Detection of shallow voids in concrete structures with impulse thermography and radar: Fid-Bau Portal

Detection of shallow voids in concrete structures with impulse thermography and radar

Maierhofer, C. / Brink, A. / Röllig, M. / Wiggenhauser, H.

For a very long time, the application of infrared thermography techniques in civil engineering was only limited to investigations of the quality of thermal insulation of building envelopes. The development of new thermography systems recording digital thermal images in real time with a frequency of 50 Hz enables also the application of active techniques, i e. the observation of the cooling down process at the surface of the structure under investigation after warming up with an external or internal heating source (i.e. a radiator). Defects like voids in concrete which have different density, heat capacity and/or heat conductivity in comparison to the bulk material are then visualised by different surface temperatures during the warming up and/or cooling down phases. The differences between temperature transient curves at surface positions above sound regions and above inhomogeneities are expected to include information about the defect parameters like depth, lateral size and type of material. In this presentation, experimental investigations with impulse thermography and radar on a concrete test specimen containing voids with different sizes and depths were analysed. Radar as a non-destructive technique for the location of inhomogeneities is used to determine the depth of the voids. The influence of size and depth of the voids on the temperature transient curves was analysed for different activation energies (variation of heating time). The transient curves were also fitted with a simple semi-empirical model. The presented results demonstrate that radar as well as impulse infrared thermography are both nondestructive testing methods which are very well suited for the detection of voids in concrete structures. With radar, the depth as well as the angular position of the voids (tilted or parallel to the surface) can be determined taking into account the well known propagation velocity of electromagnetic waves in dry concrete. Nevertheless, data recording with radar is very time consuming and the collected radar traces have to be processed to obtain time (depth) slices. Using the 1.5 GHz antenna, voids at depths between 1 and 20 cm can be localised. Using impulse thermography, voids up to a depth of approximately 10 cm can be detected.