The most familiar civil engineering structure is reinforced concrete (RC) structure. Performance of structure undergoes changes during their service life with time. Thus, it is of great concern to monitor the health of RC structure. Structural health monitoring (SHM) is the art of detecting the changes in structure that influences its performance. Various techniques to monitor the health of structure are broadly studied worldwide. PZT based smart aggregate can play an effective role as an advanced tool in the development of structural health monitoring. This research work contributes for proposing a more generous Non-Destructive Evaluation (NDE) technique for structural health monitoring by using smart materials. If performance of a structure deviates from the design parameters with time, appropriate and effective maintenance is required. Considering the relevant need of RC structures, a more sensitive and cost-effective approach by using Electro-Mechanical Impedance (EMI) technique has been proposed for implementation in real-life situations. In general, surface bonded PZT transducer is used for SHM. Since PZT transducers are of very small dimension and brittle in nature, for consistent characteristics, they should be protected from severe environmental condition and other external interruptions. For this reason, PZT transducer is embedded in structure at the time of construction and manufacturing of the embedded transducer is simple. The proposed EMI technique assesses the health of RC structures more rationally by embedding PZT transducer in the structure, whose health is to be monitored over the user specified preset frequency range. The conductance and susceptance signatures are acquired by using LCR meter. At any future point of time, when it is desired to assess the health of structure, the conductance and susceptance signatures are acquired and further utilized for damage detection and quantification. The Root Mean Square Deviation (RMSD) is used to specify damage severity.
Structures consist of an assembly of various members like columns, beams, slabs, etc. Piezoceramic transducer has been playing an important role in structural health monitoring. Any damage of the host structure reflects changes in its properties. Major damages to structures can lead to failures, causing inconvenience and safety problems. Therefore, continuous monitoring of structure is essential. Numerous structural engineers drew attention towards developing modern sensor technology for making structures, smart enough to warn the concerned authorities before their failure in the recent decade years.
Sensors like accelerometers, strain gauges, Piezo based Lead Zirconate Titanate (PZT) patches, etc. can be used to monitor the health of the structures. As compared to other sensors, piezo based sensors are becoming more popular because of their efficiency of monitoring. Smart materials are widely used in Structural Health Monitoring (SHM) systems due to the viability of them in host structures; to act as sensors and/or actuators to assess the continuous health of a structure.
Among all the health monitoring methods, Electro-Mechanical Impedance (EMI) based health monitoring using piezoelectric materials is extensively acknowledged. The conductance signature is acquired with the help of EMI active sensor which serves as an indication of damage in structure. Kaur, Gupta, Jain and Bhalla [
Damage identification is carried out by comparing the signature of structure in healthy state with the signatures obtained after a damage. The specific objective of the present study is to carry out the structural health monitoring of structures using indigenously prepared embedded PZT patches.
The Electro-Mechanical Impedance (EMI) technique using piezo elements is relatively a new technique for SHM. The EMI technique employs PZT patches as a sensor. A PZT patch is surface bonded or embedded inside the structures to be monitored, as shown in
The patch (length 2l, width w and thickness h) behaves as thin bar undergoing axial vibration. When an alternating electric field is applied to the patch; it expands and contracts dynamically in direction “1”. Two end points of the patch can be assumed to encounter equal impedance Z from the host structure. An electro-me- chanical model of the system is shown in
Equation (1) represents the complex electro-mechanical admittance Y of the coupled system.
Where, Za is mechanical impedance of the PZT patch, ω is angular frequency and kl is the wave number, d31 is the piezoelectric strain coefficient of the PZT material,
The mechanical impedance Z of the host structure is the function of the structural parameters, i.e., as stiffness, damping and mass. Any damage to the structure will cause these parameters to change, and hence changes the
drive point mechanical impedance Z. As a result, the electro-mechanical admittance Y will change and this serves an indicator of the state of health of the structure. Equation (1) is utilized in EMI technique for damage detection. The electro-mechanical 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.
There are several techniques available to quantify damages in structure. In the present work, to quantify damage in structure Root Mean Square Deviation (RMSD) technique is used which is given as:
where, M = Damage metric (Root Mean Square Deviation),
G1 = Conductance before damage,
G2 = Conductance after damage.
In order to perform structural health monitoring of concrete structures using EMI technique, a model RC beam and smart aggregate was cast. The experimental investigation is carried for damage identification and crack detection. The instrument used is digital Impedance Analyzer (LCR meter E4980A) of frequency range of 20 Hz to 2 MHz and its connecting fixture, and a USB cable and VEE Pro 9.32 for data acquisition. To monitor the health of a structure experiment was performed in two stages i.e. firstly for the healthy state of host structure and then for damaged state of structure.
The test was carried out on reinforced concrete beam. The PZT sensor was fabricated and embedded inside the RC beam. To prepare the embedded sensor SMart AGgregate (SMAG), both the surfaces of naked PZT patch with the dimensions of 10 mm × 10 mm × 0.2 mm are welded to coaxial cable wires, respectively, and the bare PZT impedance sensors is formed. The form work of card board with 20 mm diameter was first filled with cement mortar of proportion (1:2) to the half depth. After 7 days curing, the PZT (5 H) patch was bonded on the top of the surface of the cement block using epoxy (Araldite). After 24 hours, the card board was filled to full depth with the cement mortar. After 7 days of curing, the card board was removed from cement block and was ready to use in any structure under construction as shown in
The performance of sensor was studied on the R.C. beam structure whose dimensions and properties are shown in
This experiment essentially involves a simply supported RC beam with embedded SMAG at 100 mm distance from right support and 30 mm distance from bottom of beam as shown in
In the first stage, the soldered PZT patch in the form of SMAG was wired to Impedance Analyzer through Connecting Fixture of LCR meter. Then the frequency was swept through 100 kHz to 400 kHz i.e. the PZT patch transfers this vibrations to the structure through adhesive bond layer. These vibrations are transferred to structures and reflected back from the same PZT patch through waves, which will indicate the health of the structure. The required parameter i.e. Conductance (G) is directly measured through LCR meter for all the values of frequency. From the obtained data, the graph of Conductance versus Frequency is plotted. These graphs are said to be the Conductance Signature or just a signature shown by particular sensor for specimen. This data is saved in excel format and can be available as and when required. The assembly for the experiment of EMI technique is as shown in
In the second stage, damage was introduced in the form of significant visible crack by loading the specimen under Universal Testing Machine (UTM) as shown in
Dimensions/Properties | Value |
---|---|
Length (L) | 410 mm |
Cross-section | 100 mm × 100 mm |
Grade of cement (OPC) | 43 |
Grade of steel | Fe 415 |
Reinforcement bars | 2#8 mm dia at top and bottom |
stirrups | 6 mm dia @80 mm c/c |
Characteristic strength of concrete | 20 N/mm2 |
Curing period | 7 days |
For the first stage, the healthy structure response is taken, with the SMAG embedded in simply supported RC beam. Piezoelectric patch was connected via cable to LCR meter and corresponding response was recorded. The beam was subjected to frequency range of 100 to 400 kHz. In this range, corresponding conductance values were recorded by LCR meter. The conductance graph is shown in
In the second stage, the beam is laterally loaded on Universal Testing Machine until significant visible crack is seen. The same beam with embedded SMAG was connected via cable to LCR meter and corresponding response in term of conductance and susceptance was recorded as shown in
By comparing the results obtained for both the stages as shown in
The damage index recorded by embedded SMAG using root mean square deviation technique (RMSD) is found to be 5.0014% in frequency range of 100 - 400 kHz.
It is concluded from the above experimental work that continuous health monitoring of structure is possible using proposed simple low cost technique. Instead of surface bonded PZT patches, structural health monitoring with embedded PZT patches is very effective and easy to implement and the record data can be available as and when required. For health monitoring of RC structure, it is a challenging task to detect incipient damages. It is found that the incipient level damages are very quickly and effectively detected by using present EMI technique. It is again established that the conductance signature is more effective in damage detection than susceptance signature.
Suraj N.Khante,Shruti R.Gedam, (2016) PZT Based Smart Aggregate for Unified Health Monitoring of RC Structures. Open Journal of Civil Engineering,06,42-49. doi: 10.4236/ojce.2016.61005