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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME

TECHNOLOGY (IJCIET)

ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 4, Issue 6, November – December, pp. 193-201
© IAEME: www.iaeme.com/ijciet.asp
Journal Impact Factor (2013): 5.3277 (Calculated by GISI)
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IJCIET
©IAEME

A STUDY ON THE EFFECT OF ADDITION OF SILICA FUME ON
STRENGTH PROPERTIES OF PARTIALLY USED RECYCLED COARSE
AGGREGATE CONCRETE
Dr. D. V. Prasada Rao1 and G. V. Sai Sireesha2
1

(Associate Professor, Department of Civil Engineering, Sri Venkateswara University
College of Engineering, Tirupati, Andhra Pradesh, India)
2
(PG Student, Department of Civil Engineering, Sri Venkateswara University
College of Engineering, Tirupati, Andhra Pradesh, India)

ABSTRACT
This paper presents an experimental investigation on the effect of silica fume (SF) on various
strength properties of partially used recycled coarse aggregate (RCA) concrete. It was observed
from the preliminary investigation that the strength properties of partially used recycled coarse
aggregate concrete decreases with increase in the recycled coarse aggregate content. An attempt has
been made to find the effect of silica fume on various strength properties of partially used recycled
coarse aggregate concrete. In the present investigation, recycled coarse aggregate was used as 25%
and 50% replacement of natural coarse aggregate in addition to silica fume partially replaces 5%, 8%
and 12% of cement by weight. To evaluate the partially used recycled coarse aggregate concrete
containing silica fume for structural applications, various strength properties of concrete were
obtained. Compressive strength, split tensile strength and modulus of rupture test results of M20 and
M30 grades of concrete prepared using recycled coarse aggregate and silica fume are presented and
then the results are compared with the natural coarse aggregate concrete. Using the test results, it can
be concluded that a combination of 25% recycled coarse aggregate and 8% silica fume can be
recommended for the preparation of structural concrete, because of its improved strength
characteristics compared to the natural coarse aggregate concrete. It can be concluded that the
strength properties of partially used recycled coarse aggregate concrete can be improved by the
addition of silica fume. The increase in the strength of partially used RCA concrete containing silica
fume can be attributed to the availability of additional binder in the presence of silica fume. The
availability of additional binder enhances the paste-aggregate bond results the RCA concrete with
silica fume has improved strength properties. By adjusting the proportions of recycled coarse
aggregate and silica fume in the concrete mix better strength properties can be achieved. Hence,
193

recycled coarse aggregate and silica fume offers an economically viable solution to convert the waste
materials to valuable resources.
Keywords: Recycled Coarse Aggregate, Silica Fume, Partial Replacement, Structural Concrete,
Modulus of Rupture.
1. INTRODUCTION
Concrete is the most common construction material prepared using cement, natural
aggregates and water. With fast industrialization, infrastructure development and increase in the
population leads to huge construction activities. Due to increase in the various construction activities
the natural resources are fast depleting which in turn results in escalating the cost of construction
materials and then the total cost of construction [1]. During the natural calamities like earthquakes,
the collapse of reinforced concrete buildings causes a serious concern with respect to the disposal
very large quantities of demolished concrete waste material. For the disposal of concrete waste
material the availability of open land is also a problem. To preserve the natural resources for our
future generations and also to overcome the problem of disposal, the concrete waste can be used to
obtain coarse aggregate known as recycled coarse aggregate. The use of recycled coarse aggregate in
the preparation of new concrete has various environmental benefits also. Hence, recycling of the
concrete waste can be considered as an excellent source of coarse aggregate for the production of
new concrete. Hence, recycled coarse aggregate offers an economically viable solution to convert the
concrete waste material to a valuable resource.
The study on the application of recycled coarse aggregate for concrete preparation has been
going on over the last few decades [2, 3]. The properties like water absorption and specific gravity of
recycled coarse aggregate are to be determined for the concrete mix design. It has been observed that
the recycled coarse aggregate obtained from concrete waste is rough-textured, angular and has higher
water absorption than the natural coarse aggregate affecting the concrete mix proportion. The density
of recycled concrete aggregate is less than the natural coarse aggregate because of the porous and
less dense residual mortar adhering to the surface leading to the reduction in strength of the partially
used recycled coarse aggregate concrete. The preliminary studies indicated that as the quantity of
recycled coarse aggregates increased the strength of the recycled coarse aggregate concrete is
decreased.
The present research paper is focused on to understand the effect of silica fume on the
strength properties of partially used recycled coarse aggregate concrete. The recycled coarse
aggregate was used as partial replacement of natural coarse aggregate and silica fume as partial
replacement of cement. Compressive strength, split tensile strength and modulus of rupture of
partially used recycled coarse aggregate concrete with the addition of silica fume were obtained
using concrete the specimens and then the results were compared with the natural coarse aggregate
concrete.
2. EXPERIMENTAL PROGRAMME
2.1 Materials
2.11 Cement
In the present investigation Ordinary Portland cement (OPC) of 43 Grade confirming to IS
specifications was used [4]. The specific gravity of the cement is 3.15.

194

2.12 Fine aggregate
Locally available river sand (Zone - II) confirming to IS specifications with fineness modulus
of 2.7 was used as the fine aggregate in the concrete mix [5].
2.13 Recycled Coarse Aggregate
In general, to prepare a specified grade of recycled coarse aggregate concrete, it is difficult to
procure the recycled coarse aggregate from the same grade of demolished concrete waste material.
To account this practical difficulty, the recycled coarse aggregate used in the present investigation
was obtained by mixing the coarse aggregate obtained by crushing M20, M25 and M30 grade natural
coarse aggregate concrete cubes tested in the laboratory. The crushing process affects the
characteristics of the resulting recycled coarse aggregates [6]. The nominal sizes of recycled or
natural coarse aggregate adopted in the present investigation were 20 mm and 10 mm. The properties
of the recycled and natural coarse aggregate obtained by tests conducted as per IS specifications are
given in Table 1. Recycled coarse aggregate has 10 to 15% lower density and 3 times more water
absorption than the natural coarse aggregate; this is due to the porosity of the hydrated cement paste
attached to RCA. In general, recycled coarse aggregate was found to be angular and porous
compared to natural coarse aggregate. Further, recycled coarse aggregate is to be cleaned and should
be free from detrimental levels of impurities and other harmful constituents.
Table1: Properties of NCA, RCA and Sand
Property

Natural Coarse
Aggregate

Recycled Coarse
Aggregate

Fine aggregate (Sand)

Specific Gravity

2.67

2.48

2.52

Water Absorption (%)

0.46

1.52

0.44

2.14 Silica Fume
Silica fume is non-crystalline Silica, a by-product of ferro-silicon industry. Compared to
cement, the particle size of silica fume is very finer. It acts as an excellent pore-filling material.
Silica fume is usually considered as a supplementary cementitious material [7]. Generally, it can be
used in the proportions of 5 – 12% of the cement content. The properties of silica fume used in the
present investigation are shown in the Table. 2
Table 2: Properties of Silica Fume
S.No.

Characteristics

Actual Analysis Result

1

Sio2

90.20%

2

Moisture

0.20%

3

Pozzolana Activity Index

127%

4

Specific Surface Area

21m2/gm

5

Bulk density

604

6

> 45 Microns

0.20%
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2.15 Water
The water used for casting and curing concrete test specimens was free of acids, organic
matter, suspended solids and impurities which when present can adversely affect the strength of
concrete.
2.2 Concrete Mix Proportions
In the present research work the effect of silica fume as partial replacement of cement on
M20 and M25 grades of partially used recycled coarse aggregate concrete is studied. Concrete
specimens were prepared with 25% and 50% of recycled coarse aggregate in addition to 5%, 8% and
12% of silica fume content.
M20 and M25 grades of natural coarse aggregate concrete were designed based on Indian
Standard method of mix proportioning [8]. In the concrete mix, 10 mm and 20 mm coarse aggregate
were used in the ratio of 1:1.5. No superplasticiser was used in the concrete mixes. The mix
proportions of M20 and M25 grades of concrete by weight are shown in Table. 3. After obtaining the
concrete mix proportions, the recycled coarse aggregate was used as 25% and 50% by weight
replacement of the natural coarse aggregate and silica fume partially replaces 5%, 8% and 12% of
cement by weight. The mix proportions were obtained by assuming the aggregate were in a
saturated-surface dry condition and appropriate moisture adjustments were made to cater for the
different water absorption properties of the aggregates.
Table 3: Concrete Mix Proportions
Grade of
Concrete

Cement

Fine
Aggregate

Coarse
Aggregate

Water-Cement
Ratio

Cement
(kg/m3)

M 20

1

1.97

3.79

0.55

320

M 25

1

1.71

3.62

0.50

340

2.3 Preparation of Concrete Test Specimens
Two types of concrete specimens were prepared using natural coarse aggregates and recycled
coarse aggregate with the addition of silica fume. Four sets of concrete specimens for different
curing periods 3, 7, 28 and 56 days were prepared. Concrete specimens consist of 150 mm × 150 mm
× 150 mm cubes, 150 mm × 300 mm cylinders and 100 mm × 100 mm × 500 mm prisms. The cubes
and cylinders were used to find the compressive strength of concrete. The cylinders were also used to
obtain the split tensile strength of concrete and the prisms were used to get the modulus of rupture of
concrete.
2.4 Testing
The compressive and split tensile strengths of concrete were obtained using a compression
testing machine. The rate of loading for the compressive and splitting tensile tests is in accordance
with Indian Standard specifications [9]. The compressive strength tests on concrete cube and cylinder
specimens were carried out at the age of 3, 7, 28 and 56 days. The split tensile strength and modulus
of rupture tests were conducted on concrete specimens at the age of 28 days.

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3. RESULTS AND DISCUSSION
3.1 Compressive Strength
The results of the cube compressive strength of M20 and M25 grades concrete for various
combinations of RCA and SF at different curing periods is shown in Fig.1. Each value of the cube
compressive strength indicates the average of three test results. It can be observed that as the age of
concrete increases the cube compressive strength of partially used recycled coarse aggregate concrete
also increases with silica fume.

(i) 25% RCA

(ii)50% RCA
(a) M 20 Grade Concrete

25% RCA

50% RCA
(b)M 25 Grade Concrete

Fig. 1.Variation of Cube Compressive Strength of Partially Used RCA Concrete with different
percentages of Silica Fume.

197

The variation of 7 day’s and 28 day’s cube compressive strength of M20 and M25 grades
concrete containing 25% and 50% of RCA content with increase in the percentage of silica fume is
shown in Fig. 2. The compressive strength of RCA concrete initially increases up to 8% percentage
of silica fume and then decreases with increase in the silica fume content.

(a) M 20 Grade Concrete

(b) M 25 Grade Concrete
Fig.2. Variation of Cube Compressive Strength (7 Day’s and 28 Day’s) of Partially Used RCA
Concrete with different percentages of Silica Fume.

Comparison is also made between the cube compressive strength and cylindrical compressive
strength of partially used recycled coarse aggregate concrete containing various percentages of silica
fume at the age of 28 days. The test results are shown in Table.4. Similar to the natural coarse
aggregate concrete, in the case of recycled coarse aggregate concrete with silica fume the ratio of
cylindrical compressive strength to the cube compressive strength is also around 0.8

198

Table 4: Comparison between Cube and Cylinder Compressive Strength of Partially Used Recycled
Coarse Aggregate Concrete with varying percentages of Silica Fume
Recycled
28 Days
28 Days
Ratio of
Grade of
Coarse
Silica
Cube Compressive
Cylinder Compressive
Cylinder
Concrete
Aggregate Fume Strength of Concrete
Strength of Concrete
Strength to
(%)
(%)
(N/mm2 )
(N/mm2)
Cube Strength
0
0
33.00
26.28
0.80
M 20
5
30.68
24.50
0.80
25
8
34.60
27.20
0.79
12
28.05
22.13
0.79
5
28.00
22.32
0.79
50
8
33.72
27.00
0.80
12
26.31
21.12
0.80
0
0
30.96
25.42
0.82
5
29.50
24.81
0.84
25
8
33.86
27.76
0.82
12
29.36
24.50
0.83
M25
5
25.58
20.72
0.81
50
8
31.68
26.34
0.83
12
25.43
20.85
0.82

3.2 Split Tensile Strength
The variation of split tensile strength of M20 and M25 grades of partially used recycled
coarse aggregate concrete of 25% and 50% RCA content with increase in the silica fume content is
shown in Fig. 3. The split tensile strength of RCA concrete initially increases up to 8% percentage of
silica fume and then decreases with increase in the silica fume content. The split tensile strength of
M20 and M25 grades of natural coarse aggregate (control) concrete is 3.66 N/mm2 and 3.24 N/mm2
respectively. It is also found that the split tensile strength of RCA concrete with silica fume is found
to be approximately 10% of its 28 day’s cube compressive strength.

(a) M20 Grade Concrete

(b) M25 Grade Concrete

Fig.3. Variation of Split Tensile Strength of Partially Used RCA Concrete with different percentages
of Silica Fume

199

3.3 Modulus of Rupture
The variation of modulus of rupture of M20 and M25 grades of partially used recycled coarse
aggregate concrete containing 25% and 50% RCA with increase in the silica fume content is
presented in Fig.4. The modulus of rupture of RCA concrete initially increases up to 8% percentage
of silica fume and then decreases with increase in the silica fume content. It is found that the
modulus of rupture of partially used RCA concrete with silica fume is varying between 13 and 20%
the 28 day’s cube compressive strength of concrete. The modulus of rupture of M20 and M25 grades
of natural coarse aggregate (control) concrete is 4.20 N/mm2 and 4.96 N/mm2 respectively.

(a) M20 Grade Concrete

(b) M25 Grade Concrete

Fig.4. Variation of Modulus of Rupture of Partially Used RCA Concrete with different percentages
of Silica Fume

4. CONCLUSIONS
Using the test results, in it can be concluded that as the percentage of silica fume is increased
the various strength properties of partially used recycled coarse concrete are also increased up to 8%
of silica fume content and then the strength of concrete is decreased with further increase in the silica
fume content. The compressive strength, split tensile strength and modulus of rupture test results of
concrete prepared with 25% and 50% of recycled coarse aggregate in addition to 5%, 8% and 12% of
silica fume content indicates exactly the same trend.
The increase in the strength of partially used RCA concrete containing silica fume can be
attributed to the availability of additional binder in the presence of silica fume. Silica fume has high
amorphous silicon dioxide content and is a very reactive pozzalanic material. As the Portland cement
in concrete begins to react chemically, it releases calcium hydroxide. The silica fume reacts with the
calcium hydroxide to form additional binder material. The availability of additional binder enhances
the paste-aggregate bond results the RCA concrete with silica fume has improved strength properties.
The decrease in the various strength characteristics of partially used RCA concrete with increase in
the silica fume content beyond 8% is due to the fact that the strength of the additional binder formed
in the presence of silica fume decreases due to excessive silica fume content. Hence, low strength
properties of partially used recycled coarse aggregate concrete can be improved by the addition of a
specified percentage (8%) of silica fume content.
The experimental studies indicated that improved strength characteristics of RCA concrete
can be obtained with the combined use of recycled coarse aggregate and silica fume compared to the
concrete with recycled coarse aggregate only. The influence of varying percentages of recycled
coarse aggregate and silica fume on M 20 grade of concrete is studied and it is observed that at a
combination of 25% of recycled coarse aggregate and 8% of silica fume best results are obtained.
200

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