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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.