THE IMPROVEMENT OF ULTRASONIC APPARATUS FOR THE

256 Pages·2003·5.98 MB·English

IMPERIAL COLLEGE OF SCIENCE, TECHNOLOGY AND MEDICINE
UNIVERSITY OF LONDON
THE IMPROVEMENT OF ULTRASONIC APPARATUS FOR
THE ROUTINE INSPECTION OF CONCRETE
by
Robert Long
A thesis submitted to the University of London for the degree of
Doctor of Philosophy
Department of Mechanical Engineering
Imperial College of Science, Technology and Medicine
London SW7 2BX
March 2000
Abstract
The most common application of ultrasonic testing in civil engineering is to determine the
velocity of sound in concrete, which is related to concrete quality. This thesis addresses
some of the limitations of current commercial apparatus used for determining ultrasonic
pulse velocity in concrete. The intention is to recommend improvements to enhance the
reliability of measurements and make application more convenient.
The velocity of sound in concrete measured by commercial apparatus is known to vary with
the path length being tested. Attenuation of sound in concrete, commercial transducer
characteristics, and determination of signal transit times have been investigated. From this
study, a function has been derived to correct measurement errors.
Commercial equipment is calibrated by coupling the transducers to a reference bar and
setting the apparatus display to a time value stamped on the bar. To validate the time value,
an experimental and finite element study have been carried out on wave propagation in a
finite length of bar. To aid interpretation of data, signal-processing techniques have been
investigated that are suitable for the evaluation of wave velocities in dispersive systems.
Results suggest that the time value corresponds to a relatively low energy component
propagating at the longitudinal bulk wave velocity. Reliable calibration can be achieved
when the apparatus recognises the component, which is dependent on the acoustic coupling
made by the transducers to the reference bar.
Currently, viscous couplant must be applied between the transducer face and the concrete
surface under test to facilitate signal transmission. Consistent coupling is difficult to achieve
and couplant application and removal proves time consuming and inconvenient. Alternative
coupling has been investigated; one technique that looks promising is rubber coupling.
Contact models have been derived to predict the deformations of rubber coupled devices
when loaded onto rough surfaces and thereby predict signal transmission. Experiments and
predictions suggest that dry rubber coupling of transducers using a hand held device might
not be feasible. However, more convenient coupling has been achieved when wetting a
prototype rubber coupled membrane device with very little water.
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Acknowledgements
I would like to thank my supervisors, Mike Lowe and Peter Cawley, for giving me the
opportunity to study in the NDT group at Imperial College. They provided essential
guidance within a subject area that was alien to me prior to commencement of this project.
Welcome advice was often provided by other members of the non-destructive testing group
at Imperial College. I enjoyed my studies amongst you. Many thanks to Christophe Aristegui
who adapted my computer programs so that they were of some use to others.
I must thank the EPSRC for providing a 3 year scholarship that enabled me to pursue this
project.
Lastly, I thank my family and friends. Without your support I am sure I would never have
completed let alone started my studies.
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Contents
Abstract ................................................................................................................ 2
Acknowledgements .............................................................................................. 3
Contents ............................................................................................................... 4
List of Tables ....................................................................................................... 9
List of Figures ...................................................................................................... 9
Nomenclature ....................................................................................................... 20
Chapter 1
Introduction
1.1 Introduction ...................................................................................... 23
Chapter 2
Review of the inspection of concrete structures
2.1 Introduction........................................................................................ 28
2.2 Concrete material characteristics........................................................ 28
2.2.1 Material constituents....................................................... 28
2.2.2 Hardening of the cement paste........................................ 29
2.2.3 Characteristics of hardened product................................ 30
2.2.4 The need for inspection................................................... 31
2.3 Concrete non destructive inspection techniques................................. 32
2.3.1 Rebound hammer............................................................ 32
2.3.2 Infrared thermography.................................................... 33
2.3.3 Radioactive methods....................................................... 34
2.3.4 Ultrasonic techniques...................................................... 35
2.3.4.1 Pulse velocity.................................................... 38
2.3.4.2 Pulse echo......................................................... 39
2.3.4.3 Impact echo....................................................... 41
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2.3.4.4 Spectral analysis of surface waves.................... 42
2.3.5 Ground penetrating radar................................................ 44
2.3.6 Review of inspection techniques.................................... 45
2.4 The PUNDIT test equipment.............................................................. 46
2.4.1 PUNDIT apparatus description....................................... 46
2.4.1.1 Pulse generator.................................................. 47
2.4.1.2 Set reference delay............................................ 48
2.4.1.3 Receiver amplifier............................................ 48
2.4.1.4 Timing pulse oscillator, gate and counter......... 48
2.4.1.5 Power supply.................................................... 49
2.4.1.6 Outputs.............................................................. 49
2.4.1.7 Transducers....................................................... 49
2.4.2 Operation of apparatus.................................................... 50
2.4.2.1 Set reference...................................................... 50
2.4.2.2 Applying couplant, surface preparation............ 50
2.4.2.3 Inspection and interpretation of results............. 50
2.4.3 Summary......................................................................... 52
2.4.4 Conclusion...................................................................... 52
Chapter 3
Correction of measured transit times for apparatus such as the PUNDIT
3.1 Introduction........................................................................................ 58
3.2 Description of anomaly for narrow band transducers........................ 58
3.3 Signal losses due to beam spreading.................................................. 59
3.3.1 Spherical radiation approximation.................................. 60
3.4 Signal losses in concrete due to material attenuation......................... 63
3.5 Modelling received signal.................................................................. 66
3.6 Correction for the variation in measured pulse velocity..................... 69
3.7 Predicting the variation in measured pulse velocity........................... 69
3.8 Conclusion.......................................................................................... 71
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Chapter 4
Validation of PUNDIT calibration procedure
4.1 Introduction........................................................................................ 79
4.2 Reference bar description................................................................... 79
4.3 Wave propagation possibilities in reference bar................................. 81
4.3.1 Wave propagation in unbounded media......................... 81
4.3.2 Classical wave propagation in an infinitely long bar...... 85
4.3.3 Dispersion curves and for reference bar......................... 88
4.3.4 Wave propagation in a finite length of bar..................... 89
4.4 Wave velocity extraction.................................................................... 90
4.4.1 Phase Spectrum – Fourier transform............................... 91
4.4.2 Instantaneous frequency – Hilbert transform.................. 91
4.4.3 F ridge points – Wavelet transform................................ 93
4.5 Finite element study........................................................................... 94
4.5.1 Finite element model description.................................... 95
4.5.2 Finite element model results........................................... 96
4.6 Experimental study............................................................................. 99
4.7 Conclusion.......................................................................................... 101
Chapter 5
Convenient Coupling
5.1 Introduction........................................................................................ 114
5.2 Possible Alternatives.......................................................................... 114
5.3 Characteristics of rubber concrete contact ........................................ 116
5.3.1 Concrete surface roughness characteristics.................... 116
5.3.2 Real rough concrete surfaces.......................................... 119
5.3.3 Ultrasonic transmission across an interface.................... 120
5.3.4 Experimental rubber concrete contact............................ 125
5.3.5 Experimental results for rubber concrete contact........... 127
5.4 Solid contact model............................................................................ 129
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5.4.1 Contact model for a compliant solid.............................. 130
5.4.2 Contact model for indenter and surface of similar
modulus.......................................................................... 132
5.4.3 Contact model for a soft disc of finite thickness............ 133
5.4.4 Numerical solution of contact model............................. 135
5.4.5 Convergence of numerical solution............................... 136
5.4.6 Validation of solid contact model.................................. 138
5.5 Membrane contact model................................................................... 141
5.5.1 large deflections of a circular plate................................ 142
5.5.2 Constant volume function.............................................. 144
5.5.3 Membrane flat surface model........................................ 145
5.5.4 Validating membrane flat surface model....................... 146
5.5.5 Membrane rough surface model.................................... 146
5.5.6 Numerical solution of rough surface model................... 150
5.5.7 Calibrating rough surface model.................................... 152
5.6 Feasibility of dry rubber coupling when testing concrete.................. 153
5.6.1 Obtaining experimental data........................................... 153
5.6.2 Comparing model predictions to experimental data....... 154
5.6.3 Discussing dry solid coupling of transducers................. 157
5.7 Wet membrane coupling of transducers............................................. 158
5.7.1 Predicting reduction in volume of couplant required...... 159
5.7.2 Experimental investigation into the performance of wet
membrane coupled devices............................................. 160
5.8 Conclusion.......................................................................................... 161
Chapter 6
Application – Wet membrane v Conventional coupling
6.1 Introduction........................................................................................ 190
6.2 Prototype membrane shoes................................................................. 190
6.3 Experimental description.................................................................... 192
6.4 Review of results................................................................................ 194
6.5 Conclusion.......................................................................................... 195
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Chapter 7
Conclusions and future work
7.1 Review of thesis................................................................................. 202
7.2 Summary of findings.......................................................................... 204
7.2.1 Correction of measured pulse velocity........................... 204
7.2.2 Evaluation of Pundit calibration procedure.................... 205
7.2.3 Convenient coupling of transducers............................... 205
7.3 Recommended future work................................................................ 207
7.3.1 Correction of measured pulse velocity........................... 207
7.3.2 Verification of calibration technique.............................. 207
7.3.3 Contact models............................................................... 207
7.3.4 Convenient coupling of transducers................................ 207
Appendix 1
Signal processing for dispersive systems
A 1.1 Introduction...................................................................................... 208
A1.2 Simulated dispersive signals............................................................ 208
A1.3 Phase spectrum by the Fourier transform......................................... 210
A1.4 Instantaneous frequency by the Hilbert transform........................... 215
A1.5 Ridge points by the wavelet transform............................................. 219
A1.6 Conclusion....................................................................................... 229
Appendix 2
Component drawings for prototype membrane shoes
A2.1 Component drawings for membrane coupled device to suit
PUNDIT 54kHz transducers........................................................... 243
A2.2 Component drawings for membrane coupled device to suit
PUNDIT 83kHz transducers........................................................... 244
References.............................................................................................................. 245
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List of Tables and Figures
Tables
3.1 Composition of concrete samples for evaluation of attenuation coefficients......... 78
3.2 Attenuation coefficients and velocities evaluated for concrete samples................. 78
5.1 Surface roughness parameters of sample concrete surfaces. .................................. 115
6.1 Physical properties for MOSITES M1453D Silicon Rubber.................................. 186
Figures
2.1 Concrete surface hardness testing – schematic of Schmidt Hammer..................... 53
2.2 Schematic of concrete ultrasonic pulse velocity testing........................................ 53
2.3 Schematic of concrete ultrasonic pulse echo testing.............................................. 54
2.4a Impact echo technique. Determination of sound velocity for known slab depth... 54
2.4b Impact echo. Determination of reinforcement bar depth for known sound
velocity................................................................................................................... 54
2.5 Ground Penetrating Radar. Location of reinforcement bar or delaminations........ 55
2.6 PUNDIT system diagram....................................................................................... 55
2.7 PUNDIT 500 volt excitation pulse......................................................................... 56
2.8 Schematic of internals of PUNDIT 54kHz Transducers........................................ 56
2.9 Configurations for pulse velocity measurements. (a) Direct through transmission
method (b) Semi-indirect method (c) Indirect surface method............................. 57
2.10 Schematic of grid pattern marked out on concrete structure for the determination
of concrete uniformity............................................................................................ 57
2.11. Plotting signal transit times for each grid point as a 2D surface map to aid
the identification of problem areas......................................................................... 57
3.1 Signal a(t) received by Pundit apparatus (100mm thick aluminium) showing
the effect a decrease in signal amplitude a(t)/2 has on the point the threshold
is crossed............................................................................................................... 73
3.2 Predicted Pundit measured signal transit time (100mm thick aluminium) as a
function of maximum amplitude of the received first half cycle of signal a(t) ..... 73
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3.3 Spherical coordinate system................................................................................... 73
3.4 Approximation of transducer by superposition of point sources........................... 73
3.5 Application of transducers to concrete sample to obtain signals for concrete
attenuation measurement....................................................................................... 74
3.6 Application of transducers to aluminium block to obtain reference signals for
concrete attenuation measurement......................................................................... 73
3.7 Magnitude of first (circles) and second (triangles) peak of received reference
signal as a function of excitation centre frequency................................................ 74
3.8 Example received signals for 80kHz centre frequency 5 cycle tone burst for
aluminium (dashed line) and 5mm aggregate concrete (continuous) ................... 74
3.9 Mortar attenuation coefficient (cid:302)(f), evaluation by amplitude of first received
peak (circles) and second peak (triangles) ............................................................ 75
3.10 Concrete (5mm aggregate) attenuation coefficient (cid:302)(f), evaluation by amplitude
of first received peak (circles) and second peak (triangles) .................................. 75
3.11 Concrete (10mm aggregate) attenuation coefficient (cid:302)(f), evaluation by amplitude
of first received peak (circles) and second peak (triangles) .................................. 75
3.12 Schematic of transducer transfer function.............................................................. 75
3.13 Pundit 500V excitation pulse and fitted exponential decay................................... 76
3.14 Phase(f) of Pundit 500V excitation pulse............................................................... 76
3.15 Predicted output g(t) from Pundit 54kHz transducers when excited by
Pundit 500V excitation pulse f(t), to illustrate phase lag between g(t) and
impulse response h(t)............................................................................................. 76
3.16 Comparison of early part of received signal in Fig 3.1 and approximation
by equation 3.28..................................................................................................... 77
3.17 Prediction of measured bulk velocity as a function of path length tested using
Pundit transducers when testing mortar................................................................. 77
3.18 Prediction of measured bulk velocity as a function of path length tested using
Pundit transducers when testing 5mm aggregate concrete.................................... 77
3.19 Prediction of measured bulk velocity as a function of path length tested using
Pundit transducers when testing 10mm aggregate concrete.................................. 77
4.1 Pundit reference bar............................................................................................... 104
4.2 Elemental volume in Cartesian coordinates........................................................... 104
4.3 Elemental volume in cylindrical coordinates......................................................... 104
4.4 Phase velocity projection for 25mm radius aluminium bar................................... 105
4.5 Group velocity projection for 25mm radius aluminium bar.................................. 105
4.6 Axial displacements u for fundamental longitudinal mode L(0,1) ...................... 106
z
4.7 Radial displacements u for fundamental longitudinal mode L(0,1........................ 106
r
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