Muon tomography of a reinforced concrete block -- first experimental proof of concept

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.