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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1): 71- 81
© Scholarlink Research Institute Journals, 2014 (ISSN: 2141-7016)
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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1):71- 81 (ISSN: 2141-7016)
Use of Recycled Concrete Aggregates in Structural Concrete in
Mauritius
Abdus Salaam Cadersa and Mahendra Ramchuriter
Faculty of Engineering,
University of Mauritius, Reduit
Corresponding Author: Abdus Salaam Cadersa
___________________________________________________________________________
Abstract
In the face of a possible scarcity of natural aggregates in the future in Mauritius and in line with sustainable
construction, this research investigates the feasibility of the use of recycled coarse concrete aggregate of 5-20
mm fraction as an alternative to natural coarse aggregates in structural concrete. The recycled coarse concrete
aggregate used in the research was processed from waste concrete at a major local concrete batching plant.
Three pairs of grade 40 concrete mix each consisting of a control mix and a test mix, were batched. The
percentage of recycled concrete coarse aggregates by weight of all in aggregates in the test mixes were 15%,
25% and 35% respectively. The properties of both natural and recycled coarse aggregates and the fresh and
hardened properties of both control and trial concrete mixes were investigated. The results showed that the
recycled coarse concrete aggregates had poor mechanical and physical properties but the chemical contents were
within limits. Compressive and flexural strengths as well as modulus of elasticity were lowered with an increase
in recycle aggregate content. The recycled coarse aggregates (RCA) had little influence on the hardened density
of concrete and an increase in recycled aggregate content led to a decrease in bleeding capacity. The main
drawback of the RCA was the high permeability of the recycled aggregate concrete. The research concludes that
the use of RCA in structural concrete is not technically feasible in Mauritius since the most desired concrete
properties such as strength and durability are affected. However, since the properties of RCA vary highly among
sources, results of this research must not be taken as absolutes. It is recommended that more testing need to be
carried out to make sure the conclusions that have been drawn in this paper are applicable.
__________________________________________________________________________________________
Keywords: recycled coarse concrete aggregate, recycled aggregate concrete, natural aggregate replacement,
graded aggregate mixture , structural concrete,
ways. The production of aggregates from concrete
debris reduces the extraction of natural rocks, which
is a useful and practical way of protecting the
environment. The recycled aggregates produced can
be reused in construction projects and thus the
economic impact associated through purchase and
disposal cost is reduced. Besides landfill space is also
saved while using recycled concrete waste.
INTRODUCTION
The global demand for construction aggregates
exceeds 26.8 billion tons per year (Wagih et al.
2013). A critical shortage in the sources of natural
aggregates is becoming a worldwide problem,
especially in the face of the development of major
urban centres. The application of recycled aggregates
is important in providing alternative material sources
to reduce the dependence of the construction industry
on natural aggregates (Ismail et al. 2013). Indeed,
many governmental bodies throughout the world
have introduced a number of policies to sensitize
people on the importance of preserving our natural
resources and also encouraging the use of recycled
materials (Limbachiya et al. 2004).
Though recycled concrete aggregates (RCAs) have
been studied for more than 30 years, their use as a
source of aggregate in concrete has still not been
widely implemented. The chemical and physical
properties of RCAs can vary widely depending on the
source of original concrete from which they are
derived and the consequences of this variability are
not well defined. As a result, the use of such
materials in structural applications has been limited.
However, the general conclusion is that RAC has
lower properties than corresponding natural
aggregate concrete (NAC) and that this decrease is
proportional to the replacement level of NA with
RCA (Butler et al. 2013). Li (2008b) stated that an
increase in RCA content led to a decrease in
compressive strength. In addition, recycled
Recycled concrete aggregate (RCA) is generally
produced by the crushing of concrete rubble,
screening, then removal of contaminants such as
reinforcement, paper, wood, plastics and gypsum.
Concrete made with such recycled concrete aggregate
is called recycled aggregate concrete (RAC) (Wagih
et al. 2013). According to the US Environmental
Protection Agency (2011), re-use and recycling of
concrete waste provides sustainability in various
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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1):71- 81 (ISSN: 2141-7016)
aggregates from concrete require more water for the
same workability than conventional concrete.
Density, compressive strength and modulus of
elasticity are relatively lower than that of the parent
concrete; and for a given water/cement ratio,
permeability, rate of carbonation and risk of
reinforcement corrosion are higher (Padmini et al.
2009). Doming-Caboa et al. (2009) have found that
RAC with 100% replacement level of coarse
aggregate had considerably higher shrinkage and
creep than those of control NAC, being 70% and 51%
higher respectively, after 180 days.
MATERIALS AND METHODOLOGY
Selection of Materials
Cement
Ordinary Portland cement was used as binder
throughout the study.
Natural Aggregates
The natural aggregates (NA) used in the project were
obtained from a single batching plant.
The
aggregates are produced by processing locally
available basaltic rocks. The coarse aggregates were
available in three sized fractions and were single
sized.
In response to the global energy crisis in 2007, the
Government of Mauritius announced the Maurice Ile
Durable (MID) concept, which includes economic,
social and environmental aspects of development, as
being the new long term vision for making Mauritius
a sustainable island.
Table 1: Types of Natural Aggregates Used
Aggregate type
Fine NA
Coarse NA
The recycling of waste therefore forms an integral
part of this concept. However the level of awareness
towards recycling in production or application of
recycled aggregates is still generally low in Mauritius
such that most of the waste concrete is directly
landfilled and only a small amount is used as a
backfilling material.
Fraction used
(mm)
0-4
4-10
10-14
14-20
Water
Tap water was used for mixing the raw materials.
Recycled Coarse Aggregates
Concrete waste obtained at a concrete batching plant
was hammered in the laboratory to produce recycled
concrete aggregates. The concrete waste was free of
dirt and impurities such as plastic and woods and the
strength class of the concrete was unknown.
A survey carried out recently by a major local stone
crushing and concrete production company showed
that the consumption of natural aggregates in the
local construction industry increased from one
million tons in 1986 to 4 million tons in 2004. The
only source for aggregate up till now was from rocks
piled in fields or buried in the earth and stone
quarries. According to this survey, these natural
sources of aggregates are becoming scarce and other
sources must be explored to meet the increasing
demand for the future (Quatre-Bornes Town Portal,
2011).
Since the use of fine RCA is not covered in BS 8500,
only the 5-20 coarse fraction was used in this
research. Hansen (1986) has also shown that fine
RCA are also coarser and rounder than required to
produce good quality concrete. In addition, ungraded
RCA was used due to the cost implication involved
with the grading of aggregates into single sizes
(Sowerby et al. 2004).
The following steps were carried out in the laboratory
to obtain the recycled coarse aggregates;
In line with sustainable construction and in the face
of a possible scarcity of natural aggregates in the
future in Mauritius, it is imperative that the feasibility
of the use of recycled aggregates be investigated. It
is in this context that this research attempts to assess
whether recycled concrete coarse aggregates can be
used as partial replacement of natural coarse
aggregates in structural concrete in Mauritius. The
objectives include the following;
1. Assess the physical, mechanical and
chemical properties of the recycled coarse
concrete aggregates of 5-20 mm fractions.
2. Investigate the effect of the recycled coarse
aggregates of 5-20 mm fractions on the
properties of structural concrete such as
bleeding,
permeability,
compressive
strength, flexural strength, hardened density
and elastic modulus.
Stage 1: The concrete rubbles of size 300-500mm
were placed one by one in the compression testing
machine. The concrete was loaded until it crushes
into fragments of size approximately 50-100mm.
This was collected for stage 2 for further processing.
Stage 2: The fragments collected were here broken
using an ordinary hammer. The broken pieces was
then passed through two sieves (20 and 5mm),
stacked together, to remove the unwanted size
fractions. The undersize fraction was discarded
(fines) and the oversized fraction was broken and
sieved again.
Stage 3: The processed recycled concrete aggregates
retained on the 5mm sieve were washed to remove
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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1):71- 81 (ISSN: 2141-7016)
In addition, tests to determine Flakiness Index;
Aggregate Impact Value; Aggregate Crushing Value,
Los Angeles Value, Sulphate Content and Chloride
Content were carried out. Results are given at section
3.
the smaller cement particles which were attached to
the aggregate. The final product is shown in figure1.
CONCRETE MIXES
Classification of Concrete Mixes
Three pairs of grade 40 mixes namely A, B and C
were designed and batched in the laboratory. Each
pair consisted of a control mix (CM) containing 0%
of recycled coarse aggregates by weight of all in
coarse aggregates and a test mix (RACM) where
natural coarse aggregates have been partially replaced
by recycled coarse aggregates. Each test mix was
compared with its corresponding control mix. The
mixes classification is given in table 3.
Figure 1: Processed RCA (5-20mm)
Testing of Aggregates
Preliminary tests were performed on both natural and
recycled aggregates.
These tests include sieve
analysis, determination of density and water
absorption. The graphs and data obtained were then
used in the concrete mix design process. The
sampling of aggregates was done according to BS
812. The aim was to have a representative sample of
the whole mass of aggregate available and this was
done using a riffle box.
Table 3: Classification of Mixes
Pair
A
B
C
Mixture proportions of each pair were determined in
accordance to the following conditions:
•
Same concrete grade (40MPa)
•
Same targeted workability (100 ± 10mm)
•
Same maximum grain size (20mm)
•
Same type and quantity of fine aggregate
•
Same particle size distribution of all in
coarse aggregates was maintained for the control mix
and test mix as far as possible.
The following properties of the fresh and hardened
properties of concrete were investigated;
•
Fresh Concrete: Plastic Density and
Bleeding Capacity
•
Hardened Concrete: Compressive Strength;
Hardened Density; Flexural Strength; Static Modulus
of Elasticity and Initial Surface Absorption.
The test procedures were carried out according to
British Standards and ASTM only.
Table 2 gives the densities and water absorption for
both NA and RCA and figure 3 gives the grading
curves of the aggregates.
Table 2: Densities and Water Absorption for both NA
and RCA used in the Study
Relative
Density
(Oven
Dried)
Relative
density
(SSD
Basis)
Water
absorption
(%)
0–4
2.82
2.92
2.47
4 – 10
2.67
2.77
2.80
10 – 14
2.65
2.74
2.61
14 – 20
2.66
2.74
2.37
5 – 20
2.40
2.46
9.45
Aggregate Type and
Size
Fine
NA
Coarse
RCA
Mix
CM1
RACM1
CM2
RACM2
CM3
RACM3
Particle Size Distribution (PSD)
Table 4 gives the fraction of aggregates used for the
research. A continuous range of all in aggregates was
used ranging from 0-20mm.
Table 4: Aggregate Types Used
Aggregate type
Fine NA
Coarse NA
Coarse RCA
Fraction used (mm)
0-4
4-10
10-14
14-20
5-20
The coarse NA was available in three sized fractions
and was single sized. The method described in ACI
Education Bulletin (2007) for combining two or more
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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1):71- 81 (ISSN: 2141-7016)
aggregates using their respective grading was used to
determine the percentage of each fraction of
aggregates to be used so as to obtain a graded natural
coarse aggregate. It was ensured that the PSD of the
natural graded aggregates complied with the upper
and lower limits of grading as given in tables 3, 4 and
5 of BS 882.
different ratios of the four fractions were combined
such that each of the resulting three PSD obtained
corresponded to the PSD used in the respective
control mix. Table 5 gives the ratios of coarse
aggregates used for the three pair of mixes. The
grading for all-in-aggregate and fineness modulus
was also computed. The fineness modulus confirmed
the small grading variation in each pair of mixes as
shown in table 5. The percentage replacement
obtained for RACM1, RACM2 and RACM3 was 15,
25 and 35% respectively.
In order to obtain a test mix having the same PSD as
the control mix, the same method and standard code
were used for the replacement of coarse NA by
processed RCA. However, in this procedure, three
Table 5: Ratio of Coarse Aggregates Used
4-10
0.20
0.30
0.225
0.225
0.25
0.15
CM1
RACM1
CM2
RACM2
CM3
RACM3
A
B
C
10-14
0.55
0.30
0.35
0.225
0.15
0.15
14-20
0.25
0.25
0.425
0.30
0.60
0.35
Mix Design
The British method also known as the ‘DoE method’
was used for the mix design process. This method of
design comprises of tables and charts available at the
Building Research Establishment (BRE).
The following data were used for the mix design:
• Design slump: 60-180 mm
• Target Strength: 40MPa
• NA fine: Crushed rock sand with 38% passing
600 µm sieve size
• NA coarse: Crushed basaltic rock with maximum
20mm size
• RCA coarse: Crushed concrete with maximum
20mm size
• Relative densities (SSD basis) and water
absorption from table 2.2 were used.
RCA
0
0.15
0
0.25
0
0.35
Fineness Modulus
4.80
4.78
4.83
4.83
4.87
4.88
Note: No factor of safety was added to the
characteristic strength.
Trial Mix
Trial mixes were carried out in order to obtain a
workable mix. This was achieved by controlling the
amount of free water added until the targeted slump
was obtained (100 ± 10mm). The adjusted free water
refers to the amount of water remaining or extra
added to obtain the targeted slump. The modified
water content was calculated by either subtracting the
remaining water or adding the extra free water. The
wet density and yield of the resulting concrete was
also evaluated (Neville & Brooks, 2010). The wet
density was done according to BS 1881: Part 107.
Table 6: Mix Proportions for Trial MixesMix
Cement
content (kg/m3)
Fine aggregate
(kg/m3)
CM1
CM2
CM3
RCAM1
RCAM2
RCAM3
402
402
402
402
402
402
860
860
860
857
852
847
Coarse aggregate (kg/m3)
4-10
194
218
242
290
216
143
10-14
533
339
145
290
216
143
14-20
242
412
581
242
288
335
5-20
0
0
0
145
240
335
Free water
(L/m3)
Absorbed
water (L/m3)
Total water
(L/m3)
225
225
225
225
225
225
46
46
45
56
62
68
271
271
270
281
287
293
Table 7: Results of Trial Mixes
Mix
Free
remaining
water (L/m3)
Modified free
water content
(L/m3)
Slump
obtained
(mm)
Total mass of
mixture per
batch (kg)
Actual Wet
density
(kg/m3)
Yield of
concrete
CM1
CM2
CM3
RCAM1
RCAM2
RCAM3
10
15
17
0
10
8
215
210
208
225
215
217
90
90
100
105
100
90
2492
2487
2483
2507
2491
2490
2470
2460
2450
2390
2400
2400
1.009
1.011
1.013
1.049
1.038
1.038
Neville and Brooks (2010) states that trial mixes are
used to adjust the free water needed to provide the
required workability. Since the density and yield of
Actual
Ratio
0.53
0.52
0.52
0.56
0.53
0.54
concrete was adequate, only free water content for the
6 mixes was re-adjusted.
74
Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1):71- 81 (ISSN: 2141-7016)
Table 8: Re-Adjusted Mix Proportions based on Trial Mix Results
Mix
Cement content
(kg/m3)
Fine aggregate
(kg/m3)
CM1
CM2
CM3
RCAM1
RCAM2
RCAM3
402
402
402
402
402
402
860
860
860
857
852
847
Coarse aggregate (kg/m3)
4-10
194
218
242
290
216
143
10-14
533
339
145
290
216
143
Surface Texture
Coarse NA
Less angular
but irregular
in shape
Coarse
RCA
Angular
aggregate with
well defined
edges
The aggregates had a rough surface
texture but visible pores and cavities
were noted on the surface of the
smaller fraction (4-10mm)
Rough texture. Presence of mortar
covering the aggregate particles
easily distinguished
Total water
(L/m3)
215
210
208
225
215
217
46
46
45
56
62
68
261
256
253
281
277
285
5-20
0
0
0
145
240
335
Aggregate Type
Flakiness Index (%)
Coarse NA (6.3-20mm)
21
Coarse RCA (6.3-20mm)
24
It was observed that the variation in flakiness index
between the two types of aggregate was not
significant. The flakiness index for both aggregate
types was within the limit set by BS 882:1992, i.e ≤
40%. The small difference in flakiness showed that
the method used to crush concrete debris produced
aggregate of approximately the same shape as that of
aggregate produced from a crushing plant. However,
the RCA was more angular than the NA, a factor
which can increase the void content of concrete. The
rough texture of NA implies good bonding between
aggregate and cement paste. On the contrary, for the
RCA, the mortar attached on the aggregate surface
can result in weak bonds with the cement paste.
Table 9: shape and surface texture of the aggregates
Shape
Absorbed
water (L/m3)
Table 10: Flakiness Index
RESULTS AND ANALYSIS
Aggregates
Particle Shape and Surface Texture
Table 9 provides information on shape and surface
texture of the aggregates. Particle shape and texture
were assessed visually. The result of the flakiness
index test on NA and RCA used in the study is given
in table 10.
Aggregate
14-20
242
412
581
242
288
335
Free water
(L/m3)
Aggregate Crushing Value (ACV), Impact Value
(AIV) and Los Angeles Test
Table 11: ACV, AIV and Los Angeles of Aggregates
Aggregate Type
NA
RCA
% increase w.r.t NA
ACV (%)
(10-14mm)
28
31
11
AIV (%)
(10-14mm)
24
31
28
ACV tests are used to assess the strength of
aggregates. The results given in table 11 showed that
NA was stronger that the RCA. The resistance of the
RCA to wear and impact was also lower than NA.
However the Los Angeles of both aggregate types
was within the limit set by the American Standard
Test Method (ASTM C33). The results showed that
the RCA has a very low resistance to breakdown,
which can have a negative impact on the strength and
stiffness of the resulting concrete. The low resistance
of the RCA to crushing and impact was due to the
two- phase material. The attached mortar was easily
reduced to pieces upon loading.
Los Angeles (%)
(Grade B Aggregate Sizes)
25
37
48
Table 12: Chloride and Sulphate Content
Aggregate
NA (5-20mm)
Chloride
Ions (%)
0
RCA (5-20mm)
8.28 * 10-4
Sulphate Ions (%)
1.92 * 10-4
4.29 * 10-5
It can be seen that no chloride ions were present in
the coarse NA but a very small amount was present in
the RCA. However, the chloride content of the latter
was within the requirement of the first standard for
RCA in China; DG/TJ07-008, i.e ≤ 0.25%. The
sulphate content was also very low for both aggregate
types and was less than 1%. The sulphate content of
the the RCA was within the limits set in table 1 of
DG/TJ07-008.
Chloride and Sulphate Ions
The chloride and sulphate content of coarse
aggregates are given in table 12.
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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1):71- 81 (ISSN: 2141-7016)
Grading
Figure 3: Grading Curve for Aggregates
Figure 4: Coarse Aggregate Grading for Control Mixes
Figure 5: Coarse Aggregate Grading for RAC Mixes
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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1):71- 81 (ISSN: 2141-7016)
Figure 6: Grading for All In Aggregate
Table 13 Grading Curve Observations
Figures
Observations
Fine aggregate: A smooth line and a continuous range of particle sizes were observed
representing clearly the fine aggregates fraction.
Singled sized coarse aggregate: The coarse fraction was also well graded.
3
RCA: The grading curve for RCA was between the 10-14mm and 14-20mm NA fraction. A
smooth curve was obtained showing the particle size distribution between the sieve sizes
5mm to 20mm.
A smooth line and a continuous range of sizes was observed for each control mixes.
4&5
6
The difference in grading was easily distinguished whereby CM3 represented the coarser
fraction of graded aggregate.
The same trend was observed with the RACMs and here RACM3 represented the coarser
fraction.
Fine aggregates were not replaced; therefore fine aggregates grading curve was same for the
six mixes.
The difference in aggregate grading can be clearly seen between sieve sizes 5-20mm. As
specified CM1-RACM1 (A), CM2-RACM2 (B) and CM3-RACM3 (C) had approximately
the same grading and same fineness modulus.
FRESH CONCRETE
Plastic Density
The densities and calculated yield for the 6 mixes are given in table 13.
Table 13: Plastic Density and Yield of Concrete
Mix
CM1
CM2
CM3
RACM1
RACM2
RACM3
Total mass of mixture per
batch (kg)
2492
2487
2483
2507
2491
2490
Actual Wet density
(kg/m3)
2470
2460
2450
2390
2400
2400
Average Density
(kg/m3)
2460
2397
Yield of
concrete
1.009
1.011
1.013
1.049
1.038
1.038
concrete. However, since yield was satisfactory in
this study, this property of concrete was not modified
when performing the trial mixes.
The results show that the ratio of yield for all the six
mixes was above 1.0. There was a slight decrease of
2.6% in the average wet density of RACMs as
compared to that of the control mixes. Yield showed
that the volumetric quantity of concrete produced per
batch was higher than calculated. This tendency was
observed with all the three pairs of mixes. Yield was
computed to make sure that the right quantity of
materials was being used to get a given volume of
The angular shape of the RCA rendered compaction
more difficult resulting in a concrete with higher air
voids and lesser density. In addition, the higher
water/cement ratio and resulting yield values of the
RACMs showed that not all water was removed upon
77
Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1):71- 81 (ISSN: 2141-7016)
compaction. This could also affect the permeability
of the resulting recycled concretes.
Bleeding
The bleeding capacity and bleeding rate of each
concrete pairs are given in table 14.
Table 14: Bleeding Capacity
Mix
Bleeding Capacity (% Free
water)
2.33
3.94
CM1
RACM1
A
% increase w.r.t
control
CM2
RACM2
% increase w.r.t
control
CM3
RACM3
% increase w.r.t
control
B
C
69
2.32
2.87
24
Figure 7: Variation of Compressive Strength for each
Pair of Mixes
2.45
2.85
16
Results show that the strengths of the recycled
aggregate concretes are lower that their
corresponding control mixes. The strength attained
by the RACMs were below the target strength and
indicated that the RACMs were behaving more like
grade 30 concrete.
The bleeding of the RACMs was higher than the
CMs due to a higher water/cement ratio. However, in
this research, bleeding water decreased from RCAM1
to RCAM3, that is with increasing recycled aggregate
content. This discrepancy is explained by the fact that
RCAM1 contains a higher proportion of smaller sized
fraction (4-10 mm) than RCAM3 and therefore a
higher existing/attached mortar content such that
water absorbed by the latter is higher.
This behavior can be attributed to the weak bonding
in the transition zones. For the CMs, the cement paste
was bonded to the aggregate surface, whereas for the
RACs, the cement paste was bonded to the existing
mortar attached on the aggregate particles instead of
the aggregate itself. The bond was hence weak and
failure planes occurred between cement pastes and
existing mortar.
Hardened Concrete
Compressive Strength
The compressive test results on 100mm cubes are
given in table 15 and figure 4.
7days
31.13
23.50
28days
43.37
33.33
25
23
Moreover, the high water/cement ratio of RACMs
contributed to the decrease in strength observed. As
expected, the difference in strength was most
observed in pair A mixes at both 7 and 28 days due to
the high difference in water/cement ratio between
CM1 and RACM, namely 0.53 as compared to 0.56.
Please refer to table 7 above.
Flexural Strength
Flexural strength results are given in table 16 and
figure 5.
Table 16: Flexural Strength (N/mm2)
Table 15 Compressive Strength Results (N/mm2)
Mixes
CM1
RACM1
% decrease
A w.r.t control
CM2
35.10
43.50
RACM2
31.10
37.27
% decrease
w.r.t control
11
14
CM3
33.34
44.91
RACM3
29.04
38.92
% decrease
w.r.t control
12
13
Mixes
B
C
78
Flexural
Strength
(N/mm2)
Predicted Flexural Strength:
0.7
(N/mm2)
CM1
RACM1
% decrease
w.r.t control
5.5
4.2
24
4.64
4.04
CM2
RACM2
% decrease
w.r.t control
5.4
4.6
15
4.62
4.27
CM3
RACM3
% decrease
w.r.t control
5.5
4.5
18
4.69
4.36
Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1):71- 81 (ISSN: 2141-7016)
Figure 8: Flexural Strength for Control and RAC Mixes
A slight decrease in density was observed when the
RACMs were compared to their respective control
due to the lower specific gravity of RCA in the mixes
and also due to presence of trapped water that was
confirmed from yield results.
Jackson and Dhir (1988) states that as a guide,
flexural
strength
may
be
taken
as
0.7
.
Therefore,
as
given in figure 9, the flexural strengths of both CMs
and RACMs were above the predicted value.
A general decrease in flexural strength was observed
with RACMs when compared to their respective
CMs. Higher water/cement ratio and weaker bonding
at the transition zone resulted in poor flexural
strength of RACMs. However, the difference was
higher in Set A mixes (24%) due to the high
water/cement ratio and higher content of weak
aggregates (4-10 mm) fraction in the mix.
Elastic Modulus
Results of elastic modulus are given in table 18 and
variation of elastic modulus for each pair of mixes is
given in figure 6.
Table 18: Elastic Modulus (GPa)
Mixes
CM1
RACM1
% decrease w.r.t control
CM2
RACM2
% decrease w.r.t control
CM3
RACM3
% decrease w.r.t control
Hardened Density
The hardened densities for the 6 mixes are given in
table 17.
Table 17: Hardened Densities (Kg/m3)
Mix
CM1
7 Days
2483
28 Days
2550
RACM1
2475
2525
0.3
1.0
CM2
2509
2575
RACM2
2492
2549
0.7
1.0
2513
2475
1.5
2533
2500
1.3
A % decrease w.r.t
control
B % decrease w.r.t
control
C
CM3
RACM3
% decrease w.r.t
control
Elastic Modulus
31.3
29.4
6
31.0
28.3
9
29.4
25.2
14
Figure 9: Modulus Elasticity for Control and RAC
Mixes
Typical values for the modulus of elasticity of normal
concrete lie in the range of 18 - 30 GPa. A high value
indicates a stiff concrete (Neville & Brooks, 2010).
79
Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1):71- 81 (ISSN: 2141-7016)
Therefore both the CMs and the RACMs have good
elastic modulus values and are stiff concretes.
set C mixes respectively. This is firstly due to the
increase in amount of RCA and secondly due to the
increase in Fineness Modulus of aggregates from set
A to C mixes, resulting in an extension of the
interface transition zone.
However, the values of modulus of elasticity of the
RCAMs are lower than those of the CMs. This
behavior can be explained by the weak RCA content
in the mixes.
Initial Surface Absorption
The results of the permeability test are given in table
19 and figure 7.
Results also show that the decrease in modulus of
elasticity was 6%, 9% and 14% with set A, set B and
Table 19: ISAT (ml/m2/s)
Reference
CM1
RACM1
10min
(ml/m2/s)
0.82
1.56
30min
(ml/m2/s)
0.43
0.84
1hour
(ml/m2/s)
0.31
0.68
2hours
(ml/m2/s)
0.22
0.48
Total
water
absorbed (ml/m2/s)
1.78
3.56
Ratio
of
RACM/CM
CM2
RACM2
0.80
1.52
0.50
0.84
0.30
0.64
0.16
0.47
1.76
3.47
2.0
CM3
RACM3
0.84
1.44
0.43
0.94
0.32
0.66
0.17
0.47
1.76
3.51
2.0
2.0
However, the chemical composition of the RCA was
not an issue as both chlorides and sulphate content
were within limits specified in DG/TJ07-008.
Modulus of elasticity of the concrete decreased with
increasing RCA content, but nevertheless was still
within the range of typical values for stiff concrete.
Since the most desired concrete properties such as
strength and durability are affected, it is concluded in
that the use of RCA is not technically feasible in
Mauritius. However, since the properties of RCA
vary highly among different sources, more testing
should be carried out to ensure that conclusions that
have been drawn in this paper are applicable.
In addition, the research demonstrates that the
method described in the ACI Education Bulletin
(2007) can be used effectively to combine different
single aggregate fractions to produce a graded
aggregate mixture which satisfies the limits set in BS
882 for both graded aggregates and all in aggregates.
Figure 10: Initial Surface Absorption
REFERENCES
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The ISAT values given in table 19 show that the
permeability of the RACMs were twice higher than
that of the control mixes. In addition, the total water
absorbed was also almost doubled for the RCAMs.
Therefore, RACs are much more permeable and
therefore less durable that normal concrete mixes.
The water/cement ratio, aggregate porosity and voids
content all contributed to the high permeability of
RACMs.
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