STRUCTURAL CONDITION EVALUATION OF PRESTRESSED CONCRETE ... · STRUCTURAL CONDITION EVALUATION OF...

European COoperation in the field of Scientific and Technical Research

1

Group Project 5

STRUCTURAL CONDITION EVALUATION OF PRESTRESSED CONCRETE STRUCTURES

BASED ON ACOUSTIC MONITORING

Co-ordinator: Stanisław RADKOWSKI

Working Group 3 - "New assessment methods"


2

Participiants:

• Warsaw University of Technology - ��

• KWH Bautechnologen AG - ��

• AGH University of Science and Technology - ��

• Poznan University of Technology - ��ł��

• Bouwdienst Rijkswaterstaat and Advitam - � �� !��"#��

• Budapest University of Technology and Economics (BUTE) -!�$�%�&��

• National Technical University of Athens - ' ��


3

Problem:To evaluate the structural condition of prestressedconcrete structure by means of vibro-acoustic monitoring and by improving the applicability of the dynamic evaluation techniques

Basic assumption – increase and decrease of prestressing forces are accompanied by a change of stress distribution in the cross-section of concrete structure. The parameters of propagation path will change due to this phenomena.


4

1500

200

(199

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100 (98)

260

260

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What is the frequency stucture of a pulse?How it depends on: technical state and failure evolution?

WARSAW UNIVERSITY OF TECHNOLOGYThe Faculty of Automotive and Construction Machinery Engineering


5

1500

200

(199

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100 (98)

260

260

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Kielce measurement – prestressedconcrete beam, bending force (changeable) applied in middle, measuring vibrations exceeded with impulse. Various prestressed beams examined.

4200 4400 4600 4800 5000 5200 54000

0.005

0.01

0.015

0.02

0.025

0.03

0.03515 kN30 kN

Frequency [Hz]

Am

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1464

2137

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Frequency [Hz]

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6

0 10 20 30 40 50 60 70 0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

Fre

qu

en

cy [F

/Fm

ax]

Force [ kN]

Natural freq. 1500 [Hz ]Natural freq. 7000 [Hz ]

before failure -dispersion phenomena

after failure – change of natural frequency

change of phase velocity

change of Young`s modulus

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amplitude modulation of carrier function

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Without dispersion phenomena


8

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Fo

DISPERSION!


11

Phase velocity


12

Effect ofdispersion


13

change of group velocity

amplitude modulation of carrier function

before failure - dispersion phenomena


14

Load value Evnelope amplitude distribution [m/s2]

0 10 20 30 40 50 60 70 0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

Fre

qu

en

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/Fm

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Force [ kN]

Natural freq. 1500 [Hz ]Natural freq. 7000 [Hz ]

Probability distribution of amplitude of evnelope value


15

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Conclusions:

• Presented results were achieved in laboratory tests.

• Assumed models were confirmed by results achievedfor two different sets of beams.

• To make this method mature for application, method must be tested and verified on a real structure.


18

– SoundPrint ® : continuous acoustic monitoring of prestressed structures. Monitoring of internal wire break activity


19

Particular risks for pre or post tensioned structures

• Cables are very sensitive to– Corrosion– Fatigue

• These phenomenon remain hidden until final break of cable…

… and of bridge

Criticality of these risks is always high for cable based structures


20

SoundPrint detects and localizes the sounds emitted by the energy released during wire

breakage

Sensors are distributed on the structure allowing a global and continuous monitoring

Structure Post tensioned structure

Cable / strand / wire

Acoustic sensor

Library of acoustic signature of known events allow identification of captured noises


21

SoundPrint allows early detection and localization of internal wire breaks

And filtering of ambient noise

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Processing center

structure

Local DAQ Unit

Sensors

Internet

Real time Internet access

Internet

Owner


22

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• The signals were analyzed both in the time and frequency domain.

• The scope is to search for modulations in the signals

• Different parts of the signal (stationary – non-stationary) were examined with various frequency resolutions

• 2 Demodulation methods were used ( Hilbert & TeagerKaiser)

• The signals were decomposed by Empirical Mode Decomposition &

• Hilbert – Huang Transformation


23

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Three test methods were developed and verified in the frame of the researches for prestressed concrete structures for the testing and assessment of the structure condition.

The test methods are based on:

time-modal analysis method,energy method of diagnostics, load power distribution analysis.

The elaborated research methods make the use of the analysis of vibroacoustic signals.

The vibration signals are taken from the tested structure and these have a form of decaying free vibrations.

The structure was excited by impact force .

Test stand

One can distinguish the several basic steps of the investigations.

Initial investigationsBasic testsVerification tests

In parallel with the basic investigations thefollowing investigations:

• Static load tests• The ultrasonic tests• Auxiliary investigationswere carried out.

In addition, the FEM model of the beam was elaborated, and itis planned to use the BEM model as well._______________________________________A part of these investigations will be continued if the project is completed.

Two sets of beams were worked out and made:• The first set consisted of the beams with a different controlled initial

prestress

• The second set consisted of the beams with introduced initial imperfections that simulated a different degradation degree (structure integrity loss).

Basic Tests

were realized by a training system on the prestressed beams.

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Artificially introduced disintegrity of beam structures - simulation of beam crack in middle part of beam (¼ , ½ , ¾ of full integrity loss )

Tendons placed symmetrically (even stress distribution in the beam cross-section

Signal from tested object

STFTAFC

analysis

Backbone curve estimation

Nonlinearity parameterisation

conditionevaluationof tested object.(detection of:

- cracks, - integrity loss, - stress loss etc.

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F= 60 [kN]cracked beam

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load [kN]

freq

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1500

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beamwithout crack

artificially introduced

disintegrity of beam structures simulation of the

beam crack (in the middle part)

¼ full integrity loss ¾½

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We can use the time integrated average R(f) frequency in order toassess the stress loss in the structures. (highest value of the correlation coefficient).

In order to detect the integrity loss of structures the following three parameters are suitable:

• Amplitude normalized total R(f) frequency deviation• Normalized R(f) frequency deviation • Frequency deviation rate.

The determination of the representative symptoms and their sensitivity require an individual approach for each test type of the structure.

The symptoms limit values of the representative symptoms on the basis ofexperimental data should be assessed.

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Zale�no�� przepływu energii od stanu obci��enia i stanu technicznego - na

podstawie maksymalnej mody drga� gi�tnych - belka A4d, wymuszenie p. 4, pomiar drga� - czujnik p. 1

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46

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