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New Technologies for New Technologies for NDT of Concrete NDT of Concrete

Pavement StructuresPavement StructuresJohn S. PopovicsJohn S. Popovics

Department of Civil & Environmental Engineering

CEAT Seminar SeriesCEAT Seminar SeriesSeptember 8, 2005September 8, 2005

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Outline

• Motivation & background

• Current NDE techniques/applications

• New Research Directions (UIUC)

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Motivation

• US infrastructure is deteriorating: 2005 ASCE Report card for American infrastructure gave an overall grade of “D+” – estimated $1.3 trillion investment needed for improvements

• Increased use of performance-based specificationsrequire accurate in-place estimates of newpavement thickness and strength

A need for structural/pavement NDE

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Current NDE Techniques

Concrete structures and pavements

Impact-echo, GPR (RADAR), thermography

sounding/tapping, UPV and velocity tomography, electro-chemical techniques, radiography, modal analysis, acoustic emission, impulse-response, etc.

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Impact-Echo (ASTM C1383)

Propagating P-wavesgenerated by impact event. Multiply-reflectedwaves are detected bysurface sensor.

Reflected waves setup a resonancecondition having acharacteristic frequency

Analogous to a bell’s tone

FFT

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Impact-echo AnalysisThe resonant

frequency (at thepeak) is related to

distance to reflector (d or d*) and

wave velocity (VL):

f = VL/(2 d)

Thus,

d = VL/(2 f)

Reflection from slab bottom

Reflection from delamination

is a correction factorfor the shape of the element. = 0.96 for slabs

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GPR (ASTM D4748)

Wave pulses are reflectedat interfaces having

a difference in electrical properties (r )

Reflected pulses (timeand amplitude) are

monitored in thetime domain signal

antennaair: r = 1

concrete: r = 6 to 11

soil: r = 2 to 10

(water: r = 80; metal r = infinite)

(ground penetrating RADAR)

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Infra-red ThermographyMonitoring heat flow by surface temperature

Sub-surface defects disrupt heat flow. If defect near near surface, surface temperature is affected.

Temp 2

Temp 1

Air-filled voidwhere T2 < T1

heat flow(conduction)

Heat flow must be established, but direction of flow does not matter

Driven by thermal gradient

warmer zonecooler zone

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24

25

Heat flux sensorSurface

thermocouple

Surface

thermocouple

24

25

Heat flux sensorSurface

thermocouple

Surface

thermocouple

25 °C

24 °C

Flaw #5 Flaw #6 Flaw #8

25 mm

1 2 3 4

5 6 7 8

114 mm 127 mm 127 mm 127 mm 114 mm

25 mm

25 mm

25 mm

25 mm

25 mm

38 mm

38 mm

38 mm

38 mm

51 mm

25 mm 25 mm 25 mm

concrete

bonded FRP sheet

disbonds

Thermograph(disbonds are

hot spots)

Thermography Results: FRP Bond

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New Research Directions (UIUC)

• New pavement Q/A assurance work

Accurate thickness and in situ strengthestimation for new concrete pavements

• Contact-less (air-coupled) pavement inspection using stress waves

Seismic time domain approach Surface waves

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In-situ Pavement ThicknessMotivation:

* Accurate (5mm) and non-destructive thickness estimates needed for new pavement QC and pay factor application

* Best available method (standard impact-echo) does notprovide needed accuracy

Approaches:Frequency domainTime domain

sensors

x1

h

x2

pavement

wave source

Develop seismic approach Improve impact-echo

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Impact

h

ax

P-wave

S-waveθp

θs

(1-a)x

θpθs

P-wave

S-wave

Receiver

Seismic Approach for Slab Thickness

SPps C

hxa

C

haxt

2222 ))1(()(

s

p

Sin

Sin

Vs

Vp

Arrivals of mode-converted reflections (P-S and PP-SS) of short durationpulses used to back-compute wave velocity and slab thickness

P-S arrival time

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Accelerometers

bb-gun Impactor

Impact positions

Impact sensor

200 250 300 350 400 450 500 550 600 650 700 750 8000.05

0

0.05

Direct P-wavetps

tppss

Time - microseconds

Surface wave

Field Testing Set-up

Field testing setupcomprised of sensedBB-gun and multipleaccelerometer set

Arrivals of mode-converted wavesdetermined in eachsignal; velocityand thicknessthen computed

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Impact-Echo

1980s in NIST and Cornell Effective in determining

thickness of slabs and depth of flaws in plate structures

Does not work on beams & columns

2

0.96 for plate

PCD

f

Targets for improvement

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222

2222

22

22

2

22

)2(

4

)2tan(

)2tan(0

2222

22

222

22

2

22

4

)2(

)2tan(

)2tan(0

Solutions: dispersion curve

Symmetric modes Anti-symmetric modes

Lamb Wave Basis for Impact-Echo

S1-Cg=0

S1-k=0

A1-k=0

Guided waves in free plates

Resonance conditionsrepresented at zero wave number orzero group velocitylocations

Impact-echo?

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0.942

0.946

0.950

0.954

0.958

0.15 0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24 0.25 0.26

Poisson's ratio

S1 Mode ZGV

FEM resonance

β

0 500 1000 1500 2000

Time (microseconds)

Am

plit

ud

e

Experimental

FEM

0 5000 10000 15000 20000Frequency (Hz)

Am

plitu

de

Experimental

FEM

Verification

FEM (ABAQUS) model verified by experiment

Analytical (Lamb) model verified by FEM

Impact-echo frequency

Impact echo frequencyis S1 ZGV

= 0.96

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In-situ Strength EstimationMotivation:

* Accurate and non-destructive in situ concrete strength estimation needed for new pavement QC and pay factors

* Best available methods (rebound hammer, UPV, maturity) do not provide needed accuracy, reliability, or applicability

Approach:

Wave source

Surface waves

d

SensorsUse surface waves: one-sided method

Measure surface wave velocityand transmission (attenuation)and correlate to in situ strength

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Testing Set-upUltrasonic wave source

(200 kHz)

Wave sensors(accelerometers)

Field testing set-up

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Surface Wave Measurements

-1.5

-1

-0.5

0

0.5

1

1.5

0.00E+00 2.00E-05 4.00E-05 6.00E-05 8.00E-05 1.00E-04

Acceleration signals

t

A

Surface wave transmission: amplitude ratio for far sensor/near sensor

Surface wave velocity: arrival time of far sensor- near sensor

far sensor

near sensor

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In the frequency domain, we can represent wave signal sent by i and received by j (Vij) as

V12=A1d12S2, V13=A1d12d23S3, V43=A4d43S3 and V42=A4d43d32S2,

where Ai and Si are the sending and receiving response functions, dij is the transmission between points i and j.

We aim to isolate and measure d23

Self-calibrating Wave Transmission

1 2 3 4

surface waves

V12 V43

V13 V42d23 = “T”

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Correlation to Concrete Strength

2220

2260

2300

2340

2380

2420

2460

3000 4000 5000 6000 7000

compressive strength, psi

wav

e ve

loci

ty, m

/s

0.46

0.48

0.5

0.52

0.54

0.56

0.58

0.6

0.62

0.64

3000 4000 5000 6000 7000

compressive strength, psiT

ran

smis

sio

n @

hig

h f

req

uen

cies

On-going work: correlation to flexural strength

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Contact-less (air-coupled) inspection

• NDT imaging techniques provide a direct approach for assessment

• Stress-wave based NDT methods are usually less efficient due to coupling problem

• Here we aim to develop effective non-contact NDT techniques for pavements

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Wave Source

Blind zone

Air-coupled Sensing Challenge: Large acoustic impedance mismatch

between air and concrete

Use leaky R-waves?

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Air-Coupled SASW

0 5 10 15 20 250.7

0.8

0.9

1

frequency (kHz)

Co

he

ren

ce

SASW Test ( Impactor: Medium Ball), window=400s

0 5 10 15 20 252000

2100

2200

2300

2400

2500

frequency (kHz)

Ve

loci

ty(m

/s)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.52000

2100

2200

2300

2400

2500

Wavelength(m)

Ve

loci

ty(m

/s)

SASW provides surface wave velocity profiles with depth (layered system) SASW test performed on floor slab (thickness 200mm) Signals show good coherence through 22kHz Rayleigh wave velocity is about 2300m/s

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Air-Coupled MASW Multichannel analysis of surface waves (MASW)

Compared to SASW, MASW can determine phase velocities precisely using whole waveform data

Avoids spatial aliasing Distinguish fundamental mode from higher modes and body waves

Time(s)

dist

ance

(cm

)

Image of response (Mic)

0 0.5 1 1.5 2 2.5 3 3.5

x 10-3

50

100

150

200

250

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Multi-source setup

Multi-sensor setup

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Air-Coupled MASW

Phase velocity (m/s)

Fre

quen

cy (

Hz)

Vidale EB12 run1c (Mic)

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

x 104

MASW 2D spectrum image Test performed on a concrete slab with thickness 200mm, CR=2300m/s

Nils Ryden provided the MASW analysis program

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Imaging surface-opening cracks

Wave propagationdirection

Scanningdirection

surface crack

Wave propagationdirection

Scanningdirection

surface crack

Top layer concrete thickness 180-210mm Concrete R wave velocity VR=2300m/s Microphone height h = 66cm Shadow zone size: 15cm

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Imaging surface-opening cracks

0 20 40 60 80 100 120 140 160

40

60

80

100

120

140

160

180

0 100 2000

0.5

1

Hanningwindow

2ix

1 2 1

2 3 2

/

/

R A A

R A A

0 1 1

1 1 1

2 2 2

/

/

/

m

m

m

R A A

R R R

R R R

energy

ratio

normalization

1 1.5 2 2.5 3 3.5 4-1

0

1

1 1.5 2 2.5 3 3.5 4-1

0

1

1 1.5 2 2.5 3 3.5 4-1

0

1

2 2.5 3 3.5-1

0

1

2 2.5 3 3.5-1

0

1

2 2.5 3 3.5-1

0

1

1

2

3

A

A

A

Raw signals Windowed signals

0 100 2000

0.5

1

Hanningwindow

2ix

1 2 1

2 3 2

/

/

R A A

R A A

0 1 1

1 1 1

2 2 2

/

/

/

m

m

m

R A A

R R R

R R R

energy

ratio

normalization

1 1.5 2 2.5 3 3.5 4-1

0

1

1 1.5 2 2.5 3 3.5 4-1

0

1

1 1.5 2 2.5 3 3.5 4-1

0

1

2 2.5 3 3.5-1

0

1

2 2.5 3 3.5-1

0

1

2 2.5 3 3.5-1

0

1

1

2

3

A

A

A

Raw signals Windowed signals

1-D Y scan

1-D X scan 2-D scan

Energy Ratio

0 20 40 60 80 100 120 140

50

100

150

200

250

0 20 40 60 80 100 120 140

40

60

80

100

120

140

160

180

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Air-Coupled Impact-Echo

Air-coupled sensor Tested in ambient noise condition wit

hout any sound insulation Good agreement with the contact imp

act-echo test result The PCB microphone can determine t

hickness of shallow delaminations

Musical microphone: frequency response 20Hz-20kHz

PCB measurement microphone: 4Hz-80kHz

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Air-Coupled Impact-Echo – Delamination Delamination at depth 60mm Flexual mode at 2.68kHz

strong and easy to detect Impact-echo mode 33.2kHz for delaminati

ons Gives delamination depth 58mm 33.2kHz can be detected by the PCB micropho

ne

5 10 15 20 25 30 35 40 45 50 55

0

5

10

15

20

25

30

35

40

45

50

x(cm)

y(cm

)

3000 4000 5000 6000 7000 8000 9000

Frequency (Hz)

0 1 2 3 4 5 6 7 8 9

x 10-3

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0 5 10 15 20 25 30 35 400

1

2

3

4

5

6slab1_p8_b1w_R0C0_2.dat df=0.24414

f=33.2kHz

Frequency (kHz)

f=2.68kHz

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Summary Non-destructive test methods are needed for concrete pavements.

Many existing NDT methods exist for concrete pavements

New research efforts focus on improving the capability of NDT for pavements, for example for in-place pavement thickness estimation

Surface wave measurements can be carried out on concrete. The self-compensating scheme allows measurement of surface wave signal transmission. Results show correlation to in-place strength.

Contact-free methods have potential for rapid and effective NDT for pavements