Rubbercrete
Analysis of the physical and mechanical properties and durability
Prof. Ing. Giuseppe Carlo Marano, Politecnico di Bari
Dott. Ing. Cesare Marti Ph. D., Politecnico di Bari
p.i. Marcello Molfetta, Italcementi Group – Laboratorio di Mesagne (BR)
Ing. Valentina Sammarco, Libero Professionista
ABSTRACT
The build up and disposal of waste tyres is an important problem in the world and it poses a
threat to human health and increases environmental risks. According to some estimation, in
2010 the European Union (EU) was faced with the challenge of managing more than 3.3
million tones of used tyres; without considering any used tyres dumped or stockpiled illegally.
The problem is worse for less developed countries which do not have any resources to
dispose of waste tyres in a safe and effective way.
Therefore it is in the best interest of everyone to develop a safe and sustainable way of
disposing waste tyres.
The following research report analyses the characteristic of light “alternative” concrete, the
«rubbercrete», characterized by the presence of rubber particles deriving from waste tyres in
substitution of a percentage of ordinary aggregates with a variable rate from 0% to 75%,
analyzing in addition to the main physics and mechanical characteristics (such as compressive
strength, flexural strength, elastic dynamic and secant modulus, the Poisson’s ratio, shrinkage,
toughness and thermical properties) durability, too.
Not being previously examined, it is important to test on durability (such as sulphate and
chloride etching, freeze thaw resistance and water absorption) in function of aggregate
substitution ratio.
The main objective is to observe the rubber influence in cement matrix and its behavior in
diverse exposures (wet environment, rich in sulphate and chloride ions or with high thermical
range).
INTRODUCTION
Waste tyres management is an important challenge because the rubber is not biodegradable,
so the recycle represent an alternative (Guneyisi et al. 2004). The recycled rubber from tires is
currently used in various fields and for some specific applications. At first it is used as a fuel
(low specific heat) in cement kilns, as feedstock for the manufacture of carbon black, or
chopped into particles of small size, it is used as "artificial sand" or as feedstock for the
realization of artificial reefs in marine environment (Siddique e Naik,2004). Additional uses
may include the creation of artifacts for playgrounds, road safety barriers, guardrails, noise
barriers and asphalt paving mixtures (Toutanji, 1996).
During the last two decades, researches have focused on the use of the rubber from discarded
tyres as aggregate in concrete mixtures (Ganjian et all. 2009, Eldin e Senouci 1993, Toutanji
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1996, Khatib e Bayomy 1999, Siddique e Naik 2004, Batayneh et al 2008, Aiello e Leuzzi 2010
e Najim e Hall, 2010).
The rubber obtained from the recycling of waste tyres, in fact, is a promising material with
some interesting applications in the construction industry for its lightness, elasticity,
absorption capacity of energy, acoustic and thermal insulation.
In this paper we’ll describe the results of a research aimed to analyze in full the characteristics
of these concrete mixtures through a wide series of tests developed at the research laboratory
of Italcementi in Mesagne (BR).
THE EXPERIMENTATION
In general, the experimentation can be divided into two parts, one referring to the study of the
physical - mechanical characteristics of the mixtures, and one referring to the durability of the
same subject to various attacks.
In the study of the durability a set of mixtures of additived cement conglomerates have been
analyzed, containing rubber particles deriving from waste tyres in substitution of a quantity
of ordinary aggregates in percentages varying from 0% to 75%, formed on the volume of
aggregate obtained from the mix design of mixtures: in particular, we will refer to a reference
mixture, called «mix TQ» containing zero percentage of aggregates of rubber, and a set of
mixtures called «mix "X %"» in which the only variable is the percentage of rubber.
Table 1 shows the composition of the various mixtures analyzed:
mix TQ
mix
10%
mix
20%
mix
30%
mix
40%
mix
50%
mix
75%
1635,5
1292
1117
924
761
632
272
0
70
130
182
233
289
359
310
380
380
380
380
380
380
Superfluidizing
4
4
4
2,91
4,16
4
4
Aer-entraining
agent
-
0,023
0,013
0,008
0,008
0,0048
0,0030
Water
190
195
185
180
170
160
140
Entrained air
55
100
125
160
190
200
300
2137
1939,5
1815
1669
1545
1463
1154
Mixtures
Sand 0÷4mm
Rubber
2÷4mm
42,5 II-A/LL
Theorical
density
Table 1 – The composition mixtures, in [kg/m3].
The main physical-mechanical characteristics analyzed were the compressive strength, the
flexural strength, elastic dynamic and secant modulus, the Poisson ratio, the shrinkage, the
ductility, the thermal properties.
Figure 1 shows an outline of all the properties analyzed.
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FRESH
STATE
HARDENED
STATE
DURABILITY
• Density
• Consistency
• Air content
• Compressive strength
• Flexural strength
• Poisson ratio
• Elastic dynamic modulus
• Elastic secant modulus
• Shrinkage
• Ductility
• Thermal properties
• Freeze thaw resistance
• Sulphate etching
• Chloride etching
• Water absorption
Figure 1 - List of properties analyzed in the fresh state and hardened state and for the analysis of durability.
CHARACTERISTICS OF MATERIALS
The cement used is 42.5 R II-A/LL, Portland cement with limestone.
For stone aggregates was used the only fine component , that calcareous sand ranging in size
from 0 to 4 mm, already known by the abbreviation “Sand 0 ÷ 4 mm.” The aggregates used in
rubber, as a partial replacement of the aggregate stone, are of the type G1-1 (grain size
ranging between 2 mm and 4 mm) and they are obtained by mechanical grinding of used tyres
from trucks.
Figure 2 shows the aggregates of rubber used in the preparation of the mixture.
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Figure 2 – Rubber granules used.
In all the mixtures was used a super fluidizing, high power water reduction and long
workability, for concrete with low slump loss, suitable for all types of cement. These types of
additives have four main benefits on the concrete mix:
1. Encouraging strong workability with any kind of cement, preferably the pozzolanic
one;
2. Having a high degree of water reduction;
3. Improving the yield of the mixture;
4. Lowering the content in air.
In addition, we used an air-entraining agent, which has allowed an increase in Freeze thaw
resistance and de-icing salts through the introduction of air microbubbles evenly distributed
throughout the cement paste.
FRESH STATE
The density of the fresh mixture (UNI EN 12350-6), monitored for time intervals of 0 ', 30' and
60 ', has shown a reduction of the same, in the various mix increases the quantity of rubber. In
the diagram shown in Figure 3 we report the change.
2400
kg/m3
1900
0'
1400
30'
60'
900
Mixtures
Figure 3 - Variation in density as a function of the amount of rubber. Fresh state 0 ', 30', 60 '.
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Spreading [cm]
From the point of view of workability (UNI EN 11041) was obtained a slump-flow> S5. It is
necessary to highlight that the presence of the rubber reduces the workability of the mixtures:
the phenomenon is evident by comparing the consistency of the mix 10% and 20% with that
of mix 50% and 75% (the observed reduction varies from 15% to 25%). Figure 4 analyzes the
change of the consistency on the varying of the rubber percentage:
80
70
60
50
40
30
20
10
0
consistenza 0'
consistenza 30'
consistenza 60'
mixtures
Figure 4 - Change in consistency depending on the quantity of rubber
The air content (UNI EN 12350-7) influences the workability of the mixture, the density and
its resistance. The latter increases with the quantity of rubber used. In Figure 5 is shown the
percentage of air on the varying of the percentage of replacement of rubber inside the
mixtures:
Figure 5 - Variation of the air content in function of the type of mixture analyzed
It is evident as the percentage of air increases as the quantity of rubber used. Mixtures with
substitutions at 50% and 75% of rubber have equal values to 2.5 times and 5 times the
quantity of air in the mix TQ.
HARDENED STATE
The Compressive strength (EN 12390-3) decreases with the increasing amount of rubber.
Below, the results obtained from the test of resistance to compression, in function of the
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percentage of rubber, for aging times of 7, 28 and 60 days, Figure 6, and in function of time,
Figure 7:
Compressive strength [MPa]
60
50
40
30
7gg
20
28gg
10
60gg
0
Mixtures
Figure 6 - Variation of Compressive strength increases as the percentage of rubber.
Curing time: 7, 28 and 60 days.
35,00
Compressive strength [Mpa]
30,00
25,00
mix TQ
mix 10%
20,00
mix 20%
15,00
mix 30%
mix 40%
10,00
mix 50%
mix 75%
5,00
0,00
0
10
20
30
40
50
60
70
Time [days]
Figure 7 - Variation of Compressive strength as function of time.
In particular, it is evident the worsening of compressive strength due to the replacement of
ordinary aggregates with those rubber: hence the limitation in the scope of rubbercrete
applications to non-structural types.
However, it is important to note that varying percentages up to 15%, the material takes on a
very similar behavior to the concrete reference: the values of compressive strength are very
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close.
We report in Figure 8 the variation of compressive strength as function of density.
Compressive strength [Mpa]
60
50
40
7 gg
30
28 gg
20
60 gg
10
0
1000
1200
1400
1600
Density
1800
2000
2200
[kg/m3]
Figure 8 - Variation of Compressive strength as function of density. Hardened state: 7, 28 and 60 days.
For Flexural strength (UNI EN 12390-5) (60 days) there is an evident reduction in the
increase of the quantity of rubber used, (Figure 9).
Flexural strength [MPa]
9
8
7
6
5
4
3
2
1
0
Mixtures
Figure 9 – Variation of flexural strength as function of quantity of rubber used.
The elastic dynamic modulus (UNI 9524) calculated through a value of Poisson's ratio equal to
0.25, shows its variation increasing the percentage of rubber, for aging times of 7, 28 and 60
days (Figure 10). It is evident that with the increasing of the rubber, the modulus of elasticity is
reduced substantially: the reduction ranging is from 42% (20% mix) to 82% (75% mix)
because the MED of the rubber itself is lower than the one of the mix TQ and this contributes
to the reduction of the MED in rubbercrete proportionally to the quantity of ordinary
aggregate replaced. In the test, in fact, the rubber acts as a barrier against the ultrasonic
waves, thereby leading to the increase of the crossing time of the specimen and consequently
to the reduction of the MED.
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7
30
25
MED [GPa]
20
15
7gg
28gg
10
>28gg
5
0
Mixtures
Figure 10 - Variation of the MED increases as the percentage of rubber.
In the diagrams shown in Figure 11, the variation of the MED in function of the density in the
hardened state is reported, for aging times of 7 and 28 days and in Figure 12 the MED in
function of time.
35
30
MED [Gpa]
25
20
7gg
15
28gg
10
5
0
1000
1200
1400
1600
Density
1800
2000
2200
[kg/m3]
Figure 11 – Variation of the MED in function of the density in the hardened state, 7 and 28 days.
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30,00
25,00
MED [Gpa]
20,00
mix TQ
mix 10%
mix 20%
15,00
mix 30%
mix 40%
10,00
mix 50%
mix 75%
5,00
0,00
0
7
14
21
28
35
42
49
time [days]
Figure 12 - Variation of the MED in function of time
The calculation of the MED through the Erudite (UNI 9771), a tool whose purpose is to
determine, in a non destructive way, the resonance frequencies of longitudinal, torsional and
flexural that will allow us to obtain a more realistic value of the elastic dynamic modulus, has
not returned significant changes compared to the results obtained with the calculation of the
MED through the ultrasound testi.
The Secant Modulus (UNI 6556) also in this case decreases with the increasing of the quantity
of rubber (Figure 13):
25
MES [GPa]
60 days
20
15
10
5
0
Miscela
Figure 13 - MES. State hardened, 60 days
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A high elastic modulus is searched when you need low distortion, while a low elastic modulus
gives "flexibility." It is therefore clear that "flexibility" increases as the quantity of rubber in
the mixture.
Below MES related to the mass density [kg/m3], Figure 14:
25
MES [GPa]
28 days
20
15
10
5
0
1000
1200
1400
1600
Density
1800
2000
2200
[kg/m3]
Figure 14 – Changes of the MES function of density in the hardened state, 28 days.
In reference to the shrinkage (UNI 6687-73), all mixtures have a lower shrinkage to the mix
TQ in the first week, Figure 15:
tempo [giorni]
1
2
7
14
21
28
0
-200
mix TQ
Shrinkage [mm/m]
-400
mix 10%
-600
mix 20%
-800
mix 30%
-1000
mix 40%
-1200
mix 50%
mix 75%
-1400
-1600
-1800
Figure 15 – Shrinkage T=20°C, U.R.=50%
The mix 75% is the most subjected to this phenomenon as the same quantity of cement, there
is less amount of ordinary aggregate (which contrasts the shrinkage) which involves a
decrease in the aggregate/cement ratio and consequently there is the accentuation of the
phenomenon. Then, focusing on weight loss, Figure 16:
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tempo [giorni]
1
2
7
14
21
28
0,00%
-1,00%
mix TQ
weight loss [%]
-2,00%
mix 10%
mix 20%
-3,00%
mix 30%
mix 40%
-4,00%
mix 50%
-5,00%
mix 75%
-6,00%
-7,00%
Figure 16 - Weight loss in time. T=20°C, U.R.=50%
the mix 75% is also the mixture that has less weight loss between all mixtures analyzed
(approximately equal to 45% compared to the mix TQ), in fact it has the lowest water/cement
ratio.
From the analysis of the load-deflection diagram, Figure 17, in the flexural test with 4 load
points for the analysis of the ductility (ASTM C 1018-97), it is clear that increasing of the
rubber the ultimate load is reduced (as already observed for the compressive strength and
flexural strength), the behavior of the sample is not brittle type (as in the case of the mix TQ)
but tends to take a behavior of ductile type: they have large deformation under stress,
especially for percentages of rubber greater than 30%. This gives the rubbercrete good
ductility.
Figure 17 – Load-deflection curve: complete diagram.
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With regard to the thermal conductivity (UNI EN 12664), if we analyze such property in
relation to the percentage of rubber contained in the mix, there is a decay of the same with the
rubber increasing, Figure 18:
Thermal conductivity [W/mK]
1,40
1,20
1,00
0,80
0,60
0,40
0,20
0,00
mixtures
Figure 18 – Variation of the thermal conductivity as a function of the percentage of rubber.
Thermal Conductivity [W/mK]
The combined action of air and rubber creates a discreet thermal insulation that prevents the
transport of heat. If we analyze such properties in relation to density in the hardened state,
we can note an increase of the thermal conductivity with the density increasing (Figure 19):
the increase of density corresponds to a more compact structure, so to a reduction of its
porosity.
1,40
1,20
1,00
0,80
0,60
0,40
0,20
0,00
1000
1200
1400
1600
1800
2000
2200
Density [kg/m3]
Figure 19 – Variation of conductivity as function of density. State hardened, 60 days.
DURABILITY
The results obtained by subjecting the samples to freeze-thaw cycles (UNI EN 12390-9) shows
that the tyre has no influence on the strength of rubbercrete, in locations subject to extreme
temperature changes. None of the specimens of the mixtures tested has shown, at the end of
the cycle, variations in terms of mass with the exception of the mix TQ that, even if in small
quantities, has found a slight flaking surface, Figure 20:
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Figure 20 - Freeze thaw resistance: mix TQ
The total weight of scales per unit area, of the mix TQ, is equal to 0.036 kg/m2; for all the
others it is null.
In reference to the test concerning the resistance of rubbercrete to Sulphatic attack (CEN/TR
15697) it is evident that, all the blends have lower compressive strength compared to the
normal condition, consequently they are all vulnerable to attack by sulphate ions, Figure 21.
40,00
35,00
Rc [MPa]
30,00
25,00
20,00
15,00
Rc rif
10,00
Rc SO4=
5,00
0,00
mixtures
Figure 21 – Comparison between the compressive strengths of the specimens immersed in solution with
those of the specimens in normal conditions
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mixtures
0,00
Rc [MPa]
-5,00
-10,00
-15,00
-20,00
-25,00
-30,00
Figure 22 – Loss of compressive strength
The diagram in Figure 22 shows the loss of compressive strength, in function of the
replacement percentage.
The presence of the rubber has a beneficial effect on the resistance to penetration by chloride
ions (Nordtest Method ISSN 0283-7153), in percentages lower than 60% (Figure 23):
7,00E-11
diffusion coefficient
[m2/s]
6,00E-11
5,00E-11
4,00E-11
3,00E-11
2,00E-11
1,00E-11
0,00E+00
Mixtures
Figure 23 - Variation of the diffusion coefficient of chloride ions to varying the percentage of rubber.
The coefficient of penetration is reduced by 10%, 30% and 60% (maximum reduction) in
reference to mixtures containing, respectively, 10%, 20% and 30% of rubber. After passing
the latter proportion of rubber it increases the vulnerability, while remaining lower than the
mix TQ one. For values of rubber higher than 50%, finally, the resistance worsens
significantly: the penetration of ions is greater than the mix TQ one. As known, the resistance
to chloride ions is influenced by the water/cement ratio: the higher the ratio, the higher the
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porosity of the cement matrix. Because the water/cement ratios decrease as the percentage of
rubber increases, one would expect a reduction of the porosity but, exceeded 50% of
aggregate in rubber, rubber-cement matrix interface provides, probably, a preferential path to
permeation of ions chloride inside.
The analysis of the results obtained from the test concerning the absorption of water by
capillarity (NORMAL 11/85) shows that all the blends have an absorption (Figure 24), so an
absorption coefficient (Figure 25), lower than the mix TQ one with the exception of the mix
20%:
3,5
Total Absorption [%]
3,0
2,5
2,0
1,5
Total Absorption
1,0
Rif.
0,5
0,0
mixtures
Figure 24 - Total absorption to varying the percentage of rubber.
Capillary absorption coefficient
[g/(cm2* s1/2)]
0,000014
0,000012
0,00001
0,000008
0,000006
Absorption coefficient
0,000004
Rif.
0,000002
0
mixtures
Figure 25 – Change in Capillary absorption coefficient as a function of the percentage of rubber
More tests are needed to analyze the behavior of this mix.
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CONCLUSIONS
It should be noted that all the laws of interpolation obtained are usefull for the same amount
of cement and initial workability (slump-flow> 50 cm).
In the light of the results obtained it is possible to affirm the existence of merit that the
mixture could be used in some specific applications of non-structural field as in the case of
insulating screeds, light brickwork (very useful in cases where you do not want to burden
with the structural weight), curtain walls and filling materials, all applications can be used in
environments subject to extreme temperature, aggressive environments containing chloride
ions, paying greater attention to the presence of sulfate ions.
It is important to remember that these materials are highly deformable and therefore allow a
good energy dissipation and high ductility. Not least the sustainability of the product is, in
accordance with the regulatory criteria for the manufacture of concrete with aggregates from
PFU.
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