CAN CORK BE USED AS A CONCRETE AGGREGATE?
Fernando G. Branco1, António Tadeu1, Maria de Lurdes Belgas C. Reis2
1
2
– Department of Civil Engineering, Faculty of Sciences and Technology,
University of Coimbra, Portugal. e-mail: [email protected]
– Department of Civil Engineering, Polytechnic Institute of Tomar, Portugal
e-mail: [email protected]
ABSTRACT
The cork industry worldwide consumes more than 280 000 tons of cork a year. However, about
20% to 30% of the raw cork received at the processing unit is rejected, mainly as cork dust.
Finding useful applications for the rejected cork may have important economical and
environmental implications.
Given its various interesting mechanical and physical characteristics, cork is a versatile material
with a great many possible uses. In the last few decades, cork has been much more extensively
used as a material in the building industry. It has helped to enhance the quality and performance
of both existing and new products, mainly for thermal and acoustic insulation, but also as a
vibration absorber.
The work described here examines the possibilities of using cork as an aggregate in concrete
admixtures, partly replacing sand and coarse aggregates. The paper addresses the problem of
ensuring the homogeneous dispersion of cork in the fresh concrete, and analyses the influence of
the amount of cork in the admixture and cork particle size distribution on the mechanical
properties of cork concrete.
Key words: concrete, aggregate, cork, lightweight, mechanical strength.
Introduction
The material known as cork comes from the outer layer of bark of the Quercus Suber L., a type
of oak tree that is native to the western Mediterranean. Cork is composed mainly of suberin,
which accounts for about 40% of its dry weight, lignin (± 20%), polysaccharides (±. 20%) and
extractables (±15%). This chemical composition, together with its particular cellular structure,
provides cork with excellent barrier properties against polar liquids, heat and sound [1].
The cork industry worldwide consumes more than 280 000 tons of cork a year. However, about
20% to 30% of the raw cork received at the processing unit is rejected, mainly as cork dust,
which has low granulometry and is of no industrial interest [2].
Cork is a natural, light, organic product. It is dimensionally stable, and exhibits considerable
resistance to compressive loads. These characteristics make cork a product suitable for use in a
wide spectrum of applications [3]. But cork possesses other important characteristics. It is a
natural and ecological product, it is odorless, and it can be considered imputrescible and
unalterable, thus retaining its efficiency for a very long time. Given that Portugal is the world’s
largest cork producer, it may well prove to be economically interesting to find alternative uses
for the industrial waste from cork production.
In the building industry, cork’s excellent thermal behavior and its ability to absorb vibration
make it the solution of choice as a thermal insulator and acoustic absorptive material. Despite the
wide spectrum of possible applications for cork in the building industry, there are not many
published research papers on this subject. One of the few documented studies on concrete
mixtures containing cork is by Aziz et al. [4]. This work reports the results of a research project
which aimed to develop lightweight concrete using cork granules. Lightweight cork concrete was
compared with standard concrete and other kinds of lightweight concrete (aerated, no-fines,
cellular and foamed). The results indicated that although the mechanical strength of cork
concrete in compression and tension is lower than that of standard concrete, this kind of concrete
performed better than the other types of lightweight concrete analyzed. Cork lightweight
concrete also exhibited better thermal and shrinkage properties than other lightweight concretes
produced from organic materials.
The work described here is part of a more extensive research project, and presents some
preliminary results obtained so far. The project intends to evaluate the mechanical and physical
properties of cork dust, and to explore its potential advantages when used as a concrete
aggregate. The introduction of natural or expanded cork granules into concrete mixtures will
allow concrete to benefit from some of cork’s useful properties, thus leading to an improvement
in that concrete’s performance when compared with standard concrete. Thermal insulation,
acoustic behavior, particularly related to impact loads, and increased durability under freezethaw conditions are some parameters where the inclusion of cork could be a positive advantage
to concrete.
In this preliminary paper, the authors set out to analyze the impact of the presence of cork on the
compressive strength of concrete. Different concrete compositions were tested, using two types
of cork as an aggregate: natural and expanded cork granules. The effect of the size of the cork
granules and the amount of cork in the concrete mixture were also analyzed. The results obtained
for the cork concrete compositions were compared with those from a reference standard
concrete.
Experimental work
The experimental work required cork granules, which were provided by cork industry. A
reference standard concrete was designed. Then, different concrete mixtures were produced
using the reference composition as a base. In these mixtures, two types of cork granules (natural
and expanded cork) were used instead of coarse primary limestone aggregates and/or river sand.
Different replacement percentages of aggregates by cork granules were tested. In each mixture, a
given volume of coarse aggregate or sand was replaced by the same volume of cork granules,
with a similar particle size distribution [5]. This size distribution was controlled via the fineness
module, computed as
n
∑ (100 − p )
i
MF =
(1)
i =1
100
where pi represents the percentage of particles with dimensions smaller than the openings in
sieve i. Replacements of 10%, 20%, 25% and 30% were tested. In series B3 specimens, 10% of
both sand and coarse aggregate were replaced by cork granules.
The cork concrete specimens were subjected to compressive strength tests, according to the
LNEC E226:1968 test specification [6]. Tests were conducted on specimens 3, 7, 14, 21 and 28
days old, to evaluate the variation of compressive strength over time, in relation to the amount of
cork in the mixture.
Table 1 displays the mixture compositions for all types of concrete tested. In this table, NCG
stands for “Natural Cork Granules”, while ECG stands for “Expanded Cork Granules”. As
shown in the table, batches of different-sized cork granules were used for each mixture. The
amount of each granule size fraction was computed to reproduce the particle size distribution of
the aggregates replaced.
Table 1 - Concrete mix design details.
Components (kg/m3)
Batch
BR
B1.1
B1.2
B1.3
BE1.1
BE1.2
BE1.3
B2.1
B2.2
B2.3
B2.4
BE2.1
BE2.2
BE2.3
B3.1
BE3.1
Coarse
aggreg.
Sand
Cement
Water
1413
1271
1130
989
1271
1130
989
1413
1413
1413
1413
1413
1413
1413
1271
1271
498
498
498
498
498
498
498
448
398
348
373
448
398
348
448
448
300
300
300
300
300
300
300
300
300
300
300
300
300
300
300
300
180
180
180
180
180
180
180
180
180
180
180
180
180
180
180
180
NCG
(4/5)
6.90
13.80
20.69
6.90
-
NCG
(0/1)
2.25
4.50
6.75
5.61
2.25
-
NCG
(1/2)
0.58
1.15
1.73
1.43
0.58
-
ECG
(5/10)
5.16
10.33
15.49
5.16
ECG
(3/5)
5.58
11.15
16.72
5.58
ECG
(0/1)
6.54
13.08
19.62
6.54
ECG
(1/2)
2.11
4.21
6.32
2.11
Laboratory tests were carried out to evaluate the most relevant characteristics for the aggregates
used. Grading, moisture content and bulk density were determined [7, 8] for all aggregates and
cork granules used. Slump tests were performed to measure the consistency of the fresh concrete
for all mixtures.
Laboratory tests were carried out on hardened concrete to determine its mechanical strength
under compressive loads [6]. Specific density and water absorption [9] were the physical
parameters measured in specimens of hardened concrete.
In order to see whether the cork granules exhibited homogeneous dispersion within the concrete
mass, some specimens were cut in two, and the distribution of cork granules in the sawn surface
was analyzed. Figure 1 shows the cork distribution for the series B1.1 and BE1.3 specimens.
This procedure confirmed that good cork dispersion was obtained in all the concrete series
analyzed.
a)
b)
FIG. 1- Cork granule dispersion in concrete specimens: a) series B1.1; b) BE1.3.
Characterization of the materials
100
100
90
90
Cumulative mass passing (%)
Cumulative mass passing (%)
Figure 2 displays the grading test results for the aggregates and cork granules. A good
approximation between the particle size distribution of each aggregate and the corresponding
cork granule mixtures can be seen. The fineness modules for sand and coarse aggregate are 3.10
and 5.67, respectively. The cork granule replacement mixtures exhibited the same fineness
modules as the material that was replaced.
80
70
60
50
40
30
River sand
NCG replacing sand
ECG replacing sand
20
10
Coarse Limestone
NCG replacing coarse Ag.
ECG replacing coarse Ag.
80
70
60
50
40
30
20
10
0
0
Res.
200
100
50
30
16
8
4
1/4"
Sieve (ASTM series)
3/8"
1/2"
3/4"
1"
Res.
200
100
50
30
16
8
4
1/4"
3/8"
1/2"
3/4"
1"
Sieve (ASTM series)
a)
b)
FIG. 2 - Grading curves. a) River sand and replacement cork granule mixtures; b) Coarse
aggregate and replacement cork granule mixtures.
Table 2 shows the bulk densities for all the aggregates and replacement cork granules. It can be
seen that the cork granules are much lighter than the aggregates they replace, and so the concrete
made with the replacement cork granules is expected to be significantly lighter.
Table 2 – Bulk densities of aggregates and cork granules.
Aggregates
River
sand
Coarse
limestone
NCG
(4/5)
NCG
(0/1)
NCG
(1/2)
ECG
(5/10)
ECG
(3/5)
ECG
(0/1)
ECG
(1/2)
Bulk density
(kg.m-3)
2570.0
2669.5
70.1
85.5
87.8
68.0
247.8 362.8 249.0
The physical parameters evaluated for the concrete mixtures manufactured are presented in
Table 3. As expected, when a larger amount of aggregate is replaced by cork granules, there is a
perceptible drop in weight. Concrete specimens produced with natural cork granules are heavier
than those containing expanded cork granulate. One exception was batch B1.3, which is lighter
than BE1.3. When coarse aggregate was replaced by an equal volume of cork, it was possible to
obtain lower weights than when there was a similar volume replacement between sand and cork.
When natural cork granules were used, a replacement of 30% of the coarse aggregate led to a
25% reduction in concrete weight, but when cork was used to replace sand, there was only a 10%
drop in weight.
Table 3 – Bulk density and water absorption for concrete.
Batch
BR
B1.1
B1.2
B1.3
BE1.1
BE1.2
BE1.3
B2.1
B2.2
B2.3
B2.4
BE2.1
BE2.2
BE2.3
B3.1
BE3.1
Bulk density
(kg.m-3)
2450
2090
1970
2090
1930
1800
2230
2120
2140
2200
2120
2020
2060
2090
Water absorption
(%)
7.2
6.9
6.6
7.4
8.0
5.8
6.7
6.4
6.4
6.7
7.4
6.7
6.3
Mechanical behaviour of hardened concrete
Concrete cubes measuring 150x150x150mm3 were produced and subjected to compressive tests
at different ages (7, 14, 21 and 28 days). The specimens were removed from their moulds 24
hours after casting, and stored in a climatic chamber (20ºC; ±95% relative humidity) until
testing. At least three specimens from each batch were tested. The results for the compressive
tests are given in Table 4.
As expected, as the percentage of cork granulates increased, the concrete’s compressive strength
fell.
The B1 series specimens, where coarse aggregates were replaced by cork granules, exhibited
greater loss of strength than the B2 specimens, in which natural cork replaced part of the river
sand. When comparing the B1 series with the reference concrete (BR), strength losses of
between 32.6% and 74.9% were registered, as the amount of coarse aggregate replaced rose from
10% to 30%. The corresponding falls in strength when similar replacements for sand (series B2)
were made ranged between 26.5% and 52.8%.
When expanded cork was used to replace the aggregates, the strength loss was generally higher
than when natural cork was used. The exceptions were series BE1.3 and BE2.3, where a 30%
volume of aggregates was substituted by expanded cork. In these series, reductions in strength of
61.6% and 46.0%, respectively, were registered, against 74.9% and 52.8% observed in series
B1.3 and B2.3.
Table 4 – Average compressive strength of concrete specimens.
Average compressive strength (MPa)
Series
BR
B1.1
B1.2
B1.3
BE1.1
BE1.2
BE1.3
B2.1
B2.2
B2.3
B2.4
BE2.1
BE2.2
BE2.3
B3.1
BE3.1
7 days
14 days
21 days
28 days
23.40
13.86
9.30
5.95
12.17
11.03
8.68
16.68
16.05
10.11
17.21
14.88
14.39
11.31
13.62
13.53
25.25
15.98
11.44
6.77
13.51
11.86
9.83
17.79
17.13
11.09
18.66
16.93
16.21
12.83
14.99
14.73
25.66
16.68
12.18
6.51
14.33
12.80
10.26
18.38
17.78
12.02
19.26
18.32
16.92
13.69
15.96
15.78
26.14
17.62
12.55
6.56
15.16
13.24
10.05
19.21
18.32
12.33
19.15
18.28
17.65
14.11
16.54
16.61
Strength loss
against BR
(%)
0.0
32.6
52.0
74.9
42.0
49.3
61.6
26.5
29.9
52.8
26.7
30.1
32.5
46.0
36.7
36.5
Figure 3 displays the variation of the specimens’ compressive strength with their bulk density.
The results indicate that the average strength of concrete containing cork granulates as aggregate
materials is closely related to its bulk density. A linear trend line represents a good approach for
this relation. However, as only a small number of tests were performed, it is not possible to draw
definite conclusions. Further tests will be carried out on concretes with a lower bulk density, thus
containing higher percentage of cork granulates.
Average strength (MPa)
24
22
20
18
16
14
y = 0.0233x - 32.005
2
R = 0.8927
12
10
2000
2050
2100
2150
2200
2250
Bulk density (kg/m3)
FIG. 3 – Variation of the average compressive strength
with the bulk mass of the specimens.
Figure 4 shows the average compressive strength variation with the amount of cork contained in
the mixture. A loss of strength can be seen as the percentage of cork increases. Both types of
cork studied led to similar drops in strength.
Average compressive strength
(MPa)
30
25
20
BR
NCG
ECG
15
10
5
0
0
5
10
15
20
25
30
35
Cork Volume (%)
FIG. 4 – Variation of the average compressive strength with the volume
of cork contained in the mixture.
Final Remarks
This paper presents the preliminary results of a research project that aims to assess the
mechanical and physical properties of cork granules and evaluate benefits of their use as
concrete aggregates.
Several concrete mixtures, containing different amounts of cork granules, were made up. Two
types of cork (natural and expanded) were studied. Compression tests were carried out in the
laboratory on several concrete series at different ages.
Tests showed that the compressive strength of the concretes tends to fall as the amount of the
cork granulates in the mixture increases. This effect is more noticeable when coarse aggregate is
replaced by cork. The use of expanded cork led to higher losses in strength.
The compressive strength of concrete decreases as concrete bulk density decreases. The tests
carried out allowed concluding that this loss of strength, within the density range tested, follows
a linear trend. Tests on lower density concrete containing cork granules are needed to check if
this correlation is still valid outside the current test field.
The volume of cork in the concrete strongly influences its mechanical strength. This
phenomenon is particularly important when replacing the coarse aggregates. When 10% to 30%
of sand is replaced by an equivalent volume of cork granules the concrete strength drops by
between 26.5 and 52.8%, while reductions ranging from 32.6 to 74.9% were registered when
cork granules replaced the coarse aggregate. Expanded cork granules used as replacement
material usually leads to greater strength losses than the use of natural cork granules.
Acknowledgements
The authors thank the Foundation for Science and Technology, Portugal, for the financial
support granted through project POCI/ECM/55889/2004 and Grupo Amorim– Portugal, for the
cork granulate used to produce the test specimens.
References
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