International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015
Effect of Fly Ash on CBR Results of Granular Sub Base Subjected to
Modified Compaction
Ratna Prasad, R1.
Darga Kumar, N2.
1
Research Scholar, JNTU Kakinada and Professor of Civil Engineering, VVIT, Guntur, AP, India
Professor of Civil Engineering College of Engineering, Science and Technology (CEST), Fiji National University (FNU)
2
Abstract---The granular soil compacted at 5 to 10% addition
of fly ash shows higher dry density compared to all other
proportions of fly ash. The addition of percentage fly ash in
soil gravel mixture, OMC increases both in light and
modified compaction and increases slightly from 0% to 10%
and drastically increases from 10% to 25% and slightly
increases from 25% to 30% in light and
modified
compaction.. The CBR values of un soaked and soaked
conditions are decreases for each percentage fly ash soil
mixture of 0% to 30%.The percentage decrease of CBR
values from un soaked to soaked for 0% to 5% fly ash soil
mixtures are from 12.12% to 9.53% and above 15% fly ash
soil mixture, 30% to 40% reduction in CBR values were
observed for both soaked and un soaked conditions. For both
un soaked and soaked conditions the maximum percentage
decrease of CBR values in between each successive
percentages from 0% to 30% fly ash soil mixture is of
16%.From the results it can be concluded that 20% to 30%
fly ash soil mix can be used for both soaked and un soaked
conditions.
Keywords: MDD, OMC, CBR, fly ash granular soil mixtures,
modified compaction, Water Content, percentage flyash
(%FA), cohesion
1 INTRODUCTION
India has about 70 thermal power plants and coal currently
accounts for 70 per cent of power production in the country.
The process of coal combustion results in fly ash. The
problem with fly ash lies in the fact that not only does its
generated annually in India, with 65 000 acres of land being
occupied by ash ponds. Such a huge quantity does pose
challenging problems, in the form of land usage, health
hazards, and environmental dangers. Both in disposal, as well
as in utilization, utmost care has to be taken, to safeguard the
interest of human life, wild life, and environment. Various
ISSN: 2231-5381
Indian collieries supply the coal, which is known to have a
very high ash content of almost 40 to 45 per cent. Due to its
self-cementing properties, fly ash can be an effective
stabilizer for granular and fine grained materials. Fly ash by
itself has little cementitious value but in the presence of
moisture it reacts chemically and forms cementitious
compounds and attributes to the improvement of strength and
compressibility characteristics of soils. Expansive soils can
be potentially stabilized effectively by cation exchange using
fly ash. Utilization of fly ash towards engineering
applications can solve two major issues (i). Environmental
pollution problem and (ii)Wastage of land due to its dump on
the agricultural land. Nicholson presented a number of
patents (1977, 1982) for a series of investigations on cement
kiln dust (CKD) and fly ash mixtures for producing sub base
materials with different aggregates. CKD was used up to
16% by weight of the mixture, producing a durable mass by
reacting with water at ambient temperatures. The most
widely used application for self-cementing flyash is in
increasing the strength of unsuitable or unstable subgrade
materials. Generally, clay soils have soaked CBR values
from 1.5% to 5% (Rolling’s and Rolling’s 1996), which
provides very little support to the pavement structure.There
are many investigations carried out towards utilization of fly
ash. Addition of 16% self-cementing fly ash increases the
soaked CBR values of heavy clay soils into the mid-30s,
which is comparable to gravelly sands (Rolling’s and
Rolling’s 1996).especially in stabilizing the swelling soils
(Kate, 1998; Erdal Cokca, 2001; Pandian et al, 2002; Phani
Kumar and Sharma, 2004;Rao and Shivananda, 2005; Prasad
et al, 2010; Sivapullaiah and Arif, 2011). There are few
studies available in the literature relevant to the utilization of
fly ash along with granular soil in the road construction.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015
II. MATERIALS AND TEST METHODS
Table 2Properties of fly ash
A. Soil
The gravelly sand used in the present study was collected
from sekuru village near Guntur, Andhra Pradesh state, India.
The soil collected was kept in controlled conditions in the
laboratory and was used for testing as per the Indian Standard
specifications given in the respective test codes. For this soil,
the basic tests were conducted in the laboratory for its
characterization. As per the basic properties of soils are
concerned, it indicates that the soil is greyish to brown in
colour and has soil proportions of gravel, sand and little fine
fraction. The soil has 0.35% slit and clay, 92% sand and 7%
gravel fractions. The grain size distribution curve of the soil
and flyash is presented in Fig.1. The various basic properties
of soil and flyash are presented in the Table.1and 2
Property
Value
Specific Gravity
Optimum Moisture Content
OMC(%)
Maximum Dry Density, MDD
(KN/m3)
Un soaked CBR (%)
% Gravel
% Sand
% Silt and Clay
1.97
18
13.80
34
0
97.5
2.5
B. Fly Ash
The fly ash used in this investigation was collected from
Vijayawada Thermal Power Station (VTPS) Vijayawada. The
fly ash sample collected was stored in the air tight containers.
The grain size distribution curve [IS: 2720 (Part 4)-1985] for
fly ash is presented in the Fig.1 The various properties of the
fly ash obtained from the Vijayawada Thermal Power Station
(VTPS), Vijayawada, AP state, India are presented in the
Table.2. The fly ash proportions adopted in the study by dry
weight of soil are 0%, 5%, 10%, 15%, 20% , 25% and 30%.
C. Grain Size Distribution
The grain size distribution test was conducted as per the
specifications given in the IS: 2720 (Part 4)-1985. The
graphs plotted between percent passing vs. particle
size for the various proportions of flyash in granular
soil.
Fig.1 Grain size distribution curve for soil and fly ash
Table1 Basic properties of soil
Property
Specific gravity
Optimum Moisture Content, OMC
(%)i-Modified Compaction
Maximum Dry Density, MDD (kN/m3)
–Modified Compaction
Un-soaked CBR(%)Modified
Compaction
Soaked
CBR(%)
-Modified
Compaction
% Gravel
% Coarse Sand
% Medium Sand
% Fine Sand
% Silt & Clay
Effective Diameter, D10 (mm)
Coefficient of Uniformity, cu
Coefficient of Curvature, cc
Soil Classification
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Value
2.62
08.00
19.90
55.24
48.54
07.50
12.50
42.10
37.25
00.35
00.21
04.28
00.76
SW
D. Compaction Test
IS light and modified compaction tests have been conducted
on the soil with different % of flyash such as 5%, 10%, 15%,
20%, 25% and 30% and determined the Optimum Moisture
Content (OMC) and Maximum Dry Density (MDD) as per
IS:2720 (Part 7)-1980. The modified compaction tests are
adopted because; the majority highway pavements are
designed for high volume traffic loading
E.CBR Test
In the construction of pavements for low volume and heavy
volume traffic conditions, the CBR is the major design
parameter of subgrade in the estimation of thickness of
pavement. To understand CBR variations of fly ash gravelly
sand mixtures, a laboratory testing was carried out for the
conditions of modified compaction as per [IS: 2720 (Part l6)1979]. .
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015
A. Compaction Characteristics
In case of pure gravelly sand as the compactive effort
increases the water content is reducing and the maximum dry
density is increasing. Due to increase in compactive effort,
the maximum dry density is increased from 19.95kN/m³ to
20.9kN/m³ similarly the optimum moisture content is
decreased from 8 to 7.5%. In the case of pure flyash as the
compaction effort increases the MDD of fly ash is increasing
from 12.65 to 13.85kN/m³ and the OMC of fly ash is
decreasing from 19.5 to 18 %. The compaction curves of
gravelly sand are placed high as compared to the compaction
curves of flyash shown in fig.2 it can be attributed that the fly
ash has low specific gravity and is a light weight material.
The compaction curves for different proportions of flyash
added to the gravelly sand and tested under modified
compaction presents in fig.3and tabulated in table 3. From
the figure, it can be noticed that the compaction curves are
following the typical trend of granular soils.The higher MDD
is noticed for the flyash proportion of 5% added to the
gravelly sand. The MDD for 0% flyash and for 10% flyash is
observed to be almost the same. It is noticed that as the
percentage of flyash increases the affinity to absorb water is
increasing. From the figures it is further noticed that the dry
density of flyash gravelly sand mixtures is maximum in the
range of water content of 7.5% to 8.5%. To achieve the
optimum condition, about 7.5 to 8.5% water content can be
sufficient to add to the flyash gravelly sand mixtures wherein
fly ash proportions varying from 0 to 30%.
From this figure, it can be presented that the lowest and
highest dry density is 19.2kN/m³ to 21.2kN/m³ respectively
and in the range of water content of 3 to 12%. It can be
understand that highest dry density can be possible from the
modified compaction and hence for heavy traffic highway
pavement constructions the modified compaction results can
be used.
Table 3 Values of OMC and MDD with the
Percentage ofFlyash in modified compaction
Optimum
Maximum
%
Moisture
Dry
Flyash
Content(OMC) Density(MDD)
8.00
8.15
8.35
9.25
19.9
19.6
19.58
18.85
20
25
30
100
10.00
10.10
10.30
17.90
18.75
18.62
17.52
12.90
20.5
20
19.5
19
18.5
18
17.5
17
16.5
16
0%
FA
5%
FA
10%
FA
15%
FA
20%
FA
25%
FA
30%
FA
0
2
4
6
8 10 12 14
% Water Content
16
18
Fig. 3 Compaction curves for flyash gravelly sand mixtures subjected to
modified compaction
Fig. 2 Compaction curves for gravelly sand and flyash subjected to
light and modified compaction
B. CBR CHARACTERISTICS
To understand CBR variations of flyash gravelly sand
mixtures, a laboratory testing was conducted for the
conditions of modified compaction.From the load penetration
curves (Fig 4&5) of flyash gravelly sand mixtures, it can be
noticed that the penetration response is rigid in case of unsoaked condition as compared to that of soaked condition.
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0
5
10
15
Dry Density (kN/m3)
III RESULTS AND DISCUSSIONS
Also from these load penetration curves, it can be noticed
that up to about 30% of fly ash addition to gravelly sand the
response is effective both in un-soaked and soaked conditions
and suggested to use more than 30% of flyash along with
gravelly sand for the pavement construction without getting
altered the CBR properties of gravelly sand.The load
penetration behavior of flyash gravelly soils subjected to
modified compaction at OMC in un-soaked and soaked
conditions is shown in figures 4 & 5.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015
The variation of CBR with the percentage of fly ash for the
un-soaked and soaked conditions for modified compaction is
presented in Fig 6. The influence of compaction effort is not
seen in the case of un-soaked condition and where as in case
of soaked condition it can be clearly seen that there is an
influence of compaction effort on CBR values. The CBR
variation curves of un-soaked and soaked tested for modified
compaction are moving parallel with very narrow or
negligible gap. The results of ratios of CBR with the
percentage of flyash for the un-soaked and soaked conditions
tested at modified compaction is shown in the Fig.7 and
presented in table 5. From this observation, it can be noticed
that the ratio of CBR is more than one for the cases of unsoaked conditions in modified compaction as compared to
soaked condition and also it can be seen that as the
percentage of flyash increases, the ratio of CBR is almost
varying in the range of 1 to 1.5. It means that in the cases of
modified compaction, the un-soaked CBR value is in the
range of 1 to 1.5 times than the soaked CBR value. It was
understand that especially in un-soaking, the influence of
compaction effort is present. The percentage decreases of
CBR with the various percentages of flyash are shown in
fig.8 and values are presented in Table 6
1200
1000
Load (kg)
800
600
0% FA
5% FA
10% FA
15% FA
20% FA
25% FA
30% FA
400
200
0
0
2
3
4
5
6 7 8 9 10 11 12 13
Penetration (mm)
Fig.5 Load penetration curves for flyash gravelly sand mixtures subjected
tomodified compaction at OMC in soaked condition
%CBR Decrease
Fig.4 Load penetration curves for flyash gravelly sand mixtures subjected to
Modified compaction at OMC in un- soaked condition
1
45
40
35
30
25
20
15
10
5
0
US-% Decrease
S-% Decrease
0
20
40
% Flyash
Fig 6Variation of CBR% with the % fly ash tested at OMC and subjected to
modified compaction for un- soaked and soaked conditions
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Fig.8 Percentage CBR decreases in un-soaked and soaked conditions for
different percentage of Flyash with respect to CBR of Granular Soil.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015
Table 6 Percentage CBR Decrease with % of Flyash with
respect to CBR of granular soil
Fig. 7 Variation of ratio of un-soaked and soaked CBR in modified
compaction
%
Flyash
%CBR
Decrease
(Un-soaked)
%CBR Decrease
(Soaked)
00
00.00
00.00
05
10.90
08.28
10
15.76
15.16
15
20
25
30
28.49
32.74
34.00
36.98
28.30
33.10
36.13
38.30
Table 4 CBR values of fly ash Gravelly sand mixturesin
modified compaction
IV SUMMARY AND CONCLUSIONS
% Fly
ash
CBRUn soaked (%)
CBR-Soaked
(%)
00
05
10
15
20
25
30
100
55.24
49.21
46.53
39.50
37.15
36.42
34.81
48.8
48.54
44.52
41.18
34.80
32.47
31.09
29.92
03.12
Table 5 The ratios of un-soaked and soaked CBR
In modified compaction with % flyash
% Flyash
US(M)/S(M)
0
1.138
5
1.105
10
1.130
15
1.135
20
1.144
25
1.171
30
1.163
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The MDD decreases drastically from 10% to15% and 25% to
30% in light compaction with increase of % fly ash from 0%
to 30%.But from 0% to 10% and 25% to 30% fly ash, the
MDD decreases slightly in modified compaction. The addition
of % Fly ash in soil gravel mixture, OMC increases both in
light and modified compaction and increases slightly from 0%
to 10% and drastically increases from 10% to 25% and
slightly increases from 25% to 30% in light compaction and
modified compaction. The CBR values of un soaked and
soaked conditions are decreases for each percentage fly ash
soil mixture of 0% to30%.The percentage decrease of CBR
values from un soaked to soaked for 0% to 5% fly ash soil
mixtures are from 12.12% to 9.53% and above 15% fly ash
soil mixture, 30% to 40% reduction in CBR values were
observed for both soaked and un soaked conditions. For both
un-soaked and soaked conditions the maximum percentage
decrease of CBR values in between each successive
percentages from 0% to 30% fly ash soil mixture is of
16%.From the above results it can be concluded that 20% to
30% fly ash soil mix can be used for both soaked and un
soaked condition.From the results of compaction, CBR tests,
20% to 25% fly ash addition to gravelly sand do not affect the
properties of granular soil .The fly ash soil mixture is showing
encouraging results towards utilization of fly ash with gravelly
sand for low to high volume traffic in pavement construction.
The load penetration response is rigid up to 30% fly ash soil
mix and shows the CBR values more than 20% for both unsoaked and soaked condition In general, in majority flexible
pavement design, though the CBR value is more than 20%, its
value is limited to 20%. Hence, from this it can be proposed
that even up to 25% addition of fly ash can make the
economic construction of pavement without compromising
any strength aspects.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015
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