MONITORING OF CONCRETE STRUCTURES BY

ELECTRO-MECHANICAL IMPEDANCE TECHNIQUE

Presented By, Dr. S. N. Khante

Associate Professorand

Bhagyashri D. Sangai

APPLIED MECHANICS DEPARTMENT GOVT. COLLEGE OF ENGINEERING

AMRAVATI (An Autonomous Institute of Govt. of

Maharashtra)

Combination of : Foundations, Basements, Columns, Beams, Slabs etc.

Damages are bound to occur due to: Mismanagement in construction, Lack of quality in control, Temperature conditions, Loading and aggressive environment etc.

Damages such as surface cracks, holes, segregation, settlements etc.

Adversely affects the performance of the structure.

What is a Structure ?

Damages in Structure

Collapse of Bridge on Mississippi River

Structural health monitoring is the continuous monitoring of structures using integrated or applied sensor.

Aim at assuring structural integrity and periodic inspections to detect damages resulting from fatigue, corrosion, excessive loads impact , etc.

Health monitoring by smart materials is one of the best possible solution.

Smart materials involve Piezo-electric materials, Optical Fibers, Shape-Memory Alloys etc.

The primary reasons to use smart materials in Structural Health Monitoring (SHM) systems is due to the feasibility in host structures; which can act as sensors and/or actuators to present the health of a structure on continuous basis.

What is Structural Health Monitoring

Structure Sensors Data acquisition systems Data transfer and storage mechanism Data management Data interpretation and diagnosis:a)System Identification c)Structural condition assessmentb)Structural model update d)Prediction of remaining service life

SHM System's Elements

In broad sense SHM methodologies can be classified as Conventional technique and Techniques using smart materials.

Conventional Techniques Global SHM technique Local SHM technique

Global SHM technique1. Global Dynamic Technique2. Global Static Technique Local SHM technique

Techniques such as acoustic, ultrasonic,radiography, eddy currents, thermal, magnetic field or electro-magnetic impedence, etc.

Techniques in SHM

Techniques Using “Smart” Materials Low frequency technique High frequency or Electro-Mechanical Impedance (EMI)

technique Low frequency technique Also known as vibration based methods as vibrations are given to

the host structure under consideration. PZT patch perform only the role of sensor and not an actuator. Less sensible to damages as there are noise problems.

High frequency technique Also known as Electro-Mechanical Impedance (EMI) technique. PZT patch plays dual role as a sensor and as an actuator. More sensible to damages in the structures as there are no noise

nuisance

Piezoelectricity means “Electricity from pressure”.

Mechanical energy Electrical energy and vice versa.

Can be applied as a Mechatronic Impedance Transducers (MIT) Piezoelectric ceramics (Lead Ziroconate Titanate, or in short, PZT) and piezoelectric polymers (Polyvinylidene Fluoride, denoted as PVDF) are commonly used.

Piezoelectric Patch

Piezoelectricity and Piezoelectric Materials

Short film for Piezo Generator

Fig. : Modeling PZT-structure interaction

(a) A PZT patch bonded to structure under electric excitation(b) Interaction model of PZT patch and host structure

Basics of EMI technique

PZT patch is surface bonded or embedded inside the structures. When an alternating electric field is applied to the patch, it

expands and contracts dynamically in direction ‘1’. Hence, two end points of the patch can be assumed to encounter equal impedance Z from the host structure.

The patch (length 2l, width w and thickness h) behaves as thin bar undergoing axial vibration. The complex electro-mechanical admittance Y of the coupled system is derived as

Where, d31 is the piezoelectric strain coefficient of the PZT material, ȲE is the complex Young’s modules under constant electric field, (εɛ33)T is the complex electric permittivity at constant stress, Za is mechanical impedance of the PZT patch, ω is angular frequency and kl is the wave number.

Basics of EMI technique

Piezoelectric material : Sensor and actuator application Specimen under consideration : 2 Reinforced Concrete beams

(100X100X500 mm) with M20 grade concrete and Fe 250 steel. Reinforcement provided was 2 x 6 dia at top and bottom along

with stirrups of diameter 6 dia @ 80 mm c/c. Experimental study was performed in two phases – Healthy and

various damaged conductance and susceptance signatures are taken for each specimens.

LCR meter with connecting fixture, and VEE PRO software for data acquisition.

Frequency range 100 kHz – 250 kHz Material PZT 5H sample, Co-axial cable and specimens. Epoxy adhesive Araldite.

Materials and Specimens

Photograph: PZT 5-H sample

Photograph: LCR meter with the connecting fixture.

Photograph: Co-axial cable

Firstly, the PZT patch was soldered out through two electrodes present on the patch, then this patch was bonded on to the surface of specimen using a well-tested epoxy adhesive.

In this case Araldite adhesive has been used as the sensible frequency range of the PZT patch is high and the PZT patch had dual roles i.e. sensor and actuator functions.

Then, the soldered PZT patch was wired to Impedance Analyzer through Connecting Fixture of LCR meter. Then the frequency was swept through 100 kHz to 250 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 parameters i.e. Conductance (G) and Susceptance (B) are directly measured through LCR meter for all the values of frequency.

Experimental set-up

Fig. : Assembly for Experimentation of EMI technique This data is transferred to personal computer by USB cable. The

data is processed and then the graphs of Conductance Vs Frequency and Susceptance Vs Frequency are directly plotted in VEE PRO 9.32 software. These graphs are said to be the “Conductance Signature” or just a “signature” and “Susceptance Signature” for that particular specimen.

The VEE PRO 9.32 is Graphical programming language for test and measurement applications.

Experimental Set-up

Beam 1: Damage 1- crack 72 mm deep, Damage 2 – scraping 1 nearer to right support i.e. near PZT, Damage 3 – scraping 2 nearer to left support i.e. some distance away from PZT. The cracks were given by Universal Testing Machine (UTM) under flexure for all beams.

Fig. : Integral beam 1 Fig. : Damaged beam 1

Specimen and Damage details

Beam 2: The patch PZT-1 at 150 mm and PZT-2 at 450 mm from right support. The damages introduced were, Damage 1- crack 1: 32 mm deep, Damage 2 – crack 2: 60 mm deep, Damage 3 – scraping 1 nearer to right support i.e. near PZT 1, Damage 4 – scraping 2 nearer to left support i.e. near PZT 2, Damage 5 – scraping 3 between the two PZT’s.

Fig. : Healthy beam 2 Fig. : Damaged beam 2

The PZT patch was subjected to frequency range of 100-250 kHz. For this beam, the two PZT patches were employed and the responses were recorded for all the stages by impedance analyzer for both the PZT’s. PZT patch 1 peaks are more prominent and also change in signature with changes in damage level are categorically visible when compared to PZT patch 2.

Results for Beam 1

Response peaks for PZT1 are well defined even after introduction of damages.

Results for Beam 2

BEAM 1: The PZT is sensible for all the damages while, PZT2 being less sensible.

Positioning Study

BEAM 2: It is clear from the results that both PZT1 and PZT2 follow the same pattern for all the damages, while PZT 1 results are more effective because there are occurrences of peaks and little vertical shifting of conductance values for the respective damages.

Positioning Study

It can be said from positioning study that, PZT patch plays an important role in damage detection if it is placed nearer to damages

Piezoelectric material (PZT 5H grade) being smart material is sensible at high frequencies to damages occurring in the structures. Hence EMI technique can be extensively accepted for structural health monitoring systems in the form of sensor and actuator.

It can be concluded that, PZT patch is location sensitive as it shows higher damage sensitivity when positioned near the damage zone.

Conclusion

Bhalla, S, Soh, C. (a) (2004), “High frequency piezoelectric signatures for diagnosis of seismic blast induced structural damages”, Journal of NDT and E, Vol. 37, pp. 23:33.

Bhalla, S. and Soh, C. (b) (2004), “Structural Health Monitoring by Piezo Impedance Transducers. I and II: Modeling and Applications”, Journal of Aerospace Engineering, Vol.17, No. 4, pp. 154:175.

Annamdas, V., Yang, Y., Soh, C. (2010), “Impedance Based Concrete Monitoring Using Embedded PZT Sensors”, International Journal Of Civil And Structural Engineering, Volume 1, No 3, 2010 pp. 414:424.

Shanker, R. (2009), An Integrated Approach For Structural Health Monitoring, Indian Institute Of Technology, Delhi.

REFERENCES

Park, S., Inman, D., Lee, J., and Yun, C. (2009), “Reference-Free Crack Detection Using Transfer Impedances”, Journal of Sound and Vibration, Vol. 329, pp. 2337:2348.

Kim, M., Kim, E., Yan, Y., Park, H., Sohn, H. (2011), “Reference-Free Impedance-Based Crack Detection In Plates, Journal Of Sound And Vibration”, Vol. 330, pp. 5949:5962.

Du, G., Kong, Q.,Timothy, L., and Song, G. (2013), “Feasibility Study On Crack Detection of Pipelines Using piezoceramic Transducers”, International Journal of Distributed Sensor Networks, pp.1-7

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