Review Article Review of InfraRed Thermography and Ground-Penetrating Radar Applications for Building Assessment

The first appearance of concern for the good condition of a building dates back to ancient times. In recent years, with the emergence of new inspection technologies and the growing concern about climate change and people’s health, the concern about the integrity of building structures has been extended to their analysis as insulating envelopes. In addition, the growing network of historic buildings gives this sector special attention. Therefore, this study presents a comprehensive review of the application of two of the most common and most successful Non-Destructive Techniques (NDTs) when inspecting a building: InfraRed Thermography (IRT) and Ground-Penetrating Radar (GPR). To the best knowledge of the authors, it is the first time that a joint compilation of the state-of-the-art of both IRT and GPR for building evaluation is performed in the same work, with special emphasis on applications that integrate both technologies. The authors briefly explain the performance of each NDT, along with the individual and collective advantages of their uses in the building sector. Subsequently, an in-depth analysis of the most relevant references is described, according to the building materials to be studied and the purpose to be achieved: structural safety, energy efficiency and well-being, and heritage preservation. Then, three different case studies are presented with the aim of illustrating the potential of the combined use of IRTand GPR in the evaluation of buildings for the purposes defined. Last, the final remarks and future lines are described on the application of these two interesting inspection technologies in the preservation and conservation of the building sector.


Introduction
Building assessment is becoming more and more common within the construction sector because buildings are physical assets that provide habitat and comfort for people [1]. As an added value, heritage buildings evidence the evolution of humanity and provide cultural, spiritual, and aesthetic satisfaction to the people, as well as economic bene ts through tourism [2]. Proof of this importance is more than 1,100 standards published by the International Organization for Standardization (ISO) that help codify international best practices and technical requirements to ensure that buildings are safe and t for use [3]. Heritage buildings have a distinct category within the ISO standards due to their particular construction and longevity of materials [4]. In addition, all the ISO standards are periodically updated, regardless of their nature, to take into account climatic, demographic, and social changes.
ISO has more than 100 standards [3] related to the raw materials used in construction, such as concrete, cement, timber, and glass. e di erent properties (structural, mechanical, electrical, thermal, and others) of the building materials mean that the basis of the inspection is di erent depending on the structural component to be analyzed. Moreover, the boundary conditions of each case study should be taken into account due to the di erent meteorological and interior control conditions of each building. In the absence of good maintenance conditions in a building, different surface and subsurface anomalies can appear, causing impairment: (i) To the structural health of the building. e irreversible collapse of the building materials can be provoked, endangering the integrity of the building and the lives of people. (ii) To the energy demand and the well-being of the users of the building. In case of a bad state, the building materials lose their thermal capacity, which leads to a reduction of the energy efficiency level of the building and to a reduction of the thermal comfort control of each room of the building.
erefore, it is fundamental to address the continuous deterioration of the structures of buildings due to the appearance of different anomalies caused by aging, unforeseen events, environmental conditions and previous incorrect restoration treatments. is includes inspecting the building design to identify, for example, wall layers with incorrect thickness according to the regulations. Documentation of the position and nature of possible hidden targets or hidden structural elements of the building is also included for condition assessment and intervention [5]. For that, ISO standards recommend the use of non-destructive techniques (NDTs) as inspection tools for both a punctual and prolonged study over time, regardless of the nature of the building and the anomaly [6]. NDTs improve safety, sustainability, and durability in the building sector due to their lower subjectivity and faster inspection speed compared to traditional inspection tools [7], and non-intrusive and nondamaging to building integrity compared to destructive inspection tools [8].
NDTs allow the analysis with a high level of detail in any building, providing a wide range of knowledge of the structure under study. is is demonstrated by the recently published reviews on: (i) the condition evaluation of various historic monuments constructed of stone, brick masonry, or reinforced cement concrete [9]. (ii) the building envelope diagnostics for standardized energy audits [10]. (iii) the traditional procedures and futures perspectives of NDTs for the diagnosis of heritage buildings [11].
In this review article, a compilation of the state-of-theart of application of InfraRed ermography (IRT) and Ground-Penetrating Radar (GPR) for building assessment is performed, with special emphasis on applications that integrate both technologies. ese techniques are selected due to their technological maturity and their wide use as inspection tools in the building sector. However, to the best knowledge of the authors, it is the first time that a joint compilation of the state-of-the-art of both IRT and GPR for building evaluation is performed in the same work, especially in the compilation of applications that integrate both NDTs. en, the objective of this work is to demonstrate the high capabilities of these two NDTs in inspecting the design of a building, documenting its internal structure and identifying both surface and subsurface anomalies, both individually and together, and even with other NDTs. In this way, another objective is to highlight the benefits in complementarity of information and validation of results by combining both inspection tools. With this purpose, Section 2 briefly describes the performance of each technology, through their comparison and analyzing their individual and common advantages for building inspections. Next, Section 3 describes the methodology performed in the review process and Section 4 compiles the state-of-the-art IRT and GPR applications divided into three sub-sections: structural safety (Section 4.1), energy efficiency and user well-being (Section 4.2), and heritage preservation (Section 4.3). e same structure is applied for each sub-section: representation both in text and table of the most common applications according to the corresponding objective, annexing the most relevant works by means of referencing, the building materials studied, and the NDT or NDTs used. Section 5 presents three different case studies, with the aim of illustrating the potential of the combined use of IRT and GPR in building assessment from different perspectives: structural safety, energy efficiency and user well-being, and heritage prevention. Finally, Section 6 covers the final remarks after the analysis of the previous sections and describes the future perspectives of the IRT and GPR inspection in buildings.

Brief Description and Comparison of IRT and GPR
IRT uses an InfraRed (IR) camera to measure and analyze a thermal pattern based on the principle that all bodies at a temperature above absolute zero (0 K/−273.15°C) emit electromagnetic radiation. Since 0 K/−273.15°C is an unattainable limit according to the third principle of thermodynamics, all bodies are measurable with an IR camera. e electromagnetic radiation emitted by a body is detected by the IR camera and transformed into an electronic signal by means of a set of photoelectric sensors that form a plane array inside the camera. en, electronic signal is processed to produce a thermal image based on Planck's [12] and Stefan-Boltzmann's [13] Laws. Specifically, an IR camera allows the entrance through the lens of radiation emitted by the body in one of the sub-bands of the IR spectrum. e choice of the IR spectrum among all the bands of the electromagnetic spectrum is because the maximum radiation intensity emitted by a body is located in this wavelength interval, according to Wien's displacement law [13]. When the body is under normal temperature conditions, the narrow portion of the IR spectrum is between 8 and 15 μm (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37), that is, the IR spectrum sub-band of Long-Wave InfraRed (LWIR), is the optimal sub-band to measure the thermal state of the body. In addition, the atmospheric absorption level (atmospheric attenuation) of the radiation emitted by the body is low in this IR sub-band, allowing more radiation to reach the IR camera lens [14]. Heat transfer is understood as an exchange of thermal energy between a high-temperature medium to a lowtemperature medium. is process of heat propagation in different media explains the abnormal surface temperature patterns of a body. e three mechanisms of heat transfer are conduction, convection, and radiation. When dealing with transient thermal problems and building inspection using IR cameras, conduction is the most important mode of heat transfer, as this mechanism defines the transfer of thermal energy between solid bodies, and consequently their emission of radiation. Materials conduct heat at different rates, and these rates are related to their thermal properties. ese properties are different between surface and internal anomalies with respect to the sound volume of the body.
us, it is possible to make the identification of internal anomalies (qualitative analysis), and their thermal characterization and depth estimation (quantitative analysis), from the application of processing algorithms to the thermal images captured with the IR camera [15]. When the analysis of surface anomalies is the objective, their thermal footprints are detectable with thermal transient states caused by solar radiation (passive IRT). However, thermal excitation with higher intensity than solar radiation is necessary to reach the thermal footprint of internal anomalies to the surface of the body, using artificial thermal sources (flash and halogen lamps, hot and cold air guns, electrical heating, ultrasonic excitation, eddy currents, microwaves, and others) (active IRT) [16]. e GPR method uses ElectroMagnetic (EM) waves (from 10 to 6000 MHz, belonging to the radio spectrum) to acquire information from the subsurface. A transmitting antenna generates the EM wave that is propagated through the media under study. e wave is partially reflected when encountering a discontinuity in the dielectric properties of media and recorded at the surface by a receiving antenna, while the other part of the signal continues to propagate until reaching the total time window set. e intensity or strength of the reflected signals, typically called amplitude, is proportional to the dielectric contrast value between two different media. As a result, an XZ image (radargram) is obtained as a result of the GPR analysis. e values of the radargram represent the amplitude values of the different reflected signals, which are measured as the transmitting antenna moves along the surface under study. us, the X axis represents the distance (meters) of each trace (position of each echo received) and the Z axis represents the two-way travel time of the echoes through the media. If the time required to propagate to a reflector and back is measured, and the velocity of the signal propagation through media is known, the depth of the reflectors can therefore be determined. A deep theoretical background can be found in [17]. Table 1 shows a comparison between IRT and GPR techniques, indicating the advantages of one technique over the other in columns 1 and 2; and the common advantages of both when inspecting a body in column 3.

Methodology
is review article focuses on the integration between IRT and GPR for building assessments. e main criteria considered in the search method were: (i) e use of Scopus and Google Scholar databases to retrieve the related research publications. A set of keywords was used for the search. ese always included the terms IRT and GPR, together with one of the objectives to be achieved in building assessment, objectives defined in the Introduction section ("structural safety," "energy efficiency and user well-being," "heritage preservation") and the term building, in order to cover a wide area of different applications. (ii) e consideration of books, conference papers, technical notes, and manuscripts such as review articles and original research, when they matched the search criteria. en, the redundant or irrelevant publications were excluded from the initial review list based on reading the title and abstract of each publication. Journal articles were preferred to conference publications when they addressed similar topics, and experimental data were preferred to simulated data. Original research and case study articles were preferred to review articles, especially those indexed in the Journal Citation Reports (JCR) database. (iii) After that, publications that were inaccessible to the reviewers were excluded from the review list. Apart from Open Access articles (most from MDPI), the institutional publishers available and most selected were Elsevier, Springer, and Taylor & Francis. Long-range depth analysis (from 1 cm to 10 m depending on the frequency) over a couple of centimeters with active IRT Repeatable and reliable tools, high maneuverability (iv) 99 publications were relevant to the review topic. Based on these, the applications made for each of the objectives in the evaluation of buildings have been broken down into different groups, making groups by affinity. After that, the type of material studied has been analyzed for each reference, together with an evaluation of whether the IRT-GPR integration has taken place and whether other NDTs have been used in addition.
Studies combining IRT and GPR for building assessment have shown an exponential increase in interest since 2000. Figure 1 shows the number of publications relevant to this review topic per year since 2000. Most were published since 2015, with a substantial jump from previous years, with 7-8 publications per year since 2018. Most of the publications reviewed were articles, with 87 publications (87.9%), followed by conference papers and book chapters with 10 (9.9%) and 2 (2.2%) publications, respectively, as shown in Figure 2. Regarding the number of publications per objective, an equal distribution has been found over the years, with 32% in  Reinforced concrete column beyond a double wall of hollow bricks ✓ De Domenico et al. [45] Advances in Civil Engineering structural safety, with 35% in energy efficiency and user wellbeing, and with 33% in heritage preservation, respectively, as shown in Figure 3.

Structural Safety.
Good structural health is of vital importance for a building to be able to provide its performance to the user in optimal conditions. Among the requirements to be met, there is one common to all buildings, that is, safety. For that, it is essential to know the condition of each building material before a collapse of the structure happens, including factors such as the age of the material and its constitution, as well as possible existing anomalies. Given the scope of IRT and GPR, both technologies are able to provide both structural information of the building and identification of critical areas, mainly: (i) e estimation of the position of hidden targets or structural elements, such as tension cables, grade beams, conduits. (ii) e analysis of the degradation rate. As evidence of the applicability of IRT and GPR, a summary table (Table 2) of the state-of-the-art is shown below.

Energy Efficiency and Well-Being.
Energy conservation in the building sector is a fundamental aspect when it comes to mitigating the advance of global warming, which directly leads to an economic reduction of the energy bill and a better quality of life for users. A better well-being of the users of a building with high efficiency in terms of energy as it leads to a higher stability of thermal comfort.
ere are three factors to be considered in a building envelope: its thickness, its thermal mapping, and its overall heat transfer coefficient, known as U-value. ese factors are good indicators of the level of energy efficiency and thermal comfort of a building, being interrelated: (i) e thickness of the construction materials is a fundamental parameter in establishing an adequate thermal inertia of the building envelope.
(ii) e thermal mapping allows for to identify possible anomalies in the building envelope, such as any malfunction of the Heating, Ventilation, and Air Conditioning (HVAC) system integrated in the building. ermal mapping also allows the identification of the following three anomalies, which are the most common and the most detrimental in the building sector in terms of energy efficiency and wellbeing: moisture, thermal bridges, and air infiltration.
(iii) e U-value takes into account not only the thickness of the building materials (geometrical properties), but also the thermophysical properties, evaluating more precisely the thermal resistance of a building envelope to heat transfer between the interior and the exterior.
GPR and IRT can successfully address each point described above, especially IRT because temperature measurement is more related to the subject matter of this section as opposed to the radio EM wave measurement, with applicability to a wide range of materials. A summary table (Table 3) of the state-of-the-art is shown below.

Heritage Preservation.
e protection and management of cultural heritage must be particularly cautious with the preservation of its singular and historical character. IRT and GPR have also been widely used for preventive damage detection and heritage preservation. Within cultural heritage, these techniques have been successfully applied to inspect monumental buildings, statues, frescoes, and mosaics, among others. In this context, these techniques proved to be effective in obtaining valuable information, mainly including: Solla et al. [53] 6 Advances in Civil Engineering Building with lightweight material ✓ Photogrammetry Natephra et al. [75] (i) e detection of damage in façades or walls, such as delamination, fractures, cracks, detachments, and moisture. (ii) e investigation of the conservation state of timber beams in floor and ceiling systems, and the structural integrity of basement, wall foundation, floors, and soil systems. (iii) e evaluation of the internal structure and defects in structural elements or columns, and decorative elements. (iv) e inspection of the state of conservation of walls and works of art, such as the adhesion between different layers and moisture in paintings and frescoes, or stratigraphy and water content in mosaics.
A summary table (Table 4) of the state-of-the-art is shown below.

Structural Safety: Detection of Corrosion in Structures.
is case study presents the use of GPR and IRT for the detection and evaluation of corrosion in old construction, where the corrosion can lead to the collapse of the structure. In this case, the combination of techniques allows differentiating between corroded areas and areas affected by moisture, as well as to identify damages in the interior of the structure such as cracking and debonding. e construction under study is a Military Base, located at the coast in the North of Spain (at 200 m distance from the sea), and dates from the 30 s of the XXI century. is location explains that corrosion is the main threat to the stability of the structure. More information about the site can be found in [46], and its interior can be seen in Figure 4.

Methodologies.
e GPR survey was conducted using a ProEx GPR system with a 2.3 GHz antenna. e profile lines were acquired through the ceiling of the structure which consists of a reinforced concrete slab (see Figure 4). Data collection was carried out by moving the antenna perpendicular to the direction of the rebar, using the setting parameters: 1 cm spatial sampling and time window of 14 ns (composed of 292 samples per trace). Before interpretation, the 2D radargrams produced were processed in the ReflexW software to suppress the continuous component (Subtract-DC-Shift), to amplify the received signals (Gain function), to remove horizontal continuous low-frequency reflectors (Subtracting average), and to remove both low-and high-frequency noise (Butterworth) in both 1D and 2D dimensions.
Regarding the thermographic inspection, a camera NEC TH9260 was used. e inspection distance between the camera and the walls of the Military Battery was 1 meter, in such a way that the Field of View was 21.7°horizontal and 16.4°vertical. Emissivity is set to 1, in order to make noncompensated temperature measurements of apparent temperature.
is mode is selected because of the variety of materials present in the surface of the walls of the battery, which would result in multiple emissivity values to compensate. Atmospheric attenuation was compensated, considering the ambient conditions: 16°C and 70% relative humidity. e temperature profiles measured along the GPR profiles were analyzed and filtered with the aim of detecting and delimiting the pathologies present. Figure 5, the GPR data produced allowed for the identification of the following pathologies:

Results and Findings. Observing
(i) Moisture content was detected as the travel-time distance increased because the velocity of propagation of the signal in water is lower. (ii) Higher mineral salts content was interpreted in zones with severe absorption of the signals (or signal attenuation). (iii) Detachment and voids beneath detached concrete were identified as hyperbolic reflections (rebar) near the surface because of the faster velocity of propagation of the radar signal in air, while showing higher amplitude spectrum (strong reflections). (iv) Fissuration was interpreted as signal scattering (diffractions).
e evaluation of the thermographic images shows that moisture is an important factor in the walls of the battery, which covers the effect of any other pathologies that could be present. In this case, since the apparent temperature was measured, it is important to consider that some thermal anomalies can be provoked by different emissivity materials. Consequently, the emissivity factor is considered as a Controlled test chamber ✓ Gil-Valverde et al. [80]; Royuela-del-Val et al. [81] 8 Advances in Civil Engineering   parameter for evaluation, in order to determine the type of each pathology: as an example, moisture appears as an area with a different temperature than dry materials, but the temperature transition is continuous; while mineral salts also present different temperature, but because it is caused by a difference in emissivity, the temperature transition is sharp. ese effects can be seen in Figure 6, where different profiles, their thermographic images and their temperature profiles with identification of pathologies are shown. In this particular application, the combination of GPR and IRT techniques was applied to confirm the interpretation of pathologies made by each technique independently, and IRT was useful to explain some signal scattering occurring in the GPR signal: the reason for the scattering was located in the presence of moisture and mineral salts, and the possibility of failure by the GPR antenna was dismissed.

Energy Efficiency and Well-Being: Detection of Building Installations (Radiant Heating Floor).
is case study presents a combined use of GPR and IRT for the evaluation and characterization of thermal floors in residential buildings. e test site corresponds to a joint kitchen and living room, with low-reflectivity ceramic tiles as floor coating (see Figure 7).  Advances in Civil Engineering spatial sampling and a 12 ns time window. An encoder-based wheel was attached to the antenna, as a distance measurement instrument, to measure the profile length and to control the spatial sampling. A 3D data acquisition was conducted with equidistant parallel profile lines at regular intervals of 5 cm. e profile lines were collected with the antenna polarization orthogonal to the longitudinal direction of the heating pipelines. Both the 2D radargrams produced and the 3D cube were processed in the ReflexW software. e thermographic camera used was an NEC TH9260 camera, with an Uncooled Focal Plane Array (UFPA) with a size of 640 × 480, a precision of 0.2°C and a thermal resolution of 0.1°C. Images were acquired with an emissivity value of 1, in such a way that apparent temperature is measured toward a qualitative analysis of the performance of the radiant heating system. In addition, in order to minimize the appearance of reflected radiation, images were acquired from a point of view with an inclination of 10°from perpendicular to the floor, and at a distance of 2 meters for an optimal image field of view.

Results and Findings.
GPR provided information about the number of pipelines and distribution. As shown in Figure 8(a), GPR data revealed the presence of three pipelines.
e thermographic mosaic showed the presence of two pipelines (Figure 8(b)). e spatial correspondence between the 3D GPR image and thermographic mosaic leads to the conclusion that the central pipeline is not working.
Comparing both techniques, GPR gives information about all pipelines but cannot distinguish whether they are

12
Advances in Civil Engineering   Advances in Civil Engineering 13 working or not. IRT can detect only the working installations. Moreover, the different thermal print between the pipelines on the left and on the right shows a malfunctioning of the pipeline on the left, given its colder temperature distribution.

Heritage Preservation: Moisture Detection in San Julián de Moraime Church (Muxía, NW Spain).
is Church, which was declared an Asset of Cultural Interest in 1972, has more than 46 m2 of medieval wall paintings. As shown in Figure 9, these paintings are located in five sections through the northern wall, and represent the seven capital sins: section 1-sacred form, section 2-the pride and the greed, section 3-the anger and the lust, section 4-the gluttony and the envy, section 5-the sloth and the death. ere are visible signs of deterioration such as moisture, presence of fungi and algae, saline efflorescence (chlorides, sulfates, nitrates, and nitrites), calcareous formations by carbonation, and dissolution of pigments. e main factors contributing to this deterioration are: (i) moisture, which multiple origins (moisture by capillarity and infiltration, residual moisture, and moisture caused by condensation); (ii) climatic conditions (the site of Moraime has a wet and warm weather during all the year with a maximum temperature of 11°and 20°C in winter and summer, respectively); (iii) presence of marine aerosols given its proximity to the coast; (iv) the interior of the Church is below the ground level; and (v) the northern wall is the least sunny and ventilated, coinciding with the drainage ditches of the rainwater.
is situation mainly affects the first three sections. Section 4 gives to the Sacristy, which provides isolation from the outside, however, the contact with the corresponding buttresses implies entry of moisture. In the last section, the outer wall coincides with a flat ground and, therefore, where rainwater accumulates. It has no drain.
A combined IRT and GPR study was carried out to analyze the effect of moisture on the wall paintings. e IRT method focused on the paintings, while GPR was applied to the floor to investigate the water entry and possible moisture by capillarity.

Methodologies.
e GPR survey was conducted using a RAMAC system from MALÅ Geoscience, with a CUII control unit and a 500 MHz antenna. e setting parameters were 3 cm of trace-interval and a total time window of 78 ns composed of 516 samples by trace.
e equipment was mounted on a survey cart with an odometer wheel as triggering. e acquisition was based on single antenna 3D GPR methodologies. A grid of parallel profiles spaced 10 cm was collected on the floor at the northern wing (see Figure 10). A total of 19 profile lines were acquired, covering approximately 25 m 2 .
e GPR signals received were processed with the ReflexW software, using the following filters: time zero correction, subtract-mean (dewow), gain function, subtracting average, and bandpass (butterworth). e 3D cube and time-slices were elaborated with the same software. e thermographic camera used was the same as in the previous cases (NEC TH9260). e ambient conditions in the Church (20°C, 70% relative humidity) were considered due to the high humidity, which required the application of the atmospheric correction to the thermal radiation received by the camera. e area inspected was also the North wall, from the interior side. e main drawback of the inspection was the lack of sun and consequently of thermal excitation, which limited the variety of pathologies to be detected to the most severe ones. e application of thermal excitation to the walls is also not possible in order to avoid any damage to the frescoes in the walls.

Results and Findings.
Observing the 3D GPR images produced in Figure 10, the presence of water content in the subsoil is associated with a higher intensity (or amplitude) of the radar signal. Figure 10(a) shows the timeslice at 50 cm in depth, in which two reflections were interpreted (highlighted into red boxes) that most likely correspond to the foundation of the attached columns. On the other hand, Figure 10 14 Advances in Civil Engineering around the columns (red lines) that might be indicating water infiltration. e thermographic inspection corroborates the detection of moisture as the main pathology in the Church: it appears as a lower temperature area in the images, due to the higher thermal inertia of water ( Figure 11). e images of all sections show the presence of moisture in the lower part of the walls, as a sign of water capillary action. is pathology is coherent with the presence of water on the ground detected by the GPR inspection. In Sections 2 and 4, where efflorescence and mold were visible to the eye, the higher effect of the presence of water hid the thermal signs of these pathologies on the thermal images.
In this application, the combination of the GPR and IRT techniques implies the completeness of the study, including both the ground and the walls for a clearer understanding of the situation and for a more integral interpretation of the phenomena occurring in the Church and causing possible damage to the frescoes.

Conclusions and Future Perspectives
In this study, an exhaustive review of the state-of-the-art of application of IRT and GPR to building assessment is developed, with special emphasis on applications that integrate both technologies. Specifically, a brief description of each technology and a comparison between their individual and common advantages in the building sector is performed (Section 2). After that, the methodology performed in this review process (Section 3), and the most relevant references, together with the inspection tools used in each case, are detailed by indicating the building materials studied and their specific application according to one of the following three objectives to be achieved: structural safety (Section 4.1), energy efficiency and well-being (Section 4.2), and heritage preservation (Section 4.3). At the end, it is presented three different case studies, with the aim of illustrating the potential of the combined use of IRT and GPR in building assessment from the three objectives defined (Section 5).
After an analysis of the work, the following points can be highlighted as final remarks: (i) IRT and GPR are capable of detecting the position and mapping the extent of any anomaly, superficial and shallow level with IRT and deeper level with GPR, regardless of the nature of the anomaly. In structural safety, the identification of degradation, cracks, fractures, voids, delamination, detachment, and corrosion is within the reach of both NDTs. e same is true for identifying moisture when assessing building energy efficiency and well-being. Delamination, cracking, cavities and moisture in historic buildings are also covered by IRT and GPR, including small-scale elements such as frescoes, paintings, and marquetry, among others.
(ii) e combination of both inspection tools allows for both a double verification of the findings obtained with each and to complement the information acquired. For instance, in the case of the inspection of an HVAC system, the GPR detects the actual configuration of a radiant floor heating circuit system, while the IRT allows the detection of the pipes in operation. us, by combining both techniques, malfunctions can be identified [55].  Another case is moisture analysis, with GPR identifying the origin and source of moisture, and IRT providing information at surface level. Together, they determine the movement of moisture through the wall [18].
(iii) Due to the higher depth range of GPR, it is common to use the latter technique for the detection of hidden targets or structural elements and the estimation of the thickness and quality of walls, although there are some works with IRT for the same task, in case of using active IRT. is preference for the choice of GPR also applies when inspecting internal rebars and monitoring subsidence and settlement phenomenon for the same reason. e opposite would happen for the estimation of the U-value parameter for energy purposes, where the analysis of the surface behavior of the building materials versus the internal behavior prevails, using in this case IRT. e same applies when identifying thermal bridges and air infiltration.
(iv) e common practice of applying several NDTs in addition to IRT and GPR for the same case study, regardless of the building material, objective and application addressed. With this, the verification of the findings is supported, and the results are complemented for a more complete and betterquality building inspection. Laser scanning is a common NDT to use with both GPR and IRT, regardless of its application, given the geometric information provided by this technology. With the integration of laser scanning, 3D GPR and 3D IRT data are obtained. In addition to GPR, ultrasonic and ERT technologies are widely employed for walls and foundation inspections, respectively, in order to complement each other. As for IRT, surface moisture meters and heat flux meters are used for comparison and double checking of results to detect moisture as well as to estimate the U-value, respectively. e blower door test is used to support the detection of air infiltration with IRT. Photogrammetry is an NDT that is also used to generate IRT 3D data.
(v) Studies combining IRT and GPR for building assessment have shown an exponential increase in interest since 2000. Most were published since 2015, with a substantial jump from previous years and with 7-8 publications per year since 2018. Most of the publications reviewed were articles, with 87 publications (87.9%), followed by conference papers and book chapters with 10 (9.9%) and 2 (2.2%) publications, respectively. An equal distribution has been found over the years regarding the number of publications per objective, with 32% in structural safety, with 35% in energy efficiency and user wellbeing, and with 33% in heritage preservation, respectively.
(vi) e limitations found in this review process were the inaccessibility of some publications because they belong to publishers with no or restricted number of agreements for free access with universities. In addition, some publications were discarded because they were difficult to read, especially due to the lack of a clear description of the case studies analyzed and their purposes.
After the demonstration of the well-defined application of IRT and GPR in the building sector, future lines should be followed in the maturation of the joint application of both technologies. Specifically, they should be jointly directed toward the integration of the results in Building Information Modeling (BIM) and Digital Twins. BIM and Digital Twins are the current protocol being implemented as a communication bridge between inspection tools and users, and directed toward building 4.0. Devices/robots will also play a fundamental role in collecting data in areas that are difficult to access, especially in the case of GPR, and Artificial Intelligence (AI) models to process and interpret the complexity of the IRT and GPR data acquired.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.