Computational Methods for Smart Structures and Materials, C.A. Brebbia & A. Samartin (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-816-3
Health monitoring of civil infrastructure using
smart piezoceramic transducer patches
K. K.-H. Tseng', C. K. Soh% A. Gupta^ & S. Bhalla^
^ School of Civil & Structural Engineering,
Nanyang Technological University, Singapore.
'Department of Civil Engineering
Indian Institute of Technology, Delhi, India.
Abstract
In recent years, structural health monitoring has become an important
requirement in Civil Engineering, especially in the densely populated areas such
as many Asian cities. Various monitoring techniques have been proposed and
studied. This paper will focus on the application of the piezoceramic transducer
(PZT) patches on the health monitoring of civil infrastructure. The PZT patches,
attached to the surface of the structure, are electrically excited, and real part of
electrical admittance (conductance) is extracted as a function of the excitation
frequency. An Impedance Analyzer is used to scan the patches over a certain
range of frequency for the acquisition of the signature. The deviation of the
signature from that recorded for the healthy state provides an indicator for the
health of the structure. This technique is non-destructive in nature and the wide
range of the excitation frequency (from a few Hz to a few MHz) enables this
technique to capture structural damage from small to large scale. Specifically,
the results of a health monitoring study, carried out during the destructive load
testing of a prototype RC bridge, will be presented. The bridge was instrumented
with the PZT patches, which were excited at high frequencies, of the order of
kHz. The signatures of the patches located in the vicinity of the damages were
found to have undergone drastic changes, while those farther away were only
marginally affected, thus confirming the damage localization capability of the
technique for large concrete structures. The damages were quantified in nonparametric terms using root mean square deviation in signatures with respect to
the baseline signature of the healthy state.
Computational Methods for Smart Structures and Materials, C.A. Brebbia & A. Samartin (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-816-3
154
1
Computational Methods for Smart Structures and Materials II
Introduction
Regular health monitoring of civil infrastructures is of considerable importance,
in view of the immense loss of life and property that may result from their failure.
In the present study, use of surface bonded smart piezoceramic patches (PZT),
was investigated for the purpose of health monitoring during the destructive load
testing of a prototype RC bridge structure. Use of these self-sensing smart
patches on a lab sized steel truss structure was reported by Sun [4], and later, on a
prototype truss joint by Ayres[l]. In the knowledge of the authors, till date no
study has been reported to assess the damage detection ability of these smart
transducers on any prototype RC structure.
Application of the PZT patches for health monitoring is based on the principle
of electro-mechanical coupling between the bonded patch and the local structural
system in its vicinity (Giurgiutiu[2]). Sensitivity to small local damage can be
significantly increased by selecting a high frequency range of the order of a few
kHz to a few hundred kHz (Sun[4]).
2
Details of the prototype RC bridge specimen and its
instrumentation
The test structure was a single-span girder bridge make of reinforced concrete
with an effective span of 4.85m. It consisted of two longitudinal beams with
250mm in depth, which supported a deck slab of 100mm in thickness. A layout
of the bridge is shown in Fig. 1. It was constructed by the second year
undergraduate students, of the School of Civil and Structural Engineering
(CSE), Nanyang Technological University (NTU), Singapore, as a part of their
1999 In House Practical Training (IHPT 1999). Many research groups were
involved in this testing, each one monitoring the bridge with its own system of
sensors and technique. This paper only covers the details of monitoring using
PZT sensors.
The test structure was instrumented with 5 PZT patches, each of size 10mm by
10mm with 0.2mm in thickness, and attached to the structure at the locations on
the structure as shown in Fig. 1. The patches were manufactured by Pi-Ceramic,
their product designation being PI-151 (refer to product catalogue, PI Ceramic).
The patches were bonded to the concrete surface using PS adhesive
(manufactured by Tokyo Sokki Kenkyujo Co. Ltd.).
This spatial distribution of the PZT patches as shown in Fig. 1 was chosen in
order to study the effect of various locations on the sensitivity and effectiveness
of monitoring the condition of the structure. Patches 1, 2, 3 and 4 were located in
the high stress zone, a more probable location of damage initiation, while patch 5
was located at a low stress zone. Also, patches 1 and 2 were located on the
tensile stress face while the patches 3 and 4 were located on the compressive face.
Computational Methods for Smart Structures and Materials, C.A. Brebbia & A. Samartin (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-816-3
Computational Methods for Smart Structures and Materials II
1
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Fig.l: Layout and instrumentation details of the test bridge. (All dimensions
are in mm).
(a) Plan of the test bridge showing the PZT patches, (no. 1 to 5).
(b) Elevation of the test bridge showing the PZT patches (no. 1 to 5).
(c) Complete model of the bridge.
3
Experimental procedure
The experimental bridge was subjected to three load cycles, referred to as cycles
I, II, and III in this paper. The purpose of cycle I was to load the structure to the
serviceability limit followed by unloading. Cycles II and III were aimed at
reloading the structure to much higher loads, representative of the ultimate limit
of collapse, followed by unloading. Table 1 shows the loading sequence and the
Computational Methods for Smart Structures and Materials, C.A. Brebbia & A. Samartin (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-816-3
156
Computational Methods for Smart Structures and Materials II
intermediate stages at which the PZT patches were scanned for signature
acquisition.
Table 2 shows a list of frequency range for each patch, which was selected
based on a few pre-experimental trial scans. During this initial trial scanning,
patch 3 showed somewhat inconsistent and unstable signatures, indicative of
improper bonding with the host structure, and was therefore discarded. Also, the
measurements could not be recorded from patch 5 during loading cycles I and II
because of the accidental breakdown of connections. It was replaced after cycle
II and measurements could only be made from this patch during cycle III alone.
The test structure was loaded using a large-size loading frame, as shown in
Fig. 2 at the Construction Laboratory in the School of CSE. Load was
incremented in steps of 5kN using a manually operated system. The signatures of
the PZT patches were acquired using an HP 4192A Impedance Analyzer.
4
Damage quantification
In this study, Root Mean Square Deviation (RMSD), similar to that defined by
Giurgiutiu[3], but using conductance in place of real impedance, has been used to
quantify the changes in the signatures due to damage. It is defined as
RMSD(%) =
xlOO
where G] is the post damage conductance at the j* measurement point and
is the corresponding pre-damage value.
Fig 3: Test bridge (right one) ready to undergo loading.
Computational Methods for Smart Structures and Materials, C.A. Brebbia & A. Samartin (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-816-3
Computational Methods for Smart Structures and Materials II
157
Table 1: Test structure loading and signature acquisition sequence.
CYCLE
I
II
III
TEMPE-RATURE
DESCRIPTION
* Baseline signatures of the patches 1,2, and 4 acquired.
* Specimen loaded up to 35kN and unloaded.
31"C
* Signatures of the PZT patches 1,2, and 4 acquired.
* Specimen reloaded up to 40kN and unloaded.
31"C
* Specimen reloaded up to 51kN and unloaded.
* Signatures of the PZT patches 1,2 and 4 acquired.
* New baseline signatures of patches 1,2,4 and 5 acquired.
* Specimen reloaded in steps up to 60kN and unloaded.
29»C
* Signatures of the PZT patches 1,2,4 and 5 acquired.
Table 2: Frequency range selected for signature acquisition
of various PZT patches.
PATCH
MARKED
1
2
3
4
5
5
FREQUENCY RANGE (kHz)
90-115
100-125
DISCARDED
40-65
80-100
Experimental results
5.1 Signature Deviations of patches 1, 2, and 4 after cycles I and II
Fig.3 shows the load vs deflection curve for the mid-point C (see Fig. 1) during
cycles I and II. The top and bottom rebars of the beams were instrumented with
resistance strain gauges. Under the assumption that plain sections remain plain,
strains were computed at the top and bottom concrete fibres at the mid span.
During cycle I, the maximum top compressive strain at the mid-span was worked
out to be 0.000197 and the corresponding bottom tensile strain was computed to
be 0.00124. After cycle I, fine hairline cracks could be seen emerging on the
bottom surface of the beams, near the middle span. During cycle II, the cracks
initiated earlier further widened, and many new cracks also appeared. Peak
compressive and tensile strains during cycle II were 0.000868 and 0.000581,
respectively. At this stage, changes were observed in the signatures of patches 1,
2, and 4 as can be seen in Fig. 5. Fig. 6 shows the root mean square deviations of
the signatures of these patches after loading cycles I and II. The relatively higher
value of RMSD for patches 1 and 2 as compared to patch 4 (very marginal in the
Computational Methods for Smart Structures and Materials, C.A. Brebbia & A. Samartin (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-816-3
Conductance (S)
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Conductance (S)
Load (KN)
Computational Methods for Smart Structures and Materials, C.A. Brebbia & A. Samartin (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-816-3
Computational Methods for Smart Structures and Materials 11
20
.
159
16.1
12
CD
O
CO
cc
gq
After cycle I
After cycle II
PZT PATCH
Fig. 5: Root Mean Square Deviations (RMSD,%) of patches 1, 2, and 4
after cycles I and II.
5.2 Signature Deviations of patches l,2,4,and 5 after cycle III
During this cycle, the temperature conditions were slightly different from those
during cycles I and II. Moreover, patch 5, which was unserviceable during cycles
I and II, was reinstalled. Therefore, it was decided to recalibrate the PZT patches,
and to acquire fresh baseline signatures. Fig. 6 shows the load v.s. deflection
curve for the mid-point C (See Fig. 1). The strain in the concrete at the top and
bottom of the bridge during loading cycle III were calculated to be 0.000748 and
0.00493, respectively. The probable reason for the smaller values on these strains
as compared to that of cycle II could be a possible slip between concrete and the
rebars where the strain gauges were attached to.
Signatures of these patches at various stages during cycle III are shown in Fig.
7. Fig. 8 shows the variation of the root mean square deviation of the signatures
from the patches with load. The most significant observation during cycle III is
the variation in the signature of patch 1 with load. Just prior to the ultimate load,
it is observed that there is a very substantial amount of deviation in patch 1 (See
Fig. 8). Patch 2 shows a similar trend, though not as substantial as patch 1, and a
higher order of magnitude of the deviations at a much earlier stage. Patch 4 does
not show much of change. Probable reason could be that it was bonded to the
compression face, where the strain level was not high enough to cause any
damage other than micro-level cracks. Patch 5, which was bonded in a less
critical region, does not undergo much change in the signature, and the resulting
RJVISD deviations are comparatively of low order of magnitude.
80
60
40
T3
05 20
O
0
0
20 40 60 80
Displacement(mm)
Fig. 6: Load Vs deflection at the loading point (mid span) during Cycle III
Computational Methods for Smart Structures and Materials, C.A. Brebbia & A. Samartin (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-816-3
Computational Methods for Smart Structures and Materials II
160
PATCH 1
PATCH 2
0.0003
CO 0.00026
0)
c 0.00022
o
03
"O
90
95 100 105 110 115
Frequency(kHz)
Frequency (kHz)
PATCH 4
1.40E-04
40
45
50
55
60
Frequency (kHz)
Load=OkN
Load=10kN
Load=20kN
I
• 0.00018
o
O
0.00014
100 105 110 115 120 125
65
0.00027 r
0.00025
CO
0.00023
I 0.00021
I
o
3 0.00019
"O
O 0.00017
O
0.00015
80
PATCH 5
85
90
95
100
Frequency(KHz)
- Load=40kN
_ Load=49kN
- Load=55kN
- Load=58kN
Fig.7: Signatures of patches 1, 2, 4, and 5 at various loading stages during cycle III.
6
Conclusions
The results of the present health monitoring and damage detection study, carried
out on a prototype RC bridge instrumented with smart piezoceramic patches
(PZT), clearly demonstrate that these patches are smart enough in detecting
damages in RC structures at the very initial stage. In addition, the feasibility of
using PZT transducer patches for monitoring the health of large RC civilinfrastructures is well demonstrated. The conductance signatures of the surface
mounted PZT patches are very sensitive to the development of surface cracks, in
their local vicinity, and are insensitive to those farther away. Thus the damage
location can be easily identified using a distributed array of PZT on the structure.
Computational Methods for Smart Structures and Materials, C.A. Brebbia & A. Samartin (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-816-3
Computational Methods for Smart Structures and Materials II
161
In order to construct a sensitive monitoring system comprising of PZT patches,
the patches should be surface-bonded near the probable tensile crack locations.
Such suitable locations for bonding the patches can be easily determined from the
geometry and loading conditions to which the structure is likely to be subjected to
during the course of its service.
PATCH 2
PATCH 1
25
14
12
10
6
4
2
n4
20
15
JIT'
A
+
A V4
CO 10
CC 5
04
20
40
Load (KN)
60
20
40
60
Load (kN)
PATCH 4
PATCH 5
10
8
RMSD(%)
16
6
12
a
(o
cr:
8
2
4
Ow/
0
4
+*
20
40
6
Load (kN)
0
20
40
Load (kN)
60
Fig.8: RMSD(%) of patches 1,2,4, and 5 at various stages during cycle III.
References
[1] Ayres, J.W., Lalande, F., Chaudhry, Z., and Rogers, C.A., Qualitative
Impedance-Based Health Monitoring of Civil Infrastructures. Smart
Materials and Structures, 7(5), pp. 599-605, 1998.
[2] Giurgiutiu, V., and Rogers, C.A., Electromechanical (E/M) Impedance
Method for Structural Health Monitoring and Non-Destructive Evaluation.
Proc. of the International Workshop on Structural Health Monitoring, ed.
F.-K.Chang, Stanford University, Stanford, California, Technomic:
Lancaster, pp. 433-444,1997
Computational Methods for Smart Structures and Materials, C.A. Brebbia & A. Samartin (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-816-3
162
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