Presentation on 16th October 2018 by Isaak Tsalicoglou at the 3rd Japanese-Swiss workshop titled "Durability Testing of Concrete, on site and in the lab", at TFB in Wildegg, Switzerland. The state of the art of non-destructive testing applied to civil engineering has in recent years achieved many significant milestones in the field of imaging technologies for the assessment of reinforced concrete structures. The constant improvement of the available electronic components and the increased computational power of consumer-grades mobile devices are among the key factors that drove mature technologies such as Ground Penetrating Radar (GPR) and Eddy Current Testing (ECT) into products delivering new levels of accuracy and usability. At the same time, recent developments of the pulse-echo-based Ultrasonic Tomography (UT) further broadened the range of applications potentially addressed. Still, there seems to be a general lack of awareness in the user community about the real potentials and main limitations of each of the three technologies. The current work focuses on the typical scenarios and the common challenges presented by tasks of embedded object detection (structural details, steel reinforcement, utility network) and flaw detection (voids, cracks, delaminations). The complementarity of the three technologies has been proven through measurements on test blocks and real-life cases. It is noted that the proper combination of technologies naturally results in more efficient workflows, increased positioning accuracy, and a less subjective, operator-dependent interpretation of the testing results.

1. The Three (Four) Musketeers of multi-technology
concrete NDT for accuracy and productivity
3rd Japanese-Swiss Workshop - "Durability Testing of Concrete, on site and in the lab“
TFB, Wildegg, Switzerland – October 16, 2018
Isaak Tsalicoglou – Head of Product Management
Proceq SA, Switzerland
© Proceq 2018 1

2. © Proceq 2018 2
Image copyright: Disney

3. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 3
Three main technologies for concrete imaging
Eddy Current Testing
(ECT)
Ultrasonic tomography (UT) /
Ultrasonic Pulse Echo (UPE)
Ground Penetrating Radar
(GPR)
rebars
objects
back wall
rebars (limited)
objects
back wall
defects
rebars
- cover
- diameter
defects (limited)

4. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 4
ECT relies on eddy currents and electromagnetic
induction
Transmitting
coil
Receiving coil
Rebar
.
.
.
.

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GPR and UPE rely on the behavior of waves at
interfaces
Concrete 2nd material
The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity
Animation source: Wikipedia

6. © Proceq 2018 6
GPR detects “dielectric contrast” with
electromagnetic waves
Concrete 2nd material
The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity
𝜀1 𝜀2
Relative dielectric permittivity (“dielectric”)
𝑅 =
𝜀2 − 𝜀1
𝜀2 + 𝜀1
Reflection coefficient
Animation source: Wikipedia

7. © Proceq 2018 7
UPE detects discontinuities of acoustic
impedance with ultrasonic waves
Concrete 2nd material
The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity
𝑧1 𝑧2
Acoustic impedance
𝑅 =
(𝑧2−𝑧1)2
(𝑧2+𝑧1)2
Reflection coefficient
Animation source: Wikipedia

8. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 8
Relative dielectric permittivity values typical in
concrete GPR scanning
∞
Relative dielectric permittivity (“dielectric”)1 1473
SteelDry concrete Wet concreteAir
Avg. concrete

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Metallic targets in concrete cause a full reflection
of electromagnetic waves (GPR)
The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity
Reflection coefficient
𝑅 = −0.451
𝜀1 = 7
𝜀1 = 7
𝜀2 = 1
𝑅 = 1
(partial reflection)
(full reflection)
𝜀2 = ∞
Animation source: Wikipedia

10. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 10
Acoustic impedance values typical in concrete
UPE scanning
Acoustic impedance [kg/m2×106]
× 22377
× 4.84
0.000429
Air
9.6
Avg. concrete
46.5
Steel

11. 𝑧2 = 46.5
© Proceq 2018 11
Ultrasonic waves (UPE) demonstrate the
opposite behavior of GPR waves
The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity
Reflection coefficient
𝑅 = 0.9998
𝑧1 = 9.6
𝑧1 = 9.6
𝑅 = 0.433
(practically full reflection)
(partial reflection)
(All z values expressed in 106 kg/m2)
𝑧2 = 0.000429
Animation source: Wikipedia

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In the imaging of concrete structures, GPR and
UPE complement each other ideally
• Total reflection
• Better for detecting voids
• Partial reflection
• Detection below rebars is possible
• Partial reflection
• Detection below voids is possible
• Total reflection
• Better for detecting rebars
GPR UT / UPE
The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity

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Each of the three technologies offers a
different set of advantages
Single rebars very
sharply delineated
Can see beyond
an air layer
Insensitive to (non-
metallic) surface
coatings
Extensive
penetration, even in
reinforced areas
Very sensitive to
voids and cracks
Independent from
concrete composition
Concrete cover
is very precise
Determination of
rebar diameter
GPR UT/UPE ECT
The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity

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Each of the three technologies also
presents different disadvantages
Only steel elements
detected
Limited to the upper
reinforcement layer
Cannot see past
dense reinforcement
or steel plate
Cannot see small
delaminations
Limited penetration
through fresh
concrete
Requires mechanical
coupling to surface
Cannot see beyond
an air layer
Poor resolution of
single rebars
Limited penetration
through porous
concrete
The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity
GPR UT/UPE ECT

15. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 15
Profometer 650 AI
Recent implementations of the ECT, GPR and
UPE in portable NDT instruments
Proceq GPR Live Pundit Live Array Pro
ECT instrument with
Neighboring Rebar Correction
Stepped-Frequency Continuous-
Wave (SFCW) GPR
UPE array transducer
with 8 or 16 channels

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Case 1: rebars, cavities, variable profile
• Rebars Ø 20 mm in the
upper half of the block
• Two artificial voids
150x70 mm
• Thickness variation
from 400 to 550 mm
(V-shaped back wall)

17. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 17
Case 1_A: no steel reinforcement

18. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 18
SFCW GPR can detect large enough voids, and
even a deep backwall
no steel
reinforcement

19. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 19
UPE excels at concrete-to-air interfaces, i.e. the
back wall and the void
no steel
reinforcement

20. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 20
GPR and UPE scans deliver similar information
on the non-reinforced part of the block

21. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 21
Case 1_B: strong steel reinforcement

22. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 22
GPR sharply resolves the rebars – and they have
a shielding effect on other features
strong steel
reinforcement

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UPE does not image the rebars – but it does
resolve the air pocket and the back wall
strong steel
reinforcement

24. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 24
Reinforcement makes the complementarity of
GPR and UPE evident

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ECT excels at 2D mapping of the cover of first-
layer reinforcement mats

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ECT delivers accurate estimates of rebar cover
(±1 mm) / diameter (±2 mm), and spacing
±1 mm = 1/32nd of an inch

27. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 27
Case 2: detecting delaminations
Scan direction
250mm/9.8in
800 mm / 31.5 in

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GPR is blind to the delamination – but the back
wall is clear, regardless

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With UPE, the delamination is clear, and it masks
the back wall

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Case 3: embedded objects, cavities, back wall
BA

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Both GPR and UPE detect all highlighted
concrete-air and concrete-steel transitions
Rebar
Metal pipe
Back wall
Plastic pipe
Non-migrated GPR scan
Migrated UPE scanMigrated GPR scan

32. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 32
Cracks are detectable with UPE, but only faintly
so with GPR – and only beyond a critical size
Crack
Back wall
Non-migrated GPR scan
Migrated GPR scan Migrated UPE scan

33. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 33
Cracks are detectable with UPE, but only faintly
so with GPR – and only beyond a critical size
Migrated viewNon-migrated viewCrack geometry
SFCW GPR UPE

34. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 34
Ground Penetrating Radar:
versatile locator
ECT and GPR combine for comprehensive
reinforcement mapping to medium depth ranges
Ultrasonic Pulse Echo:
reliable flaw detector
Eddy Current Testing:
accurate cover meter

35. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 35
Ground Penetrating Radar:
versatile locator
GPR and UPE combine for comprehensive
feature and flaw detection to large depth ranges
Ultrasonic Pulse Echo:
reliable flaw detector
Eddy Current Testing:
accurate cover meter

36. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 36
Ground Penetrating Radar:
versatile locator
Together, ECT, GPR and UCT are “The Three
Musketeers” of structural concrete imaging
Ultrasonic Pulse Echo:
reliable flaw detector
Eddy Current Testing:
accurate cover meter

37. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 37
Data fusion of multi-technology GPR and UPE
concrete measurements is within reach
GPRUPE
Migrated Isosurface
Migrated Isosurface
Multi-technology imaging
Source: Taffe, Handbuch der Bauwerksprüfung - Mertens (Hrsg.)

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• Complementary
detection and insights
• Comprehensive
understanding
• Increased objectivity
of results
• Reduced user-
dependence
Concrete imaging requires “the right tool for the
job” and the right combinations thereof

39. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 39
Image copyright: Disney

40. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 40
Ishikawa (“fishbone”) diagram of concrete
testing with Schmidt hammers for a holistic view
Materials
Methods /
Processes
Manpower /
People
Mother Nature /
Environment
Measurements
Machines /
Equipment
ProblemCauses and effects
The business case of
concrete testing:
1. Cost-benefit ratio
2. Budget constraints

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Variability of concrete, a bulk-produced material
• Concrete will always
be of variable quality
• More tests are better
than fewer tests
• “Accurate enough” at
low total effort makes
business sense
Materials
Methods /
Processes
Manpower /
People
Mother Nature /
Environment
Measurements
Machines /
Equipment
Image source: http://www.bbrnetwork.com/

42. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 42
Schmidt hammers continue to be an integral part
of German and EU standards
Equipment accuracy, quality, repeatability
Total process productivity
Holistic view of
Schmidt hammer
testing
Materials
Methods /
Processes
Manpower /
People
Mother Nature /
Environment
Measurements
Machines /
Equipment
Image source: “Comparative investigations of
rebound hammers related to R and Q values”
since DIN 4240
since DIN EN 13791/A20

43. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 43
• Introduction of Q values as German-national appendix to
DIN EN 13791/A20
• Q-value hammer (SilverSchmidt), compared to R-value
hammer (Original Schmidt):
✓Lower dispersion of rebound values
✓High accuracy up to higher concrete strengths
• Step-wise mapping of Q values to compressive strength
classes is possible
• “Even for high-strength concrete, it is possible to estimate
the compressive strength through the Q value”
Independent research in 2018 confirmed the
accuracy benefits of R-value Schmidt hammers
Source: “Comparative investigations of
rebound hammers related to R and Q values”

44. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 44
Q value (SilverSchmidt) shows lower dispersion
for higher-strength concrete
Rebound value
Compressive
strength
Image source: “Comparative investigations of
rebound hammers related to R and Q values”
✓
Original Schmidt
SilverSchmidt

45. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 45
Only destructive testing
Real-world large-scale concrete testing…
on a budget
• 20x cores, destructive testing
• Labor-intensive; no on-site
insights possible
• Data only every few meters
• Weakened structure
Multi-fidelity testing with
multiple technologies
Only high-effort NDT
• Many efficient tests - e.g. Original
Schmidt Live / SilverSchmidt Live
• Added: few expensive tests (DT or
NDT); correlate the efficient and
expensive tests
✓ Large coverage at the right
detail, within budget
• Sophisticated NDT with non-
hammer methods
• Experienced people required; high
cost
• Low number of total tests
“Japan: total length of
expressways is 11,659 km” (!)
Image source: https://www.japan-
guide.com/e/e2354.html


✓

46. © Proceq 2018 The Three (Four) Musketeers of multi-technology concrete NDT for accuracy and productivity 46
The Schmidt hammer(s) as a valuable fourth
Musketeer of concrete NDT
Original Schmidt Live
R-value hammer
SilverSchmidt
Q-value hammer
Image copyright: Disney

47. © Proceq 2018