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Micro and macro crack sensing in TRC beam under cyclic loading
Abstract
This paper studies the ability of self-sensory carbon/glass textile reinforced concrete (TRC) beams to distinguish between micro- and macrocracking. In the proposed configuration, continuous carbon rovings knitted into the textile mesh serve both as the structural reinforcement and as the sensory system. The paper faces the challenge of detecting structural damage within the TRC structure. In this study, damage is defined as the formation of macroscopic cracks, which lead to the accumulation of significant irreversible residual deflection and to a reduction of the relative stiffness of the component. We explore experimentally the correlation between the electrical resistance change and the change of the structural properties and suggests crack detection parameters in order to identify, and mainly to distinguish, between micro- and macrostructural phenomena. Carbon rovings are found to provide electromechanical sensing capabilities, having the ability to distinguish between inner micromechanical structural phenomena and macroscopic ones. These observations are a step towards the applications of SHM techniques by intelligent carbon-based TRC elements.
... Recent studies face these visions by taking advantage of the textile reinforced concrete (TRC) technology for the development of intelligent structural elements with integrated sensory capabilities. [1][2][3][4][5][6][7][8][9][10][11][12] The concept of the self-sensory system is based on implementing electrically conductive carbon yarns within alkali resistant (AR)-glass-based textile mesh that simultaneously serve as the main reinforcement system and as the sensory agent. The studies in the literature demonstrated the self-sensory capabilities of carbon yarns to monitor the applied load, 1-10 detect the occurrence of cracks, [3][4][5][6][7][8]10 correlate the electrical properties to integrative values of strain, 1,2,5 estimate the correlation by means of gauge factor (GF) 6,7,9,10 and identify water leakage. ...
... [1][2][3][4][5][6][7][8][9][10][11][12] The concept of the self-sensory system is based on implementing electrically conductive carbon yarns within alkali resistant (AR)-glass-based textile mesh that simultaneously serve as the main reinforcement system and as the sensory agent. The studies in the literature demonstrated the self-sensory capabilities of carbon yarns to monitor the applied load, 1-10 detect the occurrence of cracks, [3][4][5][6][7][8]10 correlate the electrical properties to integrative values of strain, 1,2,5 estimate the correlation by means of gauge factor (GF) 6,7,9,10 and identify water leakage. 11,12 The measurements were taken by designated monitoring systems that were based on either direct current electrical circuits by simple two probe method, 9 four probe method, 1 and Wheatstone bridge method 2,[5][6][7][8]10 ; or alternating current circuits by taking the measurements at specific electrical current frequency 11,12 and exploring the response spectrum of the impedance. ...
... The studies in the literature demonstrated the self-sensory capabilities of carbon yarns to monitor the applied load, 1-10 detect the occurrence of cracks, [3][4][5][6][7][8]10 correlate the electrical properties to integrative values of strain, 1,2,5 estimate the correlation by means of gauge factor (GF) 6,7,9,10 and identify water leakage. 11,12 The measurements were taken by designated monitoring systems that were based on either direct current electrical circuits by simple two probe method, 9 four probe method, 1 and Wheatstone bridge method 2,[5][6][7][8]10 ; or alternating current circuits by taking the measurements at specific electrical current frequency 11,12 and exploring the response spectrum of the impedance. 3,4 The investigations correlated between the electrical measurements to various global parameters of the structural health. ...
The current study aims to handle the challenge of identifying the location of cracks in carbon-based textile reinforced concrete (TRC) elements by using their smart self-sensory capabilities. To answer this challenge, the study offers to adopt the concept of the Time Domain Reflectometer (TDR) analysis. Yet, direct implementation of the TDR analysis to smart sensory carbon yarns is not a straightforward act and requires facing some inherent challenges. Therefore, the study offers an advanced monitoring methodology that handles the various challenges. A special electrical setup is proposed that uses a pair of carbon yarns, in which one yarn is used as the electrical conductor that transmits the electrical current, and the other as insulation. Two calibration processes that consider the unique micro-structural mechanism and its correlation to the electrical characterization are developed. The efficiencies of the calibration processes are investigated by a designated identification technique. Results from this study demonstrate the potential and capabilities of the proposed smart self-sensory carbon-based TRC concept to monitor its health by detecting the cracks’ locations.
... Strain sensing by using carbon rovings: The load-carrying capacity of TRC structures is significantly influenced by the bonding mechanism between the concrete matrix and the roving, as well as the transfer of stress between the filaments [125]. When the filaments are bundled together, they form only microscopic gaps between them. ...
... The absence of tension in the inner filaments limits the load factor to approximately 30-35% [126,127]. As the tensile stress increases and the structure begins to crack, the outer filaments, experiencing the greatest stress, may also break [51,125]. When a microcrack progresses into a macrocrack, some of the inner filaments also break, along with all the outer filaments, causing the remaining filaments to shift. ...
... When a microcrack progresses into a macrocrack, some of the inner filaments also break, along with all the outer filaments, causing the remaining filaments to shift. This degradation of the structure significantly reduces its load-carrying capacity and leads to structural damage [51,118,125]. ...
Textile-reinforced concrete (TRC) is a composite material consisting of a concrete matrix with a high-performance reinforcement made of technical textiles. TRC offers unique mechanical properties for the construction industry, enabling the construction of lightweight, material-minimized structures with high load-bearing potential. In addition, compared with traditional concrete design, TRC offers unique possibilities to realize free-form, double-curved structures. After more than 20 years of research, TRC is increasingly entering the market, with several demonstrator elements and buildings completed and initial commercialization successfully finished. Nevertheless, research into this highly topical area is still ongoing. In this paper, the authors give an overview of the current and future trends in the research and application of textiles in concrete construction applications. These trends include topics such as maximizing the textile utilization rate by improving the mechanical load-bearing performance (e.g., by adapting bond behavior), increasing design freedom by utilizing novel manufacturing methods (e.g., based on robotics), adding further value to textile reinforcements by the integration of additional functions in smart textile solutions (e.g., in textile sensors), and research into increasing the sustainability of TRC (e.g., using recycled fibers).
... In TRC structures, the bonding mechanism between the concrete matrix and the roving, as well as the stress transfer mechanism between the filaments, plays an important role in the load-carrying capacity of the structure [21,22]. The bundling of the filaments results in the formation of only microscopic gaps between the filaments. ...
... However, the absence of tension in the inner filaments limits the load factor to 30-35% [23,24]. As the carried tensile stress increases and the structure begins to crack, some of the outer filaments subjected to the greatest stress also begin to break [12,21]. When a microcrack turns into a macrocrack, in addition to all the outer filaments, some of the inner filaments also break and cause the remaining ones to shift. ...
... When a microcrack turns into a macrocrack, in addition to all the outer filaments, some of the inner filaments also break and cause the remaining ones to shift. This structural degradation significantly reduces the load-carrying capacity of the structure and leads to structural damage [12,21,25]. ...
This study investigated the ability of electrically conductive carbon rovings to detect cracks in textile-reinforced concrete (TRC) structures. The key innovation lies in the integration of carbon rovings into the reinforcing textile, which not only contributes to the mechanical properties of the concrete structure but also eliminates the need for an additional sensory system, such as strain gauges, to monitor the structural health. Carbon rovings are integrated into a grid-like textile reinforcement that differs in binding type and dispersion concentration of the styrene butadiene rubber (SBR) coating. Ninety final samples were subjected to a four-point bending test in which the electrical changes of the carbon rovings were measured simultaneously to capture the strain. The mechanical results show that the SBR50-coated TRC samples with circular and elliptical cross-sectional shape achieved, with 1.55 kN, the highest bending tensile strength, which is also captured with a value of 0.65 Ω by the electrical impedance monitoring. The elongation and fracture of the rovings have a significant effect on the impedance mainly due to electrical resistance change. A correlation was found between the impedance change, binding type and coating. This suggests that the elongation and fracture mechanisms are affected by the number of outer and inner filaments, as well as the coating.
... PC based TRC structures were investigated for SHM purposes such as monitoring the load pattern [2], distinguishing between micro and macro-cracks [5] and detecting accumulated damage [7]. Further research investigated the effect of textile configuration, which change the textile-matrix bond mechanism, on the SHM capabilities [6]. ...
... The flexural and compression strengths of the matrices were tested at age of 28 days by using 40/40/160 mm and 50/50/50 mm specimens. The textile reinforcement is a generic glass-carbon bi-axial mesh with mesh size of 7-8 mm [4][5][6][7] The warp direction is composed of 6 AR-glass rovings and 2 carbon rovings, while in the weft direction only AR-glass rovings are positioned. The material properties of the textile and short fibers are given in Table 1. ...
... The study investigates three different cementitious composites: PC based TRC, textile reinforced MPC (TR-MPC) and TR-MPC with additive short fibers. The specimens were experimentally tested in monotonic flexural loading tests by using a fourpoints bending scheme [4][5][6][7]20]. The beams span length was 210 mm and the distance between the load points was 70 mm. ...
This study develops novel intelligent composite structural elements combining three advanced technologies: magnesium phosphate cement (MPC) matrix, smart-self sensory carbon-based textile reinforcement system, and additive short-dispersed fibers. In such system, the carbon rovings simultaneously serve as the main reinforcement system and the sensory agent. The material properties of the MPC matrix include minimization of environmental effects, high flexural strength and enhanced rheological properties which is an advantage in textile reinforcement system. From the sensory point of view, MPC is electrically insulated matrix which enhances the measured electrical signal from the carbon rovings. Experimental investigation demonstrates the advanced capabilities of the new hybrid structures. The investigation compares between the structural and electrical responses of textile reinforced MPC elements and TRC elements under flexural loading. The structural-electrical correlation enables to further explore new composite configurations and to develop enhanced smart self-sensory systems. The study demonstrates that by merging MPC mixture with textile and fiber reinforcement systems, it is possible to design and construct thin-walled, elements with advanced structural and self-sensing capabilities.
... The use of high strength and electrically conductive continuous carbon filaments, such as in the case of carbon rovings, enables the rovings to simultaneously serve as the main reinforcement system and as the sensory agent. The sensing capabilities of the carbon rovings within concrete structures were demonstrated in the literature by monitoring mechanical loading [6][7][8][9][10], estimating strain [11][12][13][14][15], detecting cracking [13,16] and identifying infiltration of water through cracked zones [17][18][19]. ...
... Those studies demonstrated the potential of the self-sensory carbon rovings for various monitoring applications by connecting the rovings to direct current (DC) electrical circuits by two-probes or four-probes monitoring setups [6,20], by Wheatstone bridge configurations [9,10,[13][14][15][16]21], or by a simple DC circuit [12]. A completely different sensing concept was offered by [17][18][19] that connected one end of each parallel carbon rovings to an AC circuit in order to identify water leakage within cracked zones. ...
... It was assumed that infiltration of water electrically short circuiting between the rovings and as a result change the characterization of the electrical circuit. Measurements using DC electrical circuits, such as reported in [6][7][8][9][10][11][12][13][14][15][16], are restricted to an electrical resistance change (ERC). However, since each sensory carbon roving consists of thousands of conductive filaments that are bundled together, they can be characterized by other electrical properties rather than only by the electrical resistance (ER). ...
The study electrically characterizes a smart-self sensory carbon-based textile reinforced concrete (TRC) structure and explores its sensory capabilities by various electrical properties. The hybrid system is based on implementing electrically conductive carbon rovings, within a textile mesh made of alkali resistant (AR) glass yarns, that simultaneously serve as part of the reinforcement system and as the sensory agent. The study uses an AC based electrical circuit and offers to characterize changes in the electrical properties of the sensory carbon rovings by exploring the electrical response spectrum of the impedance. It is found that, since each carbon roving consists of thousands of electrically conductive filaments that are bundled together, each roving is electrically characterized by a resistor and an inductor that are influenced by the concrete body. The AC based sensory system is experimentally investigated by monitoring changes in the measured electrical properties, that is resistance and inductance, of TRC beams under monotonic loading and correlating these changes to the micro- and macro-structural responses. It is demonstrated that a sensory system that is based on an AC electrical circuit yields additional sensitive and important sensory information on the structural health of the beams.
... The concept of the sensory carbon roving has been demonstrated in the literature for various monitoring applications, such as monitoring the applied load, [10][11][12][13] qualitatively estimating the structural response, [14][15][16] detecting cracks, 17 or water infiltration. [18][19][20] This study aims to take a significant step forward and to develop an SHM technique for evaluating the strain along the element and detecting its structural state. ...
... The sensory TRC beam specimens are investigated by monotonic and cyclic loading tests. The textile configuration as well as the production process of the sensory carbon-based TRC beams are generic and follow Goldfeld et al. 15,17 Sensory carbon-based TRC beam specimens Two identical TRC beam specimens were cast and tested (height: h = 16 mm, length: L = 300 mm, and width: b = 70 mm; see Figure 3). Each beam was reinforced with a single textile layer, positioned 4 mm from the lower face of the beam. ...
... The textile net is comprised of AR-glass rovings as the main reinforcement platform and carbon rovings used also as the sensory agents. Two AR-glass rovings were replaced by two carbon rovings in the longitudinal direction, which was implemented during the production process of the textile (see details in Goldfeld et al. 17 ). The weft direction consists only of AR-glass rovings. ...
The goal of this study is to develop a structural health monitoring methodology for smart self-sensory carbon-based textile reinforced concrete elements. The self-sensory concept is based on measuring the electrical resistance change in the carbon roving reinforcement and by means of an engineering gage factor, correlating the relative electrical resistance change to an integral value of strain along the location of the roving. The concept of the nonlinear engineering gage factor that captures the unique micro-structural mechanism of the roving within the concrete matrix is demonstrated and validated. The estimated value of strain is compared to a theoretical value calculated by assuming a healthy state. The amount of discrepancy between the two strain values makes it possible to indicate and distinguish between the structural states. The study experimentally demonstrates the engineering gage factor concept and the structural health monitoring procedure by mechanically loading two textile reinforced concrete beams, one by a monotonic loading procedure and the other by a cyclic loading procedure. It is presented that the proposed structural health monitoring procedure succeeded in estimating the strain and in clearly distinguishing between the structural states.
... The concept of the smart textile reinforcement has been mainly implemented for structural health monitoring of strain and damage sensing of carbon-based TRC beams under mechanical loading. [6][7][8][9] Goldfeld et al. 10,11 presented the feasibility of the concept on structural TRC beam level and showed that electrical resistance measured from conductive rovings, made of carbon or stainless steel fibers, can follow the structural response. It was also presented by Goldfeld et al. 12 that the smart carbon rovings-based reinforcement can sense the structural response of TRC structures along the entire range of loading process, from very early stages of loading and up to progressive failure mechanism, where traditional sensing devices usually failed to produce meaningful information. ...
... The first electrical setup is by connecting rovings to two Wheatstone bridge configurations. Wheatstone bridge configuration has been implemented successfully by Goldfeld et al. 7,9,11,12 for strain and structural damage sensing of carbon-based TRC elements; therefore, it is advantageous to use the same electrical setup to sense both straining and wetting events. The second electrical setup is based on an AC circuit that, compared to the first electrical setup (based on a DC circuit), has the ability to measure the impedance and its phase. ...
... The opposite out-of-phase connection of the rovings provides the system with a stable sensing signal. This configuration utilizes the electrical scheme proposed by Goldfeld et al. 7,9,12 for monitoring the structural state through strain measurements, providing the TRC element with a multi-functional sensory capacity, both in terms of structural strain and in terms of exposure to water. Electrical short-circuiting two adjacent carbon rovings using an AC setup ...
The study examines the use of hybrid carbon-based textile-reinforced concrete elements with self-sensing capabilities to quantitatively detect wetting events within cracked zones. The self-sensory structural element combines the advantages of AR-glass and carbon-based textile-reinforced concrete for thin-walled structural elements with those stemming from the electrical properties of reinforced carbon rovings. The article investigates the sensitivity of sensory carbon rovings to distinguish between the magnitudes of various wetting events, which is associated with the severity of the cracking, according to two electrical setups (DC and AC circuits). The sensing concept takes advantage of the continuous configuration of the carbon rovings, which enables direct connection of the roving ends to the data acquisition system, and of the manufacturing process that two carbon rovings are placed adjacent to one another. Therefore, it is assumed that wetting events electrically short-circuit the two adjacent rovings. The sensitivity of the two electrical setups is experimentally investigated and performed on a couple of bared carbon rovings and on a cracked textile-reinforced concrete beam. Test results demonstrate the sensitivity of the sensing capabilities of the carbon rovings to detect and distinguish between the magnitudes of the wetting events and consequently the severity of the cracking.
... The technology of textile reinforced cement (TRC) can answer these requirements by constructing 2D and 3D thin-walled concrete structures with self-sensory capabilities, e.g., Refs. [1][2][3][4][5][6][7][8][9]. The self-sensory capabilities are based on utilizing the piezoresistive properties of electrically conductive reinforcement rovings and correlating between the structural and electrical responses. ...
... The contribution of the textile to the internal tensile load is negligible. In the case of PC-based matrices, relatively low ERCs were observed at this state [2,4,6,7]. However, the MPC matrix can carry higher flexural stress, which is expressed by a slightly higher ERC, see Fig. 6 and Table 1. ...
... For example, replacing selected fibers in a glass textile with carbon fibers allows the use of these carbon fibers as sensor material. 193 Changes in the electrical conductivity of carbon fibers can be detected and allow their application as sensors in structural health monitoring, detecting strain and cracking [194][195][196][197] as well as water intrusion. 198,199 Another development for grid-like NCF for construction applications based on multiaxial warp-knitting technology is the development of loop-shaped anchoring points at both sides of the textile done by Rittner et al. 200 The loop-shaped anchoring points reduce anchoring lengths to 100 mm or less, enabling a reduction of overlapping lengths in large structures, where multiple textiles are required, by more than 75%. ...
The use of non-metallic, textile reinforcement structures in place of steel reinforcement is a key component in making concrete constructions more sustainable and durable than they currently are. The reason for this is the corrosion resistance of textile reinforcements, which makes it possible to reduce the thickness of the concrete cover and at the same time extend the service life of concrete structures. This reduces the amount of cement required and thus also the emission of the greenhouse gas carbon dioxide (CO2). By means of textile manufacturing technologies, customized, load-adapted reinforcement topologies can be adjusted to the requirements of highly stressed and well-designed concrete components. The objective of this paper is to give an overview of recent research literature dedicated to textile reinforcement structures that are already used for concrete applications in the construction industry as well as those currently under development. Therefore, textile reinforcement structures, which are divided into one-, two- and three-dimensional topologies, as well as common materials used for textile-reinforced concrete (TRC) are reviewed. Most research has so far been devoted to two-dimensional textile reinforcement structures. Furthermore, novel approaches to the fabrication of textile reinforcement structures for concrete applications based on robotic yarn deposition technologies are addressed.
... This paper presents studies on the suitability and accuracy of detecting structural cracks in brick masonry by exploiting the breakage of ordinary silica optical fibers bonded There is a variety of visual, mechanical, electrical, acoustical, computer vision, global dynamic behavior, and other methods that are used for damage detection in civil engineering structures [12][13][14][15][16][17][18][19][20][21]. Nevertheless, many are susceptible to the adverse effects of moisture, chemical corrosion, electromagnetic interference, and lightning discharges. ...
This paper presents a study on the suitability and accuracy of detecting structural cracks in brick masonry by exploiting the breakage of ordinary silica optical fibers bonded to its surface with an epoxy adhesive. The deformations and cracking of the masonry specimen, and the behavior of pilot optical signals transmitted through the fibers upon loading of the test specimen were observed. For the first time, reliable detection of structural cracks with a given minimum value was achieved, despite the random nature of the ultimate strength of the optical fibers. This was achieved using arrays of several optical fibers placed on the structural element. The detection of such cracks allows the degree of structural danger of buildings affected by earthquake or other destructive phenomena to be determined. The implementation of this technique is simple and cost effective. For this reason, it may have a broad application in permanent damage-detection systems in buildings in seismic zones. It may also find application in automatic systems for the detection of structural damage to the load-bearing elements of land vehicles, aircraft, and ships.
... The high tensile strength of the rovings and their high resistance against corrosion enable the production of thin-walled and structurally strong TRC elements. The use of electrically conductive rovings, such as carbon rovings, that are characterized by high mechanical performance enables the rovings to serve both as the main reinforcement system as well as the sensory agent for detecting mechanical loading Goldfeld et al., 2016aGoldfeld et al., , 2017bWen and Chung, 1999;Wen et al., 2000), strain Quadflieg et al., 2016;Teomete, 2015), cracking (Goldfeld et al., 2017a;Goldfeld and Yosef, 2019); or water infiltration Perry, 2018, 2019;Goldfeld et al., 2016bGoldfeld et al., , 2016c. ...
The study investigates the capabilities of various configurations of self-sensory carbon-based textiles to detect and distinguish between the severity of water infiltration through cracked zones along textile reinforced concrete (TRC) elements. The investigation aims to explore whether an optimal smart textile configuration can improve the structural performance while providing sensitive sensory capabilities. Such an investigation is needed for the development of intelligent TRC structures. Specifically, the study experimentally investigates the effect of two types of bindings and the effect of coating on the mutual structural-sensory performances. The sensory concept is based on changes of the electrical mechanism of two adjacent carbon rovings due to infiltration of water through cracked zones. Eight TRC beam samples were cast and mechanically loaded up to cracking. The cracked zones were monitored, and each zone was separately examined by performing a wetting event. It is demonstrated that the type of binding and coating, which significantly affect the structural response, reflect and affect the measured electrical signal. It is found that there is a tradeoff mechanism between the structural response and the sensory capabilities. While specific textile configuration improves the structural performance, it may reduce its sensory capability to distinguish between the magnitude of water infiltration.
This research addresses the challenging task of monitoring the structural integrity of fiber-reinforced composite (FRC) components under complex loading conditions. Ensuring the safety and functionality of these components is critical but economically challenging. Therefore, this study presents an innovative approach using textile-based strain sensors that are cost-effective and structurally compatible with carbon fiber-reinforced plastic (CFRP) components. The investigation includes the systematic electromechanical characterization and comparison of four different sensor materials at the yarn and composite scale in various test scenarios. Cyclic tensile, compression, and bending tests of CFRP specimens are performed and show good reproducibility of sensor signals within the elastic range, with significant agreement observed with applied strain measurement methods, particularly in tensile tests. Although there are minor deviations in compression and bending evaluations, the signals are still meaningful for in-situ detection of complex loading patterns, crack initiation, and structural failure. The study demonstrates that the integration of textile-based sensor yarns allows for continuous structural health monitoring (SHM) of CFRP components under various loading scenarios, including tensile, bending, and especially compressive loads.
Исследование посвящено возможностям мониторинга технического состояния бетонных трубопроводов, коллекторов, лотков, колодцев и контроля утечек в них посредством использования встроенных датчиков на основе углеродных нитей. Чувствительность электромеханических характеристик углеродных волокон к изменениям параметров внешней среды и механического напряженного состояния позволяет использовать углеродные нити как датчики, чувстви-тельные к повреждениям и утечкам в трубопроводах. Для проведения испытаний были изготовлены образцы изделий из текстильно-армированного бетона (ТАБ) с использованием текстильного полотна из щелочестойких стеклянных и углеродных нитей. Образцы были испытаны на трехточечный изгиб, в результате чего в бетонной матрице образовались трещины, имитирующие повреждения бетонного трубопровода. Затем был проведен мониторинг значений электрического сопротивления армирующих углеродных нитей при локальном смачивании образцов в зоне повреждений. Эксперимент выявил изменения электрического сопротивления при смачивании образцов с относительным изменением до 87% от фо-нового значения, позволяющие судить о наличии утечки в конструкции. Средние значения относительного изменения в течение всего эксперимента составили 48%, 6%, 13% и 16% для разных образцов. При смачивании изделия из ТАБ изме-нения электрического сопротивления отдельной армирующей нити неоднородны, и представляют собой чередование перепадов значения и периодов плавного убывания. Электрическое сопротивление углеродных армирующих ровингов было также предварительно измерено в фоновом режиме на краткосрочном и долгосрочном периодах для оценки наличия и влияния фоновых шумов на результаты испытаний. Было выявлено, что изменения электрического сопротивления армирующих нитей при утечках незначительны на кратковременном промежутке и существенно превышают фоновые шумы на долговременном промежутке. Углеродные нити могут быть использованы в качестве встроенных датчиков мониторинга повреждений и утечек в бетонных трубопроводах.
Preventing damage over time contributes decisively to the sustainability over the lifespan of infrastructures. The development of smart protective measures to detect structural risks early is an urgent need in sustainable construction.
Continuous carbon fibre rovings are suitable for use as reinforcing textile grids in concrete through their high modulus and alkali resistance. Additionally, their electrical conductivity allows for integration as sensors of strain change or leakage in concrete structures such as bridges and pipes. To realize a textile reinforcement for concrete structures various parameters like fibre material and orientation, grid opening, binding type and stitching length, textile dimensioning (2D, 3D) and coating are implicated. These affect not only the mechanical characteristics of the reinforcement but also the sensing properties of the carbon fibre rovings.
This study delivers an overview on the use of carbon fibre based sensors in concrete structures and their characteristic changes as a function of different textile parameters.
In civil engineering, carbon is typically regarded as a modern material to serve as reinforcement in concrete structures. Compared to steel reinforcement, it features two substantial benefits: It is not sensitive to corrosion, and has an enormously increased tensile strength. In contrast, carbon reinforcement is sensitive to lateral pressure and lacks the property of strain hardening. As a first step of establishing carbon reinforced concrete as a new building composite material, carbon reinforcement has basically served to replace the state‐of‐the‐art steel reinforcement. This target led to pioneering findings with respect to determining the material properties of the composite and developing advanced individual components. However, barely substituting steel by carbon does not allow to fully utilise the carbon's benefits while its disadvantageous properties reveal the limits of this approach. Instead, novel design principles are required to meet the material's nature aiming at appropriately using its beneficial properties.
Currently, new construction principles are being researched for high‐performance building material combinations such as textile and carbon reinforced concrete. This paper provides an overview of baselines in the preliminary stages of this research. The overview includes history, inspiration, concrete matrices, non‐metallic reinforcement, structural elements, modelling, production, tomography, and sustainability. The objective of the study is to provide a baseline for the envisaged development of principles for future construction: radically new concepts for the design, modelling, construction, manufacturing, and use of sustainable, resource‐efficient building elements made of mineral building materials with the aim of entirely benefiting from the materials’ potential.
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Structural Health Monitoring (SHM) is the automation of the condition assessment process of an engineered system. When applied to geometrically large components or structures, such as those found in civil and aerospace infrastructure and systems, a critical challenge is in designing the sensing solution that could yield actionable information. This is a difficult task to conduct cost-effectively, because of the large surfaces under consideration and the localized nature of typical defects and damages. There has been significant research efforts in empowering conventional measurement technologies for applications to SHM in order to improve performance of the condition assessment process. Yet, the field implementation of these SHM solutions is still in its infancy, attributable to various economic and technical challenges. The objective of this Roadmap publication is to discuss modern measurement technologies that were developed for SHM purposes, along with their associated challenges and opportunities, and to provide a path to research and development efforts that could yield impactful field applications.
The goal of this study is to characterise the piezoresistive capabilities of self‐sensory carbon roving reinforcements by means of integrative gauge factors (GFs). The correlation between the measured integrative electrical resistance of the carbon rovings and the distributed strain is experimentally investigated by two different textile reinforced concrete beams under monotonic flexural loading. Because the microstructural mechanism of the rovings within the concrete matrix affects the electrical resistance, the GF is a function of the structural health. Two approaches to explore the GF are suggested: the first is by considering the design and damaged states separately and accordingly defining a constant GF for each state, and the second is by considering the entire structural response which leads to a continuous non‐linear correlation. The potential of the two representations of GFs is presented by investigating the two beams. It is demonstrated that similar ranges of GFs are obtained for both beams, which further demonstrates the potential of using the proposed methodology for quantitative monitoring purposes.
The study investigates the sensitivity of hybrid coated and un-coated carbon-based textile reinforced concrete (TRC) beams to detect and distinguish between different and accumulated levels of cracking. The sensing concept is based on using the electrical conductivity of embedded carbon rovings reinforcement as the monitoring system. The focus is on the influence of coating on the structural-electrical response and its capabilities to detect the severity of cracking. Two TRC beams reinforced with uncoated and coated AR-glass/carbon-based textile were investigated under cyclic mechanical loading. The study demonstrates that the micro and macro structural mechanisms of both beams are reflected in the electrical signal and can be quantitatively correlated to the level of damage. The sensing capabilities of the textile yields stable, repeatable, sensitive and consistent electrical readings for monitoring needs. However, in case of coated textile, while coating improves the structural performance, it limits the sensing capabilities in distinguishing macro-cracking scenarios.
The study investigates the electrical mechanism of hybrid carbon-based
textile-reinforced concrete (TRC) elements with self-sensing capabilities to detect
infiltration of water within cracked zones. The concept is based on carbon rovings
that simultaneously serve as the reinforcement system as well as the sensory agent.
The main goal of the study is to characterize the mechanism of the electrical signal
obtained by exposure carbon rovings to wetting events. To meet this goal, the study
uses alternating current circuits, which yield, additionally to the resistance or
voltage changes, the characterization of the capacitance and inductance of the
system. Two sensing concepts are investigated. Both concepts take advantage of the
continuous configuration of the carbon rovings, which enables direct connection of
the roving ends to the data acquisition system. The first sensing concept assumes
that the electrical properties of a single carbon roving is affected by wetting,
while the second assumes that wetting the interface between two adjacent carbon
rovings links them electrically. The experimental investigation is performed on bare
carbon rovings, and on a cracked carbon based TRC beam. Test results characterize
the electrical mechanism of the wetting events and reveal its potential use as a
basis for smart textile-reinforced systems with integrated monitoring
functions.
This article investigates the feasibility of intelligent textile-reinforced concrete structural elements with sensing capabilities. The concept is based on dual use of glass and carbon fiber textiles as reinforcement and, at the same time, as a sensory agent. Experimental investigation demonstrates the feasibility of the concept in two applications: detecting strains in a mechanically loaded textile-reinforced concrete beam and monitoring the interaction of the structural element with a wet environment. By detecting the changes to the integrative electrical resistance of the carbon tow, the ability of the textile to sense strain and exposure to water is demonstrated. For strain sensing, the hybrid reinforcing textile provides electro-mechanical sensing with a gauge factor of the order of 1 and a detectable correlation with the load, strain, and displacement responses. For the detection of wetting, the implementation of the carbon tow in a Wheatstone bridge detects fractional resistance changes in the order of 10⁻⁵, a figure that is effectively detected by monitoring the voltage across the bridge. The response to wetting, which is conditioned by the cracking of the beam and the exposure to ionic conductive solutions, provides a mean to monitor the functionality and the structural health of the textile-reinforced concrete beam.
Piezoresitivity describes the change in electrical resistance of a conductor due to an applied strain. Previous studies showed that certain ex-PAN carbon fibres exhibit an excellent linear piezoresistive behaviour. Besides the desired strain sensitivity, k l , in the longitudinal direction, carbon fibres show a strain sensitivity, k t , transverse to the fibre direction. An experimental method was developed to empirically characterize these electromechanical material properties. The strain sensitivities k l and k t of the investigated ex-PAN fibre (Torayca T300B) were determined by varying the ratio between longitudinal strain, ε l , and transverse strain, ε t , in uniaxial tensile tests and four-point bending tests. The results show a longitudinal strain sensitivity k l in the range of 1.72–1.78 and a transverse strain sensitivity k t in the range of 0.37–0.41.
This significant transverse strain sensitivity must be considered in the case of strain measurements with carbon fibre sensors. An approach for dealing with the high transverse strain sensitivity of carbon fibre sensors is proposed, which is appropriate for orthotropic as well as isotropic materials. The approach is based on transfer functions, which includes both strain sensitivities of the carbon fibre k l and k t . The transfer function of a biaxial carbon fibre sensors element with orthogonally aligned carbon fibre sensors is given and experimentally checked for validity. Furthermore, the transfer function of a right-angled triangle carbon fibre sensor element is given. These triangle carbon fibre sensor element allows the determination of the complete state of strain of a structure including the shear strain.
This paper presents a new digital image analysis method for quantitative and online measurement of filament ruptures of a multi-filament AR-glass yarn embedded in concrete during pullout loading. The proposed method was developed based on an existing test method for determination of filament ruptures occurring during the loading called failure investigation using light transmitting (FILT) property test, which uses light transmitting property of AR-glass fibers. Artificial light is exposed on the glass filaments from one side of the specimen. On the opposite side, a charge-coupled device (CCD) camera with an optical microscope records the lighted filament cross-sections in the yarn. To detect filament ruptures during pullout loading, the light intensity time history of every individual filament of the yarn was investigated by a digital image analysis method. The number of broken filaments was also investigated by acoustic emission (AE) analysis simultaneously and the results were compared. Test results showed that the light transmitting property of AR-glass can be used to identify filament ruptures and it is possible to determine the failure of the filaments during pullout in the cross-section quantitatively by the improved FILT test.
The nano-carbon black (NCB) and carbon fiber (CF) as electric conductive materials were added into the concrete. The effect of the NCB and CF on the mechanical properties and on the fractional change in resistance (FCR) of concrete was investigated. The relationships among the FCR, the strain of initial geometrical neutral axis (IGNA) and the beam damage degree were developed. The results showed that the relationship between the FCR and IGNA strain can be described by the First Order Exponential Decay function, and the internal damage of concrete beam was reflected by the relationship between damage degree and resistance.
Self-sensing refers to the structural material sensing itself. Real-time self-sensing of damage in carbon fibre polymer-matrix composites by electrical resistance measurement is reviewed. The resistance changes irreversibly upon damage, as shown for damage inflicted by flexure, tension, fatigue, and impact. Delamination increases the through-thickness resistance. Fibre breakage increases the longitudinal resistance. The oblique resistance, as measured at an angle between the longitudinal and through-thickness directions, is particularly sensitive. Minor flexural damage causes the oblique resistance in the unloaded state to decrease. Current spread- ing enables the sensing of localized damage by measurement away from the damage, though it reduces the spatial resolution of the sensing. The resistance method is more sensitive than the potential method. Two-dimensional sensing is complicated by the anisotropic spreading of the current. Thermal damage and through-thickness (fastening) compression effect are indi- cated by the contact resistivity of the interlaminar interface. The through-thickness compression effect is alternately indicated by the longitudinal volume resistivity. The condition of a composite fastening joint is indicated by the contact resistivity of the joint interface.
Damage monitoring of the civil infrastructure is critically needed. This article provides the first report of impact damage self-sensing in cement-based materials. Cement mortar reinforced with short (5—7 mm) carbon fiber and in bulk or coating (5—10 mm thick) form is effective for sensing its own impact damage through DC/AC electrical resistance measurement, provided that the region of resistance measurement contains the point of impact. The mortar resistivity needs to be 10⁴—10⁵ Ω cm, as provided by pitch-based fiber (15 µm diameter, unsized) at 0.5% or 1.0% by mass of cement, or type A PAN-based fiber (7 µm diameter, desized) at 0.5%. Due to the low mortar resistivity of 10³ Ω cm, pitch-based fiber at 1.5% and type B PAN-based fiber (7 µm diameter, unsized) at 0.5% are less effective. Without fiber, there is no sensing ability. The surface resistance of the surface receiving the impact is an effective indicator of the damage, even for minor damage without cracking, inflicted by impact at 880 J. The oblique or longitudinal volume resistance is much less effective. The surface resistance increases abruptly upon impact, but it decreases abruptly upon impact after 5—40 impacts (number decreasing with increasing impact energy) have been inflicted.
BOTDR is one of the strain measurement technologies that is suitable for smart monitoring of civil engineering infrastructures. While the technology has the advantage of supplying spatially distributed data, it is currently limited to a spatial resolution of about 1m. This infers that the technology may lack the ability to identify the exact type and source of damage; that is, different geometrical configurations of cracking within a concrete beam may lead to similar BOTDR readings, and hence the exact nature of cracking might not be resolved by the BOTDR. This study suggests different crack indicators, and examines, both analytically and experimentally, their correlation with BOTDR readings of damaged reinforced concrete beams. The analytical part entails statistical analysis of hundreds of cracking cases in fractured reinforced concrete beams and their effect on the simulated BOTDR readings. The analysis is conducted within COMSOL-Multiphysics, and is aimed to understand the correlation between different crack indicators and the beam curvature as would be obtained by the BOTDR. The experimental part consists of a controlled load test of a reinforced beam instrumented by BOTDR fibers, and is aimed to support the analytical findings.
Atomic force microscopy and scanning electron microscopy techniques have been used to examine the surface deformation experienced by high density polyethylene during the scratch test. The scratch deformation process involves stretching of fibrils and microfibrils resulting in the formation of surface openings. At the molecular level the chains of molecules unfold and align in the direction of the moving indenter. In the scratch test, the scratch velocity may suggest that low strain rates are valid, but the local strain rates can be many orders of magnitiude higher as exemplified by atomic force microscopy. A number of modes of deformation are encountered during scratching. They include deformation bands, crazing, tearing, microcracking, regular cracking, and grooving. Crazing-tearing is the predominant mode of scratch deformation. It is envisaged that the sequence of tearing along the craze involves formation of deformation bands, development of craze, followed by tearing. Atomic force and scanning electron microscopy of scratch surface damage indicated that the nature and modes of scratch deformation are qualitatively similar to the case of uniaxial tensile deformation, implying similarities in the deformation behaviour between scratch and tensile deformation.
Piezoresistivity was observed in continuous unidirectional carbonfiber cement-matrix and polymer-matrix composites. The fiber volumefraction was 2.6–7.4% and 58% for cement-matrix andpolymer-matrix composites respectively. The DC electrical resistancein the fiber direction increased upon tension in the fiber directionfor the cement-matrix composite, due to fiber-matrix interfacedegradation, but decreased upon tension for the polymer-matrixcomposite due to increase in the degree of fiber alignment.
A prospective method for structural health monitoring of engineering materials and structures is based on embedded strain sensors in the form of electrically conductive carbon rovings. This article presents the results of the application of carbon rovings and the development of flexible textile fabrics based on these rovings for measuring the deformation in engineering materials, including concrete and polymer- and cement-based composites. The possibility of using carbon rovings as a strain sensor is demonstrated via measurements in tensile and four-point bending tests. The experimental setups and methods for measuring the electrical resistance of carbon roving as a function of strain in the roving, concrete, and composites are described. A good correlation has been found between the electrical resistance–strain curve of the carbon roving (used as a calibration curve) and the measurements in the concrete and polymer composites from tensile tests. The difference in the character of the flexural behavior and the electrical signal in the carbon roving cement-based composite, affected by the stitch type and shape of the carbon roving cross section in textile fabric, was found through four-point bending tests.
Five different carbon fiber reinforced cement matrix composites with fiber length of 13 mm were designed having different fiber volume fractions. From each mix, six samples of 5 cm cubes and three 4 × 4 × 16 cm prism samples were cast. Compression, split tensile and notched bending tests were applied to three samples from each mix with simultaneous electrical resistance measurements. Strong correlations between strain and electrical resistance change were reported. At percolation threshold, the strain caused shift of the system from post percolation to pre-percolation, changing the electrical resistance dramatically; causing highest gage factor. The microstructure controlled mechanisms were enlightened. The split tensile test and notched bending test with simultaneous electrical resistance, strain and crack length measurements have been applied as novel methods.
High-performance textiles are now widely used in civil engineering applications. This work explores the differences between the tensile properties of weft inlay warp-knitted fabrics of high-strength glass and carbon rovings as concrete reinforcements. In this study, 12 types of warp-knitted fabrics with different stitch patterns, including tricot, cord, and pillar stitches, were produced. The effect of the stitch type on the tensile properties of the fabrics was examined. The stitch type was found to significantly affect the tensile properties of the warp-knitted fabrics. The results showed that the tensile strength of fabrics with tricot and cord stitches is greater than that of fabrics with the pillar stitch. The increase in tensile strength was 14% for fabrics made of glass roving and 21% for fabrics made of carbon rovings. A similar gradation was observed for the Young’s modulus of the fabrics. The Young’s modulus was 11% and 25% higher for glass and carbon fabrics, respectively. The structural parameters of the warp-knitted fabrics, including the geometry of the stitch pattern and the yarn cross-sectional shape in a fabric that affect the tensile properties, were analyzed.
Technical textiles with embedded distributed fiber optic sensors have been developed for the purposes of structural health monitoring in geotechnical and civil engineering. The distributed fiber optic sensors are based on Brillouin scattering in silica optical fibers and OTDR in polymer optical fibers. Such “smart” technical textiles are used for reinforcement of geotechnical and masonry structures. The embedded fiber optic sensors provide online information about the condition of the structure and about the occurrence and location of any damage or degradation.
An optical-fiber sensor system for structural health monitoring of concrete elements such as beams and columns is
presented. The system employs arrays of conventional optical fibers embedded in the concrete elements as crack sensors.
Twelve types of optical fibers as well as several embedding techniques have been tested for this role. The survival rate of
optical fibers embedded in concrete could be as high as 80%. The loss of fibers during the embedding process was
acceptable provided that the number of fibers in the array had redundancy. The optical transmission of all fibers in the
array was monitored in a time-division multiplexed mode at a high repetition rate, in the kHz range. The monitoring
scheme allowed a quasi-continuous data acquisitions of large optical fiber arrays. A sharp decrease in the optical
transmission of one or more optical fibers was a clear indicator of the development of cracking in the element subjected
to flexural loads. The system was successful in detecting not only the initiation but also the propagation of cracks in
concrete elements subjected to incremental flexural loading. In this work, the relation between the mechanical properties
of the optical fibers and their behavior for the described application is discussed. Also, considerations towards a rational
design of the system are proposed. The damage detection system may be used for the mapping and monitoring of cracks
in concrete elements. The simplicity of the operation and relatively low cost of the proposed system make it a great
candidate for applications in structural health monitoring of critical elements in civil infrastructure.
This study aims to investigate the mechanical behavior of FRCM composite-strengthened concrete beams using embedded FBG sensors. FBG sensors were installed both on the tensioned surface of the concrete beam and on the PBO mesh woven, that had been applied using cementitious mortar without any epoxy resin. Conventional strain gauges were used to compare results measured from the FBG sensors. Under three-point bending, a marked difference between strains measured in the concrete and those gotten on the reinforcement net was observed. A theoretical model is presented to explain the observed discrepancy.
The influence of temperature, activation energy, and conduction on the electrical properties of carbon fiber reinforced cement mortars was investigated. Mortar specimens were made with CEM I Portland cement and a siliceous sand of maximum particle size 2 mm. Electrical measurements were taken using an Agilent 6423 LCR meter at three spot frequencies and specimens were equilibrated in a water-bath at the appropriate temperature prior to measurement, which covered the range 10-60°C. It was found that the activation energy of the carbon fiber specimens were lower than that of the plain mortar with the activation energy decreasing with increasing fiber length and increasing frequency. The volumes of fiber used within the experimental program lies within the percolation zone where a continuous fiber network can form. The results show that carbon fibers introduce dispersion to the bulk conductivity, which is not present in the plain mortar samples.
Carbon Fiber Reinforced Plastic (CFRP) is composed of electric conductive carbon fibers and electric insulator resin. Self-monitoring system has been reported utilizing electric resistance change of unidirectional CFRP due to fiber breakages and to applied strain. Piezoresistivity is electric resistance change with applied strain. Many researchers have already reported the piezoresistivity of unidirectional CFRP. There is, however, large discrepancy in the measured piezoresistivity even in the fiber direction during tensile loading: both positive piezoresistivity (electric resistance increase) and negative piezoresistivity (electric resistance decrease) are reported during tensile tests. Electric resistance change at electrodes due to poor electric contacts are reported to be a main cause of this large discrepancy. In the present study, therefore, basic properties of piezoresistivity were measured with specimens made from single-ply and multi-ply laminates using a four-prove method. Many cases of electric resistance changes in the fiber direction transverse direction were measured during tensile loading. Effect of shear loading was also investigated using a shear test. To investigate the effect of poor electric contact at the electrodes, electrodes were made without polishing specimen surface and a tensile test was performed with measuring piezoresistivity. After the test, the specimen surface was polished,. and a tensile test was performed again using the identical specimen. As a result, positive piezoresistivity was obtained for both single-ply and multi-ply specimens and negative piezoresistivity is confirmed that it was caused by the poor electric contact at electrodes.
The incorporation of a small volume of carbon fibers into a concrete mixture produces a strong and durable concrete and at the same time lends the product a smart material property. This intrinsic capability can be tapped by using simple electrical resistance techniques. There is the potential for these techniques to be used as nondestructive testing methods to assess the integrity of the composite. The results of some fundamental investigations on the bulk electrical properties of carbon fiber cement composites under compressive loading are presented. Well-defined patterns are exhibited in the electrical resistance behavior which can be correlated with the stress-strain behavior. The resistance behavior was evaluated for various fiber volume contents for both mature and early-age specimens. The response under cyclic compressive loading was studied. The effect of taking measurements both parallel and perpendicular to the axis of loading was also investigated.
In this paper, the development of carbon fiber-based piezoresistive linear sensing technique and its application in civil engineering structures is studied and summarized. The sensing mechanism is based on the electrical conductivity and piezoresistivity of different types of carbon fibers. Firstly, the influences of values of signal currents and temperature on the sensing properties are studied to decide the suitable sensing current. Then, the linear temperature and strain sensing feasibility of different types of carbon fibers is addressed and discussed. Finally, the application of this kind of sensors is studied in monitoring the health of reinforced concrete (RC) and prestressed concrete (PC) structures. A good linearity of fractional change in electrical resistance (ER) (DeltaR/R0)-strain and &DeltaR/R0-temperature is demonstrated. The &DeltaR/R0-strain and &DeltaR/R0-temperature curves of CFRP/HCFRP sensors can be well fitted with a line with a correlation coefficient larger than 0.978. All these reveal that carbon fibers reinforced polymer (CFRP) can be used as both piezoresistive linear strain and temperature sensors.
Damage of carbon fiber-reinforced concrete was revealed by an increase in the DC electrical resistance in the stress direction during dynamic compression. Minor damage that did not involve a change in modulus resulted in a partially reversible increase in resistance during loading at a stress above that in prior loading. Major damage that involved a decrease in modulus resulted in resistance increase occurring in every loading cycle irrespective of prior loading and in an irreversible increase in the baseline resistance.
Unidirectional continuous carbon fibre reinforced epoxy was found to be able to sense its own strain in the fibre direction, due to its longitudinal electrical resistance decreasing reversibly and its transverse resistance increasing reversibly upon longitudinal tension. The strain sensitivity (gauge factor) is from -35.7 to -37.6 and from +34.2 to +48.7 for the longitudinal and transverse resistances respectively. Both effects originate from resistivity changes associated with the increase in the degree of fibre alignment upon longitudinal tension. Either effect allows strain sensing. Slight irreversibility is associated with the resistance decreasing after the first strain cycle and stems from the decrease in the degree of neatness of the fibre arrangement.
Concrete containing 0.2-0.4 vol.% short carbon fibers was found to exhibit volume electrical resistivity of 103-105Omega cm and contact resistivity (between the cured concrete and stainless steel) of 103-106Omega cm at zero contact pressure. Increasing the contact pressure from 0 up to 0.05 MPa was sufficient to lower the contact resistivity to a minimum value. Increasing the fiber content to >0.4 vol.% did not decrease the contact resistivity, but decreased the volume resistivity. The values of the volume and contact resistivities depended on the non-fiber additives (i.e., latex, methylcellulose and silica fume) needed for fiber dispersion. Using latex gave a higher volume resistivity (1 × 105Omega cm) and a lower contact resistivity (5 × 103Omega cm2) than methylcellulose and silica fume; the high volume resistivity was due to the large proportion of latex used; the low contact resistivity was due to the lack of adherent on the surface of fibers protruding from the concrete containing latex.
This paper presents the results of an experimental investigation into the strength, deformation, and fracture behaviour of textile-reinforced concrete (TRC) subjected both to low and high-rate tensile loading ranging from 0.0001 to 50 s−1. High strain rates were achieved using a high-rate servo-hydraulic testing machine. The effect of the addition of short fibres on the static and dynamic response of TRC has been investigated, and the microstructure of both composite and fibre was observed after the tests using an ESEM. An increase in tensile strength, strain capacity, and work-to-fracture was observed for strain rates up to 0.1 s−1 with increasing strain rate. The addition of short glass fibres increased the tensile strength and first crack strength of the TRC. For high-speed tests (rates above 5 s−1) an increase in the tensile strength, first crack strength and work-to-fracture was also observed, but at the same time there was a decrease in the strain capacity. The tests at high loading rates showed a pronounced effect of the specimen length on the measured mechanical properties: with increasing gauge length the tensile strength and strain capacity decreased, while the work-to-fracture increased.Research highlights▶ Tensile strength, strain capacity, and work-to-fracture increased when . ▶ When a decrease in the strain capacity was observed. ▶ Different fibre-fracture morphologies were observed for high and low strain rates. ▶ The addition of short fibres increased the tensile strength and the crack stress.
Damage self-sensing (to be distinguished from strain self-sensing) by electrical resistance measurement is effective in carbon fiber rein-forced cement below the percolation threshold, as shown under uniaxial compression. Major damage that is accompanied by irreversible strain is indicated by irreversible resistivity increase ranging from 10% to 30%. Minor damage in the elastic regime is indicated by this increase ranging from 1% to 7%. The irreversible resistivity fractional change per unit irreversible strain is higher in the transverse direc-tion than the longitudinal direction. The origin of the damage self-sensing ability is attributed to the fracture of fibers that bridge micro-cracks and the consequent resistivity increase. The fracture of a bridging fiber occurs upon microcrack opening or shear.
The current status of optical fiber sensors is reviewed. The optical fiber sensors have certain advantages that include immunity to electromagnetic interference, lightweight, small size, high sensitivity, large bandwidth, and ease in implementing multiplexed or distributed sensors. Strain, temperature and pressure are the most widely studied measurands and the fiber grating sensor represents the most widely studied technology for optical fiber sensors. Fiber-optic gyroscopes and fiber-optic current sensors are good examples of rather mature and commercialized optical fiber sensor technologies. In this paper, among the various fiber-optic sensor technologies, especially, technologies such as fiber grating sensors, fiber-optic gyroscopes, and fiber-optic current sensors are discussed with emphasis on the principles and current status. Today, some success has been found in the commercialization of optical fiber sensors. However, in various fields they still suffer from competition with other mature sensor technologies. However, new ideas are being continuously developed and tested not only for the traditional measurands but also for new applications. 2003 Elsevier Science (USA). All rights reserved.
In this work we review the structural health monitoring techniques based on fiber Bragg gratings. The working principle of
the fiber Bragg gratings sensors and the most common techniques to inscribe and interrogate these sensors are described. Several
implemented examples are also presented, like the deformation monitoring of one historical building with reduced visual impact,
the unidirectional acceleration measurements in a metallic bridge structure and the bidirectional acceleration monitoring
in a 50 m mobile telecom tower. Finally, the implementation of an automated remote structural health monitoring system design
to operate with optical sensors in a highway bridge is described.
The obtained results prove the applicability of optical fiber sensors, namely fiber Bragg gratings for structural health monitoring.
The collaborative research center "Textile Reinforced Concrete (TRC) - Development of a New Technology" (SFB 532) established at Aachen University (RWTH Aachen) is investigating the basic mechanisms of this new composite material. The use of technical textiles as reinforcement material in cementitious binder systems allows the production of thin-structured elements as will be dimensioned, modelled, and produced within the research project. For this reason the material properties of the single components have to be known and will be integrated in analytical and numerical simulations of textile reinforced structures. Thus key parameters on the meso-level are introduced. These are on the one hand the tensile strength and elastic modulus of filaments and rovings, on the other hand mechanical and fracture mechanical parameters of the matrix, and finally the bonding characteristics of filaments as well as rovings embedded in the cement based matrix.
Health monitoring of concrete structures is performed by assessing the structure’s state of stress. One such method involves
monitoring electrical resistance variations as an indirect measure of stress variations. Carbon fibers were added to fresh
geopolymer mix to enhance its electrical conductivity. AC-impedance spectroscopy analyses were performed on sample specimens
to obtain their electrical resistance. Geopolymer concrete specimens entrained with carbon fibers were dynamically loaded
in bending and uniaxial compression to observe changes in electrical resistance with respect to variations in their stress
state. For beam specimens electrical resistance was found to follow a descending trend with increasing bending stresses. A
more complex relationship was noted for cylinder specimens that were loaded axially. Overall experimental results suggest
that conductive geopolymer could serve as a smart material in health monitoring applications of concrete structures.
KeywordsGeopolymer concrete–Electrical conductivity–Carbon fibers–Health monitoring–AC-impedance spectroscopy
Carbon/glass fiber hybrid textile reinforced concrete is a relatively new composite material with good mechanical capacity
and excellent electrical conductivity. Both small-scale slab heating experiments and numerical simulation are presented in
this paper. Temperature variation curves obtained during heating indicate the effects of environmental temperature, heat-conducting
layer thickness and electric heating power. Comparison of temperature rising between the situations with and without thermal
isolation layer is given as well. The results indicate that the textile can form a good conductive heating network and generate
enough heat to raise the temperature in the concrete when connected to a power supply, while the resistance of the slab remains
stable during the heating. Numerical results are in good accordance with the experiments. Real time snow-melting experiment
was conducted to verify the feasibility of deicing. The electrothermal properties of textile can be utilized for deicing and
snow melting in a safe, environmentally friendly and efficient way.
Keywordscarbon/glass fiber hybrid textile reinforced concrete–numerical simulation–electrothermal properties–deicing and snow melting
In-service structural health monitoring (SHM) of engineering structures has assumed a significant role in assessing their safety and integrity. Fibre Bragg grating (FBG) sensors have emerged as a reliable, in situ, non-destructive tool for monitoring, diagnostics and control in civil structures. The versatility of FBG sensors represents a key advantage over other technologies in the structural sensing field. In this article, the recent research and development activities in structural health monitoring using FBG sensors have been critically reviewed, highlighting the areas where further work is needed. A few packaging schemes for FBG strain sensors are also discussed. Finally a few limitations and market barriers associated with the use of these sensors have been addressed.
Measurements have been made of the effect of mechanical strain on potential distributions and resistance of unidirectional and multidirectional carbon fibre epoxy laminates. The effects of current flow direction and technique for current introduction on piezo-resistance have been studied. It was found that uniform current introduction at sample edges produced by sputtered Au–Cr contacts across the entire cross-section produced consistently low values of gauge factor of 1.75 for current flow parallel to the fibres and 2.7 for transverse current flow. Non-uniform current introduction, produced variously by local point introduction of current, or use of viscous adhesives producing intermittent contact, resulted in a wide range of apparent gauge factors ranging from 20.6 to −89. These anomalous values may be explained by a model in which the high anisotropy of resistance in unidirectional CFRP maintains initial non-uniform current throughout the sample. Under mechanical strain points of fibre contact will change, altering the distribution of current carrying fibres and leading to local changes in current. Thus changes in potential difference between two points produced by mechanical strain will not be exclusively caused by changes in local resistance. The presence of transverse plies in multidirectional laminates ensures that in plane non-uniform current distributions are largely eliminated, and the effect on piezo-resistance of non-uniform current introduction is minimised.
The self-sensing of flexural strain and damage has been demonstrated in carbon fiber polymer-matrix composite by measuring the DC electrical resistance. Upon strain in the elastic regime, the compression surface resistance decreases reversibly (due to increase in the current penetration), while the tension surface resistance increases reversibly (due to decrease in the current penetration), and the oblique resistance increases reversibly. Upon minor damage, (i) the oblique resistance after unloading decreases, (ii) the oblique resistance decreases during load increase near the start of loading, and (iii) the curve of the oblique resistance or the resistance of the tension or compression surface vs. deflection becomes nonlinear. Upon major damage, all resistances abruptly and irreversibly increase, such that the onset occurs earlier for the compression surface resistance and the oblique resistance than the tension surface resistance. The surface resistances are superior indicators of strain, whereas the oblique resistance is a superior indicator of damage.
Carbon fiber structural composites, in the form of short-fiber silica fume cement–matrix composite and crossply continuous-fiber polymer–matrix composite, were found to be thermistors due to the decrease in electrical resistivity with increasing temperature. The resistivity is the volume resistivity for the cement–matrix composite and the contact resistivity between crossply laminae for the polymer–matrix composite. The activation energy of electrical conduction is up to 0.41 and 0.12 eV for cement–matrix and polymer–matrix composites, respectively. For the polymer–matrix composite, each junction between crossply fiber groups of adjacent laminae is a thermistor, while the fiber groups serve as electrical leads.
Piezoresistivity was observed in cement-matrix composites with 2.6–7.4 vol% unidirectional continuous carbon fibers. The direct-current electrical resistance in the fiber direction increased upon tensile loading in the same direction, such that the effect was mostly reversible when the stress was below that for the tensile modulus to decrease. The gage factor was up to 60. The resistance increase was due to fiber-matrix interface degradation, which was mostly reversible. Above the stress at which the modulus started to decrease, the resistance increased with stress/strain abruptly, due to fiber breakage. The tensile strength and modulus of the composites were 88% and 84%, respectively, of the calculated values based on the rule of mixtures.
Changes of microstructure and properties in the interfacial zone of glass fibre reinforced cement (GRC) under the effect of ageing were investigated. A novel technique based on a microindentation apparatus was developed and successfully used to carry out microstrength testing in the interfacial area and push-in tests on selected individual fibres within a strand. By continuously monitoring load vs. displacement, the new technique allowed the microstrength to be measured in small, porous areas of the fibre-matrix interfacial zone, and particularly within the glass fibre strand/bundle. The results showed that the embrittlement of aged GRC was closely associated with a substantial increase of the microstrength values within the fibre bundle during the ageing process. It was also revealed that a wide range of bond properties existed within the fibre strand. The resistance to fibre sliding was much greater at the outer filaments than at the inner central filaments of the fibre strand/ bundle.
This paper presents an overview of current research and development in the field of structural health monitoring with civil engineering applications. Specifically, this paper reviews fiber optical sensor health monitoring in various key civil structures, including buildings, piles, bridges, pipelines, tunnels, and dams. Three commonly used fiber optic sensors (FOSs) are briefly described. Finally, existing problems and promising research efforts in packaging and implementing FOSs in civil structural health monitoring are discussed.
This paper introduces a fiber modification technique for enhancing the bond characteristics of AR-glass strands embedded in cement paste. AR-glass strands were modified by absorption of four different types of slurries containing sub-micron organic and inorganic particles. Two different sample preparation processes were applied prior to incorporation the modified strands in the cement paste. Bond characteristics of unmodified and modified AR-glass strands embedded in cement paste were determined by pull-out test and the failure mechanism of pulling out strands was determined by Failure Investigation by Light Transmission (FILT) test. Test results showed that the bond properties and failure type of AR-glass strands can be enhanced by the microstructure modification technique. The efficiency of the modification was highly dependent on filler type, structure and also the production method.