High volume fly ash concrete is a concrete where a replacement of about 35% or more of cement is made with the usage of fly ash.
Fly ash concrete is an eco-friendly construction material in which fly ash replaces a part of Portland cement.
3. INTRODUCTION
Fly ash concrete is an eco-friendly construction material
in which fly ash replaces a part of Portland cement.
But IS:456 – 2000 and ACI:318 allows replacement of
OPC by Fly ash up to 35% only as binding material.
High volume fly ash concrete is a concrete where
a replacement of about 35% or more of cement is
made with the usage of fly ash.
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4. WHAT IS FLY ASH?
Fly ash is a finely divided byproduct resulting from the
combustion of coal in power plants.
It contains large amounts of silica, alumina and small
amount of unburned carbon, which pollutes
environment.
It is grey in colour and alkaline in nature.
The particle size ranges between 1-100 microns.
The specific gravity of FA lies between 1.9 and 2.8
(generally 3.15 for Cement)
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5. The surface area is typically 300–500 m2/kg, although
some FA can have a surface area as high as 700
m2/kg ( around 330 m2/kg for Cement )
The mass per unit volume including air between
particles ( density ) can vary from 540 to 860 kg/m3.
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6. Classification of Fly Ash
Two classes of fly ash are defined by ASTM C618:
Class F fly ash
Class C fly ash
This classification is based on the chemical
composition of FA i.e. the sum of silica, alumina and
iron oxide percentages in the FA, being :
minimum of 70% for a Class F
minimum 50% for a Class C
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8. ISSUES WITH OPC
Conventional Portland Cement is the most consumed
commodity in the world after water.
It is also the most energy intensive material
Cement production leads to high carbon-dioxide emission.
CO2 is the primary green house gas that causes global warming
manufacturing of cement accounts for 6 to 7% of the CO2 that
humans produce.
It is produced by calcination of limestone and burning of fossil
fuels
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9. WHY TO USE FLY ASH?
being a pozzolanic, it can actually replace a part of
Portland cement
results in more durable concrete
high ultimate strength
improves workability
improves cost economy of concrete
reduction in heat of hydration
decreases density of concrete
more environment friendly concrete.
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10. EFFECT OF HIGH VOLUME OF FLY ASH
ON PROPERTIES OF FRESH CONCRETE
1. WORKABILITY
2. SETTING TIME, BLEEDING AND SEGREGATION
3. HEAT OF HYDRATION
4. DENSITY
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11. WORKABILITY
The inclusion of high volume of fly ash increases the
workability as the content of FA is increased.
The increment in the slump height is about 40%
and 54% with the inclusion of 45% and 50% FA,
respectively.
generally higher substitution of Portland cement by
fly ash reduces the water requirement for
obtaining a given workability
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12. . Effect of fly ash fineness & % on
water demand of concrete
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13. WORKABILITY
reduction in water requirement is mainly due to three mechanisms:
Fly ash gets absorbed on the surface of oppositely charged
cement particles and prevent them from flocculation, releasing
large amounts of water, thereby reducing the water-demand for a
given workability.
The spherical shape and the smooth surface of fly ash particles
help to reduce the interparticle friction and thus facilitate mobility
Due to its lower density and higher volume per unit mass, fly ash is a
more efficient void-filler than Portland cement.
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14. Contributions of enhanced workability:
Light weight concrete is easier to pump as pumping
requires less energy.
Improved finishing
Reduced segregation and bug holes
Reduced Bleeding
Less sand is needed in the mix to produce required
workability.
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15. SETTING TIME
High volume of fly ash extends both the initial and
final setting time of concrete
The impact of fly ash on the setting behavior of
concrete is dependent on:
composition and quantity of fly ash used
amount of cement
water - to – cementitious material ratio
concrete temperature
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16. Effect of FA content on penetration resistance of
setting concrete mixture
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17. Bleeding and Segregation
The inclusion of high volume of fly ash in the
mixture reduces the bleeding and segregation.
Reason:
the rate and amount of bleeding decreases due to
the reduced water demand.
The reduction of bleeding and segregation may be
related to the lubricating effect of the glassy spherical
FA particles.
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18. HEAT OF HYDRATION
Both the maximum rate of heat evolution and the
cumulative heat evolution decrease with the
inclusion of 45% FA during the first 72 hours.
The inclusion of 45% FA results in 36% reduction in
the cumulative heat evolution.
In addition, the time of reaching the maximum rate
of heat evolution delays.
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19. Effect of fly ash on heat of hydration19
Source: optimizing-the-use-of-fly-ash-in-concrete – by
Michael Thomas (www.cement.org)
20. Effect of fly ash on temperature rise20
Source: optimizing-the-use-of-fly-ash-in-concrete – by Michael
Thomas (www.cement.org) Back…
21. DENSITY
Inclusion of high volume of fly ash in the mixture
decreases its density which leads to a reduction in
the dead weight of the constructed element.
Reason:
This reduction in the density could be attributed to the
lower specific gravity of FA (1.9 to 2.8) as compared to
cement (3.15)
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22. EFFECT OF HIGH VOLUME OF FLY ASH ON
PROPERTIES OF HARDENED CONCRETE
1. COMPRESSIVE STRENGH
2. DURABILITY
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23. COMPRESSIVE STRENGH
TEST
5 mixtures were considered , 3 of them with OPC
and varying FA content, and 2 control pastes using
100% OPC or 100% BC.
conducted using 40%, 60% and 80% FA
replacement by volume of cement
maintaining the w/b by mass constant at 0.42
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24. MIXTURE PROPORTIONS OF THE PASTES
Mixture ID
Cement
(kg/m3)
Fly ash
(%)
Fly ash
(kg/m3)
Water
(kg/m3)
OPC 1359.7 0 0.0 571.1
BC 1307.5 0 0.0 549.1
40-F 864.4 40 434.5 545.5
60-F 594.0 60 671.7 531.6
80-F 306.4 80 924.0 516.7
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SOURCE: Felipe Rivera, Patricia Martínez, Javier Castro, Mauricio López “Massive volume fly ash
concrete”, Cement and Concrete Composites, Volume 63, October 2015
25. The cement pastes were prepared in a mechanical mixer
50-mm cube specimens were cast (3 specimens per age)
At the age of 24 h, the specimens were demolded and
placed in vacuum sealed plastic bags in a moist room,
(95 ± 3)% RH and (23 ± 2)˚C, until the age of testing.
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26. 26
SOURCE: Felipe Rivera, Patricia Martínez, Javier Castro, Mauricio López “Massive volume fly ash
concrete”, Cement and Concrete Composites, Volume 63, October 2015
The compressive strength gain after 3 d, 7 d, 28 d and 90 d for each mixture
27. The compressive strength of the 40-F paste is similar
to that of the OPC paste at any age
while that of the 60-F paste is very similar to the BC
paste at early ages (3 d and 7 d) and higher at 28 d
and 90 d.
The compressive strength of the 80-F paste is lower
than that of the BC paste at any age; its strength can
reach up to 22 MPa at 28 d and 35 MPa at 90 d.
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29. Rate of strength gain
The rate of strength gain in the FA-F pastes
increases with the replacement level.
Between 28 d and 90 d, the strength gain of OPC
paste is similar to that of the BC paste but lower
than that of the 80-F paste (≈0.21 MPa/d).
For the 40-F and 60-F pastes, the strength gains are
the lowest (≈0.07 MPa/d and 0.08 MPa/d,
respectively), between 28 d and 90 d.
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30. DURABILITY
Abrasion Resistance
The mix containing 70% FA exhibits a slightly lower abrasion
value than the concrete containing 50% FA and OPC
concrete
although 70% FA exhibits a higher abrasion value below a
certain compressive strength.
The abrasion resistance is mainly dependent on the
compressive strength of concrete.
It is assumed that C-S-H of 50% FA or OPC provide better
cohesion to the aggregate-sand-paste and therefore better
resist the action of surface shear forces.
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31. Permeability and Resistance to Penetration of Chlorides
Fly ash reduces permeability of concrete to water and
gas provided the concrete is adequately cured, due to a
refinement in the pore structure.
Through pozzolanic activity, fly ash chemically combines
with water and CaOH2 – forming additional cementitious
compounds, therefore :
it is not subject to leaching
it decreases bleed channels, capillary channels and void
spaces and thereby reduces permeability.
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32. With the use of fly ash, concrete becomes nearly impermeable to
chlorides and the rate of chloride penetration decreases
significantly.
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Source: optimizing-the-use-of-fly-ash-in-concrete – by Michael Thomas (www.cement.org)
33. Alkali- Silica Reaction
Class F fly ash is capable of controlling alkali – silica reaction
in concrete even at moderate levels of replacement (20% to
30%)
Reason : concentration of alkali hydroxides is reduced in the
pore solution when fly ash is present
The level of fly ash required to suppress expansion of
concrete increases with:
Increased calcium and alkali content of fly ash;
Decreased silica content of fly ash;
Increased aggregate reactivity;
Increased alkali availability from Portland cement and
environment
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34. There is low risk of concrete expansion occurring in the field
when very high volume fly ash concrete with 50% or more fly
ash is used.
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Source: optimizing-the-use-of-fly-ash-in-concrete – by Michael Thomas
(www.cement.org)
35. Sulfate Resistance
HVFA concrete specimens exhibit higher sulfate
resistance. The specimens containing 60% FA as
cement replacement exhibited better performance
in lactic/acetic and sulfuric acid
FA induces three phenomena which improves
sulfate resistance:
consumes the free lime resulting it unavailable to react
with sulfate,
reduces permeability which prevents sulfate penetration,
and
by replacing cement, the reactive aluminates are reduced.
reduced.
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36. Carbonation and Corrosion Resistance
When concrete specimens are exposed to different
methods of carbonation, the results show a reduction in
the carbonation resistance of concrete specimens with
the inclusion of 50% FA as cement replacement.
Therefore there is increase in the carbonation depth of
concrete specimens
in areas prone to carbonation, particular attention must
be paid to ensure suitable:
concrete mix proportions,
period of moist curing, and
depth of cover
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38. Resistance to Cyclic Freezing and Thawing, and Deicer Salt Scaling
Concrete can be resistant to cyclic freezing and thawing provided:
it has sufficient strength
an adequate air void system
the aggregates are frost resistant.
This holds true for fly ash concrete
concrete containing fly ash is less resistant to scaling when
subjected to freezing and thawing in presence of deicer salts.
For fly ash concrete structures exposed to de-icing salts the
following observations have been made:
scaling increases as the w/cm increases
scaling mass loss increases with fly ash content
the use of curing compounds reduces scaling
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39. CONCLUSION
Use of high volume fly ash concrete in construction is one big step
in natural resource conservation and it needs to be promoted all
over the world.
In fact, we can call high volume fly ash concrete as a green
concrete, since it can protect the environment from global warming
and at the same time from pollution.
There may be some negativity attached to it like slower
construction rates as it gains strength slowly and gives lower early
strengths.
But, the same can be ignored as the later strengths (90 days or
more) and durability of high volume fly ash concrete is much better
than plain concrete.
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40. Clearly there is no one replacement level best suited for
all applications.
The inclusion of high volume fly ash in the mixture
causes :
Reduced the heat of hydration, bleeding, segregation,
density, but increased workability and setting time.
Decreased the mechanical strength especially at early
ages with increasing FA content.
Increased durability of concrete
reduced resistance against Deicer Salt Scaling.
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41. Product Comparisons
High Volume Fly Ash
Concrete
Less energy intensive
manufacture
Higher ultimate strength
More durable
Uses a waste by-product
Less global warming gases
created
Conventional Concrete
Energy intensive
manufacturing
Weaker ultimate strength
Less durable
Uses virgin material
More global warming
gases created
42. REFERENCES
(i) Alaa M. Rashad, A brief on high-volume Class F fly ash as cement
replacement – A guide for Civil Engineer, International Journal of Sustainable Built
Environment, Volume 4, Issue 2, December 2015, Pages 278-306, ISSN 2212-6090
(ii) Felipe Rivera, Patricia Martínez, Javier Castro, Mauricio López, “Massive
volume fly ash concrete: A more sustainable material with fly ash replacing
cement and aggregates”, Cement and Concrete Composites, Volume 63,
October 2015, Pages 104- 112, ISSN 0958-9465
(iii) Vanita Aggarwal, S. M. Gupta and S.N. Sachdeva, “HIGH VOLUME FLY ASH
CONCRETE: A GREEN CONCRETE”, Journal of Environmental Research And
Development Vol. 6 No. 3A, Jan-March 2012,Pages 884-887
(iv) http://www.cement.org/docs/default-source/fc_concrete_technology/is548-
optimizing-the-use-of-fly-ash-concrete.pdf
(v) http://flyash.com/data/upfiles/resource/
TB%206%20Fly%20Ash%20Decreases%20the%20Permeability%20of%20Concrete
%202015.pdf
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