Health Monitoring Of Concrete Structures Research Articles

Intrinsic Sensing Properties of Chrysotile Fiber Reinforced Piezoelectric Cement-Based Composites.

Lead-zirconate-titanate (PZT) nanoscale powder was first synthesized by the sol-gel method, then PZT and 0–3 type PZT/chrysotile fiber (CSF)/cement composite (PZTCC) wafers were fabricated after grind-mixing PZT powder with strontium carbonate and/or cement, ductile CSF in tandem with press-sintered process, respectively. The crystal structure (XRD), microstructure (SEM), piezoelectric properties after surface silver penetration, and polarization of the PZT and PZTCC wafer were investigated. Furthermore, self-sensing responses under either impulse or cyclic loading and micro-hardness toughness of PZTCC were also investigated. Results show that the incorporation of CSF and cement admixture weakens the perovskite crystalline peak of PZTCC; reduces the corresponding piezoelectric coefficient from 119.2 pC/N to 32.5 pC/N; but effectively bridges the gap on the toughness between PZTCC and concrete since the corresponding microhardness with 202.7 MPa of PZTCC is close to that of concrete. A good linear and fast electrical response against either impulse or cyclic loading of the PZTCC is achieved with their respective sensitivity, linearity, and repeatability to 1.505 mV/N, 2.42%, and 2.11%. The sensing responses and toughness of PZTCC is encouraging as an intrinsic piezoelectric sensor for real-time health monitoring of ductile concrete structures.

Strain monitoring of concrete components using embedded carbon nanofibers/epoxy sensors

In this study, embedded strain sensors based on the principle of piezoresistivity were fabricated by epoxy-based composites filled with different contents of carbon nanofibers (CNFs). The piezoresistive performances and relevant parameters including gauge factor, linearity, repeatability and hysteresis of these sensors were investigated. A compensation circuit was proposed to eliminate the influence of temperature on sensing signals of the sensors. The CNFs/epoxy sensors were embedded into concrete cylinders to monitor their compressive strains under monotonic and cyclic loadings, thereby assessing practical applications of the CNFs/epoxy sensors as strain sensors for monitoring concrete structures. The results indicate that the sensors containing 0.58 vol% of CNFs, which have a gauge factor of 37.1, a linearity of 5.5%, a repeatability of 3.8% and a hysteresis of 6.3%, exhibited better piezoresistive performance compared to those containing 0.29 vol% of CNFs. The calibration and monitoring curves exhibited a consistent variation trend when the cylinders embedded with sensors were subjected to monotonic and cyclic loadings. This demonstrates that the CNFs/epoxy sensors have considerable potential to be used as embedded strain sensors for structural health monitoring of concrete structures.

STRUCTURAL HEALTH MONITORING OF CORROSION POTENTIAL IN CONCRETE BRIDGE DECKS

Structural Health Monitoring (SHM) of concrete structures during construction, as well as over its service life, has recently become more attractive to owners and consulting engineers. With the introduction of new materials and construction methods, various types of concrete structures are being instrumented with monitoring devices to determine their performance and response to various loading conditions. Among many other objectives, this includes monitoring concrete performance at the serviceability and durability limit states. This paper is an overview of an on-going program for the SHM of concrete bridge decks in the State of New Jersey focusing on field performance. Three types of corrosion sensors are instrumented to monitor the corrosion activities in concrete decks; one is the silver-silver chloride electrode and the other two are multi element probe (MEP) corrosion sensors. Other types of MEPs were also instrumented on bridge decks during reconstruction in late 1990s to monitor the corrosion potential of the bridge decks. Various types of sensors are installed in precast panels during fabrication as well as in-situ cast concrete decks during and after construction. Moreover, a laboratory-based accelerated corrosion testing program is also performed on concrete specimens using various types of rebars. This ongoing study is aimed at correlating laboratory-accelerated corrosion results with long-term performance of the steel in concrete bridge decks under field conditions.

Influence of Axial Load on Electromechanical Impedance (EMI) of Embedded Piezoceramic Transducers in Steel Fiber Concrete.

With the advantages of high tensile, bending, and shear strength, steel fiber concrete structures have been widely used in civil engineering. The health monitoring of concrete structures, including steel fiber concrete structures, receives increasing attention, and the Electromechanical Impedance (EMI)-based method is commonly used. Structures are often subject to changing axial load and ignoring the effect of axial forces may introduce error to Structural Health Monitoring (SHM), including the EMI-based method. However, many of the concrete structure monitoring algorithms do not consider the effects of axial loading. To investigate the influence of axial load on the EMI of a steel fiber concrete structure, concrete specimens with different steel fiber content (0, 30, 60, 90, 120) (kg/m3) were casted and the Lead Zirconate Titanate (PZT)-based Smart Aggregate (SA) was used as the EMI sensor. During tests, the step-by-step loading procedure was applied on different steel fiber content specimens, and the electromechanical impedance values were measured. The Normalized root-mean-square deviation Index (NI) was developed to analyze the EMI information and evaluate the test results. The results show that the normalized root-mean-square deviation index increases with the increase of the axial load, which clearly demonstrates the influence of axial load on the EMI values for steel fiber concrete and this influence should be considered during a monitoring or damage detection procedure if the axial load changes. In addition, testing results clearly reveal that the steel fiber content, often at low mass and volume percentage, has no obvious influence on the PZT’s EMI values. Furthermore, experiments to test the repeatability of the proposed method were conducted. The repeating test results show that the EMI-based indices are repeatable and there is a great linearity between the NI and the applied loading.

A Feasibility Study on Timber Damage Detection Using Piezoceramic-Transducer-Enabled Active Sensing.

In recent years, piezoelectric-based transducers and technologies have made significant progress towards structural health monitoring and damage evaluation for various metal and concrete structures. Timber is still commonly used as a construction material in practical engineering; however, there is a lack of research on the health monitoring of timber-based structures using piezoelectric-based transducers and methods. This paper conducts a feasibility study on timber damage detection using surface-mounted piezoelectric patches, which enable the stress-wave-based active sensing approach. Typical damage modes in timber frame structures, such as surface cracks and holes, were investigated in this study. In the active sensing approach, one piezoceramic transducer is used as an actuator to generate stress waves, which propagate along the surface of the timber structure, and other piezoceramic transducers function as sensors to detect the propagating stress waves. Defects, such as a crack or a hole, induce additional attenuation to the propagating stress wave. Based on this attenuation, the proposed method can detect the defects using the wavelet-packet-based damage index, demonstrating its implementation potential for real-time timber damage detection.

Evaluation of the parameters affecting the Schmidt rebound hammer reading using ANFIS method

As a nondestructive testing method, the Schmidt rebound hammer is widely used for structural health monitoring. During application, a Schmidt hammer hits the surface of a concrete mass. According to the principle of rebound, concrete strength depends on the hardness of the concrete energy surface. Study aims to identify the main variables affecting the results of Schmidt rebound hammer reading and consequently the results of structural health monitoring of concrete structures using adaptive neuro-fuzzy inference system (ANFIS). The ANFIS process for variable selection was applied for this purpose. This procedure comprises some methods that determine a subsection of the entire set of detailed factors, which present analytical capability. ANFIS was applied to complete a flexible search. Afterward, this method was applied to conclude how the five main factors (namely, age, silica fume, fine aggregate, coarse aggregate, and water) used in designing concrete mixture influence the Schmidt rebound hammer reading and consequently the structural health monitoring accuracy. Results show that water is considered the most significant parameter of the Schmidt rebound hammer reading. The details of this study are discussed thoroughly.

Mechanical and piezoresistive properties of self-sensing smart concretes reinforced by carbon nanotubes

ABSTRACTConcrete structures such as bridges, towers, and roads are constantly prone to damage due to development of cracks, stresses, and other defects. The present study is focused on manufacturing of self-sensing smart concretes and investigates their mechanical and piezoresistive behavior for health monitoring of concrete structures. To this end, concrete is combined with multi-walled carbon nanotubes (MWNTs) to produce an internal electrical network. Using the high piezoresistive properties of the carbon nanotubes, the concrete is able to indicate the mechanical stresses. Carbon nanotubes are also capable of reinforcing concretes and increasing mechanical strength. To this end, the concrete nanocomposite was prepared in three general cases including cement paste and concretes of grade 400 and 350 for water-cement ratios of 0.45 and 0.6. Then, the effect of different weight percentages of carbon nanotubes on the electrical and mechanical properties of the nanocomposite was experimentally investigated. In the next stage, the piezoresistive property of concrete was investigated when subject to static and dynamic loads. The results indicate that concrete nanocomposites are sensitive to the amount of applied compressive stresses so that application of compressive stresses up to 6 MPa leads to changes of up to 8 percent in the electrical resistance of concrete in the case of using cement paste nanocomposite reinforced with 1 wt.% of carbon nanotubes.

An Embedded Tubular PZT Transducer Based Damage Imaging Method for Two-Dimensional Concrete Structures

In this paper, a damage imaging method for 2-D concrete structures is proposed by employing embedded tubular piezoceramic transducers. The proposed method can image the damages when few numbers of transducers are used or less experimental data is collected. First, the time difference of arrival-based delay and sum (DAS) imaging method is adopted to improve the damage localization accuracy for concrete structural health monitoring. Compared with the conventional DAS imaging algorithm, the proposed method increases the potential number of combinations between the response signals induced by the damage in each transmitter and sensor pair, improving the signal to noise ratio of the imaging map and enhancing the performance of the defect detection and location. Second, to enable the proposed method applicable to a 2-D concrete structure (such as cement concrete pavement and shear walls), embeddable tubular lead zirconate titanate (PZT) transducers rather than conventional PZT patches are used. The new tubular PZT transducers are able to generate radially uniform stress waves in a planar concrete structure. Due to the special construction of the tubular PZT transducers, the detection aperture is significantly improved. Therefore, a few transducers are needed in contrast with the conventional PZT patches. The experimental results conducted on a concrete slab with eight embedded tubular PZT transducers demonstrate that the proposed method can predict the defect accurately and efficiently even with few transducers and less experimental data.

Monitoring of Compression and Bending Process of Reactive Powder Concrete Using MWCNTs/PDMS Composite Sensors

In the structural healthy monitoring of bridges, pavements of concrete or asphalt, flexible conductive sensors exhibit so many advantages, such as low cost, high durability, and large deformation monitoring. Herein, we report a low cost flexible composite sensor of multi-walled carbon nanotubes/Polydimethylsilosane (MWCNTs/PDMS) nanocomposites via solution casting method. Results show that MWCNTs/PDMS reveals excellent sensitivity and good stability when the fraction of MWCNTs is larger than the threshold (~2.5 wt%). The compressive strain of reactive powder concrete was lineal with the resistance change of sensor ( $\Delta R/R_{0})$ in the range of elastic deformation, while the deflection of reactive powder concrete could be predicted by the fitting exponential equation from the electric resistance change of MWCNTs/PDMS. These results highlight MWCNTs/PDMS nanocomposite as strain sensor in the structural health monitoring of reactive powder concrete structures.

Modeling of the attenuation of stress waves in concrete based on the Rayleigh damping model using time-reversal and PZT transducers

Wave-based concrete structural health monitoring has attracted much attention. A stress wave experiences significant attenuation in concrete, however there is a lack of a unified method for predicting the attenuation coefficient of the stress wave. In this paper, a simple and effective absorption attenuation model of stress waves in concrete is developed based on the Rayleigh damping model, which indicates that the absorption attenuation coefficient of stress waves in concrete is directly proportional to the square of the stress wave frequency when the damping ratio is small. In order to verify the theoretical model, related experiments were carried out. During the experiments, a concrete beam was designed in which the d33-model piezoelectric smart aggregates were embedded to detect the propagation of stress waves. It is difficult to distinguish direct stress waves due to the complex propagation paths and the reflection and scattering of stress waves in concrete. Hence, as another innovation of this paper, a new method for computing the absorption attenuation coefficient based on the time-reversal method is developed. Due to the self-adaptive focusing properties of the time-reversal method, the time-reversed stress wave focuses and generates a peak value. The time-reversal method eliminates the adverse effects of multipaths, reflection, and scattering. The absorption attenuation coefficient is computed by analyzing the peak value changes of the time-reversal focused signal. Finally, the experimental results are found to be in good agreement with the theoretical model.

A novel embeddable spherical smart aggregate for structural health monitoring: part I. Fabrication and electrical characterization

Recently developed piezoceramic-based transducers, known as smart aggregates (SAs), have shown their applicability and versatility in various applications of structural health monitoring (SHM). The lead zirconate titanate (PZT) patches embedded inside SAs have different modes that are more suitable for generating or receiving different types of stress waves (e.g. P and S waves, each of which has a unique role in SHM). However, due to the geometry of the 2D PZT patch, the embedded SA can only generate or receive the stress wave in a single direction and thus greatly limits its applications. This paper is the first of a series of two companion papers that introduces the authors’ latest work in developing a novel, embeddable spherical smart aggregate (SSA) for the health monitoring of concrete structures. In addition to the 1D guided wave produced by SA, the SSA embedded in concrete structures can generate or receive omni-directional stress waves that can significantly improve the detection aperture and provide additional functionalities in SHM. In the first paper (Part I), the detailed fabrication procedures with the help of 3D printing technology and electrical characterization of the proposed SSA is presented. The natural frequencies of the SSA were experimentally obtained and further compared with the numerical results. In addition, the influence of the components’ thickness (spherical piezoceramic shell and epoxy) and outer radius (spherical piezoceramic shell and protection concrete) on the natural frequencies of the SSA were analytically studied. The results will help elucidate the key parameters that determine the natural frequencies of the SSA. The natural frequencies of the SSA can thus be designed for suitability in the damage detection of concrete structures. In the second paper (Part II), further numerical and experimental verifications on the performance of the proposed SSA in concrete structures will be discussed.

Damage detection, localization and quantification in conductive smart concrete structures using a resistor mesh model

Interest in self-sensing structural materials has grown in recent years due to their potential to enable continuous low-cost monitoring of next-generation smart-structures. The development of cement-based smart sensors appears particularly well suited for structural health monitoring due to their numerous possible field applications, ease of use, and long-term stability. Additionally, cement-based sensors offer a unique opportunity for monitoring of civil concrete structures because of their compatibility with new and existing infrastructure. In this paper, we propose the use of a computationally efficient resistor mesh model to detect, localize and quantify damage in structures constructed from conductive cement composites. The proposed approach is experimentally validated on non-reinforced and reinforced specimens made of nanocomposite cement paste doped with multi-walled carbon nanotubes under a variety of static loads and damage conditions. Results show that the proposed approach is capable of leveraging the strain-sensing and damage-sensitive properties of conductive cement composites for real-time distributed structural health monitoring of smart concrete structures, using simple and inexpensive electrical hardware and with very limited computational effort.

Development and Application of a Structural Health Monitoring System Based on Wireless Smart Aggregates.

Structural health monitoring (SHM) systems can improve the safety and reliability of structures, reduce maintenance costs, and extend service life. Research on concrete SHMs using piezoelectric-based smart aggregates have reached great achievements. However, the newly developed techniques have not been widely applied in practical engineering, largely due to the wiring problems associated with large-scale structural health monitoring. The cumbersome wiring requires much material and labor work, and more importantly, the associated maintenance work is also very heavy. Targeting a practical large scale concrete crack detection (CCD) application, a smart aggregates-based wireless sensor network system is proposed for the CCD application. The developed CCD system uses Zigbee 802.15.4 protocols, and is able to perform dynamic stress monitoring, structural impact capturing, and internal crack detection. The system has been experimentally validated, and the experimental results demonstrated the effectiveness of the proposed system. This work provides important support for practical CCD applications using wireless smart aggregates.

Development of a capacitive sensor for concrete structure health monitoring

A novel low-cost capacitive transducer (CT) using capacitance signals, which has the potential to provide accurate health assessment and damage prediction for reinforcement concrete structures, is developed in this study. Two designs of capacitive transducers are discussed for condition assessment of rebar and concrete, respectively. Both of them consist of a supporting structure and a pair of corresponding parallel electrode plates. In order to verify the effectiveness of the designed capacitive sensor, a series of preliminary experiments are conducted. The results show that the detected CT signals rise with the increasing rebar size. Pure cement paste and fiber-reinforced concrete have demonstrated a stable dielectric constant under the same fabrication conditions. To simulate the real reinforced concrete case, various sizes of rebars were placed in fiber reinforced concrete. It is found that monitoring the CT signals is an effective way to assess the rebar diameters inside concrete. Experiments results also showed that CT signals could be applied to predict rebar positions inside the concrete. Finally, FEM simulations were implemented for the last experiment, and comparison results have well verified the effectiveness of the developed capacitive sensor. The whole work has demonstrated a promising application of the capacitive sensor in NDT of reinforced concrete in the future.

Application assessments of concrete piezoelectric smart module in civil engineering

Traditional structural dynamic analysis and Structural Health Monitoring (SHM) of large scale concrete civil structures rely on manufactured embedding transducers to obtain structural dynamic properties. However, the embedding of manufactured transducers is very expensive and low efficiency for sign...

Dielectric Characterization of Chinese Standard Concrete for Compressive Strength Evaluation

Dielectric characterization of concrete is essential for the wireless structural health monitoring (SHM) of concrete structures. Guo Biao (GB) concrete refers to the concrete mixed and cast in accordance with the Chinese standard. Currently, China is the largest producer and consumer of concrete in the world. However, minimal attention has been paid to the dielectric properties of GB concrete. This paper presents the results of the dielectric constant of GB concrete, where three regression models have been used to present the measurement data from 10 MHz to 6 GHz. The objective is to provide a data set of nominal values of the dielectric constant for ordinary GB concrete. The final goal is to facilitate a compressive strength evaluation via the measured dielectric constant. Measurements of the dielectric constant and compressive strength for five types of ordinary concrete have been undertaken, after 28 days of curing. As the main contribution in this work, the correlation model between the compressive strength and dielectric constant of GB concrete is realized.

Acoustic emission monitoring of reinforced concrete beams subjected to four-point-bending

The aim of this study is to apply acoustic emission (AE) technique and study the damage mechanism of the reinforced concrete (RC) beams under four-point bending. Laboratory experiments are performed on three types of RC beams of grade M30 with 42, 64 and 93% of longitudinal steel against balanced section. The damage in the beams are classified into four zones symbolizing formation of micro cracks, visible cracks, steel yielding and concrete crushing. The AE parameters such as amplitude, rise time, count, duration and average frequency are quantified in each damage levels and a parametric analysis is performed between average frequency and RA value. The results showed that as the level of damage increased, the values of AE parameters such as count, hits, rise time, acoustic emission energy and duration increased except for average frequency. This results coincided with the visual observation results according to crack modes. The adequacy of the crack classification is also evaluated by Gaussian mixture modelling (GMM), a probabilistic based approach. GMM is used as a parametric model to overcome the randomness found in the data set generated by AE testing. The results of the present investigation can be utilized in health monitoring of concrete structures subjected to flexural load.

Feasibility study of using smart aggregates as embedded acoustic emission sensors for health monitoring of concrete structures

Acoustic emission (AE) is a nondestructive evaluation technique that is capable of monitoring the damage evolution of concrete structures in real time. Conventionally, AE sensors are surface mounted on the host structures, however, the AE signals attenuate quickly due to the high attenuation properties of concrete structures. This study conducts a feasibility study of using smart aggregates (SAs), which are a type of embedded piezoceramic transducers, as embedded AE sensors for the health monitoring of concrete structures. A plain concrete beam with two surface mounted AE sensors and two embedded SAs was fabricated in laboratory and loaded under a designed three-point-bending test. The performance of embedded SAs were compared with the traditional surface mounted AE sensors in their ability to detect and evaluate the damage to the concrete structure. The results verified the feasibility of using smart aggregates as embedded AE sensors for monitoring structural damage in concrete. Potentially, the low cost smart aggregates could function as embedded AE sensors, providing great sensitivity and high reliability in applications for the structural health monitoring of concrete structures.

Influence of axial loads on the health monitoring of concrete structures using embedded piezoelectric transducers

Piezoceramic-based smart aggregate has been widely used to evaluate early-age concrete strength and to detect damage in concrete structures. In these structural health monitoring systems, they are generally verified and calibrated through experiments under load-free condition. However, the stress levels of actual concrete members are different. The microstructures of concrete will change with the variation of external load, and the high-frequency waves used in the monitoring system may be highly sensitive to these changes. In this study, the effects of axial compressive loading on the monitoring results are investigated. Specifically, three loading cases, that is, single cycle load, cyclic load, and step-by-step load, are employed to stress the concrete specimens embedded with smart aggregates. The amplitude and velocity of monitoring signals were measured before, during, and after each loading case. The test results show that the axial load lower than 30% of failure load still have a significant impact on the received signals. The amplitude attenuation is dependent on both frequency and load history, while the velocity is highly stress-dependent. The results indicate that the baselines of monitoring signals obtained from the same concrete structure in its healthy state can vary under different stress levels. The axial load variation should be carefully considered during the monitoring process. This study also provides a potential method to assess stress state in concrete structures using smart aggregates.

Performance indicator of the atmospheric corrosion monitor and concrete corrosion sensors in Kuwait field research station

Two field research stations based upon atmospheric corrosivity monitoring combined with reinforced concrete corrosion rate sensors have been established in Kuwait. This was established for the purpose of remote monitoring of building materials performance for concrete under Kuwait atmospheric environment. The two field research sites for concrete have been based upon an outcome from a research investigation intended for monitoring the atmospheric corrosivity from weathering station distributed in eight areas, and in different regions in Kuwait. Data on corrosivity measurements are essential for the development of specification of an optimized corrosion resistance system for reinforced concrete manufactured products. This study aims to optimize, characterize, and utilize long-term concrete structural health monitoring through on line corrosion measurement and to determine the feasibility and viability of the integrated anode ladder corrosion sensors embedded in concrete. The atmospheric corrosivity categories supported with GSM remote data acquisition system from eight corrosion monitoring stations at different regions in Kuwait are being classified according to standard ISO 9223. The two nominated field sites where based upon time of wetness and bimetallic corrosion rate from atmospheric data where metals and rebar's concrete are likely to be used. Eight concrete blocks with embeddable anodic ladder corrosion sensors were placed in the atmospheric zone adjacent to the sea shore at KISR site. The anodic ladder corrosion rate sensors for concrete were installed to provide an early warning system on prediction of the corrosion propagation and on developing new insights on the long-term durability performance and repair of concrete structures to lower labor cost. The results show the atmospheric corrosivity data of the environment and the feasibility of data retrieval of the corrosion potential of concrete from the embeddable sets of anodic ladder corrosion sensors.