Concrete admixtures_Dr D R Panchal

Presented By:
Dr. D. R. Panchal
Dr. D R Panchal,AMD, FTE, MSU,Vadodara

Copy right by Dr. D R Pa
Why Use Concrete Admixtures
 Protect Against Freeze Thaw Cycles –Improve Durability
 Water Reduction in the Mix
 Mid-Range water reducers
 High-Range water reducers – superplasticizers
 High Strength Concrete
 Corrosion Protection
 Set Acceleration
 Strength Enhancement
 Set Retardation
 Crack Control (shrinkage reduction)
 Flowability Flowability
 Finish Enhancement
 Fly Ash- Making Concrete Stronger, Durable, and Easier to Work
 Silica Fume: Early Strength and Reduced Permeability
 Produce a fluid backfill, which flows easily and is self leveling

right by Dr. D R Panchal

Chemical Admixtures
Chemical admixtures reduce the cost of construction, modify
properties of hardened concrete, ensure quality of concrete during
mixing/transporting/placing/curing, and overcome certain
emergencies during concrete operations.
 air entrainers
 water reducers
 set retarders
emergencies during concrete operations.
Chemical admixtures are used to improve the quality of concrete
during mixing, transporting, placement and curing. They fall into the
following categories:
 set retarders
 set accelerators
 superplasticizers
 specialty admixtures: which include corrosion inhibitors,
shrinkage control, alkali-silica reactivity inhibitors, and coloring.

Mineral admixtures
Mineral admixtures make mixtures more economical,
reduce permeability, increase strength, and influence other
concrete properties.concrete properties.
Mineral admixtures affect the nature of the hardened
concrete through hydraulic or pozzolanic activity. Pozzolans
are cementitious materials and include natural pozzolans
(such as the volcanic ash used in Roman concrete), fly ash and
silica fume.
They can be used with Portland cement, or blended cement
either individually or in combinations.

Protect Against FreezeThaw Cycles –Improve Durability
Air entrainment is particularly effective in providing
resistance to freeze-thaw cycles. When the moisture in concrete
freezes, these air cells relieve internal pressure by providing
microscopic chambers for the expansion of water as it freezes.
Some air entraining mixtures contain a catalyst for more
rapid and complete hydration of portland cement.rapid and complete hydration of portland cement.
To protect concrete from damage during freezing, bubbles
must have the proper size, distribution and volume.
Benefits of air-entrainment include:
 Improved resistance of concrete to severe frost action or
freeze/thaw cycles
High resistance to cycles of wetting and dryingHigh resistance to cycles of wetting and drying
High degree of workability
High degree of durability
Dosage: Typical air entrainment ranges from 5% to 8% of the
volume of concrete
Exp. Visol Resin, Natural wood Resin etc.

Water Reduction in the Mix
Water reducers have become so important in concrete, that
they could be considered the "fifth" ingredient.
They can be used to: (1) increase slump, (2) lower the water-
cement ratio, or (3) reduce cement content.cement ratio, or (3) reduce cement content.
Water reducers come as Low Range, Mid Range, and High
Range Superplasticizers. There are enough different
admixtures available that it is possible to select one that meets
the need of a particular project – whether it is tall columns
that need a mix that pumps easily, or an easy-to-finish durablethat need a mix that pumps easily, or an easy-to-finish durable
floor slab.
In general, they provide the required slump with less water
in the mix, and may provide higher strength concrete without
increasing the amount of cement.

Conventional water reducers
Are required to achieve a minimum 5% water reduction. A
conventional water reducer can reduce slump by about 1 ½ to 2 inches
without the addition of water.
Mid-Range water reducers
Can reduce water content by at least 8% and as much as 15%.
These reducers work most effectively in mixes designed to have aThese reducers work most effectively in mixes designed to have a
slump in the range of 4 to 5 inches.
 Particularly useful in hot weather concreting by counteracting the
accelerating effect of high ambient and concrete temperatures
 Slow the setting rate of concrete
 Delay initial set of concrete
 Keep concrete workable during placement
High-Range water reducers (superplasticizers)
Can reduce water content from 12% to as much as 40% and are
typically used in concretes designed to have slumps of 8 to 11 inches.
They can be used to either increase slump (by 4 to 8 inches) or lower
the water content of hot weather concrete mixes. Used to increase
flowability.

3 Types of Super Plasticizers
Ligno Sulphate based
Melamine sulphate based
Nepthalene sulphate basedNepthalene sulphate based

High Strength Concrete
Admixtures containing microsilica (condensed silica fume) are
used to meet high strength and low permeability requirements.
Benefits include reduced permeability, increased
compressive and flexural strengths, and increased durability.compressive and flexural strengths, and increased durability.
Applications include high-strength structural columns,
less permeable parking garage decks, and abrasion resistant
hydraulic structures.
Silica fume can be used in concrete to produce
compressive strengths approaching 140 N/mm2 under jobsitecompressive strengths approaching 140 N/mm under jobsite
conditions.
This admixture can be added in a slurry or in a dry form,
whichever meets the batching equipment needs. In either case,
performance is the same.

Corrosion Protection
Reinforced concrete that is exposed to deicing salts and marine
environments is particularly susceptible to chloride-induced corrosion.
A well-designed, durable, low permeability concrete mix will
provide some protection against chloride-induced corrosion of
reinforcement.reinforcement.
Corrosion inhibitors can be effective in parking structures, bridges
and marine environments. Other ways of reducing corrosion are to use
penetrating surface sealers to help prevent chloride penetration,
epoxy-coated rebar, or silica fume in the mix. Silica fume can increase
durability and reduce permeability.
Calcium chloride has been used in the past. However, the
corrosive effects of the chloride have been observed as much as 20
years after the concrete was poured. Thus, non-chloride containing
admixtures have been developed for use where potential corrosion of
embedded or stressed steel must be avoided.

Set Acceleration
Set accelerators work by accelerating cement hydration,
which results in shortened setting times and increased early
age strengths, particularly in cooler temperatures.
They increase the rate of early strength development andThey increase the rate of early strength development and
reduce time required for curing and protection
At one time, calcium chloride was the predominant
accelerating admixture. However, it is now considered by many
to be a main contributor to long-term concrete problems. In
Europe, use of calcium chloride is banned for some
applications. Chloride is considered to contribute to corrosionapplications. Chloride is considered to contribute to corrosion
of reinforcement or embedded metal in concrete. In turn this
corrosion has been associated with spalling, cracking, loss of
bond and if left uncorrected can cause eventual failure of the
element involved.
Example: Soluable Carbonate, Silicate, Fluosilicate, Organic Compound

Strength Enhancement
Concrete strength enhancement can be achieved through use of
superplasticizer admixtures to produce a low water/cement
ratio giving high performance concrete.
These admixtures promote a high slump, extremely flowable
concrete that achieves high strengths while providing superior
workability and pumpability.
High range water-reducing admixtures can also be used for
precast/prestressed structures where it is desirable to keep theprecast/prestressed structures where it is desirable to keep the
water/cement ratio to a minimum for low permeability and high
early strengths without set retardation. They are also used for
concrete requiring high-early stripping strengths.

Set Retardation
Set retarders are used where delay in setting time is required
to ensure sufficient placement, vibration or compaction time.
Set retarders permit application of higher temperature
curing of precast/prestressed concrete without negativelycuring of precast/prestressed concrete without negatively
affecting the ultimate strength.
Applications:
•Long hauls
•Trucks waiting a long time - even for a small pour•Trucks waiting a long time - even for a small pour
•Undermanned placement crew
•Slow pour rate

Things to consider:
Project location - long or short haul
•Size of pour
•Rate of pour
•Placement method•Placement method
Environment – exterior, interior
Thick sections - (if dry windy conditions, early surface
drying while the underlying concrete is still soft can make
finishing difficult and result in a wavy or cracked surface
When a set retarder may not be requiredWhen a set retarder may not be required
•Short hauls with quick placement
•Cool moist interior location
•Residential or commercial basement
• Fast pour rate

Crack Control (Drying Shrinkage Cracking)
Hydrated cement paste shrinks as it loses moisture from its
extremely small pores. As the moisture is lost in these small
pores, the surface tension of the remaining water tends to pull
the pores together which results in a loss of volume over time.
Shrinkage reducing admixtures (SRA’s) are designed toShrinkage reducing admixtures (SRA’s) are designed to
decrease the effects of drying shrinkage by reducing the
surface tension in these pores.
It should be noted that the type of aggregate and the
cement properties themselves can influence the amount of
cracking that can occur. Thus, it is important to test local
project-specific materials if doing shrinkage testing.
Curing also affects cracking. In slabs, the top tends to dryCuring also affects cracking. In slabs, the top tends to dry
out first and shrinks while the lower sections still have a
higher moisture content. This difference in moisture can be
altered by use of Shrinkage Reducing Admixtures, which alter
the way water migrates through the concrete and results in a
more uniform moisture profile.

Flowability
Superplasticizers (High-Range Water Reducers) can make a
low-to-normal slump concrete into a high-slump flowing
concrete which can be placed with little or no vibration.
However, the change in slump usually lasts only about 30 to 60
minutes depending upon the brand and dosage rate.
High-range water reducers fall into either ASTM C494 Type
F or Type G classification. In either case, they can be used to
produce very high slumps without segregation, an idealproduce very high slumps without segregation, an ideal
situation where increased flowability is necessary due to
congested reinforcement.

•difficult wall placements
•narrow forms
•sections with blockouts, penetrations, or embedded items
•pumping high vertical distances
Other applications and benefits of high-range water reducers include:
•pumping high vertical distances
•fast placement of concrete
• increased lift heights and free fall distances
Note: The increased thinness of the concrete mix means that
forms should be tight to prevent leakage even through smallforms should be tight to prevent leakage even through small
joints which can result in fins and discoloration.

Type F superplasticizers
Added at the job site and keep concrete flowable for a short
period of time. At some point, the concrete will lose slump
quickly.
Type G superplasticizersType G superplasticizers
Can be added either during batching or at the job site.
This admixture will delay setting, but cause the concrete to
be flowable for a longer period of time – which could delay
finishing. If haul times are particularly long, Type G can be
added at the plant. However, if delivery is delayed too long,added at the plant. However, if delivery is delayed too long,
the effects can be diminished. Redosing is possible to regain
the plasticity of the mix and manufacturers recommendations
should be followed closely.

Finish Enhancement
Mid-range water-reducing admixtures (Water Reduction in
the Mix) can be used as finishing enhancers for concrete -
particularly in commercial and residential flatwork andparticularly in commercial and residential flatwork and
formed concrete applications.
MRWR’s produce less permeable, more durable concrete
and concrete with significantly early and ultimate
compressive strengths.

Fly Ash - Making Concrete Stronger, More
Durable, and Easier to Work With
Fly ash aids the formation of cementitious compounds to
enhance the strength, impermeability and durability of
concrete.
Class F
Reduces bleeding and segregation in plastic concrete. In
hardened concrete, increases ultimate strength, reduces
drying shrinkage and permeability, lowers heat of hydration
and reduces creep.
Class CClass C
Provides unique self-hardening characteristics and
improves permeability. Especially useful in pre-stressed
concrete and other applications where high early strengths
are required. Also useful in soil stabilization.

Silica Fume: Early Strength and Reduced Permeability
It is not a well defined material, because it is only a by-
product (waste) in the production process of silicon and silicon alloys.
Before using its alkali, silica & carbon content be identified along
with specific surface area.
Silica fume can make a significant contribution to early-age
strength of concrete. One Kilogram of silica fume produces about thestrength of concrete. One Kilogram of silica fume produces about the
same amount of heat as one Kg. of portland cement, and yields about
three to five times as much compressive strength.
Silica fume improves concrete in two ways – the basic
pozzolanic reaction, and a microfiller effect. Addition of silica fume
improves bonding within the concrete and helps reduce permeability,
it also combines with the calcium hydroxide produced in the
hydration of portland cement to improve concrete durability.hydration of portland cement to improve concrete durability.
As a microfiller, the extreme fineness of the silica fume
allows it to fill the microscopic voids between cement particles. This
greatly reduces permeability and improves the paste-to-aggregate
bond of the resulting concrete compared to conventional concrete.

Dosage:
8% to 15% by weight of cement but as an addition not
replacement.
8% to 10% High durability / Low permeability such as bridge
decks or parking structuresdecks or parking structures
10% to 15% High strength structural columns
10% max Flatwork
The amount required is related to silica fume dosage and the water-
cementitious materials ratio. Silica fume is cementitious, but typically is
added to and not replacing the existing portland cement.
Tip: The higher percentage of silica fume used, the higher theTip: The higher percentage of silica fume used, the higher the
amount of super plasticizer needed - but mix can become "sticky".
Consider replacing about 1/3 of the super plasticizer with a mid-range
water reducer to improve workability
Uses:
•Reduces concrete permeability
•Increases concrete strength
•Improves resistance to corrosion

Produce a Fluid Backfill Which Flows Easily and is Self Leveling
Controlled low strength material (CLSM)
This material provides a fluid backfill which flows easily and
is self-leveling.
While it costs more than a dry granular material, it does not
need to be placed in lifts, spread, and compacted after each lift.
It is a fluid mixture made of portland cement, water, fine
aggregate and/or fly ash and can include an admixture to
improve flowability, lower densities, eliminate segregation and
settlement, and control strength development in applications
where future excavation is required.

ASTM Categories of Admixtures
ASTM C494 specifies the requirements for seven chemical
admixture types. They are:
•Type A: Water-reducing admixtures
•Type B: Retarding admixtures
•Type C: Accelerating admixtures
•Type D: Water-reducing and retarding admixtures
•Type E: Water-reducing and accelerating admixtures
•Type F: Water-reducing, high range admixtures
•Type G: Water-reducing, high range, and retarding admixtures

Concrete admixtures_Dr D R Panchal

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