Application of NDTs to the diagnosis of Historic Structures · 2009-04-23 · NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

Application of NDTs to the diagnosis of Historic Structures

Luigia BINDA 1 & Antonella SAISI 2

1 DIS, Politecnico di Milano, Milan, Italy, [email protected] 2 DIS, Politecnico di Milano, Milan, Italy, [email protected]

Abstract Some typical problems solved by the combination and the complementary use of different

investigation techniques such as boroscopy, radar and sonic tests, flat-jack and sonic tests, sonic and radar tomography and others will be presented.

Résumé On présente quelques problèmes typiques concernant bâtiments historiques, résolus par

l’usage de techniques d'enquête complémentaires, telles que la boroscopie, les essais radar couplés aux essais soniques, essais aux vérins plats couplés aux essais soniques, le couplage des tomographies sonique et radar, etc.

Keywords Masonry, Pulse Sonic Test, GPR, thermovision, NDT

1 Introduction In the past times “restoration” was reserved to important monuments as churches and

palaces but in the last decades more and more restoration or better “conservation” is reserved to wider entities as historic centres or groups of buildings representing an important part of the history of those centres. It is now clear that if the ancient tissue of our historic centres is taken into account it is possible to understand how much it is important also compared to the most important monuments. Without that tissue our history would be lost (Fig. 1).

Figure 1. View of Roccanolfi

The Basilica of St. Francis and the Church of St. Chiara in Assisi are two very valuable pieces of art and architecture, but they are made more precious and valuable by the preservation of the historic centre of Assisi, with the old buildings, the narrow streets, the little chapels, the dwellings and the palaces. So we have to preserve the whole historic centres and their environment as important documents of our history. Taking into account that in the Mediterranean cities these centres are usually situated in seismic areas with frequent earthquakes and that many old buildings in villages have been abandoned by their owner emigrated toward large industrial cities, the enemy for the conservation of theses structures is not only the earthquake, but mainly the lack of maintenance. Abandoned buildings become very quickly obsolete and their structures easily decay; the partial or total collapse under


earthquakes is quickly assured. Therefore if the historic centre has to be preserved preventive repair and maintenance should be assured.

Another problem is affecting the community of professionals taking care of restoration: the lack of knowledge on the materials and techniques of construction by architects and engineers who are prepared from the specific University courses on the behaviour of modern steel and concrete structures but not any more prepared on the behaviour of old masonry and timber structures.

Masonry is a composite material made with bricks and mortar, stones and mortar adjusted in many different bonding. The wall section frequently hides a complicated technique of construction in two or three leaves of different thickness; these leaves are connected in different ways or sometime are not connected at all (Fig. 2). The characteristics of masonry as a composite are frequently unknown nowadays, since the knowledge on the techniques of construction was lost decades ago, when the new construction materials as steel and concrete became in use [1].

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A correct intervention on a historic structure should start from an accurate diagnosis of the building in order to minimise the interferences of the intervention with the historicity of the architectural document, but also incompatibility between the existing structure and materials and the materials used for repair.

Furthermore, the study of the building rules and structural details, such as the wall sections, has a great importance in the mechanical behaviour of the structure [1].

The structural performance of a masonry can be understood provided the following factors are known [2]: (i) its geometry; (ii) the characteristics of its texture, if made by single or multiple leaves, (iii) the type of connection between the leaves, (iv) if the joints are empty or


filled with mortar, (v) the physical, chemical and mechanical characteristics of the components (stones, mortar); (vi) the characteristics of the masonry as a composite material.

Furthermore in many cases the structures were subjected to several earthquakes and/or lack of maintenance, so that they are damaged in various ways. In short terms we need to end up with a diagnosis on the state of damage of the structure and on its vulnerability to future damages, which should be based on the detection of damages and their causes and on a structural calculation. Then we need first of all a diagnostic investigation.

In order to fulfil these needs by overcoming the gap between our poor knowledge today and the complexity of these structures an experimental on site investigation is required and recommended also by Codes of Standards in several countries.

Non destructive techniques (NDTs) and/or Minor destructive Techniques (MDTs) can be helpful in finding hidden features (internal voids and flaws and characteristics of the wall section) which cannot be known otherwise than through destructive tests and are so much important to define the structural behaviour under the dead loads and the environmental actions [3,4]. Up to now most of the ND procedure can give only qualitative results [2]. The application to masonry of NDTs [3,5], although advanced, can be frustrating due to several factors, like the differences in masonry typologies and materials, the high non homogeneity of the materials, the interpretation of the results of each single technique but also the harmonisation of the results [6,7,8]. Furthermore, most of the NDTs come from other research fields and applications to more homogeneous materials (steel and concrete) and need a specific calibration.

A rather long experience of the authors after the application of the most appropriate NDTs to the research of hidden features and geometries in masonry, has shown that several techniques have to be applied according to the needs of the building. It is then important before choosing the appropriate NDT technique to know which type of problem has to be solved i.e. what it is necessary to find by NDT. The solution of some difficult problems cannot be reached with a single investigation technique, but with the complementary use of different techniques [3,6,7,8]. This becomes a difficult task for the designer of the conservation project, who is asked to interpret the results and use them at least as comparative values between different parts of the same masonry structure or by using different ND techniques.

To this purpose it is important the production of guidelines for the correct application of investigation techniques to diagnosis problems of different classes of masonries [1,2].

In the following some applications of NDTs to different cases will be reported.

2 Typical problems requiring investigation by NDTs The application of a NDT to the detection of hidden features or of the presence of

structural damages or other can be unsuccessful if from one side the problem to be solved is not clearly defined and from the other side the limits and advantages of the NDT are not known. From the knowledge accumulated by the authors in these last decades during the application of various investigation techniques to the study of masonry structures [2,8,9], it is possible to give a list of cases emerging from the past application, where only a complementary use of different techniques can help in solving the problem of the knowledge of hidden features.

2.1 Survey of structural damages connected to typical crack patterns. This can be a typical situation: (i) in seismic areas: in those cases a systematic survey of

the damages on load-bearing walls can be enough to recognise similar crack patterns and interpret them in terms of collapse mechanisms (Fig. 3) and even apply appropriate mathematical models based on macro-element limit analysis [4,10,11], (ii) in large areas


where soil settlement or subsidence phenomena are taking place (iii) in various cases were soil settlement under a building can occur due to local or global movements, (iv) in cases of long term damages in massive structures due to their dead load (as towers or pillars) (Fig. 4) [6,8,9].

Figure 3. Figure 4. Some examples from the collapse mechanisms abacus [10]

Crack Pattern survey of the Monza Cathedral Bell-

tower [9]

In all the mentioned cases the damage can be understood and interpreted only after the geometrical survey of the building has been carried out and the crack pattern survey has been reported on it in details. The observation of the crack pattern does in general suggest a first interpretation of the causes of the damage. So it is possible to say that the first NDT applied are the geometrical and the crack pattern survey. Only after this first phase some positions can be defined where to carry out MDTs (flat jack, etc.) and NDTs (sonic, radar, etc.) [2,8,9,12].

1.1 Presence of multiple leaf walls or detachment of layers. Most of the stone masonry walls in the historic centres or in monuments were built with

multiple leaf sections. They could be two or three leaf sections with regular stones outside and rubble inside. It is difficult to recognise these features from the prospect of the wall due either to the presence of rendering or to the multiple combination of the leaves (Fig. 2) [1]. In the cases when no destructive investigation can be applied, a combination of coring, sonic or radar tests can be used.

Layer detachment it is frequent in case of cladding or regular external leaf not collaborating with the internal ones and also in these cases alternative or combination of NDTs can be used as it will be described in the following, reporting the case of Medieval towers [7,13,14].

1.2 Presence of inclusions and voids. Inclusions and voids have to be detected in order to survey disconnection or lack of

continuity in load-bearing walls. Large voids and inclusions of different materials can be seen very easily by thermovision if they are at a small distance from the surface of the wall (Fig. 5) [15]. In the case they are deep into the wall then either sonic or radar tests (Fig. 6) [16] have to be applied which can reach deep areas in the walls. Also inclusions of other construction materials inside the masonry can be detected by NDT. It is very important to find them especially when there is the presence of timber frames or steel parts as details of tie rods are


hidden inside the masonry. Sometimes the inclusions can simply be added parts of masonry or arches closed inside it.

a) b) c) Figure 5. Traces of tie plates (a); (b) thermovision applied to detect a vertical rod

behind the plaster and (c) data after the application of a gradient filter to enhance the target [15].

a) b) c)

Figure 6. Acquisition scheme (a), 3D reconstruction of the ties position (b) and front view projection of the shallower ties (c) [16].

1.3 Determination of the masonry “quality” It is well known that the mechanical characteristics of masonry should be detected in order

to define the constitutive laws of the material for the mathematical models assumptions. Nevertheless all the parameters defining these characteristics cannot be measured on site. The only parameters that can be measured at the moment on site are the modulus of elasticity, the Poisson ratio, the value of the stress at onset of cracking on the external face of a wall. This can be obtained by carrying out the double flat-jack test, which only allows a compression test (Fig. 7) [17,18,19].

Nevertheless, this test together with the sonic pulse velocity test can be useful to define the quality of the masonry.

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Figure 7. Double flat-jack results of the tests at St. Antonio in Morgnaga, rectory (a) and church (b) St. Michele in Sabbio Chiese, church (c) [19].

1.4 Moisture detection.

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 The decay of masonry surfaces is often caused by the presence of the humidity in the wall.

The investigation on this specific problem can be subdivided in (i) detection of the presence of moisture, (ii) evaluation of the moisture content. Some techniques are very sophisticated and expensive but completely non destructive like the radar or thermography tests (Fig. 8) or others are cheaper, like the power drilling method, but not often repeatable in the time, being locally partially destructive [20].

Figure 8. Moisture distribution detected by thermovision (c) [20].

2 The use of NDTs and MDTs in the diagnostic investigation of historic masonry buildings

The diagnosis process should be based on an accurate survey, which should document the current state of the building. A preliminary in-situ survey is useful in order to provide details on the geometry of the structure and in order to identify the points where more accurate observations have to be concentrated. In the meantime the historical evolution of the structure has to be known in order to explain the signs of damage detected on the building.

The geometrical survey of the building is essential and has to be carried out accurately using simple or sophisticated techniques (e.g. photogrammetry) according to the type of geometry or of the available budget. Following this survey a more refined investigation has to be carried out, identifying irregularities (vertical deviations, rotations, etc.). In the meantime the historical evolution of the structure has to be known in order to explain the signs of damage detected on the building.

The crack pattern should be classified and accurately documented by pictures and reported on the geometrical survey (Fig. 4). The crack pattern survey can be helpful even in seismic areas where it is important to predict from known damages the mechanisms of collapse, which can occur during an earthquake (Fig. 3) [2,10,11].

The definition of the structural model can be carried out not only on the basis of the geometrical survey but also of the crack pattern.

Finally NDTs or MDTs can be applied in strategic points of the structure in order to solve the most difficult problems of hidden situations or to detect the mechanical behaviour of the structural elements (timber floors and roofs, iron tie rods, loadbearing walls).

In the case of masonry elements the most frequently utilised NDTs are certainly: thermovision, sonic and radar tests and as MDT the flat jack tests. It should be taken into account that the knowledge of the masonry construction techniques, which are so variable particularly in the Mediterranean countries with their long history of architectural works but also of destruction and re-construction caused by wars or exceptional events (earthquakes, floods, etc.) was lost at the beginning of the 20th cent. Therefore, the features of the internal parts of masonry or hidden details cannot be easily recognised from the results of the ND procedures (e.g. a radargram obtained from radar tests of the internal part of a masonry does


not immediately give the possibility of recognising if the masonry is a multiple leaf one). In those cases a direct inspection made by coring and photographing the borehole with a videocamera is frequently useful in order to check and better interpret the results of radar tests. This is true also for other NDT as sonic tests or geoelectric.

3 Combination of NDTs and MDTs In many cases NDTs have to be used in alternative as some of them are particularly

sensitive to the presence of humidity. E.g. in cases when a strong presence of humidity in a masonry wall due to capillary rise of water causes poor results of radar and geoelectric tests, in alternative thermovision or sonic tests have to be used. In some cases thermovision fails due to its poor penetration inside the tested element, then radar can be a good alternative .

But much more frequently difficult problems can be solved by the complementary use of MDTs and NDTs. Practically all the problems listed in Sec. 2 can be solved only by the complementary use of different NDTs. The first NDT is certainly the preliminary site visual inspection which can also suggest the type of problems to be solved and the type of techniques to be applied. E.g. to have information about the quality of a masonry (see Sec.2.4) a methodology has been proposed which follows several steps: (i) visual inspection allowing to choose the most representative part of the wall where to carry out the appropriate tests, (ii) sonic tests by transparency to map the density of the masonry section, (iii) single and double flat jack tests to measure the value of the compressive stress, the stress-strain behaviour of the masonry and the mechanical parameters E, ν, (iv) dismantling of small parts of the section to find the presence and thickness of multiple layers [18].

In the following section some of the most frequent problems are discussed.

4 APPLICATIONS TO SOME CASES

4.1 Damages detection

4.1.1 Structural damages caused by multiple interventions: the Syracuse Cathedral.

The Cathedral of Syracuse is the result of the transformation of an ancient Greek temple from the 5th cent b.C. with modifications that have also been consequence of the damages caused by earthquakes.

The first step of the investigation carried out [21] was in fact the study of the historic evolution of the structure due, not only to its architectural transformation but also to the earthquakes by which it was hit along the centuries. In 6th century A.D., during the Byzantine period, the temple was transformed into a church with a nave, obtained from the internal cell, and two aisles (Fig. 9). The external space between the columns was filled by stonework masonry. The walls of the temple cell were cut, obtaining arcades and pillars (Fig. 9). The works were carried out with precision taking into account the Greek masonry techniques made by large stone blocks and the necessity of the arch rhythm (Fig. 10).

At the beginning of 20th century (1924-26), several interventions were carried out as the removal of the baroque decoration inside the church. More in detail, the pillars of the central nave, obtained by cutting the walls of the temple cell, show a complex situation of damage and repairs (Figs. 11-12).

The crack patterns, with mainly vertical cracks at the lowest part and on the corners (Fig. 12), could represent a situation of progressive damage and needed a careful investigation in order to understand the causes. Due to the complex situation, the survey of the pillars was carried out with an accurate mapping of the several superficial materials, of the defects, of the cracks and of the morphology (Figs.11 and 12). The cracks have frequently a vertical pattern,


showing the presence of high compression stresses, caused not only by the dead loads but also by the fatigue effects of successive earthquakes.

Figure 9. Plan of the Syracuse Cathedral with the pillars numbering [21].

Figure 10. Scheme of the pillar assemblage: (a) original wall, (b) original shape of the

pillars and (c) shape after 1924-26 intervention [21].

(a (b

Figure 11. Figure 12. Cracks pattern survey of the pillar 26 of the

Syracuse Cathedral [21].

Damages on pillars 26 (a) and 29 (b) of the Syracuse Cathedral

[21].

The archive and on-site surveys allowed to understand a number of problems to be solved: (i) the depth of the cracks visible on the surface, (ii) the soundness of the stones, (iii) the depth of the layers of rendering and their bond to the support, (iv) the presence of inclusions, reinforcement, flaws in the stones. All this information could not be collected by a single NDT [2,3,21]. Therefore, the following techniques were chosen: (i) sonic and ultrasonic tests to detect the inside density of the material and the depth of cracks, (ii) thermovision to evaluate the extent of detachment problems affecting the rendering simulating the stone, (iii) georadar as complementary to ultrasonic test and thermovision, to find deep defects and hidden inclusions.

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 The results demonstrate that georadar was successful to investigate the masonry

morphology beyond the covering and to control the presence of internal defects and cracks of the pillars. 3D reconstructions allowed to detect the inner cracks and to explore their extension within the pillar section (Fig 13).

Figure 13. 3D mapping of the evolution of internal cracks affecting the lower part of some

pillars. The cracks were not observed from an outer inspection of the pillars. Arrows indicate the side of the radar investigations and the orientation of the

profiles [21].

A high frequency antenna (2.0 GHz) was also used to estimate the thickness of the rendering applied to reconstruct the external surface of highly damaged pillars. Finally, it was used to find expected and unexpected steel reinforcements applied during past restoration activities. In the most damaged pillars 18, 19, 29, 30, the high frequency radar investigations detected the presence of regularly spaced diffractions (about every 5-6 cm, Fig. 14a,b) at a depth of about 3-4 cm, i.e., just behind the plaster. In fact the 3D image extracted from the processed data volume at the depth of about 4cm (Fig. 14c) detects an area showing an apparent layered structure. A direct inspection in one of these areas (Fig. 15) proved that unexpected brickwork used to substitute some stones during past repairs, probably in the 1920s, produced the radar result.

Monitoring of the most important cracks was then set up in order to assess possible risks and to understand the mechanical behaviour of the structure, before planning an intervention.

a) b) c)

Figure 14. Figure 15. In pillar 18 of the Syracuse Cathedral, the radargram shows the presence of regularly spaced diffractions (about

every 5-6 cm) at a depth of about 3-4 cm [21].

The inspection proved that these

pillars were sometimes repaired

with bricks [21].

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 The first results of monitoring show a continuous movement in the Chapel walls and in

pillars 19 and 18, not fully explained by the temperature variations, showing a dangerous instability of the cracks. This situation indicates that the damage is probably caused by the insertion of a softer material (brickwork) into a large-block stone masonry. The change of the material probably made in the 20th century was worsening a situation of pre-existing damages causing the stiffer stones to assume higher stresses than the ones passing through the brickwork.

4.1.2 Structural damages caused by long term behaviour of the masonry Investigations on structural safety of ancient masonry towers have received increasing

attention in recent years probably as a consequence of some dramatic events which occurred like the sudden collapse of the Civic Tower in Pavia [22] in 1989. In order to assess the structural condition of the Bell Tower, an extensive investigation was carried out in situ by using non-destructive and slightly destructive techniques together with laboratory tests and analytical calculations. The investigated Tower (Fig. 4) is adjacent to the Cathedral of Monza (a town about 20 km far from Milan, Italy). The Tower, made from solid brick masonry, was built between 1592 and 1605 on a design by Pellegrino Tibaldi. Accurate geometrical survey of the structure was first performed; the survey included the analysis of the crack patterns and their distribution. Cracks were surveyed visually and photographically and reported on plans, prospects and sections [9,12].

Figure 16. Figure 17. Wide crack pattern on the West

prospect [9].

Crack pattern inside the Tower [9].

The detailed geometrical survey identified a complex distribution of cracks. The walls of the Tower, 74 m high and 1.40 m thick, show passing−through, large and potentially dangerous vertical cracks on the West and East sides (Fig. 4, 16-17), which are slowly but continuously opening. These cracks were already present in the 20's and they have been roughly monitored since 1927. From 1978 the cracks have been surveyed with removable extensometers showing an increasing of their opening during this time. From 1988 the rate of opening seems to be also increasing (Fig. 18).

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 Wide cracks are present also on the North and South side in the corners of the Tower up to

30 m (Fig. 16-17). Furthermore, a damaged zone at a height of 11.0 to 23.0 m with a multitude of very thin and diffused vertical cracks is present which demands structural repair. These cracks can be better seen on the internal face of the Tower walls; they are very thin, vertical and diffused along the four sides of the Tower and deeper at the sides of the entrance where the stresses are expected to be more concentrated (Fig. 17). Since the crack pattern has developed slowly along the years, a possible time dependent behaviour of the material can be postulated due to the heavy dead load, coupled to temperature variations and wind actions [23].

Since no destructive tests could be carried out on sampled masonry specimens for safety reasons, flat-jack tests were performed in selected points of the structure in order to directly measure the stresses caused by the dead load and the stress-strain behaviour of the material. Sonic pulse velocity tests were also carried out on some parts of the walls in order to define the damage extent and distribution.

Single flat-jack tests were performed, respectively at 5.4, 5.6, 13.0, 14.0, 31.5 and 38.0 m height of the Tower, to measure the value of the vertical compressive stresses, in some location, close to the safety limit, and the stiffness of the material. In addition, some double flat-jack tests were carried out in order to check the mechanical behaviour of the masonry under compression; specifically, the Young's modulus was generally ranging between 985 and 1380 N/mm2 while a Poisson ratio of 0.07-0.20 was detected [9]. The results helped to explain the diffused crack patterns of the Tower and to calibrate the repair intervention.

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Figure 19. Stress distribution and double flat results [9]

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 4.2 Presence of multiple leaf walls or detachment of layers.

4.2.1 Use of georadar for the detection of masonry sections The wall of the Avio Castle at the basement level (Fig. 20), shows an evident out of plumb

and some cracks which are monitored [24] (Fig. 21). Due to the large deformation, it was important to inspect the internal characteristic of the masonry. Radar tests were carried out on parallel profiles in order to reconstruct the internal morphology [24] (Figs 22-23).

A relevant deformation of the western upper corner of the south wall (Fig. 21a) is visible. This inflection did not produce wide cracks and could date back to the collapse of the closer Tower, at the end of the 19th century since there floors where missing and the corner did not have any connection and protection. Fig. 21b shows a large out of plumb of the outside wall with stone expulsion, which produces a relevant crack pattern in the internal room. The cracks have similar slope. The damage probably is not connected with a deformation due to the height of the wall. The masonry seems very inhomogeneous, with round stones, also of small dimensions, and damaged mortar joints.

Figure 20. Figure 21. Position of the

investigated area [24]. Deformation of the South facade

toward the valley [24].

Figure 22. Reconstruction of the radargrams respectively at the depth of: 15, 32, 39, 60

and 88 cm. The elaboration enhances the presence of internal voids (white spots). Boxes identify the positions of scaffolding holes and missing stones as

observed from the outer inspection [24].

Figs 22-23 show the reconstruction of the radar profiles, enhancing the targets in a sequence of depth slices of the wall 90 cm thick. Several voids are readable in the


radargrams in correspondence of the leaves interface. This conclusion was confirmed also by direct visual inspection. The material found inside the wall is very poor with weak or even locally missing mortar. This can explain the appearance of bulging only on the outer surface as a detachment of the two leaves of the walls.

a) b) c)

Figure 23. Radar profiles (a),(b) indicating the presence of a two leaves section and inspection detail which indicates the presence of poor infilling (c) [24]

4.2.2 Presence of the external leaf detachment: Isso Tower at Castelleone An extensive diagnostic investigation is being carried out on the Isso Tower in Castelleone

(Figs 24-25) [14]. Beside other damages, the Tower is showing a large loss of the external masonry leaf, a sort of cladding (Figs 25-26). A similar problem was already studied by the Authors in another case study, the Tower of the Cremona Cathedral, called “Torrazzo”, reported in [7,13]. Interesting results concerned the detection of the presence, in some parts of the Torrazzo, of an external thin leaf, one brick thick which was partially detached from the rest of the wall section [7].

Figure 24. Mobile platform for the on-site

survey of the prospects [14].

Figure 25. Isso Tower: details of the detached

cladding in the external and internal sides of the bearing walls [14].

Figure 26. Isso Tower: view of the

South-East and North-East prospects [14].

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 In the case of the Isso Tower the detachment represents a rather dangerous factor, as a

collapse of parts of this leaf might hit the passers-by in the square beneath the tower. The authors have applied non destructive and slightly destructive techniques (radar, sonic

tests, boroscopy and flat jack) in order to investigate the situation. Georadar, using different frequency antennas, were applied to recognise the wall morphology and the presence of local defects [5,12] (Figs 27-28). This presence was confirmed by local inspections and a number of local tests as boroscopy and flat jacks; nevertheless, the use of NDTs was necessary to investigate the extension of the defect over large surfaces [7].

Figure 27. Figure 28. Introducing underneath the

detached cladding a rigid metal ruler to calibrate

radar tests [24].

Radargram showing a detached area at the beginning of the

profile [24].

The tests detected in some parts of the Tower an external thin leaf, one brick thick, which was partially detached from the rest of the wall section. The presence of the detachment was noticed in several parts of the Tower, with an advanced state of damage.

Although the historic documentation does not give any description, it seems clear that when the bearing walls of the medieval tower (1.40 m thick) was made by solid brick masonry but with rough construction technique, their external and internal surfaces were covered by a 12 cm thick brick masonry leaf made with regular bricks and mortar joints. It is evident that this leaf grew up with the wall and is connected to the rest of the wall by a course of header bricks every 12 or 13 other courses.

The other courses are only partially connected with header and hence in many parts only the mortar is a contact between the leaf and the internal walls.

Georadar investigations detected the existence of the external masonry leaf and its detachment from the rest of the wall, therefore subjected to possible dangerous local failure; on the external prospects, the accurate survey of the damages and the GPR investigation were possible by using a mobile platform (Fig. 24). Evident damages are correlated to the lightning which struck the Tower during a storm in 1845 causing a large part of the external cladding to collapse (Figs. 25-26). The mobile platform allowed an accurate survey of the damage, often not visible from the ground level, but also to evaluate directly the detachment of the cladding. It was possible to introduce underneath the detached leaf a rigid metal ruler up to more than 80 cm (Fig. 27). This demonstrates the danger of the potential instability of the masonry.

Since the potential of high frequency radar investigations for the control of leaf detachments had already been tested in the Cremona Torrazzo [7,16], it was decided to systematically apply this method to the areas suspected to be seriously affected by the problem. The wall was surveyed with a 2GHz bipolar antenna (Aladdin system provided by IDS SpA) by executing more than one hundred horizontal profiles on each side of the Tower. The data analysis was calibrated with specific tests to estimate the velocity of the radar signal and with local tests on selected points where it was possible to insert and extract a metal


shield to enhance the reflection from the detachment (Fig. 27). An example of the radar result is shown in Fig. 28. The detachment is revealed on the radar data by a strong reflection that appears at a constant time of approximately 1.5-2.0 ns.

After preliminary tests, GPR investigations were carried out to define the extent of the detachment of this leaf. The results were compared with local inspections and were correlated with the crack pattern (Fig. 29) to understand the causes of the damage

Fig. 30 shows as a map, the results of the data elaboration and the localisation of the leaf detachment, particularly concentrated and extended in the North prospect.

Figure 29. Figure 30. Crack pattern survey of the

internal and external prospect of the North-East

side [24].

Map of the detached areas in the Isso Tower prospects [24].

4.3 Presence of inclusions and voids

4.3.1 Presence and position of tie rods of the porch vaults. During 17th and 18th century tie rods were frequently positioned in such a way to be not

visible; they were situated on the extrados side of the vault also for esthetical reasons. Fig. 31 shows, as an example, one of the possible configurations, extracted from the Breymann treatise [25]. In the studied case, the vaults have a depressed multi-centred profile (Fig. 32), which can lead to considerable thrust actions, which should be balanced.

The knowledge of the presence and the exact location of tie rods is a very important factor for the future design for conservation and reuse. The inspection does not give information on the state of conservation of the tie rods, their stress or strength. Nevertheless knowing the location and the geometry of the ties, only a very limited direct visual inspection will be required to see the state of conservation of the rods and the destructive inspection can be concentrated only in the most important area.

In some location of the porch prospect of the studied building, traces of the plates connecting the ties to the walls were visible (Fig. 5a) but it was not known if the rods were still there or had been removed during some repair intervention carried out in the fifties.


Both radar and thermovision were applied to solve the problem without using any scaffolding. Thermovision was used for inspection of the vault surface (Fig. 5) from the porch area and for inspection of the wall above the columns (Fig. 33). Radar was used to investigate the vaults from top to bottom with profiles executed on the floor of the library hall above the vaults (Figs 34-35).

Fig. 5a shows an example of the images produced by thermovision observing the wall above the columns. A vertical rod connecting the two plates is clearly detected. Fig. 5b shows again the result after the application of a gradient filter to improve the contrast. Fig. 33b also demonstrates the presence of an inclined rod hidden by the plaster of the vaults in the position indicated in Fig. 33a.

From Fig. 5 and 33 it seems that a previous tie rod set at the spring of the vault had been cut and substituted with the hidden tie rods.

These examples demonstrate that thermovision is very effective to detect near-surface rods. The method produces a very quick inspection also from the point of view of the data elaboration. Furthermore, the test equipment does not have any direct contact with the wall surface. This is a great advantage in the test of large surfaces in not easy accessible position or with non-smooth and geometrically irregular surfaces and shapes. The scaffoldings are, then, not necessary, which is a remarkable economical benefit.

Figure 31. Example of tie rods

reinforcement from the Breymann Treatise [25]

Figure 32. Profile of the inspected vaults [15].

Figure 33. Thermovision applied to detect a inclined rod hidden by the plaster

of the vault [15].

The limit of thermovision is connected to the penetration depth, usually limited to a few centimeters. If the tie rods were embedded in the wall section, scaffoldings would have been necessary to perform down-up radar inspections.

Instead, the radar was needed only to investigate the floor of the library hall above the porch in order to complete the map of the tie-rod elements.

Fig. 34 is a long radar profile over 5 vaults where it is possible to observe the shape of the vaults and many diffraction hyperbolas produced by elements orthogonal to the radar profile. Specifically, the presence of the horizontal rods between the vaults is confirmed, whose existence was already suspected because of the upper plates above the columns (Fig. 5);


furthermore, some utilities (rainwater pipes or electrical cables) can be detected below the floor.

A 3D reconstruction was also produced by processing a dense subset of parallel radar profiles. Fig. 35 shows the focused data above one of the vaults. The horizontal rods and the vault shape are quite evident.

At the end of the thermovision and radar application it was possible to reconstruct the typology of the tie rods, definitely similar to the one shown in Fig. 31.

Figure 34. Radar profile produced with a 1GHz antenna above 5 vaults of the porch.

Diffractions are produced by horizontal tie rods orthogonal to the profile and by cables or rainwater pipes below the library floor. The high energy

diffraction on the right side is an artefact produced by a metal bar fixed on the floor on the line of a door that was crossed by the profile [15].

Figure 35. 3D radar volume from an experiment executed with a 1GHz antenna above

one of the porch vault. The horizontal section at 22cm shows the rods on both sides of the vault. The vertical section shows the vault shape [15].

4.3.2 Masonry discontinuity and infill: Isso Tower at Castelleone The safety assessment of historic buildings is a strategic issue, especially for Towers sited in historical centres. In these cases any risk factor that may affect the integrity of the buildings has to be systematically taken into account. Ancient buildings often show diffused crack patterns, which may be due to different causes in relation to their load history and to their construction technique, but also due to their function changes and historical vicissitudes. At the base level of the Isso Tower [14], the masonry shows several discontinuities caused by numerous past interventions. Until 1919 several buildings were adjacent to the base of the Tower (Figs 36-37).

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 Sonic tests were carried out before the flat-jack tests, to control the masonry characteristics

along the whole perimeter of the base. The tests were very precious to evaluate the compactness of the masonry section and the effects of the past repair. E.g. in the South-East and North-West side a low velocity area revealed several filled openings (Fig. 38).

Figure 36. Figure 37. Layout of the Isso Tower area

in a map of Castelleone of 1901 and in a recent one [14].

Masonry irregularity due to the dismantling of the

adjacent buildings [14].

The localisation of the flat-jack tests was chosen to calibrate the FE model and to control the stress distribution caused by the eccentric position of the heavy tank. In detail, flat-jack tests were carried out at the base and at the tank level (Table 1).

Table 1. Stresses measured by single flat-jack test.

Test Stress tests

[N/mm2]

Stress numerical [N/mm2]

Height

[m]

Side

TCL-J8S 0.37 0.29 28.00 S-W TCL-J7S 0.36 0.29 23.00 S-E TCL-J5S 0.75 0.72 4.60 S-E inside TCL-J3S 0.37 0.73 1.00 S-E inside TCL-J9S 1.22 0.72 2.00 S-E outside TCL-J4S 1.25 1.00 1.75 S-W TCL-J2S 0.71 0.71 1.80 N-E TCL-J1S 1.56 0.72 1.50 N-W inside TCL-J6S 0.74 0.74 1.55 N-W outside

Figure 38. Distribution of the pulse sonic velocity measured at the base by transmission on the South-East wall [14]

The distribution of EM and pulse sonic velocity at the base of the tower (Figs 39-40) gives similar results. Usually, materials present complementary behaviours with respect to sonic and EM velocity (i.e., slow sonic materials are generally fast materials for EM waves, and vice-versa). In both cases, a less compact area was found in the South-East wall.

Furthermore, the high inhomogeneity of masonry could explain also the irregular distribution of the stresses measured by single flat-jack tests at the base of the Tower (Fig.


41). In the N-W side, a significant difference between the measurements in the internal an external side was found (Fig. 41).

Figure 39. Distribution of the EM velocity measured by transmission at the Tower base

[14].

a) b)

Figure 40. Distribution of the pulse sonic velocity at the Tower base respectively at 1.5 (a) and 1.8 m (b) [14].

Figure 41. Local state of stress found by single flat-jack tests at the Tower base [14].

4.3.3 Concrete frame supporting the bells: Cremona Tower The objective of this investigation was the reconstruction of the concrete floor

reinforcements and of the concrete frame supporting the bells of the Cremona Civic Tower, called “Torrazzo”at the level of 58 m, hidden by the floor [7].

A video inspection with a camera through a 5 cm hole in the concrete floor (Fig. 42) shows a system of concrete beams supporting the floor. Inspections of the beam system from the bottom were not possible because a masonry vault exists below the concrete frame.

The ND investigation was executed by surveying the concrete floor with a number of orthogonal radar profiles.

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 The position and length of the profiles were severely limited by many obstacles on the

concrete floor (bells stored on the floor, concrete columns belonging to the structure that support the active bells, a large opening in the floor used to install the bells). Nevertheless, the investigation was sufficiently dense (Fig. 43) to achieve the objective. Fig. 44 shows a typical example of one of the radar profiles. The high frequency antenna (1.6 GHz) was able to detect the reinforcement system (diffractions on the left side) consisting of rebars 3–5 cm deep and 12 cm spaced and the concrete beams (larger and deeper diffractions on the right side).

Figure 42. Figure 43. Video inspection of the structure through a hole in

the floor [7].

Map of the radar profiles on the concrete floor and

preliminary interpretation of the results [7].

Figure 44. Typical radar image obtained by surveying the concrete floor with a 1.6 GHz

antenna [7].

The final map of the beam system was imported in the geometric survey and is shown in Fig. 45. On the basis of the intensity and the size of the radar diffractions produced by the beams, the map indicates some main beams and some secondary beams. A map of the reinforced areas was also produced.

According to the radar images it seems that the diffraction intensity and the rebar spacing is sometimes lower especially in the most lateral areas of the concrete frame near the external walls. This might indicate that thinner and less dense rebars were used for the concrete areas closer to the walls.

The beam map is consistent with the position of the beams observed with the video inspection and thus the whole result is expected to be reliable. According to the beam reconstruction in Fig. 45, it seems that the main beams rest on the whole masonry thickness for three sides of the tower, whereas on the west side they rest on a thinner masonry because of the presence of the staircase inside the wall. The vertical section of the west side wall in Fig. 46 shows the shape of the staircase, the position of the two main beams and a crack


pattern observed on the external wall that might be correlated with the stress concentration produced by the beams.

Figure 45. Figure 46. Map of the beams and

supporting columns according to the interpretation of the

radar profiles and to the video inspection [7].

Vertical section of the west side of the tower at the level

of the concrete floor. The main beams resting on the west side wall above the

staircase are indicated [7]

4.4 Masonry quality The authors have more then fifteen years experience on the application of NDTs on

historic masonry buildings and especially on the application of flat jack tests. On historic buildings it is usually not allowed to carry out other on site, perhaps more significant tests (shear and compression on large scale), since they are destructive and invasive.

The authors experience [1,2,17,18] acquired on several types of Italian and European masonry structures, suggests that coupling flat-jack test with sonic test and with the observation of the masonry section by sampling, is a good and not too much invasive methodology to mechanically qualify the masonry [6,8,10,11]. A systematic investigation campaign was carried out on a sample of ten churches with the aim of supplying a good level of knowledge necessary to the structural analysis and to the proper strengthening and repair interventions [19].

According to the low budget allocated by the single churches, only few points could be chosen to detect the masonry quality, i.e. its morphology and stress-strain behaviour (Fig. 47). Systematically, the testing points were chosen in the most representative parts of the bearing walls: taking into account that the façade of a church is usually made of better masonry, the lateral bearing walls were chosen. The tests were carried out mainly on the outer face of the wall, since the inner one was usually decorated with frescos and paintings which could not be damaged.

A complete characterization of the masonry in the chosen points was achieved by measuring: the sonic velocity, the state of stress, the modulus of elasticity, the coefficient of lateral deformation, the mortar and stones chemical, physical and mechanical properties. The morphology of the wall cross section was also surveyed in order to understand whether the masonry was made of one or two leaf and the leaves were connected in some way [17,18]. This survey was carried out by sampling few stones in order to visually investigate the wall texture, redraw the inner aspect of the wall (Fig. 48), sample stones and mortars for laboratory testing.

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 In particular, four subsequent steps were followed in the same area: (i) sonic tests by

transparency on a grid of 75x75cm, (ii) single and double flat-jack test, (iii) survey of the masonry morphology and material sampling, (iv) repositioning of the stones in the wall.

Fig. 49 shows the relationship between the sonic velocity and the modulus of elasticity, compared to other values previously obtained from tests on different stone-masonry walls in historic centres.

Local State of Stress

εl εv

-0.40 -0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40

Strain [μm/mm]

0.00

0.40

0.80

1.20

Stre

ss [N

/mm

2 ]

SAJD2LVDT 1, 2, 3, 4

LVDT 5

Figure 47. St. Antonio Abate in Morgnaga: double Flat-Jack test to characterize masonry of the lateral wall of the church.

Left side of the section

Figure 48. Study of the masonry morphology

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200Sonic Velocity (m/s)

0

1000

2000

3000

4000

5000

6000

Elas

tic M

odul

us (N

/mm

2 )

SPA

SC1SC2SA-C

A2 SBN-1SBN-2 SGP

PSMA-SA2 SA-C

H1

SM

PSMA-CH1

SAM-1

SAM-2

SAI

Monumental Churches from BresciaMinor Churches from BresciaChurches out from CorrelationOther tested Buildings in Italy

Elastic Modulus vs Sonic Velocity

ln(Y) = 1.88 * ln(X) - 6.04Coef. of determination: R2 = 0.46

Figure 49. E vs. sonic velocity

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 The calculated values of the elasticity modulus allow to subdivide the churches into three

categories: 1) from 189 to 803 N/mm2, corresponding to a very week masonry with internal voids and no connection between the masonry leaves, 2) from 1063 to 1732 N/mm2 , corresponding to a better masonry but with poor connections between the leaves, 3) from 2860 to 5260 N/mm2 a definitely compact masonry well connected transversally. Figs 50-52 show some typical diagrams obtained with the double flat-jack test, as examples of the previously mentioned categories: Fig. 50 low stiffness, Fig. 51 medium stiffness, Fig. 52 high stiffness.

Other tests were carried out in order to check the presence of cracks in stone columns (by ultrasonic tests) and the state of tension in metal tie rods (by dynamic tests).

0

0.4

0.8

1.2

1.6

-4.0 -2.0 0.0 2.0 4.0 6.0 8.0

Strain [mm]

Stre

ss [

N/m

m2 ]

LVDT 3LVDT 4 HLocal state of stress

0

0.4

0.8

1.2

1.6

-1.0 0.0 1.0 2.0 3.0 4.0

Strain [mm]

Stre

ss [N

/mm

2 ]LVDT 1, 2, 3LVDT 4Local state of stress

0

0.4

0.8

1.2

1.6

-1.0 0.0 1.0 2.0 3.0 4.0

Strain [mm]

Stre

ss [

N/m

m2 ]

LVDT 2, 3LVDT 4Local state of stress

Figure 50. Figure 51. Figure 52. St. Antonio in Morgnaga,

rectory

St. Giovanni Battista in Pavone, church

St. Michele in Sabbio Chiese, church

4.5 Moisture detection: St. Maria Rossa Church The decay of masonry surfaces is often caused by the presence of the humidity in the wall.

The investigation on this specific problem can be subdivided in (i) detection of the presence of moisture, (ii) evaluation of the moisture content. Some techniques are very sophisticated and expensive but completely non destructive like the thermovision or radar test [20], [26] others are cheaper, like the power drilling method, but not often repeatable in the time, being locally partially destructive. Furthermore, the radar and thermovision methods can scan large portions of the walls while the drilling test is very local.

The choice of the investigation methodologies, then, is a function of the adopted strategies and necessities, but also of the available budget.

For the decay prevention, it is often enough a general mapping of the moisture. In fact, the most decayed areas are usually the ones affected by dry-wet or frost-defrost cycles, with salt deposits.

In the following, different tests are compared and applied on a case history: the S. Maria Rossa Church [20] (Figs 53-54). Radar microwave borehole measurements, thermovision and the power drilling method were combined for the characterisation of the moisture content and


distribution on the historic brickwall within the framework of a BRITE EC contract.

Figure 53. The Church of S. Maria Rossa.

Figure 54. Plan of the S. Maria Rossa Church. The three areas investigated are marked

with arrows.

The thermovision returns a general mapping of the area, displaying the surface temperature and qualitatively the wet area (Fig. 55). The advantage of the technique is the rapidity of the test on a large portion of the masonry but only on the surface and without any quantitative correlation with the water content.

Similar observation could be done for the radar application. The technique gives information on the moisture presence within the masonry, not only on the surface as thermovision; moisture is expected to be revealed by a decrease of the EM wave velocity and by an increase of the EM wave attenuation. This is clearly demonstrated by laboratory tests (Fig. 56) where a 1GHz antenna is moved down along a vertical profile, approximately 2m long; moisture is rising from the wet soil and as a result the EM velocity is expected to decrease as the antenna is approaching the base of the masonry. The effect is a gradual increase of the reflection time from the opposite side of the masonry. Some years ago, some correlation was attempted between the EM parameters and the water content [26].

The microwave borehole measurement is a technique developed by BAM [27], able to detect the distribution of the moisture in the masonry section, at present only qualitatively. In fact, the technique, evaluating the properties of the material by the EM wave transmission, needs correlation curves between the measured parameter, permittivity, and water content, for each tested material (Fig. 57).

The method requires the drilling of two thin holes with a diameter of 12 mm, a distance of 50 mm and a depth corresponding to the thickness of the wall, thus this method is only quasi non-destructive. Leaving the holes accessible, the technique can be repeated as monitoring tool.


Figure 55. Figure 56. Thermovision applied to the area 2 of the S. Maria Rossa

Church [20].

GPR data from a profile on a partially wet brick masonry. The

effect of moisture content is observed on the right side of the

section.

468

101214161820

0 10 20 30 40 50 60 70

depth in cm80

real

par

t ε

8.4 GHz

0 80 160 240 320 400depth [mm]

0

5

10

15

20m

oist

ure

cont

ent [

%]

Figure 57. Figure 58. Depth resolved presentation

of the real part of permittivity (area 2 of the S. Maria Rossa Church). The mean value is shown as a solid line while the mean

value obtained by radar as a dashed line [20,27].

Moisture content in Mass% measured by powder drilling (area 2 of the S. Maria Rossa

Church [20]).

The last method, the powder drilling test is a low cost technique often used, which gives the water content of the sampled material (Fig. 58), also at different depth, but it is local and not widely repeatable over time. The comparison of these methods could not give analytical correlation, but only some qualitative results showing the limits and advantages of each method.

5 CONCLUSIONS

The use on non destructive and minor destructive techniques in the investigation for the diagnosis of historic buildings is not an easy task. In order to avoid inappropriate and useless applications clear methodologies should be assumed, i.e. the problems to be solved and the appropriate types of techniques to be used must be known. This means that of each technique


advantages and limits have to be clear to the designer of the eventual restoration intervention. In fact, very seldom a single technique can solve the problem but the complementarity of NDTs and MDTs in the diagnostic investigation must be assumed. Some typical problems have been presented and their solution through case history has been shown.

Acknowledgement The authors wish to thank M. Antico, L. Cantini, E. Cardarelli, P. Condoleo, M. Cucchi,

M. Iscandri, N. Ludwig, C. Maierhofer, E. Rosina, C. Tiraboschi and L. Zanzi for their precious contribution to the experimental part of the research. The research was partially supported by the National projects RELUIS funds, Research Line 1, 2005-2008.

References 1. Binda, L., Penazzi, D., Saisi, A. (2003) “Historic masonry buildings: necessity of a

classification of structures and masonries for the adequate choice of analytical models”, Sixth International Symposium on Computer Methods in Structural Masonry STRUMAS VI, Roma, 22nd-24th September 2003, Ed. T.G.Hughes & G.N. Pande, Computers & Geotechnics Ltd, pp. 168-173.

2. Binda, L., Saisi, A., Tiraboschi, C. (2000) "Investigation procedures for the diagnosis of historic masonries", Construction and Building Materials, vol. 14, n.4, pp. 199-233.

3. Binda, L., Lualdi, M., Saisi, A., Zanzi, L. (2003) “The complementary use of on site non destructive tests for the investigation of historic masonry structures”, Proc. 9th North American Masonry Conference, Clemens, USA, pp. 978-989.

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6. Binda, L., Saisi, A., Zanzi, L. (2003). “Sonic tomography and flat jack tests as complementary investigation procedures for the stone pillars of the temple of S.Nicolo' l'Arena (Italy)”, NDT&E International, vol. 36, n.4, pp. 215-227.

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12. Gentile, C., Saisi, A., Binda, L. (2002) “Dynamic investigation of a historic masonry Bell Tower”, 6th International Masonry Conference, London, pp. 192-199.

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Application of NDTs to the diagnosis of Historic Structures · 2009-04-23 · NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009