Strength and Durability Studies on Silica Fume Modified High Volume Fly Ash Concrete

8/10/2019 Strength and Durability Studies on Silica Fume Modified High Volume Fly Ash Concrete

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976

6480(Print), ISSN 0976 6499(Online) Volume 5, Issue 10, October (2014), pp. 55-68 IAEME

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STRENGTH AND DURABILITY STUDIES ON SILICA

FUME MODIFIED HIGH-VOLUME FLY ASH CONCRETE

M. Nazeer1, P.S. Anupama

2

1Associate Professor, Dept. of Civil Engineering, TKM College of Engineering, Kollam 5.

2Asst. Professor, Dept. of Civil Engineering, St. Josephs College of Engineering and

Technology, Palai

ABSTRACT

Portland cement, as an ingredient in concrete, is one of the widely used construction

materials, especially in developing countries. The CO2 emission during its production and theutilisation of natural resources are important issues for the construction industry to participate in

sustainable development. These limitations led to the search for alternative binders or cement

substitutes. Approximately 100 million tonnes of fly ash is produced in India annually and this is

increasing rapidly due to the growth in demand for energy. Unused fly ash in large quantities leads to

environmental issues and its storage will be expensive. Fly ash improves the quality and durability of

concrete, leading to the increased service life of concrete structures. Concretes having large amounts

of fly ash (usually above 50% v/v) are termed as high-volume fly ash (HVFA) concrete. Due to the

slow strength development of fly ash concrete caused by the slow pozzolanic reaction of fly ash, the

early strength of fly ash concrete is significantly reduced. Silica fume, which is found to be more

reactive than the fly ash and which significantly, improves the mechanical properties of concrete. In

the present investigation an attempt is made to study the effect of variation of the cementreplacement by silica fume in high-volume fly ash concrete on the mechanical and durability

properties of concrete. The compressive strength development of silica fume modified high-volume

fly ash mixes immersed in water over a period of 90 days is reported. Other tests to evaluate the

penetration resistance of concrete to aggressive chemicals-such as Cl-and CO2are also conducted at

laboratory conditions. The effect of oxide composition of the binder material used, on the strength

and durability properties of concrete is also investigated. Few correlations and mathematical models

are also developed and presented in this report.

Keywords: Fly Ash, Silica Fume, Strength, Durability, High-Volume Fly Ash Concrete,

Oxide Composition.

INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING

AND TECHNOLOGY (IJARET)

ISSN 0976 - 6480 (Print)

ISSN 0976 - 6499 (Online)

Volume 5, Issue 10, October (2014), pp. 55-68

IAEME: www.iaeme.com/IJARET.asp

Journal Impact Factor (2014): 7.8273 (Calculated by GISI)www.jifactor.com

IJARET

I A E M E


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cement is a beneficial practice for sustainable, durable and economic concrete. HVFA concrete with

50% - 60% fly ash can be designed to meet the workability strength and durability requirements of

concrete. [4-9].

The main features of silica fume are a high silica content, high specific surface area and

amorphous structure. These characteristics account for the substantial pozzolanic activity of silicafume, in terms of both its capacity of binding lime and rate of reaction. The effects of silica fume on

properties of the fresh concrete include improvement of the cohesiveness and reduction of bleeding.

The main contribution of the silica fume to the strength development in hardened concrete at normal

curing temperatures takes place from about 3 days onwards. At 28 days the strength of silica-fume

concrete is always higher than the strength of the comparable Portland cement concrete. As the

proportion of silica fume increases, the workability of concrete decreases nevertheless its short term

mechanical properties such as 28-day compressive strength improves [10-13].

II. EXPERIMENTAL

MaterialsMaterials used in the present investigation was carefully selected and tested in the laboratory

to assess the quality and suitability in making concrete of required strength.

Cement: Ordinary Portland Cement (OPC) confirming to IS 12269 [14] (53 Grade) was used for the

present experimental work. The reason for selecting high grade cement is that the replacement of

cement with other supplementary cementitious materials should not cause undue reduction in

strength at early ages. The physical properties of cement used is presented in Table 1.

Table 1: Properties of Cement

Grade OPC 53 Grade

Manufacturer Coromandel King

Specific gravity 3.14

Fineness 5

Standard consistency 26.75%

Initial setting time 95 minutes

Final setting time 375 minutes

Density, g/cc 1.64

Fly Ash: Fly Ash used in the present study was obtained from Tuticorin Thermal Power Plant. From

the laboratory tests, the specific gravity was obtained as 1.84 and density as 1.23 gm/cc.

Silica Fume: Silica fume was supplied by ELKEM Materials. From the laboratory tests, the specific

gravity was obtained as 2.25 and density as 0.784 gm/cc.

The chemical composition of cement, fly ash and silicafume is presented in Table 2.

Fine aggregate: Locally available good quality river sand having specific gravity 2.50 and fineness

modulus 2.41 was used as fine aggregate. Fine aggregate used conforms to IS 383:1970 [15]

specifications (Zone II).


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Coarse aggregate: Crushed stone aggregate of size between 20mm and 4.75mm and specific gravity

2.62 and fineness modulus 6.56 was used as coarse aggregate.

Water: Clean drinking water available in the college water supply system was used for mixing and

curing of concrete.

Table 2: Chemical composition of Cement, Fly Ash, Silica Fume

Oxide Cement Fly Ash Silica Fume

CaO 63.48 0.81 2.94

SiO2 19.13 62.27 84.28

Al2O3 4.26 30.79 1.54

Fe2O3 5.17 1.22 3.47

SO3 4.10 0.15 2.34

MgO 0.67 0.43 2.09

P2O5 0.62 0.51 0.60

TiO2 0.22 0.92 0.04

Na2O 0.60 1.75 1.23

K2O 1.75 1.15 1.47

Mix Proportion

The grade of concrete prepared for the experimental study was M30. The mix design was

done as per ACI 211 method [16]. The design basically involves the determination of water-binder

ratio for a given compressive strength. After selecting the suitable water content, the cement

requirement was determined. The coarse aggregate content was fixed depending on max aggregate

size and fineness modulus of fine aggregate. The fine aggregate content was calculated on the

absolute volume basis. In the design, the volume of entrapped air was assumed to be 2 percent. The

final proportion was 1:1.75: 2.54 (cement: fine aggregate: coarse aggregate) with w/b of 0.48. The

cement content in concrete was 400 kg/m3. Five different mixes were prepared: conventional

concrete mix, HVFA mix and three HVFA + SF mixes. In High Volume Fly Ash mixes, 50%

volume the cement is replaced by Fly Ash. In other mixes, the cement is further replaced by Silica

fume at 5, 10 and 15% by mass of total binder. The cementitious material content in different mixes

is shown in Table 3.

For all mixes other than conventional concrete, only the cementitious materials will change

and the quantity of fine aggregate, coarse aggregate, water content and water to binder ratio remains

constant. (Fine aggregate 700 kg/m3, Coarse aggregate 1016.4 kg/m

3, Water 192 kg/m

3)


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Table 3: Binder Proportion for 1m3Concrete

Mix designation Cement (kg) Fly Ash (kg) Silica fume (kg)

CONV 400 0 0

HVF 228 172 0

HVFS5 216.6 172 11.4

HVFS10 205.2 172 22.8

HVFS15 193.8 172 34.2

Methods

Compressive strength: Compressive strength of concrete is the mostly valued property, which is

used in both design and quality control. In the present study, compression tests were carried out on

100mm cube specimens immediately on removal from the curing water. The specimen was loaded at

the rate of 14 N/mm2per minute. The test was conducted to determine the 3, 7, 28, 56 and 90 day

compressive strength of conventional mix, high volume fly ash mix and three mixes containing silica

fume as the third binder material. For each test-age of these mixes, three specimens were tested and

their average is reported.

Rapid chloride permeability test: The rapid chloride permeability test (RCPT) was conducted

according to ASTM C 1202 in order to determine the resistance of concrete to the penetration of

chloride ions [17]. The resistance to the chloride-ion penetration was measured at the ages of 56 and90 days. 100 mmx 50 mm disc specimens were cast for conventional, high volume fly ash and all

silica fume replaced mixes. For the specimens to be tested at 90 days, steam curing was done for a

period of 2 hours and then immersed in curing tank till the test age is reached. Another set of normal

cured specimens were also tested at 90 days. For the specimens tested at 56 days, only normal curing

was done.

Bulk diffusion test: The depth of chloride ion penetration in concrete can be assessed by bulk

diffusion test. This test method was based on Italian Standard (UNI) in which a chemical manifests a

colour change boundary in response to the quantity of chloride ions present. For conducting the test,

100mm x 200mm cylinder specimens were cast from all mixes. Six specimens were cast for each

mix. The specimens were tested at ages of 56 days and 90 days. Three curing regimes were adopted:

curing in water for 3 days and immersing in 5% sodium chloride solution till test age is

reached,

steam curing for 2 hours and then curing in water for 3 days and dipping in 5% NaCl solution

till test age is reached, and,

curing in water for 7 days and then dipping in 5% NaCl solution till test age is reached.

The specimens were taken out and split when test age is reached. To the split face is sprayed

with 0.1 M AgNO3solution. A white precipitate formed on the edges of split cylinder indicates the

presence of chlorides. The depth of penetration is measured from the edges and the diffusion

coefficient is calculated by the formula [18];


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From the plot, it is clear that the conventional mix attains higher compressive strength values

than other mixes at all ages. It is observed that the silica fume modified mixes show better strength

than high volume fly ash mix after an age of 28 days. Maximum compressive strength is observed

for high volume fly ash concrete with 10% replacement of cement with silica fume from the age of

28 days. It may also be observed that the rate of strength development is more for conventional, highvolume fly ash and 5% silica fume added mixes when compared to the other mixes after 28 days.

This could be due to the reduced workability of concrete containing higher percentage of silica fume.

The strength-age envelops of all mixes follow a linear logarithmic equation in the form:

Wherefcu cube compressive strength at the age of t days in MPa, andA andB are constants.

An attempt is also made to express the above constants in terms of the percentage silica fume

content (Sf) in the high volume fly ash concrete mixes. Thus the equation may be modified as under:

Using the derived equation, compressive strength values of silica fume replaced mixes are

calculated. The calculated values are very close to the actual values obtained and a plot showing

actual values vs calculated values is shown in Fig. 2.

Fig. 2: Compressive strength actual vs model


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In the plot, the equality line indicates the case of calculated strength value equal to the

average compressive strength measured in the laboratory. The points appearing above the equality

line corresponds to the condition that, the suggested model under-estimate the strength.

Rapid Chloride Permeability Test (RCPT): The RCPT was conducted on 100mm x 50 mm discspecimens at the age of 56 days and 90 days as explained in the previous session. The charge passed

in 6 hours is calculated from the experimental data and is plotted against silica fume content in the

mix(Fig. 3).

The charge passed decreases as the test age increases which indicate better resistance to the

penetration of chloride ions. Maximum resistance to chloride ion penetration was reported for steam

cured specimens. It may also be observed that the charge passed decreases continuously with

increase in silica fume content irrespective of testing/curing conditions. In both test ages of 56 and

90 days, addition of 5% silica fume resulted in a decrease in the charge passed. But as the

replacement level reaches 10%, a slight increase in the charge passed is noticed. With further

increase in silica fume content, again a decreasing trend is seen.

Fig. 3: Total Charge Passed vs Silica fume Content

Referring to Fig. 3, it may be concluded that the variation of total charge passed can beexpressed as a function of silica fume content in high volume fly ash mixes. A more realistic model

may be developed considering the variation as a parabolic equation. The equation may be written as

follows:

where QHV is the total charge passed through high volume fly ash concrete (without silica fume) and

Sf is the silica fume content.

For specimen initially steam cured and then water cured and tested at 90 days, the variation

of total charge can also be related to the silica fume content in the mix. For this condition, theequation is:


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While investigating the effect of oxide composition of binders present in each mix on the

durability of concrete, it was observed that the oxides such as CaO, SiO2, Al2O3 and Fe2O3 havemarked influence on the RCPT values. Thus an attempt is made to develop a multiple linear

regression model to predict the charge passed knowing the percentage of CaO,Al2O3and the silica

ratio (SR) of the total binder. The predicted values appear much closer to the experimental values.

The mathematical model is as indicated below:

Where SR is the silica ratio defined as below [19]:

As per the recommendations of ASTM C1202-97 the concrete mixes investigated in this

study may be categorized based on the chloride ion permeability as indicated in Table 4.

Table 4: Chloride permeability rating of different concrete mixes

Mix

designation

Total charge passed,

Coulombs

ASTM C1202

classificationCONV 4400 High

HVF 850 Very low

HVFS5 575 Very low

HVFS10 680 Very low

HVFS15 440 Very low

One of the disadvantages of RCPT is the longer test duration. An attempt has been made here

to correlate the total charge passed through the specimen for 6 hours with the initial current observed

at the commencement of test. A graph showing the variation of total charge passed in 6 hours with

initial current for various mixes at the ages of 56 days and 90 days (normal cured and steam cured

specimens) is shown in Fig. 4. It may be observed that a linear relationship exists between the charge

passed and initial current. However a plot of variation of total charge passed with initial current

value, without considering the curing conditions given to concrete, is shown in Fig. 5. From this

graph, the total charge passed in 6 hours can be expressed as a function of initial current as;

where Q6is the total charge passed (C) in 6 hours andI0is the initial current (mA).


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Fig. 4: Total Charge vs Initial Current for different test ages

Fig. 5: Total Charge vs Initial Current

Bulk Diffusion Test: The chloride penetration depth observed based on the method outlined in the

previous session was used to calculate the diffusion coefficients. The results obtained are presented

inFig. 6.


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Fig. 6: Diffusion Coefficients

It may be noted that the diffusion coefficient is maximum for conventional concrete at all test

ages and curing conditions adopted. In most cases, the conventional concrete mixes yield thediffusion coefficient value greater than 5 x 10

-12m

2/s which means the concrete is highly permeable.

The diffusion coefficient values of most of the other mixes at all ages lies between 1 x 10-12

m2/s and

5 x 10-12

m2/s which indicates that the addition of supplementary cementitious materials has reduced

the permeability of concrete from high to the average permeability range. In mixes with minimum

cement content, steam cured specimens and specimens immersed in solution after water curing for 7

days, when tested at the age of 56 days gave diffusion coefficient values less than 1 x 10-12

m2/s

which indicates that its permeability is low.


durability of concrete, it was observed that the oxides such as CaO, SiO2, Al2O3 and Fe2O3 have

marked influence on the diffusion coefficient values. Thus an attempt was made to develop a

multiple linear regression model to predict the diffusion coefficient knowing the percentage of CaO,

Al2O3and the silica ratio (SR) of the total binder. The predicted values appear much closer to the

experimental values. The mathematical model is presented below:


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Carbonation test: The depth of carbonation measured was plotted against silica fume replacement

level as shown in Fig. 7. From the plot, it may be observed that the depth of carbonation goes on

increasing with increase in silica fume content until the percentage of silica fume replacement

reaches 10%.

Fig. 7: Carbonation Depth vs Silica fume Content in HVFAC

With further increase in silica fume content, the depth of carbonation decreases. This trend

was seen for both test ages of 56 and 90 days, but can be clearly noticed in the curve for 90 days.

The minimum depth of carbonation was noted for conventional mix followed by high volume fly ash

mix. There exists a polynomial relation connecting the carbonation depth (mm) with the percentage

of silica fume content in the HVFA mix. In these equations CHV indicate the carbonation depthobserved in the HVF mix at the designated ages.


durability of concrete, it was observed that the oxides such as CaO, SiO2, Al2O3 and Fe2O3 have

marked influence on the carbonation depth values. Thus an attempt was made to develop a multiple

linear regression model to predict the carbonation depth knowing the percentage of CaO,Al2O

3and


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the silica ratio (SR) of the total binder. The predicted values appear much closer to the experimental

values. The mathematical model is as follows:

IV. CONCLUSIONS

From the present investigation, the performance of High Volume Fly Ash and its

modification by partial replacement of cement with Silica fume was studied and they were compared

to the performance of ordinary concrete. The strength and Durability properties of concrete were also

examined in this study.

Following conclusions are drawn from the present investigation based on the limited

observations made during the study period.

Silica fume added mixes shows higher strength values compared to their high volume fly ash

counterparts at later ages (after 28 days).

A linear logarithmic relation was developed for co-relating the compressive strength with age

and silica fume content in various mixes. Using this correlation equation compressive

strength values for various mixes are calculated and compared with the experimental results

obtained.

The addition of supplementary cementitious materials improves the resistance of concrete to

chloride penetration.

Mathematical models for predicting the diffusion coefficient, total charge passed in 6 hours

and carbonation depth by knowing the oxide composition of the binder material for various

mixes were developed and compared with the experimental values. The models gavesatisfactory results.

Equation for predicting the total charge passed in 6 hours knowing the initial current during

the beginning of RCPT is formulated to overcome the disadvantage of longer test duration.

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Strength and Durability Studies on Silica Fume Modified High Volume Fly Ash Concrete

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