The role of Computed Tomography

The role of computed tomography

György L. Balázs (Budapest University of Technology and Economics)

Computed Tomography ‒ often abbreviated as CT ‒ has been developed to detect modifications – that is illnesses - in the human body that can be an indication of any kind of diseases. In engineering we can say that this is a non-destructive method of diagnostics.In order to assess the health of our engineering structures the internal structure of the elements are needed. Can we inspect the internal structure of the material? Can we recognize pours and voids? Can we observe any deterioration of the material as we do with illnesses for human body?
In engineering we often would like see what is inside a structural element. Are there any cracks inside, is there any corrosion of reinforcement etc. It is all possible with computed tomography: to realize cracks and voids inside, anything that is based on differences in densities.

Rontgen invented X-ray for medical applications in 1895 that is used in our everyday life to detect cracking in our bones or modification in or lounge. Hounsfield and Cormark were awarded Nobel prize in 1979 developing the Computed Tomography based on the X-ray technique after successful clinical applications.
By applying the Computed Tomography technology and evaluating the measurements we are able to distinguish differences in density in 3 dimensions. This is true for both the medical as well as the engineering applications. The physical measurement observes the differences in attenuation levels of different materials. We should call the attention here that the result what is provided by a camera is a photo, however, the result of an evalutation provided by CT measurement is an image. The measured CT values are numerically expressed in Hounsfield (HU) units which is a function of a K constant (being 1000), µ is attenuation coefficient of investigated point µwater is te attenuation coefficient of water.
Experimenal results indicated the efficency of CT measuring technique not only to detect corrosion of steel reinforcement but also to freeze -thaw resistance of concrete and level of deterioration after fire of concrete by the evaluation of the modification of pore structure of concrete.
Whenever fibres are mixed to concrete, the fibre reinforced concrete (FRC) must be mixed as long as the fibre distribution is random. Unfortunately, it is very difficult to check in practical cases. With CT analysis the fibres can be distinguished in orientation and in position. Assessment of existing or new reinforced concrete constructions is often used especially to localize the steel reinforcements. The equipment – which is often called as Concrete scanner - is shifted in two directions of a horizontal slab by taking the measurements. The observed pattern of reinforcement can be presented in view, or in section or even in 3D. Not only the position of the reinforcement in the view is an important information, but also the depth of the concrete cover can be measured if the diameter of reinforcement is known.

In the nautical and aviation fields, an air radar is used to recognize other objects by sending out pulses of radio waves which are reflected off the other object back to the source. In case of the application of Acoustic Emission (AE) technique we observe that happens in the specimen with very sensitive microphones and evaluate the acoustic signals. The number of microphones is at least 4 to be able to make localization of signals. The Acoustic Emission signals can be considered as micro earthquakes, because they both represent the appearance of crack surface. AE signals indicate the deterioration.

As conclusion we can say that the efficiency of Computed Tomography has been indicated for checking cracking and corrosion of reinforcement as well as freeze-thaw resistance of concrete, influence of fire to concrete as well as fibre distribution in concrete. In additional to, the potentials of radar and Acoustic Emission techniques were mention for non-distructive testing of structural material. In principle CT evaluation gives the possibility for engineers: to characterize cracking, to realize uneven distribution of a specimen and to observe deterioration in the construction materials. These all contribute to: judge the level of safety of the element and provide possibilities for quality control.