Around the world, health monitoring of concrete structure has been playing an important role in the development of civil engineering. Structural Health Monitoring (SHM) is a process of monitoring the present condition of engineering structures. Many techniques for SHM have been reported in recent years. Moreover, there are several conflicts while monitoring health of concrete structure. This paper presents a new approach of health monitoring of structure based on electromechanical impedance (EMI) technique using embedded PZT sensors.

Earlier several authors have also proposed the SHM by using PZT smart sensors. Shanker [1] proposed the experimental study on embedded PZT as a sensor by using EMI technique. A simple low cost experimental technique was developed to extract the experimental strain mode shapes of the structure directly by using a surface bonded and embedded PZT sensor. It was found that damage ranging from incipient to near failure (severe) can be located and quantified by using the EMI technique and the experimental mode shapes with desired accuracy. Annamdas et al. [2] proposed a method of embedding PZT sensor in concrete for monitoring concrete. The implementation was verified on various lab sized concrete cubes and the observations were examined by statistical analysis. Wang et al. [3] proposed a health monitoring method based on EMI measurements, which used the electromechanical coupling property of embedded PZT transducers. Khante and Gedam [4] experimentally monitored the health of RC structure by embedding indigenously prepared PZT based smart aggregate in two phases i.e. for healthy and damaged state of RC specimen. Dumoulin et al. [5] studied crack propagation in a reinforced concrete beam by using PZT sensor. Two different types of excitation signals were used (pulse and chirp) and the resulting waves were recorded on the receivers. Based on these signals, different damage indicators were investigated and compared. Kaur et al. [6] investigated the integrity of the low-cost EMI technique and the global vibration technique for health assessment. Fourteen CVS were embedded inside a RC beam, while casting, to obtain the first curvature mode shape of the RC beam. A mode shape curvature based algorithm was adopted for damage detection and severity assessment. Song et al. [7] proposed the smart aggregates as a multifunctional tool for health monitoring. A damage index based on the wavelet packet analysis was used to determine the structural health status. Negi et al. [8] oriented PZT patches in different configuration in host structure. Visalakshi and Bhalla [9] proposed a comparison between the sensing capabilities of surface bonded and embedded piezoceramic (PZT) patches in corrosion assessment for RC structures. Accelerated corrosion tests were performed on RC specimens, and the statistical index and the equivalent parameters were compared for the two types of sensor configurations. B. Xu et al. [10] proposed a PZT based active interface debonding defect detection approach for multi-chamber Steel Reinforced Concrete (SRC) columns and validated experimentally with an irregular multi-chamber SRC specimen. A number of embedded piezo-based functional elements (EPFEs) installed close to the steel plates and piezoelectric ceramic (PZT) patches bonded on the surface of the steel plates were used as actuators and sensors respectively. Based on the amplitude of the measurement of the PZT patches under sinusoidal excitations and the wavelet packet energy under sweep sinusoidal signals, the interface debonding defects was detected successfully. Zhu et al. [11] presented the design and fabrication processes of EMI sensors embedded into concrete structures. Based on the electrical admittance and strain measurements, the health statuses of the continuous rigid frame bridge was monitored and evaluated successfully in the construction and operation stages by using a root-Mean-Square Deviation (RMSD) index. Gupta et al. [12] discussed a newly developed approach for detecting and quantifying corrosion of steel bars utilizing a piezoelectric ceramic (PZT) patch surface bonded on the rebar employing equivalent structural parameters using the Electro-Mechanical Impedance (EMI) technique.

By using conventional and EMI techniques, a lot of experimental and analytical work has been done by many researchers for health monitoring of structures. Electro-me- chanical impedance technique is considered more reliable and convenient for structural health monitoring. In previous studies, a limited experimental work using embedded PZT was carried out on the structures. Hence, in this paper, an attempt is made to study preparation of indigenous embedded SMAG and the feasibility of EMI technique for in situ SHM. This research article presents EMI approach using a new variant of the embedded SMAG for non-destructive monitoring of concrete structures. The conductance and susceptance signatures acquired over input frequency range are the basis for assessing the health of real-life RC structures.

The piezoelectric patch has a unique property to act as both sensor and actuator and can be easily surface bonded or embedded inside host structure. When mechanical stress is applied on PZT patch it produces an electrical charge, which is called as direct piezoelectric effect of PZT. The governing principle of EMI based health monitoring method is to monitor the changes in structure by measuring admittance signature of EMI active sensor bonded onto or embedded into the structure as shown in Figure 1.

Equation (1) below represents the complex electro-mechanical admittance Ȳ of the system.

where, l, w and h are the length, width and thickness of the PZT patch respectively, j being imaginary number equal to (), ω is angular frequency and kl is the wave number, (e₃₃)^T is the complex electric permittivity at constant stress, Z_a is mechanical impedance of the PZT patch, d₃₁ is the piezoelectric strain coefficient of the PZT material, Ȳ^E is the complex Young’s modulus under constant electric field, Z is the mechanical impedance of the host structure.

In EMI based method, high frequencies in the order of few kHz to hundred kHz are used. Mechanical impedance of the host structure is the function of its mass, stiffness, and damping properties. Any changes in admittance signature indicate the presence of structural damage and simultaneously structural properties also changes. The electro- echanical admittance Y consists of the real and the imaginary component called conductance and susceptance respectively. Hence, the magnitude of complex admittance can be calculated as given in Equation (2):

where, G = Conductance; real part of admittance,

B = Susceptance; imaginary part of admittance.

Damage characterization is accomplished by using root mean square deviation method (RMSD). Electrical impedance signatures are compared with each other in which one of the signature known as baseline signature is acquired when the structure is considered to be healthy. The RMSD index compares baseline signature and signature obtained in damaged condition of structure measured over preset frequency range respectively. RMSD of the signature is given by Equation (3),

where, M = Damage metric (Root Mean Square Deviation),

G₁ = Conductance before damage,

G₂ = Conductance after damage.

Smart Aggregates (SMAGs) are thin PZT transducers casted in small mortar pieces. The PZT transducer is very fragile and can be easily damaged by the vibrator during the casting of concrete structures. The robust PZT patch should be protected in the form of smart aggregate and embedded at the desired position in the concrete structure before casting, eliminating the risk of damaging the transducer during the vibrating process. The method of fabrication was carried out as explained below.

Step 1: A typical PZT patch of size 10 × 10 × 0.2 mm and 5H grade manufactured by Central Electronics Limited, Sahibabad, Delhi was selected to embed into structure. One of the ends of coaxial head wire was soldered to the electrodes of PZT patch. The soldered wires were then connected to impedance analyzer via connecting fixture for recording admittance signature over the user specified frequency range. Admittance signature of free PZT patches was recorded to ensure proper working.

Step 2: Encapsulation of waterproofed PZTs i.e. SMAG were formed in two ways as follows:

SMAG Brick: A hollow rectangular shaped card board mould of 50 × 25 × 25 mm size was prepared and a five sided rich mortar rectangular box with 5 mm side thickness maintaining its top portion open was casted as shown in Figure 2 and allowed to cure for 3 days. The free PZT was then positioned inside one of the side of box using epoxy adhesive to make it waterproof and surface from outside was marked for identification and allowed to (air) cure for 24 hours. Finally, the top layer of box was closed, still maintaining inner portion as hollow. After 24 hours of casting the mould was then allowed to (water) cure for 23 days.

Metal Wire SMAG Cylinder: Firstly two perpendicular galvanised steel wires of length 100 mm and diameter 1 mm were fixed on top of PZT patch using epoxy adhesive and also remaining portion of PZT patch was coated with epoxy adhesive to make it waterproof and allowed to (air) cure for 24 hours as shown in Figure 3(a). After curing its admittance signatures were obtained. A card board mould of 20 mm diameter was prepared and filled to its half depth with mortar and the waterproofed PZT patch with galvanised steel wire was then placed over it. Then the second half layer over PZT was filled with mortar and mould was allowed to (water) cure for 28 days as shown in Figure 3(b).

The admittance signatures were again recorded to check any damage incurred during this fabrication process. The robust encapsulated PZT was referred as Smart Aggregate (SMAG) to embed into host structure. If there was any abnormality other than appearances of new modes in signature occurs it would be regarded as failed SMAG. Again the same procedure needs to be performing to fabricate a new SMAG. After completion of these steps the SMAG was ready to embed inside concrete specimen. A typical concrete vibration sensor (CVS) made by Central Electronics Limited, Sahibabad, Delhi shown in Figure 4 was also used to embed in host structure with the fabricated SMAG.

To perform SHM using EMI technique, the instrument used was digital Impedance Analyzer (LCR meter E4980A) having frequency range of 20 Hz to 2 MHz and its connecting fixture, along with a USB cable and VEE Pro 9.32 for data acquisition system as

shown in Figure 5. The tests were performed for the healthy state of host structure and then for damaged state of structure. 100 mm × 100 mm × 410 mm reinforced concrete beams with M20 grade concrete and Fe 415 steel were monitored. The reinforcement provided was 2 × 8 mm ϕ Fe 415 steel bars at top and at bottom along with stirrups of Fe 250, 6 mm ϕ @ 80 mm c/c. The Maximum Size of Aggregate (MSA) was limited to 20 mm. Test specimen was cured for 7 days. Two different experimental studies were performed on two RC beams in which first study was the comparative study of RC beam with CVS and indigenously prepared SMAG brick and metal wire SMAG cylinder and the second study was conductance sensitivity investigations on SMAG embedded in varying orientation and surface bonded PZT.

The Conductance, susceptance and RMSD index were plotted for all the embedded SMAG, for healthy and damaged condition. VEE Pro 9.32 software was used for data acquisition and data processing.

This experimental work has presented a method to monitor the damages in concrete structure using embedded SMAG. It was verified from the results that the incipient level damage could be easily identified by lateral and vertical shifting of peaks. Since the same SMAG act as actuator and sensor, this saves the number of transducers and the associated wiring. Being high frequency technique, signatures have no interference with noise. It is well established from the first experiment that damage sensitivity of embedded SMAG depends on its location from damage and varies, i.e. nearer the location of embedded SMAG from damage, more is the sensitivity. By comparing the conductance and susceptance signatures obtained from indigenously prepared SMAG with the standard CVS, it was well established that the SMAG was also equally effective in detecting damage as damage detected by CVS for the same structure.

The second investigational study presented the comparison of SMAG embedded in two different orientation i.e. horizontal (0˚) and vertical (90˚) for their sensitivity towards conductance of RC beam and it was found that conductance peak was the highest in vertical (90˚) SMAG within less frequency range. Hence, it can be advised to ensure the orthogonal positions of SMAG while using in embedded positions in real life struc- tures. Health monitoring of structure using embedded SMAG is a better alternative for conventional tedious and time consuming SHM techniques with economy.

Gedam, S.R. and Khante, S.N. (2016) Experimental Investigation on Sensitivity of Smart Aggregate Embedded in Reinforced Concrete Beam. Open Journal of Civil Engineering, 6, 653-669. http://dx.doi.org/10.4236/ojce.2016.64053