Facades inspection with infrared thermography: cracks evaluation
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Infrared thermography is an evaluation technique that helps to identify anomalies and can provide information on their incidence and severity. The aim of this study is to apply quantitative thermography to study a case of severe cracks in facade analyzing the evolution of temperatures and Delta-T. The temperature evolution was studied over a day. There was sun incidence occurring during part of the day, allowing to calculate and assess the evolution of the associated temperatures. Thermograms were acquired hourly during the study period. The highest temperatures were observed at 12:00 noon, and the larger Delta-T between 10:00 and 12:00. There were differences between the Delta-T profiles among the three regions studied. The best time for analysis is when Delta-T is higher. Cracks with higher Delta-T were considered more degraded, and generalized branched cracks did not allow a comparative conclusion.
KeywordsInfrared thermography Facade Crack Delta-T
Infrared thermography is a technique used to study the pathology and anomalies of buildings. This technique applied to the identification and mapping of faults allows a new inspection approach that enables classifying the damage areas regarding the occurrence intensity and severity of anomalies.
Thermography is a non-destructive technique that provides a thermogram, a thermal image of the target object surface, which in this case is a region of the facade. This inspection, made without physical contact, can reach distances of tens of meters and causes no damage to the element. The result is given in real time, i.e., the temperatures observed in the thermographic camera are those occurring on the target object at the time .
The thermogram of a facade allows analyzing the surface temperatures of the elements and materials that compose it. Obviously, temperatures are a result of the action of various factors, including the action of climatic agents such as the incidence of the sun and ambient temperature. Elements in thermal or hygroscopic equilibrium with the environment are difficult to study in thermography since significant differences in surface temperature become more difficult to identify . There must be a heat flux from the external environment to the facade (direct) or from the facade to the environment (inverse) to identify the anomalies. These fluxes occur naturally by the action of climatic agents on the facade. The defects or anomalies cause disturbances to the heat flux, leading to a distribution of different temperatures on the surface of the facade or element, which is captured by the thermogram. The main difficulty, in thermographic inspection, is to identify at which heat flux condition the defects are visualized, and how they manifest themselves [3, 4].
Different types of inspection may be used when applying the thermography. If the heat flux used to study the thermography has a natural origin, for example, derived from sunlight on the facade, this study technique is named passive thermography. If the heat flux is induced on the element by heating, vibration, ultrasound, among others, the technique is known as active thermography [5, 6]. Thermography can also be considered qualitative when it is based only on the identification of hot and cold spots in the thermographic image. In this case, the analysis consists of comparing the images with standards, this type of study is usually used in restorations and diagnosis of historic buildings . Otherwise, the quantitative thermography is used when the objective is to classify the importance of the defect, and the greatest concern is the accuracy of the obtained temperatures, as well as using the proper techniques to obtain thermograms and analyze the relevant information . This approach requires accurate measurement and assessment of the material parameters such as emissivity and reflected apparent temperature. These parameters should be properly introduced in the thermogram analysis software to be able to work quantitatively with the required accuracy [9, 10, 11].
In the quantitative analysis, Delta-T is the initial parameter, corresponding to the temperature difference between the defective area and the defect-free area. Comparatively, for the same exposure conditions, a greater Delta-T may indicate a more severe condition for the anomalies . Bauer et al.  correlated the depth of cracks with the Delta-T values measured in laboratory heating cycles and reported that higher Delta-T values are associated with deeper cracks. Thus, the thermogram obtained from the quantitative thermography study can indicate the severity degree of the anomaly.
The cracks of the facade may be due to anomalies in the coating material, or associated with the substrate elements (masonry) . The diagnosis needs to establish the causes of cracks, identify the existing typology and the degree of damage. The use of thermography to study cracks, detachment and humidity are still being performed on an experimental basis, even the field studies. The studies have shown a strong dependence on the variations of surface temperature and sun incidence, with no agreement about the criteria regarding the best time to make the thermographic inspection (day or night) [15, 16]. It can be said, obviously, that the weather conditions in the country where the studies are being performed are an important variable in the processes and results since they involve differentiations of heat flux for each study [4, 14].
Few field and laboratory studies have been made regarding the degradation of facades by cracking. Most often, thermography has been used to observe the location of cracks , and not as quantitative thermography, i.e., without evaluating the severity of the anomaly.
The aim of this study is to apply quantitative thermography to study a case of severe cracks in a building facade. The temperature evolution throughout the day is investigated using thermography. The incidence of the sun during part of the studied period allowed calculating and evaluating the evolution of the associated temperatures and classifying the severity of the anomalies.
Development of experimental methodology
The thermographic monitoring consisted of studying the thermograms and obtaining the temperature at specific spots in the cases of linear cracks (A1, A2, A3, B1, and B2), and the average temperature of the cracked area in the case of branched cracking (B3, C1, C2), as shown in Fig. 1a.
The evolution of the thermal behavior of the cracks is evaluated by comparing the temperature of the defective region (TD) with the temperature of defect-free region (TND). Thus, defect-free spots, where TND was determined initially in the thermogram, were kept fixed throughout the study.
The research procedure consists of thermography monitoring of the facade in the studied region employing passive thermography. The east facade was exposed to the sun in the morning, from 7:00 to 12:00 noon. The study was conducted in July, period with no rainfall and clear skies in Brasilia. The temperature evolution throughout the day was defined by performing a hygrothermal simulation of the facade using the Wufi® Pro 5.3. Data on existing materials and a typical meteorological year (TMY) file specific to the climate of Brasilia were used for the simulation. The absorbance of 0.3 was considered.
The corrugated aluminum sheet was attached to the surface of the facade, and the image was obtained at a distance of 2.0 m. This step is necessary to obtain the TAR, as this parameter is an input data requested by the infrared camera.
Positioning of the thermographer at the point of image acquisition (distant 18.5 m from the facade), and determining the relative humidity (RH) and ambient temperature (T). Inserting the data in the camera software.
Adjustment of the thermal focus and acquisition of three consecutive images.
Subsequent analysis of the images in Flir QuickReport 1.2 to obtain the temperatures on the studied spots.
Results and discussion
The studies conducted showed that the visual analysis of thermograms (qualitative) provided simple information, which are best understood by quantitative analysis. The anomalies can be classified according to Delta-T results regarding type, and as a preliminary inference regarding severity. Region A had linear cracks and the largest Delta-T values. The thermograms show that the “hottest” area is actually a band along the fissure. This result seems to suggest that perhaps this anomaly has emerged as an initial crack, and as it evolved over time due to the action of rain and temperature, it has propagated even as adhesion failures along the fissure. This is corroborated by the high Delta-T values observed. In region B, the crack studied at B1 and B2 spots shows a smaller width band (compared to region A) and lower Delta-T values. Most likely, in these cases, the damage is smaller compared to the previous.
The branched cracks appear sharp in the thermograms. The B3 crack behaved consistently with the analysis of the region. On the other hand, the C1 and C2 cracks had no significant Delta-T evolution. Obviously, it raises the question about the possible superficiality of these fissures. However, it is clear that further analysis should be performed. Perhaps defining the degree of branching of the cracks or the use of other parameters, such as the functions of thermal contrast [4, 6, 18], would allow a more definitive analysis.
The study performed to assess the behavior of cracks in the facade using the quantitative passive thermography, allowed the main conclusions that are listed below.
The use of passive thermography in the field requires knowing the temperature evolution of the studied element. In this case, the hygrothermal simulation is a very applicable tool.
The maximum temperatures occurred near 12:00 noon, and decreased after that due the lack of sun incidence. Thus, the best period for the thermographic inspection is the morning, when the largest Delta-T values are recorded.
Linear cracks in regions A and B showed the highest Delta-T values, possibly showing associated neighboring degradations.
The branching cracks are difficult to analyze quantitatively, but they can be visually identified in regions with higher Delta-T values. Other approaches are suggested, such as the functions of thermal contrast to better understand and quantify the degree of degradation of elements.
Field tests are complex, either by the variability of the climate action or by constructive variations and degradation of the studied elements. Thermographic inspection is an auxiliary tool of great application potential, but it must be used with appropriate care by the thermographer, by being careful when taking the pictures and in the subsequent analysis of the thermograms.
The authors thank the Materials Testing Laboratory of the Universidade de Brasilia for providing the equipment and support to this study, CNPq and CAPES for supporting the research, and L. E. B. Bauer for reviewing and editing the text.
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