  Quality assurance and condition assessment of concrete structures is an
important topic world-wide due to the ageing 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 resolution much higher 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 for
contraband or the assessment of the contents of radioactive waste containers.
But 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 show more resolution and less
distortion compared to ultrasonic and radar imaging. 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 the tremendous potential of muon
imaging for concrete inspection. The next steps include the development of
mobile detectors and optimising acquisition and imaging parameters.
