Locating Hidden Elements in Walls of Cultural Heritage Buildings by Using Infrared Thermography

2019, Buildings

The structure of Tvrđa and its buildings date back to the Middle Ages. Tvrđa represents the Old Town of the city of Osijek and the best-preserved and largest ensemble of Baroque buildings in Croatia. After the withdrawal of the Ottomans in 1687, during the 18th century, the Austro-Hungarian administration systematically formed a new fortification system, regulated streets and squares and built a large number of military objects. Tvrđa took its present form in the 19th century and has kept it since then. Investigating the historical development of individual buildings, in addition to archival sources and existing architectural documentation, the obvious source of information are the buildings themselves. The aim of this paper is to explore the possibilities of using infrared thermography to find structural elements and hidden openings in historic buildings in Osijek’s Tvrđa. This paper describes the exploration of the 18th century openings on the facades of the former Kostić houses. The facades were bricked into the walls in the 19th century because houses were reused and their purposes changed from commercial to residential. Infrared thermography is often a starting, nondestructive testing method (NDT) for building analyses. This paper presents thermographic analyses of two buildings. The analyses were carried out in December 2017 and January 2018. Using a steady-state thermographic analysis of a building envelope as the first step, the audit was continued with step heating (SH) of an interest point where changes in a thermal pattern were expected due to additional bricking. Heat flux was generated by the usage of a heat gun for paint removal. View Full-Text Keywords: cultural heritage buildings; nondestructive testing method; passive thermography; active thermography

buildings Article Locating Hidden Elements in Walls of Cultural Heritage Buildings by Using Infrared Thermography Hrvoje Glavaš 1 , Marijana Hadzima-Nyarko 2, *, Ivana Haničar Buljan 3 and Tomislav Barić 1 1 2 3 * Faculty of Electrical Engineering, Computer Science and Information Technology Osijek, J. J. Strossmayer University of Osijek, Kneza Trpimira 2B, 31 000 Osijek, Croatia; hrvoje.glavas@ferit.hr (H.G.); tomislav.baric@ferit.hr (T.B.) Faculty of Civil Engineering and Architecture Osijek, J. J. Strossmayer University of Osijek, Vladimira Preloga 3, 31 000 Osijek, Croatia Institute of Art History, Ulica grada Vukovara 68, 10 000 Zagreb, Croatia; ihanicar@ipu.hr Correspondence: mhadzima@gfos.hr Received: 31 December 2018; Accepted: 25 January 2019; Published: 28 January 2019 Abstract: The structure of Tvrđa and its buildings date back to the Middle Ages. Tvrđa represents the Old Town of the city of Osijek and the best-preserved and largest ensemble of Baroque buildings in Croatia. After the withdrawal of the Ottomans in 1687, during the 18th century, the Austro-Hungarian administration systematically formed a new fortification system, regulated streets and squares and built a large number of military objects. Tvrđa took its present form in the 19th century and has kept it since then. Investigating the historical development of individual buildings, in addition to archival sources and existing architectural documentation, the obvious source of information are the buildings themselves. The aim of this paper is to explore the possibilities of using infrared thermography to find structural elements and hidden openings in historic buildings in Osijek’s Tvrđa. This paper describes the exploration of the 18th century openings on the facades of the former Kostić houses. The facades were bricked into the walls in the 19th century because houses were reused and their purposes changed from commercial to residential. Infrared thermography is often a starting, nondestructive testing method (NDT) for building analyses. This paper presents thermographic analyses of two buildings. The analyses were carried out in December 2017 and January 2018. Using a steady-state thermographic analysis of a building envelope as the first step, the audit was continued with step heating (SH) of an interest point where changes in a thermal pattern were expected due to additional bricking. Heat flux was generated by the usage of a heat gun for paint removal. Keywords: cultural heritage buildings; nondestructive testing method; passive thermography; active thermography 1. Introduction Croatia, as a part of the Mediterranean zone of the Alpine-Himalayan seismic belt, is located in an area of high seismicity, as confirmed by earlier catastrophic earthquakes in Zagreb (1880) and Dubrovnik (1667). Old buildings, built from stone and masonry blocks, do not follow any provisions and are not in accordance with earthquake-resistant design. Therefore, it is necessary to evaluate the level of seismic risk to these old buildings. Vulnerability assessments of the cultural heritage located in seismic areas has been actively researched [1,2]. In the papers by [1,2], the authors proposed a nondestructive and relatively fast but accurate seismic vulnerability assessment of heritage buildings in Tvrđa, the old core of Osijek city. However, performing a detailed analytical design can be of great significance in order to validate this fast seismic vulnerability method. The factor that complicates seismic analysis is that heritage buildings are highly anisotropic and have complex Buildings 2019, 9, 32; doi:10.3390/buildings9020032 www.mdpi.com/journal/buildings Buildings 2019, 9, 32 2 of 20 geometries and heavy masonry mass. The analytical seismic vulnerability method implements and takes into account more detailed analyses, such as geometrical, material and other uncertainties. It is of extreme importance to consider each detail of the structure and the difference between the structural and non-structural elements in order to generate a more realistic and accurate numerical model, which will represent a real structure. A number of parameters affect the result, such as hidden openings, which were closed in the past. This causes a large scattering effect of the results, as well as stiffness degradation. The aim of this research is to explore the possibilities of using passive and active infrared thermography in detecting structural elements and hidden openings in historical buildings in the old city Tvrđa located in Osijek [3]. As the fourth largest city in Croatia, Osijek represents the administrative, economic, and cultural center of the Osijek-Baranja County. The old city core of Osijek, Tvrđa, is an eighteenth-century complex of cobbled streets, grand buildings and open squares, and is the most conserved set of Baroque buildings anywhere in Croatia. The two analyzed buildings are located at the addresses 23 Kuhačeva and 2 Markovićeva Street in Tvrđa. More precisely, the facades that were analyzed for the 18th century openings are oriented towards Kuhačeva Street, which is the main and most representative street in Tvrđa from the earliest days. The structure of Osijek’s Tvrđa and its buildings date back to the Middle Ages. After the withdrawal of the Ottomans in 1687, the Austro-Hungarian administration systematically formed a new fortification system, regulated the streets and squares in the 18th century and built a large number of military objects. The current form of Tvrđa has not changed since the 19th century. Due to the long period of construction of Tvrđa, it can be assumed that a large number of buildings have architectural elements from different historical periods that are often hidden or invisible today. Apart from archival research, an indispensable source of data is the buildings themselves. Apart from interpreting the present state, different types of research can be carried, the most important of which are conservative and restorative techniques for historical development. These methods can be divided into invasive, which include restoration probes, and non-invasive methods. Previous research on historical buildings has, in most cases, been destructive. By removing the original face and plaster, the wall structure would be opened in order to determine the existence of architectural elements from earlier periods. However, contemporary research on historical building development tends to use non-invasive techniques. 2. Methodology Historical buildings, under a conventional thermographic survey, must have a continuous thermal flow through the outer envelope. Therefore, thermographic analysis was carried out during winter in December 2017 and January 2018. It is expected that on the structural elements of different wall thicknesses and structures, slightly different thermal patterns will be noticeable. The initial idea is to conduct a steady-state classical thermographic investigation followed by step heating of an individual location of specific interest, i.e., where hidden elements are expected to exist. Infrared thermography presents a mature, nondestructive investigation technique that is widely used. That being said, in addition to the cost of the required camera, the main factors that led to developing a new approach to analysis are presented in this article. Thermograms were recorded with a FLIR E6 camera with a resolution of 160 × 120 pixels, field of view 45◦ × 34◦ , temperature ranges from −20 ◦ C to 250 ◦ C, resolution difference of <0.06 ◦ C, refresh rate of 9 Hz and accuracy ±2% or 2 ◦ C. Although the resolution plays a big role in choosing a camera, it is not crucial, because the spatial resolution depends on a Field of View (FOV) and Instantaneous Field of View (IFOV). FOV depends on a lens embedded in the camera. If one is planning to record power lines, it is necessary to take a camera with narrow FOV, while a wide FOV is needed if a whole building is to be analyzed. IFOV depends on the resolution because it provides information about the spatial angle of each sensor element. In order for the measurement to be accurate, the object image on the sensor must occupy a surface that is at least equal to the size or greater than one pixel of the sensing network. In the case of Buildings 2019, 9, 32 3 of 20 our camera, a pixel would, at a distance of 1 m, cover an area of width 4.91 mm and height 4.95 mm. Thus, in order to measure accurately, the object of interest at a distance of 1 m should be greater than 9.82 × 9.9 mm, which is fulfilled in this paper. In order to fulfill the required temperature difference between the internal and external temperatures that result in a steady state thermal flux needed for the basic thermographic analysis, the Buildings 2019, 9, 32 3 of 20 measurements were conducted during the winter period, when external expected temperatures result in a temperature more 10of◦ C. On theatplaces of theof facade where hidden elements were× order to measuredifference accurately, the than object interest a distance 1 m should be greater than 9.82 expected, step heating was in order to obtain different thermal patterns, which indicated a 9.9 mm, which is fulfilled inapplied this paper. difference in the wall structure, for example, air gaps.difference Heat flux between was generated with Einhell Hot Air In order to fulfill the required temperature the internal and external ◦ C with an air flow of 500 liters per minute. Gun of 2000 W adjusted to 550 temperatures that result in a steady state thermal flux needed for the basic thermographic analysis, the measurements were conducted during the winter period, when external expected temperatures 3. Nondestructive Testing Methods of Buildings result in a temperature difference more than 10 °C. On the places of the facade where hidden elements wereNDT expected, 3.1. Methodsstep heating was applied in order to obtain different thermal patterns, which indicated a difference in the wall structure, for example, air gaps. Heat flux was generated with Nondestructive ofto historical buildings, should issues such as the Einhell Hot Air Gun testing, of 2000 especially W adjusted 550 °C with an air flow of address 500 litersbasic per minute. role of a building, constructing material characteristics as well as characteristics of the structure that is modified by upgrades and changes building over time. In the absence of detailed documentation, 3. Nondestructive testing methodsofofthe buildings information on the basic elements can be obtained using NDT. For the inspection of a building structure, NDT is very important for delamination, rebar location and corrosion detection [4]. NDT is also the 3.1. NDT methods first step in the analysis of cultural heritage based on planned further research strategies. The basic Nondestructive testing, especially of historical buildings, should address basic issues such as the classification of the NDT test method over the past two decades has undergone changes as infrared role of a building, constructing material characteristics as well as characteristics of the structure that thermography has become more available and gradually has become a standalone discipline. is modified by upgrades and changes of the building over time. In the absence of detailed The general division of the NDT method shown in Figure 1 can be further expanded, depending on documentation, information on the basic elements can be obtained using NDT. For the inspection of specific applications. Basic nondestructive testing (NDT) methods and techniques, useful in assessing a building structure, NDT is very important for delamination, rebar location and corrosion detection concrete structure durability, were applied for testing concrete structures in the work of [6]. One of [4]. NDT is also the first step in the analysis of cultural heritage based on planned further research the early Laboratory Testing Concrete Elements is available in [7]. The selection of a particular NDT strategies. The basic classification of the NDT test method over the past two decades has undergone method depends heavily on the object to be tested. In the article of [4], the appropriate methods for changes as infrared thermography has become more available and gradually has become a each test were provided, as presented in Table 1. standalone discipline. Figure 1. Categories of nondestructive testing (NDT) and evaluation of techniques [5]. Figure 1. Categories of nondestructive testing (NDT) and evaluation of techniques [5]. The general division of the NDT use method shown in Figure 1 canthe be authors further expanded, It is common to simultaneously multiple methods. Thus, in [8] useddepending radar and on specific applications. Basic nondestructive testing (NDT) methods and techniques, in ultrasound in the analysis of overlapped bridge constructions where NDT is useful, since useful it causes assessing concrete structure durability, were applied for testing concrete structures in the work of [6]. only a limited interruption to the everyday use of the facility. Infrared thermography is often used One of the earlywith Laboratory Testing Concrete Elements is available in [7]. The selection of of a particular in combination the Ground Penetrating Radar (GPR) technique. The combination GPR and NDT method depends heavily on the object to be tested. In the article of [4], the appropriate methods IR thermography is described in [9] for an example of a 16th century building located in Urla, Izmir. for each test were provided, as presented in Table 1. The building is an Ottoman structure that currently houses the Urla primary school. The studies published in [10] indicate that the combined need for different NDT methods gives the best results with IR thermography due to its speed and simple interpretation, which is the first step in testing. Buildings 2019, 9, 32 4 of 20 Bar corrosion Bar size Bar location Lamination Honeycombing - voids Crack development Crack distribution Crack width Crack depth Thickness NDT methods Elastic modulus Items Strength Table 1. List of items and possibilities of a nondestructive testing method usage [4]. Rebound hammer Penetration resistance Pull-out Ultrasonic Radar Thermography Radiography Acoustic emission Magnetic or eddy current Half-cell potential Photography The GPR technique is gaining increasing recognition in the investigation of historic buildings. The GPR technique was initially developed for geological and ground engineering research. It has proven to be a very useful means to rapidly—and nondestructively—locate metal structures such as cramps, beams, dowels and bolts within the structure of historic buildings. Particular success was also recorded in the measurement of material thickness [11]. A comparison of combined NDT techniques in civil engineering applications can be found in the following paragraphs. A laboratory and real test for concrete construction work has investigated the efficiency of GPR also applied in tandem with IR thermography and Electrical Resistivity Tomography (ERT) for the characterization and monitoring of building structures in laboratory and in-situ conditions [12]. An example from the study [10] points to finding niches using IR thermography and GPR in “Sala Delle Nicchie”, which is, by age, the closest to the analyzed structures in this paper. 3.2. Infrared Thermography Infrared thermography is a contactless method for determining the temperature distribution on the surface of the observed object by measuring the intensity of radiation in the infrared region of the electromagnetic spectrum. Infrared thermography is a nondestructive testing method which, due to technical advancements and the lower cost of equipment, has been extensively used. Infrared thermography is a mature technique which has become more attractive in an ever more increasing number of application fields [13]. Numerous investigations have been undertaken, some of which are mentioned for the purpose of this article. The investigation of historic structures with the use of IR thermography in order to assess the physicochemical behavior of conservation treatments such as stone cleaning, stone consolidation, repair mortars, as well as to disclose any substrate features, such as tesserae on plastered mosaic surfaces, can be found in [14]. IR thermography has been successfully applied for the knowledge of wall bonding, moisture mapping and the measure of the thermal diffusivity of bricks and plaster [15]. Mortar testing from different decades shows different behavior, as can be seen in [14]. Bonding materials come from different areas that have a material surplus, for example Spain, Portugal, Romania and Germany [16], and with their application, they can result in different thermal patterns. The detection of delamination and structural cracks of water leakage surface evaporation is described in [15]. It can be said that by using an IR camera for NDT of reinforced concrete structures, the amount of time needed to inspect the structure is significantly Buildings 2019, 9, 32 5 of 20 reduced. This is because the result of IRT, thermograms, which screen potential concrete defects in a concrete subsurface, can pinpoint defected areas and thus reduce the amount of time to inspect compared to the sounding test, since there is no need to inspect spot by spot [17]. Other methods of NDT testing based on potential and resistivity measurements [18] require more time. Since 2014, thermography has become widely available thanks to models placed on the market by firms FLIR, Thermal Seek and Therm App. Essentially, these are the third generation of thermographic cameras that have been developed since 1995. According to ISO 20473, thermography performs a radiation analysis in three areas (NIR 0.78–3 µm, MIR 3–50 µm and FIR 50–1000 µm), but most commonly, the classification is divided into five areas, namely a near infrared area (0.7–1,4 µm), short-wave IR (1.4–3 µm), medium-wave IR (3–8 µm), long-wave IR (8–15 µm) and far infrared area (15–1000 µm). Almost all cameras for civil use work in a long-wave IR region, except for cameras used in a gas analysis, which work in a medium-wave IR area. With respect to the end-user, thermography can be classified as qualitative and quantitative when taking into account the information the camera provides, or passive and active when taking into account the excitement type. This paper aims to show the advantages of thermography as a nondestructive research method and to encourage the use of active thermography. The technical characteristics of infrared cameras and the importance of using infrared thermography are most pronounced in Hong Kong [19]. Hong Kong has many buildings that are more than 40 years old, so they introduced a Mandatory Building Inspection Scheme (MBIS) in 2012. The primary step in the inspection is a visual inspection. It is followed by crack mapping, deflection measurement, settlement measurement and observations for signs of water leakage and steel corrosion. On the other hand, the condition assessment deals with sample material testing, in situ temperature measurement, moisture, half-cell electrical potential, vibration and delimitation, and occasionally even continuous monitoring. Thermography is widely used because it is contact free, can be used on large areas, is quick and can be done in real time. The major problem is qualitative, i.e., when it is applied for a surface analysis, delimitation thickness cannot be assessed, surface temperatures depend on human activities and weather, solar radiation interferes with equipment, thermal radiation can be obstructed, accuracy deteriorates with distance, the viewing angle distorts the image, and it is difficult to interpret the results due noise and variation in emissivity. 3.3. Qualitative/Quantitative Thermography Qualitative thermography provides basic information about the temperature distribution on the surface of analyzed structures. The actual temperature values may differ significantly from those read on the camera. Figure 2 shows a residential building with the areas for commercial usage on the ground floor, where horizontal ties and structural elements are clearly visible, as well as a part of the Buildings 2019, 9, 32 right corner of the building) which is not a heated space. 6 of 20 staircase (bottom Figure An example example of of highly-visible highly-visible structural structural elements heated and and an an unheated unheated space. space. Figure 2. 2. An elements as as well well as as aa heated Based it is Based on on the the thermogram, thermogram, it is possible possible to to install install new new openings openings without without jeopardizing jeopardizing the the loadbearing static of the structure. Compliance with the requirements of the Technical loadbearing static of the structure. Compliance with the requirements of the Technical Regulation Regulation on on Rational Use of Energy and Thermal Protection in Buildings can be analyzed by infrared thermography. Rational Use of Energy and Thermal Protection in Buildings can be analyzed by infrared thermography. Another example of an outer envelope can be seen in Figure 3, with the area where the insulation is missing colored in blue. It is evident that the wall temperature in the areas with and without insulation vary in the range of 2 °C. Figure 2. An example of highly-visible structural elements as well as a heated and an unheated space. Figure 2. An example of highly-visible structural elements as well as a heated and an unheated space. 20 Based on the thermogram, it is possible to install new openings without jeopardizing6 ofthe Based on the thermogram, it is possible to install new openings without jeopardizing the loadbearing static of the structure. Compliance with the requirements of the Technical Regulation on loadbearing static of the structure. Compliance with the requirements of the Technical Regulation on Rational example Use of Energy and Thermal in Buildings can area be analyzed by infrared Another an outer envelope canProtection be seen in Figure 3, with the the insulation is Rational Use of of Energy and Thermal Protection in Buildings can be where analyzed by infrared thermography. Another example of an outer envelope can be seen in Figure 3, with the area where missing colored in blue. It is evident that the wall temperature in the areas with and without insulation thermography. Another example of an outer envelope can be seen in Figure 3, with the area where the insulation is missing in blue. It is evident that the wall temperature in the areas with and vary in the range of 2 ◦ C.colored the insulation is missing colored in blue. It is evident that the wall temperature in the areas with and without insulation vary in the range of 2 °C. without insulation vary in the range of 2 °C. Buildings 2019, 9, 32 Figure 3. 3. Absence Absence of of the the outer outer envelope envelope insulation insulation through through aa homogenous homogenous thermal thermal flow Figure flow analysis. analysis. Figure 3. Absence of the outer envelope insulation through a homogenous thermal flow analysis. The lack lack of on the The of adequate adequate insulation insulation on the overhead overhead wall wall of of the the industrial industrial facility facility suggests suggests the the The lack ofomission adequate insulation on the overhead wallloss of the industrial facility suggests the existence of an that resulted in increased thermal (Figure 4). The cooler areas on on the existence of an omission that resulted in increased thermal loss (Figure 4). The cooler areas the existence of anthe omissioncovered that resultedwater in increased thermal loss (Figure 4). The cooler areas on the pavement are pavement are the parts parts covered with with water that, that, by by its its evaporation, evaporation, takes takes away away thermal thermal energy energy and and pavement are the parts covered with water that, by its evaporation, takes away thermal energy and ostensibly represents ostensibly represents an an area area of of lower lower temperature. temperature. ostensibly represents an area of lower temperature. Figure Figure 4. 4. Lack Lack of of insulation insulation of of the the outer outer envelope envelope in in front front of of the the radiator. radiator. Figure 4. Lack of insulation of the outer envelope in front of the radiator. The values on The task task of of quantitative quantitative thermography thermography is is to to provide provide accurate accurate temperature temperature values on the the surface surface The task of quantitative thermography is to provide accurate temperature values on the surface of an analyzed analyzed object. object. In complete aa quantitative quantitative analysis, necessary to to enter enter accurate accurate of an In order order to to complete analysis, it it is is necessary of an analyzed object. In order to complete a quantitative analysis, it is necessary to enter accurate emissivity values, apparent apparent reflected reflected temperatures temperatures and and weather weather conditions conditions when when recording. recording. The The emissivity values, emissivity values, apparent reflected temperatures and weather conditions when recording. The procedure of of quantitative quantitative thermography thermography requires person with with experience experience and and knowledge knowledge gained gained procedure requires a a person procedure of quantitative thermography requires a person with experience and knowledge gained by training. TheThe description of the for anfor operator of the IC by adopting adoptingthe thefirst firstdegree degreeofof training. description ofrequirements the requirements an operator of by adopting the first degree of training. The description of the requirements for an operator of the IC camera is best described in ISO 18436-7: 2014 "Condition monitoring and diagnostics of machines the IC camera is best described in ISO 18436-7: 2014 “Condition monitoring and diagnostics ofcamera is best described in ISO 18436-7: 2014 "Condition monitoring and diagnostics of machines Requirements for qualification and assessment of personnel Part 7: Part Thermography". When machines—Requirements for qualification and assessment of personnel 7: Thermography”. Requirements for qualification and assessment of personnel Part 7: Thermography". When performing the quantitative thermography of buildings and building openings, it should be taken When performing the quantitative thermography of buildings and building openings, it should be performing the quantitative thermography of buildings and building openings, it should be taken into that the emissivity changes with with the angle of recording, and and that that humidity affects the takenaccount into account that the emissivity changes the angle of recording, humidity affects into account that the emissivity changes with the angle of recording, and that humidity affects the measured values of of temperature, wind, and infrared the measured values temperature, wind, andatmospheric atmosphericconditions. conditions.InInthe thefield field of of infrared measured values of temperature, wind, and atmospheric conditions. In the field of infrared thermography, there is no single standard. The researchers [20] used qualitative IRT tests as defined by EN 13187: 1998 (International Organization for Standardization, 1998) and RESNET Interim Guidelines for Thermographic Inspections of Buildings (Residential Energy Services Network, 2010). With respect to buildings, the most commonly used is ISO 6781-3: 2015 Performance of buildings—determination of heat, air and moisture irregularities in buildings by infrared methods—Part 3: Qualifications of equipment operators, data analysts and report writers. 3.4. Passive/Active Thermography Passive thermography corresponds to the use of natural heat sources, such as solar radiation or slowly varying microclimate temperatures, whereas active thermography uses a noncontact thermal With With respect respect to to buildings, buildings, the the most most commonly commonly used used isis ISO ISO 6781-3: 6781-3: 2015 2015 Performance Performance of of buildings buildings -determination determination of of heat, heat, air air and and moisture moisture irregularities irregularities in in buildings buildings by by infrared infrared methods methods -- Part Part 3: 3: Qualifications Qualificationsof ofequipment equipmentoperators, operators,data dataanalysts analystsand and report reportwriters. writers. 3.4. 3.4.Passive/active Passive/active thermography Buildings 2019, 9, 32 thermography 7 of 20 Passive Passivethermography thermography corresponds corresponds to tothe the use use of of natural natural heat heatsources, sources, such suchas assolar solarradiation radiation or or slowly varying microclimate temperatures, whereas active thermography uses aanoncontact thermal slowly varying microclimate temperatures, whereas active thermography uses noncontact thermal inputs placed on the surface of the inspected body by the means of lamps, hot or cold air guns, or inputs on the surface of inspected inputs placed placed surface of the the[21]. inspected body body by by the the means means of of lamps, lamps, hot hot or or cold cold air air guns, guns, or or devices makingon thethe surface vibrate devices making the surface vibrate [21]. devices making the surface is vibrate [21].of a thermographic analysis of structures that are in a steady Passive thermography a process Passive thermography isisaaprocess of aathermographic analysis of that are in Passive thermography process thermographic analysis ofstructures structures thatnot arechange. inaasteady steady state for a long period of time, i.e., in anofenvironment where the temperature does Of state for aa long period of time, i.e. in an environment where the temperature does not change. Of state for long period of time, i.e. in an environment where the temperature does not change. Of course, in the case of the structure in Figure 4, a temperature difference of at least 10 ◦ C between the course, in the case of the structure in Figure 4, a temperature difference of at least 10 °C between the course, in theinternal case of temperature the structure is innecessary Figure 4, atotemperature of at least 10the °Cthermogram between the external and form a heat difference flow. Figure 5 shows external and internal temperature isis necessary to form aa heat flow. Figure 55 shows the thermogram external and internal temperature necessary to form heat flow. Figure shows the thermogram of the PVC opening on the outer envelope. The temperature of the structure/building is 22.9 ◦ C; the of on the outer envelope. The temperature of structure/building is 22.9 °C; of the the PVC PVC opening opening onof the outer envelope. Theinfiltration temperature of the the structure/building 22.9 °C; the the temperature of the part the ceiling under the of the outer air is 18.7 ◦ C, andisthe reflected temperature of the part of the ceiling under the infiltration of the outer air isis18.7 °C, and the reflected temperature of the part of the ceiling under the infiltration of the outer air 18.7 °C, and the reflected ◦ temperature from the glass is 19.9 C because the glass is impervious to IC radiation, and it is difficult temperature from the 19.9 °C the glass impervious to radiation, and ititisisdifficult temperature from theglass glassisis °Cbecause because the glassisisreflection impervious toIC IC radiation, difficult to measure the temperature on19.9 it correctly. The thermal is best seen on the and mirror image of to measure the temperature on it correctly. The thermal reflection is best seen on the mirror image of to measure the temperature on it correctly. The thermal reflection is best seen on the mirror image of the central heating pipe which is reflected on the surface of the glass. The line analysis on the PVC the central heating pipe which isis reflected on the surface of the glass. The line analysis on the PVC the central heating pipe which reflected on the surface of the glass. The line analysis on the PVC ◦ section of the carpentry shows an average temperature of 21.3 C. section sectionof of the thecarpentry carpentryshows showsan anaverage averagetemperature temperatureof of21.3 21.3 °C. °C. Figure 5. Thermogram of the openings in the outer envelope of the building. Figure Figure5. 5.Thermogram Thermogramof ofthe theopenings openingsin inthe theouter outerenvelope envelopeof ofthe thebuilding. building. If the outer envelope is made of various materials, the heat flow will be different with different IfIf the isis made of materials, the heat will with the outer outer envelope envelope of various various materials, heat flow flowconcrete will be be different different with different different structural elements. Figure 6made shows the outer wall of athe reinforced multi-storey building structural elements. Figure 6 shows the outer wall of a reinforced concrete multi-storey building structural elements. Figure 6 shows the outer a reinforced concrete building whose walls are made of aerated concrete units.wall Theofaverage temperature onmulti-storey the aerated concrete whose walls are made of aerated concrete units. The average temperature on the aerated concrete whose walls are made of aerated concrete units. The average temperature on the aerated concrete ◦ ◦ units is 21.1 C, while the difference between the unit and the binder is 0.9 C. The temperature on the units isis 21.1 °C, while the difference between the unit and the binder is 0.9 °C. The temperature on units 21.1 °C, while the difference between the unit and the binder is 0.9 °C. The temperature on ◦ ◦ reinforced concrete bearing wall is 19.4 C, with a temperature difference of 1.2 C. the reinforced concrete bearing wall is 19.4 °C, with a temperature difference of 1.2 °C. the reinforced concrete bearing wall is 19.4 °C, with a temperature difference of 1.2 °C. Figure 6. 6. Thermal pattern pattern of the the outer wall. wall. Figure Figure 6.Thermal Thermal patternof of theouter outer wall. A third example of passive thermography is shown in Figure 7, illustrating the section of the partition wall in the stationary state the analysis was performed in. The mean measured temperature is 29.6 ◦ C, with a difference of 0.7 ◦ C between the maximum and the minimum reading value being attributed to the difference in the emissivity of the wall, switches and thermostats. Reflection of the surrounding sources exists as well, but to a lesser extent. The partition wall has no heat flow, and therefore, the temperature distribution over the surface is homogeneous. In the case of an inspected body that has the same temperature as the environment, as can be seen in Figure 7, active thermography is needed. is 29.6 °C, with a difference of 0.7 °C between the maximum and the minimum reading value being third of passive thermography is shown in Figure and 7, illustrating the section of theof the attributedAto the example difference in the emissivity of the wall, switches thermostats. Reflection partition wall in the stationary state the analysis was performed in. The mean measured temperature surrounding sources exists as well, but to a lesser extent. The partition wall has no heat flow, and is 29.6 °C, with a difference of 0.7 °C between the maximum and the minimum reading value being therefore, the temperature distribution over the surface is homogeneous. In the case of an inspected attributed to the difference in the emissivity of the wall, switches and thermostats. Reflection of the Buildings 2019,has 9, 32 the same temperature as the environment, as can be seen in Figure 7, active 8 of 20 bodysurrounding that sources exists as well, but to a lesser extent. The partition wall has no heat flow, and thermography is needed. therefore, the temperature distribution over the surface is homogeneous. In the case of an inspected body that has the same temperature as the environment, as can be seen in Figure 7, active thermography is needed. Figure 7. The partition wall and thermogram of the area where the hidden junction box is located. Figure 7. The partition wall and thermogram of the area where the hidden junction box is located. Figure 7. The partition wall and thermogram of the area where the hidden junction box is located. Infrared thermal measurements, taken by applying thermal excitation to an analyzed object Infrared thermal measurements, by applying excitation tothermography. an analyzed object to to achieve a temperature difference, taken are considered as thermal athermal process of active Active Infrared thermal difference, measurements, taken by applying excitation to anthermography. analyzed object to achieve a temperature are considered as a process of active Active thermography produces results only are if the properties an analyzed object are different from the achieve a temperature difference, considered as aofprocess of active thermography. Active thermography produces results only if the properties of an analyzed object are different from the properties of the environment where located. In the of active thermography, excitation thermography produces results onlyit ifisthe properties of case an analyzed object are different the from the properties of the environment where it is located. In the case of active thermography, the excitation can be ultrasonic, electric, thermal and it mechanical. are recorded in time, and on their properties of the environment where is located. InThermographs the case of active thermography, the excitation can be ultrasonic, thermal and Thermographs recorded in time, on their can bewith ultrasonic, electric, thermal andmechanical. mechanical. areare recorded in time, and and on their basis and theelectric, application of software support, Thermographs information regarding material properties can be basisbasis and and with the application ofofsoftware informationregarding regarding material can be with application softwaresupport, support, information material obtained. Figure 8 the provides active thermography classification that can be foundproperties in properties [22]. can be obtained. Figure 8 provides active classificationthat that can found in [22]. obtained. Figure 8 provides activethermography thermography classification can be be found in [22]. Figure of active activethermography. thermography. Figure8.8.Classification Classification of Figure 8. refers Classification active thermography. Active thermography nowadays to computer-aided thermography. Connecting the the infrared Active thermography nowadays refers to of computer-aided thermography. Connecting infrared with an excitation source via a computer and adequate softwaresupport support increases increases the camera withcamera an excitation source via a computer and adequate software Active thermography nowadays refers to computer-aided thermography. ◦ C10upthe the sensitivity the measurement setup on average, average, one times (from(from 0.06Connecting °C up to sensitivity of the of measurement setup upup to,to,on onehundred hundred times 0.06 to infrared camera with an excitation source via a computer and adequate software support increases ◦ μ°C). The disadvantage of this approach is that the measuring time (from seconds up to hours) 10 µ C). The disadvantage of this approach is that the measuring time (from seconds up to hours) the sensitivity the the measurementthermal setup up to, on average, one hundred times (from 0.06 °C up to 10 increases of with sensitivity. The mathematical background active increases with the desireddesired thermal sensitivity. The mathematical background for active for thermographic analyses the approach analyzed algorithms been seen time in [23]. Usingseconds our example, μ°C).thermographic The disadvantage of inthis is that thecan measuring (from up tothe hours) analyses in the analyzed algorithms can been seen in [23]. Using our example, the active lock in active lock in thermography, with optical external excitation, could provide information about deeper increases with the desired thermal sensitivity. The mathematical background for active thermography, with Pulse optical external excitation, could provide informationthermal about conductivity, deeper layers of layers of facade. also applicable, but due thermographic analyses inthermography the analyzedisalgorithms can been seentoinlower [23]. Using our example, the facade. Pulse thermography is also applicable, but due to lower thermal conductivity, thermal impulse is longer. An example of pulse thermography can be seen in [24]. In our case, the activethermal lock inimpulse thermography, with optical external excitation, could provide information about deeper is longer. An example of pulse thermography can be seen in [24]. In our case, the possibility of applying layers of facade. Pulse thermography is also applicable, but due to lower thermal conductivity, active thermography is reduced because of the large analyzed area, its frequent use by pedestrians and thermal impulse is longer. An example of pulse thermography can be seen in [24]. In our case, the the absence of the required equipment. Based on the previous experience of the application of step heating results, we were encouraged to replicate the procedure on historical buildings. For example, the case of a hidden box, in which a thermostat connection is performed, is presented on the thermogram in Figure 9. The box position is not known, but it is assumed that it is near the switch. In order to determine its exact position, a hair dryer can be used to heat the wall. The thermographic camera is used to monitor the response to the heat excitation. The surface behind which the box is hidden has increased its thermal resistance and heat retention, which is longer in duration due to the difference in thermal resistance. The initial frequent use pedestrians and the absence the requiredofequipment. Based onbythe previous experience of theofapplication step heating results, we were encouraged Based on previous experience of the application step heating results, were encouraged to replicate thethe procedure on historical buildings. For of example, the case of awe hidden box, in which a to replicate the procedure on historical buildings. For example, the case of a hidden box, in which a is thermostat connection is performed, is presented on the thermogram in Figure 9. The box position thermostat connection is performed, is presented on the thermogram in Figure 9. The box position is not known, but it is assumed that it is near the switch. In order to determine its exact position, a hair not known, but it istoassumed it is near the switch. In camera order tois determine its exact the position, a hair Buildings 2019,be 9, 32 9to ofthe 20 dryer can used heat thethat wall. The thermographic used to monitor response dryer can be used to heat the wall. The thermographic camera is used to monitor the response to the heat excitation. The surface behind which the box is hidden has increased its thermal resistance and heat excitation. The surface behind which the box is hidden has increased its thermal resistance and heat retention, which is longer in duration due to the difference in thermal resistance. The initial heat retention, which is longer in duration due9a to while the difference in thermal The as initial location of the hair dryer is presented in Figure the indication of theresistance. box position, well as location of the hair dryer is presented in Figure 9a) while the indication of the box position, as well location of the hairbearing dryer isswitch presented in Figure whileinthe indication of the boxthe position, the structure of the elements, can 9a) be seen Figure 9b. Over time, outlineas ofwell the box as the structure of the bearing switch elements, can be seen in Figure 9b). Over time, the outline of as the structure of theseen bearing switch9c. elements, can be seen in Figure 9b). Over time, the outline of contour can be clearly in Figure the box contour can be clearly seen in Figure 9c). the box contour can be clearly seen in Figure 9c). (a) (a) (b)(b) (c) (c) Figure 9.9.Finding Finding box byby means of of thermal incentive. Figure hidden distribution box means incentive. Figure9. Findingaaahidden hiddendistribution distribution box by means ofthermal thermal incentive. Thetime timeduration duration and the are shown inin Figure 10.10. TheThe measured values, The temperature values are shown measured values, The time durationand andthe thetemperature temperaturevalues values are shown inFigure Figure 10. The measured values, marked with squares, represent the point which is located between the box and the switch, while the the marked markedwith withsquares, squares,represent representthe thepoint pointwhich whichisislocated locatedbetween betweenthe thebox boxand andthe theswitch, switch,while while the line,marked markedwith withdiamonds, diamonds, indicates indicates the the junction box. line, thetemperature temperatureininthe thearea areaofof the junction box. line, marked with diamonds, indicates the temperature in the area of the junction box. °C 39.33 39.33 36.63 36.63 38 33 28 8:42:43 8:41:49 8:42:43 8:41:33 8:41:33 8:41:49 8:41:16 8:41:16 8:41:08 8:41:08 8:40:53 8:40:53 8:40:42 8:40:42 8:40:15 8:40:15 8:39:41 8:39:41 8:39:00 8:39:00 29.3 35.43 36.33 34.53 34.33 34.73 35.43 33.83 34.53 34.33 34.73 35.33 32.53 33.83 34.83 34.53 33 32.53 34.83 35.33 34.53 33.03 32.63 32.23 33.03 32.63 31.73 32.23 30.73 29.23 31.73 30.73 28 29.23 8:39:14 8:39:14 29.3 38 36.33 °C Figure 10. The temperature values in the junction box and its immediate area. Figure 10. The temperature values in the junction box and its immediate area. Figure 10 shows the precise temperature values in the analyzed spot point, but not their spatial 10. The temperature values in the junction box individual and its immediate area. measurement distribution. DueFigure to camera accuracy and slight camera movement, temperature Figure 10 shows the precise temperature values in the analyzed spot point, but not their spatial readings over time tend to showaccuracy variations, the actual temperature values are continuously distribution. to the camera andalthough slight movement, Figure 10Due shows precise temperature valuescamera in the analyzed spotindividual point, but temperature not their spatial falling. This is one of the basic problems associated with classical thermography. The special temperature measurement Due readings timeaccuracy tend to show althoughmovement, the actual temperature are distribution. to over camera andvariations, slight camera individualvalues temperature distribution is presented in is Figure 11,the where hidden associated junction box is located, clearly illustrates continuously falling. This one of basicthe problems with classicalwhich thermography. The measurement readings over time tend to show variations, although the actual temperature values are Buildings 2019, 9, 32 10 of 20 the location of the connection line isbypresented which theinthermostat is connected to thejunction junctionbox box. special temperature distribution Figure 11, where the hidden is located, continuously falling. This is one of the basic problems associated with classical thermography. The which clearly illustrates the location of the connection line by which the thermostat is connected to special temperature distribution is presented in Figure 11, where the hidden junction box is located, the junction box. which clearly illustrates the location of the connection line by which the thermostat is connected to the junction box. Figure 11. 11. Thermogram Thermogram of of the the hidden hidden junction junction box. box. Figure In Figure 11, the position of the conductor under plaster is visible due to the material variation. It should be taken into account that, during the recording, the observed image is monochromatic and that, in most cases, it represents the sum of radiation in the IR spectrum. The advantages of using an IR thermal camera in finding a power supply cable can be seen in Figure 12, where materials of similar Buildings 2019, 9, 32 10 of 20 Figure 11. Thermogram of the hidden junction box. In Figure 11, the position of the conductor under plaster is visible due to the material variation. It In Figure 11, the position of the conductor under plaster is visible due to the material variation. should be taken into account that, during the recording, the observed image is monochromatic and It should be taken into account that, during the recording, the observed image is monochromatic and that, in most cases, it represents the sum of radiation in the IR spectrum. The advantages of using an that, in most cases, it represents the sum of radiation in the IR spectrum. The advantages of using an IR thermal camera in finding a power supply cable can be seen in Figure 12, where materials of similar IR thermal camera in finding a power supply cable can be seen in Figure 12, where materials of similar characteristics were used, namely plaster and gypsum. characteristics were used, namely plaster and gypsum. Figure 12. Finding a power supply cable with active thermography. (a) (b) The position of theFigure power cansupply be often observed bythermography. applying thermal excitation. 12.supply Findingcable a power cable with active When installing a lighting fixture, the danger accidental mechanical damage of a power supply cable position of to thea power supply cable can by be the often observed described by applying thermal excitation. is, in thatThe case, reduced minimum. Encouraged previously experiments with When installing a lighting fixture, the danger accidental mechanical damage of a power supply thermal step excitation, the idea for research regarding finding hidden elements in historical buildingscable in that case, reduced to a minimum. Encouraged by the previously described experiments with was is, initiated. thermal step excitation, the idea for research regarding finding hidden elements in historical 3.5. buildings Parameterswas Affecting Radiometric Accuracy of IR Thermography initiated. There are a number of parameters that ultimately define the accuracy and repeatability of a 3.5. Parameters affecting radiometric accuracy of IR thermography radiometric measurement. These include an object, surface, thermal environment, atmosphere, and instrument. Consideration should also be given to the measurement or instrument that isof a There are a number of parameters that ultimately define themethod accuracy and repeatability establishing the deviation, as it These may beinclude the source of an error rather than the infrared instrument. radiometric measurement. an object, surface, thermal environment, atmosphere, and Surface temperature considerations depend upon a type of a thermal generator. There are three instrument. Consideration should also be given to the measurement method or instrument that is fundamental types of internal thermal generators, namely (i) a constant temperature generator example establishing the deviation, as it may be the source of an error rather than the infrared instrument. electricalSurface fault, (ii) a constant power generator example building, and (iii) a finite energyThere source, forthree temperature considerations depend upon a type of a thermal generator. are example, a motor that has been switched off. Heat transfer from these fundamental types of thermal fundamental types of internal thermal generators, namely (i) a constant temperature generator generators can be complicated a period by thermal capacitance(s) of the material example electrical fault, (ii)over a constant power generator example building, and (iii)between a finite the energy energy source the surface. Anthat overview of switched the material and the typical emissivity source, forand example, a motor has been off.characteristics Heat transfer from these fundamental types necessary for the implementation of a thermographic analysis is given in Table 2 [14]. of thermal generators can be complicated over a period by thermal capacitance(s) of the material between the energy source and the surface. An overview of the material characteristics and the typical Table 2.for Thermo-physical-optical properties of various materials [14]. emissivity necessary the implementation of a thermographic analysis is given in Table 2 [14]. Material Density (kg m−3 ) Specific Heat Thermal Conductivity Thermal Diffusivity Thermal Effusivity Emissivity (λ = 8–12 µm) Limestone Plaster 2600 920 2.1 877.92 2241.25 0.93 1440 800 0.5 434.03 758.95 White marble 0.91 2695 870 3.14 1339.22 2713.33 0.95 0.95 (J kg 2. K Thermo-physical-optical ) (W m K ) (×10 mof s various ) (Ws m K [14]. ) Table properties materials −1 −1 −1 −1 −9 2 −1 0.5 2 −1 Grey marble 2650 870 6.7 2906.09 3930.25 Cement marble 3100 840 0.85 326.42 1487.75 0.86 Concrete 2400 1008 1.65 682.04 1997.92 0.94 Red brick 2025 800 0.6 370.37 985.90 0.90 Air 1.16 1007 0.026 22257.98 5.51 - Water 1000 4193 0.586 139.76 1567.51 0.96 The characteristic behavior of the masonry wall described in [25] provides an initial estimate of the optimal time during the year to begin testing, taking into account the necessary difference in temperature to achieve a homogeneous thermal flow. For analytical purposes, the test was carried out Emissiv (λ=8-12 μ Therm effusivi (Ws0.5 m2 Therm diffusiv (x10-9 m2 Therm conducti (W m-1 Specific (J kg-1 K Densit (kg m-3 Material Buildings 2019, 9, 32 Limestone 11 of 20 2600 920 2.1 877.92 2241.25 0.93 Plaster 1440 800 0.5 434.03 758.95 0.91 White marble 2695 870 3.14 1339.22 2713.33 0.95 with a temperature difference (∆T) over the building facade of at least 10 to 15 ◦ C to allow measurable Grey marble 2650 870 6.7 2906.09 3930.25 0.95 heat exchange through the element [20]. Cement marble 3100 840 0.85 326.42 ◦ 1487.75 0.86 As shown in Figure 13, the wall temperatures varied by up to 55 C or more on the surface, Concrete 2400 1008 1.65 682.04 1997.92 0.94 decreasing below freezing on cold winter nights. This wide discrepancy should be compared with a Red brick 2025 800 0.6 370.37 985.90 0.90 12 ◦ C seasonal variation in the wall center and 2 ◦ C in the interior the latter, which probably receives Air in winter. Daily 1.16 variations 1007 0.026 22257.98 5.51in summer, -e.g., some heating of the surface temperature will be greater Water 1000 4193 0.586 139.76 1567.51 ◦ ◦ 42 C, compared with winter, e. g. 25 C. The dew point for interstitial condensation is located0.96 near the outer surface. Consequently, this can result in freezing in the outer layers, since a great deal of heat The to characteristic behavior of masonry wallwith described in [25] ani.e., initial estimate it takes time of is required raise the temperature of the a heavy building thick walls byprovides even 1 ◦ C, the optimal time during the year to begin testing, taking into account the necessary difference for the heat to flow through the brick and mortar. There is a tendency for the outer face to be heated in to achieve homogeneous flow. Forthe analytical purposes, the test uptemperature prior to a response the acore can providethermal and long before inner face is affected. Thiswas timecarried lag out with a temperature difference (ΔT) over the building facade of at least 10 to 15 °C to allow would affect the theoretical temperature gradient. measurable heat exchange through the element [20]. Figure 13. Theoretical temperature changes in a thick wall [25]. Figure 13. Theoretical temperature changes in a thick wall [25]. 4. Study Area As shown in Figure 13, the wall temperatures varied by up to 55 °C or more on the surface, Analyzed Elements of Kostić House decreasing below freezing on cold winter nights. This wide discrepancy should be compared with a 12 On °C seasonal variation in the wall center with and 2the °C in the interior latter, 1733 whichand probably receives the basis of the purchase contract Stekić family, the between 1747, trader some heating in winter. Daily variations of the surface temperature will be greater in summer, e. g. Mihovil Kostić became the owner of the house in 23 Kuhačeva Street and the house on the corner of 42 °C, compared with Street winter, g. 25 °C. The dew point for14a) interstitial condensation is two-storey located near Kuhačeva and Marković (2e.Markovićeva Street) (Figure [26]. Both buildings are the outer surface. Consequently, this can result in freezing in the outer layers, since a great houses with gabled roofs, masonry brick and stucco. In the architectural layout from 1774, whendeal the of heat is required to raise the temperature of a heavy building with thick walls by even 1 °C, i.e. it takes military government of Tvrđa bought the Kostić houses, spatial organization and facade partition are time for to flow the shopping brick andspaces mortar. There is windows/openings; a tendency for the outer faceand to be evident. Onthe theheat ground floor,through there were with shop the door heated up prior to a response the core can provide and long before the inner face is affected. the window were joined by a unique segmental overhang. There were two shop openings according This to time lag would affect the theoretical temperature gradient. the drawing, east and west of the centrally located entrance on the ground floor. The position of these openings in 23 Kuhačeva Street was confirmed by the restauration probes (Figure 14b). 4. Study area After the military government bought Kostić houses in 1774, several restructurings in the late 18th and early 19th century were carried out. On that occasion, shop openings were closed because 4.1. Analyzed elements of Kostić house of the reuse of the store spaces, i.e., into a dwelling on the ground floor of 23 Kuhačeva Street and theoffice basisinof2 Markovićeva the purchase Street. contract withon thedrawings Stekić family, between 1733 1747, trader into theOn post Based from 1774 (Figure 15)and which depict Mihovil Kostić the owner of houses the house in 23 Kuhačeva Street andand the house on the corner shop openings onbecame the facades of brick oriented to Kuhačeva Street, the discovery of the of same openings by a construction probe on the ground floor of the house in 23 Kuhačeva Street, the aim of the article was to investigate the existence of shop opening from the 18th century on the house in 2 Markovićeva Street through non-invasive methods. houses with gabled roofs, masonry brick and stucco. In the architectural layout from 1774, when the military government of Tvrđa bought the Kostić houses, spatial organization and facade partition are evident. On the ground floor, there were shopping spaces with shop windows/openings; the door and the window were joined by a unique segmental overhang. There were two shop openings according to the drawing, east and west of the centrally located entrance on the ground floor. The Buildings 2019, 9, 32 12 of 20 position of these openings in 23 Kuhačeva Street was confirmed by the restauration probes (Figure 14b). (a) (b) Figure 14. Kostić house (a) at the corner of Kuhačeva and Marković street (2 Markovićeva Street) and (b) in 23 Kuhačeva Street. After the military government bought Kostić houses in 1774, several restructurings in the late 18th and early 19th century were carried out. On that occasion, shop openings were closed because of the reuse of the store spaces, i.e., into a dwelling on the ground floor of 23 Kuhačeva Street and into the post office in 2 Markovićeva Street. Based on drawings from 1774 (Figure 15) which depict shop openings on the facades (a) of brick houses oriented to Kuhačeva Street, and the (b)discovery of the same openings by a construction probe on the ground floor of the house in 23 Kuhačeva Street, the aim of Figure 14.14. Kostić house (a)(a) atthe thethe corner ofof Kuhačeva and Marković (2th(2 Markovićeva and Figure Kostić house at corner and Marković street Markovićeva Street) andin 2 the article was to investigate existence ofKuhačeva shop opening from street the 18 century on Street) the house (b) in 23 Kuhačeva Street. (b) in 23 Kuhačeva Street. Markovićeva Street through non-invasive methods. After the military government bought Kostić houses in 1774, several restructurings in the late 18th and early 19th century were carried out. On that occasion, shop openings were closed because of the reuse of the store spaces, i.e., into a dwelling on the ground floor of 23 Kuhačeva Street and into the post office in 2 Markovićeva Street. Based on drawings from 1774 (Figure 15) which depict shop openings on the facades of brick houses oriented to Kuhačeva Street, and the discovery of the same openings by a construction probe on the ground floor of the house in 23 Kuhačeva Street, the aim of the article was to investigate the existence of shop opening from the 18th century on the house in 2 Markovićeva Street through non-invasive methods. Figure 15. Architectural drawings of Kostić houses in 23 Kuhačeva Street from the 1774 [27]. Figure 15. Architectural drawings of Kostić houses in 23 Kuhačeva Street from the 1774 [27]. 5. Results Thermographic Analysis of Kostić Houses A thermographic diagnosis of historical buildings requires a multidisciplinary approach. For a high quality thermographic analysis of the buildings, it was necessary to carry out a survey of the building of interest. The complexity of the thermographic analysis process is highlighted in the provided investigation of the older buildings [17]. So far, the presence of concrete deterioration, water seepage, delamination and significant cracks was investigated in the of [9]. In Figurecover 15. Architectural drawings of Kostić houses in 23 Kuhačeva Street from the work 1774 [27]. addition, the existence of delamination and detachments of the murals were performed as well. The thermographic analysis of Kostić house on the corner of Marković and Kuhačeva Street was carried out on 19th December 2017 between 2:15 PM and 2:45 PM. The outer temperature ranged from 1.7 ◦ C to 1.5 ◦ C with a relative humidity ranging from 63% to 75%. Figure 16 shows a thermogram of the outer envelope. The analysis was performed with the setting of the emissivity 0.9 according to the preposition put forward in [28]. When the exact temperature value of the analyzed thermal radiation Concrete: rough 0.92-0.97 Mortar 0.87 Mortar: dry 0.94 Plaster 0.86-0.90 Buildings 2019, 9, 32 13 of 20 Plaster: rough coat 0.91 The wall thickness of the analyzed building is 74 cm, and the thickness of the parapet below the window, where the heating elements of the city's central heating are located, is 36 cm. From the has to be determined, the important factor is emissivity. But, in this case, that was the aim of the thermogram in Figure 16, it can be seen that the building is heated at the project temperature. The analysis. For the purpose of finding discontinuity in building elements, the temperature difference is thermogram also shows the position of the radiators on parapet walls under the window. In Figure important. Although some basic guidelines for emissivity values are given in Table 2, more detailed 16, the thermal image shows that the interior windows on the ground floor are open, since the space data can be found in Table 3. Table 3 shows typical emission values of building materials. is used for the post office, and a greater amount of fresh air is required. Figure 16. Thermogram of the whole building. Figure 16. Thermogram of the whole building. Table 3. Emission values for typical building materials [29]. The outer envelope (i.e. the facade) has not been renewed since the 1980s. An additional analysis Brick: Common 0.81–0.86 on the parts of the envelope with visible damage was carried out, but the conclusion that can be Brick: common, red 0.93 Concrete 0.92 Concrete: dry 0.95 Concrete: rough 0.92–0.97 Mortar 0.87 Mortar: dry 0.94 Plaster 0.86–0.90 Plaster: rough coat 0.91 The wall thickness of the analyzed building is 74 cm, and the thickness of the parapet below the window, where the heating elements of the city’s central heating are located, is 36 cm. From the thermogram in Figure 16, it can be seen that the building is heated at the project temperature. The thermogram also shows the position of the radiators on parapet walls under the window. In Figure 16, the thermal image shows that the interior windows on the ground floor are open, since the space is used for the post office, and a greater amount of fresh air is required. Buildings 9, 32envelope (i.e., the facade) has not been renewed since the 1980s. An additional analysis 14 of 20 The2019, outer on the parts of the envelope with visible damage was carried out, but the conclusion that can be provided is is that thatthe theshape shapeofofthe thedamage damagedoes does not strictly correlate with thermal pattern (Figure provided not strictly correlate with thethe thermal pattern (Figure 17). 17). Figure 17. 17. Analysis Analysis of of facade facade damage. damage. Figure A weak dot heat bridge on the first floor was noticed (Figure 18). This was probably a result of the metallic anchorage (structural element) covered with plaster during the last facade maintenance. Buildings 2019, 9, 32 14 of 20 Figure17. 17. Analysis Analysis of Figure of facade facadedamage. damage. A weak dot heat bridge on the first floor was noticed (Figure 18). This was probably a result of weak dot heatbridge bridgeon onthe thefirst first floor floor was was noticed (Figure 18). was a result of of AA weak dot heat (Figure 18).This This wasprobably probably a result the metallic anchorage (structural element) coverednoticed with plaster during the last facade maintenance. the metallic anchorage (structural element) covered with plaster during the last facade maintenance. the metallic anchorage (structural element) covered with plaster during the last facade maintenance. Figure pointheat heatbridge. bridge. Figure18. 18.Discovered Discovered weak weak point Figure 18. Discovered weak point heat bridge. In order to determine the existence of discontinuity in the outer envelope, heating was applied, as In order to determine the existence of discontinuity in the outer envelope, heating was applied, is shown in Figure 19. order to as In is shown in determine Figure 19. the existence of discontinuity in the outer envelope, heating was applied, n as is shown in Figure 19. Figure 19. Initial application moment of thermal excitation. Figure 19. Initial application application moment moment of of thermal thermal excitation. Figure 19. Initial excitation. There was not any discontinuity by the analysis of the thermal pattern change. In Figure 20, a linear trace of not the heating body, whichby is the continuous uninterrupted, can bechange. observed. There was any discontinuity analysisand of the thermal pattern In Figure 20, a There was not any discontinuity by the analysis of the thermal pattern change. In Figure 20, a linear trace of the heating body, which is continuous and uninterrupted, can be observed. linear trace of the heating body, which is continuous and uninterrupted, can be observed. Figure 20. Homogeneous thermal pattern on excitation along the wall. Figurethe 20. left Homogeneous thermal pattern on excitation excitation the The detail below window inthermal the lower left corner of thealong thermogram Figure 20. Homogeneous pattern on along the wall. wall.reveals that spot facade damage, due to a difference in emissivity, shows a higher apparent temperature (Figure 21). The detail below the left window in the lower left corner of the thermogram reveals that spot The detail below the left window in the lower left corner of the thermogram reveals that spot facade 21). Buildingsdamage, 2019, 9, 32 due to a difference in emissivity, shows a higher apparent temperature (Figure 15 of 20 facade damage, due to a difference in emissivity, shows a higher apparent temperature (Figure 21). Figure 21. 21. Influence Influence of of damage damage to to the the thermal thermal pattern. pattern. Figure Increasing the area under the thermal excitation did not affect the result. The thermal pattern caused by the excitement was homogenous and uniformly disappeared over time (Figure 22). Buildings 2019, 9, 32 15 of 20 Figure 21. Influence of damage to the thermal pattern. Figure 21. Influence of damage to the thermal pattern. Increasing the area under the thermal excitation did not affect the result. The thermal pattern Increasing the area under the thermal and excitation did not affect theover result. The thermal caused by the excitement was homogenous uniformly disappeared time (Figure 22).pattern Increasing the area under the thermal excitation did not affect the result. The thermal pattern caused by the excitement was homogenous and uniformly disappeared over time (Figure 22). caused by the excitement was homogenous and uniformly disappeared over time (Figure 22). Figure 22. 22. Increasing Increasing the the area area of of applied applied thermal thermal excitation. Figure excitation. Figure 22. Increasing the area of applied thermal excitation. As no discontinuity in structural elements of a building was discovered, the conclusion was that As no discontinuity in structural elements of a building was discovered, the conclusion was that nono discontinuity in structural elements of method a building wasisdiscovered, the conclusion that thereAs was change of the analyzed area, or the itself not applicable because of was the wall there was no change of the analyzed area, or the method itself is not applicable because of the wall there was thermal no change of the In analyzed or thethe method itselfprotection is not applicable because of the wall mass and capacity. the casearea, of heating lightning grounding line, the heating mass and thermal capacity. In the case of heating the lightning protection grounding line, the heating mass andofthermal capacity. the case of heating theapplied lightning protection line, the23). heating capacity the wall and the In surface character of the method weregrounding observed (Figure capacity of the wall and the surface character of the applied method were observed (Figure 23). capacity of the wall and the surface character of the applied method were observed (Figure 23). Figure 23. Thermal excitation of lightning protection installation. Figure 23. 23. Thermal Thermal excitation excitation of of lightning lightning protection protection installation. installation. Figure The measurement results were obtained based on nine thermograms and the analysis of 39 The measurement were obtained basedbased on nine and the analysis 39 measuring The measurementresults results were obtained onthermograms nine thermograms and theofanalysis of 39 measuring areas, yielding the maximum, minimum and average values of the various elements of areas, yielding the maximum, minimum andminimum average values the various elements the outer envelope measuring areas, yielding the maximum, and of average values of the of various elements of the outer envelope and the heated area. The results, presented in Table 4, represent the average values and the heated area. The results, presented in Table 4, represent the average values of maximal, minimal the outer envelope and the heated area. The results, presented in Table 4, represent the average values of maximal, minimal and average values for all multiple measurements. The parapet wall is separated and average minimal values forand all average multiplevalues measurements. The parapet wall is separated as the thinest part of of maximal, for all multiple measurements. The parapet wall is separated as the thinest part of the envelope, which contains the heating elements connected to the town central the envelope, which contains the heating elements connected to the town central heating system. as the thinest part of the envelope, which contains the heating elements connected to the town central heating system. heating system. Table 4. Characteristic values of the measured temperature. Table 4. Characteristic values of the measured temperature. Table 4. Characteristic values of the measured temperature. ◦ Heading Max. Value ◦ C Min. Value ◦ C Average C Heading Heading Wall Wall Wallparapet Wall Wall parapet Wall parapet Window Window Window Heated area Heated area Heated area Max. value °C Max. value 5.5 °C 5.5 5.5 6.7 6.7 6.7 8.5 8.5 8.5 14.4 14.4 14.4 Min. value °C Min. value °C 4.9 4.9 4.9 6.1 6.1 6.1 6.7 6.7 6.7 11.8 11.8 11.8 Average °C Average °C 5.2 5.2 5.2 6.56.5 6.5 7.77.7 7.7 13.3 13.3 13.3 It was decided to proceed with the analysis on the second building where the probes were made and there was existing documentation of the space reusage. Testing at Kostić house in 23 Kuhačeva Street was conducted on 7th February 2018 between 12:51 AM and 1:04 PM. The temperature at the time of the analysis ranged from 9.1 ◦ C to 9.9 ◦ C, and the humidity from 81% to 83%. The initial state is visible on the thermogram in Figure 24. The registered thermal radiation corresponds to the air temperature. ItItwas was decided totoproceed proceed with the analysis on the second building where the probes were made It wasdecided decidedto proceedwith withthe theanalysis analysison onthe thesecond secondbuilding buildingwhere wherethe theprobes probeswere weremade made and there was existing documentation of the space reusage. Testing at Kostić house in 23 Kuhačeva and andthere therewas wasexisting existingdocumentation documentationofofthe thespace spacereusage. reusage.Testing TestingatatKostić Kostićhouse houseinin23 23Kuhačeva Kuhačeva th February 2018 between 12:51 AM and 1:04 PM. The temperature at the Street was conducted on Street Streetwas wasconducted conductedon on777ththFebruary February2018 2018between between12:51 12:51AM AMand and1:04 1:04PM. PM.The Thetemperature temperatureatatthe the time of the analysis ranged from 9.1 °C to 9.9 °C, and the humidity from 81% to 83%. The initial state time ranged timeofofthe theanalysis analysis rangedfrom from9.1 9.1°C °Ctoto9.9 9.9°C, °C,and andthe thehumidity humidityfrom from81% 81%toto83%. 83%.The Theinitial initial state Buildings 2019, 9, 32 16state of 20 isisvisible visible on the thermogram ininFigure Figure 28. The registered thermal radiation corresponds totothe the air is visibleon onthe thethermogram thermogramin Figure28. 28.The Theregistered registeredthermal thermalradiation radiationcorresponds correspondsto theair air temperature. temperature. temperature. Figure 24. 24. Termogram Termogram of Kostić house in 23 Kuhačeva Street. Figure ofofKostić Kostić house inin23 23 Kuhačeva Street. Figure Figure24. 24.Termogram Termogramof Kostićhouse housein 23Kuhačeva KuhačevaStreet. Street. By analyzing the thermal pattern shown in in Figure 24, 24, the the difference in the probing area, as a result By analyzing the thermal pattern shown Figure difference inin the probing area, asasaaa By the Byanalyzing analyzingthe thethermal thermalpattern patternshown shownin inFigure Figure24, 24,the thedifference differencein theprobing probingarea, area,as of the difference in emissivity, is evident. The expected discontinuity observed by probing did not result ofofthe the difference ininemissivity, emissivity, isisevident. evident. The expected discontinuity observed by probing did result resultof thedifference differencein emissivity,is evident.The Theexpected expecteddiscontinuity discontinuityobserved observedby byprobing probingdid did appear during the heat excitation of the parapet wall (Figure 25a). As can be seen from Figure 25b, a not appear during the heat excitation ofofthe the parapet wall (Figure 25a). As can be seen from Figure not notappear appearduring duringthe theheat heatexcitation excitationof theparapet parapetwall wall(Figure (Figure25a). 25a).As Ascan canbe beseen seenfrom fromFigure Figure further line heating processprocess raised the wallthe temperature but did but not result inresult a change the thermal 25b), further line heating raised wall temperature did not ininaaain change ininthe the 25b), change 25b),aaafurther furtherline lineheating heatingprocess processraised raisedthe thewall walltemperature temperaturebut butdid didnot notresult resultin changein the pattern. During the heating and during the time, the thermal pattern seemed to be inhomogeneous, thermal pattern. During the heating and during the time, the thermal pattern seemed to be thermal thermal pattern. pattern. During During the the heating heating and and during during the the time, time, the the thermal thermal pattern pattern seemed seemed toto be be albeit in a manneralbeit that corresponds to thecorresponds presence of moisture, rather of than non-discontinuity innonthe inhomogeneous, ininaaamanner manner that totothe the presence moisture, rather than inhomogeneous, inhomogeneous,albeit albeitin mannerthat thatcorresponds correspondsto thepresence presenceof ofmoisture, moisture,rather ratherthan thannonnonstructural elements. By analyzing the smaller parts of thethe wall, no additional information (asadditional shown in discontinuity ininthe the structural elements. By analyzing smaller parts ofof the wall, no discontinuity the discontinuityin thestructural structuralelements. elements.By Byanalyzing analyzingthe thesmaller smallerparts partsof thewall, wall,no noadditional additional Figure 25c) was obtained. information (as shown ininFigure Figure 25c) was obtained. information information(as (asshown shownin Figure25c) 25c)was wasobtained. obtained. (a) (a) (a) (b) (b) (b) (c) (c) (c) Figure 25. Changing the thermal pattern due totoheating. heating. Figure Figure25. 25.Changing Changingthe thethermal thermalpattern patterndue dueto heating. After looking atatthe the bricked up openings asaswell well asaswall wall probing (Figure 26) and the associated After looking at the bricked up openings as well as wall probing (Figure 26) After and the associated Afterlooking lookingat thebricked brickedup upopenings openingsas wellas wallprobing probing(Figure (Figure26) 26)and andthe theassociated associated thermal pattern, it is obvious that the thermographic analysis was not as effective in finding the thermal pattern, it is obvious that the thermographic analysis was not as effective in finding the bricked thermal the thermalpattern, pattern,ititisisobvious obviousthat thatthe thethermographic thermographicanalysis analysiswas wasnot notasaseffective effectiveininfinding finding the bricked up openings in the massive homogeneous structures of the brick wall. up openings in the massive homogeneous structures of the brick wall. bricked brickedup upopenings openingsininthe themassive massivehomogeneous homogeneousstructures structuresofofthe thebrick brickwall. wall. Figure 26. Thermogram of the probing building part of a known wall structure. Figure 26. Thermogram ofofthe the probing building part ofofaaaknown known wall structure. Figure Figure26. 26.Thermogram Thermogramof theprobing probingbuilding buildingpart partof knownwall wallstructure. structure. In order to confirm the conclusions, the examined place was checked from the inside of the building. Significant thermal masses and continuous heating have resulted in a constant thermal pattern, presented in Figure 27. Buildings 2019, 9, 32 17 of 20 In order to confirm the conclusions, the examined place was checked from the inside of the building. Significant thermal masses and continuous heating have resulted in a constant thermal Buildings 2019, 9, 32 17 of 20 pattern, presented in Figure 27. Figure 27. Thermal pattern on the inside of the wall. Figure 27. Thermal pattern on the inside of the wall. The additional measurement results are obtained on eight thermograms and the analysis of 22 measuring areas measurement which resultedresults in theare maximum, and average and values the various The additional obtained minimum on eight thermograms theof analysis of 22 elements the outer envelope and area. The results,and presented Table 5,ofrepresent the measuringofareas which resulted inthe theheated maximum, minimum averageinvalues the various average values of maximal, minimal and average values for all multiple measurements. elements of the outer envelope and the heated area. The results, presented in Table 5, represent the average values of maximal, minimal and average values for all multiple measurements. Table 5. Characteristic temperature values of the outer envelope. Table 5. Characteristic temperature values of the outer envelope. Heading Max. Value ◦ C Min. Value ◦ C Average ◦ C Heading Wall outside Wall outside Wall inside WallWindow inside Window Heated area Heated area Max. value 9.6 °C 9.6 22.3 22.3 12.7 12.7 27.9 27.9 Min.9.0value °C 9.0 21.5 21.5 11.5 11.5 14.1 14.1 Average °C 9.2 21.69.2 21.6 12.0 12.0 18.8 18.8 Unlike Unlike the the first first building building in in 23 23 Kuhačeva Kuhačeva Street, Street, there there is is no no town town central central heating. heating. Therefore, Therefore, the the table does not have the temperature value of the parapet wall. table does not have the temperature value of the parapet wall. Based Based on on the the provided provided experimental experimental results results presented presented in in this this article, article, conclusions conclusions are are similar similar to to those in [30], where the authors concluded that the conventional thermographic techniques are useful those in [30], where the authors concluded that the conventional thermographic techniques are useful in in superficial superficial and and subsurface subsurface testing testing up up to to several several millimeters millimeters in in depth depth and and that that anomalies anomalies occurring occurring in deeper layers may be effectively located and identified by means of GPR. Dynamic thermographic in deeper layers may be effectively located and identified by means of GPR. Dynamic thermographic methods reveal the present in in an methods reveal the outlines outlines of of anomalies anomalies present an inner inner structure structure of of walls, walls, located located even even under under 1.5 1.5 cm cm of of plaster. plaster. However, However, no no voids voids were were found found in in this this case, case, as as in in [31], [31], and and the the temperature temperature pattern pattern shows shows homogeneous homogeneous behavior. behavior. 6. Further Investigations 6. Further investigations Due to institutional equipment availability, using GPR, further tests will be performed as in [32]. Due to institutional equipment availability, using GPR, further tests will be performed as in [32]. The research carried out by [32] demonstrates the suitability of GPR for identifying void spaces The research carried out by [32] demonstrates the suitability of GPR for identifying void spaces when when running across a rough-surfaced wall facade. This is done by comparing three commonly-used running across a rough-surfaced wall facade. This is done by comparing three commonly-used antennas (1.2 GHz, 1.6 GHz and 2.3 GHz). GPR can identify features within the blocks; however, antennas (1.2 GHz, 1.6 GHz and 2.3 GHz). GPR can identify features within the blocks; however, without having supplementary information from secondary sources, it is impossible to confirm the without having supplementary information from secondary sources, it is impossible to confirm the identity of the features. identity of the features. 7. Conclusions 7. Conclusion IR thermography, as a nondestructive testing method, can easily be applied to help detect IR thermography, as a nondestructive testing method, can easily be applied to help detect nonnon-homogeneous outer shell elements, hidden openings and structural elements. The main homogeneous outer shell elements, hidden openings and structural elements. The main characteristic characteristic of IR thermography is the detection of radiation on the surface of an object. In order of IR thermography is the detection of radiation on the surface of an object. In order to analyze the to analyze the deeper layers, it is necessary to use active thermography or some other NDT method. deeper layers, it is necessary to use active thermography or some other NDT method. Therefore, it Therefore, it can be concluded that conventional steady-state IR thermography can be useful to perform the first initial test method, which is then followed by other testing methods. Proper interpretation of the thermogram depends on the knowledge and experience of the operator during the process of analysis. The operator is very important since, by understanding Buildings 2019, 9, 32 18 of 20 the physical behavior of the object, he/she relates information about the state of the object and thermographic patterns. Based on this, it is obvious that for a quality thermographic analysis, a good understanding and knowledge of the historical development of the analyzed object is needed, alongside a basic knowledge of the thermal camera’s operation. With the arrival of cost-effective thermocameras on the market, classical thermography is becoming far more widespread, but active quantitative thermography still requires significant financial investment in the equipment and training of operators. Currently, various active thermography methods represent the latest developments in the field of IR thermography; however, even if we neglect the financial needs for the equipment, they require more time to conduct measurements. Due to limited financial sources, an affordable thermography camera with heat flux step excitation was used to implement this basic idea to find discontinuities inside structural walls. As preparation for the described building analysis, finding a junction box, as a typical example of this technique, was performed. Due to the different heat resistance and heat capacity of the analyzed part of the wall surface, in a relatively short time interval, it was possible to detect hidden elements in the wall. In the aforementioned example, a hair dryer was used as an exciter, but a variety of heat sources are used in everyday practice. In the case of the heat flux applied to the analyzed buildings, classical IR thermography did not prove effective in the process of finding structural elements in the historic structures of the old city of Tvrđa. From the recorded measurements, it was found that the main reason that classical IR thermography failed was the similar amount of thermal conductivity of the materials used, as well as the wall thickness, which results in thermal haze and the equalization of thermal patterns. The test was carried out in a steady state with a constant thermal flow resulting from a temperature difference of 20 ◦ C. No discontinuities, due to air gap, significantly different materials or elevated humidity differences, were found by heating the surface with a heat gun and analyzing the thermal conductivity over time. By heating the lightening protection installation, the thermal capacity of the wall was confirmed. It can be concluded that, as a final remark of this paper’s aims, the conducted tests corresponded to the experiences presented in the analyzed literature. Author Contributions: Conceptualization, H.G. and M.H.N.; Methodology, H.G.; Validation, H.G., M.H.N. and T.B.; Formal Analysis, H.G., M.H.N. and T.B.; Investigation, H.G., M.H.N., I.H.B. and T.B.; Resources, I.H.B. and H.G.; Data Curation, H.G. and M.H.N.; Writing—Original Draft Preparation, H.G.; Writing—Review & Editing, M.H.N.; Project Administration, I.H.B. 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Locating Hidden Elements in Walls of Cultural Heritage Buildings by Using Infrared Thermography

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