Retrofit and monitoring of an historical building using “Smart” CFRP with embedded fibre optic Brillouin sensors

Construction

Construction and Building Materials 19 (2005) 525–535

and Building

MATERIALSwww.elsevier.com/locate/conbuildmat

Retrofit and monitoring of an historical building using ‘‘Smart’’CFRP with embedded fibre optic Brillouin sensors

Filippo Bastianini a, Marco Corradi b,*, Antonio Borri b, Angelo di Tommaso a

a Department of Construction, University of Architecture of Venice, S. Croce 191, Venice 30135, Italyb Department of Civil and Environmental Engineering, University of Perugia, Via Duranti 93, Perugia 06125, Italy

Received 25 March 2004; received in revised form 24 October 2004; accepted 25 January 2005

Abstract

This paper presents the results of a real-scale experimental work regarding innovative seismic retrofitting technique for masonry

walls and vaults by epoxy-bonded composite strengthenings. Palazzo Elmi-Pandolfi in Foligno (Italy), an historical building dated

1600 that was seriously damaged in the earthquake of 1997, has been repaired and retrofitted including carbon FRP (CFRP)

strengthenings, whose effectiveness has been evaluated through dynamic and static tests.

Brillouin technology is an ideal complement for similar retrofit applications, since the low cost of the sensor makes monitoring all

the critical areas rather affordable, while the distributed sensing feature allow to detect anomalies in load transfer between FRP and

substrate and the location of eventual cracking patterns. Furthermore, Brillouin sensitivity to both strain and temperature can be

used also to ensure that the glass transition temperature of the composite matrix is never approached. In this work, preliminary tests

were performed in order to assess Brillouin monitoring effectiveness in real applications for strain monitorage and crack detection.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Masonry; FRP materials; Testing; Monitorage; FOS; Brillouin

1. Introduction

Fibre reinforced polymers (FRP) obtained saturating

carbon, glass or aramid fibres with polymeric matrixes

are characterized by a variety of advantages over otherstructural materials, such as lower density, high stiffness

and strength, adjustable mechanical properties, resis-

tance to corrosion, solvents and chemicals, flexible man-

ufacturing and fast application. At present composite

diffusion in civil engineering is increasing for repairing,

upgrading and seismic retrofit of bridges [1,2], buildings

[3,4] and other infrastructures [5]. Manufacturing de-

fects such as diffused small bubbles (micro-bubbles),voids of bigger dimension (macro-bubbles), fibre mis-

alignments and improper bonding surface preparation/

0950-0618/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2005.01.004

* Corresponding author. Tel.: +39 74 440 23030; fax: +39 75 585

3897.

E-mail address: [email protected] (M. Corradi).

levelling are quite common in hand lay up applications

[6] and might have considerable influence on strength

properties [7]. A variety of semi-destructive and non-

destructive testing techniques, such as pull-off, shear-

stripping, ultrasonic testing, thermography and acoustictesting have been demonstrated to be effective to assess

the quality of application [8–10]; however, since com-

posite applications in civil engineering are generally

more recent than other FRP applications, several ques-

tions regarding their long term behaviour and durability

remain still unsolved. Monitorage is therefore a topic of

interest.

2. Seismic retrofitting intervention

Elmi-Pandolfi building (Fig. 1) is a clustered complexthat takes almost half of a single-standing block in the

Fig. 1. Geometric survey of Elmi-Pandolfi building. CFRP strengthenings have been used for the drawing-room vault (upper left) and of the external

bearing masonry walls (left, bottom and right periphery).

526 F. Bastianini et al. / Construction and Building Materials 19 (2005) 525–535

historical centre of Foligno (Italy). The building, that in-

cludes various different structural nuclei affected by

changes and modifications during centuries, gained al-

most stable configuration around the XVII century asa noble house provided with representative halls, resi-

dential quarters for the owner�s family and for the

housekeepers, and several rooms dedicated to store-

houses and commercial activities. The building was seri-

ously damaged by the earthquake of 1997 and many of

its structures were repaired including some carbon FRP

(CFRP) strengthenings. The retrofit design is described

in detail in [11].The peripheral masonry facing Agostini street and

Rutili street (respectively left and bottom of Fig. 1)

was strengthened with three horizontal belts of CFRP

ribbon placed at different heights (Fig. 2). On the side

facing Agostini street some vertical sheets were included

too. While CFRP reinforcement gives to the masonry

Fig. 2. One horizontal CFRP belt with Brillouin sensing fibres placed

on the external surface of the bearing masonry wall.

structure the ability to bear substantial tensile stress, it

is not capable of avoiding cracking. This type of rein-

forcement was designed in order to prevent out-of-plane

collapse mechanisms as illustrated in Fig. 3. An out-of-plane mechanism of a peripheral masonry wall is often

induced by a lack of connection between the different

walls and it is typically present in historical buildings.

With regard to a single wall (Fig. 4), the force induced

in the CFRP material may be calculated:

F ¼ P2� cR �

bh

� �ð1Þ

where cR is the collapse coefficient; P, b and h are respec-

tively the weight, the height and the thickness of the

wall. If a vertical force N is added, Eq. (1) becomes:

Fig. 3. Typical out-of-plane collapse mechanism due to a lack of

connection between masonry walls.

cP

Ph

b

A

F

Fig. 4. Design concept of a single masonry walls reinforced with

CFRP horizontal belts.

Fig. 6. The masonry vault and reinforcement arches of the drawing-

room dated 1600 during surface preparation for CFRP application.

F. Bastianini et al. / Construction and Building Materials 19 (2005) 525–535 527

F ¼ P2� cR �

bh

� �þ N � cR �

di

h

� �ð2Þ

where di is the lever arm of the vertical force N.

The vault of the drawing-room was reinforced with

CFRP sheets at the extrados intended to lock-out some

of the most probable failure mechanisms, and was

dynamically tested before and after reinforcement appli-

cation. Cracking represents the basic failure phenome-non of an unreinforced vault as well as a reinforced

one. Before cracking occurs, the unreinforced vault car-

ries the load thanks to the masonry tensile strength.

Once cracked, the vault changes its behaviour to that

of a system of arches that, however, carries the load

without any substantial tension stress, i.e., in a safe con-

dition. Even if various types of failures associated with a

vault structure are possible, the most common one isconsequence of a mechanism that undergoes the forma-

tion of cylindrical hinges. The application of CFRP

strengthening sheets at the extrados of the arch is able

to prevent cylindrical hinges formation and it can ham-

per many collapse mechanisms.

The length of the solid-brick masonry vault is

10.3 m, width is 7.4 m, while its average thickness is

Fig. 5. Geometric survey of vault extrados and reinforcement arches before

and hollow-brick wall position (right).

12 cm. The vault rehabilitation started with removal

of the filling material up to the haunches, where the so-

lid clay bricks of the arched lintel are inserted into the

outer wall. At the vault extrados six reinforcement so-

lid-brick arches have been found during filling material

removal (Fig. 5). After surface cleaning by sanding andwater based solvents (Fig. 6) and then levelling the sur-

face of the outer vault area, bedding bands were created

using suitable epoxy putty. Before putty application,

surface was prepared with a suitable epoxy primer. A

first layer of CFRP was laid with epoxy-resin over the

bedding bands. An accurate surface preparation was

necessary considering that CFRP sheet is very sensitive

to local effects connected with the irregularity of the lay-ing surface. It should be noted that, despite careful

preparation, areas with abrupt variations in curvature

may occur. In these cases experimental tests showed

high degree of weakness of thin CFRP sheets. After

the CFRP sheets have been laid, a small amount of

epoxy putty was cast over each lintel, onto which a steel

plate fitted with a steel wedge were placed (Figs. 7 and

8). The latter was designed to house four anchoringrods, inserted diagonally, long enough to reach the

height of the springer. Sheets of mono-directional car-

bon fabric, 200 mm wide, bonded at the vaults extrados

where used for all strengthenings. The fabric was

CFRP application (left) and schematic representation of CFRP sheet

Fig. 7. Detail of the connection between CFRP sheet (glued to the

vault) and peripheral wall of drawing room.

Fig. 8. Detail of the connection between CFRP sheet (glued to the

hollow brick wall) and peripheral wall of drawing room.

Fig. 9. Hollow-brick walls under construction (CFRP materials were

applied between pre-existing reinforcement arches and hollow brick

new walls).

Fig. 10. A possible collapse mechanism for a masonry vault: the filling

material over the vault only stabilizes the structure.

Fig. 11. The substitution of the filling material with hollow brick walls

produces a benefit in terms of dead load and of vault strength.

528 F. Bastianini et al. / Construction and Building Materials 19 (2005) 525–535

impregnated by an appropriate epoxy resin furnished by

the fibre producer (producer: Mapei, product denomi-

nation: Mapewrap C UNI-AX 300/20) (Table 1).

The work was completed traditionally with the con-struction of hollow-brick walls (Fig. 9), arranged at a

distance apart equivalent to that of the overlaying hol-

low floor slab (�80–150 cm). The hollow brick walls

were constructed over CFRP sheets at vault extrados.

The substitution of filling material with hollow brick

walls has positive effects thanks to the dead load de-

crease. When the connection between masonry vault

and hollow brick walls is effective, their constructionalso prevents the formation of cylindrical hinges at vault

extrados and it causes high increases of inducing mech-

anism activation loads. A further CFRP reinforcement

was applied over the hollow-brick walls (Figs. 10–12).

Table 1

Mechanical properties of the carbon fibre used

Fibre type Carbon, mono-directional

Superficial density (kg m�2) 0.300

Equivalent thickness (mm) 0.167

Tensile Strength (MPa) 4800

Tensile Young�s modulus (MPa) 230,000

Elongation at failure (%) 2.1Fig. 12. The application of CFRP materials and hollow brick walls

can prevent the formation of some collapse mechanisms.

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

1 2 3 4 5 6 7 8

Frequency (Hz)

Dis

plac

emen

ts (

m)

Un-reinforced vault

CFRP reinf.

CFRP+walls

Fig. 14. Displacements vs. frequency recorded at target point 4.

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

1 2 3 4 5 6 7 8

Frequency (Hz)

Dis

plac

emen

ts (

m)

Un-reinforced vault

CFRP reinf.

CFRP+walls

Fig. 15. Displacements vs. frequency recorded at target point 3.

4.E-05

5.E-05

6.E-05

7.E-05

8.E-05

9.E-05

1.E-04

plac

emen

ts (

m)

Un-reinforced vault

CFRP reinf.

CFRP+walls

F. Bastianini et al. / Construction and Building Materials 19 (2005) 525–535 529

3. Dynamic testing

The masonry vault was tested imposing an horizontal

dynamic load located at vault abutment. The tests al-

lowed to study the dynamic behaviour under different

configurations: un-strengthened vault, CFRP strength-ened vault, CFRP and hollow-brick wall strengthened

vault. A vibrodyne was used for harmonic load applica-

tion, sweeping frequency 1.0 and 8.00 Hz in 0.42 Hz

increments, and sweeping the load force as well accord-

ing to Eq. (3), where Fmax is maximum load induced by

the vibrodyne (declared by the manufacturer) and x is

the angular frequency.

F ðtÞ ¼ F max sinðxtÞ ð3Þwhere Fmax is maximum load induced by the vibrodyne

and x is the angular frequency,

F max ¼ kx2 ð4Þwhere k = 4.644 kg s�2 is a characteristic value of the

used vibrodyne.

A dynamic analysis was also carried out in order to

find natural frequencies and modal deformed shapes

for the vault, but it resulted to be ineffective. In fact,comparison between testing and analysis showed a very

high error. This can be explained considering that the

real unreinforced vault was seriously damaged with

cracks, defects and its behaviour cannot be considered

as monolithic and elastic.

Acceleration data were recorded using piezoelectric

accelerometers both along tangential and normal direc-

tions with respect to the vault surface. Displacementswere measured using a LASER vibrometer pointing at

different target positions at vault abutment, haunch

and crown (Fig. 13).

A first analysis has to be done considering the maxi-

mum displacement measured at the target points, as a

function of the load frequency. In Figs. 14–16, positive

displacements are plotted comparing un-strengthened

1

432

Fig. 13. Survey of vault intrados with vibrodyne position (1) anchored

to the external wall, and LASER vibrometer target points (2, 3, 4).

0.E+00

1.E-05

2.E-05

3.E-05

1 2 3 4 5 6 7 8

Frequency (Hz)

Dis

Fig. 16. Displacements vs. frequency recorded at target point 2.

and strengthened vault, while negative displacements

showed the same structural behaviour and therefore

have been omitted. Tests carried out on the un-rein-

forced vault were executed after removing the filling

material from vault extrados. Then CFRP was applied

to vault extrados and the displacement peak amplitude

decreased compared to the ones measured for

unstrengthened vault, while their resonance frequencyremained almost constant. After the construction of

Table 2

Maximum displacements at laser vibrometer target points

Max displacement (mm)

Point 2 Point 3 Point 4

Before reinforcement 0.0930 0.0250 0.0240

After CFRP reinforcement 0.0087 0.0140 0.0280

After CFRP and hollow-brick

wall reinforcement

0.0067 0.0055 0.0063

530 F. Bastianini et al. / Construction and Building Materials 19 (2005) 525–535

low hollow-brick walls over the CFRP strengthened

vault a further strengthening effect was detected in terms

of a very high decrease of displacement peak amplitude

(Table 2) and of a notable increment of its resonance fre-

quency, that are both to be clearly considered as an in-

dex of the effectiveness of reinforcement in terms of

stiffening and strengthening. However a serious crack

pattern was present in the masonry vault before rein-forcement (Fig. 13). This crack pattern was located be-

tween target points 3 and 4 and its presence may

explain the reason because maximum displacements

after reinforcement measured at target point 4 were

higher compared to maximum displacements before

reinforcement. In fact, the application of reinforcement

caused the restoration of continuity between the two

parts of masonry vault divided by the crack.

Fig. 18. Detail of an inspection box of the cable raceway where is

placed temperature sensing fibre and of the strain sensing fibre bonded

to CFRP.

4. Discrete fibre optic sensors and distributed Brillouin

system

Fibre optic sensors (FOS) are often preferred for

embedding in FRP materials due to a variety of advan-

tages that include longer durability in harsh environ-ments, no electro-magnetic interference (EMI), higher

material compatibility and smaller dimension. Fabry–

Perot cavities [12], Bragg gratings [13], fibre interferom-

eters [14] and other FOS technologies have been used for

strain and temperature sensing of composite members

both in laboratory tests and in monitorage applications.

Fig. 17. Optical fibre for Brillouin sensing embedded into CFRP

strengthenings on the external masonry.

The most part of the mentioned FOS, with the excep-

tion of fibre interferometers, are characterized by small

sensitive areas (usually limited to some mm) and re-

quires complex wiring or multiplexing systems for multi-

ple point sensitivity. Fibre-optic interferometers have

gage lengths that can range up to some tenths of meters,but they are capable to give only an integral displace-

ment information over the whole length, and cannot

therefore be considered as distributed systems.

Brillouin distributed strain sensing is substantially

different from other FOS technologies being based on

an optical effect that spontaneously arises during light

propagation in common telecom optical fibres. Brill-

ouin scattering is a non-linear process that affects afraction of the incident ‘‘monochromatic’’ photons that

are travelling along the fibre, frequency shifting them

trough inelastic collisions in which the photon energy

Fig. 19. Smart CFRP sheet placed on vault.

F. Bastianini et al. / Construction and Building Materials 19 (2005) 525–535 531

difference is spent in the production/annihilation of

phonons of acoustic vibration. Since the phonon en-

ergy is influenced by the mechanical strain and by

the medium density, which varies with temperature,

the relation between Brillouin shift and fibre strain

(or temperature) can be used for sensing purposes[15]. Brillouin optical time-domain reflectometers

(BOTDR) can provide distributed sensing combining

time domain analysis of light pulse propagation with

spectrum analysis of Brillouin scattered photons.

BOTDR have been experimentally applied for large

structure temperature monitorage [16], tunnel damage

sensing, ground landslides monitoring and under-

ground water level sensing [17], and other similar appli-cations. In comparison with other FOS technologies,

Brillouin technique seems characterized by lower sensi-

tivity, repeatability and accuracy, but the advantage of

a real distributed sensing can easily overcome these de-

fects when a huge amount of measurement points can

Fig. 20. Strain distributions on CFRP strengthened ma

Fig. 21. Strain distributions on CFRP strengthened mas

help to understand the whole structure behaviour much

better than a limited number of high precision data, or

when long sensing networks allow the detection of lo-

cal phenomena whose location is impossible to be pre-

dicted a priori.

Recent considerations tend to reconsider and down-scale the importance of most FOS monitorage technolo-

gies, especially taking into account the price/

performance ratio that is usually extremely higher than

those of traditional electronic sensors, and present ten-

dency is to restrict their use only in the applications

where their peculiar advantages are indispensable.

The perspectives are radically different for Brillouin

FOS technology, which is characterized by the followingintrinsic advantages:

– real distributed sensing;

– low cost sensing fibre;

– capability to manage huge length of sensor.

sonry vault loaded with 7 kN at haunch location.

onry vault loaded with 10 kN at haunch location.

Fig. 22. Schematic representation of the tested vault where are

evidenced the smart tape layout, the load applied by the standing

workers and the deformation distribution reconstructed.

Fig. 24. Detailed view of the flat jack embedded into the transversal

masonry panel.

532 F. Bastianini et al. / Construction and Building Materials 19 (2005) 525–535

These peculiarities are easily translated into the pos-

sibility to install the sensor over almost the entire struc-

ture that has to be monitored still keeping competitive

budgets, and retrieving data capable to disclose theoverall structural behaviour. This seems extremely inter-

esting especially in all the situations where the structural

members are inhomogeneous and less known for their

age or nature, such as historical masonries, old struc-

tures and also FRP strengthenings. Among the defects

related to Brillouin technique, it is not possible to ignore

the high equipment cost, and the capability to evaluate

only quasi-static deformation phenomena.

5. Evaluation of the Brillouin fibre-optic monitorage

network

Single mode optical fibre has been applied on most of

vault and masonry strengthenings. On external walls,

the fibre circuit has been arranged in order to have astrain sensing section bonded to the CFRP (Fig. 17)

and a temperature sensing section loosely placed in a

raceway located nearby the bonded fibre (Fig. 18). This

Fig. 23. Detailed schematic for the test

arrangement has been designed in order to provide dis-

tributed thermal compensation data and to assess that

the glass phase transition temperature of the polymeric

matrix is never approached, being the wall surface dark

painted and directly exposed to sunlight heating. Dis-

tributed thermal compensation has instead not been in-cluded in the vault monitoring system, since great

thermal inhomogeneities are not expected in the closed

environment.

Preliminary testing with the scope to evaluate Brill-

ouin effectiveness for strain monitoring have been con-

ducted on the vault using 32 m of a ‘‘smart’’ CFRP

ribbon (Fig. 19) that was bonded on the strengthening

using MapeWrap 31 epoxy. The smart 10 cm wide car-bon/glass tape includes 9 different 9/125 lm Corning sin-

gle mode silica fibres, positioned at groups of three at 1,

5 and 9 cm from tape side edge. Each group has fibres

with different external diameter of the secondary PVA

buffer coating, respectively, 900, 600 and 125 lm, in or-

der to assess the difference in strain sensitivity and the

survival to weaving and installation. After the installa-

tion of the tape, the fibres with same diameter were con-nected in series by fusion splices, obtaining three total

sensing circuits 96 m long. Actually only the 900 lm fi-

circuit on the external masonry.

Fig. 25. Out-of-plane load applications to the external bearing

masonry with a flat jack. Door opening was shored with steel

propping to enhance bucking stiffness.

Fig. 26. Identification of the different sensing areas

F. Bastianini et al. / Construction and Building Materials 19 (2005) 525–535 533

bres survived completely to both weaving and installa-

tion and where used for successive tests.

The ribbon was manufactured on the purpose substi-

tuting the optical fibres to some strands, and optimising

the manufacturing parameters in order to reduce the

optical loss induced by the waviness. A volatile lubricantwas used too, in order to avoid jamming and preserve

fibre coating from scratching. Thanks to the procedure

followed, the optical loss in the embedded fibre has been

contained between 0.09 and 0.27 dB/km.

Sensor interrogation has been performed trough a

well known AQ8603 BOTDR manufactured by Yokog-

awa Ando Corp. (Japan), with a declared strain accuracy

of 40 le and resolution of 1 m within a 2 dB optical loss.It has however to be considered that both sensitivity and

resolution overcome the strict declared specifications

when the strain distribution under evaluation has no

on the analyzer display and on the structure.

Fig. 27. Comparative plot of the strain data measured by the three different fibres.

534 F. Bastianini et al. / Construction and Building Materials 19 (2005) 525–535

steep discontinuities [18] such as in case of flexural phe-

nomena associated with quasi-distributed loads. De-

clared accuracy/resolution are in fact determined for aflat top rectangular window strain distribution.

Since the strain levels obtained with the highest load

considered safe for such structure, taking into account

its age and the damages caused by 1997 earthquake,

was of the same order of magnitude of the declared

accuracy of the BOTDR, a patent pending data process-

ing was developed averaging the local strain data pro-

duced by the three different fibres that run parallelalong the smart strengthening. This allowed a further

enhancement of the signal/noise ratio estimated between

4 and 6 dB, but that is however difficult to compute ex-

actly for the lack of a reference gauge at many locations.

Improvements could be achieved using more parallel fi-

bres, but the issue of the optical loss and of the length

accuracy error limit the maximum achievable advanta-

ges. Following this approach, strain distribution plotswith good correspondence to the expected patterns have

been obtained during a load test of the vault (Figs. 20

and 21), and considering optical fibre layout, strain data

provided by Brillouin system have been used for a qual-

itative vault deformation reconstruction (Fig. 22) that is

in agreement with the expected behaviour. Due to the

severe access difficulties to the vault roof, the vault

was loaded having a number of preventively weightedworkers standing in the designated load area.

A further test was performed to assess the crack detec-

tion capability of the sensing fibre installed on the exter-

nal walls. For the purpose a 15 m long smart CFRP strip

was horizontally bonded on the external masonry wall

facing Rutili street, bridging a small pre-existing vertical

crack (Fig. 2). The ribbon, analogous to the one de-

scribed above, carried only three 9/125 lm single modefibres with secondary coating respectively of 900, 600

and 125 lm that where series-spliced obtaining an ‘‘ac-

tive’’ length of 14 meter for each fibre (Fig. 23).

A 30 cm diameter half-circular flat jack was then in-

serted into a transversal masonry (Figs. 24–26) realizing

a configuration capable to produce a controlled openingof the crack monitored by the smart CFRP.

Brillouin strain distributions were collected at load

steps of 10, 15 and 20 MPa, and the crack deformation

was evaluated through a difference plot between the

absolute strain profile at each load step and the refer-

ence one taken before loading. Residual strains were re-

corded 2 h after the load had been removed completely.

In Fig. 26 the various areas of the sensing circuit areidentified and coupled to the respective strains profile

distributions; it has to be noticed that the crack position

is well detected by all three fibres in correspondence of

the flat jack position.

A direct comparison between the different fibres

embedded in the smart-CFRP tape (Fig. 27) shows how

the higher sensitivity was obtained with the 125 lm bare

fibre, while the smoother response was given from the900 lm fibre; the latter is however less difficult to handle.

6. Conclusions

Strengthening technique based on the application of

CFRP sheets to vault�s extrados in order to lock-out

the most probable failure mechanisms has been demon-strated to be effective through dynamic testing, and it

was noticed that it produces notable reductions on the

displacement peak values but not on the resonance

frequencies.

Optical fibres for Brillouin distributed strain sensing

were demonstrated to be effectively embeddable into

wet lay-up FRP materials maintaining reasonable opti-

cal losses, allowing the simultaneous installation of thecomposite strengthening and of the monitoring sensor

and at the same time simplifying the handling of the

optical fibres.

F. Bastianini et al. / Construction and Building Materials 19 (2005) 525–535 535

The ‘‘smart’’ composite was tested on the field in a

real historical heritage monitoring application. Thanks

to the original multi-fibre data averaging approach,

and furthermore considering that real strain distribu-

tions are usually less severe than those required by the

declared specifications, the effectiveness of Brillouinstrain monitoring has been verified even for reasonably

weak strain levels. Furthermore, the effectiveness of

the ‘‘smart’’ composite for crack opening detection has

been successfully verified.

Acknowledgements

The financial support of ISRIM the Italian Ministry

of University and Scientific Research (MURST) – Cofin

2002, and of the industrial partner Federal Trade S.p.A.

are gratefully acknowledged. Special thanks are due to

Prof. E. Speranzini, Dr. A. Giannantoni, Dr. A. Annun-

ziata (I.S.R.I.M.), Mr. R. Toigo, Mr. M. Toffanin (Fed-

eral Trade S.p.A.) Mr. R. Ali (Ando Europe Ltd.), Mr.

M. Parente, Mr. P. Grati and Mr. W. Toscano (SEALS.p.A.) for their friendly cooperation.

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