Quasi-quantitative infrared thermographic detection of moisture variation in facades with adhered ceramic cladding using principal component analysis
Introduction
Moisture increases for various reasons such as rain water infiltration, condensation or capillary rise of ground water and it is a common problem of facades with adhered ceramic cladding [1]. It may cause further defects such as biological growth, efflorescence, cracking or detachment of tiles and can affect users' indoor comfort. The heating energy demand of a building may also rise because moist material has higher thermal conductivity. If increased moisture is detected before visible defects occur then the necessary (preventive) maintenance measures can be taken. This will also help to maintain the integrity of the facade, preserve users' indoor comfort conditions and prevent excessive energy use for heating.
Infrared thermography (IRT) is a remote, non-destructive inspection method used in the field of construction/building for various purposes; commonly to investigate the thermal performance/energy efficiency of buildings/building elements [2], [3], [4], [5], and for the diagnosis of historic buildings from different aspects [6], [7], [8]. In addition, research studies are still made to improve/adapt it. For instance, Fox et al. [9] studied on the time-lapse thermography, in other words taking various time-dependent thermograms of the object and evaluating the thermal response over time, through in situ inspections for the detection of building defects such as cracks. Theodorakeas et al. [10] made a quantitative analysis of active IRT results through laboratory experiments and numerical simulations to detect plastered mosaics. González-Aguilera et al. [11] studied on the automatic three dimensional thermographic modelling of buildings to be used in energy efficiency studies. Jo and Lee [12] made a quantitative modelling and mapping of blistering zones by active IRT through in situ inspection. Bauer et al. [13] studied the reproducibility of the IRT findings in delamination detection of renderings through laboratory tests.
IRT can also detect damp building components non-destructively, but its use is not yet fully standardised for all building elements and/or materials. IRT using natural sources/boundary conditions (e.g. solar radiation, air temperature, wind) to provoke surface temperature variations helps the qualitative detection of wet insulation in roofs [14] and of moisture within porous materials [8], [15], [16], [17], [18]. Because its accuracy in adhered glazed ceramic claddings is limited, it is best used as a preliminary inspection technique to decide whether further inspection with sophisticated and usually destructive techniques is necessary [19]. Where IRT applications use artificial sources to stimulate surface temperature change, the evolution of surface temperature is analysed quantitatively to detect and sometimes characterise moisture change [20], [21], [22]. However, these artificial sources require close contact with the inspected object, and thus the advantages of the remote nature of IRT are undermined in the exterior inspection of medium-to high-rise buildings.
When natural sources/boundary conditions are used in IRT, time-dependent and/or quantitative analysis methods have sometimes been tried, mainly to increase accuracy in detecting different types of defects [23], [24], [25], [26]. Considering the good results reported in the IRT literature, a research study that evaluated the use and performance of time-dependent IRT and quantitative analysis methods in the inspection of adhered glazed ceramic claddings was completed in 2013. Their use to detect delamination, which is another common problem of adhered ceramic claddings that was studied during the research, has been presented and discussed elsewhere [27], [28], and the scope of this paper is limited to detecting changes in moisture content. In this work, the use of quantitative methods is called a quasi-quantitative IRT (q-QIRT) application, since the purpose was to increase the accuracy of IRT applications, not to characterise defects. For this, a building block in Lisbon (Portugal) with rising damp problem was inspected by time-dependent IRT, and the resulting data were analysed by simple image subtraction (SIS), nonnegative matrix factorization (NMF) and principal component analysis (PCA). Surface moisture measurement and hammer-tapping were used as alternative non-destructive techniques to check the q-QIRT results. In this paper, we assess and discuss the inspection of this building block and the q-QIRT results obtained by PCA in detail, since the most promising results were achieved by PCA.
Section snippets
Moisture detection by IRT
Detecting moisture change with IRT may depend on the different physical phenomena, affected by the characteristics of the object in question. In porous materials, evaporative cooling causes variation in the surface temperature of the moist area and so IRT inspections should be performed when there is some wind but no solar exposure [29]. Evaporation is restricted in assemblies with a non-porous layer such as waterproofing membrane or glazed ceramic tile, and therefore the increased heat storage
In situ inspection of an adhered ceramic cladding
In a research study completed in 2010 [19], a building block consisting of seven attached buildings was found to have rising damp in its west facade. The buildings have a reinforced concrete structural frame with hollow clay brick infill walls, and no basements. Their ground floors are used as unconditioned garage spaces and their west facing facades are clad with white glazed ceramic tiles. In this study, the areas of interest (AOI) that were investigated in the previous study, i.e. areas A1,
Quasi-quantitative analysis of IRT data
SIS, NMF and PCA was used for the quasi-quantitative analysis of IRT data, given the good results reported by Grinzato et al. [21], Lerma et al. [25], Marinetti et al. [38], and Rajic [35] for the detection of moisture increase and/or delamination problems. The inspection of the delamination problem detected in one AOI of this block and in another building has been discussed in detail elsewhere [27], [28]. In relation to the detection of moisture increase, nearly all SIS analyses performed in
Discussion on the potential of q-QIRT using solar heat gain and PCA to detect moisture variation
The appropriateness of the methods/techniques selected for the verification of the results, the comparative performance/accuracy of the proposed approach and optimum inspection timing, its performance in eliminating false indications, some important factors in the interpretation of results and its adaptation potential are discussed separately in the following subsections.
Conclusion
The detection of increased moisture content in building facades before any signs become visible makes it possible to take the necessary (preventive) maintenance measures in good time. The use of q-QIRT was therefore studied in the inspection of adhered glazed ceramic claddings, since moisture-related problems are common in this type of cladding. The main objective was to evaluate the potential of a q-QIRT approach using solar heat gain to induce surface temperature variation, and of different
Acknowledgement
The authors thank the IST-DECivil Building Construction group, ICIST and ISTAR for allowing the use of equipment. The first author acknowledges the financial support of ITU and TUBITAK for the research visit. The other authors would like to thank FCT for financial support.
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