Keywords: diagnosis, auscultation, non destructive testing, infrared thermography, delaminations, moisture, natural sunning.

The auscultation of civil engineering structures by non-destructive testing methods makes it possible to diagnose and assess a large variety of inherent problems which, if left uncured, will continue to develop and aggravate with time. These problems can furthermore trigger new problems, thus causing pathologies leading to an accelerated ageing of the structures. So, with the increasing number of civil engineering structures throughout the world, the need for non-destructive testing methods is becoming urgent.

This article presents the possibility of using a non-destructive method of measurement based on infrared technology [1] for the diagnosis of civil engineering structures. By the use of a compact portable infrared camera, it is possible to detect subsurface details such as : (i) heterogeneities in the form of delaminations [2] or moisture movements, and (ii) reinforcement bars.

The study was carried out on mortar samples. Laboratory experiments of localisation of defects and reinforcements were carried out on mortar slabs, whereas experiments of monitoring of moisture rise by capillarity were carried out in mortar piles. In the case of delaminations, the experimental results were confronted with the results predicted by a finite element model. The active thermography technique [3] was employed for the observation of delaminations and reinforcement bars hidden under the mortar surface. For the delamination experiment, the mortar sample received a thermal excitation generated by an electric hotplate controlled by an electronic device. The excitation consisted of the transmission of heat energy homogeneously dispersed throughout the sample so as to create a temperature field pattern of sufficient amplitude to visualize the defective zones. The delaminations were simulated inside the samples by the introduction of polymer parts of known dimensions and positions. This precise knowledge of the position and size of each simulated defect enabled us to monitor with an infrared camera the surface temperature of the defective as well as of the sound zones during the thermal excitation. The analysis and processing of the infrared images on computer programs then made it possible to extract the exact temperature values of the healthy and defective zones. These measured experimental values were then plotted according to time to produce temperature profiles which were then compared with the corresponding profiles given by a numerical model. With the numeric model, it was possible to study the parameters influencing subsurface flaw detection by infrared camera. Other experiments showed the possibility of localisation of metallic reinforcement bars by the use of thermal excitation generated by electric heaters embedded in the sample.

Experiments for the monitoring of humidity movements in mortar piles were also done under natural thermal conditions (i.e. passive thermography technique, without any source of external energy). During the experiment, a portion of the initially dry mortar samples piles were continuously immersed in a constant height of water. The capillary absorption and rise of the water in the samples was then subject to a follow-up by punctual weighing throughout time. The surface of the samples was simultaneously scanned by the infrared camera. The analysis of the recorded infrared images clearly showed the progression of the moisture front with time.

The natural sunning as a source of excitation was the subject of a numerical study in order to investigate the potential use of the infrared thermography technique for the diagnostic of civil engineering structures exposed to natural climatic conditions. This study was carried out for the two extreme seasonal winter and summer periods with realistic thermal conditions. It was possible to evaluate the notable differences in temperature between healthy and defective zones on the surface of a structure. The numerical model is actually being validated thanks to the implementation of an experimental in situ study. Once this numeric model will be calibrated and validated, it will be possible thereafter to obtain, by simple visualisation of the infrared images, some informations on the defects found in the structure.

Finally, the infrared technique of thermography proved to be fast, non destructive and contactless. It can thus be used for the follow-up and the diagnosis of works. The positive results of these experiments justified its use in future work like the study of the diagnosis under natural thermal stresses (sunning) or the coupling with other techniques (deflectometer, videometry, ...) for the large-scale follow-up of defects in the bituminous roadways.

1

N. Avdelidis, A. Moropoulou, "Applications of infrared thermography for the investigation of historic structures", Journal of Cultural Heritage, 5, 119-127, 2004. doi:10.1016/j.culher.2003.07.002

2

C.A. Balars, A.A. Argiriou, "Infrared thermography for building diagnostics", Energy and Buildings, 34(2), 171-183, 2002. doi:10.1016/S0378-7788(01)00105-0

3

G.A. Washer, "Improving Bridge Inspections", Public Roads, FHA, 67(3), 2003.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £135 +P&P)