Acoustic Emission Monitoring of Conventionally

Reinforced Concrete Highway Bridges Under Service

Conditions

Thomas Schumacher, University of Delaware

Christopher Higgins, Oregon State University

Stephen Lovejoy, Oregon Department of Transportation

presented by

Thomas Schumacher

Assistant Professor

May 18, 2011

Civil & Environmental Engineering

• BACKGROUND

• IN-SERVICE LOAD TEST

• FULL-SCALE LABORATORY TEST

• B-VALUE MONITORING

• COMPARISON

• SUMMARY

• QUESTIONS

1

OVERVIEW

• USA: 27% of bridges structurally

deficient or functionally obsolete

(ASCE Report Card, 2009)

• Deficient design code provisions

• Increasing traffic volume

• Changing environment

• Structural as well as non-structural components affected

• Acoustic Emission (AE) monitoring

- Short-term monitoring � E.g. to support in-service load testing

- Long-term monitoring � Continuous, real-time feed-back2

CHALLENGES AND SOLUTIONS

Source: www.matcoinc.com

• ‘The term used for transient elastic waves generated by the release of energy

within a material or by a process ’ (EN, 2000).

• Stress wave emissions or nano-seismic activity

• Source time, location, and mechanism unknown

• Sensing typically via

surface-mounted piezo-electric

transducers

• Frequency range for R/C:

approx. 10 to 500 kHz

3

WHAT ARE AE?

Source signal, S(t)

�

Stress wave

�

Propagation

�

Surface motion � voltage

�

Amplification

�

Filtering

� Response Function Digitization/storage on PC

�

Response signal, R(t)4

Pre-amplifier, tfDAQ(ω)

Source, S(ω)

Stress wave front, p-wave

Sensor , tfS(ω)

Data acquisition system , tfDAQ(ω)

Test specimen, tfM(ω)

( ) ( ) ( ) ( ) ( )M S DAQR tf tf tf Sω ω ω ω ω=

MEASUREMENT SYSTEM

ANALYSIS: QUALITATIVE VS. QUANTITATIVE

Stored AE signals, R(t)

AE event forming

Qualitative Quantitative

Source parameters5):

- Location

- Time

AE parameters:

- Hit rates/energy/…

- Historic-severity1)

- Load-Calm ratio2)

- b-Value analysis3)

Waveform analysis:

- Comparisons4)

1)Fowler et al., 1989, 2)Ohtsu et al., 2002, 3)Gutenberg & Richter, 1949, 4)Grosse, 1996, 5)Geiger, 1910,6)Aki & Richards, 1980

Moment tensor

analysis6)

b

Cum

ulat

ive

Hits

(lo

g-sc

ale)

• Goal

- Employ and adapt method used in seismology

- Generate baseline data from full-scale R/C bridge girders

• Method

Gutenberg & Richter (1949)

relationship describes temporal

distribution of earthquakes

where N is number of EQ ≥ M,

a and b are unknown parameters,

and M is the earthquake magnitude

• b-value represents slope in magnitude-frequency distribution plot6

B-VALUE ANALYSIS

10log ( )N a bM= −

b

• Interstate Highway 5 Bridge in Cottage Grove, OR

7

IN-SERVICE LOAD TEST

AE sensors

Strain gageCrack displ.

• Loaded truck (22.7 t) at 3 different speeds (16/64/84 km/h)

• 5 locations (Loc. 3 directly over instrumented girder)

8

LOADING PROCEDURE

9

DATA ANALYSIS

Qualitative Quantitative

• Spurious AE:

Studded tires

(Schumacher et al., 2009) 8

NOISE

Source: www.nokiantires.com

- 2 realistic full-scale R/C girders

- Span length = 6.60 m

(Schumacher et al., 2008 & 2011)11

FULL-SCALE LABORATORY EXPERIMENT

12

INSTRUMENTATION & LOADING PROTOCOL

P

P/2P/2

Example Specimen #2

OL = Overload = Load the specimen has never previously seen

TT = Simulated test trucks (≤ previous OL)13

ACTUAL LOADING PROTOCOL

Experiment Time, t [h]

App

lied

For

ce, P

[kN

]

App

lied

For

ce, P

[kip

s]

-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 300

250

500

750

1000

1250

1500

1750

2000

0

50

100

150

200

250

300

350

400

Dead load, DL

Pult = 1658 kN (373 kips)

1st OL

2nd OL

3rd OL

4th OL

Phase 1, Pmax/Pult = 0.38Phase 2, Pmax/Pult = 0.54Phase 3, Pmax/Pult = 0.70Phase 4, Pmax/Pult = 0.88Phase 5, Pmax/Pult = 1.00

TT

TT

TT TT

LR3 = Previous applied OL / Ultimate capacity 14

LOAD PHASES

(1) LR3 = 0.38 (2) LR3 = 0.54

(3) LR3 = 0.70 (4) LR3 = 0.88

ESTIMATION OF SOURCE LOCATIONS

Source Locations, Phase 1 (DL) Estimated Source Locations, Phase 3 (TT)

• Estimation of b-values from AE hit data

- Linear least-squares curve fit over µA-σA to µA+σA as suggested by Rao et al. (2005).

- Replace EQ magnitude M by max. AE amplitude/20

- (1) continuous monitoring or (2) analysis of discrete loading

events16

B-VALUE ANALYSIS

2 2.5 3 3.5 4 4.5

0

0.5

1

1.5

2

AE Magnitude [AdB/20]

log(

Cum

ulat

ive

AE

Hits

) [-

]

Frequency distribution of hit amplitudes

Estimated b-value (slope of this line)± one standard deviation of data

Data mean value

50 hits

Amax

Example sensor 4

17

(1) CONTINUOUS MONITORING

Example for proposed Minimum b-Value Analysis

18

(2) DISCRETE LOADING EVENTS

• Definition: Mean of lowest b-values from all sensors for one

complete loading event

LR1 = Applied force / Ultimate capacity

LR2 = Applied force / Previous applied OL 19

PROPOSED MINIMUM B-VALUE ANALYSIS

20

EVALUATION OF LR1

21

EVALUATION OF LR2

LR1 = Applied force / Ultimate capacity

LR2 = Applied force / Previous applied OL

LR3 = Previous applied OL / Ultimate capacity 22

EVALUATION CHART

• Minimum b-values vs. measured rebar strains

23

COMPARISON

• Overloads reliably detected, i.e. b-value << 1.0

• Discrimination of different damage mechanisms difficult

• In-service load conditions difficult to estimate

• Qualitative methods simple but only work for specified conditions

• Very few AE events formed during in-service loads (expected)

• b-Values sensitive to noise and prior history

24

CONCLUSIONS

• New set of experiments on R/C

- Quantitative

- Small-scale

- Large-scale

• Use of broad-band sensors

(Glaser/NIST point-contact)

25

COMING UP…

26

THANK YOU!

Civil & Environmental Engineering