Applications of ground penetration radar geophysical technology together with infrared thermography, ground electromagnetic survey and other complementary technologies, in construction. Limitations and possibilities

3.1. The georadar (GRP)

Ground Penetrating Radar technology is based on the transmission and reflection of electromagnetic waves, that is, on their emission and subsequent reception of the different reflections that the different materials which make up the object studied.

This technology uses a balanced matrix for a pair of bistatic antennas, which allows combining the results on a grid at a fixed distance. The matrix results in a high resolution definition of the structures of different materials (8(8) Linck, R.; Fassbinder, J.W.E. (2013). Determination of the influence of soil parameters and sample density on ground-penetrating radar: a case study of a Roman picket in Lower Bavaria. Archaeol Anthropol Sci 6, 93-106. https://doi.org/10.1007/s12520-013-0145-4.
), which grants, together with geopositioning systems, greater precision in the situation of the results in plane when joining it with CAD systems or even BIM. In order to initially estimate what is to be analyzed, we must first take into account some parameters of great importance, such as the dielectric constants of the materials that we can expect to make up the sample, the size of the analysis area, the depth of the area that we intend to study and the resolution that we will need to achieve.

Once the general parameters have been established, such as the dielectric constants of the materials that we can expect the sample is made up, the size of the analysis area, the depth of the area that we intend to study and the resolution that we will need to achieve, we must proceed to choice of the most suitable antenna frequency for the objective we intend to obtain, since the lower frequency counts greater depth of penetration, but at the cost of a decrease in the resolution of the results.

When finding a zone with different electromagnetic characteristics, part of these waves are reflected and captured by the antenna. Given the cone shape of the pattern emitted by the transmitting antenna, the areas that extend with respect to the vertical will arrive with a greater delay, reflecting data that will appear to be of deeper than its reality (9(9) Benedetto, A.; Pensa, S. (2007). Indirect diagnosis of paviment structural damages using GPR reflection techniques. Journal of Applied Geophysics. 62(2), 107-123. https://doi.org/10.1016/j.jappgeo.2006.09.001.
). This is the case of the cylindrical objects reflection, which are represented in the trace with a hyperbolic image, in which the extremes are the records of the rebounds around the path or the antena.

The horizontal resolution of the GPR will depend fundamentally on the wavelength of the antenna used, in such a way that a 200Mhz antenna achieves a low resolution at depths of up to 10m, while a 2.4 Ghz antenna achieves a very high resolution, which reaches 20,000 points per square meter, although its penetration capacity decreases, being in this case below 0.5m (10(10) Annan, A.P. (2005). GPR Methods for Hydrogeological Studies. In: Rubin, Y., Hubbard, S.S. (eds) Hydrogeophysics. Water Science and Technology Library, vol 50. Springer, Dordrecht. https://doi.org/10.1007/1-4020-3102-5_7.
). In these traces the diagrams of the response or rebound times are shown, which is directly related to the speed at which the waves are transmitted in the material, the depth of the detected discontinuity, as well as the difference between the electromagnetic parameters of the material and those of the one that motivates the discontinuity. In recent years, there has been an advanced development in high-resolution antennas, reaching equipment that works in frequency ranges of up to 6 GHz. However, said equipment maintains a minimum penetration capacity, around 20 cm, for what commercial equipment maintains the maximum range between 30-50 MHz.

Regarding the representation, the georadar uses electromagnetic pulses of very short duration, 1-60 nanoseconds (ns = 10-9 seconds), in the VHF / UHF band (20-1000 MHz), which are repeated with a frequency of 50 KHz. These impulses are grouped into wave packets made up of 1,000-15,000 of them. When these impulses are generated by means of the transmitting antenna, these impulses, in their trajectory through the subsoil, may encounter a change in the geological stratum, a construction, cavities, objects, humidity or water table levels, etc.; In short, although other factors such as density and relative humidity also influence , what they detect is mostly a change in the electrical properties (dielectric constant) of the media in which they propagate. This causes part of the energy to be reflected and collected by the receiving antenna, while the rest continues its way through the interior of the analyzed sample.

Residual moisture is also important, because it affects the speed of the wave (the higher the humidity, the lower the speed) which can distort the results. In metallic elements, density is also important, given that being high, the radar wave does not penetrate it. It also influences if there is salinity in the subsoil or clay, the worst enemies of the radar wave.

This energy is represented by color-amplitude windows that are usually chosen so that they are representative of the objectives of each job. The positive amplitudes correspond to the passage to a medium with a higher dielectric constant, the target being the maximum positive amplitude recorded. The negative amplitudes correspond to the passage to a medium with a lower dielectric constant, gray being the maximum negative amplitude. Since it can be considered that one works with a normal incidence when the reflecting surface is flat and in the case of working in non-magnetic media, the reflection coefficient of the georadar is given by the simplified expression (Equation 1)

Where ε2 corresponds to the dielectric constant of medium 2, while ε1 corresponds to the dielectric constant of medium 1. Applying the principle to studies of reinforced concrete, the existing media will be concrete and the rebar of the reinforcements, these acting as short-circuiting for the emission of the georadar. for which they are characterized by maximum amplitude. In the examples studied, the decrease in this maximum amplitude reflection that must occur in the metallic elements (rebar) should be indicative of the degree of oxidation / corrosion.

Amplification of data obtained in the trace. As the delay time is prolonged, there is a decrease in the characteristics of the signal since the energy of the electromagnetic pulse decreases as it propagates through the material, which means that as we increase the depth it will be necessary to amplify the data that are received (11(11) Rodríguez Abad, I. (2009). Evaluación de la técnica no destructiva del georradar para la inspección, diagnóstico y análisis resistente de la madera. Doctoral Thesis. Polytechnic University of Valencia.
).

Filter application. In most cases, signal processing will be required in order to eliminate the “noise” data that clouds the trace data and makes it difficult to identify and interpret the data that is really significant for the result that We want to obtain, for for which two main types of filters are applied, band filters for the elimination of unwanted frequencies and filters for the elimination of the background (12(12) Lualdi M.; Znzi L.; Binda L. (2003). Acquisition and processing requirements for high quality 3D reconstruction from GPR investigations. Acts of the International Symposium of Non-Destructive Testing in Civil Engineering, Berlin, Germany.
).

Velocity of propagation and characteristics of the materials. For the correct interpretation of the delay times in the rebound of the signals, it is essential to know the propagation speeds of the electromagnetic waves in the material to be analyzed (Figure 1). Although these speeds can be calculated empirically, as a general rule they are estimated based on the lithological characteristics of the materials that we estimate that the sample to be analyzed contains. Among these, the most relevant characteristics for the propagation of electromagnetic waves in non-magnetic materials are the Dielectric Permittivity (ξ) and Conductivity (σ), which have been used to determine the parameters that characterize the different materials (13(13) Maierhofer, C.; Brink, A.; Röllig, M.; Wiggenhauser, H. (2003). Detection of shallow voids in concrete structures with impulse thermography and radar. Ndt & E International - NDT E INT. 36, 257-263. https://doi.org/10.1016/S0963-8695(02)00063-4.
).

A concept that we must bear in mind is that of Electrical Resistivity (Equation 2). The electrical resistivity (ρ) of a certain material is defined as its specific electrical resistance, and it depends on the electrical resistance (R), the surface (S) and the length (l):

The electrical resistivity of a particular conductor material is a measure of how strongly the material opposes the flow of electric current through it, and is inversely proportional to conductivity (σ). This variable parameter depending on the material and its density, is a concept used in GPR measurement, being the more effective the greater the difference between the different materials that make up the sample analyzed. In this way, sands and sandstones have resistivity values between 100 and 5000 Ωm and clays between 3 and 100 Ωm, compared to 0.0172 Ω.mm2/m for copper or 0.0971 Ω.mm2 /m for iron.

3.2. Infrared thermography (TIR)

This technology is widely proven as a non-destructive test of special efficiency for its use in the construction sector, from the verification of the thermal conditions of the building envelopes to the detection of various deterioration processes caused by physical and chemical agents in the structures and their elements. They are included as analysis tools to raise planimetries identifying their injuries (14(14) Guerrero Mena, C.N. (2013). Evaluación de la Aplicabilidad de la Técnica de la Termografía Infrarroja al Reconocimiento del Estado de Elementos de hormigón. (Master Thesis). Polytechnic University of Catalonia. Spain
).

The development of easy-to-use portable equipment and its popularization at affordable costs has allowed its use to be generalized. Likewise, it makes it possible to show the real temperature of an object and observe the variations in temperature and emissivity on its surface by measuring the variations in infrared radiation. The image we obtain is called a thermogram, which instantly shows the temperature at each point on the surface of the object, whether it is stationary or moving. The system can be applied in two main ways, the active and the passive technique.

Active Thermography. It uses a stimulation by means of an external infrared radiation source that heats the surface of the médium, producing a heat radiation that depends on the temperature distribution, allowing the detection of anomalies on the surface or under the surface.

Passive Thermography. In this case, the system analyzes the object’s own emissions without the need for external stimulation, measuring and interpreting among other parameters the surface temperature patterns.

3.3. Infrared aerial photography

Although aerial photography systems have been used to identify and survey land uses since the mid-nineteenth century (15(15) Serra, W.; et al. (2002). Fotointerpretación, fotogrametría y teledetección. Retrieved from http://www.efn.uncor.edu.
), nowadays technological development has made it possible to process huge amounts of data, giving rise to GIS information systems. geographic.

There are several types of aerial photos (16(16) González Vázquez, X.; Marey Pérez, M.; Vicente Rodríguez, V. (2006). Aplicación de sistemas de información geográfica y fotografía aérea infrarroja para la planificación de dehesas. International Congress of Project Engineering. Valencia. Spain.
):

Among the film typology, in addition to panchromatic, which provides images similar to those captured by the human eye, and color, infrared photographs distinguish between black and white photographs, and color infrared, or false color, which have many advantages since in addition to being sensitive to the same spectrum band as black and white, they assign assigned colors to objects. with certain properties or characteristics.

In the technology of infrared color photography, the blue color is completely eliminated, thus filtering the effect of atmospheric light scattering, providing information beyond the spectrum of visible light, between 700 and 1200 nanometers. This type of photography has its origins in 1910, when the American physicist Robert W. Wood published his research to demonstrate the reflection of chlorophyll, by including cryptocyanin as a dye applied to photographic plates. Later, during the First World War, the United States used plates sensitive to the infrared spectrum to detect camouflaged elements. At present, for use with a digital camera we will only have to use an infrared filter.

3.4. Multispectral photography

The evolution of systems that traditionally allowed capturing images for cartographic uses now allows us to analyze time series of physical phenomena that occur on earth. The development of complex algorithms and specific techniques and processes now allow a large amount of data about the interrelation of objects with electromagnetic radiation.

The spectrum is the distribution of the intensity of radiation as a function of a characteristic quantity, such as wavelength, energy, frequency, or mass. In this way, the spectral image is the one that reproduces an object depending on the wavelength that it is emitting or reflecting. This radiation allows a substance to be identified. Electroscopes, in addition to allowing the observation of spectra, make it possible to carry out measurements on their wavelength, frequency and intensity of radiation. The electromagnetic spectrum ranges from the lowest wavelengths of radiation, such as gamma rays, to the highest, such as radio waves. Within this spectrum, the range of radiation visible to the human eye is between 400 nm and 700 nm. Below this band is ultraviolet radiation, and above infrared.

The colors that the human eye appreciates correspond to the wavelengths that objects transmit or reflect, and not those that they absorb, being these complementary to the former. For this reason, if it is intended to observe the radiation absorbed by an object, a suitable source must be used, and study the image band covering the region of the spectrum closest to the used source.

We can then deduce the differences between a multispectral and a hyperspectral image (17(17) Hyperspectral. Retrieved from https://www.astrofisicayfisica.com/2012/06/que-es-el-espectro-electromagnetico.html.
), which focus on the number of spectral bands they include. The former capture images in the two spatial directions, while the latter add a Z spatial dimension to the former. The multispectral images are made up of a number of between 3 and 20 bands, not necessarily contiguous, with which we can obtain intensity values at the discrete wavelengths of the object.

While to take color photos as seen by the human eye, RGB cameras (red, green, blue) are mounted, multispectral cameras allow us to visualize radiation that goes beyond RGB, with this type of camera we will be able to capture the red edge (0.68 to 0.75 microns) and the near infrared (0.75 to 1.7 microns).

There is a wide variety of multispectral cameras and NIR (near infrared) cameras on the market. The choice will depend on the possible use that is going to be given. The use of these systems is wide, and they go as far as the search for the location of hydrocarbon deposits through the exploration of small superficial leaks, constituting a direct method of exploration (Hörig, Kühn, Oschütz and Lehmann 2001).

3.5. The ground electromagnetic survey (GEM-2)

Also call Electromagnetic Profiler, the GEM-2 is an active method that uses an electromagnetic signal (EM) to detect variations in the electrical conductivity of the surface being analyzed, providing a fast multi-frequency technique for surface geophysical explorations that allows to visualize the geomorphological and geological structure of the subsoil, and the spatial and volumetric positioning of internal anomalies, provided they have a certain entity (Figure 2). The principle is based on the detection of each conductive coefficient of the different elements that make up the sample. A receiving coil in the EM instrument allows the secondary electromagnetic field caused by the currents to be measured at the same time as the original signal being transmitted (18(18) Bim Yaccup, R.; Brabham, P.J. (2012). Ground Electromagnetic Survey (GEM-2) technique to map the hydrocarbon contami-nant dispersion in the subsurface ay Barry Docks, Wales, UK. Retrieved from https://www.researchgate.net/publication/266679587.
).

This secondary field is separated into two orthogonal components, which provide, on the one hand, a measure of the apparent conductivity of the analyzed area, and on the other hand, variations that indicate anomalies of different consideration.

A big advantage of the system is its portability, which linked with active GPS tracking allows an operator to collect around 20,000 data points per hour at five frequencies. The depth of penetration into the material that is achieved depends, among other factors, on the conductivity of the material and the chosen EM wave frequencies

4.1. Geophysical survey of buildings in the Residential Complex in La Moraleja, Madrid (Spain)

Scope of the work carried out: In this project, geophysical prospecting was carried out using infrared thermography (TIR,) infrared aerial photography (IR), Ground Penetration Radar (GPR) and Electromagnetic Profiler (GEM) on the roofs of the buildings of the complex in order to determine possible existence of water under the horizontal facing of the upper terraces, and its spatial and volumetric positioning.

The buildings presented some deficiencies in the waterproofing of the roofs caused by a concatenation of mistakes in their construction, which caused repeated pathologies. During the development of the repair work, atmospheric phenomena of an explosive cyclogenesis type occurred, which caused the formation of water pockets between the various layers of roof formation. That problem occurred in a greater and different number of buildings than those reflected in the project. of construction.

These bags produced pathologies of diverse consideration because of the leaks into the houses. The property made, with little success, an attempt to locate and eliminate the bags by opening on site test holes. The method was unsuccessful so, the work was entrusted to a specialized company with the aim of locating, spatially identifying, and, where appropriate, quantifying the possible existence of dammed rainwater.

To carry out the work, the company used the following coordinated systems:

Specific GPR and GEM Methodology: Fundamentally, it consisted of making a gridded mesh in X and Y coordinates, for detailed prospecting the hole Surface of the chosen areas. So, the data collection was carried out with a high mesh density, practically the maximum that can be done in a geo-morphological environment as complicated as a roof, through a GPR on a crutch provided odometer and GEM equipped with DGPS. Given the need for high resolution of the radagrams and electromagnetic profiles, the theoretical quality of the surveyed envelopes and the prior technical evaluations, it was agreed to use the 1.0 GHz antenna (maximum range 1 meter deep in concrete with these dielectric characteristics) and three frequencies, high, medium and low for electromagnetic profiles, with a maximum range of 2 meters deep.

The GPR equipment was used manually following the same grid mesh methodology, with the advantage of its resolution greater than 20,000 information points per square meter.

The use of the multifrequency electromagnetic GEM during data collection was carried out by means of a sweep at an approximate height of 0.20 cm above the surface. The equipment allowed to carry out an exact positioning of the anomalies that were detected, since it was equipped with a GPS differential and three frequencies, in addition the data obtained was reliable even at depths greater than 1 m for this investigation. Both technologies were carried out under very complex conditions, given the location of the prospecting.

Methodology used with the Thermography (TIR) and Infrared Aerial Photography (IR) equipment: The methodology consisted of taking a battery of aerial photographs by means of UAV equipment, with an infrared (IR) and thermographic (TIR) digital camera, following the grid determined, under pre-programmed working conditions (IR, sensitivity, emissivity, etc.). Likewise, a specific oblique orientation was determined, following previous parameters and in a preconceived daylight schedule, morning in this specific case, to take advantage of thermal dissipation.

The thermal differences in the analyzed grid were an indicator of the existence of deficiencies inside different geo-morphological structures (contamination, cavities, water, decay, etc.), since the facing acts as a heat collector during the day and from dusk it radiates the energy by thermal dissipation to the colder air. As there is a layer of different density and composition, a variation in the temperature of the facing could be detected in these areas. The presence of moisture on the surface and / or shallow levels would delay thermal dissipation and, in any case, would considerably lower the detected temperature (Figure 3).

During the collection of thermographic data, the daytime atmospheric temperature remained at very low thermal values, so there was no great difference in thermal dissipation, which slightly impaired the thermal differentiation capacity of the structure of the area.

Data collection process: The data collection with the GPR and GEM equipment was carried out according to an imaginary grid established in the field, managing to prospect 86% of the surface by means of GPR, 94% by GEM and 100% by infrared sensors. In this way, the total area analyzed covered an area of 1,520 square meters.

TIR analysis: Once thermograms of shadows and surface elements had been filtered by removing the basic primary anomalies, slight anomalies of negative category were detected (radiation shielded by the presence of water in the structure), in several areas of the grid, associated with the existence of high residual humidity in the roof structure compared to thermograms in other areas.

The primary anomalies were verified using high-resolution ground geophysical methods. In general, the surfaces presented a very low profile gradient and a heterogeneous thermal level.

IR analysis: The infrared image analysis of the spectrum closer to the visible was processed to detect geomorphological surface changes as density, corrosion or presence of water due to exogenous conditions, positioning the sub and superficial structure of the area, where significant surface contrasts were detected, clearly associated with the presence of high residual humidity on the surface. and inside the enclosure. The different surface composition of the roofs, with areas welded with insulating extruded polystyrene slabs with surface protection of porous mortar and others only with asphalt layer, significantly impaired the ability to differentiate infrared. In asphalt layer and enclosures, shades of yellow-green with points of maximum concentration were appreciated, in the area soaked with insulating plate, dark blue and magenta tones were appreciated.

GEM analysis result: In this analysis, points with maximum internal accumulation of water were detected, with indication AE (in yellow) and AP (in red color) concentrated in the areas closest to the three internal enclosures identified with the exits to the exterior of the three stair cores.

Final Result and Conclusion of the joint data analysis: Infrared thermography and infrared photography provided primary but basic data to visualize the extension of the areas with probable involvement by residual humidity, with the verification of thermal signs of existence on the surface and structure of different thermal and infrared anomalies associated with the current presence of water inside the composition of the envelope roof, but the massive presence of residual moisture on the surface and the existence of a differentiated geomorphological structure, with areas with tiles differentiated from the areas with only asphalt layer, prevented the clarification of accumulation areas (Figure 4).

Likewise, both the GEM and the GPR detected electromagnetic anomalies associated with the existence of water inside the roofs of the building envelopes, most of them grouped in the upper and central layers, between 0 and -30 centimeters deep, medium-sized and current in nature, very difficult to identify due to the presence of rainwater on the surface. The areas with the greatest accumulation were positioned, showing maximum accumulations of water in the free phase of up to 2 cm.

However, the accumulation of constitutive layers of the roofs, with a very marked difference in their dielectric constants and their geophysical properties, significantly hindered the interpretation of the final data, and therefore the determinations of the conclusions.

4.2. Geophysical Prospecting for the National Highway Directorate on the bridge over Arroyo Saladino, RN9 highway. Buenos Aires

Scope of the work carried out. The objective of the project was the verification by non-intrusive techniques of the structural situation of the reinforced concrete bridge over the Arroyo Saladino (Argentina), and the detection and spatial and volumetric positioning of the different pathologies that could be detected. The structure showed visual signs of deficiencies, such as cracks and deformations, necessitating its detection, evaluation and analysis. Likewise, it was intended to detect possible anomalies in the geological layers found in the subsoil of the bridge and stream and the preparation of a location plan, so that later, they serve as a guide for subsequent repairs and / or in the event that If the highly relevant detected were interpreted, the rectification works considered appropriate by Dirección Nacional de Vialidad, which is a State company for management of the highway network, were planned. Next, a summary of the work carried out was developed that illustrates both the methodology and the results obtained.

Specific methodology. The fundamental methodology consisted of planning and making a grid in X and Y coordinates, to prospect in detail the areas chosen by the national company that owns the structure. Given the importance of the fact to be investigated, data was collected with a high mesh thickness, practically the maximum that can be done in a geo-morphological environment as complicated as a bridge with a circulation of 3,000 vehicles / hour, without disturbing the circulation, located in restricted access areas of an International Highway, through a radar equipped with an odometer and GEM with DGPS. The need for high resolution in the radagrams and electromagnetic profiles, together with the theoretical quality of the surveyed soil and the previous technical evaluations, led to the decision to primarily use the 900 MHz antenna (maximum range between 1.5 and 2 meters of depth in concretes with these dielectric characteristics) and assess deeper layers also with the profilometer and five frequencies (maximum range between 9 and 12 meters deep in subsoils with these dielectric characteristics).

Once the field work was finished, the data was dumped into a computer for subsequent export, treatment, representation and interpretation, using specific software for each task. The GEM-2 software: WinGEMv3 was used for the data dump and export, while the Surfer9.0 and Matlab6.5 programs were used for the treatment and graphic representation. Within the data processing, several processes were carried out. Depending on the results that were obtained, decisions were made in the processing of the records, that is. Ranges of values were eliminated, certain contrasts or zones were accentuated, filters, operators were applied, using various color scales, etc. In this way, maps of magnitudes were obtained in which we will be able to appreciate, in the best possible way depending on the sedimentary environments, the most interesting contrasts or anomalous areas according to the analysis objectives.

Ground Penetrating Radar: The GPR used was the GSSI SIR-3000 with a SmartCar trolley equipped with an odometer, a radarteam crutch, and as auxiliary material, a measuring tape, signaling cones and an independent photographic sketch of each section. It was intentionally chosen not to know the data or hypotheses about its constructive structure prior to carrying out this survey by means of georadar so that its knowledge did not influence the layout, volume and spatial positioning of the possible pathologies that were detected.

Infrared Photography: The methodology consisted of taking a series of photographs by means of a digital infrared (IR) thermographic (TIR) and visual spectrum (RGB) camera of the bridge structure, under previously programmed working conditions (IR, sensitivity, emissivity, etc.) and with an oblique orientation determined by previous parameters and in a preconceived sunlight schedule, specifically at dusk, so that it was possible to take advantage of the thermal dissipation. This action was designed to detect and position possible internal anomalies associated with pathologies on the surface or within the structure and is included solely for the purpose of demonstrating the capabilities of thermography in this area.

GEM and Side Scan Sonar. To analyze the geo-morphological and geological structure of the terrain. As important as auditing the structure of the bridge itself was analyzing its integration with the terrain on which it sits, including the flooded area when in doubt about the possible existence of a current in the subsoil of the stream, given the close relationship in the detection of internal moisture in the structure, and its internal corrosion problems. The geological structure of the subsoil was extremely important so the work included detecting the possible existence of discontinuities or internal cavities, which could affect the stability of the construction, as well as its degree of hardness and flexibility to avoid resonance problems. For this purpose, the analysis was carried out using an acoustic torpedo drawn from a small inflatable boat. This system is capable of detecting and positioning any object and / or geo-morphological structure at the bottom of rivers / swamps, graduated with pre-established parameters and without the need for dives. The profilometer, is placed on the rubber band and equipped with various frequencies, penetrates under the river bottom, showing in plan the geological structure of the ground under water up to a maximum level of approximately -15 m depth.

The data obtained was treated by the specialized company with its own system for its processing, mostly by using the following commercial software:

Results Obtained by the different systems.

Aerial Infrared Thermography (TIR) and Aerial Infrared Photography (IR): The exploration area was carried out using a microdrone between 6 and 90 meters high, from various points located in front of the surveyed area (red dots), using a FLIR VUE PRO 336 camera, DJI 20 Mpx, and Canon IXUS 180 IR. On the ground, using a FLIR T-400 thermal imager and a FUJI IS-1 infrared camera. The entire structure of the bridges and even attached areas have been completely covered (Figure 5).

TIR analysis: The result showed a heterogeneous thermal level, once shadows and vegetation thermobars were filtered, inhomogeneities were detected in the central left area of the bridge deck “O”, as well as a change in longitudinal density, and a large thermal difference between areas N and S Saladillo stream margin marked area structurally altered by geomorphological change.

IR analysis: The infrared processed by stratigraphic surface changes, positioning the sub and surface structure of the area, detecting exogenous radiometric changes, contributed from inside bridge decks (dark blue color) of different entities. However, they were primary anomalies, which required verification with higher resolution geophysical means, as the angle and distance were insufficient.

GPR: 2D and 3D radagrams were made. In the set of tests, the internal structure was clearly detected, more importantly the negative repositioning and deficient internal density in the connection areas with isostatic section, due to poor construction and water entry and subsequent internal corrosion and decay. It includes heavily damaged areas in the lower part of the bridge, of an irregular shape and very poor internal density, with radar “shadow” points caufsed by a high degree of residual humidity.

In one of the supports of the isostatic central slabs, a serious accumulation of residual moisture was detected that caused advanced internal corrosion and delamination, recommending the adoption of immediate safety measures, as well as the realization of intrusive analysis, since it could not be ruled out the sudden collapse of that point, given the degree of its structural affection (Figure 6).

Conclusions

As a brief summary of the study’s conclusions, we can point out that the analyzes carried out are conclusive in that both slabs are clearly differentiated in their degree of internal density, and even in the internal composition of concretes and rebar. Significant electromagnetic anomalies were detected in both bridges, very marked in the decks, and less important in the piers and abutments, associated with internal corrosion, delamination and fractures, due to the erosive action of residual moisture.

In relation to the main points with anomalies and / or pathologies, the main problem detected was internal corrosion, a decrease in the density of the concretes due to the massive entry of residual moisture, particularly serious in three of the four connection areas between the concreting and the two isostatic zones. These isostatic areas are in good condition except for the almost superficial layer, where corrosion is beginning to act.