Cork in the sector of construction
Renxiang Lu
DECivil, Instituto Superior Técnico, Lisbon University, Lisbon, Portugal
October 2014
Abstract
Portugal is the biggest producer of cork in the world. It is an important material for the economic
development of the country being associated with the wine bottling production. Due to the high
demands of quality control in this sector, a great amount of waste is produced. This waste can be
reused by alternative sectors like the construction’s industry. Cork, as a biological material, has unique
properties in terms of thermal and acoustic insulation which makes it more and more appreciated and
an alternative to traditional materials in the construction. The most used manufactured cork materials
in construction are: the expanded cork agglomerated, the cork agglomerated composite and the
rubbercork. Every company that specializes in cork marketing, apart from focusing on the production
of stoppers, has subholdings that are responsible for the fabrication of their own solutions by
combining these manufactured products with others. Despite the disadvantages of natural materials
and some unknown behaviours, some existing buildings applied with cork and past experiences from
engineers and architectures show that with further research, it can surely revolutionize the era of new
and sustainable buildings.
Keywords: cork, suberin, viscoelasticity, cork agglomerated composite, expanded cork agglomerated,
thermal insulation, acoustic insulation, floor covering, wall covering.
1. Introduction
The cork’s industry in Portugal is an important business for the economic development of the country
which represents about one per cent of national’s GDP, especially with wine bottling market and
exportations. However, facing the strict quality control methods in the fabrication of spotters, the cork
that is accepted only regenerates in a period of nine years. Within this cycle a great amount of waste
(cork with less quality) is produced which can be reused in less demanding areas such as the
construction’s sector. Nowadays, in order to apply this material in the construction, it must be
processed and manufactured properly depending on the type of application.
The aim of this study is to demonstrate why cork can be used in construction, which products and
solutions exist and how it can contribute to the development of civil engineering. This work is divided
in several sections starting with the characterization of properties and explanation of different
behaviours when tested under specific conditions. We describe what cork really is and why cork can
be used in this sector. In the following sections we describe the manufactured products that were
mentioned before, how they are made, how and where they are more suited for application, and which
are the benefits and precautions to consider. In the final part of this work we describe some case
studies that reveal the reality of existing cork buildings and solutions that are applied in practical
situations. So far, despite the unknown behaviours demonstrated by this material, more and more
research has been done, and hopefully, it will succeed for future innovation of the use of this material.
In terms of existing studies, intense research has been performed in areas such as Biology or Organic
Chemistry that details the constitution and different properties of cork. The most acknowledgeable
works are from [Natividade, 1950] and [Gibson and Ashby, 1982]. There is still a lack of studies about
manufactured products, however the main contribution was done by [Gil, 1990-2005] who explained
many relevant aspects. The rest of the information is dispersed in sources form different cork
companies. The lack and dispersion of information were the biggest obstacles to the accomplishment
of this work. We believe, nevertheless, that the goal of collecting and compiling all the spread
information in order to be available for future works was accomplished.
2. General consideration of cork
Cork is a natural material extracted from the cork trees (quercus suber L.). Cork trees are typical from
Mediterranean regions due to its specific weather conditions. Portugal is the largest producer of cork
in the world followed by other countries like Spain, France, Morocco and Algeria. In the national
territory the most cultivated area concentrates on Alentejo and Algarve regions. Normally cork trees
can live about 150 to 200 years and reach in average 10 to 15 metres height (figure 1).
The
cultivation of these trees contributes to the economic development of Portugal as well as the
conservation of nature especially in preventing against desertification [1].
Cork, as a raw-material, is similar to wood but shows a different chemical constitution and
consequently different properties. Its main property is the capacity of insulation that makes it special
and used in wine bottling industry (figure 2). However, not every cork has the power of insulation
required for the stoppers. Cork with the required quality is only regenerated in cork trees in cycles of
nine years. All the rejected material is considered waste and not available for this industry, but they
can be processed and reused in less-demanding (in terms of properties) areas like fashion,
mechanical engineering and especially in civil engineering [2].
Figure 1: Cork tree (quercus suber L.)
[1].
Figure 2: Stoppers made by cork
[S1].
3. The structure of cork
The characterization of cork is done in two ways: the description of its anatomical structure and its
chemical composition. This characterization will help to understand why it is able to be applied in the
construction.
3.1 Anatomical structure
In cork trees there are two layers of coating: the primary layer denominated by epidermis and the
secondary coating called periderm. The periderm holds protective properties and substitutes the
epidermis when it dies. The botanical tissue that is responsible for the generation of cork is named
phellogen. It is a meristematic cell that locates between the epidermis and periderm that grows
inwards or outwards. Cells that develop outwards are termed as phellem which is the cork that is
extracted from the trees.
In this section the main component is suberin, a substance with low
permeability and works as a wall against the penetration of liquid and gases. The portion that develops
inwards is termed phelloderm which has nutrient storage functions. The phellogen develops with
successive growth and degeneration cycles, the new formed layers push the old ones to the periphery.
These become dry, harden and present cracks due to the shear stress motivated by the rapid cellular
increment in this region (figure 3) [1].
Figure 3: Constitution of a cork tree trunk. 1 – Xylem; 2 – Cambium; 3 – Phloem; 4 – Phelloderm; 5 –
Phellogen; 6 – Phellem (cork); 7 – Outer bark [1].
3.2 Chemical composition
The main difference between cork and other cellulosic materials is the presence of suberin in the cell
walls. The constitution of cork depends on its age and external factors but, in general, for the ones
extracted from adult cork trees, the chemical composition is approximately this: 40% suberin, 25%
lignin, 20% polysaccharides, 14% extractives and 1% ashes. The suberin is a component that grants
mostly the capacity of insulation to the material but also some mechanical resistance. Microscopically,
it is compound by a complex mixture of aliphatic monomers (mostly) linked by long and non-branched
chains. The lignin is a substance that works as a support for the structural components and confers
also some resistance against microbiological attacks. If lignin is removed, all the cell wall would
collapse. It has an amorphous structure, and is isotropic and dissolvable in water. The
polysaccharides are composed by carbohydrates and when subjected in hydrolysis reactions, it
originates a great amount of monosaccharides. They can be divided in two groups: cellulose and
hemicellulose. Both of them have energetic storage and structural functions. Finally, the extractives
are non-structural organic substances and appear in a greater amount in corks from the first
extractions. The most important component of the extractives is calcium but they also contain sodium
and magnesium [1].
4. Properties and defects
The most important properties to consider of any material in civil engineering are physical and
mechanical properties, because the material is always subjected to forces and deformations which are
fundamental to quantify in order to guarantee the safety of the building and the conditions services.
However, there are some defects that can occur eventually and must be taken into account as well.
4.1 Physical properties
Cork’s properties have been studied for many years but still, there are unknown behaviours to be
discovered. In this work some these properties will be briefly described in order to better understand
why cork can be applied in construction [1].
Relative density is extremely low (about 0.12 to 0.24) which is an advantage for the reduction of the
structures’ self-weight. Relative density values are the result of the weight of cell wall and plus the
contribution of different gases. The density varies with the part of the tree from which cork is extracted
and with the season when the tree rings are generated [1].
Thermal conductivity is another important property that has to be mentioned. Cork is acknowledged by
its low conductivity (almost insulation) used as stoppers in wine bottling industry in order to prevent the
leakage of the liquid and the smell of wine. In construction it is usually applied as wall of floor
insulation. The reason why cork has insulation properties is due to the fact that the air and other gases
inside the cork’s pores, which have a high conductibility, do not make any contribution during the
energy exchange process. Normally the thermal conductivity of a cellular material is due to several
contributions from different natures: the conduction through the solid part, the conduction and
convection of gases and the radiation between the cell walls. In cork the contribution from the gases is
despised due to the small size of cork pores (less than 1 cm ) and the non-connected porosity
structure that difficults the circulation of the gases’ convection fluxes. Cork can be applied as acoustic
insulation as well. However its workability can only be tested combining other materials in series. In
addition to that it must be processed previously into expanded agglomerated which has opened pores
(with connection) so that the sound scatters in the air [1].
4.2 Mechanical properties
The viscoelasticity of cork is an important mechanical property that must be taken into account,
especially in the fabrication of anti-vibratic insulations. When a load is applied on cork, its behaviour
can be described in two parts: one is the elastic fraction, the deformation is recovered when the load is
removed. The other is the viscid part, the deformation cannot keep up while the variation of the
imposed tension varies due to the friction and relative movements between molecules that prevents
the instant displacement. The second fraction is responsible for the cork’s damping properties [1].
The compression is the most important mechanical property because cork is usually applied as
insulation inside the walls which is compressed by other layers or works as floor covering which has to
support vertical loads. The experiment that is taken to describe this behaviour is the uniaxial test. Due
to the anisotropic properties of this material, the test must be carried out in all the principal directions.
The obtained results can be represented in a chart divided in three regions. The first is the viscoelastic
phase which was explained before. The elastic part recovers immediately, whereas the viscid part
offers a gradual resistance to the applied force. This phenomenon is due to the internal friction of
molecules that prevents the instant recovery. Despite the non-linearity of the curve and heterogenetic
properties of cork, it can be simplified in a Hooke’s law. In the second phase, the curve approximates
to a horizontal level. When the values of load tend to this order of magnitude, the phenomenon of
buckling occurs on cells because they enter in contact with each other. With the enhancement of the
load’s intensity, cells can also collapse and originate a huge displacement. The third and last step is
the densification of cells which is represented by a huge slope on chart. The cells are even more
compressed and decrease in size (figure 4) [1].
Figure 4: Cork’s compression chart. a) The simplified curve which emphasizes the three different
regions. b) Obtained charts by experimental tests of radial (R) and non-radial (NR) compression [1].
In this same chart the values of young modulus can be calculated as well by considering an elasticlinear model. Each cell’ structure has to be also simplified as a hexagonal “honeycomb”. The same
pattern can also be used to the quantification of Poisson coefficient. The obtained values are about 18
to 32 Mpa and 0.1 to 0.2 respectively [1].
4.3 Defects
There are several defects that cork can suffer caused by deficiency of growth or related with external
factors. The existence of big cracks is a frequent problem on cork’s industry. This anomaly is due to
dysfunction of the phellogen when cork trees are in an increasing growth period. Sometimes there are
regions of cork with excessive amount of moisture caused by soil and weather conditions to which the
cork trees are exposed. The liquid in the pores can be removed but the cork can no longer be used in
the wine bottling industry [1].
There are extrinsic factors that have to be taken into account such as insects and funguses that can
infest cork plants stored in warehouses in certain environmental conditions. Insects dig through the
planks and lay the eggs inside the tunnels. The hatched worms fed themselves by eating the cellular
tissues of cork (figure 5). These cork planks can be only used for the fabrication of agglomerates.
Funguses use their hyphae to penetrate inside the cork cell walls so that they can absorb the needed
nutrients to live and expand. They often appear in places with high humidity and lack of solar
exposition. It can be detected by the change of colour of the planks and the releasing of a mold smell
(figure 6) [1].
Figure 5: Cork sample attacked by insects
[1].
Figure 6: Cork sample attacked by
funguses [1].
5. Cork products
According to surveys carried out in 2010, the market of cork products is really important to portuguese
economy reaching about 180 million euros. Only the cork’s waste is manufactured to these products
while the ones with better quality are used in the wine bottling industry. There are essentially two types
of products: the expanded cork agglomerated (figure 7) and the agglomerated cork composite to
which the rubbercork belongs to. The first type is produced based on cork of bad quality and subjected
to extreme conditions of heat and vapour (figure 8). Under these conditions, it provokes an expansion
of each particle which releases an inner resin that aggregates every grain as a whole. The material
possessed and darken colour so it is also termed as black cork agglomerated. This product is usually
used as internal insulation but also exterior covering. The cork agglomerated composite is made by
waste of better quality that is aggregated by applying thermosetting or thermoplastic glue. After the
junction of each particle, it can be pressed on moulds or fabricated in an extrusion process. It can be
applied as wall, floor or ceiling coating [2].
5.1 Applications
Expanded cork agglomerated, also termed as insulation cork board in construction (ICB), is mostly
used as thermal and acoustic insulation not only because of its own insulation properties but also
because of its dimensional stability when facing variations of temperature. It can be applied inside the
wall with an air gap, as an ETICS (external thermal insulation composite system) in pavement and
roofing, as internal insulation or even as an external insulation. The density of the product and the
grain size distribution of the material (granulometry) are important parameters that depend on the
given role. Frequently, for thermal insulation, the density is about 120kg/ m while for acoustic
insulation the value is only about 90kg/m which has more empties. The modulus of elasticity of both
is between 1 to 2MPa [1].
Figure 7: Expanded cork
agglomerated [S1].
Figure 8: Fabrication of expanded cork
agglomerated [S1].
Tests were made to prove the truthfulness of this material by comparing with other traditional solutions
such as expanded polystyrene (EPS) and stone wool (MW) (table 1).
Table 1: Properties comparison of different materials with ICB.
Thermal conductivity
Materials/Units
W/
Density
kg/
ICB
0,038
100
EPS
0,035
20
MW
0,04
70
Table 1 demonstrates that ICB is the heaviest among all which is bad in terms of structure’s selfweight. In terms of thermal conductivity it is better than MW and slightly worse than EPS. However,
the greatest advantage of ICB is its fireproof characteristics which prevents the spread of the flames
whereas EPS burns away instantly (figures 9 and 10).
Figure 9: ICB subjected to fire.
Figure 10: EPS subjected to fire.
Fire seems to be a big threat for biological materials, however cork shows some resistance against it
due to its self-chemical constitution. As explained before, suberin is the main compound of cork’s cell
wall. When the flames reach the cork, the bark protects the inner core and burns out. In the interior
there is a great amount of suberin that insulates the material and prevents the spreading of fire. The
cork’s bark has also a certain percentage of suberin which helps delaying the penetration to inner
structure. In addition to that the smoke and gases that are released during the combustion are not
toxic which is unharmful to human’s health when applied in construction [3].
In terms of acoustic properties ICB is as good as the best traditional insulation materials.
Another application of ICB is its use as an outer covering. It not only grants the capacity of insulation
but can also expose its natural beauty and architectonic authenticity. ICB can also be used as antivibratic insulation as well as the rubbercork. ICB must have a higher density (180kg/m or more) and
be more stiff (modulus of elasticity about 4 to 8MPa), in order to absorb the tensions and, at the same
time, play a protective role [1].
The agglomerated cork composite (figure 11) is mostly used as covering on pavements, walls and
ceilings. For pavements the agglomerate must have certain stiffness in order to resist the impact by
responding with a little deformation. So the density and modulus of elasticity are higher, about 450 to
600kg/ m and 20MPa respectively. For walls and ceilings the agglomerate does not have any
demands, so they are softer and the density and the modulus of elasticity are much lower. The
interaction with the glue that aggregates all the grains is another factor to take into account. Generally
the fabrication is based on materials with an extended granulometry, consequently creating less empty
spaces in order to spend a smaller amount of glue since it is more expansive. Another material that
belongs to this category is the rubbercork (figure 12). It is made by combining cork and rubber so that
is possible to take the advantage of both. The binding of two materials is due to chemical reactions of
vulcanization. It is normally applied as anti-vibratic insulation in industrial pavements [1].
Figure 11: Cork agglomerated composite
[S1].
Figure 12: Rubbercork [S1].
For pavements there are essentially two types: the traditional ones and non-traditional ones. On the
first type the agglomerate work as base layer (can be one or two layers) on which is applied a
finishing surface that can be waxed or varnished (figure 13). This type of coverings is usually sold in
plates or tiles with dimension of 300×300m and 4 to 6mm of thickness. Amongst the non-traditional
types, the most acknowledged is the floating floor. In this solution there is a middle layer of MDF
(medium density fibre) or HDF (high density fibre) between the agglomerated boards which confers
more mechanical resistance to the solution. Before the finishing surface, which grants the protection
against wear, there is another layer that has decorative functions (figure 14). The thickness of the
agglomerate base is 1 to 3mm, the middle layer of MDF or HDF is 6 to 7mm, the intermediate layer of
cork is 2.5 to 3mm and the rest can vary. The dimensions of each plate are 900×300m and they are
usually joints in male-female format [4].
Figure 13: Traditional pavement solution. 1
– Finishing surface 2 – Middle layer of cork;
3 – Base layer of agglomerate [4].
Figure 14: Non-traditional solution.1 – Finishing
surface; 2 – Decorative layer; 3 – Middle layer
of cork; 4 – MDF or HDF; 5 – Base layer of
agglomerate [4].
Relatively to walls and ceilings the cork agglomerated composite is applied not only because of its
good insulation capacities but mainly to exhibit the aesthetics of this solution. . The thicknesses of the
finishing layer and the plates (that are usually glued on the support) as well as the other dimensions
are similar to the pavements’ thicknesses (figures 15 and 16) [2].
Figure 15: Internal wall coating [S2].
Figure 16: Ceiling coating [S2].
In Portugal the biggest seller of these products is undoubtedly the Grupo Amorim and its sub-holdings:
Wicanders for coverings and Amorim Isolamentos for insulations. This company started its activity in
1870 and works not only on the wine bottling industry but also contributes for the investigation of new
cork products. There are more solutions related with construction products developed by Grupo
Amorim, but all of them are based on the original ones described before. Other improvements were
developed such as other kinds of finishing surface or different combination of layers which improves
the mechanical behaviour of the solutions and makes them unique on this market [2].
6. Case studies
The first traces of constructions that used cork or its derivate materials were in Alentejo regions in the
XVII century, due to the existence of a large number of cork tress and scarcity of traditional materials
of construction. In this region the walls of the dwellings were built with clay with incorporation of pieces
of cork that worked as insulation, also called “cork grout”. However, the ceilings were made by wood
(figure 17) [5].
Based on these traces and with the development of cork products, more and more cork buildings have
appeared. In 2000, the Portugal Pavilion in Expo – Hannover was the first building in which expanded
cork agglomerate was applied as outer covering (figure 18). It was designed by two well-known
Portuguese architectures Eduardo Souto Moura and Álvaro Siza Vieira. In the exposition the pavilion
was much conceptualized because it emphasized the cultural and sustainability aspects of the product
at the same time. The pavilion was presented in an “L” shape and covered a total area of 2052m . The
plates of agglomerate were fixed mechanically on a metallic support. Each steel profile had 1cm of
thickness and 17cm of width that covered the entire facade. To improve the adherence between plates,
on the interface of the joints was applied a string of mastic. After the exposition, due to the capacity of
disassembly, the plates were transported and set again in Coimbra where some musical concerts took
place. Since then, many architects and engineers travel to Portugal and come to see this building that
is served as a worthwhile pattern to the development of future cork buildings [6].
Figure 17: “cork grout” dwellings [5].
Figure 18: Portugal Pavilion 2000
Hannover [6].
In terms of internal coating applications there are examples from all over the world that are publicized
on Grupo Amorim internet source which applied their fabricated solutions. One of them is the Cape
Town City Hotel in South Africa. The project was designed by M&B Arquitecture&Interiors company
which is specialist in applying natural and eco-efficient solutions. In the hotel, all the pavements are
adopted by different kinds of Wicanders solutions with different decorations. The total covered area is
about 800m (figure 19) [S2].
Other example is the restoration project of Nezu Museum in Japan. The responsible of this project
was the architect Kengo Kuma who is known by contrasting different styles combining with the
presence of nature. To emphasize the tranquillity of Japanese harmony, the pavement of main hall is
covered Wicanders solutions in an area of 1600m (figure 20) [S2].
Figure 19: Cape Town City Hotel [S2].
Figure 20: Nezu Museum [S2].
7. Conclusion
In this work we concluded the following:
- Portugal is the biggest producer of cork that is used mainly in the wine bottling industry. However,
due to the high demands of quality, a great amount of waste is produced. This waste can be used in
alternative industries especially in the area of construction.
- The main compound of the cell walls of cork is the suberin. This substance confers thermal and
acoustic insulation properties that are appreciated when applied in buildings. Other important property
is its viscoelasticity which make it able to work as anti-vibratic insulation.
- The most used manufactured products applied in construction are: the expanded cork agglomerated
which works as insulation or outer covering and the agglomerated cork composite which works as
internal coating. In Portugal the main company that marketed these solutions is the Grupo Amorim
and sub-holdings.
- Some existing buildings with incorporation of cork or its derivative materials and past experiences
from specialists, show that this material is increasingly distinguished by its unique values. Despite the
occurrence of several explainable behaviours, with further studies about cork, this material will
certainly be a worthwhile contribution to the development of forthcoming constructions. About the
future works the recommendations are the following:
-There is still some insecurity about some properties of cork as a raw-material as well as a
manufactured product. Besides that, the behaviour of cork against fire is not much documented which
would be important for the development of new buildings.
- Improvements must be made to guarantee the conditions of services and the durability of the
material on the new applications of cork products such as the application of expanded cork
agglomerate as an outer covering. Also some research is needed about the conservation and
maintenance at work.
References
[1] Fortes, M.A., Rosa, M.E. & Pereira, H. (2004). aCortiça. 1ª edição, IST Press. Lisboa.
[2] Gil, L. 2007. Manual Técnico: Cortiça como Material de Construção. Ed. APCOR, Stª Mª Lamas.
[3] Silva J. S. & Catry F., Forest fires in cork oak stands in Portugal, International Journal of
Environmental Studies, vol. 63, 2006.
[4] Reis A. M. P. L. (2011) Revestimentos de pisos em aglomerado de cortiça. Tese de Mestrado em
Engenharia Civil – Instituto Superior de Engenharia de Lisboa, Lisboa.
[5] Silva, J.G., Vale, C. P. (2010) A utilização da cortiça em paredes de adobe – Contexto histórico e
perspectivas futuras. Terras em Seminário. Porto.
[6] Chiebao, F. (2011). Cortiça e Arquitectura. 1ª edição, Euronatura. Coimbra.
Internet sources
[S1] http://www.amorim.com/
[S2] http://www.amorimisolamentos.com/