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CONCRETE ADMIXTURES
Paper Presented By
MAHENDERAN (Final B.Tech civil) [email protected]
L.SIREESHA (Final B.Tech civil) [email protected]
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ABSTRACT:
Admixtures are ingredients other than water, aggregates,
hydraulic cement, and fibers that are added to the concrete batch
immediately before or during mixing, in nominal quantities. A
proper use of admixtures offers certain beneficial effects to
concrete, including improved quality, acceleration or retardation
of setting time, enhanced frost and sulphate resistance, control of
strength development, improved workability, and enhanced
finishability. Admixtures vary widely in chemical composition, and
many perform more than one function. Two basic types of admixtures
are available: chemical and mineral. All admixtures to be used in
concrete construction should meet specifications; tests should be
made to evaluate how the admixture will affect the properties of
the concrete to be made with the specified job materials, under the
anticipated ambient conditions, and by the anticipated construction
procedures.
Materials used as admixtures included milk and lard by the
Romans; eggs during the middle ages in Europe; polished glutinous
rice paste, lacquer, tung oil, blackstrap molasses, and extracts
from elm soaked in water and boiled bananas by the Chinese; and in
Mesoamerica and Peru, cactus juice and latex from rubber plants.
The Mayans also used bark extracts and other substances as set
retarders to keep stucco workable for a long period of time.
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CONTENTS
Abstract
Chemical Admixtures
Air Entraining
Water reducing
Set retarding
Accelerators
Super Plasticizers
Mid range water reducing admixtures
Corrosion inhibiting
Shrinkage reducing admixtures
Mineral Admixtures
Fly Ash
Silica Fume
Ground Granulated Blast Furnace Slag (GGBFD)
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STUDY ON CONCRETE ADMIXTURES
CHEMICAL ADMIXTURES:
Chemical admixtures are added to concrete in very small amounts
mainly for the entrainment of air, reduction of water or cement
content, plasticization of fresh concrete mixtures, or control of
setting time.
Air-Entertainers
Water-Reducing
Set-Retarding
Accelerators
Super plasticizers
Mid-range water-reducing admixtures
Corrosion-inhibiting admixtures
Shrinkage reducing admixtures AIR ENTRAINMENT is the intentional
creation of tiny air bubbles in concrete. The bubbles are
introduced into the concrete by the addition to the mix of an
air-entraining agent, a surfactant. The air bubbles are created
during mixing of the plastic concrete, and most of them survive to
be part of the hardened concrete. The primary purpose of air
entrainment is to increase the durability of the hardened concrete,
especially in climates subject to freeze-thaw; the secondary
purpose is to increase workability of the concrete while in a
plastic state. A water: cement ratio (w/c) of approximately 0.25 is
required for all the cement particles to hydrate. Water beyond that
is surplus and is used to make the plastic concrete more workable
or flowable. Most concrete has a w/c of 0.45 to 0.60, which means
there is substantial excess water that will not react with cement.
Eventually
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the excess water evaporates, leaving little pores in its place.
Environmental water can
later fill these voids. During freeze-thaw cycles, the water
occupying those pores expands and creates stresses, which lead to
tiny cracks. These cracks allow more water into the concrete and
the cracks enlarge. Eventually the concrete breaks off. The failure
of RCC is most often due to this cycle, which is accelerated by
moisture reaching the reinforcing steel. Steel expands when it
rusts, and these forces create even more cracks, letting in more
water. These air bubbles that are created improve the resistance of
the concrete structure against Freeze and Thaw cycles.
WATER REDUCERS or PLASTICIZERS Water-reducers generally reduce
the required water content of a concrete mixture for a given slump.
These admixtures disperse the cement particles in concrete and make
more efficient use of cement. This increases strength or allows the
cement content to be reduced while maintaining the same strength.
The basic role of water reducers is to
deflocculate the cement particles agglomerated together and
release the water tied up in these agglomerations, producing more
fluid paste at lower water contents.
Water-reducers are used to increase slump of concrete without
adding water and are useful for pumping concrete and in hot weather
to offset the increased water demand. Some water - reducers may
aggravate the rate of slump loss with time. Water-reducing
admixtures are used to improve the quality of concrete and to
obtain specified strength at lower cement content. They also
improve the properties of concrete containing
marginal- or low-quality aggregates and help in placing concrete
under difficult conditions. Water reducers have been used primarily
in bridge decks, low-slump concrete overlays, and patching
concrete. Water-reducers should meet the requirements for Type A in
ASTM C 494 Specification for Chemical Admixtures for Concrete.
Composition. Water-reducing admixtures can be categorized according
to their active ingredients. There are the following:
Salts and modifications of hydroxylized carboxylic acids (HC
type); Salts and modifications of lignosulfonic acids (lignins);
and Polymeric materials (PS type).
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RETARDERS or SET-RETARDERS They are chemicals that delay the
initial setting of concrete by an hour or more. Retarders are often
used in hot weather to counter the rapid setting caused by high
temperatures. Most retarders also function as water reducers.
Retarders should meet the
requirements for Type B or D in ASTM C 494. Effect on Concrete
Properties and Application. Because of retarding action, the 1-day
strength of the concrete is reduced. However, ultimate strength is
reported to be improved by using set-controlling admixtures. One of
the most important applications of retarding admixtures is
hot-weather concreting, when delays between mixing and placing
operation, may result in early stiffening. Another important
application is in prestressed concrete, where retarders prevent
the
concrete that is in contact with the strand from setting before
vibrating operations are completed. Set retarders also allow use of
high-temperature curing in prestressed concrete production without
affecting the ultimate strength of the concrete.
Accelerating admixtures or Accelerators
They are added to concrete either to increase the rate of early
strength development or to shorten the time of setting, or both.
Chemical compositions of accelerators
include some of inorganic compounds such as soluble chlorides,
carbonates, silicates, fluosilicates, and some organic compounds
such as triethanolamine. Among all these accelerating materials,
calcium chloride is the most common accelerator used in concrete.
However, growing interest in using "chloride-free" accelerators as
replacement for calcium chloride has been observed because they
form the root cause for the steel corrosion in RCC. Calcium nitrite
accelerates the hydration of cement, as shown by the larger amounts
of heat developed in its presence.
Super plasticizers or High Range Water Reducers (HRWR)
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These chemical admixtures can be added to concrete mixtures to
improve workability. Strength of concrete is inversely proportional
to the amount of water added or water-cement (w/c) ratio. In order
to produce stronger concrete, less water is added, which makes the
concrete mixture very unworkable and difficult to mix,
necessitating the use of plasticizers, water reducers, super
plasticizers or dispersants. Super plasticizers are linear polymers
containing sulphonic acid groups attached to the polymer backbone
at regular intervals. Most of the commercial formulations belong to
one of four families:
Sulphonated melamine-formaldehyde condensates (SMF) Sulphonated
naphthalene-formaldehyde condensates (SNF) Modified
lignosulphonates (MLS) Polycarboxylate derivatives
Mid-range water-reducing admixtures Water-reducing admixtures
that provide moderate water reduction without significantly
delaying the setting characteristics of concrete are also
available. Because these admixtures provide more water reduction
than conventional water-reducers but less water-reduction than high
range water reducers, they are referred to as mid-range
water-reducing admixtures. These admixtures can help reduce
stickiness and improve inishability and pumpability of concrete
including concrete containing silica fume, or manufactured or
coarse sand. Mid-range water-reducing admixtures are typically used
in a slump range of 125 to 200 mm and may entrain additional air.
Therefore, evaluations should be performed to establish
air-entraining admixture dosage for desired air content.
Corrosion inhibiting admixtures Chlorides are one of the causes
of corrosion of steel in concrete. They can be introduced into
concrete from deicing salts that are used in the winter months to
melt snow or ice, from seawater, or from the concrete mixture
ingredients. There are several ways of combating chloride-induced
corrosion, one of which is the use of corrosion-inhibiting
admixtures. These admixtures are added to concrete during batching
and they protect
embedded reinforcement by delaying the onset of corrosion and
also reducing the rate of corrosion after initiation. There are
several commercially available inhibitors include an inorganic
formulation that contains calcium nitrite as the active ingredient
and
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organic formulations consisting of amines and esters. Calcium
nitrite and calcium thiosulphate usually increase the strength
development of concrete at early ages.
Shrinkage-reducing admixtures The loss of moisture from the
concrete as it dries results in a volume contraction termed drying
shrinkage. Drying shrinkage tends to be undesirable when it leads
to cracking due to either internal or external restraint, curling
of floor slabs, and excessive loss of prestress in prestressed
concrete applications. Drying shrinkage can be reduced
significantly by using shrinkage-reducing admixtures. These are
organic-based
formulations that reduce the surface tension of water in the
capillary pores of concrete, thereby reducing the tension forces
within the concrete matrix that lead to drying shrinkage.
MINERAL ADMIXTURES Mineral admixtures (fly ash, silica fume
[SF], and slags) are usually added to concrete in larger amounts to
enhance the workability of fresh concrete; to improve resistance of
concrete to thermal cracking, alkali-aggregate expansion, and
sulphate attack; and to enable a reduction in cement content.
Fly Ash
Silica Fume
Ground Granulated Blast Furnace Slag(GGBFS) FLY ASH
Fly ash is comprised of the non-combustible mineral portion of
coal consumed in a coal-fueled power plant. Chemically, fly ash is
a pozzolan. When mixed with lime (calcium hydroxide), pozzolans
combine to form cementitious compounds. Concrete containing fly ash
becomes stronger, more durable, and more resistant to chemical
attack. Fly ash particles are glassy, spherical shaped ball
bearings typically finer than cement particles that are collected
from the combustion air-stream exiting the power plant. There are
two basic types of fly ash: Class F and Class C. Both Class F
and
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Class C fly ashes undergo a pozzolanic reaction with the lime to
create the same binder (calcium silicate hydrate, C-S-H gel) as
cement. The main benefit of fly ash in concrete is that it not only
reduces the amount of non-durable calcium hydroxide (lime), but in
the process converts it into calcium silicate hydrate (CSH), which
is the strongest and most durable portion of the paste in concrete.
Fly ash also makes substantial contributions to workability,
chemical resistance and the environment.
Fly Ash Contributes to Concrete Durability and Strength There is
a huge difference between durability and strength. Durability is
the ability to maintain integrity and strength over time. Strength
is only a measure of the ability to sustain loads at a given point
in time. Cement normally gains the great majority of its strength
within 28 days. Concrete made with fly ash will be slightly lower
in strength than straight cement concrete up to 28 days, equal
strength at 28 days, and substantially higher strength within a
years time. Conversely, in straight cement
concrete, this lime would remain intact and over time it would
be susceptible to the effects of weathering and loss of strength
and durability.
Fly Ash Contributes to Concrete Workability First, fly ash
produces more cementitious paste. It has a lower unit weight,
which
means that on a pound for pound basis, fly ash contributes
roughly 30% more volume of cementitious material per pound versus
cement. The greater the percentage of fly ash ball bearings in the
paste, the better lubricated the aggregates are and the better
concrete flows. Second, fly ash reduces the amount of water needed
to produce a given slump. Water demand of a concrete mix with fly
ash is reduced by 2% to 10%, depending on a number of factors
including the amount used and class of fly ash. Third, fly ash
reduces the amount of sand needed in the mix to produce
workability. Because fly ash creates more paste, and by its shape
and dispersive action makes the paste more slippery, the amount of
sand proportioned into the mix can be reduced. Since sand has a
much greater surface area than larger aggregates and therefore
requires more paste, reducing the sand means the paste available
can more efficiently coat the surface area of the aggregates that
remain.
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Fly Ash Protects Concrete Fly ash concrete is less permeable
because fly ash reduces the amount of water needed to produce a
given slump, and through pozzolanic activity, creates more durable
CSH as it fills capillaries and bleed water channels occupied by
water-soluble lime (calcium hydroxide). Fly ash improves corrosion
protection. By decreasing concrete permeability, fly ash can reduce
the rate of ingress of water, corrosive chemicals and oxygen thus
protecting steel reinforcement from corrosion and its subsequent
expansive result. Fly ash also increases sulphate resistance and
reduces alkali-silica reactivity. In reducing alkali-silica
reactivity, fly ash has the ability to react with the alkali
hydroxides in Portland Cement paste, making them unavailable for
reaction with reactive silica in certain aggregates.
Fly Ash Reduces Heat of Hydration in Concrete The hydration of
cement is an exothermic reaction. Heat is generated very quickly,
causing the concrete temperature to rise and accelerating the
setting time and strength gain of the concrete. Warm weather will
naturally raise the temperature of concrete
aggregates, which make up the majority of the mass in concrete.
This natural heating of the aggregates, coupled with solar heating
at the construction site, can cause thin concrete slabs to suffer
the damaging effects of thermal cracking, leading to reduced
concrete strength and durability. In such cases, replacing large
percentages of cement with fly ash can reduce the damaging effects
of thermal cracking and provide the time needed for desirable
finish.
Conclusions of using Fly Ash in Cement Ease of Pumping
Reduced Bleeding Reduced Segregation Improved Finishing
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SILICA FUME
Silica fume, also known as micro silica, is a byproduct of the
reduction of high-purity quartz with coal in electric furnaces in
the production of silicon and ferrosilicon alloys such as
ferrochromium, ferromanganese, Ferro magnesium, and calcium
silicon. With an of 15 percent, the potential exists for very
strong, brittle concrete. It increases the water demand in a
concrete mix; however, dosage rates of less than 5 percent will not
typically require a water reducer. High replacement rates will
require the use of a high range water reducer. Effects on Air
Entrainment and Air-void System of Fresh Concrete. The dosage of
air-entraining agent needed to maintain the required air content
when using Silica Fume is slightly higher because of high surface
area and the presence of carbon. This dosage is increased with
increasing amounts of Silica Fume content in concrete. Effects on
Water Requirements of Fresh Concrete. Silica Fume added to concrete
by itself increases water demands, often requiring one additional
pound of water for every pound of added Silica Fume. This problem
can be easily compensated for by using HRWR.
Effects on Consistency and Bleeding of Fresh Concrete. Concrete
incorporating more than 10% Silica Fume becomes sticky; in order to
enhance workability, the initial slump should be increased. It has
been found that Silica Fume reduces bleeding because of its effect
on rheologic properties.
Effects on Strength of Hardened Concrete. Silica Fume has been
successfully used to produce very high-strength, low-permeability,
and chemically resistant concrete.
Addition of Silica Fume by itself, with other factors being
constant, increases the concrete strength.
Effects on Freeze-thaw Durability of Hardened Concrete. The use
of Silica Fume has no significant influence on the production and
stability of the air-void system. Freeze-
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thaw testing (ASTM C 666) on Silica Fume concrete showed
acceptable results; the average durability factor was greater than
99%. Effects on Permeability of Hardened Concrete. Addition of
Silica Fume to concrete reduces its permeability .Also, addition of
Silica Fume (8% Silica Fume) significantly reduces the chloride
permeability. This reduction is primarily the result of the
increased density of the matrix due to the presence of Silica
Fume.
GROUND GRANULATED BLAST FURNACE SLAG (GGBFS)
It is a recyclable material created when the molten slag from
melted iron ore is quenched rapidly and then ground into a powder.
This material has remarkable cementitious properties. Several
problems with GGBFS include slow strength gain and decreased
surface quality. Countering these problems, GGBFS concrete has
higher late strength and lower permeability. Effect on Strength of
Hardened Concrete. The strength development of concrete
incorporating slags is slow at 1-5 days compared with that of the
control concrete. Between 7 and 28 days, the strength approaches
that of the control concrete; beyond this period, the strength of
the slag concrete exceeds the strength of control concrete.
Flexural strength is usually improved by the use of slag cement,
which makes it beneficial to concrete paving application where
flexural strengths are important. It is believed that the increased
flexural strength is the result of the stronger bonds in the
cement-slag-aggregate system because of the shape and surface
texture of the slag particles. It is not suitable for Cold weather
applications. Effects on Permeability of Hardened Concrete.
Incorporation of granulated slags in cement paste helps in the
transformation of large pores in the paste into smaller pores,
resulting in decreased permeability of the matrix and of the
concrete. A significant reduction in permeability is achieved as
the replacement level of the slag increases from 40 to 65% of total
cementitious material by mass.
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Effects on Freeze-Thaw Durability of Hardened Concrete.. The
resistance of air-entrained concrete incorporating Ground
granulated blast furnace slag is comparable to that of conventional
concrete. The results of freeze-thaw tests on concrete
incorporating
25-65% slag indicate that regardless of the water-to- (cement +
slag) ratio, air-entrained slag concrete specimens performed
excellently in freeze-thaw tests, with relative durability factors
greater than 91%.
CONCLUSIONS Air-entraining and other chemical admixtures have
become a very useful and integral component of concrete. Admixtures
are not a panacea for every ill the concrete producer, architect,
engineer, owner, or contractor faces when dealing with the many
variables of concrete, but they do offer significant improvements
in both the plastic and hardened state to all concrete. Continued
research and development will provide additional reliability,
economy, and performance for the next generation of quality
concrete. Overviews of the conclusions by usin admixtures in
concrete are:
Increase workability without increasing water content or
decrease the water content at the same workability;
Retard or accelerate time of initial setting;
Reduce or prevent shrinkage or create slight expansion;
Modify the rate or capacity for bleeding;
Reduce segregation;
Improve pumpability;
Reduce rate of slump loss;
Retard or reduce heat evolution during early hardening;
Accelerate the rate of strength development at early ages;
Increase strength (compressive, tensile, or flexural); Increase
durability or resistance to severe conditions of exposure,
including
application of deicing salts and other chemicals;
Decrease permeability of concrete;
Control expansion caused by the reaction of alkalis with
potentially reactive aggregate constituents;
Increase bond of concrete to steel reinforcement;
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Increase bond between existing and new concrete;
Improve impact and abrasion resistance;
Inhibit corrosion of embedded metal
Produce colored concrete or mortar.
The following table shows the various MIXES with the use of
admixtures and their Compressive Strength after various days.
Ingredients Mix
(kg / m3) A B C D E Portland Cement 534 315 513 163 228
Silica Fumes 40 36 43 54 46
Fly ash 59 - - - -
GGBFS - 137 - 325 182
Fine Aggregate 623 745 685 730 800
Coarse Aggregate 1069 1130 1080 1100 1110
Total water 139 150 139 136 138
Water/cementitious 0.22 0.31 0.25 0.25 0.30
Slump (mm) 255 - - 200 220 Strenth (Mpa) 1 day - - - 13 19
7 days - 67 91 72 62
28 days 115 83 119 114 105
90 days - 93 145 126 121
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