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A S T U D Y O N U S E O F
R I C E H U S K A S H I N
C O N C R E T E
M . B . G S A M E E E R K U M A R
GITAM INSTITUTE OF TECHNOLOGY
2
A STUDY ON USE OF RICE HUSK
ASH IN CONCRETE A PROJECT REPORT SUBMITTED IN THE PARTIAL FULFILLMENT OF THE
REQUIREMENT FOR THE AWARD OF THE DEGREE OF
BACHELOR OF ENGINEERING IN CIVIL ENGINEERING
SUBMITTED BY
M.B.G Sameer Kumar
D. Santosh Pushparaj
P.R.D Prasad
K. Bipin Chandra Phani
G. Ramu
Under the Esteemed Guidance of
Sri. P. Chandan Kumar M.Tech (Stru.); PGDES
Assistant Professor
Department of Civil Engineering
GITAM Institute of Technology
GITAM University
Visakhapatnam
3
Department of Civil Engineering
GITAM Institute of Technology
GITAM University
CERTIFICATE
This is to certify that this is a bonafied report of the work entitled “A Study on Use of
Rice Husk Ash in Concrete” done by the following students:
1. M.B.G Sameer Kumar 2005CIE022
2. D. Santosh Pushparaj 2005CIE043
3. P.R.D Prasad 2005CIE038
4. K. Bipin Chandra Phani 2005CIE008
5. G. Ramu 2005CIE039
Of final year of B.E Civil Engineering in the partial fulfillment of the requirement for the
award of the Bachelor’s Degree in Civil Engineering by GITAM Institute of
Technology, GITAM University during the year 2005-2009.
(Prof. Y.S Prabhakar) (Sri. P. Chandan Kumar) Head of the Department Project / Thesis Guide Department of Civil Engineering Assistant Professor Department of Civil Engineering
4
DECLARATION
We hereby declare that the Project entitled “A Study on Use of Rice Husk Ash in
Concrete” being submitted by us in the Department of Civil Engineering, GITAM
Institute of Technology, GITAM University is of our own and has not been submitted to
any other University or any Institute for the award of any Degree / Diploma.
M.B.G Sameer Kumar
D. Santosh Pushparaj
P. R. D Prasad
K. Bipin Chandra Phani
G. Ramu
5
ACKNOWLEDGEMENTS
We are highly obliged to express our deep felt thanks for the initiation of the
project by Sri. P. Chandan Kumar (Project Guide) and Dr. M. Potha Raju (Professor)
of Department of Civil Engineering, GITAM Institute of Technology - GITAM
University. It is with great pleasure that we express our gratitude for their guidance &
advice with which this study has been carried out. We thank them for their valuable
suggestions and worthy counsel.
We would like to express sincere thanks to Prof. Y. S Prabhakar, Head of the
Department of Civil Engineering, for continuous help and support to us. We also
thank all other faculty members for their kind and consistent patronage.
We are profoundly grateful to Mr. Narayan P Singhania (N K Enterprises,
Jharsuguda - Orissa) for giving us insight into the subject under study in various
particulars involved along with providing us the required material for carrying out the
research.
My deep felt thanks to the Dr. V. Ramachandra, Manager – Technical Services,
Ultratech Cements, Aditya Birla Group for providing us the required cement in time at
free of cost.
We are grateful to Sri. K. Anand Rao (Lab Technician), Sri. P.V.V.S.S Murthy
(Attender), of Department of Civil Engineering, for their timely help in carrying out our
project without any constraints.
Last but not least a special word of thanks to my parents and batch mates for
their constant encouragement and immense support.
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COCOCOCONTENTSNTENTSNTENTSNTENTS
1. ABSTRACT 10
2. INTRODUCTION 11-24
2.1. General
2.1.1. Global Urbanization
2.1.2. Concrete and Environment
2.1.3. Concrete – why the tarnished image?
2.1.4. What’s wrong with modern Portland cement?
2.1.5. Modified binders – the only way forward
2.1.6. Sustainability – the ultimate challenge
2.1.7. 21st Century Concrete Construction
2.2. Scope of the Project
2.3. Objective of the Project
3. LITEREATURE ON RICE HUSK ASH 25-49
3.1. General
3.2. Publication Review on Use of Rice Husk Ash
3.2.1. Steel Industry
3.2.2. Cement and Concrete
3.2.3. Refractory Bricks
3.2.4. Lightweight Construction Materials
3.2.5. Silicon Chips
3.2.6. Control of insect pests in stored food stuffs
3.2.7. Water purification
3.2.8. Vulcanizing rubber
7
3.2.9. Adsorbent for a gold-thiourea complex
3.2.10. Ceramics
3.2.11. Soil ameliorant
3.2.12. Oil adsorbent
3.2.13. Other Uses
3.3. Technical Review on Use of Rice Husk Ash
3.3.1. Introduction
3.3.2. Overview of Husk to Ash process
3.3.3. Overview of Ash production
3.3.4. Method of Ash analysis
3.3.5. Factors influencing Ash Properties
3.3.6. Review of influence of combustion method on properties of RHA
3.4. Potential to earn Carbon Credits
3.4.1. Introduction
3.4.2. Role of RHA in reducing GHG emissions
3.4.3. Calculating the value of CERs from Portland cement Substitution
4. EXPERIMENTAL PROGRAMME 50-71
4.1. General
4.2. Materials
4.2.1. Cement
4.2.2. Rice Husk Ash
4.2.3. Aggregate
4.2.4. Super Plasticizer
4.2.5. Water
8
4.3. Mix Design – Proportions
4.4. Moulds
4.4.1. Cube Moulds
4.4.2. Beam Moulds
4.5. Casting of Test Specimens
4.5.1. Preparation of Materials
4.5.2. Proportioning
4.5.3. Weighing
4.5.4. Mixing Concrete
4.6. Compaction of Test Specimens
4.6.1. Compaction by hand
4.6.2. Compaction by vibration
4.7. Vibrating Table
4.8. Curing of Test Specimens
4.9. Test for Compressive Strength Test of Concrete Specimens
4.9.1. Apparatus
4.9.2. Procedure
4.9.3. Placing the specimen in the testing machine
4.9.4. Calculations
4.10. Test for Flexural Strength of Moulded Flexure Test Specimens
4.10.1. Apparatus
4.10.2. Procedure
4.10.3. Placing the specimen in the testing machine
4.10.4. Calculations
9
5. RESULTS AND DISCUSSIONS 72-88
5.1. General
5.2. Mix Proportioning
5.2.1. Mix proportioning of Control Concrete
5.2.2. Mix proportioning of Rice Husk Ash Concrete
5.3. Strength characteristics of concrete
5.3.1. Compressive strength
5.3.2. Flexural strength
6. CONCLUSIONS 89
7. FUTURE SCOPE 90
8. REFERENCES 91-92
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LIST OF TABLES
S.no Name of the Table Pg. no
1. Cement produced and CO2 emitted 18
2. Chemical Properties of the Procured PPC 53
3. Physical Properties of the Procured PPC 54
4. Specifications of Rice Husk Ash 54
5. Physical Properties of Rice Husk Ash 55
6. Chemical Properties of Rice Husk Ash 55
7. Mix Proportions of Rice Husk Ash concrete 5% replacement 77
8. Mix Proportions of Rice Husk Ash concrete 7.5% replacement 78
9. Mix Proportions of Rice Husk Ash concrete 12.5% replacement 78
10. Mix Proportions of Rice Husk Ash concrete 10% replacement 78
11. Mix Proportions of Rice Husk Ash concrete 15% replacement 78
12. Compressive Strength of Control concrete in N/mm2 79
13. Compressive Strength as a ratio of 28 days strength at different
ages for Control Concrete 81
14. Highest compressive strength obtained at different ages 81
15. Increase or decrease in strength of concrete at 3 days w.r.t %
replacement of RHA 82
16. Increase or decrease in strength of concrete at 7 days w.r.t %
replacement of RHA 82
17. Increase or decrease in strength of concrete at 28 days w.r.t %
replacement of RHA 82
18. Increase or decrease in strength of concrete at 56 days w.r.t %
replacement of RHA 82
19. Percentage increase in compressive strength of M20 grade Rice
Husk Ash concrete w.r.t age 83
20. Flexural strength of control concrete in N/mm2 87
21. Flexural strength of Rice Husk Ash concrete in N/mm2 89
22. Flexural strength of control and rice husk ash concrete in N/mm2 90
28 day compressive strength and flexural strength of control
concrete and rice husk ash concrete 90
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LIST OF FIGURES
S.no Name of the Figure Pg. no
1. Growth in demand for primary material 17
2. Projected Increase in the demand of cement 19
3. Projected CO2 from the global cement industry 19
4. Planting Rice Husk Ash 29
5. Rice Husk Piles being burnt 38
6. Vibrating Table 67
7. Test Specimens being cured in curing tank 68
8. Hydraulic Compressive Testing Machine 71
9. Universal Testing Machine 73
10. Universal Testing Machine testing a test specimen 74
11. Flexural test specimens after testing 74
12. Compressive Strength v/s Age of Control 76
13. Comparative bar chart for control concrete 77
14. Strength of control concrete at different ages 80
15. Compressive strength of M20 grade control concrete at different
ages 80
16. Effect of age on compressive strength of concrete w.r.t different %
replacement of rice husk ash 84
17. Effect of rice husk ash percentage on compressive strength of
concrete 85
18. Effect of % replacement of rice husk ash on compressive strength
w.r.t water binder ration for M20 grade concrete 86
19. Flexural strength v/s Age in days of control concrete 87
20. Effect of age on flexural strength of concrete w.r.t different %
replacement of rice husk ash 88
21. Effect of rice husk ash percentage on flexural strength of concrete 89
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ABSTRACT
Over 5% of global CO2 emissions can be attributed to Portland cement production.
Demand for cement continues to grow. The emissions caused by annual increases in
production exceed gains to reduce emissions through manufacturing efficiencies and
cleaner fuels. And also increase in the cost of conventional building materials and to
provide a sustainable growth; the construction field has prompted the designers and
developers to look for ‘alternative materials’ for the possible use in civil engineering
constructions. For this objective, the use of industrial waste products and agricultural
byproducts are very constructive. These industrial wastes and agricultural byproducts
such as Fly Ash, Rice Husk Ash, Silica Fume, and Slag etc can be used as cementing
materials because of their pozzolanic behavior, which otherwise require large tracts of
lands for dumping. Large amounts of wastes obtained as byproducts from many of the
industries can be the main sources of such alternate materials. The world rice harvest is
estimated at 588 million tons per year and India is second largest producer of rice in the
world with annual production of 132 million tons per year.
Thus the concrete industry offers an ideal method to integrate and utilize a
number of waste materials, which are socially acceptable, easily available, and
economically within the buying powers of an ordinary man. Presence of such materials
in cement concrete not only reduces the carbon dioxide emission, but also imparts
significant improvement in workability and durability.
In the present investigation, a feasibility study is made to use Rice Husk Ash as
an admixture to an already replaced Cement with fly ash (Portland Pozzolana Cement)
in Concrete, and an attempt has been made to investigate the strength parameters of
concrete (Compressive and Flexural). For control concrete, IS method of mix design is
adopted and considering this a basis, mix design for replacement method has been
made. Five different replacement levels namely 5%, 7.5%, 10%, 12.5% and 15% are
chosen for the study concern to replacement method. Large range of curing periods
starting from 3days, 7days, 28days and 56days are considered in the present study.
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INTRODUCTION
2.1 General:
The world at the end of the 20th century that has just been left behind was very
different to the world that its people inherited at the beginning of that century. The
latter half of the last century saw unprecedented technological changes and innovations
in science and engineering in the field of communications, medicine, transportation and
information technology, and in the wide range and use of materials. The construction
industry has been no exception to these changes when one looks at the exciting
achievements in the design and construction of buildings, bridges, offshore structures,
dams, and monuments, such as the Channel Tunnel and the Millennium Wheel. There
is no doubt that these dramatic changes to the scientific, engineering and industrial face
of the world have brought about great social benefits in terms of wealth, good living
and leisure, at least to those living in the industrialized nations of the world. But this
process of the evolution of the industrial and information technology era has also,
however, been followed, particularly during the last four to five decades, by
unprecedented social changes, unpredictable upheavals in world economy,
uncompromising social attitudes, and unacceptable pollution and damage to our
natural environment. In global terms, the social and societal transformations that have
occurred can be categorized in terms of technological revolutions, population growth,
worldwide urbanization, and uncontrolled pollution and creation of waste. But perhaps
overriding all these factors is globalization - not merely in terms of economics,
technologies and human and community lives - but also with respect to climatic
changes and weather conditions of the world.
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2.1.1 Global urbanization:
2.1.1.1 The infrastructure crisis:
The unprecedented changes that have occurred in the world and society during the
latter half of the last century have placed almost insatiable demands on the construction
industry in terms of the world's material and energy resources. Continued population
growth and evolutionary industrialization have resulted in an endless stream of global
urbanization. It took the world population until the year 1804 to reach the first one
billion; yet the increase from 5 to 6 billion has taken just 12 years. It is now estimated
that the world's population will increase from 6 billion now to 8 billion by 2036 and 9.3
billion by 2050. More than 95% of this increase will take place in the developing parts of
the world. Further, for the first time in human history, about 50% of the world lives in
and around cities rather than in rural areas. It is estimated that by the end of this year,
there will be at least 20 mega cities with 10 million or more inhabitants, and there will
be a hundred or more big cities with more than 1 million people, almost all again in the
developing nations of the world. This explosion into an urban way of life will continue
to demand enormous resources and supply of construction materials required to build
the infrastructure - such as housing, transportation, education, power, water supply
and sanitation utilities - the basic facilities needed to support life in these mega cities
and big cities.
2.1.1.2World energy demands:
The impact of global urbanization and world industrialization is not merely on the
demand for construction materials; a more insidious implication is on world energy
demands, which again impinges finally on the construction industry. In the present
context of the world, some 25% of the world's population lives in the industrialized
world, and they account for nearly 75% of the global energy consumption. This
disproportionate consumption of energy and world resources can be better understood
when one considers that whilst, on a world-wide basis, the average energy availability
15
per individual is 2 kW years p.a. (defined as one unit), the average per capita
consumption is about 11 units in N. America compared to 0.6 units in China and 0.43
units in South East Asia and Africa. A large proportion of the world's energy budget is
spent on the manufacture of materials such as cement, metals and plastics. On an
approximate basis, materials consume some 20-25% of the world's total energy budget.
If it is now assumed that a doubling of the present population will entail an increase in
the global energy consumption to only double the present level, then the demand for
construction materials will place an impossible burden on the environment. Whether
fossil fuel and/or wind, water, or nuclear energy will be capable of meeting these needs
on a global basis is a different debatable issue, but bearing in mind that the dramatic
increase in the demand for power has to be in the developing nations of the world, one
can understand and appreciate the complex and vicious interaction in this global
scenario of population growth, global urbanization, energy demand and material
resources, all of which could contribute to irredeemable environmental degradation.
2.1.1.3 Global warming:
The massive and wasteful consumption of a disproportionate share of the earth's
material and energy resources by the industrialized nations of the world has resulted in
a massive increase in the emission of greenhouse gases. In 1960, CO2 emission was
about 10 billion tonnes. In 1995, this was about 23 billion tonnes excluding those from
deforestation and fires. About 4% of the world population produces around 25% of the
world's CO2 emission! Some 60% reduction in CO2 emission is required to stabilize the
earth's eco system and climatic changes. The Kyoto agreement in 1997 was to reduce the
CO2 emission from the developed world by 5% by 2012! The Portland cement industry
accounts for some 5 to 7% of the total global emission of CO2. The direct and
unmistakable consequence of the emission of greenhouse gases is Global Warming.
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2.1.1.4 Role of Cement Industry in Global Warming
Ordinary Portland cement (OPC) consists of 95% clinker and 5% gypsum. The clinker is
produced from crushing limestone together with other minerals and then heating them
at high temperatures (900-1,450°C). During finishing, the gypsum is added to the
clinker as it is ground to a small particle size (typically 10-15 microns). The clinker is the
most energy and emissions intensive aspect of cement production, thus it is known as
“the clinker factor”; for example, OPC has a clinker factor of 0.95. The global warming
potential (GWP) of the cement is reduced by reducing the clinker factor – this is
achieved in blended cements by inter-grinding pozzolans or slags with the clinker
during finishing. Blended cements are far more popular in Europe, than in North
America, the U.K. and most of Asia.
On average about 0.9 tonnes of CO2 are emitted for every tonne of clinker
produced. Energy use is currently responsible for between 0.3 and 0.4 tonnes of this
CO2; these emissions could be reduced. The 0.53 tonnes of CO2 emitted per tonne of
clinker cannot be reduced. These are known as “process emissions”, this is the CO2
released from the calcination of limestone. When it is heated, it breaks down into quick
lime and CO2 (CaCO3 CaO+CO2). According to an independent evaluation of the
industry in 2006, in the last 25 years there have been 30% reductions in CO2 emissions,
by some companies. These are attributed mainly to the adoption of more fuel-efficient
kiln processes. The most potential for further improvement is in the increased
utilization of renewable alternative fuels and the production of blended cements with
mineral additions substituting clinker. The chart in figure 42 demonstrates the current
potential for improvement in emissions reductions. As previously noted, Europe’s low
clinker factor translates to a lower energy use per tonne of cement; in fact it is 74% of
the global average. Similarly, for CO2 emitted per tonne of cement, Europe emits only
64% of the global average. This highlights the large effect reducing the clinker factor has
on CO2 emissions.
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2.1.1.5 Growth of Cement Industry:
Global development and the real estate boom of the past two decades have sharply
affected the demand for basic materials, especially cement. Figure 43 also shows an
increased need for steel. The ominous cement emissions statistics often raise the
following question: “since steel is totally recyclable, why not just use that?” For certain a
structure, steel is the appropriate choice; however there are many project-specific
factors to consider before determining the right and most sustainable material.
Structural steel (usually 90% recycled) has an embodied energy content of 27,500,000
BTUs/ton, (compared to 817,600 BTUs/ton for typical OPC concrete) – so by energy
measures alone using steel is far from a sustainable solution. Furthermore, because
cement has a low material cost/labor intensity ratio, it will likely remain the building
material of choice for most of the developing world. This is a major reason why
greening concrete is important.
Figure 1: Growth in demand for Primary materials; Source: US Geological Survey
18
The growing demand for cement (±4.7%/yr) will outstrip all projected CO2 emissions
reductions plans. By 2050, cement demand is projected to be 5.5Gt/yr, an increase of
140% above 2005 consumption. Figure 45 illustrates an ominous “Business as Usual”
(BAU) scenario up to the year 2050. Current and future cement and CO2 emissions are
shown in table 13 with both BAU and best available practice (BAP) scenarios. The
International Energy Agency (IEA) estimates that maximizing efficiencies through best
available practices and maintaining a 0.7 clinker factor would reduce CO2 emissions to
0.8 tonnes per tonne of cement produced. It is widely accepted that by 2050 carbon
trading and capture and storage technology will be important strategies in global
emissions management. The IEA uses an estimated future value for CO2 of $25/tonne.
Under this scenario, it is projected that carbon capture and storage (CCS) will be
economically viable for only about 10% of the cement sector (figure 46). Even with
efficient improvements and maximum viable CCS, the growth in demand for cement
will mean that in 2050 the cement industry will be contributing 9% of global CO2
emissions; 70% of this CO2 will be from calcination.
Table 1: Cement Produced and CO2 emitted
2005 Production / Emission (M Tonnes) 2050
Projected (BAU)
2050 Projected (BAP)
USA Canada India China Global Global Global
Cement Produced
121 11.2 130 1064 2300 5500 5500
Total CO2
109 10 117 958 2070 4950 4400
Figure 3: Projected CO
19
Figure 2: Projected Increase in demand
Figure 3: Projected CO2 from global cement industry
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2.1.2 Concrete and the environment:
How does concrete fit into this complex world scenario of the construction industry?
The answers are simple but wide-ranging. Whatever be its limitations, concrete as a
construction material is still rightly perceived and identified as the provider of a
nation's infrastructure and indirectly, to its economic progress and stability, and indeed,
to the quality of life. It is so easily and readily prepared and fabricated into all sorts of
conceivable shapes and structural systems in the realms of infrastructure, habitation,
transportation, work and play. Its great simplicity lies in that its constituents are most
readily available anywhere in the world; the great beauty of concrete, and probably the
major cause of its poor performance, on the other hand, is the fact that both the choice
of the constituents, and the proportioning of its constituents are entirely in the hands of
the engineer and the technologist. The most outstanding quality of the material is its
inherent alkalinity, providing a passivating mechanism and a safe, non-corroding
environment for the steel reinforcement embedded in it. Long experience and a good
understanding of its material properties have confirmed this view, and shown us that
concrete can be a reliable and durable construction material when it is built in sheltered
conditions, or not exposed to aggressive environments or agents. Indeed, there is
considerable evidence that even when exposed to moderately aggressive environments,
concrete can be designed to give long trouble-free service life provided care and control
are exercised at every stage of its production and fabrication, and this is followed by
well-planned inspection and maintenance schemes.
2.1.3 Concrete - why then the tarnished image?
Inspite of this excellent known performance of concrete in normal environments, there
are two aspects of the material that have tarnished its image. The first relates to the
environmental impacts of cement and concrete, and the second, to the durability of the
material.
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2.1.3.1 Environmental impacts:
Engineers cannot afford to ignore the impact of construction technology on our
surroundings - and this applies to our environment at a regional, national and global
scale. The construction industry has a direct and visible influence on world resources,
energy consumption, and on carbon dioxide emissions. Compared to metals, glass and
polymers, concrete has an excellent ecological profile. For a given engineering property
such as strength, elastic modulus or durability, concrete production consumes least
amount of materials and energy, produces the least amount of harmful byproducts, and
causes the least damage to the environment. In spite of this, we have to accept that
Portland cement is both resource and energy - intensive. Every tonne of cement requires
about 1.5 tonnes of raw material, and about 4000 to 7500 MJ of energy for production.
The cost of energy to produce a tonne of cement is estimated to account for 40 - 45% of
the total plant production cost. Much more importantly, every tonne of cement releases
1.0 to 1.2 tonnes of CO2 into the environment by the time the material is put in place. In
the world we live in, the use of resources and energy, and the degree of atmospheric
pollution that it inflicts are most important.
2.1.3.2 Deterioration of concrete:
It is now well established that the record of concrete as a material of everlasting
durability has been greatly impaired, for no fault of its own, by the material and
structural degradation that has, nevertheless, become common in many parts of the
world (3-9). The major reasons for this apparent fall from grace are numerous - partly
out of the perceived (or sometimes planted) image of concrete as a material of enduring
quality that needs no maintenance, and as a medium that will not deteriorate; and
partly by the assumption that somehow the impermeability of concrete and protection
of the embedded steel against external aggressive agencies will be automatically and
adequately provided for by the cover thickness and the presumed quality of concrete.
Experience has shown that neither can be assumed as a normal and natural
22
consequence of the process of concrete fabrication. As a specific example, some 40% of
more than 500,000 highway bridges are rated as structurally deficient or functionally
obsolete. Some $100 billion is the estimated requirement to eliminate current backlog of
bridge deficiencies, and maintain repair levels. It can be readily seen that there is a
fundamental problem in the construction industry - with choice of materials, design,
construction, maintenance, repair and rehabilitation.
2.1.4 What's wrong with modern Portland cement?
2.1.4.1 Changes in the chemistry of cement:
The experience that even when specific building code requirements of durability in
terms of concrete cover and concrete quality are achieved in practice, there is an
unacceptably high risk of premature corrosion deterioration of concrete structures
exposed to aggressive salt-laden environments, directly points to the fact that Portland
Cement concretes are not totally resistant to penetration by aggressive ions, even when
the water cementitious materials (w/cm) ratio is as low as 0.40 (14-19). The strong
implication here is that with current design codes, premature deterioration due to steel
corrosion is likely to continue. There is thus a need for a fundamental change in
thinking about concrete and concrete quality made with Portland cement
One of the major reasons for this much lower resistance of modern Portland
cement concrete to penetration by aggressive ions is the gradual but significant changes
that have occurred in the chemical composition of Portland Cements during the last
four to five decades. The two major changes in cement composition and their
implications on engineering and durability properties of the resulting concrete can be
identified as:
i) A significant increase in the C3S/C2S ratio from about 1.2 to 3.0 resulting in
higher strengths at early ages with a lower proportion of strength developed
after 28 days. From a design point of view, this implies that structural design
23
strengths can be achieved with lower cement contents and higher
water/cement ratios.
ii) A direct result of the changes in this chemical composition of Portland cement
is an increase in the heat of hydration evolved, and more importantly, in the
evolution of heat at early ages. It is estimated that the average increase in
peak temperature is about 17%, and this peak temperature is reached in less
that half the time the high strength may appear to be attractive at first sight,
but may give misleading ideas of durability. Although strength is clearly the
result of the pore-filling capability of the hydration products, there is
considerable evidence to show that there is no direct relationship between
cement/concrete strength and impermeability, for example and hence
durability, whatever be the nature of the concrete constituents
2.1.4.2 Cracking and quality of concrete:
The three major factors that encourage the transport of aggressive agents into concrete,
and influence significantly its service behavior, design life and safety are cracking,
depth and quality of cover to steel, and the overall quality of the structural concrete.
These three factors have an interactive and interdependent, almost synergistic, effect in
controlling the intrusion into concrete of external aggressive agents such as water, air,
chloride and sulphate ions. Chloride and sulphate ions, atmospheric carbonation, and
the corrosive effects of the oxides of nitrogen and sulphur, are recognized to be the most
potentially destructive agents affecting the performance and durability of concrete
structures, whilst the depth of cover, concrete quality, and cracking are the most critical
factors in determining the electrochemical stability of steel in concrete.
2.1.5 Modified binders - the only way forward:
Extensive research has now established, beyond a shadow of doubt that the most direct,
technically sound and economically attractive solution to the problems of reinforced
24
concrete durability lies in the incorporation of finely divided siliceous materials in
concrete. The fact that these replacement materials, or supplementary cementing
materials as they are often known and described, such as Pulverized Fuel Ash (PFA),
Ground Granulated Blast Furnace Slag (Slag), Silica Fume (SF), Rice Husk Ash,
Natural Pozzolana, and Volcanic Ash are all either pozzolanic or cementitious make
them ideal companions to Portland Cement (PC). Indeed, Portland cement is the best
chemical activator of these siliceous admixtures so that PFA, slag and/or SF and PC can
form a life-long partnership of homogeneous interaction which can never end in
divorce or unhealthy association and after-effects. But more importantly, the PC +
FA/slag/SF/RHA partnership can result in high quality concrete with intrinsic ability
for high durability with immense social benefits in terms of resources, energy and
environment - the only way forward for sustainable development. There are two
fundamental reasons why this PC-siliceous materials partnership is essential for
sustainable development in the cement and concrete industry.
2.1.5.1 Environmental aspects:
Every tonne of cement clinker requires about 4000-7500 MJ total energy for production
whilst slag requires only 700 to 1000 MJ/tonne, and PFA about 150 to 400 MJ/tonne.
Replacing 65% of cement with slag having 15% moisture content, for example, will only
require 0.5 tonnes of raw material and about 1500 MJ of energy. Each tonne of cement
replaced will thus save at least 2500-6000 MJ of energy. Further, since every tonne of
cement releases 1.0 to 1.2 tonnes of CO2, for every one tonne reduction in clinker
production, there is an almost equivalent reduction in CO2 emissions. These direct
impacts on economics and environment are strong, hard-to-refute arguments for using
cement replacement materials in concrete construction.
25
2.1.5.2 Durability considerations:
It is now well-established that the incorporation of industrial byproducts such as PFA,
slag and Rice Husk Ash in concrete can significantly enhance its basic properties in both
the fresh and hardened states. Apart from enhancing the rheological properties and
controlling bleeding of fresh concrete, these materials greatly improve the durability of
concrete through control of high thermal gradients, pore refinement, depletion of
cement alkalis, resistance to chloride and sulphate penetration and continued micro-
structural development through long-term hydration and pozzolanic reactions. Further,
concrete can provide, through chemical binding, a safe haven for many of the toxic
elements present in industrial wastes; and there are strong indications that these
mineral admixtures can also reduce the severity of concrete deterioration problems
arising from chemical phenomena such as alkali silica reaction, delayed ettringite
formation and thaumasite formation.
2.1.6 Sustainability - the ultimate challenge:
A critical evaluation of the world scenario described above emphasizes the complex
but close interrelationship between three seemingly unrelated but gigantic problems
that confront the construction industry, namely
- The insatiable infrastructure needs of a rapidly growing and urbanizing world
coupled with the desire for a better quality of life of nations suffering from a lack
of availability and accessibility to world resources, global warming, and the
consequent destruction of infrastructure through natural disasters.
- The need to achieve a balance between economic development and protection of
environment
- The crises in the area of materials and durability.
Sustainability implies that the needs of the present generation are met without wasting,
polluting or damaging/destroying the environment, and without compromising the
26
ability of future generations to meet their needs. In the construction industry,
sustainable development would involve, amongst others,
- Design for durable and functional service life of structures for the duration of
their specified design life.
- Use of waste materials, reduction of waste and recycling of waste.
- Construction to cause the least harm to our environment.
2.1.7 21st Century concrete construction:
Bearing in mind the technical advantages of incorporating PFA, slag, SF and other
industrial pozzolanic byproducts in concrete, and the fact that concrete with these
materials provides the best economic and technological solution to waste handling and
disposal in a way to cause the least harm to or environment, PFA, slag, Rice Husk Ash
and similar materials thus need to be recognized not merely as partial replacements
for PC, but as vital and essential constituents of concrete. Indeed a stage has now been
reached where the use of PC alone as the binder in the concrete system would need to
be justified before such a material can be accepted for construction. Viewed in this way,
the 21st century concrete will be seen as a provider for mankind with a construction
material requiring the least consumption of energy and raw material resources, and
reduced environmental pollution through reduced carbon dioxide emissions.
Enhancement of the durability of infrastructure construction and stopping of the
desecration of the environment - the essential basis for quality of life - should thus be
the criteria for selection of material constituents for the 21st Century Concrete.
This report will introduce and explore the usage Rice Husk Ash (RHA) as a
replacement along with cement. Fly ash has been thoroughly studied and used for
several decades, yet current usage is far below its potential. The benefits of rice husk
ash (also known as rice hull ash) have been documented since the 1980’s, yet it remains
barely available in the INDIA. The addition of recovered ultra-fines (such as mineral
flours) to concrete has gotten relatively little attention, especially in the INDIA. For the
27
first time in INDIA in this project already replaced fly ash based cement has been used
along with Rice Husk Ash. The research summarized here provides the structural
engineer with some practical insights into the use of carbon-neutral mineral admixtures
and their performance benefits.
2.2 Scope of the project:
The Experimental investigation is planned as under:
1) To obtain Mix proportions of Control concrete by IS method.
2) To conduct Compression test on RHA and Control concrete on standard IS
specimen size 150 x 150 x 150 mm.
3) To conduct Flexural test on RHA and Control concrete on standard IS specimen
size 100 x 100 x 500 mm.
2.3 Objective of the project:
The aim of the present investigation is:
a) To study different strength properties of Rice husk ash concrete with age in
comparison to Control concrete.
b) To study the relative strength development with age of Rice husk ash concrete
with Control concrete of same grade.
28
LITERATURE ON RICE HUSK ASH
3.1 General:
Rice covers 1% of the earth’s surface and is a primary source of food for billions of
people. Globally, approximately 600 million tonnes of rice paddy is produced each year.
On average 20% of the rice paddy is husk, giving an annual total production of 120
million tonnes. In the majority of rice producing countries much of the husk produced
from the processing of rice is either burnt or dumped as waste. Rice husks are one of the
largest readily available but most under-utilized biomass resources, being an ideal fuel
for electricity generation. The calorific value varies with rice variety, moisture and bran
content but a typical value for husks with 8-10% moisture content and essentially zero
bran is 15 MJ/kg. The treatment of rice husk as a ‘resource’ for energy production is a
departure from the perception that husks present disposal problems. The concept of
generating energy from rice husk has great potential, particularly in those countries that
are primarily dependent on imported oil for their energy needs. For these countries, the
use of locally available biomass, including rice husks is of crucial importance. Rice husk
is unusually high in ash compared to other biomass fuels – close to 20%. The ash is 92 to
95% silica (SiO2), highly porous and lightweight, with a very high external surface area.
Its absorbent and insulating properties are useful to many industrial applications, and
the ash has been the subject of many research studies. If a long term sustainable market
and price for rice husk ash (RHA) can be established, then the viability of rice husk
power or co-generation plants are substantially improved. A 3 MW power plant would
require 31,000 tonnes of rice husk per year, if operating at a 90% capacity factor. This
would result in 5580 tonnes of ash per year. Revenue from selling the ash for beneficial
use would decrease the pay-back period for the capital needed to build the project.
Many more plants in the 2 - 5 MW range can become commercially viable around the
world and this biomass resource can be utilized to a much greater extent than at
present. Rice husk ash has many applications due to its various properties. It is an
excellent insulator, so has applications in industrial processes such as steel foundries,
29
and in the manufacture of insulation for houses and refractory bricks. It is an active
Pozzolana and has several applications in the cement and concrete industry. It is also
highly absorbent, and is used to absorb oil on hard surfaces and potentially to filter
arsenic from water. More recently, studies have been carried out to purify it and use it
in place of silica in a range of industrial uses, including silicon chip manufacture. RHA
is a general term describing all types of ash produced from burning rice husks. In
practice, the type of ash varies considerably according to the burning technique. Two
forms predominate in combustion and gasification. The silica in the ash undergoes
structural transformations depending on the temperature regime it undergoes during
combustion. At 550°C – 800°C amorphous silica is formed and at greater temperatures,
crystalline silica is formed. These types of silica have different properties and it is
important to produce ash of the correct specification for the particular end use.
Figure 4: Planting Rice in fields
30
3.2 Publication review on Use of Rice Husk Ash:
3.2.1 Steel industry:
RHA is used by the steel industry in the production of high quality flat steel. Flat
steel is a plate product or a hot rolled strip product, typically used for automotive body
panels and domestic 'white goods' products. This type of steel is generally produced by
continuous casting, which has replaced the older ingot method. In the ingot method
molten steel was poured into a large mould where it would be allowed to cool and
solidify to form an ingot. The ingot would then be rolled in primary mills, in the first
stage of its transformation into a usable steel product. In developed countries this
process has largely been superseded by the continuous casting process, although the
ingot method is retained for certain applications where it is the most suitable way of
producing the steel required. Elsewhere this is not always the case, with many of the
steel industries of Eastern Europe and Asia still relying heavily on the old ingot
method. In continuous casting, a ‘ladle’ of steel, containing more than 200 tonnes of
molten metal at 1650°C, empties into a tundish, a receptacle that holds the steel and
controls its flow in the continuous process. From the tundish the steel passes in a
controlled manner to a water cooled mould where the outer shell of the steel becomes
solidified. The steel is drawn down into a series of rolls and water sprays, which ensure
that it is both rolled into shape and fully solidified at the same time. At the end of the
machine, it is straightened and cut to the required length. Fully formed slabs emerge
from the end of this continuous process. It is in continuous casting that RHA plays a
role. RHA is an excellent insulator, having low thermal conductivity, high melting
point, low bulk density and high porosity. It is this insulating property that makes it an
excellent ‘tundish powder’. These are powders that are used to insulate the tundish,
prevent rapid cooling of the steel and ensure uniform solidification. Traditionally ash is
sold in bags which are thrown on to the top of the surface of the tundish of molten steel.
Approximately 0.5 to 0.7 kg of RHA is used per tonne of steel produced. There are
health issues associated with the use of RHA in the steel industry. Traditionally
31
crystalline ash is preferred to amorphous. This poses problems as the ash has a
tendency to explode over the operator when it is being thrown on top of the tundish,
exposing them to crystalline silica and possible silicosis. A new innovation is the
production of pellets from RHA which can be much better controlled, and are better
from an operational and safety point of view.
RCL Ricegrowers Ltd with Biocon in Australia has devised a method for making
pellets, although no details are publicly available. The National Research Development
Centre (NRDC) in India have also devised a method for making pellets, which they
claim will spread over the top of the molten steel more easily. Details of the technique
are sparse, the husk is first pulverized in a mill prior to combustion and then certain
chemicals added, the pellets formed and then dried at 350°C.
However, research by CORUS (formerly British Steel) at the Teeside Technology
Centre, has cast doubts on the safety even of pellets. They tested amorphous ash and
found that there were few health problems, as there was no crystalline silica, and the
ash proved to be equally as good an insulator as the traditionally used crystalline ash.
The problems occurred when emptying the tundish at the end of the process. It was
found that the heating of the steel for 4 hours at 1500°C had transformed the silica from
its amorphous form into cristobalite and tridymite, crystalline forms of silica with
serious health risks associated. It is likely that the same chemical transformation will
occur with pellets, and so CORUS do not see them as the ideal solution. There are also
issues of steel quality relating to the use of RHA. Although RHA is an excellent
insulator, it will oxidize with elements in steel such as aluminum to form alumina
(Al2O3). This is a non-metallic compound that remains in the steel and is a nuisance in
future use. Despite this it is still used in the production of certain steel where its
insulating properties are necessary. Approximately ten years ago some tundish
powders were produced incorporating a proportion of RHA, but these are not currently
being used.
32
3.2.2 Cement and concrete:
Substantial research has been carried out on the use of amorphous silica in the
manufacture of concrete. There are two areas for which RHA is used, in the
manufacture of low cost building blocks and in the production of high quality cement.
3.2.2.1 Introduction:
Concrete is produced by mixing Portland cement with fine aggregate (sand),
coarse aggregate (gravel or crushed stone) and water. Approximately 11% of ready mix
concrete is Portland cement. It is the binding agent that holds sand and other aggregates
together in a hard, stone-like mass. Cement is made by heating limestone and other
ingredients to 1450°C in a kiln to produce clinker; this involves the dissociation 11 of
calcium carbonate under heat, resulting in lime (calcium hydroxide) and CO2. The lime
then combines with other materials to form clinker, while the CO2 is released to the
environment. The pulverized/ground clinker mixed with gypsum is called Portland
cement. Small amounts of admixtures are often added. Admixtures are either naturally
occurring compounds or chemicals produced in an industrial process, which improve
the properties of the cement. Most admixtures are pozzolans. A pozzolan is a powdered
material, which when added to the cement in a concrete mix reacts with the lime,
released by the hydration of the cement, to create compounds which improve the
strength or other properties of the concrete. Pozzolans improve strength because they
are smaller than the cement particles, and can pack in between the cement particles and
provide a finer pore structure. RHA is an active pozzolan. RHA has two roles in
concrete manufacture, as a substitute for Portland cement, reducing the cost of concrete
in the production of low cost building blocks, and as an admixture in the production of
high strength concrete. The type of RHA suitable for pozzolanic activity is amorphous
rather than crystalline.
33
3.2.2 Low cost building blocks:
Ordinary Portland cement (OPC) is expensive and unaffordable to a large
portion of the world's population. Since OPC is typically the most expensive constituent
of concrete, the replacement of a proportion of it with RHA offers improved concrete
affordability, particularly for low-cost housing in developing countries. The potential
for good but inexpensive housing in developing countries is especially great. Studies
have been carried out all over the world, such as in Guyana, Kenya and Indonesia on
the use of low cost building blocks. Portland cement is not affordable in Kenya and a
study showed that replacing 50% of Portland cement with RHA was effective, and the
resultant concrete cost 25% less. Using a concrete mix containing 10% cement, 50%
aggregate and 40% RHA plus water, an Indonesian company reported that it produced
test blocks with an average compressive strength of 12N/mm2. This compares to
normal concrete blocks, without RHA, which have an average compressive strength of
4.5 to 7N/mm2 or high strength concrete blocks which have a compressive strength of
10N/mm2. Higher strength concrete with RHA allows lighter weight products to be
produced, such as hollow blocks with enhanced thermal insulation properties, which
provide lighter walls for steel framed buildings. It also leads to reduced quantities of
cement and aggregate.
3.2.3 Enhanced properties of RHA cement:
Portland cement produces an excess of lime. Adding a pozzolan, such as RHA,
this combines with lime in the presence of water, results in a stable and more
amorphous hydrate (calcium silicate). This is stronger, less permeable and more
resistant to chemical attack. A wide variety of environmental circumstances such as
reactive aggregate, high sulphate soils, freeze-thaw conditions, and exposure to salt
water, de-icing chemicals, and acids are deleterious to concrete. Laboratory research
and field experience has shown that careful use of pozzolans is useful in countering all
of these problems. The pozzolan is not just a "filler”, but a strength and performance
enhancing additive. Pulverized fly ash and ground granulated blast furnace slag are the
34
most common pozzolan materials for concrete. Many studies have been carried out to
determine the efficacy of RHA as a pozzolan. They have concentrated on the quantity of
ash in the mix and the improved characteristics resulting from its use.
3.3 Refractory bricks:
Due to its insulating properties, RHA has been used in the manufacture of
refractory bricks. Refractory bricks are used in furnaces which are exposed to extreme
temperatures, such as in blast furnaces used for producing molten iron and in the
production of cement clinker. The market is small, and other synthetic alternatives are
preferred. However a UK company, GORICON, is interested in using it. Commercial
details of prices and quantities are not available.
3.4 Lightweight construction materials:
There is anecdotal evidence of RHA being used in the manufacture of
lightweight insulating boards in developing countries. Research at the University of
Arkansas has also focused the manufacture of insulation from RHA. The material
produced is very low density and so lightweight it floats.
3.5 Silicon chips:
The first step in semi-conductor manufacture is the production of a wafer, a thin
round slice of semi-conductor material, which is usually silicon. Purified polycrystalline
silicon (traditionally created from sand) is heated to a molten liquid and a small piece of
silicon (seed) placed in the molten liquid. As the seed is pulled from the melt the liquid
cools to form a single crystal ingot. This is then ground and sliced to form wafers which
are the starting material for manufacturing integrated circuits. Biocon in Australia have
carried out work on purifying amorphous RHA but can only get to about 99.9% purity
at a great cost, and so Biocon consider that there are no real market opportunities with
silicon chips. However The Indian Space Research Organization has successfully
35
developed technology for producing high purity precipitated silica from RHA and this
has a potential use in the computer industry. A consortium of American and Brazilian
scientists has also developed ways to extract and purify silicon with the aim of using it
in semiconductor manufacture. A company in Michigan is purifying RHA into silica
suitable for several industries, including silicon chip manufacture.
3.6 Control of insect pests in stored food stuffs:
It is known that farmers in Asia will use RHA to prevent insect attack in stored
food stuffs. Several scientific studies have been carried out to test the efficacy of this.
The ash used is that from open fires, and so is predominantly crystalline. Indonesian
soy beans are sometimes infested by Graham bean beetles (Callosobruchus analis). RHA
has been shown to prevent this by mixing 0.5% ash to soy bean. RHA was shown to be
better than wood ash and lime, and the report concludes that RHA is ‘highly effective in
controlling C. analis beetles’. It is also thought that RHA can control beetles such as
adzuki bean weevil (C. chinensis) which attacks stored mung beans. RHA was also
shown to keep stored potatoes free of the Potato tuber moth (Phthorimaea operculelle) for
up to 5 months of storage. It is thought that the insects are irritated by the high levels of
silicon and the needle like particles.
3.7 Water purification:
The use of RHA as a water purifier is generally known, although only one
documented study could be found. Greenwich University is researching small scale
paddy milling in Bangladesh and Vietnam, an objective is to find end-uses for the ash,
and the possibility of using it for water purification. Tests so far have indicated that
RHA is inefficient in removing arsenic from water. AgriTech in USA have produced a
proto type plant for manufacturing activated carbon from RHA, and the major market
for this is in water purification.
36
3.8 Vulcanizing rubber:
There are several reports detailing the use of RHA in vulcanizing rubber. In the
laboratory RHA has been shown to offer advantages over silica as a vulcanizing agent
for ethylene-propylene-diene terpolymer (EPDM), and is recommended as diluent filler
for EPDM rubber. No analysis of the ash is given so it is not known if it is amorphous or
crystalline.
3.9 Adsorbent for a gold-thiourea complex:
Gold is often found in nature as a compound with other elements. One way it is
extracted is to leach it by pumping suitable fluids through the gold bearing strata. RHA
produced by heating rice husks at 300°C has been shown to adsorb more gold-thiourea
than the conventionally used activated carbon. Ash produced by heating husks to 400°
and 500°C was found not to absorb gold thiourea complex.
3.10 Ceramics:
There is very little information on the use of RHA in ceramic glazes, other than
that it must be pure and high quality.
3.11 Soil ameliorant:
There are reports of RHA being used as a soil ameliorant to help break up clay
soils and improve soil structure. Its porous nature also assists with water distribution in
the soil. It is not sold widely on the commercial market for this use, and is a low value
market. RHA has no fertilizing potential as it does not provide the essential nutrients
necessary for plant growth. Research in USA has also been carried out on using it as a
potting substrate for bedding plants. RHA was found to increase the pH of the soil, and
so was recommended for use with plants which require alkaline soil, or in situations
where acid irrigation water is present. Wadham Biomass Facility, California sells its ash
37
to environmental remediation companies as an ingredient in a patented environmental
process for treating metals-tainted soil and similar waste streams.
3.12 Oil absorbent:
Husks burnt slowly over a period of six months have been found to be effective as oil
absorbent and are marketed in California under the trade name ‘Greasweep’. This is a
relatively small operation, but there is potential to increase this market. It is thought it is
amorphous ash that is being used. Other research studies have examined the absorption
of vacuum pump oil and the reduction of fatty acids in frying oils.
3.13 Other uses:
There are other uses for RHA which are still in the research stages:
• In the manufacture of roof tiles.
• As a free running agent for fire extinguishing powder.
• Abrasive filler for tooth paste.
• A component of fire proof material and insulation.
• As a beer clarifier.
• Extender filler for paint.
• Production of sodium silicate films.
38
3.3 Technical review on Use of Rice Husk Ash:
3.3.1 Introduction:
Commercially, it is important to determine and control the type and quality of rice husk
ash produced. These can vary depending upon the different combustion techniques
used. For example, stoker fired boilers tend to produce higher quantities of crystalline
ash, whereas similar boilers with suspension firing produce more amorphous ash. The
additional revenue stream provided by the sale of RHA may be the key to an energy
projects’ viability. If this is the case the appropriate technology should be chosen to
produce ash of the required type and quality for the target RHA market. For example,
the colour of the ash is important for some cement markets where the ash influences the
colour of the final cementitious product, as well as being a major indicator of the
samples’ residual carbon. For example, from Thailand, ‘blackish’ and ‘whitish’ ash can
command $150 and $400 a tonne respectively. RHA can be produced from rice husks by
a number of thermal processes which are described below.
Figure 5: Rice husk piles being burnt
39
3.3.2 Overview of husk to ash process:
3.3.2.1 Rice husk as a fuel:
The husk surrounding the kernel of rice accounts for approximately 20% by weight of
the harvested grain (paddy). The exterior of rice husks are composed of dentate
rectangular elements, which themselves are composed mostly of silica coated with a
thick cuticle and surface hairs. The mid region and inner epidermis contains little silica.
In small single stage mills in developing countries, where bran (the layer within the
husk) is not fully separated from the husk, the husk plus bran stream can rise to 25% of
the paddy. For larger mills, where the husk and bran are fully separated (the type more
likely to be providing the husk for electrical generation), a husk to paddy ratio of 20% is
appropriate. Most heating values for rice husk fall in the range 12.5 to 14MJ/kg, lower
heating value (LHV). If some bran remains with the husk, a somewhat higher calorific
value results. Rice husks have low moisture content, generally in the range of 8% to
10%.
The high ash content of rice husks and the characteristics of the ash impose
restrictions on the design of the combustion systems. For example, the ash removal
system must be able to remove the ash without affecting the combustion characteristics
of the furnace (especially if the ash produced is mostly bottom ash). The temperatures
must be controlled such that the ash melting temperature of approximately 1440ºC is
not exceeded and care must be taken that entrained ash does not erode components of
the boiler tubes and heat exchangers. This influences the design of the combustion
system, a review of which is presented below.
3.3.2.2 Incineration:
Incineration is the term usually used for deliberate combustion of husk without the
extraction of energy and encompasses:
• open burning (such as deliberately setting fire to piles of dumped husk),
40
• enclosed burning (typically a chamber made from fire resistant bricks with openings
to allow air to enter and flue gases to leave).
3.3.2.3 Boilers with integral combustion:
For energy recovery from the combustion of fuels, the most common type of
combustion system incorporates heat extraction from the combustion chamber using
steel tubes through which water circulates. In so doing the water removes heat from the
combustion chamber while at the same time increasing in temperature. This type of
boiler is called a “water wall boiler”. An alternative type uses an un-cooled combustion
chamber (sometimes called a firebox) connected to a large drum of water through
which tubes are placed to carry the hot exhaust gases from the combustion chamber to
the boiler chimney. This type is called a “fire tube boiler”. Such boilers tend to be less
expensive for applications where a boiler size of less than 20tonne/hr and a pressure
below 20 bar is appropriate. A variant of the fire tube boiler configuration is one in
which the combustion chamber remains un-cooled but the hot gases go to a separate
water tube heat exchanger. Sometimes the heat exchanger is called a heat recovery
steam generator (HRSG). This configuration avoids a potential problem that can occur
with high ash fuels which can cause ash build-up in the tubes of fired tube units. For
power production using rice husks, water tube boilers are the most common choice. The
combustion chamber is normally of rectangular cross section. The walls of the chamber
are formed either by tubes welded to each other or with the interstitial space filled with
refractory. The tubes may extend to the base of the chamber or finish at a higher level
with un-cooled fire-brick walls filling the lower area. The chamber is closed at the base.
The type of closure depends on the type of boiler but there is always a means of
extracting ash from the base. This ash is called “bottom ash” to distinguish it from “fly
ash” which leaves with the hot flue gases and is removed later in the process. Generally,
the chamber tapers at the top before connection to a gas passage where the exiting hot
Gases pass over additional water or steam filled tubes before release to atmosphere.
Sometimes steam or water filled tubes are suspended from the chamber roof into the
41
central combustion zone of the chamber. Combustion boilers with water cooled tubes
for rice husk application may be further sub-divided into three main categories: stoker
fired, suspension fired and fluidized-bed.
Stoker fired:
Stoker fired boilers employ a grate at the bottom of the combustion chamber. Rice husks
are fed above the grate on which they form a pile where combustion mainly occurs.
Secondary combustion of released volatile gases occurs above the pile. Typically
temperatures vary over a wide range but are highest in the pile. As a result the fusion
temperature for ash can is reached. Most ash drops through the grate. The smaller
volume residual fly ash is carried away by the flue gases.
Suspension fired:
Suspension firing is an adaptation of the nozzle burners used to burn liquid fuels such
as oil. This arrangement avoids the need for a grate at the base of the combustion
chamber. This has several potential advantages including:
− The elimination of an expensive and high maintenance piece of equipment,
− Improved combustion using finer particles,
− Easier control of excess air to the combustion chamber,
− Improved combustion efficiency.
The solid fuel has to be prepared so that it is sufficiently fine to be blown into the
combustion chamber such that combustion occurs within the short period of time
available whilst the fuel is in suspension. Otherwise, the fuel will fall to the base of the
chamber which would then need to have a grate similar to a stoker-fired unit. For rice
husks, this means that the husks have to be ground to a fine powder before combustion.
Fluidized bed combustors:
The term “fluidized bed combustor” (FBC) encompasses a range of combustion/boiler
combinations where combustion of the fuel takes place within a bed of inert material
42
that is kept “fluid” by an upward draught of air. The combustion chamber is similar to
conventional boilers, such as stoker fired designs, except that the floor of the boiler is
covered with numerous air nozzles and some ash removal outlets. Primary combustion
air enters the boiler through the nozzles and in so doing causes the mix of fuel and inert
material to mix continuously in a manner similar to a fluid. The fuel is often fed from
apertures located some distance above the bed. Depending on the ash content of the
fuel, additional inert material may also be introduced to ensure that sufficient bed
inventory exists for stable fluidization. The mixing caused by fluidization produces a
relatively uniform combustion temperature and avoids the extremes in temperature
that occur in other types of combustion. FBCs are conveniently subdivided into
“bubbling” and “circulating” types. Bubbling FBCs have a relatively low fluidizing air
velocity. This creates a bed which remains within the lower part of the combustion
chamber (i.e., there is no deliberate entrainment of fuel and inert bed material in the flue
gas). Circulating FBCs employ a higher air velocity which causes a portion of the
fluidized bed material, the “lighter” particles, to be transported upward with the flue
gas. These particles are “caught” in a cyclone, or similar mechanical separation device,
and returned to the main bed, hence the term “circulating”. Circulating FBCs tend to be
more efficient that bubbling beds but the added complexity has resulted in their
application only for larger boiler sizes - typically for outputs greater than 150MWth.
relatively few FBCs have been used for rice husk applications. Where used, bubbling
bed types seem to have been employed.
3.3.2.4 Gasification:
Gasification is a type of combustion in which the fuel is heated to release volatiles and
convert carbon to carbon monoxide. The gaseous products in the volatile form a
producer gas which can then be used in a manner similar to gaseous fuels. The heat to
produce gasification is normally derived from the fuel itself. The producer gas contains
varying amounts of hydrogen, carbon monoxide and methane depending on the fuel
and the gasifier design. An important potential advantage of gasification is that the
43
producer gas (after cleaning) can be used as fuel for reciprocating internal combustion
engines (ICE) or for gas turbines. This avoids the Rankine steam cycle as the means to
convert thermal energy to electrical energy and avoids need for cooling water.
Theoretically, a gasifier coupled with an ICE or gas turbine can lead to significantly
higher energy conversion efficiency. This might be approximately 33% at relatively
small unit sizes (down to about 1500kWth). At the same size, a steam boiler system
might achieve only 15% conversion efficiency or less.
3.3.3 Overview of ash production:
The different types of combustion have one common characteristic. They all result in
the oxidation of most of the “combustible” portion of the husk while leaving the inert
portion. The inert portion is generally called ash or, after gasification, char. The
distinction is somewhat blurred. Originally the term “char” referred to the un-
combusted residue that had not been taken to a sufficiently high enough temperature to
change its state, whereas the term “ash” implied that a higher temperature and change
of state had occurred. However, when applied to RHA, the term ash appears to be
reserved for all processes apart from gasification irrespective of whether a change of
state has occurred. In chemical analyses of husks the term “ash” refers to the chemical
constituents of the residual from complete combustion without consideration of the
morphology of the components. The term “ash”, in this study refers to the residual of
the particular combustion or gasification process which produced the ash. The fine
particulate matter which is carried away from the combustion zone by the flue gas
produces fly ash. With stoker and suspension fired boilers this ash is close to 100%
amorphous since the crystalline portion of the ash does not seem to carry in the flue gas.
Bottom ash is denser than fly ash, and for rice husks tends to be more crystalline than
the fly ash. Where fluidized beds and gasifier are concerned the distinction is not so
readily made, since the combustion occurs at lower temperatures and thus a higher
proportion of amorphous ash would be expected in the bottom ash compared with
44
bottom ash from stoker and suspension boilers. The proportion of bottom ash to fly ash
depends upon the boiler type and operating conditions. For example, McBurney
Corporation offer a suspension fired boiler with regrinding of the husks. This produces
approximately 10% bottom ash and 90% fly ash. Suspension fired boilers by other
manufacturers, such as Fortum, are expected to produce similar proportions of ash.
Combustion units with un-cooled chambers, such as challenger units by Advanced
Recycling Equipment Inc appear to produce nearer to 50% bottom ash and 50% fly ash.
At least one manufacturer, Torftech UK Ltd has a compact bed reactor (Torbed®
reactor) which produces almost 100% fly ash. For stoker fired boilers 20-30% of the ash
is expected to be bottom ash, the remainder fly.
3.3.4 Methods of ash analysis:
Typically, the ash will contain some un-burnt components as well as inert components
of the husks. The un-burnt component is predominantly carbon. It is typically measured
by reheating a sample of the ash in an oven. The difference in mass of the sample before
and after heating is referred to as the ‘Loss on Ignition’ (LOI). The LOI value is normally
the same as the carbon content of the ash. The carbon content of RHA varies according
to the combustion process. RHA analyses from a literature search and from analyses
performed on RHA material for this study indicate carbon (or LOI) values ranging from
1% to 35%. Typically, commercial RHA combustion appears to result in RHA with 5-7%
maximum carbon. The high silica content in the husk may be responsible, in part, for
the residual carbon in RHA by ‘cocooning’ the carbon such as to prevent air circulating
around it or by bonding to the carbon at the molecular level to form silicon carbide. The
silica in the rice husks is at the molecular level, and is associated with water. It occurs in
several forms (polymorphs) within the husks. In nature, the polymorphs of silica (SiO2)
are: quartz, cristobalite, tridymite, coesite, stishovite, lechatelerite (silica glass), and
opal; the latter two being amorphous. For RHA as a potentially marketable product we
need only distinguish between amorphous silica and crystalline silica. From the
45
technical literature, two forms appear to predominate in combustion and gasification.
These are lechatelerite (silica glass), an amorphous form, and cristobalite, a crystalline
form. SiO2 can also occur in a very fine, submicron form. This form is of the highest
commercial value although it is the most difficult to extract. The major and trace
elements are conventionally expressed as their respective percentage oxides and may
not actually be present in this oxide form. SiO2 is generally determined as ‘total’ SiO2,
since the proportion of crystalline to amorphous silica requires further costly analysis,
usually by X-Ray Diffraction (XRD). Determining the quantity of these polymorphs is
fundamental to investigating a market for the ash. The colour of the ash generally
reflects the completeness of the combustion process as well as the structural
composition of the ash. Generally, darker ashes exhibit higher carbon content (with the
exception of those that may be darker due to soil chemistry/region (see below). Lighter
ashes have achieved higher carbon burnout, whilst those showing a pinkish tinge have
higher crystalline (tridymite or cristobalite) content.
3.3.5 Factors influencing ash properties:
3.3.5.1 Temperature:
XRD patterns of ash combusted at a range of temperatures from 500-1000ºC have shown
a change from amorphous to crystalline silica at 800ºC, and the peak increased abruptly
at 900ºC. The change from amorphous to crystalline silica at 800ºC was also found in
other studies. In Vietnam, a series of experiments using a laboratory oven under
conditions designed to simulate the conditions of combustion from a rural facility were
carried out. SEM analysis of the ash found that the ‘globular’ amorphous silica
increased in size from 5-10µm to 10-50µm with rising combustion temperatures from
500-600ºC. The transition to completely crystalline silica was complete by 900ºC. These
changes affect the structure of the ash. As such, the ‘grindability’ and therefore
reactivity of the ash is affected since, after grinding, a greater surface area is available
46
for chemical reactions if the ash is to be used as a pozzolan. For the steel industry, more
crystalline ash is preferred as this increases its refractory properties.
3.3.5.2 Geographical region:
It has been reported that chemical variations in husk composition (and consequently
ash composition) are influenced by such things as the soil chemistry, paddy variety and
climate. However, only one report of a change in the physical and chemical properties
of ash influenced by region was found. A variation in colour and trace metal was found
in ash from husks burnt in different regions, with ash produced from husks from
Northern India resulting in a much darker ash than husks from the US. The colour
variation was not related to differences in the carbon remaining in the ash, although it is
not known the precise regional features that affected the ash. It could be due to the
agronomy of the paddy as studies have shown that differences in mineral composition
of ash can be attributed to fertilizers applied during rice cultivation, with phosphate
having a negative affect on the quality of the ash in terms of its ability to act as a
pozzolan. It has also been said that the high K2O found in some ashes could be a
consequence of K-rich fertilizers used during the paddy cultivation.
3.3.6 Review of influence of combustion method on properties of RHA:
The main factors in the various combustion and gasification processes that determine
the type of ash produced are time, temperature and turbulence. These effect all
chemical changes that occur in the combustion process including the way the ash
morphology is altered. A broad explanation of combustion techniques was given in
Section 5.2. Specific chemical and physical properties of ash, taken from literature
accounts, are described below. Appendix A compiles the chemical analyses of rice husk
ash from the literature review, going back several decades. It also includes the analyses
of two samples of RHA (one bottom ash and one fly ash sample) obtained specifically
for this study. In most of the analyses there were no details of combustion or analytical
47
techniques, making it impossible to associate the chemistry of ash directly with a
specific combustion technique. This lack of information may be due in part to many of
the analyses having been conducted under laboratory conditions, and also due to the
commercial sensitivity of giving exacting technological specifications alongside
chemical analyses of ash. A summary of all the ash analyses is at the end of this chapter.
A study in 1972 compared a range of data for ash composition. The wide range of
values was as a result of the variation of purity of the samples and the accuracy of the
analytical procedures used. However, since there is no information on different
combustion techniques employed in the husk combustion, or analytical techniques
used, it is difficult to tell whether any of the reported ranges in chemistries seen could
be attributed to particular combustion techniques. The patent filed by P.K.Mehta, for
producing RHA of a quality ideally suited to the cement market, describes burning the
husk continuously at a low temperature to preserve the amorphous nature of the silica.
The method utilizes the fly ash after its separation from the flue gases by a multi
cyclone separator. Commonly, in the production of highly amorphous ash, low
temperatures and fairly long “burntimes” are used, as for Mehta’s patent. Other work
in India has also concentrated on this technique, and has shown how a two-stage
process of combustion could control the chemical and physical properties of the
resultant ash, increasing its pozzolanic activity by taking the husk through a
carbonizing process without “flaming”. This type of burning was shown to produce a
fine white ash which did not ‘carbonize’. By comparison, a “normal” combustion
process (taking the furnace from room temperature up to the fixed burning
temperature, where it was held until combustion was completed) produced a black
colored ash. This same study compared the RHA in terms of electrical conductivity and
compressive strength tests with concrete. The electrical conductivity is an effective
measure of the amorphousness of the ash and showed that the “slow-burn” process
produced significantly more amorphous ash. Similar results were found in a study in
Guyana to ascertain the relationship between operation conditions and ash chemistry
produced in terms of ash colour, carbon content and ‘silica activity index’ (a measure of
48
its pozzalanicity). Comparing ‘5-hour’ with ‘7-hour’ burn times showed higher LOI in
the shorter burn-time experiments (~6%) compared with ~3% LOI for longer burn
times. In addition, higher percentages of silica were produced over longer combustion
periods although no details were given concerning the percentage of amorphous and
crystalline ash.
3.3.6.1 Fixed grate boilers:
None of the reports in the literature made specific reference to conventional grate
(fixed- or moving-grate) technology, and although reference to “normal” or
“conventional” boilers may well be a reference to a grated boiler we cannot assume this
in terms of the reported ash properties. However, a sample of ash (“Patum”) from a
fixed grate boiler in Thailand was analyzed, the results of which are given in analysis 31
in Appendix 1. A significant difference between this and other ash samples is the large
grain size, with 50% of the sample larger than 0.425mmsq/hole sieve. Compared with
the circulating fluidized bed RHA (see “Fortum” ash analysis below) the Patum RHA
showed a higher LOI (4.1% versus 2.2%), a higher total carbon content (3% versus 0.5%)
and higher crystalline silica content as one would expect comparing the two
technologies. The coarseness of the ash samples has market significance, because for the
majority of marketable purposes (steel, cement, absorbent etc) a fine material is
preferred, and the grinding of husks before combustion or RHA after combustion adds
a significant cost to the process.
3.3.6.2 Fluidized bed:
Since FBCs have a more uniform and lower combustion temperature than stoker
boilers, it is possible that such boilers produce less crystalline ash. Burning husk in a
fluidized bed burner has been found to give mainly amorphous ash with a high specific
surface area. Best results have been obtained by controlling the temperature of the
burner via fuel feed rate, with the air supply set at an optimum velocity of 15m/s and
the temperature set at an optimum 750ºC. Comparing the properties of this ash for
49
pozzolanic reactivity with Portland cement, with ash obtained from conventional
combustion techniques, (although no description of “conventional” combustion
techniques was given) gave excellent results in terms of its compressive strength. There
was no information giving of the proportion of amorphous to crystalline ash although
the silica ‘recovery’ was high (97.6%) and the carbon content ranged from 1-4%.
3.3.6.3 Circulating Fluidized Bed (CFB):
The only RHA sample available that can be unequivocally assigned to combustion by a
circulating fluidized bed is “Fortum”, a sample obtained specifically for this study. The
sample is a very fine material, with approximately 50% by volume passing through a
0.150mm sq/hole sieve. It is a pale grey ash (see Plate 12b) compared with the coarser
bottom ash from Patum (Plate 12a). It has a low carbon content of 0.5% as is often,
although not always, the case with pale colored ash. Generally one would expect more
amorphous ash from CFB combustion since the time spent at higher temperatures tends
to be short, and due to the suspended nature of the fuel the temperature is evenly
distributed and does not result in extremely high temperature “hot-spots”. However,
the analysis of the Fortum ash reveals a fairly high crystalline silica content of 33%
crystobalite and 20% (transitional amorphous to crystalline) quartz. In terms of trace
elements the Fortum and Patum samples exhibit similar concentrations.
3.3.6.4 Grate versus ‘conventional’:
The National Research Institute in Chatham, UK is conducting a two year long
investigation into improving the boiler efficiency of rice furnaces in Bangladesh, with a
view to producing RHA of a consistent quality to sell to the cement industry. The NRI
have conducted a series of experiments both on the RHA itself and also on blocks made
with varying proportions of RHA, substituting for cement, to examine changes in its
strength properties. The NRI obtained several samples, mainly from two types of boiler
(grate and conventional), however no additional information about the exact type and
operating conditions were taken. The results so far show a clear correlation between the
50
types of ash produced, in terms of crystalline vs. amorphous silica content, and the
boiler type. The average percentage of crystalline silica in the ash was 75.1% and 17.45%
for grated and conventional furnaces respectively.
3.3.6.5 Gasification:
Literature sources reviewed to date focus more on the relatively high un-burnt carbon
content of char/ash from gasifier without providing data on the relative amounts of
amorphous to crystalline ash. The un-burnt carbon can exceed 40%. This would
preclude use of the ash for other than low cost uses and may explain why no extensive
beneficial use of gasifier ash has been found. Joseph investigated the combustion
processes necessary to burn husks under controlled conditions such that the ash
remains mainly amorphous and that the C content is reduced to below 15%, in order
that it can be used as an additive in lime bricks or cement. The findings from this fairly
early research concluded that combustion through gasification, rather than through a
vortex furnace produced the better quality ash, and the quality of the ash was further
“improved” by varying the gasification conditions. Significantly and so far unreported
from other publications, they found that variations in collection methods and ash
cooling significantly affected the properties and characteristics of the ash. Once
collected from the gasification system carbon burnout occurred over the proceeding
four days in the concrete ash pit, the carbon content of the ash after four days had
dropped to 7-10% from 26% immediately after collection.
3.3.6.6 Additional Technology:
Torftech, a Canadian based company, supplies Torbed® reactor based rice hull
combustion systems. The technology provides ash with a high percentage of
amorphous silica for use in the concrete and cement industries. They are able to
produce low carbon, high surface area low crystalline ash by maintaining the
temperature of their expanded bed reactors below 850ºC (at 830ºC with an estimated
51
residence time of approximately 5 minutes, no crystallization occurs). The technology
has been a joint venture between Torftech and the University of Western Ontario.
3.4 POTENTIAL TO EARN CARBON CREDITS:
3.4.1 Introduction:
The Kyoto Protocol is part of the UN’s Framework Convention on Climate Change and
has set an agenda for reducing global greenhouse gas emissions. If CO2 emissions can
be shown and verified to be reduced due to different practices, then Certified Emission
Reductions (CERs) can be generated. If RHA is used in concrete manufacture as a
cement substitute then there is the potential to earn CERs. Cement manufacturing is a
major source of greenhouse gas emissions, accounting for approximately 7% to 8% of
CO2 globally. There is an emerging market globally for CER’s, with current prices
around US$5/tonne of CO2. It is hard to predict the size and future prices within the
market, but using RHA as a cement substitute can generate CERs, and one company
(Alchemix) has already investigated selling these on the international market.
3.4.2 Role of RHA in reducing GHG emissions:
The cement industry is reducing its CO2 emissions by improving manufacturing
processes, concentrating more production in the most efficient plants and using wastes
productively as alternative fuels in the cement kiln. Despite this, for every tonne of
cement produced, roughly 0.75 tonnes of CO2 (greenhouse gas) is released by the
burning fuel, and an additional 0.5 tonnes of CO2 is released in the chemical reaction
that changes raw material to clinker (calcinations). The potential to earn CERs comes
primarily from substituting Portland cement with RHA. There are other environmental
benefits of substituting Portland cement with RHA. The need for quarrying of primary
raw materials is reduced, and overall reductions in emissions of dust, CO2 and acid
gases are attained.
52
3.4.3 Calculating the value of CERs from Portland cement substitution:
The World Bank Prototype Carbon Fund provides examples of acceptable CERs from
substituting Portland cement. Their guidelines have been adapted to show the potential
income from CERs for the generic 3MW rice husk to energy power plant used for the
Cost Benefit Analysis.
A 3MW suspension fired boiler plant would typically produce 5550 tonnes
annually of RHA. Assuming 50% of RHA produced is sold for cement substitution:
Rice Husk Ash Produced RHA sold for cement substitution (5555 tonne/year) x 50% = (2775 tonne/year)
Emission reductions from substitution of Portland cement are calculated as totaling 1.25
tonnes of CO2 per tonne of cement substituted derived as follows:
0.75 tonnes of CO2 per tonne of cement from energy use
0.50 tonnes of CO2 per tonne of cement from calcinating limestone
Thus the total annual emission reduction for cement with RHA substitution in cement
is:
RHA sold for cement 1.25 tonnes of CO2 Annual Emissions substitution x per tonne of cement = (3469 tonnes CO2/year) (2775 tonne/year)
Using current estimates of approximately US$5/tonne of CO2, this could provide an
additional annual income stream of US$17,345. This is not significant compared to the
potential income from sales to the steel market of $694,000 and to the cement market of
$315,200, but could make a difference in a marginal project.
53
EXPERIMENTAL PROGRAMME
4.1 General:
This chapter deals with the Mix design procedure adopted for Control concrete and the
studies carried out on properties of various materials used throughout the Experimental
work. Also the details of method of Casting and Testing of Specimens are explained.
4.2 Materials:
4.2.1 Cement:
Cement used in the experimental work is PORTLAND POZZOLONA CEMENT
conforming to IS: 1489 (Part1)-1991. The physical properties of the cement obtained on
conducting appropriate tests as per IS: 269/4831 and the requirements as per IS 1489-
1991 are given in Table.
Table 2: Chemical properties of procured PPC
Particulars Test Results Requirements of IS: 1489-1991
Loss on Ignition 1.79 5.0 Max
Magnesia (% by mass) 1.86 6.0 Max
Sulphuric anhydride (% by mass) 1.55 3.0 Max
Insoluble Material ( % by mass) 24.44 27.464 Max
Chloride (%) 0.011 0.1 Max
54
4.2.2 Rice Husk Ash:
Rice Husk Ash used in the present experimental study was obtained from N.K
Enterprises Jharsuguda, Orissa. Specifications, Physical Properties and Chemical
Composition of this RHA as given by the Supplier are given in Table:
Table 3: Physical properties of procured PPC
Particulars Test Results Requirements of IS: 1489-1991
Specific Gravity 3.15
Fineness (m2/kg) 369 300 Min
Normal Consistency 32%
Setting Time (Minutes):
• Initial
• Final
175
265
30
600
Soundness
• Le-Chatlier Expansion
• Autoclave Expansion
1 mm
0.06 %
10 mm Max
0.8% Min
Compressive Strength (Mpa)
• 72 + 1 hr (3 days)
• 168 + 2 hr (7 days)
• 672 + 4 hr (28 days)
27
38
56.5
16 Min.
22 Min.
33 Min.
Drying Shrinkage % 0.06 % 0.15 Max
% of Fly Ash addition 29
Table 4: Specifications of Rice Husk Ash
Silica 90% minimum
Humidity 2% maximum
Mean Particle Size 25 microns
Color Grey
Loss on Ignition at 8000C 4% maximum
55
4.2.3 Aggregates:
4.2.3.1 Fine Aggregate:
Fine aggregate was purchased which satisfied the required properties of fine
aggregate required for experimental work and the sand conforms to zone III as per the
specifications of IS 383: 1970.
a) Specific gravity = 2.7
b) Fineness modulus = 2.71
Table 5: Physical Properties of Rice Husk Ash
Physical State Solid – Non Hazardous
Appearance Very fine powder
Particle Size 25 microns – mean
Color Grey
Odour Odourless
Specific Gravity 2.3
Table 6: Chemical Properties of Rice Husk Ash
SiO2 93.80%
Al2O3 0.74%
Fe2O3 0.30%
TiO2 0.10%
CaO 0.89%
MgO 0.32%
Na2O 0.28%
K2O 0.12%
Loi 3.37%
56
4.2.3.2 Coarse Aggregate:
Crushed granite of 20 mm maximum size has been used as coarse aggregate. The
sieve analysis of combined aggregates confirms to the specifications of IS 383: 1970 for
graded aggregates.
a) Specific gravity =2.64
b) Fineness Modulus = 6.816
4.2.4 Super Plasticizer:
Super plasticizers are usually highly distinctive in their nature, and they make possible
the production of concrete which, in its fresh or hardened state, is substantially different
from concrete made using water-reducing admixtures.
4.2.4.1 Conplast SP430A2:
This is the name of the super plasticizing admixture manufactured by “FOSCROC
Chemicals” used in this project. The main objectives of using this super plasticizer are:
• To produce high workability concrete requiring little or no vibration during
placing.
4.2.4.2 Standards Compliance:
Conplast SP430A2 complies with IS: 9103 and BS: 5075 and ASTM-C-494 Type
‘G’ as a high range water reducing admixture.
4.2.4.3 Description:
Conplast SP430A2 is based on Sulphonated Naphthalene Polymers and is
supplied as a brown liquid instantly dispersible in water. Conplast SP430A2 has been
specially formulated to give high water reduction up to 25% without loss of workability
or to produce high quality concrete of reduced permeability.
57
4.2.4.4 Properties:
Specific gravity: 1.265 – 1.280 at 270C
Chloride content: Less than 0.05%
Air entrainment: Less than 1% over control
4.2.5 Water:
Water is an important ingredient of concrete as it actively participates in the chemical
reaction with cement. Since it helps to form the strength giving cement gel, the quantity
and quality of water is required to be looked in to very carefully. Mixing water should
not contain undesirable organic substances or inorganic constituents in excessive
proportions.
In this project clean potable water was obtained from Department of Civil
Engineering, GIT – GU for mixing and curing of concrete.
58
4.13 Mix Design for M20-Grade Concrete:
DESIGN STIPULATIONS:
Characteristic Compressive Strength required at the end of 28 days: 20 N/mm2
Maximum size of Aggregate: 20mm (Angular)
Type of Exposure: Moderate
Degree of Workability: 0.90 (compacting factor)
Degree of Quality Control: Good
TEST DATA FOR MATERIALS:
Specific Gravity of Cement: 3.15
Specific Gravity of Coarse Aggregate: 2.64
Specific Gravity of Fine Aggregate: 2.70
Assumed slump to be achieved: 50-100 mm
Super Plasticizer: Conplast – SP 430A2
SIEVE ANALYSIS:
Coarse Aggregate: Confirming to Table 2 of IS: 383
Fine Aggregate: Confirming to Zone II of Table 4 of IS: 383
TARGET MEAN STRENGTH OF CONCRETE:
For a tolerance factor of 1.65 and using table 1, the obtained target mean strength for the
given grade of concrete = 20 + 4.6 x 1.65 = 27.6 N/mm2
59
SELECTION OF WATER CEMENT RATIO:
From the fig: 1 the free water cement ratio for the obtained target mean strength is 0.50.
This is equal to the value prescribed for Moderate conditions in IS 456.
SELECTION OF WATER AND SAND CONTENT:
From table 4, for 20 mm nominal maximum size aggregate and sand confirming to
grading zone II, water content per cubic meter of concrete = 186 kg and sand content as
percentage of total aggregate by absolute volume = 35 percent.
REQUIRED ADJUSTMENTS:
Water content % % sand in total agg.
For increase in compacting Factor (0.9-0.8) that is 0.1 +3 0 For sand confirming to Zone II Of table 4 of IS 383 0 -3.0 ------------------- ------------------ +3 -3.0 ------------------- ------------------ Therefore required sand content as percentage of total aggregate by absolute volume = 35- 3 = 33% Required water content = 186 + 186x3/100 = 186 +5.58 = 191.61 /m3
60
DETERMINATION OF CEMENT CONTENT:
Water cement ratio = 0.50
Water = 191.61 /m3
Cement = 191.61/0.50 = 383 kg/m3
This content is adequate for mild exposure condition, according to appendix A of IS 456
DETERMINATION OF COARSE AGGREGATE AND FINE AGGREGATE:
From table 3, for the specified maximum size of aggregate of 20mm, the amount of
entrapped air in the wet concrete is 2 percent.
0.98 = (191.61 + 383/3.15 + 1/0.33 x fa/2.7) x 1/1000
0.98 = (186 + 372/3.15 + 1/0.77 x ca/2.64) x 1/1000
Fa = 594kg/m3
Ca = 1356 kg/ m3
THE MIX PROPORTION THEN BECOMES:
Water Cement Fine Aggregate Coarse Aggregate
191.61 /m3 383 kg 594kg 1356 kg
0.50 1 1.55 3.54
61
4.4 Moulds: (As per IS: 516 – 1959)
4.4.1 Cube Moulds:
The mould shall be of metal, preferably steel or cast iron, and stout enough to
prevent distortion. It shall be constructed in such a manner as to facilitate the removal
of the moulded specimen without damage, and shall be so machined that, when it is
assembled ready for use, the dimensions and internal faces shall be accurate within the
following limits:
The height of the mould and the distance between opposite faces shall be the
specified size + 0.2mm. The angle between adjacent internal faces and between internal
faces and top and bottom planes of the mould shall be 900 + 0.50. The interior faces of
the mould shall be plane surfaces with a permissible variation of 0.03 mm. Each mould
shall be provided with a metal base plate having a plane surface. The base plate shall be
such dimensions as to support the mould during the filling without leakage and it shall
be preferably attached to the mould by spring or screws. The parts of the mould when
assembled shall be positively and rigidly held together, and suitable methods of
ensuring this, both during the filling and on subsequent handling of the filled mould,
shall be provided.
In assembling the mould for use, the joints between the sections of mould shall
be thinly shall be thinly coated with mould oil and a similar coating of mould oil shall
be applied between the contact surfaces of the bottom of the mould and the base plate
in order to ensure that no water escapes during the filling. The interior surfaces of the
assembled mould shall be thinly coated with mould oil to prevent adhesion of the
concrete.
62
4.4.2 Beam Moulds:
The mould shall be of metal, preferably steel or cast iron, and stout enough to prevent
distortion. It shall be constructed in such a manner as to facilitate the removal of the
moulded specimen without damage, and shall be so machined that, when it is
assembled ready for use, the dimensions and internal faces shall be accurate within the
following limits:
a) The height of the mould shall be either 15.0 + 0.005 cm or 10.0 + 0.005 cm, and the
corresponding internal width of the mould shall be 15.0 + 0.02cm or 10.0 + 0.02
cm respectively. The angle between the interior faces and the top and bottom
planes of the mould shall be 900 + 0.050. The internal surfaces of the mould shall
be plane surface with a permissible variation of 0.02 mm in 15.0 cm and 0.1 mm
overall.
b) Each mould shall be provided with a metal base plate and two loose top plates of
4.0 X 0.6 cm cross section and 5.0 cm longer than the width of the mould. The
base plate and the top plate shall have plane surfaces with a permissible
variation of 0.05 mm. The base plate shall support the mould without leakage
during the filling, and shall be rigidly attached to the mould.
c) The parts of the mould when assembled shall be positively and rigidly held
together, and suitable methods of ensuring this, both during the filling and on
subsequent handling of the filled mould, shall be provided.
d) In assembling the mould for use, the joints between the sections of the mould
shall be thinly coated with mould oil and a similar coating of mould oil shall be
applied between the contact surfaces of the bottom of the mould and the base
plate in order to ensure that no water escapes during the filling. The interior
faces of the assembled mould shall be thinly coated with mould oil to prevent
adhesion to concrete.
63
4.5 Casting of Test Specimens: (As per IS: 516-1959)
4.5.1 Preparation of Materials:
All materials shall be brought to room temperature, preferably 270 + 30 C before
commencing the results.
The cement samples, on arrival at the laboratory, shall be thoroughly mixed dry
either by hand or in a suitable mixer in such a manner as to ensure the greatest possible
blending and uniformity in the material, care is being taken to avoid the intrusion of
foreign matter. The cement shall then be stored in a dry place, preferably in air-tight
metal containers.
Samples of aggregates for each batch of concrete shall be of the desired grading
and shall be in an air-dried condition. In general, the aggregate shall be separated into
fine and coarse fraction and recombined for each concrete batch in such a manner as to
produce the desired grading. IS sieve 480 shall be normally used for separating the fine
and coarse fractions, but where special gradings are being investigated, both fine and
coarse fractions shall be further separated into different sizes.
4.5.2 Proportioning:
The proportions of the materials, including water, in concrete mixes used for
determining the suitability of the materials available, shall be similar in all respects to
those to be employed in the work. Where the proportions of the ingredients of the
concrete as used on the site are to be specified by volume, they shall be calculated from
the proportions by weight used in the test cubes and the unit weights of the materials.
64
4.5.3 Weighing:
The quantities of cement, each size of aggregate, and water for each batch shall
be determined by weight, to an accuracy of 0.1 percent of the total weight of the batch.
4.5.4 Mixing Concrete:
The concrete shall be mixed by hand or preferably in a laboratory batch mixer, in
such a manner as to avoid loss of water or other materials. Each batch of concrete shall
be of such a size as to leave about 10 percent excess after moulding the desired number
of test specimens.
4.5.4.1 Hand Mixing:
The concrete batch shall be mixed on a water-tight, non absorbent platform with
a shovel, trowel or similar suitable implement, using the following procedure:
a) The cement and fine aggregate shall be mixed dry until the mixture is
thoroughly blended and is uniform in color.
b) The coarse aggregate shall then be added and mixed with the cement and
fine aggregate until the coarse aggregate is uniformly distributed
throughout the batch, and
c) The water shall then be added and the entire batch mixed until the
concrete appears to be homogenous and has the desired consistency. If
repeated mixing is necessary, because of the addition of water in
increments while adjusting the consistency, the batch shall be discarded
and a fresh batch made without interrupting the mixing to make trial
consistency tests.
65
4.6 Compaction of Test Specimens: (As per IS: 516-1959)
The test specimens shall be made as soon as practicable after mixing, and in such a way
as to produce full compaction of the concrete with neither segregation nor excessive
laitance. The concrete shall be filled into the mould in layers approximately 5 cm deep.
In placing each scoopful of concrete, the scoop shall be moved around the top edge of
the mould as the concrete slides from it, in order to ensure a symmetrical distribution of
the concrete within the mould. Each layer shall be compacted either by hand or by
vibration as described below. After the top layer has been compacted, the surface of the
concrete shall be finished level with the top of the mould, using a trowel, and covered
with a glass or metal plate to prevent evaporation.
4.6.1 Compaction by Hand:
When compacting by hand, the standard tamping bar shall be used and the
strokes of the bar shall be distributed in a uniform manner over the cross section of the
mould. The number of strokes per layer required to produce specified conditions will
vary according to the type of concrete. For cubical specimens, in no case shall the
concrete be subjected to less than 35 strokes per layer for 15 cm cubes or 25 strokes per
layer for 10 cm cubes. The strokes shall penetrate into the underlying layer and the
bottom layer shall be rodded throughout its depth. Where voids are left by tamping bar,
the sides of the mould shall be tapped to close the voids.
4.6.2 Compaction by Vibration:
When compacting by vibration, each layer shall be vibrated by means of an electric or
pneumatic hammer or vibrator or by means of a suitable vibrating table until the specified
condition is attained.
66
4.7 Vibrating Table: (As per IS: 7246-1974 & IS: 11389-1985)
4.7.1 Scope:
This standard deals with the use of vibrating tables for the consolidation of concrete
and gives recommendations regarding placing of concrete and its consolidation by
vibration.
4.7.2 Suitability of Table Vibrators:
Vibrating tables are used for the consolidation of concrete in moulds for the
manufacture of plain and reinforced concrete or prestressed concrete elements. In case
of lightweight concrete prepared from admixtures and lightweight aggregates, the
degree of vibration shall be suitably controlled since excessive or over vibration may
lead to floating of aggregates to the surface where thorough consolidation is not
desirable, table vibrators may be used for improving the cohesion among the grains of
concrete. With the table vibrators, the vibration of concrete can start from the moment
the concrete is placed on the base of the mould, so that the expulsion of air facilitated
and compaction continues steadily with the addition of each batch of concrete in the
mould.
4.7.3 Recommended Practice for Vibration of Concrete:
4.7.3.1 Placing moulds on the vibrating table:
The moulds may be rigidly clamped to the vibrating table in such a manner that they
have contact with the support in as many and in the most suitable places possible, so
that the vibration amplitude is fairly uniform over the whole range of the support and
moulds. With the rigid and uniform clamping of the moulds the frequency and
amplitude of vibration of the table are uniformly transmitted to the mould as well as the
fresh concrete. For smaller units when the moulds are not rigidly clamped on the table,
they are repeatedly thrown into the air in a haphazard manner owing to the vibration
acceleration of the tables, which is generally considerably greater than the acceleration
67
due to gravity. During this process the concrete may be subjected to impacts with quite
high acceleration but there may be considerable loss of energy transmitted to the
concrete and there may be damage to the concrete, moulds and table.
4.7.3.2 Period of Vibration:
The period of vibration depends on the efficiency of the vibrating table, the consistency
of fresh concrete and the height of the filled concrete. The appropriate vibration time
will have to be determined in each case. The vibrating time is considered adequate
when the laitenance layers is about to form at the top surface. The adequacy of
compaction due to vibration is also indicated by the movement of the whole mass in the
form while the top surface of concrete is pressed strongly by hand and moved. On
adequate compaction, there is cessation of escape of air bubbles and the top surface of
concrete appears smooth with greasy white appearance.
Figure 6: Vibrating Table
68
4.8 Curing of Test Specimens: (As per IS: 516-1959)
The test specimens shall be stored on the site at a place free from vibration, under
damp matting, sacks or other similar material for 24 hours + ½ hour from the time of
adding the water to the other ingredients. The temperature of the place of storage shall
be within the range of 220 to 320C. After the period of 24 hours, they shall be marked for
later identification, removed from the moulds and, unless required for testing within 24
hours, stored in clean water at a temperature of 240 to 300C until they are transported to
the testing laboratory. They shall be sent to the testing laboratory well packed in damp
sand, damp sacks, or other suitable material so as to arrive there in a damp condition
not less than 24 hours before the time of test. On arrival at the testing laboratory, the
specimens shall be stored in water at a temperature of 270 + 20C until the time of test.
Records of the daily maximum and minimum temperature shall be kept both during the
period of the specimens remain on the site and in the laboratory.
Figure 7: Test specimens being cured in curing tank
69
4.9 Test for Compressive Strength of Concrete Specimen: (As per IS: 516-1959)
4.9.1Apparatus:
4.9.1.1 Testing Machine:
The testing machine may be of any reliable type, of sufficient capacity for the
tests and capable of applying the load at the specified rate. The permissible error shall
be not greater than + 2 percent of the maximum load. The testing machine shall be
equipped with two steel bearing platens with hardened faces. One of the platens shall
be fitted with a ball seating in the form of a portion of a sphere, the centre of which
coincides platen shall be plain rigid bearing block. The bearing faces of the both platens
shall be at least as large as, and preferably larger than the nominal size of the specimen
to which the load is applied. The bearing surface of the platens, when new, shall not
depart from a plane by more than 0.01 mm at any point, and they shall be maintained
with a permissible variation limit of 0.02 mm. The movable portion of the spherically
seated compression platen shall be held on the spherical seat, but the design shall be
such that the bearing face can rotated freely and tilted through small angles in any
direction.
4.9.2 Procedure:
Specimens stored in water shall be tested immediately on removal from the
water and while they are still in the wet condition. Surface water and grit shall be
wiped off the specimens and any projecting fins removed. Specimens when received
dry shall be kept in water for 24 hours before they are taken for testing. The dimensions
of the specimens to the nearest 0.2 mm and their weight shall be noted before testing.
70
4.9.3 Placing the Specimen in the Testing machine:
The bearing surfaces of the testing machine shall be wiped clean and any loose
sand or other material removed from the surfaces of the specimen which are to be in
contact with the compression platens. In the case of cubes, the specimen shall be placed
in the machine in such a manner that the load shall be applied to opposite sides of the
cubes as cast, that is, not to the top and bottom. The axis of the specimen shall be
carefully aligned with the centre of thrust of the spherically seated platen. No packing
shall be used between the faces of the test specimen and the steel platen of the testing
machine. As the spherically seated block is brought to bear on the specimen, the
movable portion shall be rotated gently by hand so that uniform seating may be
obtained. The load shall be applied without shock and increased continuously at a rate
of approximately 140 kg/sq cm/min until the resistance of the specimen to the
increasing load breaks down and no greater load can be sustained. The maximum load
applied to the specimen shall be recorded and the appearance of the concrete and any
unusual features in the type of failure shall be noted.
4.9.4 Calculation:
The measured compressive strength of the specimen shall be calculated by
dividing the maximum load applied to the specimen during the test by the cross-
sectional area, calculated from the mean dimensions of the section and shall be
expressed to the nearest kg per sq cm. Average of three values shall be taken as the
representative of the batch provided the individual variation is not more than + 15
percent of the average. Otherwise repeat tests shall be made.
71
Figure 8: Hydraulic Compressive Testing Machine
4.10 Test for Flexural Strength of Moulded Flexure Test Specimens (As per IS: 516-1959 & IS: 9399-1979)
4.10.1 Apparatus:
The testing machine may be of any reliable type of sufficient capacity for
the tests and capable of applying the load at the rate specified. The permissible errors
shall be not greater than + 0.5 percent of the applied load where a high degree of
accuracy is required and not greater than + 1.5 percent of the applied load for
commercial type of use. The bed of the testing machine shall be provided with two steel
rollers, 38 mm in diameter, on which the specimen is to be supported, and these rollers
shall be so mounted that the distance from centre to centre is 60 cm for 15.0 cm
specimens or 40 cm for 10.0 cm specimens. The load shall be applied through two
similar rollers mounted at the third points of the supporting span that is spaced at 20 or
13.3 cm centre to centre. The load shall be divided equally between the two loading
rollers and all rollers without subjecting the specimen to any torsional stresses or
restraints.
72
4.10.2 Procedure:
Test specimens stored in water at a temperature of 240 to 300C for 48 hours
before testing shall be tested immediately on removal from the water whilst they are
still in a wet condition. The dimensions of each specimen shall be noted before testing.
No preparation of the surface is required.
4.10.3 Placing the Specimen in the Testing Machine:
The bearing surface of the supporting and loading rollers shall be wiped clean,
and any loose sand or other material removed from the surfaces of the specimen where
they are to make contact with the rollers. The specimen shall then be placed in the
machine in such a manner that the load shall be applied to the uppermost surface as
cast in the mould, along two lines spaced 20.0 or 13.3 cm apart. The axis of the
specimens shall be carefully aligned with the axis of the loading device. No packing
shall be used between the bearing surfaces of the specimen and the rollers. The load
shall be applied without shock and increasing continuously at a rate such that the
extreme fiber stress increases at approximately 7 kg/sq cm/min, that is, at a rate of
loading of 400 kg/min for the 15.0 cm specimens and at a rate of 180 kg/min for 10.0 cm
specimens. The load shall be increased until the specimen fails, and the maximum load
applied to the specimen during the test shall be recorded. The appearance of the
fractured faces of concrete and any unusual features in the type of failure shall be noted.
4.10.4 Calculation:
The flexural strength of the specimen shall be expressed as the modulus of rupture fb,
which, if ‘a’ equals the distance between the line of fracture and the nearer support,
measured on the centre line of the tensile side of the specimen, in cm, shall be calculated
to the nearest 0.5 kg/sq cm as follows:
73
fb = p x l b x d2
when ‘a’ is greater than 20.0 cm for 15.0 cm specimen, or greater than 13.3 cm for a
10.0 specimen, or
fb = 3 p x a b x d2
when ‘a’ is less than 20.0 cm but greater than 17.0 cm for 15.0 cm specimen or less than
13.3 cm but greater than 11.0 cm for a 10.0 specimen
Where
b = measured width in cm of the specimen,
d = measured depth in cm of the specimen at the point of failure
l = length in cm of the span on which the specimen was supported, and
p = maximum load in kg applied to the specimen
If ‘a’ is less than 17.0 cm for a 15.0 cm specimen, or less than 11.0 cm for a 10.0 specimen,
the results of the test shall be discarded.
Figure 9: Universal Testing Machine
74
Figure 10: Universal Testing Machine with test specimen
Figure 11: Flexural Test Specimens after testing
75
RESULTS AND DISCUSSIONS
5.1 General:
This chapter deals with the presentation of test results, and discussions on
Compressive and Flexural strength development of Control concrete and Rice husk ash
concrete at different curing periods.
The present investigation is based on the IS method for Control concrete. For Rice
husk ash (RHA) concrete, replacement method is considered. Trial mix proportions
have been obtained for M20 grade Control concrete from the mix design. By conducting
trail mixes, an optimized proportion for the mix is obtained for M20 grade Control
concrete.
Compressive strength behavior of RHA concrete designed by the replacement
method are studied, where in the effect of age and percentage replacement of cement
with RHA on Compressive strength is studied in comparison with that of M20 grade
Control concrete. In addition Flexural strength studies are also carried out.
5.2 Mix Proportioning:
5.2.1 Mix proportioning of Control concrete:
According to IS method of mix design, the proportions of Control concrete were
first obtained; trial mixes were carried out to determine the strength at 3, 7 and 28 days,
and the results obtained are shown in figure, where in the compressive strength
obtained for M20 grade trial mixes are represented against age. The target mean
strength required by M20 grade concrete is also marked in the figure.
As the cube compressive strength at 28 days obtained was higher than the target
mean strength as shown in figure, the trials were conducted based on reduced cement
content. The compressive strength at different ages of M20 grade concrete under trial
mix and final mix are dissipated through bar chart in the figure. The final mix
proportions arrived at is shown in table.
76
The slump was measured to know the range of workability, which was desired
to be between 50 to 100 mm. But the slump obtained was 0 mm in the trial mix; hence
super plasticizer was used to obtain the required slump. Different mixes were tested for
slump and optimum dosage, which gave the required slump, was noted and same was
used in the final mix.
Comparison of compressive strength at 28 days of trial and final mix are shown
in figure 12, where in the target mean strength required is also indicated. It can be seen
how closely the compressive strength of the final mix at 28 days correlates with the
target mean strength for the M20 grade concrete.
Figure 12: Compressive strength v/s Age of control concrete
0
5
10
15
20
25
30
35
40
3 7 28
Co
mp
ress
ive
Str
en
gth
in
N/m
m2
Age in Days
Concrete Trial Mix
M20
Target Mean Strength
77
Figure 13: Comparative bar chart for Control concrete
5.2.2 Mix proportioning of Rice husk ash (RHA) Concrete:
In this method, three replacements of cement i.e., 5%, 7.5%, 10%, 12.5% and 15%
with Rice husk ash (RHA) are done, where as the total binder content remains the same.
The mix proportions considered for each replacement by replacement method
with RHA are presented in tables
TABLE 7: MIX PROPORTIONS OF RICE HUSK ASH CONCRETE FOR 5% REPLACEMENT
GRADE OF
CONCRETE CEMENT IN
KGS RICE
HUSK
ASH IN
KGS
FINE
AGGREGATE
IN KGS
COARSE
AGGREGATE
IN KGS
WATER IN
LTRS SUPER
PLASTICIZER IN
LTRS
M20 0.95 0.05 1.55 3.54 0.5 1.2
IN CUM 363.85 19.15 594 1394 191.61 4.36
0
5
10
15
20
25
30
35
40
Trail Mix Final Mix
Co
mp
ress
ive
Str
en
gth
in
N/m
m2
28 Days
Age in Days
Trial and Final Mix
Trail Mix
Final Mix
78
TABLE 8: MIX PROPORTIONS OF RICE HUSK CONCRETE FOR 7.5% REPLACEMENT
GRADE OF
CONCRETE CEMENT IN
KGS RICE
HUSK
ASH IN
KGS
FINE
AGGREGATE
IN KGS
COARSE
AGGREGATE
IN KGS
WATER IN
LTRS SUPER
PLASTICIZER IN
LTRS
M20 0.925 0.075 1.55 3.54 0.5 1.2
IN CUM 354.27 28.72 594 1394 191.61 4.25
TABLE 9: MIX PROPORTION OF RICE HUSK CONCRETE FOR 10% REPLACEMENT
GRADE OF
CONCRETE CEMENT IN
KGS
RICE
HUSK
ASH IN
KGS
FINE
AGGREGATE IN
KGS
COARSE
AGGREGATE IN
KGS
WATER IN
LTRS
SUPER
PLASTICIZER IN
LTRS
M20 0.90 0.1 1.55 3.54 0.5 1.2
IN CUM 344.70 38.3 594 1394 191.61 4.13
TABLE 10: MIX PROPORTION OF RICE HUSK CONCRETE FOR 12.5% REPLACEMENT
GRADE OF
CONCRETE CEMENT IN
KGS RICE
HUSK
ASH IN
KGS
FINE
AGGREGATE IN
KGS
COARSE
AGGREGATE IN
KGS
WATER IN
LTRS SUPER
PLASTICIZER IN
LTRS
M20 0.875 0.125 1.55 3.54 0.5 1.2
IN CUM 335.125 47.8 594 1394 191.61 4.02
TABLE 11: MIX PROPORTIONS OF RICE HUSK CONCRETE FOR 15% REPLACEMENT
GRADE OF
CONCRETE CEMENT IN
KGS
RICE
HUSK
ASH IN
KGS
FINE
AGGREGATE IN
KGS
COARSE
AGGREGATE IN
KGS
WATER IN
LTRS
SUPER
PLASTICIZER IN
LTRS
M20 0.85 0.15 1.55 3.54 0.5 1.2
IN CUM 325.55 57.45 594 1394 191.61 3.90
79
5.3 Strength characteristics of Concrete:
5.3.1 Compressive Strength:
Most concrete structures are designed assuming that concrete processes
sufficient compressive strength but not the tensile strength. The compressive strength is
the main criterion for the purpose of structural design. To study the strength
development of Rice husk ash (RHA) concrete in comparison to Control concrete,
compressive strength tests were conducted at the ages of 3, 7, 28 and 56 days. The tests
results are reported in table for control concrete are in table for RHA concrete
respectively.
5.3.1.1 Control Concrete (CC):
a) Effect of Age on Compressive Strength:
Table 11 gives the test results of Control concrete. The 28 days strength obtained
for M20 grade Control concrete is 30.3 Mpa. The strength results reported in table 11 are
presented in the form of graphical variation (fig: 12), where in the compressive strength
is plotted against the curing period.
Table 12: Compressive strength of Control concrete in N/mm2
Grade of Concrete 3 days 7 days 28 days 56 days
M20 14.51 20.58 30.3
36.36
The strength achieved at different ages namely 3, 7, 28 and 56 days for Control
concrete are also presented in bar chart in figure 13. From the figure, it is clear that as
the age advances, the strength of Control concrete increases. The rate of increase of
strength is higher at curing period up to 28 days. However the strength gain continues
at a slower rate after 28 days.
80
Figure 14: Strength of control concrete at different ages
Figure 15: Compressive strength of M20 grade control concrete at different ages
Strength achieved by M20 grade control concrete at different ages as a ration of
strength at 28 days is reported in table 12. From the table, it can be seen that 3 days
strength is found to be 0.47 times that of 28 days strength, for 7 days, the strength is
0
5
10
15
20
25
30
35
40
3 7 28 56
Co
mp
ress
ive
Str
en
gth
in
N/m
m2
Age in days
Strength of control concrete on ageing
M20
0
5
10
15
20
25
30
35
40
3 7 28 56
Co
mp
ress
ive
Str
en
gth
in
N/m
m2
Age in Days
Representation of Strength of M20 grade Control Concrete at
different ages
81
found to be 0.67 times that of 28 days strength, for 56 days, the strength is found to be
1.2 times that of 28 days strength.
Table 13: Compressive strength as a ratio of 28 days strength at different ages for control concrete
Grade of Concrete 3 Days 7 Days 28 Days 56 Days
M20 0.47 0.67 1 1.2
5.3.1.2 Rice Husk Ash (RHA) Concrete:
a) Effect of age on Compressive Strength of Concrete:
Figure 13 to figure 14 represents the variation of compressive strength with age
for M20 grade RHA concrete, in each figure, variation of compressive strength with age
is depicted separately for each replacement level of RHA considered namely 5%, 7.5%,
10%, 12.5% and 15%. Along with the variations shown for each replacement, for
comparison similar variations is also shown for control concrete i.e., for 0%
replacement.
In each of these variations, it can be clearly seen that, as the age advances, the
compressive strength also increases. The highest strength obtained at a particular age
for different replacement levels with RHA is reported in table 13 for the ages of 3 days,
7 days, 28 days and 56 days respectively.
Table 14: Highest Compressive strength obtained at different ages
Age in days 0% 5% RHA 7.5% RHA 10% RHA 12.5% RHA 15% RHA
3 14.51 12.96 13.32 12.7 10.7 8.88
7 20.58 19.3 19.7 18.96 18.58 16.22
28 30.3 31.5 31 30 30.14 21
56 36.36 35.84 37.62 36.15 32.88 25.88
Percentage increase in strength with respect to control concrete strength (i.e. 0%
replacement) at 3 days, 7 days, 28 days and 56 days are calculated and presented in
table 14 to 17.
82
Table 15: Increase of decrease in strength of concrete at 3 days w.r.t % replacement of RHA
Percentage Replacement Increase or decrease in strength
0-5% -11.95
0-7.5% -8.93
0-10% -14.25
0-12.5% -35.60
0-15% -63.40
Table 16: Increase or decrease in strength of concrete at 7 days w.r.t % replacement of RHA
Percentage Replacement Increase or decrease in strength
0-5% -6.63
0-7.5% -4.46
0-10% -8.54
0-12.5% -10.76
0-15% -26.88
Table 17: Increase or decrease in strength of concrete at 28 days w.r.t % replacement of RHA
Percentage Replacement Increase or decrease in strength
0-5% 5.0
0-7.5% 2.31
0-10% -1.0
0-12.5% -0.53
0-15% -44.28
Table 18: Increase or decrease in strength of concrete at 56 days w.r.t % replacement of RHA
Percentage Replacement Increase or decrease in strength
0-5% -1.45
0-7.5% 3.46
0-10% -0.58
0-12.5% -2.27
0-15% -40.49
83
In each of the above table, the change in strength for M20 grade RHA concrete is
presented separately and the following observations are made,
� The maximum increase in the compressive strength of RHA concrete i.e., 5.0%
has occurred at 28 days with 5% replacement, whereas the compressive strength
of RHA concrete is found to be decreased by 63.40% at 3 days with 15% RHA
replacement.
� It can be clearly observed that at the age of 28 days, there is gradual increase in
the compressive strength of RHA concrete for all the replacement levels with
respect to control concrete.
Strength development of concrete for different percentage replacements with RHA is
presented in table 18. In each table, by what percentage the compressive strength
increases with respect to previous age is reported.
Table 19: Percentage increase in compressive strength of M20 grade Rice husk ash concrete w.r.t age
CRL
% Increase between
3 days – 7 days
%Increase between
7 days – 28 days
% Increase between
28 days – 56 days
0% 41.83 47.23 20
5% 48.91 63.21 13.77
7.5% 47.89 57.36 21.35
10% 49.29 58.22 20.5
12.5% 42.41 62.22 17.94
15.% 82.65 54.13 23.23
84
From the above table it can be clearly seen that, the strength is higher for control
concrete (i.e 0% replacement) for initial period up to between 3-7 days up to 10%
replacement with Rice husk ash, and for 15% replacement with RHA, the strength is
very much higher when compared to that of control concrete. The rate of strength
development between 7-28 days is maximum when cement is replaced with 5% RHA.
Thus from the above table it is clear that the rate of strength development is maximum
up to the age of 28 days at all the replacement levels with RHA, and as the age advances
from 28 – 56 days, the rate of strength development gradually decreases at all the
replacement levels.
Figure 16: Effect of age on compressive strength of concrete w.r.t different % replacement of rice husk
ash
0
5
10
15
20
25
30
35
40
3 7 28 56
Co
mp
ress
ive
Str
en
gth
in
N/m
m2
Age in days
Variation of Compressive strength with age and percentage
of rice husk ash
0% RHA
5% RHA
7.5% RHA
10% RHA
12.5% RHA
15% RHA
85
b) Effect of percentage replacement of cement with Rice husk ash (RHA) on compressive
strength of concrete:
Figure 14 represents the variation of compressive strength with percentage
replacement of RHA for M20 grade concrete.
Figure 17: Effect of Rice Husk Ash percentage on compressive strength of concrete
Comparison between different replacements is made possible if the water cement
ration is common. For better pictorial representation, the variations are also represented
in the form of bar charts in the figure 15. The graph is so developed that a common
water cement ratio is considered for different replacement, so that for a particular water
cement ratio how the variation is observed with different replacement.
0
5
10
15
20
25
30
35
40
0% 5% 7.50% 10% 12.50% 15%
Co
mp
ress
ive
Str
en
gth
in
N/m
m2
Percentage of Rice Husk Ash
Percentage of Rice Husk Ash v/s Compressive strength
Strength at 3 Days
Strength at 7 Days
Strength at 28 Days
Strength at 56 days
86
Figure 18: Effect of % replacement of rice husk ash on compressive strength w.r.t water binder ratio for M20 grade concrete
5.3.2 Flexural Strength:
It is seen that strength of concrete in compression and tension (both tension and
flexural tension) are closely related, but the relationship is not of the type of direct
proportionality. The ratio of the two strengths depends on general level of strength of
concrete. In other words, for higher compressive strength, concrete shows higher tensile
strength, but the rate of increase of tensile strength is of decreasing order. The use of
pozzolanic material increases the tensile strength of concrete. The results of flexural
strength test are tabulated in table 19 and the corresponding graph is shown in fig: 16
0
5
10
15
20
25
30
35
40
0% 5% 7.50% 10% 12.50% 15%
Co
mp
ress
ive
Str
en
gth
in
N/m
m2
% Replacement of RHA
M20 Grade 0.55 w/c
3 Days
7 Days
28 Days
56 Days
87
5.3.3.1 Control Concrete:
Fig: 16 show the variation of flexural strength of control concrete with respect to age for
M20 grade. It is clear from the figure 16 that, the flexural strength increases at a greater
rate up to 28 days and the increase is gradual for further increase in age.
Table 20: Flexural strength of Control concrete in N/mm2
Curing Period 3 Days 7 Days 28 Days 56 Days
M20 1.01 1.17 4.21 4.95
Figure 19: Flexural strength v/s Age in days of Control concrete
0
1
2
3
4
5
6
3 7 28 56
Fle
xura
l S
tre
ng
th i
n N
/mm
2
Age in Days
Flexural Strength of Control Concrete
M20 Control Concrete
88
5.3.3.2 Rice Husk Ash (RHA) Concrete:
Table 20 gives the details of flexural strength of M20 grade Rice Husk ash
concrete at different curing periods and at different cement replacement levels with
Rice husk ash. Variation of flexural strength with respect to age and percentage of RHA
and effect of RHA percentage on Flexural strength of M20 grade concrete is depicted in
the figure 17 and 18 respectively. The rate of development of flexural strength is higher
at 7 days to 28 days. At the later age between 28 days to 56 days only a marginal
increase is observed. At 28 days, there is very less variation in flexural strength of RHA
concrete at the replacement levels, where as there is a comparative increase in flexural
strengths of RHA concrete at higher curing periods.
Table 21 gives the flexural strength of control concrete and rice husk ash with
respect to different age of curing. Flexural strength of the concrete keeps on increasing
with increase in curing period, which is clearly depicted if figure 17. Both the strength
values of control concrete and rice husk ash concrete for M20 grade and plotted in the
figure.
Figure 20: Effect of Age on Flexural Strength of concrete w.r.t different % replacement of rice husk ash
0
1
2
3
4
5
6
1 2 3 4
Fle
xura
l S
tre
ng
th i
n N
/mm
2
Age in Days
Variation of Flexural Strength with Age and Percentage of
RHA
0% RHA
5% RHA
7.5% RHA
10% RHA
12.5% RHA
15% RHA
89
Figure 21: Effect of Rice Husk Ash percentage on Flexural Strength of concrete
Table 22 gives the details of 28 days compression and flexural strength of Control
concrete and Rice Husk ash concrete with different cement replacement levels for M20
grade. All the percentage replacement levels considered are compared in bar char in
figure 19 for both Rice Husk ash concrete and Control concrete and for all the three
percentage replacement levels considered.
Table21: Flexural strength of Rice Husk Ash concrete in N/mm2
Curing Period 3 days 7 days 28 days 56 days
5% 1.22 1.36 3.62 4.21
7.5% 1.44 1.62 3.84 4.62
10% 1.34 1.41 2.75 3.29
12.5% 1.22 1.44 2.24 2.76
15% 1.04 1.25 2.08 2.35
0
1
2
3
4
5
6
0% 5% 7.50% 10% 12.50% 15%
Fle
xura
l S
tre
ng
th i
n N
/mm
2
Percentage of Rice Husk Ash
Percentage of Rice Husk ash v/s Flexural Strength
Strength at 3 days
Strength at 7 days
Strength at 28 days
Strength at 56 days
90
Table 22: Flexural strength of Control and Rice Husk ash concrete in N/mm2
Curing Period 3 days 7 days 28 days 56 days
0% 1.01 1.17 4.21 4.95
5% 1.22 1.36 3.62 4.21
7.5% 1.44 1.62 3.84 4.62
10% 1.34 1.41 2.75 3.29
12.5% 1.22 1.44 2.24 2.76
15% 1.04 1.25 2.08 2.35
Table 23: 28 Day Compressive and Flexural Strength of Control Concrete & Rice Husk Ash concrete
Strength
Type Compressive Strength in N/mm2 Flexural Strength in N/mm2
Percentage
Replacement 0% 5% 7.5% 10% 12.5% 15% 0% 5% 7.5% 10% 12.5% 15%
Control
Concrete 30.3 4.21
Rice Husk
ash Concrete 31.5 31 30 30.14 25 3.62 3.84 2.75 2.24 2.08
91
CONCLUSIONS
Based on the limited study carried out on the strength behavior of Rice Husk ash, the
following conclusions are drawn:
1. At all the cement replacement levels of Rice husk ash; there is gradual increase in
compressive strength from 3 days to 7 days. However there is significant increase
in compressive strength from 7 days to 28 days followed by gradual increase
from 28 days to 56 days.
2. At the initial ages, with the increase in the percentage replacement of both Rice
husk ash, the flexural strength of Rice husk ash concrete is found to be decrease
gradually till 7.5% replacement. However as the age advances, there is a
significant decrease in the flexural strength of Rice Husk ash concrete.
3. By using this Rice husk ash in concrete as replacement the emission of green
house gases can be decreased to a greater extent. As a result there is greater
possibility to gain more number of carbon credits.
4. The technical and economic advantages of incorporating Rice Husk Ash in
concrete should be exploited by the construction and rice industries, more so for
the rice growing nations of Asia.
92
FUTURE SCOPE
� Other levels of replacement with Rice husk ash can be researched.
� Some tests relating to durability aspects such as water permeability, resistance to
penetration of chloride ions, corrosion of steel reinforcement, resistance to
sulphate attack durability in marine environment etc. with Rice husk ash and
Silica fume need investigation.
� The study may further be extended to know the behavior of concrete whether it
is suitable for pumping purpose or not as present day technology is involved in
RMC where pumping of concrete is being done to large heights.
� For use of Rice husk ash concrete as a structural material, it is necessary to
investigate the behavior of reinforced Rice husk ash concrete under flexure,
shear, torsion and compression.
93
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