation results at different stages (fresh, early-aged and dry) of its life. Firstly, it is developed and tested for a single flanged open-ended waveguide sensor with a hardened concrete specimen, and then the methodology is modified for the developed sensors embedded in concrete-based composite structures with fresh, early-age and dry concrete. Modern computational tool CST Microwave Studio and a performance network analyser are used for simulation and measurement, respectively, throughout this research work. The second contribution is a dual waveguide sensor, which is proposed, designed and applied for the detection and monitoring of a small gap in concrete-metal composite structures. It consists of two waveguide sections and a metal plate and uses the transmission of electromagnetic waves along gap when it occurs between the metal plate and concrete surfaces. It provides more measurement data than the single waveguide sensor for characterising concrete-metal structures such as transmission properties of guided waves along the gap and reflection properties of the metal–concrete interface at two different places at the same stage of concrete. As a result, the proposed sensor increases the size of the interface under inspection and sensitivity to the gap using the magnitude of reflection coefficient and magnitude of transmission coefficient together and/or independently. The third contribution is the design and application of a dual waveguide sensor with rectangular dielectric insertions that is proposed and tested for the characterisation of concrete–metal structures at different stages of the concrete life including its fresh stage. The dielectric insertions are designed and implanted in the waveguide sections in such a way that they create the resonant response of the sensor and prevent water and concrete entering the sections. The resonant properties of the sensor allow long-term monitoring of the concrete hydration, including the detection of the transition from fresh to hardened concrete on its first day. The proposed sensor along with the modified algorithm provides the determination of the complex dielectric permittivity of fresh concrete. The fourth contribution is a dual waveguide sensor with tapered dielectric insertions. Each tapered dielectric insertion is designed with a tapered part and rectangular part to reduce wave reflection from the insertions over an entire frequency band. The proposed sensor has improved performance at the resonant responses of a quarter-wavelength resonator formed by an open end at the tapered part and shorted end at the rectangular part of each insertion. The last contribution is the development of dual waveguide sensors with attached dielectric layer and their application for the detection and monitoring of gap between dielectric materials and concrete in metal-dielectric layer-concrete composites as well as the determination of complex dielectric permittivity of concrete at different stages of its life. One of the most attractive designs is the sensor with empty waveguide sections due to its simplicity and robustness, and capability of the layer for preventing penetration of the obstacles and water, and for optimization of the sensor. On the other hand, the sensors with dielectric insertions and the layer demonstrate a significantly higher magnitude of transmission coefficient. The proposed DWSs can be applied to characterise fresh concrete in a dielectric mould or on-line, and to investigate the shrinkage of different categories of concrete

    Non-destructive Evaluation of Damage in Concrete with Applications in Shallow Foundations

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    The most widely used material for civil infrastructure is reinforced concrete. The concrete deteriorates over time because of several reasons, and therefore, inspection of concrete is necessary to ensure its compliance with the design requirements. Decision makers often have insufficient data to implement the appropriate corrective measures in the face of infrastructure failure. Better assessment methods are essential to obtain comprehensive and reliable information about the concrete elements. Although, different methods exist to inspect concrete members, there is no comprehensive technique available for condition assessment of concrete of shallow foundations. To ensure the integrity of shallow foundations during construction and during its service life, it is necessary to monitor their conditions periodically. To achieve this goal a new NDT methodology is developed to reliably evaluate the conditions of new shallow foundations without changing their future performances. Recently, there is a trend to overcome coupling issues between the transducers and the object under investigation, by installing sensor networks in concrete to assess its integrity. Although many NDT approaches are designed to evaluate the integrity of concrete structural elements, shallow foundations, which are concrete elements embedded in soil, have received less attention. The challenging aspect of characterizing shallow foundations is limited accessibility for in-service foundation inspections because of structural restrictions. Even when accessibility is possible, the NDT methods (ultrasonic pulse velocity, UPV) used may produce measurements with high uncertainties because of inconsistent coupling between the transducer and the surface of the material being tested. In the current research project, a new NDT procedure is developed based on design of new transducers embedded at the base of lab-scale concrete foundation models, and these transducers are waterproof and used as receivers. The transducers consist of radial-mode piezoceramics that can detect waves from different orientations. The developed methodology relies mainly on two methods to emit the transmission pulse; either using a direct contact method by gluing the transducer to the concrete surface or using a plastic tube partially embedded in concrete and filled with water. The first procedure is used when the accessibility to the top surface of the foundations is possible; otherwise, the second option is employed to reach the concrete surface of foundations. The new methodology can be used in different stages: during construction of foundations to monitor the uniformity and quality of the concrete, and during in-service life to periodically assess the condition of the foundations, specifically after an event that may cause severe damage in concrete such as earthquake and overloading. To verify the applicability of the methodology, unreinforced and reinforced shallow foundation lab-scale concretemodels were tested in the laboratory under uniaxial compression loads. In this work, all ultrasonic measurements are averaged 16 times to ensure the consistency of the results and to eliminate high frequency noise. The average coefficient of variance obtained is less than 3.5%; which is considered acceptable in this type of measurements (typical measurement error ~5%). Also, different tests were repeated more than three times by removing and putting back all the ultrasonic transducers to enhance the statistical significance of the results. The main contributions of the research presented in this thesis are: Characterization of low and high frequency transducers using laser vibrometer to characterize their responses for better ultrasonic measurements. Characterization of a single fracture growth in a homogenous material based on wave velocity and wave attenuation. Characterization of cement-based materials using ultrasonic pulse velocity and laser vibrometer methods. Evaluation of freeze/thaw damage and monitoring progressive damage in concrete specimens subjected to uniaxial compression load using ultrasonic pulse velocity and laser vibrometer methods. Fabrication of thirty-six new radial ultrasonic transducers to embed in concrete models for quality control purposes and to monitor progressive damage using new transmission pulse methodology

    Mechanical characteristics of organically modified fly ash-kaolinite mixtures

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    Fly ash is a hazardous waste material to the environment. Beneficial reuse of fly ash, however, brings economic and environmental benefits, i.e., decreases the cost of disposal, reduces greenhouse gas emission, and improves mechanical properties of construction materials. Due to the lack of pozzolanic properties, the reuse rate of Class F fly ash in geotechnical engineering is relatively low (ACAA 2013). In order to promote the effectiveness and sustainability of high volume reuse of class F fly ash in geotechnical engineering, biopolymers (Chitosan, and Xanthan gum) and synthetic polymers (Polyethylene Oxide, PEO) were used instead of the traditional stabilizers to improve the engineering behaviors of fly ash-kaolinite mixtures. The effects of polymers on microstructure development and geotechnical properties (i.e., stiffness, stiffness anisotropy and strength) of fly ash-kaolinite mixtures were measured in the laboratory. Shear wave velocity (Vs) and Vs anisotropy of organically modified fly ash-kaolinite mixtures were investigated by a self-developed floating wall-type consolidometer bender element testing system. Vs results showed that PEO and chitosan increased Vs of the fly ash-kaolinite mixtures, while xanthan gum decreased Vs. Vs anisotropy of the fly ash-kaolinite mixtures was decreased by the addition of polymers. Critical-state friction angles of fly ash-kaolinite mixtures increased with the addition of PEO and chitosan, while decreased with xanthan gum. Polymer bridging and columbic interactions, and the subsequent changes in size and fabric of kaolinite are the major influencing mechanisms. Physicochemical effects and the fly ash ratio also contributed to the observed changes --Abstract, page iii

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

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    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

    Acoustic and Elastic Waves: Recent Trends in Science and Engineering

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    The present Special Issue intends to explore new directions in the field of acoustics and ultrasonics. The interest includes, but is not limited to, the use of acoustic technology for condition monitoring of materials and structures. Topics of interest (among others): • Acoustic emission in materials and structures (without material limitation) • Innovative cases of ultrasonic inspection • Wave dispersion and waveguides • Monitoring of innovative materials • Seismic waves • Vibrations, damping and noise control • Combination of mechanical wave techniques with other types for structural health monitoring purposes. Experimental and numerical studies are welcome

    International RILEM Conference on Materials, Systems and Structures in Civil Engineering Conference segment on Service Life of Cement-Based Materials and Structures

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    Vol. 2O volume I encontra-se disponível em: http://hdl.handle.net/1822/4341

    Proceedings of the International RILEM Conference Materials, Systems and Structures in Civil Engineering segment on Service Life of Cement-Based Materials and Structures

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    Vol. 1O volume II encontra-se disponível em: http://hdl.handle.net/1822/4390
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