Non-Destructive Testing Techniques Applied for Diagnostic Investigation: Syracuse Cathedral in Sicily, Italy

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Non-Destructive Testing Techniques Applied forDiagnostic Investigation: Syracuse Cathedral in Sicily,ItalyLuigia Binda a; Maurizio Lualdi a; Antonella Saisi aa Department of Structural Engineering, Politecnico of Milan, Milan, Italy

Online Publication Date: 01 October 2007To cite this Article: Binda, Luigia, Lualdi, Maurizio and Saisi, Antonella (2007)'Non-Destructive Testing Techniques Applied for Diagnostic Investigation: Syracuse

Cathedral in Sicily, Italy', International Journal of Architectural Heritage, 1:4, 380 - 402To link to this article: DOI: 10.1080/15583050701386029URL: http://dx.doi.org/10.1080/15583050701386029

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NON-DESTRUCTIVE TESTING TECHNIQUES APPLIEDFOR DIAGNOSTIC INVESTIGATION: SYRACUSECATHEDRAL IN SICILY, ITALY

Luigia Binda, Maurizio Lualdi, and Antonella Saisi

Department of Structural Engineering, Politecnico of Milan, Milan, Italy

The long-term research experience of the authors has highlighted the importance of gaining

knowledge of the building through experimental investigation. Recently the authors inten-

sively studied the Syracuse Cathedral (Sicily, Italy) to evaluate the structural state of the

preservation of the pillars. The Cathedral of Syracuse was built in different phases on an

ancient Greek temple from the fifth century BC and modified throughout later centuries. The

pillars of the central nave, obtained by cutting the temple cell walls, show a complex situation

of damage and repairs. An investigation program (including radar, sonic, and ultrasonic

tests, for example) has been recently planned, aimed to design the preservation and restora-

tion actions. An accurate geometric survey of the surface problems and defects allowed the

localization of the most damaged area, suggesting the need for further control by non-

destructive testing (NDT). The results obtained from this survey were compared with the

results of the other type of tests, and the elaborated data will be used to implement an

analytical model for the study of the seismic vulnerability. The preliminary results of the

experimental investigation carried out by the authors are presented in this article.

KEY WORDS: masonry, diagnosis, NDT&E, radar test, thermovision

1. INTRODUCTION

In the past decade, the term restoration has more often been replaced by the term

preservation, meaning that the historic buildings should first of all be preserved as

much as possible as historic documents of our past. This approach also requires in-

depth knowledge of the construction to understand the role of all its features and

details, such as the properties of the materials and the structure.

Prevention and rehabilitation can be successfully accomplished only if a diag-

nosis of the state of damage of the building has been formulated. In addition to the

preliminary damage controls before the intervention, the effectiveness of the repair

techniques should be controlled during and after the repair work. It is known that a

correct intervention on a historic structure should start from an accurate diagnosis of

the building to minimize the intervention’s interference with the authenticity of the

architectural document. The investigation also may require long-term monitoring of

the structure.

The structural performance of a historic masonry building can be understood

provided the following factors are known: 1) its geometry; 2) the characteristics of its

International Journal of Architectural Heritage, 1: 380–402, 2007

Copyright � Taylor & Francis Group, LLC

ISSN: 1558-3058 print / 1558-3066 online

DOI: 10.1080/15583050701386029

Address correspondence to Luigia Binda,Department of Structural Engineering, Politecnico ofMilan,

Piazza Leonardo da Vinci, 32, 20133 Milan, Italy. E-mail: [email protected]

Received 29 September 2006; accepted 09 April 2007.

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masonry texture (single- or multiple-leaf walls; connection between the leaves; joints

empty or filled with mortar; physical, chemical, and mechanical characteristics of the

components such as bricks, stones, and mortar; 3) characteristics of masonry as a

composite material; and 4) damage and crack pattern distribution (Binda et al., 2000).

The diagnosis should result from: 1) an experimental investigation on site and in the

laboratory aimed to define the characteristics of the materials and of the structure

itself, and 2) the structural analysis based on appropriate mathematical models (Rossi

1997). The investigation on site must be as non-destructive as possible and provide

information with good precision. The Italian Seismic Code (OPCM, 2005) now

requires the on-site experimental investigation.

Several investigation procedures have been applied in the past decades, most

from other research fields (e.g., medicine, aerospace engineering) or from application

to the study of new materials (e.g., steel, concrete, composites). Nevertheless, because

of difficulty interpreting the collected data, application of non-destructive testing

(NDT) techniques, although advanced, to masonry, can be frustrating as masonry is

a composite, highly non-homogeneous material (Binda et al., 1999; Suprenant and

Schuller, 1994). Furthermore, when a complex investigation is carried out using

different techniques, the highest difficulty is represented not only by the interpretation

of the results of the single technique but also by the harmonization of all the collected

data (Binda and Tiraboschi, 1999; Binda and Saisi, 2001; Binda et al., 2003a).

The solution of very difficult problems, such as the detection of the morphology

of multiple-leaf masonry sections, the presence of voids and cracks in masonries, and

their mechanical characteristics, cannot be reached with a single investigation techni-

que, but instead with the complementary use of different techniques (Binda et al.,

2003a). All the techniques must always be appropriately applied to the different

materials; therefore, a project of investigation must be prepared in advance and

involve the designer, who must pose clear questions if good answers are desired.

The Cathedral of Syracuse in Sicily, Italy ( Figure 1), a Greek temple of the fifth

century BC, was transformed into a Christian church in the sixth century AD and was

subsequently submitted to many modifications throughout the centuries due to the

damage caused by various earthquakes. The pillars of the central nave show a diffuse

crack pattern with mainly vertical cracks at the lowest part and on the corners. This

damage requires interpretation to decide whether repair is needed and how to

choose appropriate repair techniques. Therefore, the Cultural Heritage Syracuse

Superintendent, M.Muti, asked to the authors to conduct an investigation campaign.

A design for the investigation was set up after the first study of the damage detected on

the pillars, including findings such as deep cracks, delaminations, detachments of

thick plasters, empty joints, and out-of-plumb alignment.

2. INVESTIGATION PROCEDURES FOR THE DIAGNOSIS OF DAMAGED

STRUCTURES

The necessity of establishing the building integrity or the load-carrying capacity

of a masonry building arises for several reasons, including: 1) assessment of the safety

coefficient of the structure (before or after an earthquake, or following accidental

events such as a hurricane or fire), 2) change of use or an extension of the building, 3)

assessment of the effectiveness of repair techniques applied to structures or materials,

and 4) long-term monitoring of material and structural performance. The flow chart

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in Figure 2 (Binda et al., 2000) schematically represents the needs to be fulfilled by the

experimental investigation with the techniques adequate to address these needs.

Most NDT techniques can give only qualitative results; therefore the designer is

asked to interpret the results and use themat least to provide comparative values between

different parts of the same masonry structure or to use multiple different NDTs. It must

be clear that, even if consultation of experts in the field is required, the designer or design

team member is responsible for the diagnosis and must: 1) set up the on-site and

laboratory survey project, 2) constantly follow the survey, 3) understand and verify the

results, 4) make technically acceptable use of the results, including their use as input data

for structural analyses, 5) choose appropriate models for the structural analysis, and 6)

arrive at a diagnosis at the end of the study. These operations can be accomplished with

the help of experts in the field. Therefore information is needed for architects and

engineers on the availability and reliability of the investigation techniques.

3. THE SYRACUSE CATHEDRAL

The Greek temple of Athena in Syracuse, built the fifth century BC, was trans-

formed into a Christian church in the sixth century AD, and successively became the

Figure 1. Photograph of the Baroque facade of Syracuse Cathedral (Sicily, Italy).

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Cathedral of Syracuse; the building was frequently modified throughout the centuries

until the present configuration (Privitera, 1863; Agnello, 1950 and 1996; Russo, 1991

and 1992; Giovannini, 2005).

Several styles and structural details belonging to the different times can be recog-

nized: 1) in the external walls, the ancient Greek columns and the filling wall between

them of the Byzantine era, 2) the Baroque facade (Figure 1), 3) the apse added in sixth

Figure 2. Flowchart of the information required and corresponding investigation techniques (Sicily, Italy)

(Binda et al., 2000).

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century AD and modified in the twelfth and seventeenth centuries AD. Furthermore,

because the city of Syracuse is in a seismic area, the Cathedral was damaged, repaired, or

partially rebuilt several times (Agnello, 1950 and 1996). The Cathedral pillars had been

obtained by cutting out the stonework walls of the internal cell of the Greek temple; the

pillars show several repaired areas and replacements, but also several serious cracks. The

pillars seem to be suffering progressive damage over a long time period.

To evaluate the state of preservation, the extension and depth of the replace-

ments, and the presence of internal defects, an investigation program was planned. As

a first step, a survey of the pillars was conducted with an accurate mapping of the

superficial materials, defects, cracks, and morphology. On the base of this detailed

survey, NDT tests, such as sonic, ultrasonic, and radar tests, with thermovision

inspection, were performed to investigate the depth of the damage (Suprenant and

Schuller, 1994; Binda et al., 2000). The complementarity of NDTs could be success-

fully exploited to diagnose the state of damage of the pillars (Binda et al., 2001 and

2003b; Giovannini, 2005).

3.1. Historical Evolution of the Building

The investigation was carried out by analyzing the structure from different

points of view (Binda et al., 2000). First, historical documentation was found both

on the ancient period and the most recent transformation (Giovannini, 2005). The

iconographic research gives important information on the general evolution of the

building (Binda et al., 2000). Nevertheless, interesting information was mainly col-

lected by stratigraphic survey and on-site observation of the masonry texture and

structural details (Giovannini, 2005). To understand the structural complex and the

reasons for the past damage, especially after each earthquake (Agnello, 1950 and

1996), the historic research focused on the main steps of the spatial transformation,

based both on indirect and direct survey of the building.

The building, in fact, is the result of a stratification of several expanding, repair,

and strengthening interventions from its origin in the 480 BC up to the current date

(Privitera, 1863; Russo, 1991). The most ancient building is the Greek temple, while

the existing building is a cathedral with a dome, lateral chapels, and a Baroque facade

(Giovannini, 2005). In the sixth century AD, during the Byzantine period, the temple

was transformed into a church with a nave, obtained from the internal cell, and two

side aisles (Russo, 1992). The external space between the columns was filled by

stonework masonry. The temple cell walls were cut, obtaining wide arcades and

pillars. The transformation of the Greek masonry made by large stone blocks required

precise works to obtain a sequence of regular arches

Information is not available for the following period between the sixth and the

twelfth centuries, except that the building was not used during the Arab occupation

of Syracuse (878–1040 AD). With the Norman invasion, and after some strong

earthquakes, the church was modified with a new facade, a new apse, windows in

the perimeter walls, a new roof, and increased wall height in 1085 AD (Russo, 1992).

In the 1542, a strong earthquake struck the city of Syracuse and caused the collapse

of the Norman facade, which was successively rebuilt, as well as serious damage to

the lateral walls. Figure 3a shows a survey of the damage that occurred in 1542 on the

north side, with a shift of the column drum still visible. The wall was strengthened as

shown in Figure 3b.

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The 1693 earthquake destroyed again the facade that was rebuilt as seen today

(Agnello, 1950). There is no information about the effects on the internal walls, even if

it is reasonable to assume that serious effects could have affected the first pillars and

the spans toward the collapsed facade. At the beginning of the twentieth century

(1924–1926), several interventions were carried out, such as the removal of the

Baroque decoration inside the church.

Figures 4 and 5 show the evolution of the Cathedral plan and volume in different

times. The effects of the subsequent transformation of the central nave could be

Figure 3. Syracuse Cathedral (Sicily, Italy): (a) schematic illustration of a detail of a damaged column on the

north side, and (b) drawing from the eighteenth century of a view of the intervention following the

earthquake.

Figure 4. Schematic illustrations of the evolution of the Syracuse Cathedral (Sicily, Italy) from 470 BC

through today.

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documented only by a detailed geometric survey and by the survey of the masonry

texture in the area (Figure 6).

3.2. Survey of the State of the Pillars

The monument shows signs of damage in several parts, including in the lateral

walls and columns (Figure 3) and in the facade; long cracks and damage are present in

the apse. Nevertheless, since the first inspection, the pillars’ situation seemed very

complex. The pillars, octagonal with a square base (Figures 7 and 8), seem the most

damaged elements of the church. Probably their configuration dates back to the

intervention in 1924–1926. The first step of the investigation was the geometric survey

of the walls and pillars of the central nave. Subsequently the different materials

appearing on the pillar surfaces were mapped. Finally, the crack pattern survey and

an accurate survey of the out-of-plumb alignment of the pillars were carried out. In

this way, it was possible to identify the state of damage of the pillars, as it can be

determined from visual inspection.

3.2.1. Geometry and Material Survey Obtained from the cell walls, the pillars

are built by large stone blocks according to the ancient Greek technology, as seen in

Figure 8. The pillars show: 1) disconnection between the blocks, 2) cracks, 3)

repaired cracks, and 4) cracks repaired and later reopened. In several situations,

damage was repaired with different mortars to regularize the pillar shape (Figure 9).

The removal of the existing Baroque decoration is recognizable due to the surface

irregularity, especially in the corners, which were cut during the intervention in the

1920s.

3.2.2. Crack Pattern Survey The crack pattern was accurately documented by

photographs and reported on the geometric survey (Figures 10–12). Cracks were

classified according to their thickness. In particular, the survey localized the most

frequent damage on the pillar base and on the corners of the pillars. The cracks have

Figure 5. Axonometric projections of the evolution of the Syracuse Cathedral (Sicily, Italy). Shown from

left to right are the interventions before the earthquakes in years 1169, 1542, 1693, and 1800.

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frequently a vertical pattern, probably generated by the compressive stresses due to

compressive and eccentric loads increased by the earthquake effects. In some cases, the

corners and part of the stone blocks were expelled. The mortar traces in these cases are

trials made in the past to locally repair the damage. In the survey, the repaired cracks

were enhanced to evaluate the evolution of the damage. The differences of the surface

appearance were also reported to stress the sequence of the interventions.

3.2.3. Survey of the Geometry Variation The out-of-plumb alignment of the

pillars was measured both in the aisles and in the nave and reported in Figure 13.

Figure 6. Drawings showing areas with similar masonry textures (areas noted 1–4) that correspond to

undated building phases for the Syracuse Cathedral (Sicily, Italy).

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The graph in Figure 14 details the differences between these values. The horizontal

displacement measured on top of each pillar on the right side are substantially higher

then those on the left side. The maximum values concern the pillars numbered 25, 26,

and 27 and reach 14 cm. The right side pillars are the most inclined. Generally,

considering the geometry of the church, this finding could be partially explained by

the fact that the left aisle is less restrained than the right (Figure 15). Nevertheless, the

presence of the short-span vaults in the left aisle could produce an increase of stiffness

for the left-side pillars of the nave.

Figure 7. Syracuse Cathedral (Sicily, Italy): (a) plan of the church with the pillars numbered, and (b)

photographic view of the nave.

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Figure 8. Schematic illustration of the masonry assemblage and of the pillars: (a) original wall, (b) original

shape of the pillars, and (c) shape after the 1924–1926 intervention in Syracuse Cathedral (Sicily, Italy).

Figure 9. Photographs and schematic illustrations of details of the pillars in Syracuse Cathedral (Sicily,

Italy) showing parts made with different materials: (a) pillar 15, and (b) pillar 21.

Figure 10. Schematic illustration of the survey of pillar 19 in Syracuse Cathedral (Sicily, Italy).

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3.3. First Interpretation of the Damage and Detection of the Problems to be

Studied

The results of the previous steps of the investigation (historical documentation

and survey) allowed a first interpretation of the damage. Clearly, the pillars were

submitted during their long life to a number of modifications, which influenced their

present situation. Particularly during the time of the Norman invasion, the height of

the walls of the central nave was increased (Figures 5 and 6); hence, the state of stress

under compression was also increased.

Figure 6 shows also the modifications that occurred to the walls and pillars of

the nave toward the altar, where also arches had been completely reconstructed.

Figure 11. Schematic illustration of the crack-pattern survey of (a) pillar 19, and (b) pillar 26 in Syracuse

Cathedral (Sicily, Italy).

Figure 12. Schematic illustration and photograph of detail from the crack-pattern survey for pillar 26 in

Syracuse Cathedral (Sicily, Italy).

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Figure 14. Graph of the horizontal displacement measured on top of each pillar in Syracuse Cathedral

(Sicily, Italy).

Figure 13. Schematic illustrations of (a) plan of the general out-of-plumb pillar alignment, and (b) detail of

pillar 30 in Syracuse Cathedral (Sicily, Italy).

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Figures 9–11 show the mapping of the different materials used in the reconstruction.

Some of the external parts were clearly a sort of thick plaster made of cement-based

material; other parts seemed to be different types of stones. The basic stone blocks

were made with the typical limestone from Syracuse.

To avoid destroying any part of the original stones, it was impossible to sample

or to drill any core samples before better knowing the real situation. Investigators

were only allowed to take to the laboratory in Milan (Italy) a small sample of the

original stone found on the ground at the level of the foundation excavation. The tests

performed on cylinders (diameter 5 mm, height 120 mm) obtained from the stone

(Figure 16) show that the strength varies from an average value of 9.79 N/mm2 in dry

condition to 7.67 N/mm2 in the water-saturated state. The Poisson ratio is 0.02, and

the elastic modulus is 14.68 N/mm2.

The collected data were not useful enough to gain a better understanding of the

damage and its causes. Therefore, further investigation with a NDT was necessary to

better study the pillar morphology and materials used in the different interventions.

Figure 16. Syracuse Cathedral (Sicily, Italy): (a) schematic illustration of the cylinders obtained formechan-

ical tests of sampled stones, and (b) photographs of sampled stones.

Figure 15. Schematic illustrations of the deformation in the church section of Syracuse Cathedral (Sicily,

Italy).

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To characterize the mechanical properties of the masonry, an on-site mechanical test

such as a flat jack test is usually carried out. In this case, the tests could not be

performed for two main reasons: 1) to avoid spoiling the stones, the single flat-jack

test was not carried out because of the very thin joints; 2) the double flat-jack test was

not performed because of the large dimension of the pillar stones. Thus, the state of

stress could be calculated only analytically. The elastic parameters of the masonry

could not be measured directly because of the joint and stone dimensions.

NDT can be helpful in finding hidden characteristics (e.g., internal voids, flaws,

and characteristics of the wall section) that cannot be known otherwise than through

destructive tests. As noted previously, most NDT can give only qualitative results.

Nevertheless, the solution of very difficult problems cannot be reached with a single

investigation technique, but rather with the complementary use of different techniques

(Binda et al., 2003a and 2003b). In these cases, the choice has to be made by the

designer on the basis of his or her hypothesis. The designer also is asked to interpret

the results.

In the case of Syracuse Cathedral, the archive and on-site investigation allowed

the understanding of a number of problems to be solved: 1) the depth of the cracks

visible on the surface, 2) the soundness of the stones, 3) the depth of the layers of

rendering and their bond to the support, and 4) the presence of inclusions, reinforce-

ment, and flaws in the stones. All this information could not be collected by a single

NDT. Therefore, the following techniques were chosen: 1) sonic and ultrasonic tests to

detect the depth of cracks, 2) thermovision to evaluate the extent of detachment

problems affecting the rendering simulating the stone, and 3) georadar, as comple-

mentary to ultrasonic test and thermovision, to find deep defects and hidden

inclusions.

3.3.1. Sonic and Ultrasonic Tests The testing methodology is based on the gen-

eration of sonic or ultrasonic impulses at a point of the structure. An elastic wave is

generated by percussion or by an electrodynamic or pneumatic device (transmitter)

and collected through a receiver, usually an accelerometer, which can be placed in

various positions. The elaboration of the data consists in measuring the time that the

impulse takes to cover the distance between the transmitter and the receiver. The use

of sonic tests for the evaluation of masonry structures has the following aims (Binda

et al., 2001):

� to qualify masonry through the morphology of the wall section, to detect the

presence of voids and flaws, and to find crack and damage patterns; and

� to control the effectiveness of repair by injection technique

For a given velocity, the wavelength decreases as the frequency increases,

providing the possibility for greater resolution in the final velocity reconstruction.

It is beneficial therefore to use a high frequency to provide the highest possible

resolution. However, there is also a relationship between frequency and attenuation

of waveform energy. As frequency increases, the rate of waveform attenuation also

increases, thus limiting the size of the wall section that can be investigated. The

optimal frequency is chosen considering the attenuation and resolution require-

ments to obtain a reasonable combination of the two limiting parameters. In general,

it is preferable to use sonic pulse with an input of 3.5 kHz for inhomogeneous

masonry.

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It is important to stress that the pulse sonic velocity is characteristic of each

masonry typology and thus generalization of the values is impossible. Velocity values

will be typical of each type of masonry. Sonic pulse velocity tests by transmission give

a qualitative evaluation of the masonry consistency and of the presence of the

detachment. Pulse sonic velocity is, in fact, higher in homogeneous and compact

materials (Binda et al., 2000, 2001, and 2003b).

Sonic pulse velocity tests by transparency were carried out on the most damaged

pillars to globally check their response in term of velocity values. Figure 17 shows two

cases of irregular distribution of the velocity. As expected, the higher values are

measured in the large stones, whereas decreasing values could indicate an internal

detachment between the stones or the presence of cracks and damage or of a weaker

and less dense material. The low values detected in the upper area of Figure 17a

showed a peculiar situation, which was later found to have been caused by the

inclusions of weaker material.

Ultrasonic tests, carried out across the main cracks of all the pillars, allow

estimation of the depth of the cracks (Bungey, 1982). Ultrasonic equipment (with

40-kHz transducers) was used to estimate the penetration of all the important cracks

observed through the crack pattern survey (Figure 18). Also, surface measurements

were carried out. The emitting device was placed at the same distance as the receiving

device with respect to the crack. The tests showed that the cracks could have a great

depth, up to 40 cm.

3.3.2. Thermovision Thermovision is a NDT that has been applied since several

years ago to works of art and monumental buildings. The thermographic analysis is

based on the thermal conductivity of a material and may be passive or active. The

passive application analyzes the radiation of a surface during thermal cycles due to

Figure 17. Schematic illustrations of sonic tests carried out on (a) pillar 18 and (b) pillar 20 in Syracuse

Cathedral (Sicily, Italy).

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natural phenomena (insulation and subsequent cooling). In an active thermographic

survey, forced heating is applied to the analyzed surfaces. A camera sensitive to

infrared radiation collects the thermal radiation data. In fact, each material emits

energy (electromagnetic radiation) in this field of radiation; this radiation is charac-

terized by a thermal conductivity, which is the capacity of the material itself of

transmitting heat, and its own specific heat. The result is a thermographic image in a

color or black-and-white scale. Each tone corresponds to a temperature range.

Usually the differences in temperatures are a fraction of degree.

Thermovision can be very useful in diagnostic surveys; in fact, it is used to

identify areas under renderings and plasters that can hide construction anomalies.

thermovision is particularly interesting for studies on frescoed walls. Other applica-

tions are: 1) survey of cavities, 2) detection of inclusions of different materials, 3)

detection of water and heating systems, and 4) detection of moisture presence. In the

presence of moisture, the camera will find the coldest surface areas where there is

continuous evaporation (Binda et al., 2003c).

In the Syracuse Cathedral, detachments of previous surface repairs were fre-

quently observed during the crack-pattern survey. Thus, a thermocamera was used to

study the extension of the detached areas. Being a superficial test, the technique was

sensitive for detection of cracks up to approximately 4–5 cm, according to the material

characteristics.

Thermovision was mainly applied to estimate the detachment of the several

types of rendering or re-making of the stone surface applied in the past interventions.

In these cases, the reasons for such intervention and which type of material could be

under the covering were unclear. In most cases, the covering seems to be detached

from the support, revealing the poor compatibility between them (Figure 19). This

situation was found in several positions.

3.3.3. Radar Tests Among the techniques and procedures of investigation that have

been proposed in the past years, georadar seems in one regard to be most promising

and in another regard to need a great deal more study and research (Binda et al., 1998,

1999, and 2003a). When applied to masonry, the target of radar investigations can be:

1) to locate the position of large voids and inclusions of different materials, such as

steel and wood; 2) to qualify the state of conservation or damage of the walls; 3) to

define the presence and the level of moisture; and 4) to detect the morphology of the

wall section in multiple-leaf stone and brick masonry structures (Binda et al., 1999,

Figure 18. Syracuse Cathedral (Sicily, Italy): (a) photographs of the ultrasonic test procedure, and sche-

matic illustrations of the results for (b) pillar 31, and (c) pillar 32.

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2003a, and 2005). Georadar seems to be a powerful tool to detect the presence of voids

and structural irregularities, the presence of moisture, and hopefully the presence of

multiple leaves in stone masonry.

The georadar method is based on the propagation of short electromagnetic

impulses, which are transmitted into the building material using a dipole antenna. The

impulses are reflected at interfaces between materials with different dielectric or con-

ductive properties (e.g., at the surface and backside of walls, at detachments, at voids).

When the transmitting and receiving antennas, which are often contained in the same

housing, are moved along the surface of the object under investigation, radar images are

produced as color or gray-scale maps giving the intensity of radar echoes as a function of

antenna position and wave travel time. By measuring the time range between the

emission of the wave and the echo, and knowing the velocity of propagation in the

media, it would be possible to know the depth of the obstacle in the wall. In the real cases,

the velocity is unknown and must be estimated with specific measurements or data

analyses because it changes depending on the material and in the presence of voids.

Furthermore, the velocity is higher in drywalls and lower inwetwalls (Binda et al., 1999).

The interpretation of the radar data involves the identification of significant

anomalies. It should be a recognition process detecting features on the records, which

should be characteristic of known signatures. Identifiable features on a radar record

are continuous reflections from layers or cracks or diffractions from discontinuities

such as voids and local inhomogeneities in the masonry. Radar tests need always a

preliminary calibration to verify if the emitted signal is powerful enough to penetrate

the material for the expected thickness. The radar signal is strongly attenuated,

especially at higher frequencies, in poor dielectric materials (as moist masonries),

and countermeasures such as selecting a lower-frequency antenna must be taken to

preserve the desired penetration (Padaratz and Forde, 1995). When the penetration is

enough to detect the opposite side of the wall, the average radar velocity through the

masonry can be estimated and it is possible to calibrate the relationship between the

time and space scales.

Georadar was used for the Syracuse Cathedral to investigate the masonry

morphology beyond the covering and to control the presence of internal defects of

Figure 19. Schematic illustrations and photographs of thermovision carried out on (a) pillar 13 and (b) pillar

29 in Syracuse Cathedral (Sicily, Italy). Higher temperatures (shown in white and light grey) observed after

artificial heating are associated with covering detachment problems.

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the pillars. The internal crack distribution was studied with high-frequency (2 GHz),

three-dimensional (3D) radar surveys for some important cracks that ran parallel and

close to one of the pillar sides. The high-frequency antenna was selected to ensure the

best resolution needed to produce accurate images of internal cracks. Preliminary tests

proved that the selected frequency was appropriate to penetrate approximately 0.5 m

in these pillars so that any internal target was potentially detectable from the nearest

side. This favorable situation was exploited to produce very effective 3D reconstruc-

tions of the internal geometry of these cracks. In addition, some new alarms were given

by these radar investigations since new internal cracks were found that showed no

evidence on the pillar surfaces.

As an example, Figure 20 clearly shows a 3D representation through different

sections of a defect in the second part of the profiles. The defect appears as a crack 40–

45 cm long located internally approximately 20 cm from the surface of the pillar that

was investigated with a dense grid of parallel horizontal radar profiles. The crack is

approximately parallel to the investigation surface with a slightly changing slope. The

defect is readable in almost all the profiles, except for the most external one.

The high-frequency antenna 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 some cases, the radar investigations detected the

presence of regularly spaced diffractions (approximately every 5 to 6 cm, as shown in

Figure 21) at a depth of approximately 3–4 cm (i.e., just behind the plaster). A direct

inspection in one of these areas (Figure 21) proved that the unexpected brickwork used

to substitute some stones during past repairs, probably in the 1920s, produced the

diffractions. The discovery of this unusual feature suggested checking other pillars

with the same type of thick plaster. Figure 22 shows the situation found on a side of

pillar number 18 after the removal of the thick plaster.

3.3.4. Monitoring of Cracks The experimental investigation showed a very com-

plex situation. Monitoring of cracks was then set up to assess possible risks and to

Figure 20. Pillar 17 in Syracuse Cathedral (Sicily, Italy): (a) photograph of the radar acquistion for the

three-dimensional elaboration carried out at the pillar base, and (b) radar image of an internal crack

(highlighted) approximately 20 cm from the investigation surface.

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understand the mechanical behavior of the structures before planning an intervention.

To evaluate the differential movement of the Syracuse Cathedral, the most serious

cracks have been monitored since July 2005. Figure 23 shows the position of the

measurement bases on different pillars. The displacements are measured by removable

extensometer every month. Monitoring is executed using simple low-cost instrumen-

tation but requires regular intervention for reading the data. It is important to stress

that the monitoring should be continued for 4 to 5 years (at least 18 months being the

minimum significant period) to calculate the influence of the temperature variation.

Figure 24 shows the localization of some monitored cracks.

3.4. Discussion of the Results and Conclusions

This research highlights the problems of the pillars of the Syracuse Cathedral

and shows that the complementarity of the tests allows a deep knowledge of materials,

structures, and special features of the masonry. The investigation could contribute,

even partially, to ‘‘flesh out’’ the story of the building evolution. The visual inspection

allowed the localization of the most damaged area, suggesting the need for further

Figure 21. Pillar 19 in Syracuse Cathedral (Sicily, Italy): (a) photograph of radar technique; (b) radar image

shows areas characterized by a near-surface undulating response generated by regularly spaced diffractions

(approximately every 5–6 cm) at a depth of approximately 3–4 cm; and (c) photograph, the inspection

proved that these pillars were sometimes repaired with bricks as shown.

Figure 22. Photographic views (a)–(c) of pillar 18 in Syracuse Cathedral (Sicily, Italy).

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control. The typical vertical cracks show the presence of high stresses, probably caused

both by constant loads and by the earthquakes effects. The crack depths, investigated

by the ultrasonic tests, demonstrate some serious damage.

The removal of the Baroque decoration in the 1920s probably changed the

geometry of the pillars, reducing the section and then increasing the effects of the

vertical actions. In fact, many stone blocks in the pillar basement show vertical cracks

around the corners. Mapping of the repairs places the most damaged area, toward the

altar, close to the lateral entrance, and toward the facade. In many cases, the inter-

ventions were not carried out organically and appear to be without any documenta-

tion. Only a detailed analysis of the building and of the building techniques, evaluating

the main damage reasons and the survey of the state of preservation, could give an

answer to some questions.

Figure 24. Photographs of examples of monitored cracks in (a) pillar 19, and (b)–(c) pillar 26 in Syracuse

Cathedral (Sicily, Italy).

Figure 23. Plan of the localization of the monitoring for Syracuse Cathedral (Sicily, Italy).

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The investigation carried out on pillars 13, 14, 15, 24, 25, and 26 shows damage

and internal discontinuity that does not appear alarming. These pillars support two

couples of arches, built by shaped blocks; due to the several collapses of the facade

they were probably reconstructed. Pillar 26 is one of the most damaged, with a very

low adhesion of the surface layer. The radar investigation shows the presence of metal

elements used to join several cracks at the base. Some cracks are extended inside the

section.

In pillars 18 and 19, toward the altar, damage could be observed. It is important

to stress that the effects of the 1542 earthquake, which produced a great deformation

of the perimeter wall close to the pillars. This finding could justify the bad state of

preservation of these two pillars, characterized by the presence of detached covers and

deep cracks on all the prospects. Despite the several interventions, the section is

reduced. The cracks in these pillars also show a larger movement than elsewhere.

They also have been subjected to the substitution of some stonework by brickwork,

creating areas with different stiffness.

Pillars 29 and 30 have serious cracks, surface cover, and reconstruction in brick

and stone, with a low adhesion to the support. The detachments are not only between

the covering and the support but also in the stone, due to cracks and to the presence of

the expulsive parts. Metal elements, probably ties, demonstrate the tentative of con-

fining the pillar.

Toward the altar, pillars 20, 21, 31, and 32 support two arches higher and with a

larger span than the other arches. The masonry texture is hardly readable due to the

presence of a surface covering. Apparently, the masonry seems built by smaller units.

The radar investigation has confirmed the difference in the morphology.

The first results of monitoring reported in Figure 25 show a continuous move-

ment in the chapel wall and in pillars 19 and 18. It must be noted that the temperature

Figure 25. Graph of the results of pillar crack monitoring in Syracuse Cathedral (Sicily, Italy); pillars are

numbered in the box on the right.

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varied from 15–31�C. The continuous movement measured suggests that help with a

provisional structure is necessary as soon as possible for the most damaged pillars.

4. CONCLUSIONS

The results of the investigation allow some remarks on the application of NDT:

� The solution of difficult problems, such as internal decay of materials, presence of

voids, inclusions, and humidity, cannot be found by a single NDT, but only by a

complementary application of different techniques.

� The different techniques were successfully applied because each technique was

carefully chosen to solve a specific problem (e.g., ultrasonic tests for depth of cracks,

thermovision for detachment of external parts).

� The structural engineer should be familiar with the use of the techniques, but also

know deeply the problems to be solved.

� Results of the investigation show that the state of damage on some pillars (numbers

18 and 19) is progressing rather quickly and some decision must be made to avoid

future failure. On the most damaged parts, acoustic emission tests are being carried

out to detect the rate of progression and some provisional confinements of pillar 18

and 19 are being provided. Meanwhile, static and dynamic analyses are being

carried out to check the overall safety of the cathedral.

ACKNOWLEDGMENTS

Authors wish to thank L. Cantini, P. Condoleo,M. Cucchi, andD.N. Khanh for

their contribution in the experimental work in-situ and the students N. Giovannini,

M. Marangio, A. Pavesi, andM.Mariotti. Special thanks for their collaboration goes

to the Arch. M. Muti, to the Eng. R. Meloni and to the Arch. L. Regalbuto of the

Cultural Heritage Syracuse Superintendence. Radar investigations were performed

with a high-frequency antenna kindly provided by IDS S.p.A. The experimental

research was supported by the Soprintendenza ai Beni culturali ed Ambientali of

Syracuse (Cultural Heritage Syracuse Superintendence).

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