Integrated Global Vibration and Low-Cost EMI Technique for Structural Health Monitoring of RC Structures Using Embedded PZT Patches

Abstract

The electro mechanical impedance (EMI) technique facilitates capturing of
incipient damage. Further advantage of EMI techniques ceases after damage severity
increases from incipient to moderate level, which can be more realistically quantified by
global vibration techniques. Lot of research has already been done in the field of structural
health monitoring (SHM) of reinforced concrete (RC) structures. Concrete vibration sensor
(CVS) is a new sensor developed at the Smart Structures and Dynamics Laboratory (SSDL)
of IIT Delhi. CVS is a ready to use packaged sensor for dynamic response measurement of
RC structures such as buildings and bridges. These sensors provide the additional advantages
of higher longevity and negligible decay of the sensing element. Also, the use of low-cost
adaptations for EMI technique provides a cutting edge for SHM. In this paper, the integrated
application of the global vibration technique and the low-cost EMI technique is carried out on
a lab-based RC. The specimen (a simply supported RC beam) has the provision to introduce
controlled damage severity levels. Fourteen CVS are embedded inside a RC beam, while
casting, to obtain the first curvature mode shape of the RC beam. The damage location is
detected by comparing the curvature mode shapes of the undamaged and the damaged beams
using global vibration technique. A mode shape curvature based algorithm is adopted for
damage detection and severity assessment. The admittance signature in the frequency range
50-150 kHz of all CVS is acquired for low-cost EMI technique. In the EMI technique, the
RMSD values are used to locate and quantify the damage. Both the global vibration
technique and the EMI technique successfully locate and quantify the damage in the RC
beam using CVS. These changes would then present necessary warnings for proper and
timely coactive action.

Full text

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INTEGRATED GLOBAL VIBRATION AND LOW-COST EMI
TECHNIQUE FOR STRUCTURAL HEALTH MONITORING
OF RC STRUCTURES USING EMBEDDED PZT PATCHES
Naveet Kaur1, Nikit Gupta2 and Neshan Jain2, Suresh Bhalla3
Indian Institute of Technology (IIT) Delhi, New Delhi, India
ABSTRACT. The electro mechanical impedance (EMI) technique facilitates capturing of
incipient damage. Further advantage of EMI techniques ceases after damage severity
increases from incipient to moderate level, which can be more realistically quantified by
global vibration techniques. Lot of research has already been done in the field of structural
health monitoring (SHM) of reinforced concrete (RC) structures. Concrete vibration sensor
(CVS) is a new sensor developed at the Smart Structures and Dynamics Laboratory (SSDL)
of IIT Delhi. CVS is a ready to use packaged sensor for dynamic response measurement of
RC structures such as buildings and bridges. These sensors provide the additional advantages
of higher longevity and negligible decay of the sensing element. Also, the use of low-cost
adaptations for EMI technique provides a cutting edge for SHM. In this paper, the integrated
application of the global vibration technique and the low-cost EMI technique is carried out on
a lab-based RC. The specimen (a simply supported RC beam) has the provision to introduce
controlled damage severity levels. Fourteen CVS are embedded inside a RC beam, while
casting, to obtain the first curvature mode shape of the RC beam. The damage location is
detected by comparing the curvature mode shapes of the undamaged and the damaged beams
using global vibration technique. A mode shape curvature based algorithm is adopted for
damage detection and severity assessment. The admittance signature in the frequency range
50-150 kHz of all CVS is acquired for low-cost EMI technique. In the EMI technique, the
RMSD values are used to locate and quantify the damage. Both the global vibration
technique and the EMI technique successfully locate and quantify the damage in the RC
beam using CVS. These changes would then present necessary warnings for proper and
timely coactive action.
Keywords: Structural health monitoring, low-cost EMI technique, global vibration
technique, embedded PZT patches
Naveet Kaur is a research scholar in Department of Civil Engineering at Indian Institute of
Technology (IIT) Delhi, India. Her research interest includes structural health monitoring of
concrete structures with a focus on embedded PZT patches in reinforced concrete structures.
Nikit Gupta and Neshan Jain are undergraduate pass-outs from Department of Civil
Engineering at IIT Delhi, India. They have extensively worked on structural health
monitoring using low-cost EMI technique and global vibration technique of concrete
structures.

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Dr Suresh Bhalla is an Associate Professor of Department of Civil Engineering and
Incharge of Structural Engineering Section at IIT Delhi. His research interests are structural
health monitoring, system identification and non-destructive evaluation of civil structures.
INTRODUCTION
Civil infrastructures, such as long span bridges, offshore structures, large dams, nuclear
power stations, tall buildings, large space structures etc. have a long service life of several
decades to over hundred years, during which they are inevitable to suffer from environmental
corrosion, long term loading or fatigue effects, material aging or their coupled effects with
extreme loading, resulting in damage accumulation, performance degeneration and reduced
capacity [1]. Therefore, its accurate diagnosis in protecting the structures is a vital step.
Properly monitored structures thus benefit in terms of greater durability and lesser economic
losses caused by structural malfunctioning.
This paper focuses on the objective to develop a Structural Health Monitoring (SHM)
System which would include (a) detecting damage in a concrete beam using PZT (low-cost)
sensors by comparing the first mode shape of the damaged and undamaged beam (b)
integrating the procedure in objective (a) with low-cost EMI technique.
SHM is the art of estimating the state of structural health, in other words it is the art of
detecting the changes in structure that affect its performance. Two major categories of SHM
are disaster response (earthquake, explosion, etc.) and continuous health monitoring (ambient
vibrations, wind, etc.). This work is focused on continuous health monitoring. There are two
SHM approaches: direct damage detection (visual inspection, X-ray, etc.) and indirect
damage detection (change in structural properties/behaviour). Objectives of health monitoring
are (a) to ascertain that damage has occurred or to identify damage (b) to locate the damage
(c) To determine the severity of damage (d) to determine the remaining useful life of the
structure. The various conventional techniques found in literature [2] for SHM are (a) Static
response based techniques (b) Local SHM techniques (c) Impedance Based Technique (EMI
technique) (d) Global vibration techniques. This study uses global vibration and EMI
technique in synergy.
Electro-Mechanical Impedance (EMI) Technique
The electro-mechanical impedance (EMI) technique employs piezoelectric ceramic Lead
Zirconate Titarate (PZT) patches as sensors. These patches, owing to the inherent direct and
converse mechatronic effects, can be utilized as impedance transducers for SHM [3]. These
patches measure the admittance as a function of frequency and act as impedance transducers.
This technique is quite similar to the global vibration technique, the difference being the
frequency range employed (30-400 kHz in EMI technique against less than 100 Hz in global
vibration technique). Major features are summarized below:
(i) It bridges the gap between global and local damage detection techniques.
(ii) It is used for both strength estimation and damage detection of concrete structures.
(iii) It is a more cost effective and hassle free alternative for strength analysis and damage
detection.
(iv) It has far greater sensitivity (to the order of the local ultrasonic techniques) to
structural damages than the conventional global vibration techniques.
(v) It employs low-cost transducers, which can be permanently bonded to the structure
and can be interrogated without removal of any finishes or rendering the structure
unusable

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(vi) No complex data processing or expensive hardware is required and the data
acquisition is simpler.
Several proof-of-concept applications of the EMI technique have been reported in the
literature. The use of the EMI technique for SHM of a lab sized truss structure was reported
by Sun et. al. [4]. The damage detection and localization capability of the EMI technique on
real-life concrete structures was established through a destructive load test on a prototype
reinforced concrete (RC) bridge [5]. Other Significant proof-of-concept applications of the
technique on structures such as composite reinforced masonry walls, steel bridge joints and
pipeline systems can also be found [6,7]. The most significant observation was that the
technique is tolerant to mechanical noise, giving it a leading edge over the conventional
global dynamic methods [6].
The sensing zone PZT patch is limited to 0.4 to 2m only, hence, large numbers of
PZT patches are required for real life monitoring of engineering structures like bridges and
multi storeyed buildings. This technique does not give the overall stability of structure in case
of civil engineering structures. Since, civil engineering structures are of indeterminate in
nature, occurrence of cracks at some places may not affect their overall stability [8].
Low-Cost EMI Technique
The conventional EMI technique requires impedance analyzer or LCR meter, which typically
costs in the range of $20,000 to $41,000. A low-cost electrical admittance measurement
technique based on Fast Fourier Transform (FFT) analyzer (which typically costs $10,000) in
place of the impedance analyzer was proposed by Peairs et. al.[9]. It involved the FFT of the
time domain data, unlike the steady state measurements of impedance analyzer or LCR
meter. Therefore, it faced bandwidth restrictions and could not be relied upon for frequencies
greater than 100 kHz. Xu and Giurgiutiu [10] made further improvement by using a pair of
function generator (FG) and a data acquisition (DAQ) card in place of the FFT analyzer and
employing a sweep signal in place of chirp. However, their technique still necessitated FFT
of time domain data. A major improvement (circuit shown in Fig. 1) has recently been done
by Bhalla et. al. [11] aiming at further cost cuts while ensuring steady state quasi sweep
measurements similar to an impedance analyzer. Here, the overall hardware cost is less than
$2000 and the measurements are steady state and thus more consistent.
Figure 1: Circuit for low-cost approach [11].
Health of the structure can be known by comparing the healthy signature and
signature obtained after some period when health monitoring is needed. But to quantify
changes in signature due to damage, there are several techniques such as wave form chain
code (WCC) technique, signature assurance criteria (SAC), adaptive template matching

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(ATM), equivalent level of degradation system, root mean square deviation technique
(RMSD) etc. The RMSD of the signature was defined by Bhalla [2] as:
(
)
()
100(%) 2
0
2
0
×
−
=∑
∑
i
ii
G
GG
RMSD (1)
where, 0
i
Gdenotes the baseline conductance value and i
Gthe conductance value after damage
Global Vibration Techniques
These techniques involve subjecting the structure under consideration to low frequency
excitations so as to obtain the first few natural frequencies and extract the corresponding
mode shapes. These are then processed to obtain information pertaining to the location and
severity of the damages. Several ‘quick’ algorithms have been proposed to locate and
quantify damages in simple structures (mostly beams) from the measured natural frequency
and mode shape data. The change in curvature mode shape method [12], the change in
stiffness method [13], the change in flexibility method [14] and the damage index method are
some of the algorithms in this category, to name a few. The main drawback of the global
dynamic techniques is that they rely on relatively small number of first few structural modes,
which, being global in character, are not sensitive enough to be affected by localized. It could
be possible that damage large enough to be detected might already be detrimental to the
health of the structure. Another limitation of these techniques is that owing to low frequency,
typically less than 100 Hz, the measurement data is prone to contamination by ambient noise,
which too happens to be in the low frequency range. Recently, Shanker et. al. [15] integrated
global vibration technique with EMI technique. The benefit of global technique is especially
felt when damage grows to moderate/severe levels.
EXPERIMENTAL SETUP
In order to perform modal analysis of a structure using global vibration technique, a model
RC beam was cast and embedded with PZT sensors were installed at equal intervals. The
beam was cast in such a way that it could undergo repeatable damages of different severity
levels. The data was acquired simultaneously from all the sensors via eight channel data
acquisition system. The low-cost EMI technique requires a function generator, digital
multimeter and a resistive circuit [11]. The complete experimental set up for global vibration
technique and low-cost EMI technique is shown in Fig. 2. CVS are ready to use packed
sensors designed for dynamic response measurement of especially RC structures such as
bridges and buildings, developed at the Smart Structures and Dynamics Laboratory (SSDL)
[16] of IIT Delhi. These sensors were cast by a proprietary technique and embedding the
sensor into the concrete. A layer of seven sensors at both top and bottom was embedded in
the beam. Two sensors (one in top layer and other in bottom layer) were joined in series and a
connector was attached to them for data acquisition. Fig. 3 shows the location of the seven
sensors in plan embedded in the beam and also the embedded PZT sensor, supporting system
and connection plates for varying damage severity levels. These sensors provide excellent
signal to noise ratio, higher longevity, and negligible decay of the sensing element and cost
benefit.

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Figure 2: Experimental set-up for (a) global vibration technique (b) low-cost EMI technique.
Embedded PZT Patches
Supporting
System
Location of Seven Embedded PZT Patches in RC Beam
1 2 3 4 5 6 7
Embedded PZT Patch
Connector
Connection plates
to creat e damage
Figure 3: Embedded PZT Patches in RC, supporting system and connection plates for
varying damage severity levels.
Concrete Beam
The beam was casted with M-30 concrete in such a way that repeatable damage can be
produced. During casting, one of the sensors (of a single pair) was kept at the bottom and side
plates were bolted inside the mould used for casting. After casting, the wheels were attached
Personal
Computer
Digital Multimeter
Function Generator
Resistive Circuit
Eight channel data
acquisition system
(QDA1008)
Personal
Computer
Connection
Impact Point
Concrete Beam
(b)(a)
Concrete Be
am

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on top. All sensors (of the same pair) were embedded. Fig.4 shows the beam set up before
casting and Fig.5 shows the set up after casting. After casting the beam was inverted and
joined together through the plates attached at the ends.
Figure 4: Concrete beam setup before casting.
Figure 5: Concrete beam setup after casting.
Data Acquisition System
An eight channel data acquisition system (QDA1008) manufactured by Quazar Technologies
[17] was used for data acquisition and is shown in Fig. 6. The data was collected at an
interval of 10 milliseconds and the duration of the data acquisition was kept 1 second. The
acquired response from QDA1008 in time domain was converted to frequency domain by
performing FFT in the MATLAB. It has eight (multiplexed) analog inputs with a sampling
frequency of 1kHz/channel and sampling jitter of 50μSec with a ADC-resolution of 16-bit.
Figure 6: Eight channel data acquisition system (QDA1008).
7 Embedded sensors Connectio
n Plate
Wooden Ply
for Separation
Connectio
n Plate
Connection
Plate
Beam
Reinforcement
Wheels
7 Embedded Sensors each in
Top and Bottom layer
Wooden Ply for
Separation
Connection Plate Connection Plate

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EXPERIMENTAL OBSEVATIONS
The beam was hit at the centre of the beam using a hammer and the sensors were attached to
the data acquisition system. So, by hitting once, the voltage response of all the sensors was
taken simultaneously in time domain.
In the undamaged stage the bolts were tightened to the maximum. The voltage
responses of all the sensors were acquired in time domain. Using MATLAB, the time domain
data was converted to voltage response in frequency domain using Fast Fourier
Transformation (FFT). The absolute values of response of all the sensors were considered in
order to plot the first mode shape of the beam.
A damaged beam was created by loosening the bolts of connection plates and hence
introducing a controlled damage. The methodology of plotting the mode shape remained the
same as followed for the undamaged beam.
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Figure 7: FFT plot for undamaged beam. Figure 8: FFT plot for damaged beam.
The natural frequency of the beam was determined from FFT plots of the responses
and compared for damaged and undamaged stage (Fig. 7 and Fig. 8) which clearly shows that
it has reduced from 27 Hz (undamaged stage) to 19 Hz (damaged stage). In order to identify
the location of damage in the beam, it was divided into eight sections formed by seven sensor
locations. The ordinate values of each section between ith and (i+1)th sensor was calculated by
(
)
(
)
2
11
U
i
D
i
U
i
D
iVVVV
Y++ −+−
= (2)
where, U
i
V is the FFT amplitude of the ith sensor in undamaged stage and D
i
V is the FFT
amplitude of the ith sensor in damaged stage at the first natural frequency. Fig.9 shows a plot
of Y depicting the position of damage in the beam. It clearly shows that the damage is
between the sensor location 4 and 5, which is actually true
Frequency (Hz) Frequency (Hz)
FFT Voltage
FFT Voltage
Peak at
27Hz
Sensor 4 Peak at
19Hz
Sensor 4

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Figure 9: Damage location identification.
The low-cost EMI technique was also applied using Agilent 34411A multimeter [18]
to find out the damaged location. The admittance value of the damaged and the undamaged
beam was measured and the RMSD value was calculated using Eq. (1) to determine the
damage location. Fig. 10 shows the RMSD values of all the sensors. RMSD value is largest
(2.69%) for PZT patch no. 5 which was nearest to the damage location.
Figure 10: RMSD values for sensors
CONCLUSIONS
In this paper, the embedded PZT sensors in coupled form have been demonstrated
successfully for SHM. CVS sensors can be used in global vibration technique as well as EMI
technique simultaneously. Embedded PZT sensors are very cost-effective with no
compromise with the resolution. Both RMSD and curvature based index are able to provide
accurate location of the damage. Whereas, the low-cost EMI technique is suitable for
incipient damage, global technique is beneficial when damage grows to moderate/severe
levels.
Damage Section
Y

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