Keyword: Laser Scanner, Non destructive testing, UAV, remote sensing, LARS

ABSTRACT The paper describes a multidisciplinary and integrated methodology related to material and structural elements tests, Laser Scanner surveys and Remote Sensing technology, applied to Historical Heritage. The knowledge process was aimed at defining the rehabilitation project of an historic building in Caporciano, a small old town centre damaged by the Abruzzo (Italy) earthquake which occurred on April 6th, 2009. The building analyzed is characterized by an extremely complex morphology, developed on different levels and making surveys using traditional methods particularly difficult. Therefore the laser scanning technique, which provides a complete 3D model of the entire structure, has been used. Due to the effects of the earthquake, the testing of the building’s structure had to take several aspects into consideration. An extensive non-destructive and semi-destructive tests survey has been performed, in order to obtain historical-architectural knowledge and in situ materials characterization. The main techniques used were thermograph, ground penetrating radar, endoscopy, masonry analysis and mortar penetration test. In this case study, UAV remote sensing was employed, to collect data thanks to sensors mounted on carriers operating at low altitude. I was able to survey the roofs of the buildings: captured images were processed to identify any damage even on inaccessible areas. 1 INTRODUCTION

The occurrence of a catastrophic earthquake in Abruzzo (April 2009), brought the problem of preservation and protection of historical buildings to the attention of the scientific community.

Italian old town centres are inexorably exposed to seismic risk and always lacking in methodical maintenance activity, necessary to ward off deterioration and destruction of buildings, and the construction of Abruzzo’s small villages is no exception.

In the post-seismic intervention phase, the local institutions drew up guidelines and city plans for each village’s old town centre, in order to lead the reconstruction towards the ultimate aim of preserving their architectonical and urban values, and, at the same time, of improving the buildings’ safety standards, in conformity with

national Building Regulations (Norme Tecniche per le Costruzioni 2008).

The arduous rehabilitation of the damaged buildings starts from the understanding of their features, deficiencies and potentiality.

In Abruzzo’s reconstruction process, particular importance was given to collecting structural data through on site diagnostic methods, providing a reserved item specifications for it. It brought the importance of this delicate phase, preparatory and necessary to develop any further structural analysis, to the attention of each technician (engineer, architect, etc.).

The case-study presented explains a multidisciplinary and integrated methodology related to material and structural elements tests, supported by a thorough survey operation. The collected data synergic analysis provided a

Fabio Armillotta, CASa Associati, Viale Bovio 64, 65123 Pescara.

Dante Abate, Elena Candigliota, Francesco Immordino, Samuele Pierattini ENEA, Via Martiri di Monte Sole 4, 40129 Bologna.

Cristina Santacroce, Structural tests consultant, Via C. Ciglia 56, 65128 Pescara.

Integrated technologies for structural assessment of historic buildings: case-study in Caporciano (AQ)

complete view of the buildings’ situation, essential to start the rehabilitation project.

2 TERRESTRIAL LASER SCANNING (TLS) SURVEY

Due to the complexity of the building, which is composed of a dense labyrinth of different rooms and passages, and thus does not allow an accurate survey using traditional methods, the metrical and morphological description of the structure was realized exploiting the terrestrial laser scanner technique (TLS).

This well-known technology is able to acquire a large amount of three dimensional data and therefore able to describe the object and its main metrical features.

A high definition Laser Scanner survey creates a 3D model (point cloud) where the object’s shapes and dimensions are described by millions of points.

Due to its multi-resolution, this model allows a multi-scale approach, linking the whole complex (main spaces) to its detailed parts (valuable features).

2.1 Terrestrial Laser Scanning (TLS) technique The laser scanning technique exploited in this

case study is based on the Time of Flight law. TLS systems can record the coordinates of

millions of points measured on the surface of an object without any direct contact.

Basically the instrument emits a laser pulse which travels through the air, hits the object and comes back to the receiver which calculates the time elapsed.

Since the time of travel through the air of the laser pulse is known, and the speed of light is known as well, the software is able to compute the distance between the scanner and the object, and then the z coordinate:

V= d \ T à d=V*(2)T where V is the speed of light, d is the distance

and T is the time. The instrument accuracy certified by the

manufacturer is about 6 mm @ 50 m. However sources of errors in measuring coordinates are diverse and can be grouped in four classes: instrumental errors (in the laser rangefinder, beam deflection, unit and axes errors); object errors (the offset deriving from the surface reflectivity); environmental errors (propagation, atmospheric conditions, interfering radiation, etc.); methodological errors.

The output data are represented by a point cloud. Each point has the x, y and z coordinates, the reflectance value and the RGB value.

The reflectance value indicates for each point of a surface hit by the laser beam, the response factor, i.e. the ratio between the irradiance, or incident radiant flux density, and the emission, or outgoing radiant flux density.

However, some parameters like angle of incidence, sun radiation and distance between the object and the instrument, must be evaluated to consider the acceptable and correct value.

The RGB value is acquired by an internal digital camera and mapped directly onto the point cloud. Such a scanning system is provided with a rotating head and one high speed rotating mirror, which allow the acquisition of a scene with a large field of view (FOV= 360° H x 270° V)

2.2 The Survey The survey took place over two weeks of

intense work involving two operators. The equipment was composed of:

− Terrestrial Laser Scanner. − HDS Targets. − Tripod. − UPS. − Laptop.

The Laser Scanner was mounted over a tripod and powered by the UPS since the electricity was shut down right after the earthquake.

To digitize the entire buildings (inner and outer surfaces) 91 point clouds were acquired (Figures 1 & 2).

The final unified database consists of ca. 28 million points and almost 190 targets acquired for the registration of all the clouds in a unique reference system

A resolution of 2cm x 2cm was set for each scan station, which allows representation of the main features of the complex in CAD drawings (Scale 1:100) and the performance of accurate deformation analysis.

Due to the peculiar case study (a building abandoned in a hurry during a seismic event) the final point cloud also shows all the furniture which is still inside each room, causing occasional occlusions.

The complete coverage of spatially complex objects has been guaranteed by collecting data from different viewpoints.

One room was not digitized due to the collapse of the ceiling which made the room unsafe for both the operators and the equipment.

As known, the building object of this study is situated in a small historic centre. The streets

surrounding the structure are narrow and do not allow appropriate angles, from the bottom of the building, in order to also acquire its roofs which were not in sight from the instruments’ positions. Few stations took place on the hill facing the complex. However due to the distance between the laser scanner and the building the acquisition resulted in a partial lack of data over the roofing which weren’t totally digitized.

Figure 1. Scan position inside the building.

Figure 2. Scan positions outside the building.

Figure 3. Total Point Cloud ca. 28 million points (front view).

2.3 Post Processing Commercial packages for processing point

clouds obtained by TLS perform pre- and post - elaboration to obtain 2D CAD drawings and 3D models of high metric quality and detail.

For this project the point cloud data were processed with the software Cyclone.

The first task was the registration of all the single point clouds into the same reference system. HDS targets and ICP cloud constraints were used to align all the point clouds. The final RMS error was minimized to ca.0.5 mm.

After the registration the point cloud was filtered and optimized.

According to the team responsible for the restoration projects, different plans and sections were extracted in order to describe properly each part of the damaged building.

For each element needed, a User Coordinate System (UCS) was created and a reference plane.

The final 3D model shows the complexity of the building with rooms embedded one among the other (Figures 3 & 4).

By analysing the point cloud, it was possible to visualize the connection between neighbours’ rooms whose comprehension was hampered by the stratified layers.

Six different layers, according to the “up” direction, were identified, starting from the cave excavated from the solid rock at the bottom, to the last floor of the building.

Figure 4. Total Point Cloud ca. 28 million points (side view)

The point cloud sections were exported into dxf file and processed manually into Autocad (Figure 5).

The CAD drawings were made by including details and components outside the section plane and possibly occluded by other elements. This means that all openings or recesses inserted in the walls, not noticeable directly from the cutting plane, are also represented. They are visible and measurable whether they belong to the domain below or above the projection plane.

This has been possible thanks to the ability to query the 3D database and to display those parts useful for each representation.

Together with CAD drawings it is possible to obtain ortho-photo of the structures and high resolution images (Figure 6).

Figure 5. CAD drawing (Plan level 0)

Figure 6. Ortho Photo.

2.4 Future works The morphological database can be queried for

further data analysis since it represents the “historical memory” of the building at the time of the survey.

Besides CAD drawings, the point cloud data will be used for FEM analysis of the masonry vaults which could have been seriously affected by the seismic waves. Indeed one of the vaults of the upper floor collapsed during the earthquake. For this task the vaults point cloud will be triangulated in order to obtain mesh models (Figure 7).

Figure 7. Vault Mesh model and its photo.

These files will then be analysed to check their static features, geometrical and morphological

deformation and finally plan any potential conservation action.

2.5 Low altidute remote sensing LARS (Low Altitude Remote Sensing) or

UAV (Unmanned Aerial Vehicles) remote sensing is a set of techniques for collecting data through the use of various kinds of sensors mounted on carriers operating at low altitude. This technology has been used to survey and check the state of conservation of the building’s roofs. 2.5.1 Acquisition tools

Images of the building were acquired through a high definition camera GoPro mounted on a quadricopter using a remotely controlled system with the possibility of movements along three axes X, Y and Z.

The quadricopter used, was equipped with a tool for automatic control of positioning and stabilization, both of the aircraft as well as that of the camera, so as to have the entire system perfectly orthogonal to the scene to be captured.

Figure 8. Quadricopter with high resolution digital camera.

2.5.2 Acquired digital images Images at different angles of the building

(Figures 9 & 10) were acquired in order to get different views of the roofs and check their condition.

Figure 9. Quadricopter during the flight acquisition.

Figure 10. Quadricopter during the flight acquisition.

Figure 11. Detail view by quadricopter camera.

Figure 12. Panoramic view by quadricopter camera.

Figure 13. View of roofs by quadricopter camera

The acquired images with resolution 1920x1080 allowed views with high detail of the roofs’ features (Figures 11, 12 & 13).

The UAV remote sensing is a low-cost technology regarding equipment and maintenance. It also allows acquisitions even in situations where space is limited, inaccessible or in dangerous areas.

3 STRUCTURAL DIAGNOSTIC SURVEY A structural diagnostic survey was planned according to the requirements of the team of technicians. The aims of the investigation were: to recognize and characterize the masonry and its component, in order to define characteristic parameters (strength, deformability, etc.); to identify materials used in the vaults and construction techniques; to detect metallic tie rods hidden beneath the floor surface. Due to the complexity of the building and in order to obtain a greater amount of information, preserving as many architectural elements as possible, it was preferable to apply extensive non-destructive testing methods. Localized semi-destructive tests (double flat-jack test) were nevertheless carried out.

3.1 Thermography general check Thanks to continous innovation and

experimentation, the application of infrared thermal analysis to Architectural Heritage field is, nowadays, has assisted in solving several problems concerning the detection of critical situations in buildings’ structures and surfaces.

This non destructive investigation method has the merit of supplying information on the building studied, covering vast areas of its surfaces in spite of needing any contact with it. It has to be considered as a qualitative analysis but necessary to attain a complete knowledge of the building, especially in the case of historic buildings, characterized by a complex morphology and structural layout due to ‘stratifications’ and modifications.

Infrared thermographical techniques were applied to investigate the surface area of all of the building’s façades, with the aim of identifying possible non-homogeneous portions of the structure, using a Flir B620 camera.

Regarding the building investigated, it was possible to detect two different masonry texture types and to have a clear picture of their distribution. It was also possible to exclude the presence of stopped up windows or particular pre-existing elements hidden under the plastered

surface. Furthermore, it was possible to exclude the existence of any reinforced concrete ring beam.

This first diagnostic recognition guided all the further decisions about structural investigations.

Figure 14. IR image of SW façade, the presence of relieving arches above the windows is clear.

3.2 Masonry characterization techniques In order to supply a wide range of information,

about morphological and mechanical characteristics of masonries, different kinds of investigative methods were carried out.

To analyse the masonry texture, mechanical plaster removal was operated in previously planned specific building points. In some cases free-plaster surfaces were preferred, in order to preserve the integrity of the building’s surface.

The main critical aspect of masonry structures is the bonding condition between their elements (stone, mortar) and the presence of voids and cavities across the section. Several focused endoscopic inspections have been carried out to investigate masonry cross-sections.

Two main masonry types were recognized. The first one referred to the lower building levels and the second covered the rest of the building above ground.

The first masonry type texture (Figure 15) is characterized by the presence of medium and large size limestone elements, mainly rough-hewn and some roughly squared in the lower wall portion. The mortar joints are irregular or quasi-regular (sub-horizontal). The cross-section presents few cavities and fairly good connection between the two external leaves.

The features of this masonry type correspond to those of the masonry type classified as “Muratura a conci sbozzati, con paramento di limitato spessore e nucleo interno” by the NTC 2008, with better connection condition (Table 1).

The second masonry type texture (Figure 16) is composed of small and medium sized

limestone elements, simply rough-hewn or boasted. Clay elements, such as bricks and roof tiles are used as wedges in the external masonry. The mortar joints are irregular and thick. The cross-section is characterized by the presence of cavities and voids with a fairly good connection between the two external leaves.

The features of the second masonry type correspond to those of the masonry type classified as “Muratura in pietrame disordinata (ciottoli, pietre erratiche e irregolari)” by the NTC 2008 (Table 2).

Figure 15. First masonry texture type.

Figure 16. Second masonry texture type.

Table 1. First masonry type mechanical parameters.

fm τ0 E G w [N/cm2] [N/cm2] [N/mm2] [N/mm2] [kN/m3] 198 3.47 918 306 20 297 5.05 1296 432

Table 2. Second masonry type mechanical parameters.

fm τ0 E G w [N/cm2] [N/cm2] [N/mm2] [N/mm2] [kN/m3] 99 1.98 621 207 19 178 3.17 945 315

A significant deficiency noted in the investigated building’s masonry structures, is the absence of efficient connections between orthogonal walls. This widespread problem was noticed, thanks to plaster removal and does not depend only on the realization of different construction phases but mainly on an erroneously adopted construction method.

Both of the masonry types present crumbly or powdery mortars, with calcareous-based binder and characterized by a non-homogeneous granulometry and advanced deterioration conditions. Evaluation of the mortar mechanical response was carried out employing the Pin System Penetrometer test, collecting the result data for each measurement point, and the Felicetti Penetrometer test on each previous penetration imprint. The resulting mortar resistance Class, according to NTC 2008 classification, is M2.5.

The result of the semi-destructive localized test, the double flat-jack test, operated in two different points of the building, related to the recognition of the two main masonry types, confirmed the correspondence to the respective masonry NTC 2008 classification (Figure 17 - 18).

The numerous data, collected and obtained by the non-destructive tests, permitted to extend the results to the whole building masonry structure.

Figure 17. Result of double flat-jack test for the first masonry type .

Figure 18. Result of double flat-jack test for the second masonry type .

3.3 Ground penetrating radar techniques for foundation and floor inspection

GPR investigation technique was employed to investigate the consistency of building foundations, to define floor and vault features and to verify the possible presence of underground cavities or voids next to the building structure.

GPR investigations in Civil engineering contests are normally carried out using single fold technique and using a monostatic radar antenna consisting of modules, with functions of transmission and reception, at nominal frequencies equal to:

− 600 MHz, sampled on 1024 samples/scan and full scale set to 128 ns, in a favourable situation, it is able to explore to a depth of 4-6 m, with a level of detail up to 5-10 cm. This antenna is ideal for investigations of floor and masonry structures.

− 1600 MHz, sampled on 1024 samples/scan and full scale set to 40 ns, achieved a high level of detail (about 1 cm), with investigation depth up to 1.5-2 m. This antenna is ideal for both structural investigations on reinforced concrete structures and on masonry structures.

Thanks to the employment of GPR, it was possible to characterize the vaults’ structures,

recognizing the constituent materials (stone or bricks) and the thickness and the consistence of upper vault surface filling materials (if present). There was no damage on the lower surfaces of painted and decorated vaults.

The presence of hidden metallic tie rods was detected, in spite of non-visible end elements on external façades (Figure 19).

The exact extension of an old cistern was also detected at the lower building level (Figure 20).

Figure 19. GPR section across the vault between 1st and 2nd level. The brick “in folio” vault section and the presence of two tie rods, are clear.

Figure 200. GPR section across the floor of the -3rd level, the arch below describes the vault section of the underground cistern structure.

4 CONCLUSIONS The use of integrated methodology with

innovative technology represents a valid method of providing information for the assessment of the vulnerability of architectonical heritage, when considering the restoration and the consolidation of buildings, especially in complex situations within old town centres.

Each technique and method contributes to the understanding of building, taking different and complementary aspects into consideration.

The data extracted, using the methodologies applied in this analysis, therefore represent a safeguarding tool for monitoring the state of conservation of historical heritage.

ACKNOWLEDGEMENTS The authors would like to thank the structural diagnostic survey team members: Engineers N. Aretusi, M. Pascale, M. Pesaresi, D. Ranalli and M. Scozzafava.

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