# The XTRA Option at the NEUTRA Facility--More Than
## 3 Examples of Applications
### Investigations of Construction Materials  
The investigations of water uptake/transport in concrete and cement-based materials are one of the oldest and still also the most frequent [20] applications of transmission neutron imaging. The addition of the X-ray imaging modality allows for a significant extension of such investigations. One of our first applications of bi-modal N/X imaging focused on the investigation of the inner structure of fiber-reinforced concrete [21], which allowed for the segmentation of aggregates and air voids from the neutron tomographic dataset, while the distribution of fiber reinforcement (0.1 mm stainless steel fibers) was based on the corresponding X-ray tomography (see Figure 9).  
Figure 8: Results of tomography studies with neutrons and X-rays of the ceramic samples from Augusta Raurica containing both metallic and organic remains at different positions of the samples: above—virtual slices; below—3D view on the material distribution.  
Figure 9: Tomography data of a steel reinforced concrete sample (**left**) (3 cm \(\times\) 3 cm \(\times\) 15 cm) obtained with X-rays (**middle**) and thermal neutrons (**right**).  
Note that the spatial resolution of the neutron images in [21] was more than an order of magnitude worse than it is today, and thus X-ray tomography provided better resolution of the steel fiber network.  
Concrete is one of the porous materials where the benefit of bimodal neutron/X-ray imaging is established by the high sensitivity and resolution of X-rays for the porous matrix, while neutrons provide superior contrast for the observation of a liquid hydrogenous phase distributed within the matrix. Only the combination of both provides meaningful insights. For example, the interfaces between light weight aggregate internal curing reservoirs and the adjacent cement paste can be detected by X-ray imaging while neutron imaging hardly enables a distinction of these phases. Thus, while the morphologies of such interfaces is examined by X-ray imaging, their impact on the water transport and function for internal curing reservoirs [22] can only be illuminated by the added sensitivity of neutrons for the water distribution, see Figure 10.  
Another concrete-related field of investigation in which such a bi-modal approach has shown invaluable potential is the issue of corrosion of metal reinforcements in concrete. Corrosion of reinforcements is a threat to the sustainability and safety of our basic infrastructure and causes significant cost to society. Neutron imaging provides unique insights on corrosion of reinforcements within the concrete environment and the non-destructive nature allows for continued observations of corrosion processes in the context of the surrounding material. An example of an investigation at NEUTRA showing the 3D distribution of corrosion products and thus the extent of corrosion in a reinforced concrete specimen is shown in Figure 11[24].  
High resolution bi-modal neutron/X-ray studies enable yet more detailed insights, not only identifying individual corrosion pits on the rebar surfaces but correlating their respective position with different phases in the surrounding concrete matrix.  
Figure 10: Core-drill of plain concrete—(from **left** to **right**) section from X-ray tomography 120 kV, photograph, corresponding section based on neutron tomography. The sample diameter is 40 mm (image reprinted from ref. [23]).