Vega Pérez-Gracia

Assessment of Complex Masonry Structures with GPR Compared to Other Non-Destructive Testing Studies

Columns are one of the most usual supporting structures in a large number of cultural heritage buildings. However, it is difficult to obtain accurate information about their inner structure. Non-destructive testing (NDT) methodologies are usually applied, but results depend on the complexity of the column. Non-flat external surfaces and unknown and irregular internal materials complicate the interpretation of data. This work presents the study of one column by using ground-penetrating radar (GPR) combined with seismic tomography, under laboratory conditions, in order to obtain the maximum information about the structure. This column belongs to a “Modernista” building from Barcelona (Spain). These columns are built with irregular and fragmented clay bricks and mortar. The internal irregular and complex structure causes complicated 2D images, evidencing the existence of many different targets. However, 3D images provide valuable information about the presence and the state of an internal tube and show, in addition, that the column is made of uneven and broken bricks. GPR images present high correlation with seismic data and endoscopy observation carried out in situ. In conclusion, the final result of the study provides information and 3D images of damaged areas and inner structures. Comparing the different methods to the real structure of the column, the potential and limitations of GPR were evaluated.

Characterization of a Romanesque Bridge in Galicia (Spain)

This article presents the characterization of a mediaeval bridge located in Fillaboa, Galicia (northern Spain). The study of this bridge involves data acquisition about the structure (geometry, visual inspection of damages, and nondestructive testing), the evaluation of the possible damage mechanisms compatible with the observed cracks and fissures, and the dynamic evaluation of the structure. This bridge is a masonry four-lancet arches bridge, with damage on the piers and abutments. Two non-invasive methodologies are applied to obtain information about the bridge: ground-penetrating radar (GPR) survey and ambient vibration noise measurements. The drawings of the structure were created using close-range photogrammetry (CRP). A finite elements model of the structure was obtained prior to the vibration field measurements, as a preliminary evaluation. Data obtained from GPR and the geometry determined with CRP were the information used in this preliminary model of the bridge. This model was improved using the dynamic field test to compare model behavior and to validate the numerical results. A second and more accurate model was then obtained by using finite elements according to the experimentally measured modal frequencies (the possible first three transversal vibration modes of the bridge).

Nanozonation in Dense Cities: Testing a Combined Methodology in Barcelona City (Spain)

Microzonation is widely used in seismic risk evaluations to define the predominant period values, which are usually associated with extended areas of a few hundred meters. However, the representative values corresponding to these areas are obtained from few measurements in each area. Thereby, results are accurate only in the case of depth-dependent soils. However, not detected narrow and sharp lateral changes in soil are potentially the cause of imprecision and could be a source of specific errors. This article aims to present several tests conducted in order to emphasise the importance of accurate selection of points, to underscore the necessity of more precise and detailed evaluations, and to suggest a possible methodology to select the most appropriate data acquisition points. Results highlight the need to divide microzonation areas into smaller zones for a precise evaluation in locations where sudden changes in soil characteristics exist. Therefore, in such sites the requirement of nanozonation appears; defining zones with the same soil response. Distance between vibration measurements could be the main problem for nanozonation; data acquisition in areas with irregular geology can be time consuming when a precise analysis is required. In the most complicated environments or in dense cities, it could even be unfeasible. Consequently, it is necessary to establish a functional methodology to adequately distribute the measurement points throughout the area. On this occasion, three sites in Barcelona city were studied. This city is surrounded by mountains at NW, W, and S, and by the Mediterranean Sea at N and E. As a consequence, the shallow geology is characterized by many paleochannels and streams that are currently buried. These geological structures most likely affect the soil response. Several tests were carried out to determine this dependence. The tests were based on Ground Penetrating Radar (GPR) surveys to define the paleochannels position and on vibration measurements in order to define properly the soil response. The results from both methods were compared to the known geology to accurately define the effect of the shallow geological structures in the predominant period and in the GPR images. Areas with the same geological unit but different materials were identified in the GPR images, allowing the selection of the most appropriate distance between vibration measurements in each place. As a final result, predominant periods that were measured over the same geological unit but over different material showed changes higher than the 40% in short distances. This procedure could improve the soil response maps, including nanozonation.

GPR applications in dense cities: Detection of paleochannels and infilled torrents in Barcelona GPR applications in dense cities

Barcelona is placed in a basin delimited by the Mediterranean Sea (E), the Collcerola Mountains (W) and the rivers Besos (N) and Llobregat (S). The city was built on Quaternary alluvial deposits and on the Tertiary and Palaeozoic materials of the surrounding mountains. The Quaternary materials are preponderant in the plane of the city, presenting a high lateral variability due to the paleochannels and infilled torrents existing between the Sea and the Mountains. The city was built on these heterogeneous materials. In order to prepare a detailed vulnerability map of the Barcelona city, a GPR survey provides information about the position of these geological structures. Results were compared to H/V spectral ratio measurements and soundings.

Geophysical exploration of columns in historical heritage buildings

Columns are one of the most usual supporting structures in cultural heritage buildings. This work presents the study of different columns using ground-penetrating radar (GPR) combined with seismic tomography. These columns belong to a Gothic Cathedral and to a Modernista Building. In the first case, columns are constructed with regular ashlars. GPR provide images that could define the internal ashlar distribution and shape, and the possible existence of inner cracks. However, the images must be supported with additional information from a seismic survey that could provide evidences about the stone quality. In the second case, columns are built with irregular and fragmented clay bricks, and mortar. The internal irregular and complex structure causes complicate 2D images. However, 3D images provide valuable information about the condition of the internal tube and indicate the existence of irregular elements. Results present high correlation with seismic data. In conclusion, results provide information and 3D images defining damaged areas and inner structures. Also, comparing the different methods with the structural knowledge, it is possible to define potential and limits of non-destructive testing applied to these elements.

Inspection Procedures for Effective GPR Surveying of Buildings

A considerable number of studies about GPR applications in building inspection can be found in the literature. New advances in software development, laboratory tests under controlled conditions and numerous cases studies are representative works in this field of knowledge. Some applications are focused on rebar detection, on concrete building assessment, and in modern masonry structures. However, the majority of the works are focused on cultural heritage buildings evaluations, presenting interesting and diverse cases studies. Remarkable results can be found about cracks detection and inspection of masonry walls and columns. Software development has been focused, in many cases, to the enhancement of radar images to facilitate data interpretation. In other cases, synthetic models have been developed to compare results with GPR images from complex scenarios. Evaluations of quantitative properties of constructive materials have been developed based on laboratory tests. Other special works have been also based on laboratory tests: damp measures, concrete degradation due to corrosion, and damages due to tree roots are tested in laboratory specimens under controlled conditions. Although it is a promising subject, few studies have been applied in buildings, revealing the difficult inherent to these complex scenarios. Open issues have been defined as a final conclusion based on the revision of different works. Developments of radar imaging, models and new applications seem to be the most relevant future lines in the GPR building inspection, probably based in a proper and complete definition of casuistic and requirements in structures evaluations.

Study of wood beams in buildings with ground penetrating radar

A large number of buildings in Barcelona city were built during the XIX Century, using wooden beams and masonry. Nowadays, these structures, some of them declared architectural heritage, suffer damages and important deterioration. Geophysical surveys by means of ground-penetrating radar could provide valuable information on the most damaged areas of the beams, and could supply useful data to improve restoration policy and heritage protection. The knowledge of the structure is needed in many cases, but often there is not information about the internal structure and the constructive techniques. However, a large number of supporting structures use to be embedded in the floor and covered by mortar, obstructing the direct visual inspection. Therefore, the access to the structure can be one of the highest difficulties in those inspections because the direct access is not possible. Hence, GPR was applied to determine internal constructive wooden elements and its state of conservation. In this work, several buildings were studied to locate damaged beams and to identify internal structures. Differences in reflections due to damaged and healthy beams appear are clearly highlighted in GPR images in several cases, while in other, differences are not so apparent.

Geological structures evaluated by means of scattering noise in ground penetrating radar images

Coastal geology in Barcelona City is formed by Quaternary sediments overlying Tertiary strata. The Quaternary plane is crossed by paleochannels and streams, all of them under the city structures and constructions. Boreholes demonstrate differences in the grain size distribution of the materials from the different geological structures. In this work, Ground-penetrating radar was proved as a tool to obtain quick information about these changes. The results from more than 20 km of GPR profiles in Barcelona city, seems to indicate that, depending on the relationship between the wavelength and grain size, GPR signal scattering increases significantly. Therefore, the analysis of the noise in GPR signals supplies information to determine the possible existence of geological changes in the quaternary deposits. Several tests and measures in well-known emplacements, as well as models and simulations, were used to determine the ability of this method in the study of the Barcelona plain shallow geology. Correlation between paleochannels and infilled streams and higher scattering noise is observed in this study. The analysis of the scattering effect on the amplitude of the radar data could help to locate paleochannels and subterranean infilled streams, being a valuable tool to improve the knowledge of the city subsoil.

GPR resolution in cultural heritage applications

The non-destructive study of historical buildings, archaeological sites and other Cultural Heritage structures requires high resolution methodologies and a good knowledge of the potential of the different methods. Laboratory measurements provide valuable information about the ability to detect different targets and to determine structural problems, but these data must be compared to the results obtained in real and complex structures. In this work, we present experimental GPR measurements made in order to determine the spatial resolution under laboratory conditions. These results were compared to the data obtained in different GPR surveys applied to Cultural Heritage. The information obtained in drillings, in visual inspections and in old documentation about the historical buildings and archaeological sites is used to determine the resolution in each case.

Analysis of the attenuation in soils and water content in remote sensing surveying

Ground-penetrating radar (GPR) is a high resolution surveying method applied to civil engineering, surface geology, archaeology and other disciplines. Mainly it is used solving the direct problem and obtaining a model of the studied medium. Otherwise, the study of the inverse problem could provide other valuable information: the electromagnetic properties of the medium. These parameters are obtained from the changes of the velocity, attenuation and frequency of the recorded wave. The physical properties of the medium related to those wave parameters are, mainly, the water content and the porosity. Several lab experiences are performed in order to obtain these parameters from different soil samples. Porosity and water content are measured and controlled. Velocity is obtained by measuring the two-way travel time of the reflected wave and comparing wave reflected amplitudes on the surface of the samples. Attenuation coefficients are determined from the analysis of the amplitude of the wave traveling in different thickness samples. Frequencies velocities and wave attenuation are analyzed in the different cases in order to characterize those different media and to relate its water content and its porosity with these measured parameters. The experimental results were also compared with the complex refraction index model (CRIM).