Journal of Nondestructive Evaluation (2021) 40:65
https://doi.org/10.1007/s10921-021-00797-3
Muon Tomography of the Interior of a Reinforced Concrete Block: First
Experimental Proof of Concept
Ernst Niederleithinger1 · Simon Gardner2 · Thomas Kind1 · Ralf Kaiser2,3 · Marcel Grunwald1 · Guangliang Yang2,3 ·
Bernhard Redmer1 · Anja Waske1 · Frank Mielentz1 · Ute Effner1 · Christian Köpp1 · Anthony Clarkson2,3 ·
Francis Thomson2,3 · Matthew Ryan4 · David Mahon2
Received: 7 August 2020 / Accepted: 4 July 2021 / Published online: 24 July 2021
© The Author(s) 2021
Abstract
Quality assurance and condition assessment of concrete structures is an important topic world-wide due to the aging infrastructure and increasing traffic demands. Common topics include, but are not limited to, localisation of rebar or tendon ducts,
geometrical irregularities, cracks, voids, honeycombing or other flaws. Non-destructive techniques such as ultrasound or
radar have found regular, successful practical application but sometimes suffer from limited resolution and accuracy, imaging
artefacts or restrictions in detecting certain features. Until the 1980s X-ray transmission was used in case of special demands
and showed a much better resolution than other NDT techniques. However, due to safety concerns and cost issues, this method
is almost never used anymore. Muon tomography has received much attention recently. Novel detectors for cosmic muons
and tomographic imaging algorithms have opened up new fields of application, such as the investigation of freight containers. Muon imaging also has the potential to fill some of the gaps currently existing in concrete NDT. As a first step towards
practical use and as a proof of concept we used an existing system to image the interior of a reference reinforced 600 kg
concrete block. Even with a yet not optimized setup for this kind of investigation, the muon imaging results are at least of
similar quality compared to ultrasonic and radar imaging, potentially even better. The data acquisition takes more time and
signals contain more noise, but the images allowed to detect the same important features that are visible in conventional high
energy X-ray tomography. In our experiment, we have shown that muon imaging has potential for concrete inspection. The
next steps include the development of mobile detectors and optimising acquisition and imaging parameters.
Keywords Muon tomography · Non-destructive testing · Reinforced concrete · Ultrasound · Radar · X-ray
1 Introduction
The continuous availability of the European road transport
network is one of the essential prerequisites for mobility
and economic growth in the EU and world-wide. EU road
infrastructure is getting older and suffers from aging issues
* Ernst Niederleithinger
ernst.niederleithinger@bam.de
1
Bundesanstalt für Materialforschung und -prüfung (BAM),
Berlin, Germany
2
School of Physics & Astronomy, University of Glasgow,
University Avenue, Glasgow G12 8QQ, UK
3
Lynkeos Technology Ltd., University of Glasgow, No 11 The
Square, Glasgow G12 8QQ, UK
4
National Nuclear Laboratory, Central Laboratory, Sellafield,
Seascale, Cumbria CA20 1PG, UK
with a large part of it already approaching the end of its
life. According to the European Union Road Federation,
the network had a length of 5.5 million km and a value
of 8000 billion Euros in 2018, the latter declining [1]. In
Germany, 10% of the bridges (bridge deck area considered)
under federal administration were rated with a condition
“less than sufficient” [2]. In France, about 25,000 bridges
are prone to structural health issues which affect both safety
and accessibility [3]. They are subject to serious fatigue
problems, due to the increase of freight volumes (and traffic) with ever greater overall vehicle weights. Bridges and
roads allow individual mobility and the supply of private
households and the economy, but their aging and upcoming
fatigue problems lead to progressive degradation of bridge
structures and thus to safety and reliability problems. These
effects are further intensified by technological developments
in heavy goods vehicle traffic (e.g. road trains, platooning
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etc.). Damage to structures that is usually only detected at
a late stage, can have far-reaching consequences for traffic. In the worst case, the total failure of a structure can
lead to the complete inaccessibility of entire road sections
in the traffic network, as illustrated by the collapse of the
Polcevera Viaduct (aka Morandi Bridge) in Genoa in August
2018, leading to multiple deaths and resulting in a significant loss of gross domestic product. Traditional approaches
for assessing the condition of transport infrastructure are
based on structural inspections at fixed or adjustable time
intervals. They are inadequate for an efficient inspection of
the transport infrastructure assets, which after all amount to
about 40% of the total European assets [1]. Only an efficient
inspection, preferably permanently under flowing traffic, will
give infrastructure owners and managers the right picture to
prioritize their maintenance operations.
There are already a number of Non-Destructive Testing (NDT) methods that provide engineers with tools to
inspect aging infrastructure [3, 5–8]. Standard technologies for structure assessments are ultrasonic methods and
ground penetrating radar. For specific tests, e.g. reference
measurements with very high resolution, X-ray radiography is also used. However, all of these techniques have their
limitations. Ground penetrating radar is a very rapid and
effective inspection method and is very sensitive for metal
detection, but in concrete constructions the penetration depth
and resolution of ground penetrating radar depend on the
frequency of the radar used: low frequencies can penetrate
up to 1.5 m with resolutions of several cm and high frequencies can reach resolutions of several mm but the penetration
depth is limited to about 40 cm. Ultrasonic echo instruments
show greater penetration depths (around 1 m in commercial
applications) but have a resolution of at best 1 cm. In addition, it is often not possible to inspect beyond the first reinforcement layers due to reflections. Ultrasonic echo methods
are excellent to detect voids, cracks or delaminations, but
cannot image features behind these obstacles. X-ray radiography (including variants such as X-ray tomography and
laminography) can provide images with excellent resolution.
Depending on the radiation energy, the penetrated thickness
of a concrete structure can be up to 1 m with a spatial resolution of a few mm. However, when using X-ray radiography,
attention must always be paid to compliance with the radiation protection regulations. In practice this often means that
X-ray radiography cannot be applied.
Muon tomography, a purely passive technique using
natural cosmic background radiation as a source, has the
potential to overcome some of these issues [9]. Showers of
high energy particles, including muons, are constantly created by collisions between cosmic rays and the upper atmosphere. The muons from these showers are highly penetrative
and can pass through tens and hundreds of meters of rock
before coming to rest and decaying. As cosmic muons are
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Journal of Nondestructive Evaluation (2021) 40:65
a naturally occurring radiation there are no costs or energy
requirements in generating them, and because no additional
radiation is generated there is no safety concern. It is a passive imaging system. There are two types of muon imaging
techniques. The first is muon absorption imaging (or muon
radiography), and the second is muon multiple scattering
imaging (or muon tomography). While muon radiography
uses one detector (or a set of detectors on the same plane)
to detect muons after passing the object of interest, muon
tomography uses two detectors (or two sets of detectors on
different planes) to detect the muons before and after passing
through the object of interest. The latter allows volumetric
reconstruction of the object’s scattering properties, resulting
in high resolution 3D images [10].
The absorption of naturally occurring cosmic-ray muons
was first used to investigate complex structures over 60 years
ago by British physicist George, when he determined the
weight of ice above a mining tunnel in Australia [11]. Imaging the interior of a volcano by muography was shown in
2001 [12]. Fifteen years ago, researchers at Los Alamos
National Laboratory demonstrated that the Coulomb-scattering of the muon could be exploited to identify high-density, high-atomic number (Z) material within large, shielded
transport containers [13, 14]. Since this discovery, the field
of non-destructive testing using cosmic ray muography has
developed and, in recent years, has experienced an exponential growth with more than 40 research groups and projects
active in over 20 countries throughout the world. In 2017
the topic received great attention with the publication of the
high-profile measurements from within the great pyramid of
Khufu in Egypt that indicated the presence of a previously
unknown chamber [15]. In recent years, half a dozen companies have formed to commercialise muography imaging
technology for a variety of different applications including
nuclear contraband detection for national security, brownfield mineral exploration and nuclear waste characterisation.
Recently, an experiment using muography for the detection
of animal burrows in river embankments was reported [16].
The application of muon tomography to nuclear waste
containers includes the investigation of objects or voids in
concrete as this is used for stabilizing and shielding waste
objects inside the containers. However, in the experiments
published so far, the size of the detected objects (several
cm) is larger than what would be required for reinforced
concrete in civil engineering. The diameter of rebar is typically between 8 and 28 mm. The idea to use muon tomography to inspect civil engineering concrete structures is e.
g. reported by Durham et al. in 2016 [18]. The same paper
describes a successful experiment mapping section of a
concrete panel with different thicknesses. However, there
is no practical demand for thickness measurements on real
constructions if both sides are accessible. The idea of using
muon tomography as a tool for the inspection of interior
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Journal of Nondestructive Evaluation (2021) 40:65
features of reinforced concrete structures including a concept for developing a suitable mobile detecting system was
submitted to the EC research program Horizon 2020 in early
2019 by the authors. Civil engineering, especially bridge
inspection, was also highlighted as a key future application
in an overview article by one of the authors in 2019 [9].
The same idea was developed by another research group
and successfully explored by simulations [19]. Other similar applications reported have been limited to simulations
or conceptual designs due to the lack of suitable detectors
[20, 21].
As a proof of concept experiment, muon tomography of a
reference reinforced concrete block produced by BAM (German Federal Institute for Materials Research and Testing)
was carried out at the University of Glasgow in September
2019 using a detector system originally developed to examine radioactive waste containers. To our knowledge, this is
the first ever experiment of this kind. The results have been
compared to several state-of-the-art techniques for concrete
NDT at BAM. We have chosen the ultrasonic echo and radar
echo techniques as the most used, state-of-the-art methods
for non-destructive investigation of the internal geometry of
concrete structures. X-ray laminography was performed as
a reference. Note, that we are aiming for a qualitative evaluation only, serving as a prof, that further developments and
investments in this technology are justified.
The paper is organized as follows: in the Sect. 2, the
reference concrete block is introduced as well as the various methods and devices used for examination, including
techniques for data processing and imaging. In Sect. 3 the
images produced by the various techniques are shown and
compared for three different horizontal cross sections of the
reference block. In Sect. 4 the advantages and limitations are
compared. Section 5 finalise this paper.
2 Materials and Methods
2.1 The Reference Concrete Block “Radarplatte”
The reference concrete block “Radarplatte” (radar slab)
was produced for training purposes. In its volume of
1.2 m × 1.2 m × 0.2 m four different targets were placed,
which are typical for reinforced concrete structures (Fig. 1).
Near to the top and the back, reinforcement bar mats were
placed, each covering just about 50% of the area and overlapping on about 25% of the area. Depth of rebar is between
about 30 mm (bottom) and about 50 mm (top). The mat at
the top has a mesh size of 150 mm, diameter of 10 mm, and
the one on the bottom mesh of 100 mm, diameter of 6 mm.
In between these two-reinforcement bar mats, an empty tendon duct with an outer diameter of 65 mm and a concrete
cover of 90 mm was placed. Finally, a Styrofoam block of
600 mm × 300 mm × 50 mm was inserted at the bottom to
simulate a flaw at the backwall of the block.
2.2 Muon Tomography
Muon tomography is a technique, which is used to reconstruct 3D density maps of volumes using the Coulomb scattering of muons [9, 10]. By measuring the tracks of muons
as they enter and exit the volume, an estimate of the average
magnitude of scattering occurring in discrete volume elements can be calculated. Due to their high average energy
of several GeV, i.e. 10,000 times higher than the typical
X-ray energy, and due to the way muons interact with matter,
they are highly penetrating and can pass through hundreds
of meters of rock (or concrete).
The primary advantages of using muon tomography
over other methods are penetration depth and the fact that
it is entirely passive and non-destructive method. The comparatively long time it takes to make a measurement using
cosmic-ray muons, can be considered its main detractor;
millions of muons are required to create a high-resolution
image and the flux of muons at sea level is around 170 Hz/
m2. This means that in practice data needs to be collected
continuously for days or even weeks. The flux of muons also
has a strong angular dependence, characterised by cos2θ to
the vertical, which leads to a better imaging resolution in the
horizontal plane than in the vertical direction.
Note, that the term tomography is used differently in
muon imaging and X-ray radiography related literature. In
NDT standards, including those for X-ray imaging, tomography refers to imaging methods using 360° ray coverage,
generated by rotating the object or the source/detector setup.
The correct term for imaging planar objects with limited
ray coverage (access just from two sides of the object) is
laminography. However, to be consistent with the respective
literature we are staying with the term tomography for the
muon imaging method used in this research.
The Lynkeos Muon Imaging System (MIS), used for the
investigation of the “Radarplatte”, consists of 4 detector
modules each containing 2 orthogonal layers of scintillating fibres read out with 64-channel multi-anode photomultipliers. From the fibre hits in each module a space point of
the muon hit can be determined (Fig. 2, Table 1) [13]. Two
modules placed above the volume are used to reconstruct
the incident muon tracks and two below for the outgoing,
scattered tracks.
The active area of the MIS modules is 1 m by 1 m. The
horizontal resolution of the MIS is limited by the 2 mm
diameter of the scintillating fibres used in the detectors;
these are triangularly packed in two sublayers allowing an
effective resolution of less than 2 mm where muons pass
through neighbouring fibres. The vertical resolution of
the reconstructed image is of the order of 4 cm due to the
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Fig. 1 The reference concrete block “Radarplatte”. Left: Design cross-section and view from above. Right: Pictures before and after concreting
angular acceptance of the detector being limited to near vertical tracks. The most important parameters are compiled
in Table 1.
The first two algorithms that were developed and published for the reconstruction of muon tomography data are
known as POCA (point of closest approach) and MLEM
(maximum likelihood expectation maximization) [20]. They
are well known and widely used. A detailed overview of
the different algorithms in use has been published by some
of the authors [10]. The image reconstruction for the muon
tomography data used in this paper is the MLEM algorithm.
A detailed description of the MLEM algorithm can be found
in a paper published by the Los Alamos group [23]. As part
of the reconstruction, the volume between the top and bottom detectors is divided into voxels. The reconstructed value
attributed to each voxel is calculated based by MLEM on
the average measured scatter of the set of muons which pass
through the voxel. The voxel value is expected to increase
with the density of the volume it relates to.
In total 23 million muon tracks were used to reconstruct
the tomographic image of the concrete sample. These muons
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were detected during the continuous running of the MIS for
1203 h between 23rd September and 12th November 2019.
2.3 Radar
Radar is a non-destructive testing tool for the investigation
of Civil Engineering structures and is based on the transmission and reception of electromagnetic waves [24–27].
The received signal gives information about the internal
structure of an investigated object by reflecting the transmitted electromagnetic wave back from objects which are
conductive like metals or have different dielectric properties like concrete and air. The distance and position
of objects can be derived from the received signal and
material properties by analysing the size and shape of the
received signal. The equipment for radar is designed for
using different broadband antennas with a relative bandwidth of about 100%. The typical centre frequency of these
antennas, which are applied for the investigation of reinforced concrete buildings, lies in a range of 1 to 3 GHz.
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Fig. 2 Muon Imaging. Left: principle of muon tomography. Two detectors above and two detectors below the object were used to trace muon
flight paths and scattering, Right: the “Radarplatte” test object inside the Lynkeos Muon Imaging System (MIS)
Table 1 Experimental parameters for muon tomography
Source
Detector
Reconstruction
Cosmic ray muons (1 to 100 GeV)
Lynkeos Muon Imaging System (MIS)
1024 × 1024 Fibres (Resolution < 2 mm)
Exposure time = 1203 h
Trigger rate = 11 Hz
Voxel size 3.4 mm × 3.4 mm × 10 mm
Volume size 300 × 300 × 178
Voxels (1060 × 1060 × 1780 mm3)
The penetration depth decreases with higher frequencies
and the resolution increases simultaneously.
The radar data were collected by guiding a radar antenna
manually along parallel lines, where the distance between
lines was 5 cm. Lines were parallel to the sides of the
“Radarplatte” (Fig. 3). A distance wheel encoder was connected to the antenna to collect A-scans (amplitude scan,
recorded time series for a specific transmitter–receiver
configuration) every 2.5 mm along the line. Data were
acquired both in x- and y-direction.
An antenna with a centre frequency of 2 GHz was used
and connected to a radar control unit (GSSI SIR3000,
Fig. 3). The collected data were analysed using the proprietary software of the manufacturer. The main processing steps
included the application of a travel time dependent gain, 2D
reconstruction by Kirchhoff migration and the calculation of
the envelope function by Hilbert transform. As the response
of rebar is highly dependent on the antenna polarization,
all processed profiles (x- and y-direction) were assembled
to a three-dimensional data cube by adding the respective
amplitudes for the two measurements for each voxel. This
allows the visualization of rebar independent of its orientation. The travel time axis of the three-dimensional data cube
was transformed to a depth axis by using a constant wave
propagation speed.
In general, a characterization of objects is possible
with radar (and as well with ultrasound) by evaluating the
phase information in the reflected signals, but this involves
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Fig. 3 Radar data acquisition and the orientation of the two sets of measurement lines on top of the “Radarplatte”
additional non-standard processing steps and has not been
used here.
Two dimensional projections at three depths were generated. The depths were 5 cm, 12 cm and 17 cm. Each depth
slice was averaged over 1 cm in z-direction.
2.4 Ultrasound
Ultrasonic echo measurements have been established for the
investigation of concrete constructions for about 25 years.
Point contact shear wave transducers, without the need of
a coupling agent, were introduced into practical application
in the mid-1990s and are now almost exclusively used in
the testing of concrete components. Today’s commercial
devices consist of two to sixteen arrays of three to twelve
coupled transducers. The frequency range is in between 40
and 60 kHz (3 dB attenuation), leading to a resolution in
the centimeter-range. Reflections are recorded from elastic impedance contrasts within the object (e.g. concretesteel, concrete-air) and from its boundaries. At interfaces
to air, the energy is almost totally reflected, shadowing all
features behind such interfaces. This means that ultrasonic
echo techniques cannot image features behind e.g. delamination. Aggregates and larger pores are causing scattering of
ultrasonic waves, leading to an inherent level of structural
noise. Depth of penetration is limited to values around 1 m,
depending on the degree of reinforcement, porosity, aggregate size, and other factors. The method is mainly used for
thickness measurement, geometry evaluation, detection of
larger rebar, tendon ducts, voids, cracks and delaminations.
The state of the art is described e.g. in [28–30].
The ultrasonic data were collected using an automated
scanning system (Fig. 4) developed by BAM. A transducer
array (“ultrasonic probe” in Fig. 4) with 12 shear wave point
contact transducers, each for transmitting and receiving, was
13
Fig. 4 BAM NDT scanner with ultrasonic shear wave probe (Acsys
M2503) mounted on the “Radarplatte”
used. The probe was connected to an ultrasonic setup based
on a custom-made pulse generator and commercial data
acquisition equipment. Two data sets were collected on a
2 cm by 2 cm grid using different orientations of the probe
(x- and y-polarization). The data sets were processed and
imaged using the software InterSAFT developed by University of Kassel [31]. The most important parameters are
shown in Table 2. Other than with radar, the resulting data
volumes for x- and y-direction were kept separate. In the
results section, the data set showing the greater response to
the features in question, is displayed.
2.5 X-ray Laminography
While X-ray computed tomography (CT) has been widely
used as a common non-destructive imaging technique, it is
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Table 2 experimental parameters for ultrasound
synchronized and parallel to the object, whereas the object
remains stationary. The detector size is smaller than the
“Radarplatte”. Thus, the detector had been moved several
times, forming six overlapping frames in a 3 by 2 array).
For each frame, the object is irradiated by X-rays from
various source positions (usually several hundred) and the
radiation that penetrates the object is recorded by the detector. The digitally stored projections contain 3-dimensional
information about the object. A 3D volume data set of the
studied object is reconstructed from the projections by a
filtered backpropagation algorithm (FBP) that is adapted to
the specific geometry of the laminography arrangements. In
the volume data set of the concrete plate, different features
can be detected, e.g. reinforcement, concrete matrix, cracks,
and air inclusions [28]. Typically, X-ray laminography data
is not free of artefacts that result from irregular illumination
or the high pass filter used by the reconstruction algorithm
to enhance the edges in the projection data. These artefacts
may complicate the quantitative analysis of the internal features of the object.
The experimental parameters of the planar tomography of
the “Radarplatte” are described in Table 3. The pixel resolution of the digital detector was 400 µm. The total number of
single projections was approx. 7300.
Probe
Data Acquisition
Pulse center frequency
Sample rate
Samples
Point (A-scan) distance
Number of A-scans
Polarization
Reconstruction
Acsys M2503
BAM proprietary setup using
NI components, BAM NDT
scanner
50 kHz
1 MHz
1000
2 cm by 2 cm
51 × 51 (10 cm offset to the edges)
x- and y-direction
SAFT (Software InterSAFT)
not well-suited to visualize internal structures of large and
flat objects. This is because CT requires a full rotation of
the object and the object must fit in the field-of-view of the
digital X-ray detector. Laminography or tomosynthesis are
applied on laterally extended planar objects with large aspect
ratios for example pipelines, large concrete samples, and
rotors of wind power plants, where the 360° image acquisition around the object is not physically possible.
In classical laminography, which is based on a relative
motion of the X-ray source, the detector and the object can
be set up in different geometrical arrangements. The X-ray
source and the detector are either moved synchronously on
circular paths around the object, a so-called rotational laminography, or are simply moved in opposite directions in the
case of translational laminography.
Planar tomography (PT) as a special case of coplanar translation laminography was used to investigate the
concrete plates using the HEXYTech equipment of BAM
(Fig. 5). The radiation source (X-ray tube, gamma source,
accelerator) and detector (e.g. matrix detector) are moved
Fig. 5 Left: HEXYTech-equipment for X-ray laminography of large
and thick-walled test object. Right: sketch of the horizontal scanning
procedure. For each frame position the source and detector move syn-
3 Experimental Results
The experimental results have been acquired and processed
as described in the previous section, resulting in 3D voxel
datasets of the investigated volume. The voxel datasets have
been geometrically referenced to the upper main surface of
the “Radarplatte” (z = 0 m). The x and y axis are along the
longer edges (Fig. 1). To evaluate and compare the results of
chronously along the marked paths (red) relative to the object movement and several hundred projections are recorded for each (horizontal) path
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Table 3 Experimental parameters for X-ray planar tomography
Source
Detector
Reconstruction
Betatron JME 7.5 MeV, focus spot
size = 0.3 mm ∙ 3 mm
Perkin Elmer XRD
1024 × 1024 Pixel (Resolution 400 µm)
Exposure time: 1000 ms
Number of frames: 6, arranged in 3 by 2 array
Voxel size: 0.5 mm × 0.5 mm × 1.5 mm
Volume size: 2460 × 1170 × 200
Voxels (1230 × 585 × 300 mm3)
the investigations, three depth sections parallel to the upper
surface have been extracted at depths of 5 cm, 12 cm, and
17 cm below the upper surface.
Note, that these depth sections are not necessarily produced by the respective values at the precise depth but may
be averaged over a certain depth interval. This is a usual
procedure for ultrasonic and radar images acquired on concrete to smooth the structural noise caused by the inherent
inhomogeneous nature of concrete. Here, muon tomography
is an exception where only single slices of the reconstructed
volume are shown. Some of the datasets are limited to certain parts of the object due to experimental limitations of the
devices used. X-ray tomography misses 10 cm of the upper
part of the block (y = 110 to 120 cm). The muon tomography
is currently limited to 1 m by 1 m and misses a small part
of the left top corner, and a larger part of the bottom right
corner. In addition, the object was inserted into the muon
detection system with an offset of the edges of 0.6 degrees,
which was not corrected in the imaging process. These limitations can be overcome by optimizing the setup.
The uppermost of the three depth levels discussed here,
intersects with the upper reinforcement mesh (Fig. 6a).
Muon tomography (Fig. 6b) shows all rebars clearly, proving a horizontal resolution of at least about 1 cm of this
technique. Some of the features at larger depth show up
slightly (tendon duct and Styrofoam plate) as bright shadows, which is typical for experiments of the transmission
tomography type. Shadow artefacts from the support structure of the sample table appear as dark vertical lines at x
between 10 and 20 cm and above x = 105 cm. Radar (Fig. 6c)
shows all rebars clearly, but the signatures are wider than the
actual bars (about 20 mm in this visualization). Ultrasound
(Fig. 6d) is not able to image the reinforcement in this case
as it is beyond the resolution limit for the setup used here.
X-ray laminography (Fig. 6e) shows the clearest picture of
all presented technologies. Shadows from deeper features
and boundary effects are similar to the ones seen in muon
tomography. Interestingly, the images of the reinforcement
grid show non-equidistant spacing and other distortions for
all applicable technologies. These deviations from the design
drawings were visible as well in a detailed photograph
(Fig. 6f) of the reinforcement grid before concreting, thus
13
not being associated to distortions of the images but to the
actual internal geometry of the object under investigation.
The second depth level (12 cm) intersects with the center
of the tendon duct (Fig. 7a). All technologies are able to
image this feature, alas, with different clarity and level of
detail. While X-ray laminography (Fig. 7e) shows even the
undulations of the corrugated pipe, all other methods images
are more or less straight shaped and show a significant level
of noise. Radar (Fig. 7c) shows artefacts from the reinforcement layer above, which is typical for echo techniques. Muon
tomography shows the tendon duct with a clarity comparable
to radar and ultrasound. As in X-ray laminography, artefacts
from features above and below are present.
The third depth level studied intersects with the lower
reinforcement and the Styrofoam block (Fig. 8a). The Styrofoam block is imaged by all techniques, while the reinforcement is missed by ultrasound (Fig. 8d) for the same reasons
as the upper reinforcement. The radar image (Fig. 8c) is partially distorted by artefacts caused by the features above this
depth level. These features show up in the muon tomography
(Fig. 8b) and X-ray laminography (Fig. 8e) images as well,
but with lesser effect on the image of the objects, which are
actually at this depth level. Radar and X-ray-laminography
pick up the irregular spacing between the two y-oriented
rebar at the left edge, which is visible in the close-up photograph (Fig. 8e). In the muon tomography image this is hard
to verify due to an imaging artefact. Note, that the rebar
diameter of the lower mat is just 6 mm, verifying the potential of muon tomography for sub-cm resolution.
4 Discussion
The muon tomography images, acquired with a setup which
is not yet optimized for concrete inspection, have shown that
high density and low density features in concrete objects
can be imaged by this emerging technology and be distinguished without further data processing (low density/air:
bright, high density/steel: dark). The resolution of muon
tomography might exceed the one of ultrasound and radar,
at least for the scenario investigated here. Some artefacts
are present in the images. Some (e.g. the shadows of objects
above and below the depth level under consideration) are
due to two inherent limitations of the technology: first,
tomographic reconstruction algorithms may produce artefacts and are”smearing” anomalies in case of limited angular
coverage. Second, the angle of the incident muons varies
just between − 30° and + 30° from the vertical axis due to
the geometrical acceptance of the detector system. This is
limiting the vertical resolution.
In addition to all the features present in the concrete slab,
an additional high-density vertical band is present towards
the left edge of all the muon tomography images. This is the
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Fig. 6 The horizontal cross-sections, depth of 5 cm (upper reinforcement). a Design with upper reinforcement, b muon tomography, c radar, d
ultrasound (y-Polarization), e X-ray laminography and f detail of upper reinforcement (slightly irregular mesh)
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Fig. 7 The horizontal cross-sections, depth of 12 cm (tendon duct). a design with position of tendon duct, b muon tomography, c radar, d ultrasound (x-polarization) and e X-ray laminography
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shadow of part of the support structure which held the concrete sample in position during the measurement. The image
edges are noisier and more sensitive to misalignment due to
the limited acceptance of events only from vertical muons.
The comparison of muon tomography images with
those of X-ray laminography show similarities (shadowing
effects from above and below, edge effects, different signature of low- and high-density objects). The resolution of the
images as well as the low noise level are distinct advantages
of active X-ray technologies. Still, the quality of the muon
tomography images is even in this early stage (first-ever
experiment on reinforced concrete using a new technology
in a non-optimized setup) fully sufficient for an assessment
of the internal geometry of the object under investigation, e.
g. for deriving positions where it is safe to drill.
The distortions of the reinforcement grid in the NDT
images compared to the design drawings was shown to be
due to actual misalignment of some rebars. This hints to the
capability of these techniques, including muon tomography,
to provide detailed, high resolution checks of the actual position of features compared to the design in practical applications. However, the degree of accuracy must be determined
in further research.
Page 11 of 14
65
5 Conclusion and Outlook
Our first-ever experiment with muon tomography of a reinforced concrete block was successful. All built-in features
were detected and correctly identified. The clarity of the
images matches the one of radar and ultrasound while the
resolution might even be better. The detection limit for rebar
is smaller than 1 cm diameter rebar. However, a thorough
quantitative assessment and validation of the results is still
pending. Note, that the data shown here required a recording
time in the order of weeks while the acquisition of ultrasonic
data was performed within about two hours and of the radar
data in less than 30 min. Equipment costs and requirements
regarding operator skills are currently much lower for radar
and ultrasound as well. Muon tomography in its current
state does not reach the image quality of X-ray tomography.
However, it does not require any radiation safety measures
on site.
We are optimistic that muon tomography can be developed into a useful tool for non-destructive structural investigations to fill the gaps in technologies currently used on site.
The highest priority to progress this exciting technology is
the development of an efficient and affordable mobile muon
detector, which could be mounted above and below bridge
decks or inside and outside of box girders. Measurement
and processing parameters still must be optimized. A thorough validation of the technology and its possibilities and
limitations has to follow. We foresee that a combination of
different technologies using methods from data fusion and/or
improvements of the reconstruction software using machine
leaning will be important steps on the way to becoming a
standard technology.
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Journal of Nondestructive Evaluation (2021) 40:65
Journal of Nondestructive Evaluation (2021) 40:65
Page 13 of 14
◂Fig. 8 The horizontal cross-sections, depth of 17 cm (lower rein-
forcement and Styrofoam plate). a design with position of reinforcement and Styrofoam plate, b muon tomography, c radar, d ultrasound
(y-Polarization), e X-ray laminography and f detail of lower reinforcement (distance between leftmost bars in y-direction smaller than
usual)
6.
7.
Acknowledgements The work of the University of Glasgow has been
supported by funding from STFC and EPSRC via the University of
Glasgow Impact Accelerator Account.
8.
9.
Funding Open Access funding enabled and organized by Projekt
DEAL.
10.
Data Availability The data are available from the authors on request.
The reference concrete block is accessible at BAM.
Code Availability No special code has been developed for this study.
11.
12.
Declarations
Conflict of interest The authors state no conflict of interests.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
13.
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15.
16.
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