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Active infrared thermography in civil engineering - quantitative analysis by numerical simulation
(2003)
The cooling-down process of building structures after heating-up with an external radiation source was analysed to detect voids inside and below the surface. Quantitative results of concrete test specimen containing voids with different sizes at various depths will be presented here. The experimental results were compared to numerical simulations performed with a Finite Difference program developed at BAM.
Today's civil engineering NDT needs are for quantitative, fast to apply and easy to interpret methods. They are required on site both for evaluation of existing structures/infrastructure and for quality control in new construction. With this purpose, BAM has carried out in-situ feasibility studies, using radar, impact-echo and ultrasonic-echo methods. Two examples are given of their combined application on different railway slab–track constructions, using high frequency radar, scanning impact-echo and ultrasonic array methods. The aim was to evaluate limitations and advantages, comparability and complementarity of these methods. The on-site measurements demonstrated the possibility to investigate, with good accuracy via NDT, the bonding conditions between sleepers and slab, to locate voids and measure layer thickness. The acoustic and electromagnetic methods used, were found to be complementary.
The application of impulse radar for structural investigation of concrete elements has been increased during the last years related to technical developments, i. e. of high frequency antennas. In this paper, case studies will be presented related to the location of reinforcing bars, tendon ducts and repaired concrete areas in concrete bridges, anchors and dowels in concrete highways and delaminations of layered structures as they are used for non-ballasted railway tracks. It is shown that impulse radar can be applied in case of regular inspection and for searching the cause of damages but also for quality assessment in civil engineering.
Impulse-thermography is an active method for quantitative investigation of the near surface region of various structures which has recently been applied to civil engineering. It is well suited for the detection of voids and honeycombing in concrete up to concrete covers of 10 cm and more. For quantitative analysis, a computer program for numerical simulation of the heating up and cooling down processes was developed based on Finite Differences. With this program parameter studies have been performed for investigating the influence of environmental conditions, material parameters and geometry on the thermal behaviour. The comparison between experimental and simulated results enables the Inverse Solution.
The applicability of pulse phase thermography (PPT) for the investigation of structures is studied systematically on concrete test specimens and on a plastered sandstone column. In the test specimens, voids and delaminations are implemented in different depths and with different sizes, modelling real voids, honeycombing and debonding. Delaminations of plaster in concrete and masonry and behind tiles on concrete are investigated. PPT is based on the frequency analysis of the cooling down process of actively heated surfaces. Therefore, it is contactless and thus completely non-destructive (if overheating of the surface is prevented), fast and allows the inspection of large surface areas. The interpretation of amplitude and phase images gives semi-quantitative information about the observed defects. The phase images provide a deeper probing up to 1015 cm in relation to the interpretation of the thermograms and to the amplitude images. In addition, the influence of surface inhomogeneities and non-uniform heating is reduced.
An advanced quantitative approach of pulse-phase-thermography for nondestructive
testing in civil engineering is described in this contribution. The
characteristic frequency of the maximum phase-contrast between defects and sound
areas is used as a means for the characterization of its depth.
The new approach is tested in the laboratory on concrete structures with defects of
polystyrene. The surfaces of the structures were heated with IR-radiators for varying
time periods.
The presented investigations were funded by the Deutsche Forschungsgemeinschaft
(DFG) and were carried out in co-operation with the Technical University of Berlin
(TUB).