An Automated Procedure for Assessing Local Reliability Index and Life-Cycle Cost of Alternative Girder Bridge Design Sol...Published: 20 January 2019 by Hindawi Limited in Advances in Civil Engineering
Stakeholders of civil infrastructures have to usually choose among several design alternatives in order to select a final design representing the best trade-off between safety and economy, in a life-cycle perspective. In this framework, the paper proposes an automated procedure for the estimation of life-cycle repair costs of different bridge design solutions. The procedure provides the levels of safety locally guaranteed by the selected design solution and the related total life-cycle cost. The method is based on the finite element modeling of the bridge and uses design traffic models as suggested by international technical standards. Both the global behavior and the transversal cross section of the bridge are analyzed in order to provide local reliability indexes. Several parameters involved in the design, such as geometry and loads and materials’ characteristics, are considered as uncertain. Degradation models are adopted for steel carpentry and rebars. The application of the procedure to a road bridge case study shows its potential in providing local safety levels for different limit states over the entire lifetime of the bridge and the life-cycle cost of the infrastructure, highlighting the importance of the local character of the life-cycle cost analysis.
Earthquake-induced damage detection and localization in masonry structures using smart bricks and Kriging strain reconst...Published: 21 December 2018 by Wiley in Earthquake Engineering & Structural Dynamics
The intrinsic vulnerability of masonry structures to seismic events makes structural health monitoring of the utmost importance for the conservation of the built heritage. The development of piezoresistive bricks, also termed smart bricks, is an innovative technology recently proposed by the authors for the monitoring of such structures. Smart bricks exhibit measurable variations in their electrical properties when subjected to external loads or, alternatively, strain self‐sensing capabilities. Therefore, the deployment of a network of smart bricks into a masonry structure confers self‐diagnostic properties to the host structure. In this light, this paper presents a theoretical investigation on the application of smart bricks to full‐scale masonry structures for seismic assessment. This includes the study of the convenience of providing electrical isolation conditions to the sensors, as well as the effectiveness of smart bricks when installed into either new constructions or in pre‐existing structures. Secondly, numerical results are presented on the seismic analysis of a three‐dimensional masonry building equipped with a network of smart bricks. Finally, in order to map the strain field throughout the structure exploiting the outputs of a limited number of sensors, an interpolation‐based strain reconstruction approach is proposed.
<p>The seismic monitoring of masonry structures is especially challenging due to their brittle resistance behavior. A tailored sensing system could, in principle, help to detect and locate cracks and anticipate the risks of local and global collapses, allowing prompt interventions and ensuring users’ safety. Unfortunately, off-the-shelf sensors do not meet the criteria that are needed for this purpose, due to their durability issues, costs and extensive maintenance requirements. As a possible solution for earthquake-induced damage detection and localization in masonry structures, the authors have recently introduced the novel sensing technology of “smart bricks”, that are clay bricks with self-sensing capabilities, whose electromechanical properties have been already characterized in previous work. The bricks are fabricated by doping traditional clay with conductive stainless steel microfibers, enhancing the electrical sensitivity of the material to strain. If placed at key locations within the structure, this technology permits to detect and locate permanent changes in deformation under dead loading conditions, associated to a change in structural conditions following an earthquake. In this way, a quick post-earthquake assessment of the monitored structure can be achieved, at lower costs and with lower maintenance requirements in comparison to traditional sensors.</p> <p>In this paper, the authors further investigate the electro-mechanical behavior of smart bricks, with a specific attention to the fabrication of the electrodes, and exemplify their application for damage detection and localization in a full-scale shaking table test on a masonry building specimen. Experimental results show that smart bricks’ outputs can effectively allow the detection of local permanent changes in deformation following a progressive damage, as also confirmed by a 3D finite element simulation carried out for validation purposes.</p> <p><strong>Related video presentation available <a href="https://www.dropbox.com/s/6mqmyw5ag12he0y/ECSA18%20Pres%20D%27Alessandro8p.mp4?dl=0">here</a></strong>.</p>
In recent years, self-sensing structural materials have drawn enormous attention of scientific community due to their potential to enable continuous monitoring of the integrity of structures. The new paradigm of smart condition-based maintenance advocates the use of next-generation structures completely or partly constituted by self-sensing materials, that is, the structure or part of it also behaves as a sensor. In this context, the remarkable mechanical and electrical properties of Multi Walled Carbon NanoTubes (MWCNTs) have fostered an increasing number of applications as fillers for composites with multifunctional properties. Among a wide spectrum of potential applications, the development of skin-type piezoresistive distributed strain sensors shows great promise. Such sensors can be deployed onto large-scale structures, enabling a continuous monitoring of the strain state in the global area of the structure. In this paper, a theoretical study on the potential application of smart MWCNT/epoxy strip-like strain sensors for damage detection/localization/quantification in Reinforced Concrete (RC) beams is presented. A micromechanics-based finite element model is proposed for the electromechanical analysis of MWCNT/epoxy strips. Furthermore, a damage detection algorithm through model updating approach is introduced. To do so, an Euler-Bernoulli model for beams equipped with a smart MWCNT/epoxy strip is developed. Finally, two numerical case studies are presented including: a 2D concrete beam with multiple prescribed crack-like damages, and a 3D RC beam under four-point flexural conditions with non-prescribed cracking. Results show that the proposed smart strips are capable of exploiting the damage-induced variations in the electrical output to locate and quantify damages for real-time distributed structural health monitoring of RC beam structures.
Two-step hierarchical micromechanics model of partially saturated porous composites doped with ellipsoidal particles wit...Published: 01 September 2018 by Elsevier BV in Composites Part B: Engineering
Recent advances in the manufacture of micro- and nano-composites have made it possible to produce new multifunctional materials. However, the development of theoretical models that assist their design still remains an open research issue. This paper presents a two-step hierarchical micromechanics approach for the mechanical homogenization of particle-reinforced porous composites, including particle/matrix interfacial bonding and porosity saturation effects. Firstly, the particle-reinforced matrix is homogenized by means of a double-inclusion approach. The interfacial bonding effect is accounted for by both compliant and hard interphases surrounding the particles. Secondly, another homogenization step is conducted by considering the particle-reinforced composite as a homogeneous matrix and voids as embedded inclusions. Pores saturation is also taken into account by means of homogeneous equivalent pores. Comparative analyses against experimental data are presented to demonstrate the effectiveness of the present approach, followed by detailed parametric analyses to illustrate the influence of the major micromechanical variables, including interphase thickness and stiffness, filler aspect ratio, porosity and saturation degree.
Life cycle monitoring of structural health of civil constructions is crucial to guarantee users’ safety. An optimal structural health monitoring system allows to automatically detect, locate, and quantify any damage in structural elements, thus anticipating major risks of local or global failures. Critical issues affecting traditional monitoring systems are sensors’ placement, hardware durability, and long-term reliability of the measurements. Indeed, sensors’ deployment is crucial for an effective investigation of the static and dynamic characteristics of the structural system, whereby durability and long-term stability of sensing systems are necessary for long-term monitoring. A very attractive solution to some of these challenges is developing sensors made of the same, or similar, material of the structure being monitored, allowing a spatially distributed and long-term reliable monitoring system, by the use of self-sensing construction materials. Within this context, the authors have recently proposed new “smart clay bricks” that are strain-sensing clay bricks aimed at embedding intelligent monitoring capabilities within structural masonry buildings. While previous work focused on smart bricks doped with titanium dioxide and using embedded point electrodes, this work proposes an enhanced version of smart bricks based on the addition of conductive micro stainless steel fibers that possess higher electrical conductivity and a more suitable fiber-like aspect ratio for the intended application, as well as plate copper electrodes deployed on top and bottom surfaces of the bricks. The paper thus presents preparation and experimental characterization of the new smart bricks. The influence of different amounts of fibers is investigated, allowing the identification of their optimal content to maximize the gauge factor of the bricks. Both electrical and electromechanical experimental tests were performed. Overall, the presented results demonstrate that the new smart bricks proposed in this paper possess enhanced strain-sensing capabilities and could be effectively utilized as sensors within structural masonry buildings.
Effect of PCM on the Hydration Process of Cement-Based Mixtures: A Novel Thermo-Mechanical InvestigationPublished: 23 May 2018 by MDPI in Materials
The use of Phase Change Material (PCM) for improving building indoor thermal comfort and energy saving has been largely investigated in the literature in recent years, thus confirming PCM’s capability to reduce indoor thermal fluctuation in both summer and winter conditions, according to their melting temperature and operation boundaries. Further to that, the present paper aims at investigating an innovative use of PCM for absorbing heat released by cement during its curing process, which typically contributes to micro-cracking of massive concrete elements, therefore compromising their mechanical performance during their service life. The experiments carried out in this work showed how PCM, even in small quantities (i.e., up to 1% in weight of cement) plays a non-negligible benefit in reducing differential thermal increases between core and surface and therefore mechanical stresses originating from differential thermal expansion, as demonstrated by thermal monitoring of cement-based cubes. Both PCM types analyzed in the study (with melting temperatures at 18 and 25 ∘C) were properly dispersed in the mix and were shown to be able to reduce the internal temperature of the cement paste by several degrees, i.e., around 5 ∘C. Additionally, such small amount of PCM produced a reduction of the final density of the composite and an increase of the characteristic compressive strength with respect to the plain recipe.
Renewable energy production has become a key research driver during the last decade. Wind energy represents a ready technology for large-scale implementation in locations all around the world. While important research is conducted to optimize wind energy production efficiency, a critical issue consists of monitoring the structural integrity and functionality of these large structures during their operational life cycle. This paper investigates the durability of a soft elastomeric capacitor strain sensing membrane, designed for structural health monitoring of wind turbines, when exposed to aggressive environmental conditions. The sensor is a capacitor made of three thin layers of an SEBS polymer in a sandwich configuration. The inner layer is doped with titania and acts as the dielectric, while the external layers are filled with carbon black and work as the conductive plates. Here, a variety of samples, not limited to the sensor configuration but also including its dielectric layer, were fabricated and tested within an accelerated weathering chamber (QUV) by simulating thermal, humidity, and UV radiation cycles. A variety of other tests were performed in order to characterize their mechanical, thermal, and electrical performance in addition to their solar reflectance. These tests were carried out before and after the QUV exposures of 1, 7, 15, and 30 days. The tests showed that titania inclusions improved the sensor durability against weathering. These findings contribute to better understanding the field behavior of these skin sensors, while future developments will concern the analysis of the sensing properties of the skin after aging.
Condition assessment of civil infrastructures is difficult due to technical and economic constraints associated with the scaling of sensing solutions. When scaled appropriately, a large sensor network will collect a vast amount of rich data that is difficult to directly link to the existing condition of the structure along with its remaining useful life. This paper presents a methodology to construct a surrogate model enabling diagnostic of structural components equipped with a dense sensor network collecting strain data. The surrogate model, developed as a matrix of discrete stiffness elements, is used to fuse spatial strain data into useful model parameters. Here, strain data is collected from a sensor network that consists of a novel sensing skin fabricated from large area electronics. The surrogate model is constructed by updating the stiffness matrix to minimize the difference between the model’s response and measured data, yielding a 2D map of stiffness reduction parameters. The proposed method is numerically validated on a plate equipped with 40 large area strain sensors. Results demonstrate the suitability of the proposed surrogate model for the condition assessment of structures using a dense sensor network.
Crack detection in RC structural components using a collaborative data fusion approach based on smart concrete and large...Published: 27 March 2018 by SPIE-Intl Soc Optical Eng in Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2017
Recent advances in the fields of nanocomposite technologies have enabled the development of highly scalable, low-cost sensing solution for civil infrastructures. This includes two sensing technologies, recently proposed by the authors, engineered for their high scalability, low-cost and mechanical simplicity. The first sensor consists of a smart-cementitious material doped with multi-wall carbon nanotubes, which has been demonstrated to be suitable for monitoring its own deformations (strain) and damage state (cracks). Integrated to a structure, this smart cementitious material can be used for detecting damage or strain through the monitoring of its electrical properties. The second sensing technology consists of a sensing skin developed from a flexible capacitor that is mounted externally onto the structure. When deployed in a dense sensor network configuration, these large area sensors are capable of covering large surfaces at low cost and can monitor both strain- and crack-induced damages. This work first presents a comparison of the capabilities of both technologies for crack detection in a concrete plate, followed by a fusion of sensor data for increased damage detection performance. Experimental results are conducted on a 50 50 5 cm3 plate fabricated with smart concrete and equipped with a dense sensor network of 20 large area sensors. Results show that both novel technologies are capable of increased damage localization when used concurrently.
Monitoring a building’s structural performance is critical for the identification of incipient damages and the optimization of maintenance programs. The characteristics and spatial deployment of any sensing system plays an essential role in the reliability of the monitored data and, therefore, on the actual capability of the monitoring system to reveal early-stage structural damage. A promising strategy for enhancing the quality of a structural health monitoring system is the use of sensors fabricated using materials exhibiting similar mechanical properties and durability as those of the construction materials. Based on this philosophy, the authors have recently proposed the concept of "smart-bricks" that are nanocomposite clay bricks capable of transducing a change in volumetric strain into a change in a selected electrical property. Such brick-like sensors could be easily placed at critical locations within masonry walls, being an integral part of the structure itself. The sensing is enabled through the dispersion of fillers into the constitutive material. Examples of fillers include titania, carbon-based particles, and metallic microfibers. In this paper, experimental tests are conducted on bricks doped with different types of carbon-based fillers, tested both as standalone sensors and within small wall systems. Results show that mechanical properties as well as the smart brick’s strain sensitivity depend on the type of filler used. The capability of the bricks to work as strain monitoring sensors within small masonry specimens is also demonstrated.
An Experimental Study on Static and Dynamic Strain Sensitivity of Embeddable Smart Concrete Sensors Doped with Carbon Na...Published: 09 March 2018 by MDPI in Sensors
The availability of new self-sensing cement-based strain sensors allows the development of dense sensor networks for Structural Health Monitoring (SHM) of reinforced concrete structures. These sensors are fabricated by doping cement-matrix mterials with conductive fillers, such as Multi Walled Carbon Nanotubes (MWCNTs), and can be embedded into structural elements made of reinforced concrete prior to casting. The strain sensing principle is based on the multifunctional composites outputting a measurable change in their electrical properties when subjected to a deformation. Previous work by the authors was devoted to material fabrication, modeling and applications in SHM. In this paper, we investigate the behavior of several sensors fabricated with and without aggregates and with different MWCNT contents. The strain sensitivity of the sensors, in terms of fractional change in electrical resistivity for unit strain, as well as their linearity are investigated through experimental testing under both quasi-static and sine-sweep dynamic uni-axial compressive loadings. Moreover, the responses of the sensors when subjected to destructive compressive tests are evaluated. Overall, the presented results contribute to improving the scientific knowledge on the behavior of smart concrete sensors and to furthering their understanding for SHM applications.
Various nondestructive evaluation techniques are currently used to automatically detect and monitor cracks in concrete infrastructure. However, these methods often lack the scalability and cost-effectiveness over large geometries. A solution is the use of self-sensing carbon-doped cementitious materials. These self-sensing materials are capable of providing a measurable change in electrical output that can be related to their damage state. Previous work by the authors showed that a resistor mesh model could be used to track damage in structural components fabricated from electrically conductive concrete, where damage was located through the identification of high resistance value resistors in a resistor mesh model. In this work, an automated damage detection strategy that works through placing high value resistors into the previously developed resistor mesh model using a sequential Monte Carlo method is introduced. Here, high value resistors are used to mimic the internal condition of damaged cementitious specimens. The proposed automated damage detection method is experimentally validated using a $500 x 500 x 50 $ mm reinforced cement paste plate doped with multi-walled carbon nanotubes exposed to 100 identical impact tests. Results demonstrate that the proposed Monte Carlo method is capable of detecting and localizing the most prominent damage in a structure, demonstrating that automated damage detection in smart-concrete structures is a promising strategy for real-time structural health monitoring of civil infrastructure.
An Experimental Study on Static and Dynamic Strain Sensitivity of Smart Concrete Sensors Doped with Carbon Nanotubes for...Published: 07 February 2018 by MDPI (Preprints) in ENGINEERING
The availability of new self-sensing cement-based strain sensors allows the development of dense sensor networks for Structural Health Monitoring (SHM) of reinforced concrete structures. These sensors are fabricated by doping cement-matrix materials with conductive fillers, such as Multi Walled Carbon Nanotubes (MWCNTs), and can be embedded into structural elements made of reinforced concrete prior to casting. The strain sensing principle is based on the multifunctional composites outputting a measurable change in their electrical properties when subjected to a deformation. Previous work by the authors was devoted to material fabrication, modeling and applications in SHM. In this paper, we investigate the behavior of several sensors fabricated with and without aggregates and with different MWCNTs content. The strain sensitivity of the sensors, in terms of fractional change in electrical resistivity for unit strain, as well as their linearity are investigated through experimental testing under both static and dynamically varying compressive loadings. Moreover, the responses of the sensors when subjected to destructive compressive tests are evaluated. Overall, the presented results contribute to improving the scientific knowledge on the behavior of smart concrete sensors and to furthering their understanding for SHM applications.
Multifunctional smart concretes with novel phase change materials: Mechanical and thermo-energy investigationPublished: 01 February 2018 by Elsevier BV in Applied Energy
Energy performance in buildings and integrated systems represents a key aspect influencing anthropogenic emissions worldwide. Therefore, novel multifunctional materials for improving envelope thermo-energy efficiency through passive techniques are presently attracting notable researchers’ effort. In this view, the integration of phase change materials (PCMs) into structural concrete showed interesting effects in enhancing the material thermal capacity while keeping proper structural strength. This work presents a multiphysics thermo-mechanical investigation concerning innovative concretes incorporating paraffin-based PCM suitable for structural-thermal multifunctional applications in high-energy efficiency building envelopes. Both classic microPCM-capsules and the novel more pioneering macroPCM-capsules with 18 °C phase transition temperature are used for the new composite preparation. Results confirm the thermal benefits of PCM and demonstrate that the addition of PCM reduces the mass density of concrete by almost twice PCMs weight. Average compressive strength decreases with increasing the amount of PCM, but its coefficient of variation is not as negatively affected, which is promising in terms of structural reliability. Indeed, a 1% weight content of microPCM and macroPCM results in reduced coefficients of variation of the compressive strength, determining an increase in characteristic compressive strength. This benefit might be associated to both a filler effect of the PCM and to a positive thermal interaction between inclusions and cement hydration products. The multifunctional analysis showed promising performance of PCM-based macro-capsules as aggregates, even if their concentration is relatively minor than the classic micro-capsules already acknowledged as effective additives for high energy efficient cement-based materials.
Ageing and degradation of concrete civil structures and infrastructural systems are becoming a matter of primary concern worldwide. The definition of appropriate strategies for continuously monitoring the integrity of concrete structures is therefore an urgent priority. Self-sensing concrete (SsC), a concrete engineered at the nanoscale through the addition of functional fillers enabling strain and damage sensing, is a rapidly emerging technology that promises to be a brilliant solution to this monitoring challenge. SsC is based on the incorporation of micro- and nano-fillers into cementitious matrices, mostly using carbon nanoinclusions, to provide electrical conductivity. The strain sensitivity of SsC originates from the property of the material of exhibiting variations of its internal resistivity and impedance under an applied mechanical deformation or following a damage. This chapter gives an overview on the current state of development of the technology of SsC, also highlighting the most fruitful research directions for the full development of its potential. Topics covered in the chapter include composition and processing of SsC, strain sensing methods and models, main fields of applications, research trends and open problems.
Experimental analysis on slamming reduction in rectangular liquid tanks subjected to harmonic motionPublished: 01 January 2018 by AIP Publishing in INTERNATIONAL CONFERENCE OF NUMERICAL ANALYSIS AND APPLIED MATHEMATICS (ICNAAM 2017)
This paper investigates the reduction of slamming effects related to liquid sloshing in moving tanks by using internal vertical perforated screens. The research is framed in the context of the evaluation of energy dissipation in Tuned Sloshing Dampers (TSD), generally applied in civil engineering for the structural vibration control of tall buildings. A deep understanding of the physical problem and the mechanisms generating and suppressing waves within the tank in relation to the dissipation energy which the device could be able to provide is much needed, yet not achieved in the literature. The paper presents some recent advances in this framework through an experimental campaign carried out on tanks with different dimensions subjected to harmonic motion.
The paper proposes the novel concept of smart bricks as a durable sensing solution for structural health monitoring of masonry structures. The term smart bricks denotes piezoresistive clay bricks with suitable electronics capable of outputting measurable changes in their electrical properties under changes in their state of strain. This feature can be exploited to evaluate stress at critical locations inside a masonry wall and to detect changes in loading paths associated with structural damage, for instance following an earthquake. Results from an experimental campaign show that normal clay bricks, fabricated in the laboratory with embedded electrodes made of a special steel for resisting the high baking temperature, exhibit a quite linear and repeatable piezoresistive behavior. That is a change in electrical resistance proportional to a change in axial strain. In order to be able to exploit this feature for strain sensing, high-resolution electronics are used with a biphasic DC measurement approach to eliminate any resistance drift due to material polarization. Then, an enhanced nanocomposite smart brick is proposed, where titania is mixed with clay before baking, in order to enhance the brick's mechanical properties, improve its noise rejection, and increase its electrical conductivity. Titania was selected among other possible conductive nanofillers due to its resistance to high temperatures and its ability to improve the durability of construction materials while maintaining the aesthetic appearance of clay bricks. An application of smart bricks for crack detection in masonry walls is demonstrated by laboratory testing of a small-scale wall specimen under different loading conditions and controlled damage. Overall, it is demonstrated that a few strategically placed smart bricks enable monitoring of the state of strain within the wall and provide information that is capable of crack detection.
Recent Advances on SHM of Reinforced Concrete and Masonry Structures Enabled by Self-Sensing Structural MaterialsPublished: 14 November 2017 by MDPI AG in Proceedings
<p><span>Structural Health Monitoring is aimed at transforming civil structures into self-diagnosing systems able to automatically reveal the occurrence of a fault or a damage after a critical event such as an earthquake. While data science is presently experiencing a tremendous development, leading to the availability of powerful tools and algorithms that extract relevant information by effectively fusing data provided by different types of sensors, one of the main bottlenecks still limiting the development of SHM in the filed of civil engineering is the general lack of reliable sensing technologies that are effectively applicable to the large scale. A very promising solution to such a large scale challenge would be using the same construction materials for strain sensing and direct damage detection. In this view, the authors have recently proposed smart concretes and smart bricks that are piezoresistive concretes and clay bricks obtained by doping traditional construction materials with conductive nano- or micro inclusions. These novel multifunctional materials have the ability to provide measurable electrical output under application of a mechanical load and to provide information useful for damage detection, localization and quantification. The paper introduces both technologies, discusses their potentials and illustrates their application to paradigmatic structural elements arranged in the laboratory. The presented results contribute to showing the revolutionary impact that smart concretes and smart bricks may have in the near future on SHM of concrete and masonry structures.</span></p>
Experimental wind tunnel study of a smart sensing skin for condition evaluation of a wind turbine bladePublished: 30 October 2017 by IOP Publishing in Smart Materials and Structures
Condition evaluation of wind turbine blades is difficult due to their large size, complex geometry and lack of economic and scalable sensing technologies capable of detecting, localizing, and quantifying faults over a blade's global area. A solution is to deploy inexpensive large area electronics over strategic areas of the monitored component, analogous to sensing skin. The authors have previously proposed a large area electronic consisting of a soft elastomeric capacitor (SEC). The SEC is highly scalable due to its low cost and ease of fabrication, and can, therefore, be used for monitoring large-scale components. A single SEC is a strain sensor that measures the additive strain over a surface. Recently, its application in a hybrid dense sensor network (HDSN) configuration has been studied, where a network of SECs is augmented with a few off-the-shelf strain gauges to measure boundary conditions and decompose the additive strain to obtain unidirectional surface strain maps. These maps can be analyzed to detect, localize, and quantify faults. In this work, we study the performance of the proposed sensing skin at conducting condition evaluation of a wind turbine blade model in an operational environment. Damage in the form of changing boundary conditions and cuts in the monitored substrate are induced into the blade. An HDSN is deployed onto the interior surface of the substrate, and the blade excited in a wind tunnel. Results demonstrate the capability of the hybrid dense sensor network and associated algorithms to detect, localize, and quantify damage. These results show promise for the future deployment of fully integrated sensing skins deployed inside wind turbine blades for condition evaluation.
Damage detection, localization and quantification in conductive smart concrete structures using a resistor mesh modelPublished: 01 October 2017 by Elsevier BV in Engineering Structures
Assessment of a monumental masonry bell-tower after 2016 Central Italy seismic sequence by long-term SHMPublished: 05 September 2017 by Springer Nature in Bulletin of Earthquake Engineering
The response of the San Pietro monumental bell-tower located in Perugia, Italy, to the 2016 Central Italy seismic sequence is investigated, taking advantage of the availability of field data recorded by a vibration-based SHM system installed in December 2014 to detect earthquake-induced damages. The tower is located about 85 km in the NW direction from the epicenter of the first major shock of the sequence, the Accumoli Mw6.0 earthquake of August 24th, resulting in a small local PGA of about 30 cm/s2, whereby near-field PGA was measured as 915.97 cm/s2 (E–W component) and 445.59 cm/s2 (N–S component). Similar PGA values also characterized the two other major shocks of the sequence (Ussita Mw5.9 and Norcia Mw6.5 earthquakes of October 26th and 30th, respectively). Despite the relatively low intensity of such earthquakes in Perugia, the analysis of long-term monitoring data clearly highlights that small permanent changes in the structural behavior of the bell-tower have occurred after the earthquakes, with decreases in all identified natural frequencies. Such natural frequency decays are fully consistent with what predicted by non-linear finite element simulations and, in particular, with the development of microcracks at the base of the columns of the belfry. Microcracks in these regions, and in the rest of tower, are however hardly distinguishable from pre-existing ones and from the physiological cracking of a masonry structure, what validates the effectiveness of the SHM system in detecting earthquake-induced damage at a stage where this is not yet detectable by visual inspections.
Algorithm for damage detection in wind turbine blades using a hybrid dense sensor network with feature level data fusionPublished: 01 September 2017 by Elsevier BV in Journal of Wind Engineering and Industrial Aerodynamics
Earthquake-Induced Damage Detection in a Monumental Masonry Bell-Tower Using Long-Term Dynamic Monitoring DataPublished: 11 August 2017 by Informa UK Limited in Journal of Earthquake Engineering
This work investigates the use of an advanced long-term vibration-based structural health monitoring tool to automatically detect earthquake-induced damages in heritage structures. Damage produced in a monumental bell-tower at increasing values of the Peak Ground Acceleration (PGA) of the seismic input is predicted by incremental nonlinear dynamic analysis, using a Finite Element model calibrated on the basis of experimentally identified natural modes. Then, predicted damage effects are artificially introduced in the monitoring data to check for their detectability. The results demonstrate that a very small damage, associated to a low intensity and low return period earthquake, is clearly detected by the monitoring system.
Enhanced lumped circuit model for smart nanocomposite cement-based sensors under dynamic compressive loading conditionsPublished: 01 June 2017 by Elsevier BV in Sensors and Actuators A: Physical
Biphasic DC measurement approach for enhanced measurement stability and multi-channel sampling of self-sensing multi-fun...Published: 02 May 2017 by IOP Publishing in Smart Materials and Structures
Investigation of multi-functional carbon-based self-sensing structural materials for structural health monitoring applications is a topic of growing interest. These materials are self-sensing in the sense that they can provide measurable electrical outputs corresponding to physical changes such as strain or induced damage. Nevertheless, the development of an appropriate measurement technique for such materials is yet to be achieved, as many results in the literature suggest that these materials exhibit a drift in their output when measured with direct current (DC) methods. In most of the cases, the electrical output is a resistance and the reported drift is an increase in resistance from the time the measurement starts due to material polarization. Alternating current (AC) methods seem more appropriate at eliminating the time drift. However, published results show they are not immune to drift. Moreover, the use of multiple impedance measurement devices (LCR meters) does not allow for the simultaneous multichannel sampling of multi-sectioned self-sensing materials due to signal crosstalk. The capability to simultaneously monitor multiple sections of self-sensing structural materials is needed to deploy these multi-functional materials for structural health monitoring. Here, a biphasic DC measurement approach with a periodic measure/discharge cycle in the form of a square wave sensing current is used to provide consistent, stable resistance measurements for self-sensing structural materials. DC measurements are made during the measurement region of the square wave while material depolarization is obtained during the discharge region of the periodic signal. The proposed technique is experimentally shown to remove the signal drift in a carbon-based self-sensing cementitious material while providing simultaneous multi-channel measurements of a multi-sectioned self-sensing material. The application of the proposed electrical measurement technique appears promising for real-time utilization of self-sensing materials in structural health monitoring.
Novel dynamic thermal characterization of multifunctional concretes with microencapsulated phase change materialsPublished: 19 April 2017 by SPIE-Intl Soc Optical Eng in Smart Materials and Nondestructive Evaluation for Energy Systems 2017
Concrete is widely applied in the construction sector for its reliable mechanical performance, its easiness of use and low costs. It also appears promising for enhancing the thermal-energy behavior of buildings thanks to its capability to be doped with multifunctional fillers. In fact, key studies acknowledged the benefits of thermally insulated concretes for applications in ceilings and walls. At the same time, thermal capacity also represents a key property to be optimized, especially for lightweight constructions. In this view, Thermal-Energy Storage (TES) systems have been recently integrated into building envelopes for increasing thermal inertia. More in detail, numerical experimental investigations showed how Phase Change materials (PCMs), as an acknowledged passive TES strategy, can be effectively included in building envelope, with promising results in terms of thermal buffer potentiality. In particular, this work builds upon previous papers aimed at developing the new PCM-filled concretes for structural applications and optimized thermalenergy efficiency, and it is focused on the development of a new experimental method for testing such composite materials in thermal-energy dynamic conditions simulated in laboratory by exposing samples to environmentally controlled microclimate while measuring thermal conductivity and diffusivity by means of transient plane source techniques. The key findings show how the new composites are able to increasingly delay the thermal wave with increasing the PCM concentration and how the thermal conductivity varies during the course of the phase change, in both melting and solidification processes. The new analysis produces useful findings in proposing an effective method for testing composite materials with adaptive thermal performance, much needed by the scientific community willing to study building envelopes dynamics. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Continuous and embedded solutions for SHM of concrete structures using changing electrical potential in self-sensing cem...Published: 19 April 2017 by SPIE-Intl Soc Optical Eng in Nondestructive Characterization and Monitoring of Advanced Materials, Aerospace, and Civil Infrastructure 2017
Interest in the concept of self-sensing structural materials has grown in recent years due to its potential to enable continuous low-cost monitoring of next-generation smart-structures. The development of cement-based smart sensors appears particularly well suited for monitoring applications due to their numerous possible field applications, their ease of use and long-term stability. Additionally, cement-based sensors offer a unique opportunity for structural health monitoring of civil structures because of their compatibility with new or existing infrastructure. Particularly, the addition of conductive carbon nanofillers into a cementitious matrix provides a self-sensing structural material with piezoresistive characteristics sensitive to deformations. The strain-sensing ability is achieved by correlating the external loads with the variation of specific electrical parameters, such as the electrical resistance or impedance. Selection of the correct electrical parameter for measurement to correlate with features of interest is required for the condition assessment task. In this paper, we investigate the potential of using altering electrical potential in cement-based materials doped with carbon nanotubes to measure strain and detect damage in concrete structures. Experimental validation is conducted on small-scale specimens including a steel-reinforced beam of conductive cement paste. Comparisons are made with constant electrical potential and current methods commonly found in the literature. Experimental results demonstrate the ability of the changing electrical potential at detecting features important for assessing the condition of a structure. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Damage location and quantification of a pretensioned concrete beam using stochastic subspace identificationPublished: 19 April 2017 by SPIE-Intl Soc Optical Eng in Nondestructive Characterization and Monitoring of Advanced Materials, Aerospace, and Civil Infrastructure 2017
Stochastic subspace identification (SSID) is a first-order linear system identification technique enabling modal analysis through the time domain. Research in the field of structural health monitoring has demonstrated that SSID can be used to successfully retrieve modal properties, including modal damping ratios, using output-only measurements. In this paper, the utilization of SSID for indirectly retrieving structures’ stiffness matrix was investigated, through the study of a simply supported reinforced concrete beam subjected to dynamic loads. Hence, by introducing a physical model of the structure, a second-order identification method is achieved. The reconstruction is based on system condensation methods, which enables calculation of reduced order stiffness, damping, and mass matrices for the structural system. The methods compute the reduced order matrices directly from the modal properties, obtained through the use of SSID. Lastly, the reduced properties of the system are used to reconstruct the stiffness matrix of the beam. The proposed approach is first verified through numerical simulations and then validated using experimental data obtained from a full-scale reinforced concrete beam that experienced progressive damage. Results show that the SSID technique can be used to diagnose, locate, and quantify damage through the reconstruction of the stiffness matrix. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
The authors have recently proposed a hybrid dense sensor network consisting of a novel, capacitive-based thin-film electronic sensor for monitoring strain on mesosurfaces and fiber Bragg grating sensors for enforcing boundary conditions on the perimeter of the monitored area. The thin-film sensor monitors local strain over a global area through transducing a change in strain into a change in capacitance. In the case of bidirectional in-plane strain, the sensor output contains the additive measurement of both principal strain components. When combined with the mature technology of fiber Bragg grating sensors, the hybrid dense sensor network shows potential for the monitoring of mesoscale systems. In this paper, we present an algorithm for the detection, quantification, and localization of strain within a hybrid dense sensor network. The algorithm leverages the advantages of a hybrid dense sensor network for the monitoring of large scale systems. The thin film sensor is used to monitor strain over a large area while the fiber Bragg grating sensors are used to enforce the uni-directional strain along the perimeter of the hybrid dense sensor network. Orthogonal strain maps are reconstructed by assuming different bidirectional shape functions and are solved using the least squares estimator to reconstruct the planar strain maps within the hybrid dense sensor network. Error between the estimated strain maps and measured strains is extracted to derive damage detecting features, dependent on the selected shape functions. Results from numerical simulations show good performance of the proposed algorithm. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Damage detection of wind turbine blades is difficult due to their large sizes and complex geometries. Additionally, economic restraints limit the viability of high-cost monitoring methods. While it is possible to monitor certain global signatures through modal analysis, obtaining useful measurements over a blade's surface using off-the-shelf sensing technologies is difficult and typically not economical. A solution is to deploy dedicated sensor networks fabricated from inexpensive materials and electronics. The authors have recently developed a novel large-area electronic sensor measuring strain over very large surfaces. The sensing system is analogous to a biological skin, where local strain can be monitored over a global area. In this paper, we propose the utilization of a hybrid dense sensor network of soft elastomeric capacitors to detect, localize, and quantify damage, and resistive strain gauges to augment such dense sensor network with high accuracy data at key locations. The proposed hybrid dense sensor network is installed inside a wind turbine blade model and tested in a wind tunnel to simulate an operational environment. Damage in the form of changing boundary conditions is introduced into the monitored section of the blade. Results demonstrate the ability of the hybrid dense sensor network, and associated algorithms, to detect, localize, and quantify damage. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Multipurpose experimental characterization of smart nanocomposite cement-based materials for thermal-energy efficiency a...Published: 01 March 2017 by Elsevier BV in Solar Energy Materials and Solar Cells
Micromechanics modeling of the uniaxial strain-sensing property of carbon nanotube cement-matrix composites for SHM appl...Published: 01 March 2017 by Elsevier BV in Composite Structures
This article newly proposes the application of the stretching method, that is used in geophysics for detecting variations in the velocity with which waves propagate in the earth's crust from seismic noise recordings, in the context of vibration-based Structural Health Monitoring (SHM) of civil structures. The result is a computationally efficient long-term vibration-based SHM tool, that follows the current trend of using a very limited number of sensors permanently installed on site to measure operational structural responses for the purpose of damage detection. In the SHM setting, the proposed method aims at a direct identification of small permanent shifts in the natural frequencies of the structure in a changing environment, which is achieved by maximizing the correlation coefficient between a reference waveform, computed in a training reference period in which the structure is assumed to be undamaged, and a stretched version of the same waveform evaluated at the current time. The comparison is performed in the frequency domain and the waveform of interest is obtained from cross-correlations of the ambient vibration measurements. More specifically, in the case of multiple sensors, the waveform can be either the cross-power spectral density of the signals recorded by a pair of sensors, or the largest singular value of the spectral matrix of the measurements. It follows that the method can be regarded as an extension of the classic Frequency Domain Decomposition (FDD). A key feature of the proposed stretching method is mitigating the effects of environmental fluctuations by time domain averaging of cross-correlations over a proper period of time, before taking their Fourier transform to estimate the spectral densities. Such a time domain averaging is carried out in a relatively long period of time for estimating the reference waveform, whereas it is carried out in a shorter time for estimating the current waveform. The main features of the proposed methodology are its very low sensitivity to environmental fluctuations, resulting in a quite short training period length, and its low computational cost, which could be compatible with a direct integration within smart sensors with embedded electronics. The performance of the method is illustrated in the case study of an Italian historical monumental bell tower that has been monitored by the authors for more than 1 year.
Structural health monitoring of cylindrical bodies under impulsive hydrodynamic loading by distributed FBG strain measur...Published: 12 January 2017 by IOP Publishing in Measurement Science and Technology
Environmental effects on natural frequencies of the San Pietro bell tower in Perugia, Italy, and their removal for struc...Published: 01 January 2017 by Elsevier BV in Mechanical Systems and Signal Processing
Highlights•Hygro-thermal effects on natural frequencies of a masonry bell-tower are investigated.•Temperature variations produce significant changes in natural frequencies.•Three temperature-driven types of frequency variations are observed including freezing.•Linear regressive models allow to accurately remove temperature effects for SHM.•Five environmental sensors provide the most accurate estimation of natural frequencies. AbstractContinuously identified natural frequencies of vibration can provide unique information for low-cost automated condition assessment of civil constructions and infrastructures. However, the effects of changes in environmental parameters, such as temperature and humidity, need to be effectively investigated and accurately removed from identified frequency data for an effective performance assessment. This task is particularly challenging in the case of historical constructions that are typically massive and heterogeneous masonry structures characterized by complex variations of materials' properties with varying environmental parameters and by a differential heat conduction process where thermal capacity plays a major role.While there is abundance of documented monitoring data highlighting correlations between environmental parameters and natural frequencies in the case of new structures, such as long-span bridges, similar studies for historical constructions are still missing, with only a few literature works occasionally reporting increments in natural frequencies with increasing temperature of construction materials due to the closure of internal micro-cracks in the mortar layers caused by thermal expansion.In order to gain some knowledge on the effects of changes in temperature and humidity on the natural frequencies of slender masonry buildings, the paper focuses on the case study of an Italian monumental bell tower that has been monitored by the authors for more than nine months. Correlations between natural frequencies and environmental parameters are investigated in detail and the predictive capabilities of linear statistical regressive models based on the use of several environmental continuous monitoring sensors are assessed. At the end, three basic mechanisms governing environmentally-induced changes in the dynamic behavior of the tower are identified and essential information is achieved on the optimal location and minimum number of environmental sensors that are necessary in a structural health monitoring perspective.
A Simplified Parametric Study on Occupant Comfort Conditions in Base Isolated Buildings under Wind LoadingPublished: 01 January 2017 by Hindawi Limited in Advances in Civil Engineering
Vibrations in buildings can cause occupant discomfort in the form of annoyance, headache, or sickness. While occupant comfort is considered in international standards regarding the design of high rise buildings against wind loading, it is neglected in the design of buildings with seismic protective base isolation systems. Nevertheless, due to their low flexibility, base isolated buildings can be prone to wind-induced vibrations, which makes occupant discomfort a potentially significant serviceability limit state. This paper presents a study on occupant comfort conditions in wind-excited base isolated buildings. A numerical simplified parametric procedure is proposed in order to evaluate the return period of wind events causing human discomfort. A parametric investigation is then presented to evaluate the effects of salient parameters on comfort conditions. The procedure is based on (i) the nonlinear dynamic analysis of the structure modeled as a single-degree-of-freedom oscillator with hysteretic base isolators, (ii) the digital generation of time histories of turbulent wind velocity, and (iii) comfort evaluations based on international standards. Results demonstrate that discomfort conditions can occur a few times in a year, depending upon the wind-exposure of the site, what suggests considering this serviceability limit state in the design of base isolated buildings.1. IntroductionOccupant discomfort caused by wind-induced vibrations is a major serviceability limit state in the performance-based design of high rise buildings , whereby humans have subjective sensitivity to floor acceleration that, in turn, can cause discomfort in the form of annoyance, headache, or sickness.While it is almost accepted that discomfort is caused by intense floor acceleration, there is no agreement in the scientific literature and in international standard codes, on how to carry out comfort evaluations, as the problem involves aspects that are difficult to model, including physiological and psychological aspects . In this regard, Kwok et al.  reviewed existing studies on human perception and associated tolerance thresholds of vibrations in tall buildings, highlighting the need for the development of internationally accepted practical occupant comfort serviceability criteria, accounting for the subjective nature of the problem. Kwon and Kareem  compared major international standards in regard to wind effects on buildings and highlighted that most of the technical standards use comfort criteria based on along-wind acceleration. Lamb et al.  investigated motion sickness in tall buildings highlighting the importance of considering differences in individual perceptions when examining the effects of building motion on personal comfort, which could also affect work productivity. The same authors reported that the effects of building motions on individual comfort are difficult to assess, because most of the occupants do not complain when they feel discomfort. Bernardini et al.  proposed a probabilistic framework for the performance-based design of tall buildings for occupant comfort, providing a computational tool that allows the designer to meet a certain acceptable probability of exceedance of comfort conditions by a certain number of occupants.While the importance of occupant comfort is well-understood in the design of high rise buildings, specific design criteria are considered to limit discomfort caused by wind-induced vibrations.The same issue is typically neglected in the design of buildings with base isolation devices, with the exception of some sporadic attempts that were made at the very beginning of the development of the technology . The comfort conditions assume lesser relevance in earthquake resistant systems such as walls structures , while assuming increased relevance for irregular structures .This is motivated by the general understanding that vibration effects caused by wind loading in the case of base isolated buildings are negligible. Nevertheless, base isolated buildings , even though often low-rise, have low natural frequencies of vibration, thus being potentially wind-sensitive and prone to wind-induced vibrations [11, 12]. Thus, the lack of international criteria and standards that account for occupant comfort in base isolated buildings under wind loading does not appear to be fully justified at the present state of the knowledge and the investigation of this problem is a worth research effort, as addressed in this study.The present paper is a contribution towards a more aware understanding of the role that human comfort conditions may play in the performance-based design of base isolated buildings under wind loading. To this aim, a simple general methodology to evaluate the short term return period of wind events determining discomfort in wind-excited base isolated buildings is proposed, at first. This methodology, presented in Section 2, considers a simple single-degree-of-freedom nonlinear hysteretic oscillator representing a base isolated building under turbulent wind loading and is based on comfort analysis carried out according to widely accepted international standards [13–15]. The model is then used for carrying out some parametric investigations in Section 3, highlighting the major design parameters affecting the critical wind velocity above which discomfort is expected to occur. These results demonstrate that discomfort conditions may occur a few times in a year depending on the wind characteristics of the site and on the characteristics of the base isolated structures. Finally, the paper is ended with proper conclusions in Section 4.2. Simplified Analytical ModelIn this section we present the simplified parametric numerical model finalized at rapidly evaluating the return period of wind events determining occupants’ discomfort in buildings with base isolation devices. The procedure comprises the following steps:(1)definition of input parameters regarding both the site and the building;(2)digital generation of a time history of turbulent wind velocity;(3)nonlinear dynamic analysis by time integration;(4)evaluation of the response of the structure in terms of horizontal acceleration;(5)occupants’ comfort analysis according to acknowledged international standards;(6)assessment of the critical gust intensity determining human discomfort in the building;(7)evaluation of the return period of wind events causing occupant discomfort.Steps from (2) to (5) are iterated by considering increasing intensities of the wind gust, so as to determine, in step (6), the critical condition determining discomfort and its corresponding return period in step (7).2.1. Structural ModelAssuming, without any loss of generality, that the base isolated building has an approximately rectangular plan, with one dimension that is much greater than the other, and that the wind blows along the shorter building dimension, , at a first level of approximation the dynamic behavior of the structure can be modeled by a single-degree-of-freedom nonlinear oscillator (Figure 1(a)), whose equation of motion is where is the lateral displacement, denoting time, is the mass per unit length of the building, is the damping coefficient, and is the wind force per unit length. In (1), the nonlinear restoring force of the base isolation devices is written aswhich is formally similar to an elastic restoring force, but with a nonlinear stiffness coefficient . This last is computed by modeling the constitutive behavior of the base isolators through an elastic hardening plastic hysteretic model, also called bilinear hysteretic model , as illustrated in Figure 1(b), where is the yielding displacement, is the ultimate displacement, is the initial preyielding stiffness, and is the postyielding stiffness. These quantities are calculated aswhere is the secant stiffness and is a characteristic parameter of the isolators that for elastomeric ones assumes the typical value of 4 . The secant stiffness of the isolators is chosen to meet a certain value of the fundamental period, , of the isolated mode of the equivalent linear system as follows:However, it should be noted that, in preyield conditions, occurring in the case of low wind intensities, the system behaves as a relatively rigid linear one, with a fundamental period of the isolated mode, , given byFigure 1: Simplified mechanical system for comfort parametric analysis under wind loading: sketch of the wind-excited base isolated building (a) and constitutive behavior of the base isolation devices (b).2.2. Wind Loading ForceThe wind loading force is expressed aswhere is the air density, is the total height of the building from the ground, is the pressure coefficient of the building, is the ten-minute mean wind velocity at a height from the ground, and is the turbulent wind velocity. Considering that , the term , in (6), can be neglected with a good approximation. According to Eurocode 1 , the ten-minute mean wind velocity is written as a function of the height from the ground through the classic logarithmic profile that models the atmospheric boundary layer aswhere is the basic wind velocity, which depends upon the geographical location of the site and its altitude above the sea level, is the terrain factor, is the height below which the logarithmic profile loses its significance and the mean wind velocity is considered as constant, and is the roughness length of the site.These quantities assume the values summarized in Table 1, which are taken from the Italian technical standard according the definitions of terrain categories reported in Eurocode 1 . The basic wind velocity, , is instead defined as the ten-minute mean wind velocity with a return period of 50 years, irrespective of wind direction, at a height of 10 m above flat open country terrain (terrain category II).Table 1: Terrain categories as defined by Eurocode 1  and corresponding roughness length , terr
Static and Dynamic Strain Monitoring of Reinforced Concrete Components through Embedded Carbon Nanotube Cement-Based Sen...Published: 01 January 2017 by Hindawi Limited in Shock and Vibration
The paper presents a study on the use of cement-based sensors doped with carbon nanotubes as embedded smart sensors for static and dynamic strain monitoring of reinforced concrete (RC) elements. Such novel sensors can be used for the monitoring of civil infrastructures. Because they are fabricated from a structural material and are easy to utilize, these sensors can be integrated into structural elements for monitoring of different types of constructions during their service life. Despite the scientific attention that such sensors have received in recent years, further research is needed to understand (i) the repeatability and accuracy of sensors’ behavior over a meaningful number of sensors, (ii) testing configurations and calibration methods, and (iii) the sensors’ ability to provide static and dynamic strain measurements when actually embedded in RC elements. To address these research needs, this paper presents a preliminary characterization of the self-sensing capabilities and the dynamic properties of a meaningful number of cement-based sensors and studies their application as embedded sensors in a full-scale RC beam. Results from electrical and electromechanical tests conducted on small and full-scale specimens using different electrical measurement methods confirm that smart cement-based sensors show promise for both static and vibration-based structural health monitoring applications of concrete elements but that calibration of each sensor seems to be necessary.1. IntroductionStructural health monitoring (SHM) is a topic of growing scientific interest in various fields of engineering, with potential to increase engineering systems’ safety and lead to the optimization of repair, maintenance, and restoration activities [1, 2]. Strategically designed monitoring systems can detect damage or variations of structural behavior during the service life of a structure [3–5]. SHM is also useful for fast screening of structural conditions after severe events, such as blasts and earthquakes. The choice of the monitoring sensors and of their proper placement is critical for accurately analyzing the structural performance and for the measurement of damage-sensitive features and their statistical investigation. A spatially distributed dense sensing system increases the probability of finding and characterizing a given amount of damage. An ideal solution is to transform the entire structure into a self-sensing system, analogous to the biological nervous system.Recent developments in the field of nanotechnology have led to cement-based sensors doped with piezoresistive nanofillers, which can be used as a potential solution for distributed sensing in reinforced concrete (RC) structures [6–8]. Nanomodified cement-based sensors can be easily fabricated and embedded in a structure at critical locations. In addition, they possess approximately the same durability as the materials to be monitored and exhibit lower maintenance costs with respect to traditional sensors. Promising conductive nanoinclusions for cement-based sensors are carbon-based particles [9–12]. Among them, carbon nanotubes are particularly suitable owing to their particular aspect ratio. They consist of concentric cylindrical graphene sheets of nanometric diameters and lengths up to some micrometers [13–15]. Their dispersion into a cementitious matrix enhances the electrical properties of the original materials, providing them with self-sensing capabilities [16–18]. The self-monitoring ability is achieved through the correlation of strains or stresses of the material to electrical features, such as electrical resistance or impedance [19–22]. Different electrical effects contribute to the strain-sensing mechanism: the piezoresistivity of the conductive nanofillers, the contact resistance of the electrodes, the intrinsic resistance of the different materials, and the tunneling and the field emission effects due to the nanosize of the fillers [16, 23]. Although several studies were recently devoted to investigating challenges related to the dispersion of the conductive nanoparticles in the cement matrix [24, 25], the fabrication of nanocomposites with different amounts of fillers [26, 27], and electromechanical testing [28–30], the repeatability and accuracy of the electrical behavior upon dynamic sensing still require in-depth investigations. The authors have researched a new cement-based sensor doped with multiwalled carbon nanotubes, termed carbon nanotube cement-based sensor (CNTCS) [15, 24, 31, 32]. The CNTCSs were fabricated with different types of cement matrices (pastes, mortars, and concretes) and various amounts of carbon nanoinclusions.Building on previous work, the objective of this paper is to investigate the use of CNTCSs as smart embedded sensors for dynamic strain monitoring of RC elements. A meaningful number of samples are first characterized and the application to a full-scale RC beam is studied. The rest of the paper is organized as follows. Section 2 presents materials and preparation procedures of the cementitious sensors with multiwalled carbon nanotubes (MWCNTs). Section 3 describes the experimental methodologies. Section 4 discusses results of electromechanical tests under application of slowly varying cyclical loads and sinusoidal dynamic loads with increasing frequencies. The sensitivity of the samples and their frequency response functions are analyzed and the electrical and electromechanical properties of the sensors are investigated through both two- and four-probe measurement methods. This section also includes test results from the embedment of CNTCSs in a full-scale RC beam. Section 5 concludes the paper.2. Materials and Preparation of SamplesTen cube samples of 5 cm side were fabricated for the experimental campaign. The cementitious matrix was a cement paste doped with 1% of MWCNTs with respect to the weight of cement (Figure 1). Before solidification, four stainless steel meshes were embedded symmetrically through the sample. The inner meshes were placed at a mutual distance of 2 cm, while the outer ones were at a distance of 4 cm. Figure 1 shows the geometry of the samples and of the electrodes.Figure 1: Geometry of the samples and electrodes (dimension in cm) (a); picture of the fabricated samples with strain gauges installed onto lateral surfaces (b).The carbon nanotubes were type Arkema Graphistrength C100. Their dispersion in the cementitious matrix was achieved using a physical dispersant, preliminary mechanical mixing, and sonication (Figures 2(i)–2(iii)). A stable water suspension was obtained and then mixed with cement and 0.5% of a plasticizer with respect to the weight of cement (Figure 2(iv)). The cement was type 42.5, pozzolanic. The water/cement ratio was 0.45. The smooth mixture was poured into oiled molds and the steel nets were embedded (Figure 2(v)). After solidification, the samples were unmolded for curing in laboratory conditions for the next 28 days (Figure 2(vi)). Each sample was then instrumented with two 2 cm long electrical strain gauges, with a gauge factor of 2.1, placed onto the center of opposite lateral sides, as shown in Figure 1.Figure 2: Preparation process of the cement paste samples with 1% of MWCNTs.3. Methodology3.1. Electrical TestsElectrical tests were performed on the ten sensors using a data acquisition system consisting of an NI PXIe-1073 chassis with dedicated modules. Both two-probe and four-probe measurement configurations in DC current were adopted. The chassis was equipped with two modules: the electric power was supplied through an NI PXI-4130, capable of providing a four-quadrant ±20 V and ±2 A output in a single isolated channel, while electrical measurements were conducted through a high speed digital multimeter, model NI PXI-4071. This last module acquired voltage in the two-probe measurement setup and current in the four-probe measurement setup. The temperature of the sensors was controlled before and during the tests using a climatic chamber to avoid temperature-induced drifts.The electrical resistance of each sensor was evaluated after 6000 s of polarization in order to mitigate the signal drift due to the dielectric nature of the cementitious matrix. In the two-probe method, the sensors were subjected to an electrical voltage of 5 V with a current measurement range of 1.0 mA. The inner electrodes with a mutual distance of 20 mm were used as the active electrodes. In the four-probe configuration, the sensors were subjected to a current level equal to 15 mA with a voltage measurement range of 10 V. In this case, the current was applied to the external electrodes placed at a mutual distance of 40 mm, while the voltage drop was measured across the internal electrodes, as in the two-probe method. The electrical resistance was calculated, in both cases, based on the first Ohm’s law. Equations (1) and (2) refer to the computation of the electrical resistance for two- and four-probe configurations, respectively:where and are the applied constant voltage and current, and are the measured variations of voltage and current intensity over time, and is the polarization time.3.2. Electromechanical TestsAxial compression tests were conducted to assess the strain-sensing capability and repeatability of the measurements taken from the cement paste samples. Cyclical and dynamic axial compression tests were performed on the ten sensors using the chassis NI PXIe-1073 for data acquisition. The chassis was equipped with three modules: the electric power generator, the multimeter used for the electrical tests, and the data acquisition system for the strain gauges, model PXIe-4330, 8 channels, 24-bit resolution, 25 kHz maximum sampling rate, antialiasing filters. The compression loads were applied using a servo-controlled pneumatic universal dynamic testing machine, model IPC Global UTM14P, with a controlled temperature chamber. For the electrical tests, both two-probe and four-probe measurem
Detecting earthquake-induced damage in historic masonry towers using continuously monitored dynamic response-only dataPublished: 01 January 2017 by Elsevier BV in Procedia Engineering
DYNAMIC MONITORING AND NONLINEAR ANALYSIS OF THE DOME OF THE BASILICA OF S.MARIA DEGLI ANGELI IN ASSISIPublished: 01 January 2017 by ECCOMAS in Proceedings of the 6th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering (COMPDYN 2015)