Abstract
Maintenance, repair and rehabilitation of existing cement
concrete structures involve a lot of problem leading to significant
expenditure. In the recent past, there has been considerable attention for
improving the properties of concrete with respect to strength and durability,
especially in aggressive environments .High performance concrete (HPC) appears
to be better choice for a strong and durable structure. Suitable addition of
mineral admixtures such as silica fume (SF), ground granulated blast furnace
slag and fly ash in concrete improves the strength and durability of concrete
due to considerable improvement in the microstructure of concrete composites, e
specially at the transition zone .Very few studies have been reported in India
on the use of SF for development of HPC and also durability characteristics of
these mixes have not been reported. In order to make a quantitative assessment
of different cement replacement levels with SF on the strength and durability
properties for M60, M7O and M110 grades of HPC trial mixes and to arrive at the
maximum levels of replacement of cement with SF, investigations were taken.
This paper reports on the performance of HPC trial mixes having
different replacement levels of cement with SF. The strength and durability
characteristics of these mixes are compared with the mixes without SF.
Compressive strengths of 60 MPa, 70 MPa and 110 MPa at 28days were obtained by
using 10 percent replacement of cement with SF. The results also show that the
SF concretes possess superior durability properties.
Introduction
Silica
fume (SF) is a by-product of the smelting process in the silicon and
Ferro-silicon industry. The reduction of high- purity quartz to silicon at
temperatures up to 2,000 C produces SiO2 vapours, which oxidizes and condense
in the low- temperature zone to tiny particles consisting of non-crystalline
silica. By-products of the production of silicon metal and the ferrosilicon
alloys having silicon contents of 75% or more contain 85–95% non-crystalline
silica. The by-product of the production of ferrosilicon alloy having 50%
silicon has much lower silica content and is less pozzolanic. Therefore, SiO2
content of the silica fume is related to the type of alloy being produced
Silica fume is also known as micro silica, condensed silica fume, volatilized
silica or silica dust.
The
American concrete institute (ACI) defines silica fume as a ‘‘very fine non-
crystalline silica produced in electric arc furnaces as a by product of
production of elemental silicon or alloys containing silicon’’. It is usually a
grey coloured powder, somewhat similar to Portland cement or some fly ashes. It
can exhibit both poz-zolanic and cementitious properties.
Silica
fume has been recognized as a poz-zolanic admixture that is effective in
enhancing the mechanical properties to a great extent. By using silica fume
along with superplasticizers, it is relatively easier to obtain compressive
strengths of order of 100– 150MPa in laboratory. Addition of silica fume to
concrete improves the durability of concrete through reduction in the permeability,
refined pore structure, leading to a reduction in the diffusion of harmful
ions, reduces calcium hydroxide content which results in a higher resistance to
sulphate attack
The
silica fume is collected in very large filters in the baghouse and then made
available for use in concrete.
Silica
fume has historically been available in three basic product forms: undensified,
slurried, and densified. There is no data available, after many years of
testing, to show that any one of the product forms will perform better in a
concrete mixture than any of the others. Slurried silica fume is no longer
available in the U.S. market. Undensified silica fume is available, but it is
not frequently used in ready-mixed or precast concrete. Undensified silica fume
is primarily used in pre-bagged products such as grouts or repair mortars.
Densified
silica fume is produced by treating undensified silica fume to increase the
bulk density up to a maximum of
About 400 to 720kg/m3.
This increase in bulk density is usually accomplished by tumbling the
silica-fume particles in a silo, which causes surface charges to build up.
These charges draw the particles together to form weak agglomerates. Because of
the increased bulk density, this material is more economical for truck
transportation
Densified
silica fume works very well in concrete. However, one caution when working with
this product form is to ensure that the mixing is adequate to break up the
particle agglomerations. Mixing in some types of mixers such as those that are
used in dry mix shotcrete, roof tiles, or other applications where coarse
aggregate is not present may not be adequate to break up the agglomerations. In
those situations, an undensified silica fume may be more appropriate.
2.1.1
Availability and Handling
Silica fume is available in two
conditions: dry and wet. Dry silica can be provided as produced or densified
with or without dry admixtures and can be stored in silos and hoppers. Silica
Fume slurry with low or high dosages of chemical admixtures are available.
Slurried products are stored in tanks.
2.2
Properties of Silica Fume
2.2.1
Physical Properties
Silica fume particles are extremely
small, with more than 95% of the particles finer than 1 lm. Its typical
physical properties are given. Silica fume colour is either premium white or
grey
2.2.2
Chemical Composition
The
fumes generally contain more than 90 percent silicon dioxide, mostly amorphous.
Other constituents are carbon, sulphur and the oxides of aluminium, iron,
calcium, magnesium, sodium and potassium. The loss on ignition of some
Norwegian and American products runs from 0.7 to 2.8 percent. The chemical
composition of the fume varies according to the type of alloy or metal being
produced. For example, the fume from a ferrosilicon furnace will generally
contain more iron and magnesium oxides than that from a furnace producing
silicon metal.
2.2 Silica fume
Vitreous
silica, mainly of cristobalite form. Silica fume has a very high content of
amorphous silicon dioxide and consists of very fine spherical particles. Silica
fume generally contains more than 90% SiO2. Small amounts of iron, magnesium,
and alkali oxides are alsofound.
2.3
Reaction Mechanism
Because of its extreme fineness and
very high amorphous silicon dioxide content, silica fume is a very reactive
pozzolanic material. As the Portland cement in concrete begins to react
chemically, it releases calcium hydroxide. The silica fume reacts with this
calcium hydroxide to form additional binder material called cal- cium silicate
hydrate which is very similar to the calcium silicate hydrate formed from
Portland cement. It is an additional binder that gives silica-fume concrete its
improved properties. Mechanism of silica fume in concrete can be studied
basically under three roles:
(i) Pore-size Refinement and Matrix
Densification:
The presence of silica fume in the
Portland cement concrete mixes causes considerable reduction in the volume of
large pores at all ages. It basically acts as filler due to its fineness and
because of which it fits into spaces between grains in the same way that sand
fills the spaces between particles of coarse aggregates and cement grains fill
the spaces between fine aggregates grains.
(ii)
Reaction with Free-Lime
CH
crystals in Portland cement pastes are a source of weakness because cracks can
easily propagate through or within these crystals without any significant
resistance affecting the strength, durability and other properties of concrete.
Silica fume which is siliceous and aluminous material reacts with CH resulting
reduction in CH content in addition to forming strength contributing
cementitious products which in other words can be termed as ‘‘Pozzolanic
Reaction’’.
(iii)
Cement Paste–Aggregate Interfacial Refinement
In
concrete the characteristics of the transition zone between the aggregate
particles and cement paste plays a significant role in the cement-aggregate
bond. Silica fume addition influences the thickness of transition phase in
mortars and the degree of the orientation of the CH crystals in it. The
thickness compared with mortar containing only ordinary Portland cement
decreases and reduction in degree of orientation of CH crystals in transition
phase Hence with the addition of silica fume. Mechanical durability properties
and y is improved because of the Enhancement in interfacial or bond strength.
Mechanism behind is not only connected to chemical formation of C–S–H (i.e.
pozzolanic reaction) at interface, but also to the microstructure modification
(i.e. CH) orientation, porosity and transition zone thickness) as well.
2.4
Heat of Hydration
Silica fume is amorphous in nature
and may contain some crystalline silica in the form of quartz or cristobalite.
The higher surface area and amorphous nature of silica fume make it highly
reactive. The hydration of C3S, C2S, and C4AF are accelerated in the presence
of silica fume. Grutzeck et al.concluded that silica fume experiences rapid
dissolution in the presence of Ca (OH)2
and a supersaturation of silica with
respect to a silica-rich phase. This unstable silica-rich phase forms a layer
on the surface of the silica fume particles. The layer is then partly dissolved
and the remainder acts as a substitute on which conventional C–S– H is formed.
2.5
Silica Fume Efficiency
Silica fume efficiency in concrete
is not constant at all percentages of replacement. The ‘‘overall efficiency
factor’’ of silica fume can be assessed in two separate parts; ‘‘general
efficiency’’ which is constant at all percentages of replacement and the
‘‘percentage efficiency factor’’ which varies with the replacement percentage.
The activity of silica fume in concrete is obtained in terms of the amount of
cement replaced through its ‘‘cementing efficiency factor’. Efficiency factor
for silica fume in concrete can be defined as the number of parts of cement
that may be replaced by one part of the silica fume, without changing the
property being investigated generally the compressive strength. It is generally
more efficient in concretes having high water-cement ratios. Research in Norway
and Canada indicates that in concretes with a water-cement ratio of about 0.55
and higher, the silica fume has an efficiency factor of 3-4.
Fineness
Silica
fume consists of very fine vitreous particles with a specific surface area in
the order of 20,000 square meters per kilogram. The extreme fineness of silica
fume is best illustrated by the following comparison with other fine materials.
Most particles of a typical silica fume are smaller than 1 micron. The high reactivity
of silica fume with Portland cement is primarily due to its very high specific
surface and its high content of amorphous silicon dioxide.
Material
|
Fineness,
|
|||
square meters
|
||||
Silica fume
|
about 20,000
|
|||
Tobacco smoke
|
about 10,000
|
|||
Fly ash
|
400 to 700
|
|||
Normal portland
|
300 to 400
|
2.6
METHODS OF USING SILICA FUME IN CONCRETE
As an admixture
Small
quantities of silica fume, 5 to 10 percent by weight of cement, can be added to
concrete. The resulting loss in slump is compensated for either by the addition
of more water or the use of superplasticizers. In either case, there is a
marked increase in compressive strength as compared with the control mix. This
is particularly so with the use of superplasticizer.
As a partial replacement for cement
Silica fume can be used as a partial
replacement for cement. The percentage replacement may vary from 0 to 30
percent. Though this does not change the weight of the cementitious materials,
there is an increase in the water demand because of the extreme fineness of
silica fume. In order to maintain the same water- (cement plus silica fume)
ratios, superplasticizers are used to maintain the required slump. This
approach also results in an increase in compressive strength at the age of 3
days and thereafter.
POSSIBLE APPLICATIONS
To conserve cement
Because of its very high efficiency,
the judicious use of silica fume can help conserve cement, especially in
concretes with a water-cement ratio around 0.55. A number of ready mixed
concrete producers in Norway are using silica fume in this way. A small ready
mixed concrete producer in Quebec has also started using silica fume to
conserve cement.
To produce ultra-high-strength concrete
Silica fume has been used with
superplasticizers to produce ultra -high-strength concrete. Compressive
strengths on the order of 15,000 psi and greater have been reported this of
course is a very specialized area with limited applications.
To reduce alkali-aggregate reaction
Like fly ashes and natural
pozzolans, silica fume can be used to counter alkali-aggregate reactions.
Silica fume has the added advantage that relatively small quantities may be
needed, by comparison with the former materials. This is a very promising area
for the use of silica fume.
2.7 Applications of Silica Fume
• High Performance Concrete (HPC)
containing silica fume— for highway bridges, parking decks, marine structures
and bridge deck overlays which are subjected to constant deterioration caused
by rebar corrosion current, abrasion and chemical attack. Silica fume will
protect concrete against dicing salts, seawater, road traffic and freeze/thaw
cycles. Rebar corrosion activity and concrete deterioration are virtually
eliminated, which minimizes maintenance expense.
• High-strength concrete enhanced
with silica fume—provides architects and engineers with greater design
flexibility. Traditionally used in high-rise buildings for the benefit of
smaller columns high- strength concrete containing silica fume is often used in
precast and prestressed girders allowing longer spans in structural bridge
designs.
• Oil Well Grouting—whether used for
primary (placement of grout as a hydraulic seal in the well-bore) or secondary
applications (remedial operations including leak repairs, splits, closing of
depleted zones); the addition of silica fume enables a well to achieve full
production potential. Besides producing a blocking effect in the oil well grout
that prevents gas migration, it provides these advantages such as (i) Improved
flow, for easier, more effective application; (ii) dramatically decrease
permeability, for better control of gas leakage; and (iii) lightweight
• Repair Products—silica fume is used
in a variety of cementitious repair products. Mortars or grouts modified with
silica fume can be tailored to perform in many different applications—overhead
and vertical mortars benefit from silica fume’s ability to increase surface
adhesion. Silica fume significantly improves cohesiveness making it ideal for
use in underwater grouts, decreases permeability in grouts used for
post-tensioning applications and increases the resistance to aggressive
chemicals
• Refractory and Ceramics—the use of
silica fume in refractory castables provides better particle packing. It allows
for less water to be used while maintaining the same flow characteristics. It
also promotes low temperature sintering and the formation of mullite in the
matrix of the castable. This produces a castable that has a low permeability to
avoid gas, slag and metal penetration. Castables incorporating silica fume are
stronger than non-silica fume containing castables especially at high
temperatures with higher density they attain lower porosity and are more volume
stable.
2.7.1 PROBLEMS WITH THE USE OF
SILICA FUME
Availability
In some areas silica fume is still
regarded as a waste product and is not actively marketed for use in concrete.
Some users have found it practical to send a cement- hauling truck to a plant
that has furnaces making sili- con or ferrosilicon, and haul loads of fume to
their own silos.
Handling problem
Because of its extreme fineness,
silica fume is very light and does present handling problems. In Norway, the
problems have been overcome to a degree by transporting and using silica fume
in slurry form. In Canada, patents have been taken out on a process to densify
the fume for transportation purposes. The densification would be done at the
plant where the fume is produced. The process increases the bulk density by a
factor of about 3 and makes the particles slightly coarser than cement or fly
ash but, it is claimed, without any sacrifice of its beneficial properties.
Some ready-mixed concrete producers are currently using the fume as produced,
without densification.
Difficulty in entraining air
Investigations at the Canada Center
for Mineral and Energy Technology have indicated some difficulty in entraining
5 to 7 percent air in concretes with high cement contents and 20 to 30 percent
of silica fume. At lower percentage replacements, no such problems were
encountered, although the dosage of air-en traning agent required to entrain a
given percentage of air did increase markedly as compared with the control
concrete.
Cost
In the past two years, people have
begun to recognize the value of silica fume. Consequently the price has sky-
rocketed. It was only a waste product a few years ago. Now the price of fume
varies from half to twice the price of normal portland cement. Further
increases in the price may limit the use of fume to specialized applications.
2.8 Effect
of Silica Fume on Fresh Properties of Cement
Rheological properties of a fresh
cement paste play an important role in deter- mining the workability of
concrete. The water requirement for flow, hydration behaviour, and properties
of the hardened state largely depends upon the degree of dispersion of cement
in water. Properties such as fineness, particle size distribution, and mixing
intensity are important in determining the rheological properties of cement
paste. Due to the charges that develop on the surface, cement particles tend to
agglomerate in the paste and form flocs that trap some of the mixing water.
Factors such as water content, early hydration, water reducing admixtures and
mineral admixtures like silica fume determine the degree of flocculation in a
cement paste.
Fresh concrete containing silica
fume is more cohesive and less prone to segregation than concrete without
silica fume. Concrete containing silica fume shows substantial reduced
bleeding. Additionally silica fume reduces bleeding by physically blocking the
pores in the fresh concrete. Use of silica fume does not significantly change
the unit weight of concrete.
2.8.1 Consistency
Rao determined the influence of
silica fume on the consistency of cement pastes and mortars. Specific gravity
and specific surface
of the silica fume were 2.05 and
16,000 m2/kg, respectively. Silica fume was varied from 0 to 30% at
a constant increment of 2.5/5% by weight of cement. Since the SF is finer than
the cement, the specific surface increased with increase in SF content. The
standard consistency of pure cement paste was found out to be 31.50%; while at
30% SF, it was 44.25%. It was observed that the consistency of cement increased
with the increase in SF content. As much as 40% of additional water requirement
was observed for cement pastes containing 20–30%
Variation of consistency of cement pastes containing
different percentages of silica fume
Qing et al examined the influence of
nano-SiO2 (NS) addition on consistency of cement paste incorporating NS or
silica fume. The influence of NS or silica fume addition on consistency and
setting time of fresh pastes is given It was found that with increasing the NS
content, fresh pastes for sample A-series grew thicker gradually and their
penetration depths (consistency value) decreased gently as compared with that
of control sample CO. While with increasing the silica fume content, the pastes
for sample B-series grew thinner and their depths increased. They concluded
that silica fume makes cement paste thinner as compared with NS.
2.8.2Setting Times
Lohtia and Joshi concluded that the
addition of silica fume to concrete in the absence of water-reducer or superplasticizer
causes delay in setting time, compared to non-silica fume concrete of equal
strength, especially when the silica fume content was high. The additions of
5–10% silica fume to either super-plasticized or non-superplasticizer concrete
with W/(C : SF) ratio of 0.40 did not exhibit any significant increase in
setting time. However, when 15% silica fume was added with superplasticizer,
both the initial and final setting times were delayed by approximately 1 and 2
h, respectively. The observed delay was attributed to the relatively high dose
of superplasticizers needed for the high amount of silica fume added to
concrete. Studies the influence of fume on the setting cilica time of paste.
Specific gravity specific surface of cement and the silica fume were 2.05 and
16,000 m2/kg, respectively He observed that initial setting time
decreased with the increase in silica fume content. At smaller contents, the
setting time of cement paste did not affect much.
However, at higher silica fume
contents, the initial setting time was significantly decreased. At 30% silica
fume, the initial setting time had been only 30 min. The final setting time
seem to be not influenced by the silica fume. The pozzolanic action of silica
fume seems to be very active at early hours of hydration. Therefore, he
concluded that silica fume contents result in quick setting of cement. It was
observed that the setting of fresh pastes was slightly accelerated but the
difference between initial and final setting time decreased with increase in NS
content. While the setting of fresh pastes was obviously retarded and the
difference was also decreased with increasing the silica fume content. They
concluded that silica fume makes cement paste thinner and retards the cement
setting process
2.8.3 Workability
The physical properties of
micro-silica are known to reduce workability mainly due to small particle size
that leads to higher water demand. The workability of concrete mix containing
mineral admixture is considerably improved by using chemical admixture. The
combination of a superplasticizer and a mineral admixture (silica-fume) is
desirable, since silica fume in the amount exceeding 5% from the mass of cement
considerably increases the fine fraction volume and hence the water requirement
of the binder.
MIX PROPORTIONS FOR STANDARD
CONSISTENCY
MIX
|
OPC
|
MICROSILICA
|
WATER
|
ADMIXTURE
|
1
|
100
|
0
|
27.5
|
0
|
2
|
95
|
5
|
30
|
0
|
3
|
90
|
10
|
32
|
0
|
4
|
85
|
15
|
37.5
|
0
|
5
|
80
|
20
|
43
|
0
|
6
|
95
|
4
|
27.5
|
3
|
7
|
90
|
10
|
27.5
|
5
|
8
|
85
|
15
|
27.5
|
6.5
|
9
|
680
|
20
|
27.5
|
8
|
2.9
Effect of Silica Fume on the Hardened Properties of Cement
2.9.1 Compressive Strength
When silica fume is added to
concrete, it results in a significant change in the compressive strength of the
mix. This is mainly due to the aggregate-paste bond improvement and enhanced microstructure.
2.9.1.1Compressive Strength of
Cement Paste/Mortar
Huang and Feldman found that mortar
without silica fume has lower strength than cement paste with the same
water–cement ratio, while mortar with 30% of cement replaced with silica fume
has a higher strength than cement–silica fume paste with the same
water–cementitious ratio.
They concluded that the addition of
silica fume to mortar resulted in an improved bond between the hydrated cement
matrix and sand in the mix, hence increasing strength. This improved bond is
due to the conversion of the calcium hydroxide, which tends to form on the
surface of aggregate particles, into calcium silicate hydrate due to the
presence of reactive silica.
Cong et al. observed that the
replacement of cement by silica fume (up to 18%) and the addition of
superplasticizer increased the strength of cement paste. Concrete containing
silica fume as a partial replacement of cement exhibited an increased compressive
strength largely because of the improved strength of cement paste matrix. But,
changes in paste aggregate interface caused by the incorporation of silica fume
had little effect on the compressive strength of concrete Motar compressive
strength
Age (days)
|
0% Silica fume
|
10% Silica fume
|
7
|
3.26 ± 0.12
|
2.93 ± 0.13
|
28
|
6.58 ± 0.19
|
7.11 ± 0.25
|
2.9.1.2 Compressive Strength of
Normal Strength/HPC
Bentur et al. reported that the
strength of silica fume concrete is greater than that of silica fume paste
which they attributed to the change in the role of the aggregate in concrete.
In cement concrete, the aggregate functions as inert filler but due to the presence
of weak interfacial zone, composite concrete is weaker then cement paste. But,
in silica fume concrete, the presence of silica fume eliminates this weak link
by strengthening the cement paste aggregate bond and forming a less porous and
more homogenous microstructure in the interfacial region. Thus, silica fume
concrete is stronger than silica fume cement paste, taking into account that
the strength of aggregate exceeds the strength of cement paste.
Table 2.11
|
Development of compressive
strength with age (MPa)
|
||||||||||
Concr
|
Silica
|
Compressive strengths (MPa)
|
|||||||||
Ete
|
fume
|
||||||||||
mixes
|
(%)
|
7
|
14
|
28
|
42
|
90
|
365
|
400
|
|||
d
|
da
|
da
|
da
|
da
|
day
|
days
|
|||||
OPC
|
0
|
4
|
52
|
58
|
62
|
64
|
73
|
74
|
|||
SF 6
|
6
|
5
|
58
|
65
|
69
|
71
|
73
|
73
|
|||
SF
|
10
|
5
|
61
|
67.
|
71
|
74
|
73
|
73
|
|||
SF
|
15
|
5
|
63
|
70
|
73
|
76
|
75
|
76
|
|||
The silica fume content was 0, 6,
10, and 15%, and water– cementitious ratio being 0.35. From the results it can
be seen that
(i) at the age of 28 days, the
silica fume concrete was 21% stronger than control concrete; (ii) compressive
strength development of concrete mixtures containing silica fume was negligible
after the age of 90 days; however, there was 26% and 14% strength increase in
the control concrete after 1 year compared to its 28 and 90 days strength,
respectively. Also the tests showed that at the age of 400 days, the
compressive strength of control concrete and concrete mixes containing
different proportions of silica fume were the same.
According to Wild et al. this
difference in strength development in OPC concrete and silica fume concrete can
be attributed to the rapid formation of an inhibiting layer of reaction product
preventing further reaction of silica fume with calcium hydroxide beyond 90
days.
Sobolev studied the compressive
strength of high performance concretes. It was observed that (i) increase in
superplasticizer dosage from 8 to 18% led to a reduction of w/c from 0.31 to
0.26 and improved the concrete compressive strength from 86 to 97 MPa;
(ii)maximum compressive strength of 91 MPa was obtained at 15% silica fume.;
(iii) lower strength value of 90 MPa occurred at 10 and 20% silica fume; and
(iii) reduction of w/c 0.32–0.19 increased the compressive strength of cement
concrete and resulted in super high strength concrete having strength up to 135
MPa.
Table 2.12 Details of HPC mixtures
|
|||||
Proportions
|
SF (5%)
|
SF (10%)
|
SF (15%)
|
SF
|
|
Cement
|
426
|
449
|
468
|
478
|
|
Silica fume
|
22
|
50
|
83
|
120
|
|
Age
|
Compressive strength (MPa)
|
||||
1 day
|
16.8
|
24.1
|
34.4
|
45.1
|
|
3 days
|
28.6
|
42.2
|
63.0
|
84.9
|
|
7 days
|
50.1
|
67.2
|
84.8
|
102.5
|
|
28 days
|
60.0
|
80.0
|
100.0
|
120.0
|
|
Table 2.13
|
Cube compressive strength Mixture
|
Compressive strength(MPa)
|
||||||||||
1
|
3 days
|
7 days
|
28 days
|
56 days
|
90 days
|
180
|
||||||
w/cm 0.27
|
39
|
68
|
72.5
|
84
|
86.5
|
87.5
|
90
|
|||||
SF 5
|
35
|
63
|
75.5
|
88.5
|
93
|
96.5
|
97.5
|
|||||
SF 10
|
25
|
61
|
79
|
95.5
|
100
|
104
|
107s
|
|||||
SF 15
|
24.5
|
59.5
|
76.5
|
101
|
103.5
|
106
|
109
|
|||||
w/cm 0.30
|
48
|
63.5
|
72
|
83.5
|
84.5
|
85.5
|
87.5
|
|||||
SF 5
|
46
|
62
|
81
|
91
|
95.5
|
95.5
|
97
|
|||||
SF 10
|
42
|
61.5
|
78.5
|
95
|
97
|
99
|
103
|
|||||
SF 15
|
38
|
57.5
|
74.5
|
98.5
|
101.5
|
104
|
106.5
|
|||||
w/cm 0.33
|
41.
|
58.0
|
62.5
|
75
|
78
|
79
|
81.5
|
|||||
SF 5
|
35.
|
55.0
|
69.5
|
83.0
|
85.0
|
90.0
|
90.0
|
|||||
SF 10
|
32.
|
53.0
|
70.5
|
89.5
|
90.5
|
92.0
|
93.5
|
|||||
SF 15
|
31.0
|
47.5
|
70.5
|
88.5
|
93.0
|
95.5
|
100.5
|
|||||
2.9.
Effect of Curing on the Compressive Strength of Concrete
Bentur and Goldman studied the
effect of water and air-curing in mild environmental conditions on the
compressive strength at the age of 90 days. The air curing resulted in a
somewhat lower strength compared to continuous water curing. This was
attributed to the observations that the strengthening influence of the silica
fume takes place quite early during the period 1–28 days and possibly slower
rate of drying from within the silica fume concrete, which apparently developed
a tight micro-structure after 7 days of water curing. Similar trends were
2.9.1.4 Compressive Strength of
Recycled Aggregate Silica Fume Concrete
González-Fonteboa and
Mart9inez-Abella studied the properties of concrete using recycled aggregates
from Spanish demolition debris (RC mixes) and the impact of the addition of
silica fume on the properties of recycled concrete (RCS mixes)
A comparison was made between both
these materials and standard conventional concrete (CC mixes), which was also
modified by adding silica fume (CCS mixes). It also aimed to study the effect
of addition of silica fume on the basic properties of recycled concrete. For
the test four series of mixes were made. They reported that (i) Pozzolanic
effect of silica fume was seen between 7 and 21 days which tends to increase the
compressive strength of the concrete; and (ii) concrete containing 8% silica
fume displayed greater compressive strength than concretes that did not contain
this admixture, at all ages.
Almusallam et al.investigated the
effects of silica fume on the compressive strength of concrete made with
low-quality coarse aggregates. Four types of low quality coarse aggregates,
namely calcareous, dolomitic, and quarzitic limestone and steel slag were used,
and silica fume content was 10 and 15% as partial replacement of cement. The
concrete specimens had a w/c ratio of 0.35 and a coarse aggregate to fine
aggregate ratio of 1.63.They observed that compressive strength increased with
age in all the concrete specimens. After 180 days of curing, highest
compressive strength was noted in the 15% silica fume cement concrete specimens
followed by those prepared with 10% silica fume and plain cement concrete. The
higher compressive strength noted in the silica fume cement concrete, compared
to plain cement concrete, may be attributed to the reaction of the silica fume
with calcium hydroxide liberated during the hydration of cement. Khatri et al.
stated that it results in formation of secondary calcium silicate hydrate that
fills up the pores due to the hydration of the initial calcium silicate
hydrate.
Babu and Babu studied the use of
expanded polystyrene (EPS) beads as lightweight aggregate both in concrete and
mortars containing silica fume as a supplementary cementitious material. Three
percentages of silica fume—3, 5 and 9% (by weight of the total cementitious
materials) were used. They concluded that the rate of strength development was
greater initially and decreased as the age increased A comparison of strengths
at 7 days reveals that concretes with 3% silica fume developed almost 75% of
its 28-day strength, while that with 5 and 9% silica fume developed almost 85
and 95% of the corresponding 28-day strength. They concluded that rate of
strength gain was increasing with an increasing per- centage of silica fume.
2.9.2 Tensile Strength
Hooton reported the splitting
tensile strength of silica fume concretes up to the age of 182 days It can be
seen that except at 28 days, the splitting tensile strength was not improved
for silica fume concrete mixes. Also it was observed that with increasing
replacement of silica fume split tensile strength decreased.
Bhanja and Sengupta studied the
isolated contribution of silica fume on the tensile strengths of
high-performance concrete. Five concrete mixes, at w/cm ratios of 0.26, 0.30, 0.34,
0.38 ,0.42 were prepared by partial replacement of cement by equal weight of
silica fume. The dosage of silica fumes were 0% , 5, 10, 15, 20 and 25% of the
total cementitious materials. For all the mixes, tensile strengths were
determined at the end of 28 days. Studies clearly exhibited that very high
percentages of silica fume did not significantly increase the splitting tensile
strength and increase was insignificant beyond 15%.
Splitting
tensile strength of concrete
Test age (days)
|
Concrete
|
||||
Control
|
10% SF
|
15% SF
|
20% SF
|
||
28
|
5.2
|
6.3
|
6.2
|
4.6
|
|
91
|
6.8
|
6.7
|
6.2
|
5.6
|
|
182
|
7.1
|
6.2
|
6.5
|
5.6
|
|
2.9.3 Flexural Tensile Strength
Bhanja and Sengupta studied the
contribution of silica fume on the flexural strength of high performance
concrete (HPC). Five series of concrete mixes, at w/ cm ratios of 0.26, 0.30,
0.34, 0.38 and 0.42 were made with partial replacement of cement by equal
weight of silica fume. The dosages of silica fumes were 0, 5, 10,15, 20 and 25%
of the total cementitious materials. The variations of flexural tensile
strength with silica fume replacement percentage at different w/cm ratios are
studied . They stated that silica fume seemed to have a pronounced effect on
flexural strength in comparison with splitting tensile strength. For flexural
strengths, even very high percentages of silica fume significantly improve the
strengths. Also it was found that there was a steady increase in the flexural
strength with increase in the silica fume replacement percentage.
Köksal et al. evaluated the flexural
strength of concrete incorporating hooked steel fibres and silica fume. Aspect
ratios (l/d) of fibres were 65 and 80 and volume fractions (Vf) of steel fibres
were 0.5 and 1%. Silica fume was added to concrete directly as the percentages
of 0, 5, 10 and 15% by weight of cement. Significant increases in the flexural
strengths of the concretes were observed by adding silica fume and steel fibres.
The increases in the flexural strengths of the concretes without steel fibres
were 7,42.1 and 64.9% for the 5, 10 and 15% silica fume, respectively. Also
they found that the flexural strengths of concretes containing 1% steel fibre
were found to be greater than that of the concrete with 0.5% steel fibre for
each of the silica fume content.
2.9.4 Modulus of Elasticity
Hooton reported the modulus of
elasticity of silica fume concretes up to the age of 365 days. It can be seen
that elastic modulus of the Portland cement concrete was approximately equal to
silica fume concretes at 28 days but continued to increase at later ages.
Mazloom et al. investigated the effect of silica fume on the secant modulus of
elasticity of high performance concrete. The percentages of silica fume were:
0,
Table 2.22 Flexural strengths of concrete at different
curing times
|
||||||||
Curing
|
Flexural tensile strength (MPa)
|
|||||||
time
|
||||||||
(days)
|
||||||||
Gabb
|
Basalt
|
Quartsite
|
Limestone
|
Sandstone
|
||||
ro
|
(132)
|
(160)
|
(110)
|
(52)
|
||||
3
|
12.6
|
11.4
|
12.9
|
7.9
|
3.2
|
|||
7
|
16.1
|
15.4
|
14.9
|
12.5
|
4.5
|
|||
28
|
17.3
|
16.7
|
16.2
|
12.8
|
5.2
|
|||
90
|
18.4
|
17.9
|
16.9
|
13.9
|
5.6
|
|||
Table 2.23 Modulus of elasticity of silica fume concrete
|
|||||
Testing age (days)
|
Concrete
|
||||
Control
|
10% SF
|
15% SF
|
20%
|
||
28
|
43.2
|
43.7
|
42.8
|
43.4
|
|
91
|
48.0
|
46.2
|
45.0
|
45.7
|
|
182
|
49.2
|
46.7
|
46.1
|
46.1
|
|
385
|
51.8
|
48.4
|
48.1
|
48.1
|
|
Table 2.25 Modulus of elasticity of concrete after 28 days
of curing [6]
Aggregate
|
Modulus of elasticity (GPa)
|
|||
0% SF
|
10% SF
|
15% SF
|
||
Calcareous limestone
|
21.6
|
26
|
29.3
|
|
Dolomitic limestone
|
24.5
|
25.9
|
32.8
|
|
Quartzitic limestone
|
28.8
|
36.2
|
38
|
|
Steel slag aggregates
|
29.6
|
32.9
|
40.4
|
|
2.9.5 Toughness
Köksal et al.studied the effect of
silica fume (0, 5, 10, and 15%) on the steel fibre reinforced concrete. Steel
fibres with hooked ends were used. Aspect ratios (l/d) of fibres were 65 and 80
and volume fractions (Vf) of steel fibres were 0.5 and 1%. the relations
between toughness of concrete, evaluated up to a 10 mm deflection, and silica
fume content for each aspect ratio. It was concluded that steel fibres in
matrixes with a high strength can exhibit a broken fracture down behaviour
without being pulled-out from matrix due to since strong bond between fibres
and matrix. However, for low silica fume content or low matrix strength, the
common failure type at the fracture plane appeared as the pulling-out of fibres
from matrix, demonstrating the adverse effect of relatively resulting in a
weaker bond.
2.9.6 Absorption
Demirbog˘ a and Gül studied high
strength concretes using blast furnace slag aggregates(BFSA). Silica fume and a
superplasticizer were used to improve BFSA concretes. They concluded that water
absorption values were somewhat less than those of control specimens. Silica
fume and BFSA were considered responsible for this behaviour.
Gonen and Yazicioglu studied the
capillary absorption performance of concrete by adding mineral admixtures,
silica fume and fly ash in the concrete mixes, the replacement of fly ash and
silica fume were kept at the level of 15 and 10% as the weight of cement,
respectively. It can be seen that the capillary absorption of concrete sample
with FA was increased by as much as 47%; however, this increasing trend was
reversed in specimens with fly ash and silica fume. Since silica fume is very
fine, pores in the bulk paste or in the interfaces between aggregate and cement
paste is filled by these mineral admixtures, hence, the Capillary pores are
reduced.
2.9.7 Porosity
Gleize et al. investigated the
effect of silica fume on the porosity of mortar.10% of Portland cement was
replaced with silica fume in a 1:1:16(cement/lime/ sand mix proportion by
volume) masonry mortar. The porosity results are given in Table 2.27. They found that the silica fume lowered the
porosity only at 28 days and the pore structure of mortar with silica fume was
found to be finer than that of non-silica fume mortar. But this refinement in
pore size was more pronounced at 28 days than 2 days due to silica fume
pozzolanic reaction.
Igarashi et al. evaluated the
capillary porosity and pore size distribution in high-strength concrete
containing 10% silica fume at early ages. They concluded that
silica-fume-containing concretes were found to have fewer coarse pores than the
ordinary concretes, even at early ages of 12 and 24 h. The threshold diameter
at which porosity starts to steeply increase with decreasing pore diameter was
smaller in silica-fume-containing concretes than in ordinary concretes at 12 h.
Table 2.27 Total porosity of mortars
Silica fume content (%)
|
Age (days)
|
Total porosity (%)
|
|
0
|
7
|
30.57
|
|
10
|
7
|
32.31
|
|
0
|
28
|
28.53
|
|
10
|
28
|
27.92
|
|
This smaller threshold diameter in silica-fume-containing
concretes indicated higher packing density of binder grains in these concretes.
2.9.8 Thermal Properties
Demirbog˘ a studied the effect of
silica fume on thermal conductivity (TC) of concrete. Density decreased with
the replacement of silica fume. It can be seen that the highest value of TC of
concrete was obtained for specimens produced with 100% PC. Further, the graph
declines largely with increasing silica fume replacement for PC. For 7.5 and
15% silica fume replacement, keeping other conditions constant, the reductions
were 5 and 14%, respectively, compared to the corresponding control specimens.
Demirbog˘ a reported that silica
fume decreased thermal conductivity of mortar up to 40 and 33% at 30%
replacement of PC, respectively. Chen and Chung and Postaciog˘ lu and Maddeler
had reasoned that the reduction in thermal conductivity was primarily due to
the low density of LWAC (Lightweight Aggregate concrete) with silica fume and
fly ash content, and may be partly due to the amorphous silica content of
silica fume and fly ash
2.9.9 Creep
Khatri et al. studied the behaviour
of concretes containing silica fume having a constant water/solids ratio of
0.35 and a total Cementations materials content of 430 kg/m3. They
observed that silica fume reduced the strain due to creep compared with
Portland cement concrete. Adding silica fume to concrete containing 65% slag
did not affect the creep. Ternary mixes containing 15 or 25% fly ash and 10%
silica fume experienced greater creep than control concrete.
Mazloom et al. studied the creep of
high performance concrete having silica fume. The control mix was made with
OPC, while the other mixes were prepared by replacing part of the cement with
silica fume at four different (0, 6, 10 and 15%) replacement levels by mass.
The w/c ratio was 0.35. It was found that silica fume had a significant
influence on the long-term creep. As the proportion of silica fume increased to
15%, the creep of concrete decreased by 20–30%
Tao and Weizu carried out an
experimental study on the early-age tensile creep behaviour of high-strength
concrete (HSC) comprising of silica fume concrete under uniaxial restraining
stresses. The experiments were performed with three 0.35 w/b mixtures,
including plain concrete OPC, double-blended concrete silica fume (6%
replacement of OPC by silica fume). The compressive creep strain for silica
fume and OPC concretes during the temperature rising period are known. It was
found that about 70% of free expansion deformation was compensated by
compressive creep within the first day. After this period, the com-pressive
creep was replaced by tensile creep due to high tensile stress development in
specimens.
2.9.10 Shrinkage
Taylor identified four effects
contributing to drying shrinkage; capillary stress, surface free energy,
disjoining pressure, and movement of interlayer water. Capillary stress
describes the phenomenon of transfer of the tension from the Table 2.30
Age
of loading (days)
|
Concrete
|
||||||
OPC
|
SF 6
|
SF 10
|
SF 15
|
||||
7
|
595
|
510
|
459
|
417
|
|||
28
|
413
|
407
|
381
|
328
|
|||
Values of creep of 809270 mm high
specimens on completion f the tests meniscus of capillary pore water to the
walls of the pore as water evaporates. The pore shrinks and may even collapse,
in which case it will not expand on rehydration. The surface tension of solid
particles is reduced by the adsorption of molecules. When they are removed, the
particles tend to contract. Disjoining pressure is analogous to the phenomenon
that occurs in the swelling of clays as water is drawn between adjacent
particles forcing them apart. As the water is removed, the particles come back
together.
2.10Effect
of Silica Fume on the Durability Properties of Concrete
2.10.1 Permeability
Perratonetal. Studied the effect of
silica fume on the chloride permeability of concretes. Concretes were made with
water– cementitious ratios of 0.4 and 0.5. Silica fume dosage varied from 5 to
20% by weight of cement. Concretes were moist cured for 7 days before drying in
air at normal and low temperatures for 6 month. They observed significant
reduction in the chloride-ion diffusion in silica fume concretes which further
decreased with increasing addition of silica fume as shown in Fig. 2.16. Main reason that could be attributed to reduced
permeability is that addition of silica fume cause considerable pore refinement
i.e. transformation of bigger pores into smaller one due to their pozzolanic
reaction concurrent with cement hydration. By this process the permeability of
hydrated cement paste as well as porosity of the transition zone between cement
paste and aggregate are reduced.
2.10.2 Freezing and Thawing
Sørensen studied the effect of
silica fume on salt-scaling of concrete. He found that drying-rewetting history
of concrete prior to freezing and thawing has a significant effect on
conventional concrete, whereas silica fume concrete is relatively unaffected.
Air entrainment has a beneficial effect on both types of concrete, but
frost-resistant silica fume concretes can be made without entrained air.
Feldman investigated the effect of
silica fume and sand/cement ratio on pore structure and frost resistance of
Portland cement mortars. Silica fume-Port - land cement blend mortars
fabricated with 0, 10 and 30% silica fume at a water/ binder ratio of 0.60 and
a sand/cement ratio of 2.25 were monitored by mercury porosimetry while being
cured for 1–180 days Mortars were also made with and without 10% silica fume at
a water/cement ratio of 0.60 and sand/cement ratios of 0, 1.0, 1.5, 1.8, 2.0,
2.25 and 3.0. Mercury intrusion measurements were carried out after 14 days of
curing. In the presence of silica fume pore volume in the 0.5 to 20 9 103 nm
pore diameter range increased with sand/cement ratio. Mortar prisms were subjected
to freezing and thawing cycles according to ASTM standard test method C 666,
Procedure B. Results indicated that if the sand/cement ratio was 2.25 or over,
expansion was less than 0.02% after 500 cycles. At lower sand/cement ratios 10%
silica fume gives little protection
2.10.3 Corrosion
Berke used electrochemical tests on
concrete samples monitored for 2 years, and found that using silica fume (up to
15% addition to cement) improved the long-term corrosion resistance.
Rasheeduzzafar and Al-Gahtani reported that blending of plain cements with 10
or 20% silica fume significantly improved the corrosion resistance. They found
hardly any tangible advantage in corrosion-ini- tiation time by increasing the
silica-fume content from 10 to 20%.
Khayat and Aitcin observed that iron
oxide layer on conventional steel reinforcing bars becomes unstable when the pH
of surrounding concrete dropped to approximately 10–11 or when this layer comes
in contact with chloride ions. When silica fume was used as cement replacement,
the pH of concrete decreased because cement content is less. Also decrease in
Ca(OH)2 content due to pozzo- lanic
reaction of silica fume and reduction in alkali-pore water concentration
further reduces the pH. But these factors have small effects in destabilizing
the passive iron oxide layer since pH of concrete does not fall below 12 even
when 30% silica fume was used. Diffusion coefficient of chloride and chlorides
content in concrete are reduced significantly in presence of silica fume. Also
the use of silica fume substantially increased the electrical resistivity of
concrete hence slowing the rate of corrosion.
2.10.4 Sulfate Resistance
According to ACI Committee 234, the
effect of silica fume on sulfate resis- tance is due more to the reduction in
permeability than to dilution of the C3A content because of the relatively low doses
of silica fume used in practice.
Sellevold and Nilsen reported field studies of concretes
with and with out 15% silica fume. After 20 years’ exposure to ground water
containing 4 g/L sulfate and 2.5–7.0 pH, the performance of the silica fume
concrete was found equal to that of the concretes made with sulfate-resisting
Portland cement, even though the water/cementitious materials ratio was higher
for silica fume concrete (0.62) than for control (0.50).
Cohen and Bentur studied the effect
of 15% silica fume replacement of Types I and V Portland cement on the
resistance to sulfate attack in magnesium and sodium sulfate solutions. The
water–cementitious materials ratio was 0.3. In the sodium sulfate solutions,
the silica fume concrete specimens were resistant to sulfate attack. In the
magnesium sulfate solutions, all the specimens expanded, with the Type I cement
specimens (with or without silica fume) expanding more than Type V cement
specimens (with or without silica fume). Since specimens were then (6mm), the
authors attributed the effect of silica fume on sulphate resistance more to chemical effects than to reduced
permeability.
2.10.5 Carbonation
Skjolsvold investigated carbonation
depths of field concrete with or without silica fume. The results were
normalized to correct the differences in compressive strength and length
exposure to the atmosphere. The mean carbonation depth was greater for silica
fume concretes under these conditions, but the variation was quite high.
Laboratory study showed that for a given compressive strength, silica fume
concrete had greater carbonation rates than concretes with out silica fume.
Schubert believed that the consumption of Ca(OH)2 in the pozzolanic reaction
acts to increase the rate of carbonation, while the blocking of capillary pores
acts to decrease it.
4. Experimental Programme
Experimental
investigations have been carried out on the M60, M70 and M110 HPC specimens to
ascertain the workability, strength and durability related properties.
4.1
Materials used
•
Ordinary Portland cement, 53 Grade conforming to IS: 12269-1987.
•
Silica fume as mineral admixture in dry densified form obtained
from ELKEM INDIA (P) LTD., MUMBAI conforming to ASTM C-1240.
•
Superplasticizer (chemical admixture) based on Sulphonated
naphthalene Formaldehyde condensate - CONPLAST SP430 conforming to IS: 9103-1999
and ASTM C - 494
•
Locally available quarried and crushed blue granite stones
conforming to graded aggregate of nominal size 12.5mm as per IS:383-1970 with specific gravity
2.82 and fineness modulus 6.73 as Coarse aggregates (CA).
•
Locally available Karur river sand conforming to Grading zone II
of IS: 383-1970 with
specific gravity 2.60 and fineness modulus 2.96 as fine aggregates (FA).
•
Water : Drinking water supplied to Coimbatore city from Siruvani dam
for concreting and curing.
4.2 Mix proportions
Mix proportions are
arrived for M60, M70 and M110 grades of concrete based on Absolute volume
method of mix design by replacing 0, 2.5, 5, 7.5, 10, 12.5 and 15 percent of the mass
of cement by SF and the material requirements per
5. Mixing And Placing
Consideration
5.1 Handling the micro
silica
Because of its extreme
fineness, micro silica presents handling problems. A cement tanker that could
ordinarily haul 35 metric tons of cement accommodates only 7 to 9 tons of dry
micro silica and requires 20 to 50 percent more time for discharging. Some producers
mix micro silica with water on a pound-for-pound basis ton form a slurry that
is transportable in tank trailers designed to handle liquids. The water of the
slurry replaces part of that ordinarily added to the mix. One supplier prepares
a slurry which, used at the rate of 1 gallon per 100 pounds of cement, will
provide aboutn5 percent micro silica by weight of cement. In 1984, that
supplier was quoting a price of $1.70 per gallon at a plant in West Virginia.
In Canada, patented methods have been used to densify the micro silica for
shipment to ready mix producers. Some concrete producers also use the loose
micro silica just as it is collected.
5.1.2 Water
requirements of the mix
When no water reducing
agent is used, the addition of micro silica to a concrete mix calls for more
water to maintain a given slump. Water content can be held the same by using a
water reducer or super plasticizer along with the micro silica. Water reducing
agents appear to have a greater effect on micro silica concrete than on normal
concrete. Thus water demand for given micro silica concrete can be controlled
to be either greater or smaller than for the reference concrete.
5.1.3 Placing and
finishing, curing
The gel that forms
during the first minutes of mixing micro silica concrete takes up water and
stiffens the mixture, necessitating adjustment of the timing of charging and
placing. Scandinavian researchers have concluded that micro silica concretes
often require 1 to 2 inches more slump than conventional concrete for equal
workability. When cement content and micro silica dosage are relatively high,
the mixture is so cohesive that there is virtually no segregation of aggregates
and little bleeding. This may cause problems for floors or slabs cast in hot,
windy weather because there is no water film at the surface to compensate for
evaporation. Plastic shrinkage cracking can readily develop unless precautions
are taken. It is important to finish the concrete promptly and apply a curing
compound or cover immediately. With lean concrete mixes or mixes containing fly
ash replacement of cement, different effects have been reported. For example,
Reference 4 re ports that mixes with less than 380 pounds of cement per cubic
yard plus 10 percent micro silica are both more cohesive and more plastic so no
extra water is needed to maintain slump.
5.1.4. Concrete color
effects
Freshly mixed concrete
containing micro silica can be almost black, dark gray, or practically
unchanged, depending on the dosage of micro silica and its carbon content. The
more carbon and iron in the admixture, the darker the resulting concrete.
Hardened concretes are not much darker than normal concretes when dry.
Sometimes there is a faint bluish tinge, but when the micro silica concrete is
wet, it looks darker than normal Silicosis danger doubted Micro silica is
essentially non crystalline. Currently available data indicate it has no
tendency to cause silicosis, the lung disease associated with inhalation of
crystalline SiO2. However, because of possible cumulative long-term effects,
Norwegian standards restrict dust in the air of the workplace to the same level
as that of other dusts such as natural diatomaceous earth, mica, and soapstone of concrete.
6. DETAILS OF
EXPERIMENTAL INVESTIGATIONS
6.1
WORKABILITY AND
STRENGTH RELATED TESTS
Workability tests such
as slump test, compaction factor test and Vee-Bee consistometer test were
carried out for fresh concrete as per IS specifications, keeping the dosage of
superplasticizer as constant at 3 % by weight of binder. For hardened
concrete, cube compression strength test on 150mm size cubes at the age of 1 day,
3 days, 7 days, 14 days,28 days and 56 days of curing were carried out using
3000 KN capacity AIMIL compression testing machine as per IS:516-1959. Also,
compression strength and split tensile strength tests on 150mm x 300mm
cylinders and flexural strength tests on 100mm x 100mm x 500mm beams were
carried out on 28 days cured specimens as per IS specifications. The stress-strain graph for HPC is
obtained using compressometer fitted to cylinders during cylinder compressive
strength test for finding Modulus of Elasticity for HPC mixes.
6.2. Durability Related Tests
The durability related
tests such as Saturated Water Absorption (SWA) test, Porosity test, Sorptivity
test, Permeability test, Acid resistance test, Sea water resistance test,
Abrasion resistance test and Impact resistance test were carried out on
hardened concrete specimens at the age of 28 days of curing.
6.2.1 Test for
Saturated Water Absorption & Porosity
The water absorption
was determined on 1OOmm cubes as per ASTM C-642 by drying the specimens in an
oven at a temperature of 105° C to constant mass and then immersing in water
after cooling to room temperature. The specimens were taken out of water at
regular intervals of time and weighed. The process was continued till the
weights became constant (fully saturated) . The difference between the water saturated
mass and oven dry mass expressed as a percentage of oven dry mass gives the
SWA.The SWA of concrete is a measure of the pore volume or porosity in hardened
concrete, which is occupied by water in saturated condition. It denotes the
quantity of water, which can be removed on drying a saturated specimen. The
porosity obtained from absorption tests is designated as effective porosity. It
is determined by using the following formula.
The volume of voids is
obtained from the volume of the wsater absorbed by an oven dry specimen or the
volume of water lost on oven drying a water saturated specimen at 105° C to
constant mass. The bulk volume of the specimen is given by the difference in
mass of the specimen in air and its mass under submerged condition in water.
Sorptivity measures
the rate of penetration of water into the pores in concrete by capillary
suction. When the cumulative volume of water that has penetrated per unit
surface area of exposure 'q' is plotted against the square root of time of
exposure ''1t' , the resulting graph
could be approximated by a straight line passing
through the origin. The slope of this straight line is considered as a measure of
rate of movement of water through the capillary pores and is called sorptivity.
In this present study, the test for sorptivity was conducted on 1OOmm cubes by
immersing them in water and measuring the gain in mass at regular intervals of
30min duration over a period of 2 hrs.
6.3.1. Permeability
Test
Permeability is
related to the durability of concrete, specially its resistance against
progressive deterioration under exposure to severe climate. The tests for
permeability were carried out on 1OOmm x 100mm cylinders as per
IS:3085-1965,using a AIMIL Concrete permeability apparatus
.Cylinders are kept in
permeability mould and tightly packed and sealed. Water pressure was applied at a pressure of
10
kg/cm2over
cylinders using air compressor The water percolated through the cylinder
specimens was collected in a glass bottle for a period of 100 hours.
6.3.2. Acid Resistance
Test and Sea Water Attack Test
Cubes of 150mm size
were weighed and immersed in water diluted with 1 % of sulphuric acid by weight
of water and in water diluted with 3% of sodium chloride by weight of water for acid
resistance test and sea water resistance test respectively for 45 days
continuously and then the cubes were taken out and weighed . The percentage loss
in weight and the percentage reduction in compressive strengths were
calculated.
6.3.3. Abrasion
Resistance Test
Deterioration of concrete
surface may occur due to abrasion by sliding, scraping or action of abrasive materials
carried out by water. The tests for abrasion resistance were carried out on
specimens of 70mm x 70mm x 35mm size, using Tile Abrasion testing machine. The specimen was kept
in the abrasion testing machine after measuring the thickness accurately. The testing machine
was allowed to rotate for 300 revolutions by keeping the speed of the machine
as 30 rev/min. specimens were taken
out and weighed and final thickness were found out.
6.3.4. Impact
Resistance Test
The tests for impact
resistance were carried out on specimens of size 152mm diameter x 62.5mm
thickness, using Drop weight testing machine. The specimens were kept on the
base plate and centred . A drop hammer weighing 45 N was used to apply the impact load. The number of blows
required by dropping a hammer through a height of 457mm to cause the ultimate
failure, was recorded.
7.Test Results and
Discussion
The results of
workability, strength and durability related tests are listed in Tables 2, 4
and 6. The results of strength and durability tests have demonstrated the
superior strength and durability characteristics of the concrete mixes
containing SF.
7.1. Workability &
Strength Related Properties
It was observed that
the workability of concrete decreased as the percentage of SF content was
increased. The optimum percentage of cement replacement by SF is 10 % for
strength related tests for M60, M70 and M110 grades
of concrete. This is due to the fact that the increase of strength
characteristics is due to the pozzolanic reaction and filler effects of SF. The flexural strength
and Modulus of Elasticity values obtained experimentally are higher and lower
than the values calculated by the expression 0.7 ,/fck and 5000 ,/fck respectively as per IS:456-2000.
7.2. Saturated Water
Absorption, Porosity and Sorptivity
It has been observed
that the optimum percentage of cement replacement by SF for M60, M70 & M110
grades of concrete is 10 % for achieving lowest SWA , porosity and
sorptivity. It is also to be
noted that the SWA, porosity and sorptivity of HPC mixes containing SF are
lower compared to that of HPC mix without SF. This is due to the improvement in m icrostructure due to
pozzolanic reaction and micro filler effects of SF, resulting in fine and
discontinuous pore structure. The SWA and Sorptivity values of the concrete mixes were around
1.95% and 0.0505 mm/min°·5 respectively.
The Concrete Society,
UK, classifies the concretes with SWA of around 3 % as good concretes .Taywood Engineering
Ltd., has suggested that
good concretes have sorptivity of less than
0.1 mm I min°·5.
These comparisons prove that the HPC mixes developed in the present study could
be considered to have shown superior SWA and sorptivity performance.
7.3. Permeability
No percolation of
water has been found for M60, M70 and M110 grades of concrete trial mixes. Immediately after
this, the cylinders were
removed from. The permeability moulds and was split to measure the water
penetration depth. Water penetration was found to be negligible in all samples
of HPC trial mixes containing SF, whereas for the mixes without SF the depth of
water penetration was more. This confirmed that use of SF and low w/b ratio had resulted in
almost impermeable concrete.
7.4. Acid Resistance
and Sea Water Resistance
From the results of
percentage loss in weight and percentage reduction in compressive strengths, it
has been observed that M60, M70 and M110 grades of HPC trial mixes containing
SF were less attacked by acid and sea water compared to the HPC mixes without
SF. Hence, HPC mixes containing SF are more durable against acid and sea water
attack.
7.5. Abrasion
Resistance and Impact Resistance
The results of average
loss in thickness obtained from abrasion resistance test and the average number
of drops at failure from the impact resistance test for M60, M70 and M110
grades of HPC trial mixes showed that the concrete mixes containing SF have
higher abrasion and impact resistance. This is due to the formation of stable
C-S-H gels.
Mix
|
SF
|
w/b
|
Cement
|
SF
|
FA
|
CA
|
Water (lit)
|
(%)
|
ratio
|
(kg)
|
(kg)
|
(kg)
|
|||
C1
|
0
|
0.320
|
453.10
|
0
|
811.10
|
11.15
|
141.84
|
C2
|
2.5
|
0.320
|
441.80
|
11.30
|
807.10
|
11.15
|
141.84
|
C3
|
5
|
0.320
|
430.50
|
22.60
|
803.00
|
11.15
|
141.84
|
C4
|
7.5
|
0.320
|
419.20
|
33.90
|
798.70
|
11.15
|
141.84
|
C5
|
10
|
0.320
|
407.80
|
45.30
|
794.9
|
11.15
|
141.84
|
C6
|
12.
|
0.320
|
396.50
|
56.60
|
791.00
|
11.15
|
141.84
|
C7
|
15
|
0.320
|
385.20
|
68.00
|
786.90
|
11.15
|
141.84
|
Table 2 Strength and Durability Related Properties of HPC
Properties
|
C21
|
C22
|
C23
|
C24
|
C25
|
C26
|
C27
|
|||||||||
Silica Fume
|
0
|
2.5
|
5
|
7.5
|
10
|
12.
|
5
|
15
|
||||||||
Cube compressive
strenqth (MPa),
|
||||||||||||||||
1 day
|
18.84
|
22.76
|
24.11
|
26.34
|
29.22
|
28.50
|
27.15
|
|||||||||
3 days
|
30.50
|
34.75
|
36.11
|
40.65
|
45.33
|
41.20
|
38.85
|
|||||||||
7 days
|
41.24
|
46.20
|
48.01
|
51.65
|
55.22
|
52.11
|
49.75
|
|||||||||
14 days
|
46.33
|
49.15
|
54.20
|
54.20
|
61.33
|
58.75
|
55.65
|
|||||||||
28 days
|
54.64
|
57.92
|
62.25
|
62.25
|
72.22
|
68.84
|
65.50
|
|||||||||
56 days
|
59.44
|
63.10
|
71.33
|
78.15
|
78.15
|
75.10
|
72.15
|
|||||||||
Cylinder compressive
strength
|
||||||||||||||||
28 days
|
43.25
|
45.80
|
49.65
|
51.35
|
58.75
|
54.44
|
52.66
|
|||||||||
Split tensile strength(MPa).
|
||||||||||||||||
28 days
|
4.65
|
5.15
|
5.40
|
5.75
|
6.10
|
5.95
|
5.80
|
|||||||||
Flexural
Strength( MPa)
|
5.80
|
7.10
|
7.30
|
7.60
|
8.30
|
8.10
|
7.10
|
|||||||||
Elastic
Modulus
|
30.1
|
30.9
|
31.8
|
32.1
|
35.1
|
33.9
|
33.3
|
|||||||||
(GPa)
28 days
|
||||||||||||||||
Slump (mm)
|
82
|
74
|
68
|
61
|
55
|
45
|
34
|
|||||||||
Table 3 Details of HPC
Trial Mixes for M70 Grade
Mix
|
SF%
|
w/b
|
Cement
|
SF
|
FA
|
CA
|
Super
|
Water
|
ratio
|
(kg)
|
(kg)
|
(kg)
|
(kg)
|
plasticizer
|
(lit)
|
||
C11
|
0
|
0.30
|
483.30
|
0
|
785.70
|
1070
|
11.90
|
141.30
|
C12
|
2.5
|
0.30
|
471.30
|
12.10
|
781.40
|
1070
|
11.90
|
141.30
|
C13
|
5
|
0.30
|
459.20
|
24.20
|
777.10
|
1070
|
11.90
|
141.30
|
C14
|
7.5
|
0.30
|
447.10
|
36.30
|
772.70
|
1070
|
11.90
|
141.30
|
C15
|
10
|
0.30
|
435.00
|
48.40
|
768.40
|
1070
|
11.90
|
141.30
|
C16
|
12.5
|
0.30
|
422 .90
|
60.40
|
764.10
|
1070
|
11.90
|
141.30
|
C17
|
15
|
0.30
|
410 .80
|
72.50
|
759.80
|
1070
|
11.90
|
141.30
|
Strength and
Durability Related Properties of HPC Mixes for M70 Grade
Properties
|
C11
|
C12
|
C13
|
C14
|
C15
|
C16
|
C17
|
|
Silica
|
0
|
2.5
|
5
|
7.5
|
10
|
12.5
|
15
|
|
Cube
|
compressive
strenath 1MPa),
|
|||||||
1day
|
21.50
|
25.89
|
27.15
|
29.5
|
34.55
|
33.85
|
31.22
|
|
3 davs
|
34.44
|
39.11
|
40.24
|
45.88
|
51.0
|
46.56
|
44.66
|
|
7 days
|
47.52
|
52.33
|
56.33
|
58.56
|
63.14
|
60.1
|
57.28
|
|
14 days
|
54.74
|
57.0
|
61.1
|
62.72
|
73.07
|
69.89
|
67.76
|
|
28 days
|
62.55
|
66.11
|
69.2
|
71.22
|
82.46
|
78.88
|
75.33
|
|
56 davs
|
68.65
|
73.30
|
75.85
|
82.26
|
90.2
|
87.10
|
82.50
|
|
Cvlinder
compressive strenath (MPa),
|
||||||||
28 days
|
50.42
|
53.65
|
55.36
|
57.55
|
66.85
|
63.15
|
60.40
|
|
Split tensile
strength (MPa),
|
||||||||
28 days
|
4.7
|
5.10
|
5.25
|
5.60
|
5.85
|
5.40
|
5.30
|
|
Flexural strength
28 days
|
7.3
|
7.50
|
8.10
|
8.50
|
9.10
|
8.80
|
8.60
|
|
Elastic modulus
28 days
|
32
|
33.4
|
34.0
|
34.7
|
37.5
|
36.9
|
35.8
|
|
SLUMP (mm)
|
65
|
56
|
54
|
50
|
45
|
30
|
21
|
|
1. Table 5 Details of HPC Trial Mixes for M110
Grade
Mix
|
SF
|
w/b
|
Cement
|
SF
|
FA
|
CA
|
Super
|
Water
|
(%)
|
ratio
|
(kg)
|
(kg)
|
(kg)
|
(kg)
|
plasticizer
|
(lit)
|
|
C21
|
0
|
0.232
|
625.00
|
0
|
666.3
|
1070
|
15.37
|
138.75
|
C22
|
2.5
|
0.232
|
609.40
|
15.60
|
660.7
|
1070
|
15.37
|
138.75
|
C23
|
5
|
0.232
|
593.70
|
31.30
|
655.2
|
1070
|
15.37
|
138.75
|
C24
|
7.5
|
0.232
|
578.10
|
46.90
|
649.6
|
1070
|
15.37
|
138.75
|
C25
|
10
|
0.232
|
562.50
|
62.50
|
644 .0
|
1070
|
15.37
|
138.75
|
C26
|
12.5
|
0.232
|
546.90
|
78.10
|
638.50
|
1070
|
15.37
|
138.75
|
C27
|
15
|
0.232
|
531.0
|
93.80
|
632.9
|
1070
|
15.37
|
138.75
|
Table 6 Strength and
Durability Related Properties of HPC Mixes for M110 Grade
Properties
|
C21
|
C22
|
C23
|
C24
|
C25
|
C26
|
C27
|
|||||||||
Silica Fume
|
0
|
2.5
|
5
|
7.5
|
10
|
12.
|
5
|
15
|
||||||||
Cube compressive strenqth (MPa),
|
||||||||||||||||
1 day
|
31.78
|
38.14
|
41.2
|
43.5
|
50.66
|
49.58
|
45.1
|
|||||||||
3 days
|
49.24
|
56.89
|
57.0
|
64.2
|
72.38
|
66.96
|
65.50
|
|||||||||
7 days
|
68.87
|
75.75
|
79.2
|
88.3
|
93.66
|
88.11
|
85.38
|
|||||||||
14 days
|
78.38
|
85.07
|
88.7
|
93.5
|
106.2
|
102.6
|
101.1
|
|||||||||
28 days
|
91.22
|
97.80
|
103.22
|
108.66
|
121.2
|
115.2
|
112.4
|
|||||||||
56 days
|
99.15
|
105.4
|
109.6
|
116.75
|
126.2
|
120.8
|
118.3
|
|||||||||
Cylinder compressive strenq
|
th (MPa),
|
|||||||||||||||
28 days
|
71.06
|
77.16
|
82.35
|
86.92
|
98.14
|
91.6
|
89.11
|
|||||||||
Split tensile strength (MPa).
|
||||||||||||||||
28 days
|
6.20
|
6.72
|
7.55
|
7.94
|
8.45
|
8.2
|
8.05
|
|||||||||
Flexural
|
8.90
|
9.40
|
9.80
|
10.10
|
10.90
|
10.30
|
9.70
|
|||||||||
Strength
|
||||||||||||||||
Elastic
|
39.7
|
40.6
|
40.9
|
41.4
|
44.4
|
43.1
|
42.9
|
|||||||||
Moduluss
|
||||||||||||||||
Slump (mm)
|
29
|
28
|
27
|
25
|
24
|
17
|
14
|
|||||||||
8.
Conclusions
1.
Cement replacement level of 10 percent with SF in M60, M70 and
M110 grades of HPC mixes is found to be the optimum level to obtain higher
values of compressive strength, split tensile strength, flexural strength
2.
The IS: 456-2000 code underestimates the flexural strength and
overestimates the Modulus of Elasticity for HPC.
3.
The use of SF and low w/b ratio resulted in practically
impermeable concrete.
4.
The compression failure pattern of concrete is due to the
crushing of coarse aggregate and not due to bond failure.
5.
The results of the strength and durability related tests
demonstrated superior strength and durability characteristics of HPC mixes
containing SF. This is due to the improvement microstructure due to pozzolanic
action and filler effects of SR, resulting in fine and discontinuous pore
structure.
6.
Even a partial replacement of cement with SF in concrete mixes
would lead to considerable savings in consumption of cement and gainful
utilization of SF. Therefore, it can be concluded that replacement of cement
with SF up to 10 % would render the concrete more strong and durable. This
observation is in par with the maximum limit of 1O % for mineral admixture in
concrete mixes as recommended by IS: 456-2000.
7.
Silica
fume increases the strength of concrete more 25%. Silica fume is much cheaper
then cement therefore it very important form economical point of view. Silica
fume is a material which may be a reason of Air Pollution this is a byproduct
of some Industries use of microsilica with concrete decrease the air pollution.
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
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