IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308

STRENGTH CHARACTERISTICS OF FLY ASH CONCRETE WITH

SAME WORKABILITY

A. Krishna Rao1, D. Rupesh Kumar2

1
Assistant Professor, Department of Civil Engineering, CMR Institute of Technology, Hyderabad-504110, T.S., India
2
Associate Professor, Department of Civil Engineering, University College of Engineering (A), Osmania University,
Hyderabad-500007, T.S., India

Abstract
For making high strength concrete, cement can be replaced by fly ash (collected from Vijayawada Thermal Power Station) upto
an extent of 50% without affecting either compressive strength or flexural strength. It is proposed to investigate the effect on
strength of fly ash concrete when water-binder ratio changes with same workability as that of basic mix. The consistency of
cement decreases with increase in fly ash percentage. But no definite pattern is noticed in respect of initial and final setting times,
through in all cases. The water-binder ratio, for a compaction factor of 0.72, reduces from 0.370 for basic mix to 0.335 for mix
with 50% replacement of fly ash. The average increment in compressive strength at 28 and 90 days is 7% and 2% respectively.
However, the average increment in flexural strength at 28 and 90 days age is 8% and 19% respectively. Hence, for the fly ash
concrete with same workability, the increase in compressive strength is very marginal (2%); however, the increase in flexural
strength is commendable (19%) compared to the basic mix.

Keywords: Replacement of cement, fly ash, high strength concrete, workability, water-binder ratio.
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1. INTRODUCTION that the best pozzolans in optimum proportions mixed with

Portland cement improves many of the qualities of concrete
Cement is the back bone for global infrastructure such as: less heat of hydration and thermal shrinkage;
development. It was estimated that global production of increase of water tightness; reduction of alkali-aggregate
cement is about 4200 million metric tons in 2017. reaction; improvement of resistance to attack by sulphate
Production of every ton of cement emits carbon dioxide to soils and sea water. The IS: 456: 2000 [1] permits the use of
the tune of about 0.87 ton. It can be said that 7% of the pozzolana as it improves extensibility and workability, its
world’s carbon dioxide emission is attributable to Portland lower susceptibility to dissolution and leaching, its lower
cement industry. We cannot go on producing more and more cost. The reactivity of pozzolana is made use of to obtain
cement because of the high consumption of natural cementitious properties in three forms viz, (i) Pozzolana as a
resources like lime stone and also the environmental part replacement of ordinary Portland cement; (ii) Pozzolana
pollution it causes. There is a need to economize cement as a fine aggregate; and (iii) Pozzolana as an admixture.
utilization and one of the practical solutions is to partially
replace cement with pozzolanic materials. Pozzolona is a IS 456: 2000 permits the use of the following pozzolanic
natural or artificial material containing silica in a reactive materials: (i) Fly ash confirming to Part 1 of IS: 3812-1981
form. It is a siliceous and aluminous material, which in itself [2]; (ii) Silica fume confirming to a standard approved by
possesses little or no cementitious value but in finely the deciding authority; (iii) Fly ash giving required
divided form and in the presence of moisture chemically performance and uniformity characteristics; (iv)
reacts with calcium hydroxide at ordinary temperatures to Metakaoline having fineness between 700 to 900 m2/kg; (v)
form compounds possessing cementitious properties. Ground granulated blast furnace slag.
Pozzolanas can be categorized as natural and artificial. Vipul and Pawan [3] proved for M40 grade that replacing
While diatomaceous earth materials possessed by cement by fly ash and lime up to 75% gives about 40.78%
calcinations of soils are natural pozzolans, blast furnace cost benefit compared to that of original mix cost.
slag, rice husk ash, etc. are artificial pozzolans. During According to Padhye and Deo [4] with increase of fly ash
hydration of cement considerable quantities of calcium there is steep increase in strength from 7 to 28 days and the
hydroxide is liberated which is useless from the point of variation in early strength is more than the variation in later
view of strength or durability. If pozzolanic material is strength. Thus, fly ash has an adverse effect on early
added to the cement, the silica present in pozzolana strength of concrete. The strength increases with increasing
combines with Ca(OH)2 liberated by hydrating cement in amount of fly ash up to an optimum value, beyond which
the presence of moisture to form stable calcium silicates strength starts to decrease with further addition of fly ash.
which have cementitious properties. Since soluble Ca (OH)2 Oner, Akyuz and Yildz [5] stated that the optimum value of
is converted into insoluble cementitious products, the fly ash replacement for the four test groups is about 40% of
permeability of such concrete is improved. It is established
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IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308

cement. Gopalakrishnan et al. [6] studied the effect of 2.6251, fineness modulus is 6.5675, moisture content is
different percentages (0, 15, 20, 25 and 30%) of replacement 0.1%, loose and rodded bulk densities are 1.4481 gm/cm3
of cement with fly ash and concluded that cement can be and 1.656 gm/cm3 respectively and satisfied as per the IS:
replaced upto 25%. Rehi and Garg [7] replaced 20% of 383-1970 [14].
cement by weight with 27.5% fly ash and reported that the
28 day compressive strength is 109 to 123% of standard 2.1.3 Fly Ash
mix. Mehta [8] reported that for 25 MPa concrete, the
cement content can be reduced from 307 kg/m3 to 154 kg/m3 The composition of fly ash (collected from Vijayawada
by suitable adjustment in fine and coarse aggregate and Thermal Power Station): SiO2 is 61.32%, Al2O3 is 26.30%,
reducing water-binder ratio from 0.58 to 0.38. Langley and Fe2O3 and Fe3O4 is 6.95%, CaO is 2.41 %, Na2O is 1.02%.
Leaman [9] shown that the high volume fly ash concrete Its specific gravity is 2.0325 and bulk density is 0.8434
was having low permeability, low tendency for crack g/cm3.
formation, propagation and resistance to freezing and
thawing. Kiattikomal et al. [10] has studied that the 2.1.4 Cement and Fly Ash
fineness, not the chemical compositions, has the significant The properties of mortar with partial replacement of cement
effect on compressive strength of mortar. The mortars with by fly ash are shown in Table 1.
finer fly ashes gained higher compressive strength than
those with the coarser ones. Naik and Singh [11] shown that 2.2 Water-Binder Ratio for Fly Ash Concretes for
the times of setting were generally delayed up to a certain
level (60%) of cement replacement with fly ash, beyond this same Workability as that of basic Mix
level, rapid setting occurred. Dunstun’s [12] investigations
on high fly ash content concrete (HFCC) show that a The compaction factor for basic mix is 0.72 and w/c (water-
durable concrete exhibits increase in compressive strength cement ratio) is 0.37. For the concretes with partial
beyond 28 days, little evidence of carbonation, low to replacement of cement by fly ash, the water-binder ratio for
average permeability, and resistance to chloride penetration. same compaction factor is determined by trial and error
In this respect, it is significant that at the marine exposure method and the results are given in Table 2.
sites, the chloride concentrations decrease significantly with
depth. Strength and workability are two most important 2.3 Mix Design
factors in the design of concrete mix. Most of the research M50 mix design is prepared using the Entroy and
done in the past on strength of concrete with partial Shacklocks method (using references Shetty [15] and IS:
replacement of cement by fly ash is keeping same water- 456 – 2000). The mix proportion for the basic mix is 1:
binder ratio irrespective of the quantum of replacement of 0.94:2.36:0.37 and compaction factor is 0.72. 54 cubes are
cement by fly ash. Hence, in this paper, it is proposed to casted and cured for 7, 28 and 90 days; 36 beams are casted
investigate the effect on high strength concrete with cement and cured for 28 and 90 days.
partially replaced by fly ash when water-binder ratio
adopted is such that the workability is same as that of basic 2.4 Testing of Specimens
mix.
The concrete cube specimens are capped at both ends to
2. EXPERIMENTAL PROGRAM ensure smooth surfaces and tested for compressive strength
at the age of 7, 28 and 90 days. The reported results are the
2.1 Materials used in Manufacture of Fly Ash average of three concrete samples. The flexural strengths of
concrete are reported at 28 and 90 days.
Concrete
2.1.1 Cement 3. RESULTS AND DISCUSSIONS
Ordinary Portland cement of 53 grade is used in the study. Table 1 gives the properties of mortar with partial
The fineness of cement is 2.5% and satisfied as per the IS: replacement of cement by fly ash and also expressed as a
12269-1987 [13] and the specific gravity of the cement is percentage of strength of basic mix. The results show that
3.15. The expansion of cement is 0.5 mm. The 7, 28 and 90 for cement partially replaced with fly ash, compressive
days compressive strength on cement mortar is 41.45 strength of mortar cubes reduces at 7 and 28 days compared
N/mm2, 49.69 N/mm2 and 51.11 N/mm2 respectively. The to that of the basic mix. However, the compressive strength
standard consistency is 30%, initial and final setting times increases marginally at 90 days as compared to the basic
are 130 min and 239 min respectively. mix. Hence, this fly ash can be used for manufacturing the
high strength fly ash concrete. The consistency of cement
2.1.2 Aggregate decreases with increase in fly ash percentage. But no
definite pattern is noticed in respect of initial and final
River sand is used as fine aggregate with the following setting times, through in all cases. However, these are
properties: Specific gravity is 2.62, fineness modulus is confirming as per IS: 4031(Part 5):1988 [16].
2.0865, loose and rodded bulk density is 1.5325 gm/cm3 and
1.7210 gm/cm3, moisture content is 0.1%. For coarse
aggregate the maximum size is 20 mm, specific gravity is

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Volume: 06 Issue: 08 | Aug-2017, Available @ http://www.ijret.org 54
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308

The results of compressive strength and flexural strengths of terms of percentages in parenthesis. The experimental
fly ash concrete are given in Table 2 and also expressed in results show that for same workability as that of basic mix,
the water-binder ratio reduces as the percentage of that of the basic mix (except for a small decline of 1.57% for
replacement of fly ash increases from 0 to 50%. 10% replacement). For 30% and 40% replacements, it is in
fact higher by about 13% and 16% respectively. The
The water-binder ratio, for a compaction factor of 0.72, average increment in compressive strength at 28 days is 7%.
reduces from 0.370 for basic mix to 0.335 for mix with 50% At 90 days age, the compressive strength of concrete with
replacement of fly ash. With decrease in water-binder ratio replacement up to 50% is not less than that of the basic mix
the compressive and flexural strengths increases. Therefore, (except for a small decline of 0.58% for 40% replacement).
for correct comparison of strength of concrete with and The average increment in compressive strength at 90 days is
without fly ash, the water-binder ratio adopted should not be just 2%. The flexural strength of concrete at 28 and 90 days
the same; but, it should be based on the criteria of same is highest at 30% replacement of fly ash. The average
workability. increment in flexural strength at 28 and 90 days age is 8%
and 19% respectively.
At 7 days age, the compressive strength of concrete with
20% replacement of fly ash is higher than that of the basic From the above discussions it is seen that for the fly ash
mix, whereas with 10% replacement, it is 3.5% less. For concrete with same workability, the increase in compressive
replacements beyond 20% the compressive strength is much strength is very marginal (2%); however, the increase in
less. At 28 days age, the compressive strength of concrete flexural strength is commendable (19%) compared to the
with replacement of fly ash even upto 50% is not less than basic mix.

Table 1: Properties of mortar with partial replacement of cement by fly ash

% of cement 2
Normal Initial setting Final setting Compressive strength (N/mm )
replaced by fly
consistency (%) time (min) time (min) 7 days 28 days 90 days
ash
0 30.00 130 239 41.45 (100.00) 49.69 (100.00) 51.11 (100.00)
10 29.25 111 210 38.67 (93.29) 44.99 (90.54) 53.78 (105.22)
20 29.00 115 250 37.35 (90.10) 46.39 (93.37) 53.43 (104.53)
30 28.50 118 225 33.98 (81.90) 42.58 (85.71) 51.20 (100.17)
40 28.00 119 230 29.06 (70.11) 43.79 (88.12) 53.46 (104.60)
50 27.75 125 247 27.22 (65.69) 42.38 (85.29) 50.20 (98.21)

Table 2: Compressive and flexural strength test results of concrete

% of replacement of Water-binder Compressive strength (N/mm2) Flexural strength (N/mm2)
S. No.
cement by fly ash ratio 7 days 28 days 90 days 28 days 90 days
46.85 54.76 65.52 6.984 7.045
1 0 0.3700
(100.00) (100.00) (100.00) (100.00) (100.00)
45.24 53.90 67.63 7.265 8.654
2 10 0.3625
(96.56) (98.43) (103.22) (104.20) (122.84)
50.04 58.62 68.20 7.209 8.083
3 20 0.3500
(106.80) (107.00) (104.09) (103.22) (114.73)
43.45 61.87 66.17 8.130 8.610
4 30 0.3400
(92.74) (112.98) (100.99) (116.20) (122.21)
38.42 63.91 65.14 7.513 8.184
5 40 0.3375
(82.00) (116.71) (99.42) (107.54) (116.16)
33.98 55.83 67.19 7.693 8.457
6 50 0.3350
(72.53) (101.95) (102.55) (110.15) (120.04)

4. CONCLUSION The water-binder ratio, for a compaction factor of 0.72,

reduces from 0.370 for basic mix to 0.335 for mix with 50%
For making high strength concrete, cement can be replaced replacement of fly ash. With decrease in water-binder ratio
by fly ash (collected from Vijayawada Thermal Power the compressive and flexural strengths increases. Therefore,
Station) up to an extent of 50% without affecting either for correct comparison of strength of concrete with and
compressive strength or flexural strength. For cement without fly ash, the water-binder ratio adopted should not be
partially replaced with fly ash, compressive strength of the same; but, it should be based on the criteria of same
mortar cubes reduces at 7 and 28 days, but increases workability.
marginally at 90 days as compared to the basic mix. Hence,
this fly ash can be used for manufacturing the high strength
fly ash concrete.
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Volume: 06 Issue: 08 | Aug-2017, Available @ http://www.ijret.org 55
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308

The consistency of cement decreases with increase in fly ash [11] Tarun Naik, R., and Shiw Singh, S. (1997).
percentage; but, no definite pattern is noticed in respect of “Influence of Fly ash on setting and hardening
initial and final setting times, through in all cases. The characteristics of concrete systems.” ACI Materials
average increment in compressive strength at 28 and 90 days Journal, 355-360.
is 7% and 2% respectively. However, the average increment [12] Dunstun, M.R.H. (1992). “Investigation into the long
in flexural strength at 28 and 90 days age is 8% and 19% term in-situ performance of high fly ash content
respectively. Hence, for the fly ash concrete with same concrete used for structural applications.” ACI SP,
workability, the increase in compressive strength is very 132, 1-20.
marginal (2%); however, the increase in flexural strength is [13] IS: 12269-1987, Grade Ordinary Portland Cement
commendable (19%) compared to the basic mix. Specifications, BIS. New Delhi.
[14] IS: 383-1970, Specification for Coarse and Fine
ACKNOWLEDGEMENTS Aggregates from Natural Sources for Concrete, BIS,
New Delhi.
The authors of the present paper work would like to [15] Shetty, M.S. (2009). Concrete Technology, S. Chand
acknowledge CMR Institute of Technology, Hyderabad, and Company Ltd., New Delhi.
Telangana, for providing the laboratory facilities during the [16] IS: 4031 (Part 5):1988 Methods of physical tests for
research work. hydraulic cement: Determination of initial and final
setting times.
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