the thin concrete vault of the paraboloide of casale, italy. innovative methodologies for the...
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THE THIN CONCRETE VAULT OF THE “PARABOLOIDE OF CASALE
(ITALY)”: INNOVATIVE METHODOLOGIES FOR THE SURVEY,
STRUCTURAL ASSESSMENT AND CONSERVATION INTERVENTIONS
Clara Bertolini Cestari,
Antonia Spanò,
Tanja Marzi
Politecnico di Torino,
Dept. Architecture and Design,
Viale Mattioli 39,
10125 Torino, Italy
Stefano Invernizzi
Politecnico di Torino,
Dept. Structural, Geotechnical
and Building Engineering,
Corso Duca degli Abruzzi 24,
10129 Torino, Italy
Chiabrando Filiberto
Politecnico di Torino,
Dept. Environment, Land and
Infrastructure Engineering,
Corso Duca degli Abruzzi 24,
10129 Torino, Italy
KEYWORDS: concrete, vault, assessment, TLS, 3D metric model, Cultural Heritage
ABSTRACT
The former clinker warehouse, also known as the “Paraboloide” of Casale is an architecture symbol of the
concrete Italian industrial heritage. The constructive technology and the structural shape of the thin
concrete vault are comparable to the outstanding realizations of the beginning of the XXth century, as for
examples the works of E. Torroja and of P.L. Nervi. The metric survey has been achieved using high
resolution terrestrial laser scanning technology (TLS), in order to obtain an accurate 3D model with
uniform level of detail and precision, in addition to a continuous metric information for each portions and
elements of the structure. This procedure allowed a proper reading of the structural typology of the
building and the recognition of possible design and construction principles, essential for the conservation
project. The laser survey was carried on both intrados and extrados of the huge parabolic vault,
performing a high number of scan positions, located inside and along the outer limit of the building. The
model processing has been mainly directed to the evaluation and control of geometrical features of the
reinforced concrete membrane. The image texturing of the complex surfaces has been performed by the
Focus 3D Faro integrated camera, with principal axis coaxial with laser ray. Some other photogrammetric
stripes, focused on interesting portions of the structure, enabled to accomplish the detailed inquiries of the
shape model with a materials decay analysis. The structural assessment combined a detailed finite
element analysis. The analysis was performed with the software Diana (TNO, The Netherlands),
accounting for smeared cracking and the presence of reinforcements. Finally, the paper summarizes the
main criteria for interventions, according both to structural safety requirements and conservation
prescriptions, enhancing the extraordinary innovative value of this particular cultural heritage.
INTRODUCTION
The paper deals with the problematic of a possible rehabilitation proposal of a former clinker warehouse,
the “Paraboloide” of Casale, and its reconversion into a ‘cultural pole’.
Given the particular nature of the building in terms of structure, and in combination with the lack of
documentation, it was considered necessary to realize an accurate metric survey. In fact, the analysis of
the building in all of its aspects (documentation, survey, structural behaviour and material deterioration)
has been carried out with in-depth analysis. The metric survey has been achieved using high resolution
terrestrial laser scanning technology (TLS) also known as LiDAR (Laser Imaging Detection and Ranging),
in order to obtain an accurate 3D model with uniform level of detail and precision, in addition to
continuous metric information for each portions and elements of the structure. This procedure allowed to
obtain a detailed mapping of the degradation of the structures. In addition, the adopted approach provided
a proper reading of the structural typology of the building and the recognition of possible design and
construction principles, essential for the conservation project.
It is worth to underline that the feasibility of laser surveying in conservation project is commonly
recognized. In the case of the Paraboloide of Casale, there are additional reasons due to the necessity to
emphasize the form resistant structural shape of the thin walled rib vault, which becomes much less
effective if traditional surveying techniques are adopted.
The geometrical information of the survey, together with the results from the non-destructive
investigation, have been formatted to define a detailed finite element model to perform the structural
assessment. The analyses, which are currently under development, were performed with the software
Diana (TNO, The Netherlands), accounting for smeared cracking and the presence of reinforcements.
In the present paper, a set of general guidelines are recalled, describing the specific methods that should
be carried out in the future by various specialists, in view of an eventual rehabilitation of the building.
THE “PARABOLOIDE” OF CASALE
Industrial buildings for cement production are spread throughout the whole territory of Monferrato, with a
larger concentration around the town of Casale. Some of them still host functions related to the cement
industry, but most of them are abandoned or destined to be demolished. A special case among Casale’s
industrial buildings are the remains of the former Italcementi complex, situated at the borders of the
historic center, very close to the railway station. It is considered today as a monument of the industrial
heritage and a symbol of the town’s development. The initial complex was constructed during the 1930’s
for the company Italcementi of Bergamo. The only part of the complex that still exists today is the
horizontal silo, the so-called “Paraboloide”, while the rest of the buildings were demolished.
The structural system of the “Paraboloide” was an innovative for that period: a structural system
consisting of a reinforced concrete shell with parabolic arches, which create a unified internal space of big
height, without any intermediate floors. The series of beams that runs across the vault and are supported
in the center by a pillar, are only apparently ties. In reality those beams are posterior to the original
conception of the structure and were supposed to sustain a temporary floor that is not in place anymore.
Thanks to its simplicity, the pure and imposing architectural result and the refined of concrete, a new
material for that time, the “Paraboloide” is considered nowadays as a symbol of the industrial architecture,
of lightweight form-resistant structures, of the challenges of great spans covered by light shells,
constituting not only a symbol of the industrial past of Casale, but of a whole season of experimentation
with new materials. The “Paraboloide” of Casale is probably the first example of a building with this
specific typology to be built on the Italian territory. Recently an agreement between Politecnico di Torino,
the municipality of Casale and the Association “Il Cemento nell’identità del Monferrato Casalese”, was
constituted in order to promote the safeguard and the enhancement of the industrial heritage of the area.
The “Anchor Point” of the whole territory of Monferrato Casalese has as main headquarter the Buzzi
Unicem Spa, the only industry of Casale founded in 1907 and still operating at international level.
Fig. 1 External and internal views of the “Paraboloide” of Casale
CONSTRUCTIVE TYPOLOGY AND MATERIAL ANALYSIS
In order to understand the particular nature and the outstanding value of the “Paraboloide” shell, as a
witness of the concrete structures’ evolution history, it is necessary to carry out an analysis in terms of
typology and material. Those two aspects, the structure and the material, are merged together in the
Paraboloide’s materialistic asset. Unfortunately, there is a lack of available information about the actual
building’s physical composition, since the analysis had to be limited to the observations obtained from a
visual inspection.
Therefore, it can be useful to start from some general comments on the structural behaviour of the
parabolic shell and to illustrate the apparent degradation problems of the material. The combined analysis
of those two aspects could lead to some interesting conclusions.
The application of the parabolic form in engineering, and especially in the reinforced concrete structures,
fully interprets the structural possibilities of this material. In fact, reinforced concrete, invented at the
beginning of the XXth century, allowed the engineers to overcome the limits considered till that time
insurmountable by the so-called “heavy” structures (realized in stone or bricks), both in terms of height
and width, and to reach the grace of the metal structures.
The “Paraboloide” of Casale, covers a surface of 1150 m2 with a rectangular plan, 23 m wide and 50 m
long, with an overall height of 12,5 m, including a 4,5 m high open gallery, located on top of the shell
structure.
The vault is just 8 cm thin and is ribbed by transverse parabolic arches and with thin longitudinal beams.
The resulting structural scheme can be defined as a thin ribbed vault. Twelve hoppers, with a shape of
reversed truncated pyramids, were placed at the base of the parabolic arches.
The principal structure of the “Paraboloide” consists of eight reinforced concrete arches, with a
rectangular section and a parabolic profile. On each side of the building, 24 reinforced concrete pillars
with a trapezoidal profile can be found, 8 of which are equipped with internal buttresses – a much
different and more robust profile compared to the rest of the pillars. Those eight pillars are, in fact, the
ones that correspond to the parabolic arches, while the rest of them have a secondary role and they
contribute to the stability and the rigidness of the complex. A sort of rhythm in the continuity of the
pillars can be noticed: between two principal pillars, two secondary pillars are located, and the secondary
ones form the openings of communication with the underground galleries.
As far as the reinforced concrete beams are concerned, at first sight, they seemed to be added in order to
eliminate the thrust of the arches. However, after a careful observation of the structure, it can be assumed
that the beams and the pillars were not part of the original structure mainly because the quality of their
surface is quite different from the rest of the structure’s reinforced concrete surface;
Therefore the pillars in the middle of the beams, necessary for their support, seem to have been added
afterwards. This hypothesis is also confirmed by the necessity of the industrial spaces to have a plant with
the maximum surface available, free-from vertical structural elements.
The technique of reinforced concrete allowed the replacement of the classical barrel vault, conceived as a
series of arches, by the thin cylindrical vault, in which the loads are transmitted by the membrane both to
the transversal planes (planes of the directrices) and to the longitudinal planes (planes of the generatrices).
In such a constructional type, the structural constraining conditions can vary a lot, depending on the
specific functional needs. In this way, beyond the traditional continuous or discontinuous supports along
the generatrices, the vault can result constrained exclusively through rigid arches, or by means of
transversal diaphragms, even notably distant, laid out in the planes of the directrix.
In other words, the necessary and sufficient condition in order to verify a certain stress state, is to have a
great thinness of the spatial arching, that is its ability to resist mainly by shape, just like a “tent“ in its
formal sense.
The “Paraboloide”, a reinforced concrete spatial construction, shows the typical degradation problems
related to this typology of buildings. In order to identify the defects, damage and decay, and their causes,
the investigation, based on a preliminary visual inspection of those degradation problems, was put into
relation with the detailed results from the laser-scanning survey. The available repair options are
illustrated by suggested intervention repair-criteria in the following, integrated with the overall
rehabilitation proposal. It is of course essential to consider the adopted approach as part of a long-term
maintenance strategy for the building.
In general, the most important aspects to be considered for the durability of concrete constructions are:
chemical attack of the concrete; physical attack of the concrete; electrochemical attack of the
reinforcement steel, causing secondary deterioration of the concrete; construction errors; calamities.
Successful repair of a structure starts with a correct condition assessment and identification of the cause
of degradation. All other stages in the repair and protection process depend on these matters. ENV 1504,
part 9 explicitly stresses the importance of these issues and identifies the following key stages:
• assessment of the conditions of the structure;
• identification of the cause of the deterioration;
• deciding the objectives of protection and repair together with the structure owners;
• selection of the appropriate principle(s) of protection and repair;
• selection of methods;
• definition of properties of the products and systems;
• specification of maintenance requirements following protection and repair.
LASER SURVEY
The laser survey applied to the Paraboloide has been referenced within the National Cartographic System
by means of a topographic GNNS (Global Navigation Satellite system) network. In addition to provide an
overall Coordinate System for the survey, the topographic network allows to control the error propagation
and, at the same time, to provide a cartographic reference.
In order to guarantee a complete three dimensional covering of the structure, the plan of scan acquisition
must verify sufficient overlapping of the point acquisition clouds, as well as the resolution and precision
of the scanner which was the FaroFocus 3D (www.faro.com).
The acquisition of the internal shape was carried out with the scanner placed on the ground (7 scan
positions). The intradox scanning allowed not only to obtain the exact shape of the vault and ribs, but also
the mapping of the regions affected by spalling of the concrete.
The external shape of the building was acquired with a set of measuring observations from the ground, all
around the building (11 scan positions), and exploiting an elevator (at an height ranging between 3.8 and
5.9 m, corresponding to the height of the lateral roofing) to better acquire the above part of the vault and
the shelters. (12 scan positions)
A number of reflective targets placed on the building and measured by topographical survey, helped the
alignment of different point clouds to be more precise, in fact we obtained average residuals in each scan
registration minor than 1 centimeter.
Fig 2. Plan of the Paraboloide and its generation from a projected view of the points cloud.
Fig 3. Vertical section drawing of the Paraboloide and its generation from a projected view of the points
cloud.
Fig 4. 3D model of the three central modular spans of the Paraboloide
The overall points cloud database have been cleaned from spurious acquisitions and automatic processes
have been used to provide noise reduction. The initial density of points is reduced to avoid an excessive
calculation demand and to limit the elaboration time. The 3D continuous surface model generation and
managing, together with the subsequent extraction of profile sections have been carried out by the
modeling suite 3DReshaper, by Technodigit. (www.technodigit.com)
The resulting mesh must be carefully tuned: from one side it is necessary to retain the useful details while,
on the other side, a mesh too refined can be useless.
The observation of graphical metric results, both bi-dimensional and three-dimensional, enable a first
assessment of the structure health conditions: there are no subsidence or anomalies of the large shape
vault. The chance to analyze with an high level of detail the consecutive section profiles (the parabolic
directrixes translated along the longitudinal axis of the building), prove that the actual structure is close-
fitting the original shape.
Furthermore, the 3D model managing enables to know in detail the dimensional substance and the spatial
order of each constructive element of the structure, so a schematic model illustrating the principal and
secondary structural elements has been reconstructed with an high level of adherence to the real
constructive arrangement. Surely this kind of model allow to start the structural analyses with the chance
to assess the elements static behavior in a deeper way than earlier.
Fig. 5. A 3D view of the section profiles generated from a sample portion of the structure (equidistance
50 cm) and a front view of the meshed 3D model depicting the pure geometrical shape of cross arches.
Fig. 6. The section planes can be placed in
the exact position and spatial asset in order
to point out and measure the structural
elements dimensions and three-
dimensional arrangement.
Fig. 7. Several study models of the main
structural elements derived from the laser
model.
THE MODEL 3D TEXTURING AND SURFACE DETERIORATION ANALYSIS
The technique for the texture application, adopted by several modeling software instead of
photogrammetric approach, provides to carry out the images projection by means of several
corresponding control points adjusted in the images and in the model. The spatial position of the centre of
images projection is well estimated and the metric results are quite close to the rigorous photogrammetric
process.
Therefore it is possible to start a metric
analysis of the extension of concrete decay.
Three central modular spans of the
structure intrados have been chosen as
sample portion for the whole structure, and
we can consider the percentages of
deteriorated surface as rappresentative for
the entire building.
Four classes of surface decay have been
detected: low level and medium level of
carbonation, surfaces not yet interested by
carbonation, bio growth at the concrete
surface’s intrados.
Figg. 8-9. 3D views of the Paraboloide
textured intrados, without and with the
plotted surface concrete deterioration areas.
deterioration typologies Areas (m2) percentages
water staining and bio growth 232.53 40%
low level of carbonation 17.49 4%
medium level of carbonation 43.98 9%
surfaces not yet interested by carbonation 195.69 37%
Fig. 10. Planar view of concrete deterioration detection and conputation of total surfaces interested by
different type of material decay.
CRITERIA FOR ENHANCEMENT INTERVENTIONS
As already mentioned, the survey of the building’s current condition had to be limited at a visual
inspection that led to the aforementioned observations: both directly in situ and by means of textured
model. The deterioration pattern of the “Paraboloide’s” concrete face includes:
• extensive carbonation throughout the structure’s surface;
• extensive spalling especially at the arches, but also in other areas;
• larger and micro-cracks;
• water staining;
• efflorescences.
One simplified assumption on the deterioration mechanism of the material (except for the carbonation and
the subsequent spalling), could be as follows: the surface of the concrete’s extrados developed micro-
fissures due to creep and/or freeze-thaw cycle, which grew bigger and deeper over time; those cracks
subsequently led to rain water penetration at the interior of the “Paraboloide”, which created the stains,
efflorescences and bio growth at the concrete surface’s intrados.
Another problem, which should be taken into account in this case, is the uncleaned gutter in the north-
western facade; some considerable vegetation growth has occurred inside the gutter, and the plants’ roots
have most likely penetrated the adjacent concrete. The growth of the roots inside the concrete could cause
some severe cracking and spalling of the concrete.
Since this is a historical listed building, maximum attention should be paid during the condition survey
and emphasis should be given on non-destructive techniques. A more detailed schedule to be carried out
in the future, in view of the building’s rehabilitation, could be as follows:
• Sampling: the sampling different locations of the building should be carefully designed, so that core
samples are representative of the area of interest, while ensuring that they are useful, but that the visual
impact is minimised. A series of laboratory tests should be carried out on those samples, including
petrographic analysis (the cores’ diameter should be at least twice the maximum aggregate diameter) for
examining the silica gel formation indicative of AAR, and to look at consolidation, pore sizes and
distribution, freeze–thaw damage, cement content, aggregate types and any other suspected problems.
Core samples can also be sliced and crushed for chloride analysis to provide a profile with depth, as well
as to measure the concrete resistivity as long as the moisture is sealed in on removal and the core is not
excessively wetted during the coring procedure;
• Depth of carbonation: i.e. the location of the carbonation front and its proximity to the bars should be
measured. The measurement can be carried out using a phenolphthalein solution, either on site or in a test
laboratory on cores taken from the site;
• Chloride analysis: representative drillings of concrete, taken to different depths, e.g. in 10
mm increments, should be taken to the laboratory for chloride content analysis in order to determine the
profile of the chloride in the concrete. In the laboratory, those drillings should be dissolved in acid and
then titrated so as to measure the chloride content, or analysed using a chloride specific ion electrode;
• Radar, thermography, radiography and ultrasonics: these techniques are used to look into the concrete
non-destructively. They can find cracks, voids, prestressing tendons, reinforcement and other
embedments. Given their non-destructive character and the reliability of their results, they should
absolutely be implemented in the case of the “Paraboloide”; sampling, however, should still be locally
carried out and the two methods should be combined.
Furthermore, given that radiography can be hazardous for human health, methods such as thermography
or ultrasonics should be opted;
• Resistivity measurement: resistivity, an indicator of concrete quality and moisture content, can be
measured with a four-probe Wenner array on-site, or can be taken from a core, as described above.
Resistivity can be used to gauge corrosion susceptibility;
• Corrosion survey and measurement: the technique of half-cell survey can reveal the location and
magnitude of the anodes and cathodes, plotted with a simple device called a half-cell. This method only
shows the more anodic areas and indicates the corrosion risk. For measuring the corrosion rate of steel in
concrete, instead, the technique of linear polarisation could be implemented on areas identified as
interesting by the visual and half-cell survey.
• Corrosion and defect monitoring: the corrosion of concrete and reinforcement, as well as the progress of
deterioration should be monitored. Given that the building results already in an at least moderate state of
conservation, it is essential that at least its corrosion and deterioration rate are documented, if an appraisal
and repair program is not immediately launched. Furthermore, those monitoring processes could be
incorporated in a long-term repair/maintenance strategy as an alternative approach to a one-off repair.
CONCLUSION
The ongoing research activity carried out on the Paraboloide has been justified by the complexity of the
intervention on a historical heritage. For this reason a multi-disciplinary research group was established,
involving technological, historical, structural and geomatic competences. It was therefore possible to set
up a framework for future conservation and refurbishment interventions.
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ACKNOWLEDGEMENTS
A special acknowledgment goes to Dr. Consolata Buzzi, President of the Association “Il Cemento
nell’identità del Monferrato Casalese”, and to the technical staff of Buzzi Unicem Spa of Casale
Monferrato, for the fruitful help in every research activity.
Furthermore the students Lida Kitsaki and Carlotta Bagna are kindly acknowledged for their work, and
Paolo Abellone for modelling expert advice.