I2.2 - Application of active thermography to the detection of safety relevant defects in civil engineering structures

SENSOR+TEST Conferences 2009
2009-05-26 - 2009-05-28
Congress Center Nürnberg
Proceedings OPTO 2009 & IRS² 2009
IRS² 2 - Pyrometry/Thermography
C. Maierhofer, M. Roellig - BAM Bundesanstalt für Materialforschung, Berlin, Germany
215 - 220


During the last ten years, advanced non-destructive testing (NDT) methods like radar, ultrasonic and sonic methods have become available for the assessment of existing structures in civil engineering. These technologies are mainly suited for the detection and characterization of inhomogeneities at depths between 5 and 100 cm. In the near surface region within the first 10 cm, there is still a deficiency of information as many safety relevant cases of damage originate from defects close to the surface. E. g. voids and honeycombing among the top layer of reinforcement in concrete structures, delaminations of CFRP laminates used for subsequent strengthening of concrete and masonry structures, delaminations of protective coating systems as well as surface and subsurface cracks can be characterized with active thermography as will be shown in this contribution.
The main innovation of active thermography related to standard testing methods is a faster, non-destructive, image-guided and reliable detection of defects and inhomogeneities close to the surface of a variety of building structures. In contrary to the well known passive investigations of the quality of thermal insulation of building envelopes, this method is based on active heating by using either an internal or external heating source for a distinct time interval. Due to the resuming temperature differences, a non-stationary heat transfer is induced. Structural elements, inhomogeneities of material properties, voids and delaminations can be detected in the thermal images (thermograms) recorded with an infrared (IR) camera, if thermal properties are different related to the surrounding material. The difference between temperature distribution and change as a function of time above non-defect regions and inhomogeneities includes information about the defect parameters like depth, lateral size and type of material. Sophisticated data analysis of temporal temperature data in time and frequency domain (e. g. pulse-phase-thermography) affords the detection of flaws in concrete structures, masonry and multi-layered systems with high reliability. The combination of experimental data and numerical simulation enables the selection of optimum measurement parameters as well as quantitative information from experimental results.