MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHT-
WEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT CONCRETE
José Maria Guedes Machado Lemos de Figueiredo
Extended Abstract
Masters in Civil Engineering
Supervisor: Prof. Dr. Jorge Manuel Caliço Lopes de Brito
Co-supervisor: Prof. Dr. José Alexandre de Brito Aleixo Bogas
April 2013
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
CONCRETE
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1. INTRODUCTION
Construction is currently one of the economic sectors that generate the largest amounts of
waste, due to the nature of the materials involved.
According to Zordan (1997) the reuse of construction waste begins to be considered a good
alternative to using traditional raw materials.
Reusing solutions/waste recycling promotes sustainability of the construction sector. On the
other hand, for the implementation of this concept, besides multidisciplinary, cultural changes, envi-
ronmental education and a systemic view are also necessary [Brandon (1998); Angulo (1998); John
(2000); Zwan (1997)].
Concrete is nowadays one of the most employed materials in the construction industry. Its su-
premacy is justified by the countless advantages it presents in economic, versatility and mechanical
behaviour terms.
It is common to associate concrete to the concept of a heavy and cold material, a factor that
leads to the increasing need of solutions that seek to correct and enhance all the advantages it pre-
sents. Lightweight concrete with low-density aggregates (LWC), characterized by densities lower
than 2000 kg/m3 and thermal conductivity lower than conventional concrete, meets these require-
ments. Density reduction allows the design of lighter structures,, which is a huge advantage.
Silva et al. (2004) reported that the use of concrete with lightweight aggregates versus that of
conventional concrete presents a number of attractive features that lead to a cost reduction making
its use increasingly frequent in construction.
This kind of concrete (LWC) has been the subject of numerous studies concluding that the use
of some types of expanded clay aggregates can currently achieve lightweight concrete with similar
strength to conventional concrete with equivalent composition.
2. EXPERIMENTAL PROGRAM
2.1. Research significance
This research aims to associate recycling of lightweight concrete with its production, in order
to understand well what the performance, at the mechanical level, of concrete with aggregates from
crushed lightweight aggregates concrete. With that purpose a total of 12 concrete mixes were manu-
factured with compositions changing in term of the replacement rate (0%, 20%, 50% and 100%),
for two types of lightweight aggregates (Leca M and Leca HD) and two types of recycled light-
weight aggregates (RLHD and RLM), and subsequently the results were analyzed in terms of me-
chanical performance.
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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Simultaneously another dissertation was carried out by João Miguel Quilhó Correia Cabaço
from Instituto Superior Técnico, which analyses the “Performance of lightweight aggregate con-
crete from shredding of lightweight concrete floors in terms of durability”.
In this study, specimens were tested in order to determine the main features of concrete be-
haviour. Table 1 presents the tests conducted during the experimental campaign, as well as the
standards used.
Table 1 - Description of performed tests and standards.
Aggregate tests Test standard
Sieve analysis NP EN933-1/ NP EN933-2
Particle density and water absorption NP EN1097-6
Apparent bulk density NP EN1097-3
Crushing strength NP EN13055-2
Water content NP EN1097-5
Shape index NP EN933-4
Fresh concrete tests Test standard
Slump NP EN12350-2
Fresh density NP EN12350-6
Hardened concrete tests Test standard
Compressive strength NP EN 12390-3
Splitting tensile strength NP EN 12390-6
Modulus of elasticity LNEC E 397
Abrasion resistance DIN 52108 (2002)
2.2. Materials
During the investigation, expanded clay aggregates of two types were used, commercially
known as: Leca HD - aggregates for structural lightweight concrete; Leca M - aggregates for non-
structural lightweight concrete.
Figure 1 - Lightweight aggregate, Leca®.
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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In this study it was decided to use two types of siliceous sand in order to obtain compact mix-
es. It was also used Portland cement CEM I 42.5 provided by SECIL, located in Outão, Setúbal.
Finally, the water used in the production of the lightweight concrete was tap water.
Table 2 summarizes the main features of the aggregates used in this experimental campaign,
namely their particle dry density, 24-hour water absorption, loose bulk density, crushing strength
and grading range (di/Di).
Table 2 - Properties of aggregates
Material bulk density
(kg/m3)
Water absorp-
tion (%)
Apparent bulk
density (kg/m3)
Crushing re-
sistance (MPa)
Shape index
(%)
Fine sand 2604 0,20 1494 - -
Coarse sand 2610 0,22 1493 - -
Leca HD 1092 12,61 681 5,71 -
Leca M 595 23,22 339 1,20 -
RLHD 1735 15,72 1000 7,55 23.9
RLM 878 29,41 463 1,95 8.8
Given the unavailability of recycled lightweight aggregates, they were produced in the Con-
struction Laboratory of the Department of Construction, Architecture and Georresources of IST,
Lisbon.
The production of recycled lightweight aggregates resulted from casting, curing and subse-
quent crushing of source concrete (BO), followed by the separation of crushed aggregates by size
fractions.
Regarding the experimental program of this research, two types of recycled aggregates,
RLHD and RLM, were used, from the production and subsequent grinding of structural and non-
structural lightweight concrete, respectively (Table 3).
The compositions of the origin concretes produced are presented in table 3.
Table 3 - Composition of the two original concrete mixes (l/m3 of concrete).
Materials BOHD BOM
Sand (l/m3) 313,25 -
Leca HD (l/m3) 350 -
Leca M (l/m3) - 630
Cement (kg/m3) 350 150
Water (l/m3) 192,5 90
fcm 7 days (MPa) 34,2 0,7
fcm 28 days (MPa) 37,2 0,7
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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Crushing of the two types of source concrete blocks (Figures 2 and 3) was performed accord-
ing to a mechanical process, in a jaw crusher, in the Laboratory of Construction of the Department
of Construction and Architecture of IST, Lisbon (Figure 4). That was followed by particle size sep-
aration of the recycled lightweight aggregate generated using a vibrating sieving device (Figure 5).
Figure 2 - Structural lightweight concrete blocks Figure 3 - Non-structural lightweight concrete blocks
Figure 4 - Jaw crusher, Laboratory of Construction in the Department of
Construction, Architecture and Georresources of IST, Lisbon
Figure 5 - Vibrating sieving device, Laboratory of
Construction in the Department of Construction,
Architecture and Georresources of IST, Lisbon
2.2.1. Properties of aggregates
The dry bulk density of lightweight aggregate revealed to be consistent with the values made
available by the supplier. An analysis of this property in the recycled lightweight concrete aggre-
gates showed an increase in the dry bulk density of 60% and 50% compared to structural and non-
structural lightweight aggregates, respectively. The recycled aggregates are made of a fraction of
original lightweight aggregates and another of mortar or paste. Since the bulk density of this frac-
tion is higher than that of the original lightweight aggregate, an increase of this property is ex-
pected.
As expected, the non-structural lightweight aggregate (Leca M) recorded higher absorption
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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values than those of the structural lightweight aggregate (Leca HD). The non-structural lightweight
aggregate (Leca M) has lower density and greater inner porosity, which results in increased water
absorption. Concerning the RWLAC, similarly to the differences observed between the structural
and the non-structural lightweight aggregate, they also showed an increased absorption compared to
lightweight aggregates. This increase results from two factors: the highest content of broken parti-
cles; the mortar surrounding the lightweight aggregates.
The apparent bulk density had a remarkable increase in recycled aggregates, with a sharp in-
crease in those from structural lightweight concrete (RLHD). This was expected since this property
is directly affected by the particle size, shape and the compactness of the aggregate.
Regarding crushing strength, the results for lightweight aggregates were again in agreement
with those provided by the supplier. On the other hand, aggregates with higher density and higher
apparent bulk density (Leca HD) recorded higher values of crushing strength compared to non-
structural lightweight aggregate (Leca M).
In recycled aggregates, the amount of mortar stuck to the aggregate itself confers an increase
of the crushing strength. This is a very unique feature of recycled lightweight aggregates concrete,
since it does not happen with natural aggregates, given their greater strength against that of mortar.
Structural lightweight concrete recycled aggregate (RLHD) had higher shape index than non-
structural lightweight concrete recycled aggregate (RLM). This results from the most angular shape
found in RLHD particles. As several authors [Matias and de Brito (2005), Pereira (2010), Fonseca
(2009) and Amorim (2008)] found, this is a characteristic of recycled concrete aggregate.
Figure 6 shows the evolution of the water absorption of lightweight and recycled aggregates.
The aim of this test is to understand the evolution of the water absorption of lightweight ag-
gregates and recycled. Knowing this property allows assessing more accurately the amount of addi-
tional mixing water, since both lightweight aggregates and recycled aggregates have high porosity.
Figure 6 shows there was a rather high initial absorption for both lightweight aggregates used,
and a significant slowing-down after the first 30 minutes.
Figure 6 - Aggregates water absorption over time.
0
5
10
15
20
25
30
0 250 500 750 1000 1250 1500
Wa
ter a
bso
rpti
on
(%
)
Time (minutes)
Leca HD Leca M RLHD RLM
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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2.3. Concrete design
In order to understand the mechanical performance of concrete with recycled lightweight ag-
gregate from crushed lightweight concrete, four families of concrete were produced, resulting from
the partial or total replacement of the original lightweight aggregates (Leca HD and Leca M) with
the recycled lightweight aggregates (RLHD and RLM). The substitution rates adopted were 0%,
20%, 50% and 100%, comprising 12 concrete mixes, which are referred to as:
BRHD - control lightweight concrete with structural expanded clay (Leca HD);
BRM - control lightweight concrete with non-structural expanded clay (Leca M);
BHD20RHD - lightweight concrete with Leca HD replaced with 20% of RLHD;
BHD50RHD - lightweight concrete with Leca HD replaced with 50% of RLHD;
BM20RHD - lightweight concrete with Leca M replaced with 20% of RLHD;
BM50RHD - lightweight concrete with Leca M replaced with 50% of RLHD;
B100RHD - lightweight concrete with 100% of RLHD;
BHD20RM - lightweight concrete with Leca HD replaced with 20% of RLM;
BHD50RM - lightweight concrete with Leca HD replaced with 50% of RLM;
BM20RM - lightweight concrete with Leca M replaced with 20% of RLM;
BM50RM - lightweight concrete with Leca M replaced with 50% of RLM;
B100RM - lightweight concrete with 100% of RLM;
In order to obtain a concrete similar to the one used in structural applications, it was decided
to produce the mixes with the following features:
Strength class: LC35/38;
Consistency class: S3 (100 a 150 mm) - Target: 125 ± 10 mm;
Water/cement ratio: 0.55;
Binder: CEM II A-L 42.5 R cement from Secil, Outão, Setúbal;
Mixing water: tap water, from the public supply network;
Maximum aggregate size: 11.2 mm.
However, one must emphasize the functional difference between the two reference concrete
mixes. BRHD is a structural lightweight concrete while BRM has non-structural characteristics.
Table 4 shows the amounts adopted for the production of the two reference concrete mixess
(BRHD and BRM). Each concrete was designed for a total volume of 222.8 dm3.
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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Table 4 - Composition of the two control concrete mixes (kg/m3 of concrete).
Size grading BRHD BRM
Lightweight aggregate
12.5 - 14 4.8 2.6
11.2 - 12.5 19.5 10.6
8 - 11.2 76.6 41.7
Coarse aggregate 6.3 - 8 77.2 42.1
5.6 - 6.3 46.3 25.2
4 - 5.6 133.5 72.8
2 - 4 22.8 12.4
Fine aggregate Sand Coarse sand 565.0 565,0
Fine sand 260.3 260,3
Cement 350.0 350.0
Water 192.5 192.5
3. Results and discussion
3.1. Fresh concrete properties
3.1.1. Workability
A range of 125 ± 10 mm in the slump test was stipulated for all compositions, corresponding
to a concrete consistency class S3.
Figure 7 shows the results of the slump test by Abram’s cone for all mixes produced.
Figure 7 - Slump test results.
Figure 7 allows concluding that all mixes complied with the stipulated range for slump, with
values within the interval set, with no need to correct the ratio w/c.
According to the project EuroLightCon R12 (2000), the absorption of lightweight aggregates
100
110
120
130
140
0 10 20 30 40 50 60 70 80 90 100
Slu
mp
(m
m)
Replacement rate of LWA with RLWAC (%)
BHD
BHD20RHD
BM20RHD
BHD50RHD
BM50RHD
B100RHD
BM
BHD20RM
BM20RM
BHD50RM
BM50RM
B100RM
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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can lead to variations in the workability of concrete.
So, before producing the mixes, two tests were carried out (water content and water absorp-
tion at 30 minutes) to assess the amount of additional mixing water needed, to keep constant the
effective w/c. However, this is difficult to control and may result in variations in water quantity.
The largest slump obtained in BHD50RM (135 mm) can be associated with a variation of the water
absorption of the used aggregates, due to the difficulty of controlling the amount of mixing water.
3.1.2. Fresh concrete density
Figure 8 shows how the fresh concrete bulk density varies with the substitution rate of light-
weight aggregate (Leca HD and Leca M) with recycled lightweight aggregate (RLHD and RLM).
There is a clear growing trend of the fresh bulk density as recycled aggregates replace light-
weight aggregates. This result was expected, since the bulk density of recycled aggregates is higher
than that of lightweight aggregates. However, the replacement of lightweight aggregate Leca HD by
recycled aggregate RLM results in a decrease of this property, due to the greater density of light-
weight aggregate Leca HD, when compared to the recycled aggregate RLM.
Figure 8 - Fresh concrete density test results.
3.2. Hardened concrete properties
3.2.1. Compressive strength
This tests aims to evaluate the strength of the various concrete mixes produced when subject-
ed to an uniform compression stress. Thus, the mixes were exposed to an uniform growing com-
pression stress until collapse was reached.
Table 5 shows the average compressive strength values at 7, 28 and 90 days.
1700
1800
1900
2000
2100
2200
0 20 40 60 80 100
Den
sity
(k
g/m
3)
Replacement rate of LWA with RLWAC (%)
BHDRHD BMRHD BHDRM BMRM
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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Table 5 - Compressive strength at 7, 28 and 90 days.
7 days 28 days 90 days
Concrete mix Replacement ratio
LWA-RLWAC (%)
ρdry
(kg/m3)
(fcm / ρdry) 28
days (103 m)
fcm (MPa) fcm (MPa) fcm (MPa)
BRHD 0 1628 23.6 32.8 38.4 39.3
BH20RHD 20 1672 24.1 33.9 40.4 41.3
BH50RHD 50 1739 24.8 34.4 43.1 46.8
B100RHD 100 1852 23.6 36.1 43.7 48.5
BHD20RM 20 1612 23.9 33.1 38.5 39.2
BH50RM 50 1590 22.8 32.2 36.3 38.7
BRM 0 1453 13.2 16.0 19.2 20.7
BM20RM 20 1473 17.0 21.6 25.1 26.5
BM50RM 50 1503 18.4 23.8 27.7 30.5
B100RM 100 1552 21.5 27.7 33.4 34.6
BM20RHD 20 1533 17.2 22.6 26.4 28.6
BM50RHD 50 1653 18.6 24.2 30.7 32.8
All mixes tended to increase its compressive strength with age. However, a less sharp devel-
opment occurred after 7 days, in particular in mixes with lower recycled aggregates’ content.
Figures 9 and 10 show the influence that the replacement rate has on the compressive strength
at 28 days of series BHD and BM, respectively.
Figure 9 - Effect of the replacement rate on the compressive
strength at 28 days of series BHD.
Figure 10 - Effect of the replacement rate on the compressive
strength at 28 days of series BM.
Figure 9 shows there is a greater influence on compressive strength of series BHD when recy-
cled lightweight aggregate RLM are incorporated, compared to RLHD. This has to do with the
composition of the source concrete, since it is a non-structural concrete (devoid of fine aggregate)
and the fact that was low-strength mortars, with w/c = 0.55, were used.
y = 0,1x + 39,199 R² = 0,84
y = -0,1x + 38,953 R² = 0,96
32,0
34,0
36,0
38,0
40,0
42,0
44,0
46,0
0 20 40 60 80 100
f cm
, 2
8 d
ias (M
Pa)
Replacement rate (%)
Série BHDRHD
Série BHDRM
y = 0,1x + 20,721 R² = 0,94
y = 0,2x + 20,028 R² = 0,98
15,0
20,0
25,0
30,0
35,0
40,0
45,0
0 20 40 60 80 100
f cm
, 28
dia
s (M
Pa)
Replacement rate (%)
Série BMRM
Série BMRHD
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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Unlike with the replacement with RLHD, there was a decrease on compressive strength with
the replacement with RLM. This reduction results from the lower strength of recycled aggregate.
Nevertheless, the increase in compressive strength observed for mixes of series BHD with RLHD
results from the increase of mortar content and the higher strength of the recycled aggregate.
Unlikely series BHD, Figure 10 shows that for the BM series there is a greater influence of
the substitution of lightweight aggregate Leca HD with recycled lightweight aggregate RLM. This
results from the increase of mortar content that the use of this type of recycled lightweight aggre-
gate (RLHD) causes in the mix and the improvement of the characteristics of lightweight aggregate.
Nevertheless, the incorporation of RLM also caused a growing trend of compressive strength, due
again to the increase of mortar content and the greater strength of the recycled aggregate (resulting
from the effect of confinement of the light aggregate due to the paste).
3.2.2. Splitting tensile strength
This test consists of subjecting the cylindrical concrete specimens to a compressive force ap-
plied on a narrow area along their length, which generates orthogonal stresses to the plane of stress,
leading to the rupture of the specimen under tensile stress.
Table 6 shows the average values of splitting tensile strength for all the mixes, at 28 days.
Table 6 - Splitting tensile strength at 28days.
Splitting tensile strength
Concrete mix Replacement ratio LWA-RLWAC (%) fctm, SP (MPa)
BRHD 0 3.0
BHD20RHD 20 2.9
BHD50RHD 50 3.5
B100RHD 100 3.9
BHD20RM 20 3.0
BHD50RM 50 2.9
BRM 0 1.5
BM20RM 20 2.5
BM50RM 50 2.4
B100RM 100 2.7
BM20RHD 20 2.5
BM50RHD 50 2.8
Figures 11 and 12 show that the replacement of lightweight aggregate with recycled aggregate
improves the splitting tensile strength of mixes, at 28 days.
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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Figure 11 - Effect of the replacement rate on splitting tensile
strength at 28 days of series BHD.
Figure 12 - Effect of the replacement rate on splitting tensile
strength at 28 days of series BM.
Figure 11 shows a clear evolution of the splitting tensile strength as the aggregate replacement
of lightweight aggregate Leca HD with recycled RLHD increases, suggesting that it has a big influ-
ence on this property. The variation of this property derives mainly from the texture of the aggre-
gate, its tensile strength and the quality of the aggregate-paste transition zone. So the use of recy-
cled aggregate RLHD promotes increased tensile strength, given the higher angular shape and ten-
sile stress of lightweight aggregate Leca HD. On the other hand, lightweight aggregate Leca HD
replacement with recycled aggregate RLM shows a decrease in splitting tensile strength. This re-
duction is again due to the shape of RLM, which is less angular than the RLHD’s and its strength,
which is lower than that of Leca HD. Regarding series BM, Figure 12 shows a significant im-
provement in splitting tensile strength, as the substitution of lightweight aggregate Leca M with
recycled aggregate RLHD increases. However, the main cause of this increase is the higher tensile
strength of the recycled lightweight aggregate. On the other hand, the use of recycled aggregate
RLM led to a growth of splitting tensile strength. This growth of tensile strength is not as signifi-
cant as when replacing with RLHD, because of the lower tensile strength displayed by the RLM.
Finally, all mixes of both series showed the same mode of rupture, with the rupture surface in-
tersecting the aggregates. So, it can be concluded that this property tends to be mainly conditioned
by the tensile strength of the aggregate.
3.2.3. Modulus of elasticity
The analysis of modulus of elasticity allows assessing the stiffness characteristics or deforma-
bility of concrete based on the relationship between stress and strain (σ - ε) under elastic defor-
y = 0,01x + 2,8673 R² = 0,91
y = -0,003x + 2,996 R² = 0,93
2,6
2,8
3,0
3,2
3,4
3,6
3,8
4,0
0 20 40 60 80 100
f ctm
, sp
, 28
dia
s (M
Pa)
Replacement rate (%)
BHDRHD
BHDRM
y = 0,01x + 1,9034 R² = 0,61
y = 0,02x + 1,7574 R² = 0,94
1,4
1,9
2,4
2,9
3,4
3,9
0 20 40 60 80 100
f ctm
, sp
, 28
dia
s (M
Pa)
Replacement rate (%)
BMRM
BMRHD
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
CONCRETE
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mations. Thus, the response of structures to imposed loads is intrinsically related to this parameter,
both in terms of deformation and the distribution of stresses.
Table 7 shows the average values of the modulus of elasticity for all mixes at 28 days.
The variation of the modulus of elasticity with the rate of replacement of light aggregate with
recycled aggregate is illustrated in Figures 13 and 14.
Table 7 - Modulus of elasticity at 28 days.
Modulus of elasticity
Concrete mix Replacement ratio LWA-RLWAC (%) E (GPa)
BRHD 0 20.75
BHD20RHD 20 20.74
BHD50RHD 50 23.41
B100RHD 100 25.37
BHD20RM 20 21.26
BHD50RM 50 21.19
BRM 0 12.77
BM20RM 20 19.11
BM50RM 50 19.82
B100RM 100 20.64
BM20RHD 20 18.40
BM50RHD 50 21.79
Figure 13 - Effect of the replacement rate on the modulus of
elasticity at 28 days of series BHD.
Figure 14 - Effect of the replacement rate on the modulus of
elasticity at 28 days of series BM.
Figure 13 shows a high influence on the modulus of elasticity of the replacement rate of
the lightweight aggregate Leca HD with recycled aggregate RLHD. The elasticity modulus de-
y = 0,05x + 20,426 R² = 0,95
y = -0,003x + 21,068 R² = 0,12
18,00
19,00
20,00
21,00
22,00
23,00
24,00
25,00
26,00
0 20 40 60 80 100
E (G
Pa)
Replacement rate (%)
BHDRHD
BHDRM
y = 0,06x + 15,363 R² = 0,60
y = 0,1x + 14,593 R² = 0,91
12,00
14,00
16,00
18,00
20,00
22,00
24,00
26,00
0 20 40 60 80 100
E (G
Pa)
Replacement rate (%)
BMRM
BMRHD
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
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pends mostly on the proportion and stiffness of its elements, particularly the cement and aggre-
gates. The replacement of lightweight aggregate with RLHD implies increased mortar content
in the mix, leading to an increase in the modulus of elasticity. On the other hand, there was a
slight improvement of the initial modulus of elasticity in the mixes of series BHD composed by
RLM, linked to the increased stiffness of the recycled lightweight aggregate RLM (effect of the
envelopment of mortar around the original lightweight aggregate Leca M). Nevertheless, the
increase in the replacement rate of lightweight aggregate with RLM shows a slight reducing
trend of this property, justified by the similarity of the stiffness of recycled aggregate RLM and
the original lightweight aggregate (Leca HD). This results from the similarity of bulk densities
of the lightweight aggregate (Leca HD) and recycled aggregate RLM. On the other hand, the
paste that surrounds the recycled aggregate RLM acts as a confinement of the latter, which con-
tributes to a further increase of its stiffness. So, even though the bulk density of the RLM is
slightly lower than that of the original lightweight aggregate (Leca HD), their stiffness tends to
be identical.
Figure 14 also proves there is a big influence on the modulus of elasticity of the replace-
ment rate of lightweight aggregate Leca M with recycled aggregate RLHD. This substitution led
to a significant growth of this property that is justified by the higher strength and stiffness of
the recycled lightweight aggregate RLHD, when compared to Leca M. All mixes of series BM
made with RLM also show an improvement of this property with the increase of the replace-
ment rate. The stiffness increase results, once again, from the incorporation of a recycled aggre-
gate (RLM) with higher stiffness than the original aggregate (Leca M), not only because it has
greater bulk density but also because of the envelopment of the paste, which confines the ag-
gregate itself.
3.2.4. Abrasion resistance
The precise characterization of the abrasion resistance is a factor of great importance in
structures where the concrete is subjected to exterior actions which cause the deployment of its
elements.
Table 8 shows the average abrasion resistance values recorded for all mixes at 91 days.
Figures 15 and 16 show the influence of the replacement rate on the average wear result-
ing from the characterization of the abrasion resistance of the specimens.
In lightweight concrete, abrasion resistance depends not only on the lightweight aggre-
gates, but also on the strength of the matrix and bond between the aggregates and the cement
paste.
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
CONCRETE
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Table 8 - Abrasion resistance at 91 days.
Abrasion resistance
Concrete mix Replacement ratio LWA-RLWAC (%) Average wear (mm) Mass difference (g)
BRHD 0 4.55 9.3
BHD20RHD 20 4.22 9.0
BHD50RHD 50 4.12 8.5
B100RHD 100 4.46 9.4
BHD20RM 20 4.53 9.5
BHD50RM 50 4.17 8.5
BRM 0 5.78 12.2
BM20RM 20 4.69 9.9
BM50RM 50 3.59 7.5
B100RM 100 4.10 8.5
BM20RHD 20 4.88 10.0
BM50RHD 50 3.61 8.1
Figure 15 - Influence of the replacement rate on the average
wear at 91 days of series BHD.
Figure 16 - Influence of the replacement rate on the average
wear at 91 days of series BM.
Figure 15 shows that the replacement of lightweight aggregate Leca HD with RLHD pro-
motes a significant improvement of wear resistance as the replacement rate, increases except for the
B100RHD mix, whose result appears to be anomalous in view of the trend. This improvement re-
sults from the increase of mortar content conferred by RLHD. The result obtained for the B100RHD
mix can be partly justified by the variability associated to abrasion test.
On the other hand, the replacement of lightweight aggregate Leca HD with RLM led to signif-
icant improvements in abrasion resistance, i.e. a wear reduction with the increase of replacement
rate with RLM. This can be justified, once again, by the increase of mortar content in the mix, as
y = -0,0002x + 4,3478 R² = 0,002
y = -0,005x + 4,5779 R² = 0,86
4,0
4,1
4,2
4,3
4,4
4,5
4,6
4,7
4,8
4,9
5,0
0 20 40 60 80 100
We
ar a
vera
ge (
mm
)
Replacement rate (%)
BHDRHD
BHDRM
y = -0,016x + 5,2036 R² = 0,52
y = -0,01x + 5,2332 R² = 0,38
3,0
3,5
4,0
4,5
5,0
5,5
6,0
6,5
7,0
0 20 40 60 80 100
We
ar a
vera
ge (
mm
)
Replacemente rate (%)
BMRM
BMRHD
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
CONCRETE
15
well as by the fact that the original lightweight aggregate (Leca M) is benefited by a confinement
effect, which results in increasing its surface resistance, and therefore its wear resistance.
Regarding the replacement of lightweight aggregate Leca M with RLM, Figure 16 shows
again the increase of abrasion resistance for replacement rates up to 50%, primarily due to the
strength increase of the replaced aggregate that stems from the action of the paste that involves the
original lightweight aggregate Leca M and the increase of mortar content. However, for replace-
ment rates of 100% recycled aggregate (RLHD / RLM), there were, similarly to the BHD series,
anomalous values. In parallel, the replacement with RLHD showed a similar trend to that observed
for the replacement with RLM, even though leading to a higher degradation of abrasion resistance.
4. CONCLUSIONS
In this study, the mechanical performance of concrete with lightweight aggregates from
crushed lightweight concrete floors was analysed. Through the analysis of all the results, in the
course of this dissertation, it can be concluded that:
It is feasible to use recycled lightweight aggregate from crushed lightweight concrete, in the
production of recycled lightweight concrete (bulk density less than 2000 kg/m3);
In general, the incorporation of recycled lightweight aggregate promotes the increase of
compression strength, as well as structural efficiency. However, in mixes with structural
lightweight aggregate (Leca HD) the substitution of lightweight aggregate with non-
structural recycled lightweight aggregate (RLM) results in a decrease of this property;
Like compressive strength, splitting tensile strength showed very significant improvements
with the increase of the replacement rate of lightweight aggregate with recycled lightweight
aggregate, allowing confirming the correlation of this property with the rupture mode (the
surface rupture intersected the aggregates in both series), as well as with its tensile strength.
However, for mixes with structural lightweight aggregates (Leca HD),« the substitution with
recycled lightweight aggregates RLM led to a reduction of this property;
The modulus of elasticity showed a trend similar to that obtained in the characterization of
the splitting tensile and compressive strength, improving with the increase of the replace-
ment rate with recycled aggregate. With the exception of mixes composed by structural
lightweight aggregate (Leca HD) and recycled lightweight aggregate RLM, the results indi-
cate that there is a high relationship between the modulus of elasticity and the stiffness and
proportion of the elements of structural lightweight concrete;
Abrasion resistance also showed, in general, an improvement with the increase of the re-
placement rate. However, the variability associated to the abrasion test produced anomalous
MECHANICAL PERFORMANCE OF CONCRETE WITH RECYCLED LIGHTWEIGHT AGGREGATE FROM CRUSHED LIGHTWEIGHT
CONCRETE
16
results (by comparison with the trend observed) for mixes composed only by lightweight
aggregate recycled (RLHD and MLR), i.e. with no original lightweight aggregates.
Generally, the replacement in concrete of lightweight aggregate with recycled lightweight ag-
gregate promotes the improvement of its mechanical properties. This is promoted by the increased
bulk density, which usually translates into a negligible variation of structural efficiency (except for
mixes of series BHD composed by RLM). Nevertheless, the incorporation of 20% recycled light-
weight aggregate RLM, in the production of lightweight concrete led to very significant improve-
ments at the level of its mechanical performance.
5. REFERENCES
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CONCRETE
17
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