Fly ash and silica fume concrete mix design

CONTENTS INTRODUCTION OBJECTIVES METHODOLOGY RESULTS AND DISCUSSIONSCONCLUSION

INTRODUCTIONConcrete is a unique construction material

possessing superior strength and durability characteristics. So large number of structures come up in past few decades.

Portland cement concrete will remain a major construction material of choice in Civil Engineering construction.

Unfortunately, production of cement involves emission of large amounts of carbon-dioxide gas into the atmosphere.

Approximately 1 ton of CO2 is released into the atmosphere during the production of 1 ton of cement.

Substantial energy and cost savings can result when industrial by products are used as a partial replacement of cement.

Such as Fly ash, Rice husk ash, High Reactive Met kaolin, silica fume are some of the pozzolanic materials which can be used in concrete as partial replacement of cement.

Use of these pozzolanic materials as cement replacements and the results are encouraging. and has many advantages like high strength, durability and reduction in cement production.

OBJECTIVESTo study the effect of partial replacement of

cement with fly ash and silica fume.Evaluation of the compressive strength of high

volume fly ash concrete. To find out the optimum percentage of

replacement of fly ash. To know the strength variations with use of fly

ash and silica fume to the normal concrete.

ADVANTAGES OF Fly ash and silica fume concrete

Reducing the cement content to reduce cost.

Obtaining reduced heat of hydration.

Improving workability.

Improving durability.

Attaining required levels of strength in concrete at ages > 90 days.

 

METHODOLOGY Program of experimental work: plain concrete,

30% replacement of cement by fly ash,

40%replacement of cement by fly ash,

15% replacement of cement by silica fume,

20% replacement of cement by silica fume.

Materials used:- 53 Grade Ordinary Portland cement

Fine Aggregate

Coarse Aggregate

Water

Fly ash

Silica fume

Cement: Cement used in the investigation was 53

Grade Ordinary Portland cement confirming to IS: 12269.The Specific gravity of cement is 3.15.

Fine aggregate: Specific gravity of the sand is 2.58. The sand

obtained was sieved as per IS sieves (i.e. 4.75 mm, 2.36 mm).

Coarse aggregate: The coarse aggregate was obtained from a

local crushing unit having 20mm normal size.

Water:Portable water with PH value of 7.0 confirming to IS 456-2000 was used for making concrete and curing this specimen as well.

Fly Ash: In the present investigation, Class 'F' fly ash,

obtained from Vijayawada Thermal Power Station, was used and its properties are given in (IS: 3812-2003).

Silica fume: silica fume has been recognized as a material of

pozzolanic admixture that is highly effective in enhancing mechanical properties. By using silica fume along with super plasticizers.

EX:-Silica Fume used in Viaduc de Tulle Bridge, France.

Physical and chemical properties of fly ash and silica fumePhysical property Fly ash Silica fume

Specific gravity 2.00 to 2.05 2.2

Physical Form

Bulk density

Powder

1300

Powder

1350-1510

Chemical property Fly ash Silica fumeSilicon Dioxide

(SIO2 )Min 35% 90-96 %

Alluminium Oxide( Al2O3)

25-29% 0.5-0.8%

Ferric Oxide (Fe2O3) 4.5-4.8% 0.2-0.8%Calcium Oxide (CaO) 0.5-1.2% 0.1-0.5%Magnesium Oxide (MgO)

0.3-0.5% 0.5-1.5%

Chemical composition:fly ash is a heterogeneous material. SiO2,

Al2O3, Fe2O3 and occasionally CaO are the main chemical compone -nts present in fly ashes. The mineralogy of fly ashes is very diverse.

Two classes of fly ash are defined by ASTM C618: Class F fly ash and Class C fly ash.

Silica fume is a highly reactive pozzolanna that converts all or most of the liberated calcium hydroxide to C-S-H. (calcium silicate hydrate) gel.

Casting of specimens:To obtain the best percentages of mix proportions

in both cases (Fly Ash and Silica Fume) separate casting of the test specimens were conducted.

Blending of Silica Fume (SF) and Fly Ash (FA) were avoided as the individual effects of SF and FA were observed in this study.

Compression test is the most common test conducted on the hardened concrete.

For the determination of compressive strength 150mm x 150 mm x 150 mm cubes were used. All the specimen -s were moist cured under water until testing.

Four types of specimens were casted to conduct all sort of test regarding strength and water permeability.

Standard Sample (dimension 120mm x 200mm x 200mm) for water permeability test,

Standard Cube specimen (dimension 150mm x 150mm x 150mm) for compressive strength test,

Cylinder specimen (dimension 150mm diameter with 300mm height) for split tensile strength test and

beam specimen (100mm x 100mm x 500mm) for flexural tensile strength test were casted.

Testing of specimens:The compressive strength of concreteis one

of the most important design parameter required for the concrete.

Compressive strength of cube specimen as per ASTM standard was conducted by compression machine for 7, and 28 days.

Results are noted in N/mm2

Effect of fly ash and silica fume on concrete properties:Workability: Fly ash increases workability when compared

with conventional concrete with same water content.

The use of good quality fly ash with a high fineness and low carbon content reduces the water demand of concrete .

Fresh concrete containing silica fume is more cohesive and less prone to segregation than concrete without silica fume.

silica fume it is necessary to increase the initial slump of concrete.

Bleeding:Because of the low water content, the

bleeding is very small. It is, therefore, very important that the curing of the HVFAC commenced as soon as possible.

Concrete containing silica fume shows significantly reduced bleeding. As silica fume dosage is increased, bleeding will be reduced.

Setting time:The low cement content of HVFAC and the

slow reacting property of fly ash increases setting time.

The final time of set of HVFAC is 2 to 3 hours longer

Unlike other SCM’s such as slag and fly ash, silica fume does not significantly affect setting time.

Durability and permeability:Permeability of concrete is governed by many

factors such as amount of cementious material, water content, aggregate grading, consolidation, and curing efficiency .

Sufficiently cured concrete containing good quality fly ash shows dense structure which offers high resistivity to the infiltration of deleterious substances.

Silica fume can produce very large reductions in water permeability of up to one order magnitude or more, depending on the mix design and dosage of silica fume.

Creep:The creep of concrete is influenced by a large

number of parameters and the effect of fly ash on creep.

For example, if loaded at an early age, fly ash concrete may exhibit higher amount of creep than portland cement concrete because of lower compressive strength.

However, if concrete is loaded at an early age when they have attained the same strength, fly ash concrete will exhibit less creep because of its continuous strength gain.

The creep of silica-fume concrete should be no higher than that of concrete of equal strength.

Compressive strength:fly ash has strong effects in compressive

strength of concrete for 7 and 28 days of age. The variation of compressive strength for different replacement levels of OPC by Fly Ash.

As well as fly ash, Silica fume has also strong effects in compressive strength of concrete for 7 and 28 days of age. The variation of compressive strength for different replacement levels of OPC by silica fume.

The compressive strength results were carried out for each cube and plotted on the graphs.

RESULTS AND DISCUSSIONS:Table for compressive strength of fly ash:

Table for compressive strength of silica fume:

% fly ash added 7 days compressive

strength

28 days compressive

strengthOPC(0 %) 20.27 32.95

30% fly ash 18.51 29.8040% fly ash 17.32 28.40

% silica fume added

7 days compressive

strength

28 days compressive

strengthOPC(0 %) 20.27 32.95

15% silica fume 31.45 38.7520% silica ffume 29.67 37.40

Graph showing results of fly ash and silica fume:

30% fly

ash

40% fly

ash

OPC

15% si

lica f

ume

20% si

lica f

ume

05

1015202530354045

7 days strength28 days strength

30% fly ash

40% fly ash

OPC

15% silica fume

20% silica fume

0 5 10 15 20 25 30 35 40 45

28 days strength7 days strength

Some projects constructed using fly ash on the globe:

Bayview high-rise apartment With fly ash max of 45%(Courtesy Ecosmart Foundation)

View of tunnel of Delhi Metro

Rail Corporation where fly ash has been used

Some projects on silica fume:

Silica Fume used in Viaduc de Tulle Bridge, France

Discussions:The compressive strength of the fly ash

concrete increased with an increase in the number of days that was cured.

Early strength gain( within the first 3-7 days) generally decrease as more fly ash is added to the concrete.

Fly ash affects the early strength gain probably due to the free lime that is still reacting during the curing process.

As the concrete is further cured for the ultimate desired strength is attained at 56days.

Silica fume also decrease the voids in concrete.

Addition of silica fume reduces capillary. Absorption and porosity because fine particles of silica fume reacts with lime present in cement.

High early compressive strength. very low permeability to chloride and water intrusion.

Enhanced durability and Increased toughness.Silica fume is useful to marine structures due

to resistance to abrasion. Superior resistance to chemical attack from

chlorides acids, nitrates and sulfates.Silica fume gives high bonding strength to the

concrete.

CONCLUSIONPozzolanic materials have significant

influence on water permeability and mechanical properties of concrete.

10% by weight silica fume exhibited lowest penetration of water (11mm), where lowest water permeability (15mm) for fly ash was obtained at 20% by weight.

The water permeability and strength characteristics of high performance concrete can be improved considerably by replacing the OPC with either silica fume or fly ash.

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