Ultrasonic Pulse Velocity Gonzalo Cetrangolo

8/10/2019 Ultrasonic Pulse Velocity Gonzalo Cetrangolo

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Non-contact Ultrasonic PulseVelocity Method for Concrete

Gonzalo P. Cetrangolo

University of Illinois at Urbana-Champaign

March, 2006

CEE 498 Prof. Daniel Kuchma


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Outline

Introduction

Stress wave propagation and Methods for Structure

Maximize Energy transferQuarter wavelength matching layer

Experimental results through air

Experimental results through concrete

Conclusions


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Stress-Wave propagation in solidhalf-space elastic media

P-wave: longitudinal waves propagate parallel to thepropagation direction:

VL=[E(1-n)/(r(1+n)(1-2n))]

S-wave: shear waves propagate perpendicular to the

propagation direction: Vs= [G/r]

R-wave: surface waves propagation only near surface

(half space): VR/Vs=(0.87+1.12n)/(1+n)

E:Youngs Modulus

r: Mass density

n: Poissons ratio

G:Shear Modulus

r: Mass density

n: Poissons ratio

Stress waves occur when pressure or deformation is appliedsuddenly (such as by an impact source)


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Stress-Wave Methods for structures

Ultrasound through thicknessTime of flight

Ultrasound echo-method

Time of flight

Spectral Analysis of Surface Waves

Dispersion curves

Impact Echo method

Resonance frequency


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Ultrasonic Methods

Direct Configuration Indirect Configuration

The timer shows the time of

flight of the direct throughthickness P-wave arrival

VL= Travel path / time of fight

Pulse Velocity Method Pulse echo Method

Arrival time of a stresswave reflected from adefect (difference in

Acoustic Impedance) isdetermined

In both cases, transducers are coupled to the surface using a

viscous material such as grease.

ACI document 228-1R

ACI document 228-1R


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Ultrasonic Pulse Velocity

UPV has been used to asses the quality ofthe concrete successfully for several years

Identify areas of concrete damage

Locate defects and voids Locate delaminations

Estimate in place-strength by using

calibration curve Coupling agent is needed between

transducers and specimen


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Benefits of Air coupled

Coupling agent between the transducersand concrete surface are time and laborintensive

Results variability caused by difference inthe pressure use to keep the transducerclamp to the surface

Allow to test concrete having rough surface

Allow faster inspection


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Product information

The Ultran Group is the only producer ofNon-contact transducers for throughthickness measurements

The only technical information available ofthese transducers is in their own webpage

The cost of the equipment is U$S 20,000,which include a field computer, transducersset and a pulser unit.


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Air-Concrete reflection/transmission

R= Ar =Ai

Z2-Z1Z1+Z2

T=1+R =

Material 1 Material 2

Ai At

Ar

Normal incidence

2*Z2Z1+Z2

Z1: specific Acoustic Impedance of material 1

R: ratio of sound pressure of the reflected wave tothe sound pressure of the incident wave

Z2: specific Acoustic Impedance of material 2

Ai: Amplitude of the incident wave

Ar: Amplitude of the reflected wave

Ar: Amplitude of the transmitted wave

T: ratio of sound pressure of the transmitted wave to the

sound pressure of the incident wave

Air-Concrete interface

Zair= 0.4 kg/(m2s)Zconcrete = 9*106kg/(m2s)

R=0.9999999

T=0.0000001


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Maximize wave energy transfer

4_ * PZTairvalueltheoretica ZZZ

Wave energy transfer between two distinct media is maximizedwhen an appropriate intermediate impedance matching layer is inserted.

Vibrating Crystal

Matching Layer

Air

Waves inphase

Select and attach the optimal impedance matching layer between thevibrating piezoelectric crystal in the UPV transducer (PZT-4) and air

Quarterwavelength Matching layer

r*L

VZ


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Experimental Setup

A pair of 24 kHz modified transducers with matching layer caps

Pulser unit (UPV Pulser from James Instruments)

Oscilloscope LeCroy LT344

Matching layer caps

Oscilloscope

LeCroy LT344

Air Gap

Pulser unit

Sender Receiver


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Experimental Results through air

Unmodified contact transducer set

Time domain signal through air

Amplitude

Amplitude(Volts)

Time(ms)

FFT19.5 kHz

Time(ms)

Time domain signal through air

Amplitude(Volts)

AmplitudeFFT

19.5 kHz

Modified transducer set with matching layer

Frequency (Hz)

Frequency (Hz)


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Experimental Results through air

0

50

100

150

200

250

0 9 18 27 36 45

Amplitude(FFT)

Air gap distance (mm)

0

30

60

90

0 9 18 27 36 45Air gap distance (mm)

Unmodified transducer set

Amplitude(FFT)

Modified transducer set

Signals through different air gaps

The original transducers provide a

narrow-band ultrasonic pulse at a

center frequency of 19.5 kHz.

The transducer has a 38 mmdiameter face, giving a near-field

distance in air of 16 mm.

Maximum received Amplitude at

9mm distance betweentransducers face.

Local maximum every 9mm.


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Behavior at 9 mm air gap?

V air = 344 m/s f PZTcrystal= 19.5kHz

V air= 2 * D air gap* fPZT-4 crystal

D air gap= V air/ (2* fPZT-4 crystal)

D air gap= 9 mm

Resonance at 9 mm, 18 mm, 27 mm, 36 mm,

D = n * l/2 n = 1, 2, 3

Impedance matching layers provide 300% increase in receivedsignal amplitude through air.

Air gap resonance matches transducer crystal resonance


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Measurement through concrete

7.2 cm of air gap

Foam Foam

5.4 cm of concrete

9mm air gap

Foam

Foam

Signal through air

Signal through concrete

V concrete= d concrete/ (time concrete- time air+dconcrete/ V air)

Experimental setup (air)

Experimental setup (concrete)


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Conclusions

Impedance matching layers provide 300% increase inreceived signal amplitude through air

Impedance matching layers and 9 mm air gap between the

modified transducers and the sample enable contact-lessthrough-thickness P-wave Velocity measurements inconcrete specimens up to 10 cm thick showing similarbehavior as standard full contact UPV

Averaging of the received signal increase the SNR


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References

[1] ACI Committee 228 (2003). In-place methods to estimate concrete

strength (ACI 228.1R-03). American Concrete Institute, FarmingtonHills, Mich.

[2] Berriman, J., Purnell, P., Hutchins, D.A. and Neild, A.(2005).Humidity and Aggregate Content Correction Factors for Air-coupled Ultrasonic Evaluation of Concrete Ultrasonics 43 (4) (211-217).

[3] Gomez Alvarez-Arenas, Tomas E. (2004). Acoustic ImpedanceMatching of Piezoelectric Transducers to the Air, IEEE

Transactions on Ultrasonics, Ferroelectrics, And Frequency Control51 (5) (624-633) .

[4] Krautkramer, J. and Krautkramer, H. (1990). Ultrasonic Testing of

Materials, fourth ed., New York, Springer-Verlag.[5] Lide, David R., ed.(1998-1999). CRC Handbook of Chemistry and

Physics, 79th ed., Cleveland, Ohio.

Ultrasonic Pulse Velocity Gonzalo Cetrangolo

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