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Material which is added to concrete during mixing in order to
modify particular properties of concrete1.Accelerators - (CaCl2)
NaCl, formate triethenolamine2.Retarders-Gypsum, sugars,
lignosulphates3.Air entrainers -Wood resins/soaps, fats and
oils4.Water reducers (plasticisers) 5.Others- e.g. Corrosion
Inhibiting AdmixturesAdmixtures
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Why use admixtures?To give special properties to fresh or
hardened concreteenhance durability, workability or strength
characteristics of mixtureto overcome difficult situation -
hot/cold weather concreting, pumping requirements, early strength
requirements, or very low w/c ratio specificationsto ensure quality
of concrete during mixing, transporting, placing and
curingsuccessful usage depends on appropriate methods during
batching and concretingAdmixtures
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Chemical admixtures usually used in small quantities (2%) of
mixTwo types:AdmixturesMineral Fly ash (Pozzolan)Silica Fume Slag
(GGBS) ChemicalAir-entrainingAccelerator Water-reducingSet
Retarders
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Mineral admixture: added in larger amount to enhance workability
of fresh concreteto improve resistance of concrete thermal
cracking,alkali-aggregate expansion and sulphate attackto enable
reduction in cement contentChemical admixture: added in very small
amount forentrainment of airreduction of water or cement
contentplasticization fresh concrete mixturescontrol of setting
timeAdmixtures
- Accelerate the hardening or development of early strength
concreteneed not have any effect on the setting (stiffening) time
(BS 5075:Part 1:1982)the admixture is usually dissolved in waterthe
most common : Calcium Chloride (
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Benefit: increase the resistance of concrete to erosion and
abrasionimproves strength developmentCaCl2 reduces resistance to
sulphate attack, increases shrinkage and creepapplication : low
temp placing, urgent repair works
Accelerator
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Delay the initial setting time of concrete (BS5075:Part 1: 1982)
by an hour or morealso delay hardening time - architectural finish
of exposed aggregateoffsets the effects of hot weather - allow more
time for placing and finishing
Set Retarders (ASTM Type B&D)
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Increase plastic shrinkage (extend plastic stage) but drying
shrinkage is unaffected.compared with normal mix, the early
strength is reduced, later strength increased - so long-term
strength maintainedSet Retarders
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Purpose:Produce a concrete with lower w/c ratio, greater
plasticity and higher strengths with minimal impact on setting
timeto obtain higher slump using the same water content (10%
reduction)consist of low range, mid-range and
high-range(super-Plasticizers) types
Plasticizers - Water Reducers (ASTM Type A)
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Used to increase slump for pumping concrete and in hot weather
concreting to offset the increased water demandsuperplastisizers
reduce water content 12-25% - which increases strengthincrease
slump to produce flowing concretePlasticizers (Water Reducers)
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Principal active components: surface active agents dispersing
ability of the admixture gives greater surface area of cement
exposed to hydration - hence an increase in early strengthlong-term
strength improved from uniform distribution of the dispersed cement
throughout concreteeffective with all types of cement
Plasticizers (Water Reducers)
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to purposely produce microscopic air bubbles in concrete
objective:improve the durability and resistance to damage from
freezing and thawing and de-icing saltsimprove workability, reduce
bleeding and segregationAir-entraining Admixtures
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application: exterior flatwork (parking lots, driveways,
sidewalks, pool decks, patios)only to structures exposed to
freezing - assist in expansion and shrinking of structures due to
thermal changesAir-entraining Admixtures
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Specifications of Various Type of Admixtures (BS5075:Part
1:1982)
Type of admixture
Water Reduction, per cent
Compacting Factor
Stiffening Time
Compressive Strength
Time from completion of mixing to reach a resistance to
penetration* of
Percentage of control mix (minimum)
Age
0.5Mpa (70psi)
3.5Mpa (500psi)
Accelerating
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Not more than 0.02 below control mix
More than 1h
At least 1h less than the control mix
125
95
24h
28 days
8
-
-
-
-
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Retarding
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Not more than 0.02 below control mix
At least 1h longer than control mix
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90
95
7 days
28 days
8
-
-
-
-
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Normal-water reducing
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At least 0.03 above control mix
-
-
90
90
7 days
28 days
8
Not more than 0.02 below control mix
Within 1h of control mix
Within 1h of control mix
110
110
7 days
28 days
Accelerating Water Reducing
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At least 0.03 below control mix
-
-
125
90
24h
28 days
8
Not more than 0.02 below control mix
More than 1h
At least 1h less than control mix
125
110
24h
28 days
Retarding Water Reducing
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At least 0.03 below control mix
-
-
90
90
7 days
28 days
8
Not more than 0.02 below control mix
At least 1h longer than control mix
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110
110
7 days
28 days
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Admixtures & Functions
Type
Function
Air entraining
Improves durability, workability, reduces bleeding, reduces
freezing/thawing problems (e.g. special detergents)
Superplasticizers
Increase strength by decreasing water needed for workable
concrete (e.g. special polymers)
Retarding
Delays setting time, more long term strength, offsets adverse
high temperature weather (e.g. sugar)
Accelerating
Speeds setting time, more early strength, offsets low
temperature weather (e.g. calcium chloride)
Mineral admixtures
Improves workability, plasticity, strength (e.g. fly ash)
Pigment
Add colour (e.g. metal oxides)
************AcceleratorsThis admixture accelerates the
setting/hardening time of fresh concrete. It increases the early
strength of concrete but has little effect on the 28-days strength.
In practice, the setting time is reduced as described in BS5075:
Part 1:1982. The most common accelerator is Calcium Chloride
(CaCl2), which accelerates the primary early strength development
of concrete. This admixture is sometimes used when concrete is to
be placed at low temperatures (2 to 4oC) or when urgent repair work
is required because it increases the rate of heat development
during the first few hours after mixing. Calcium Chloride probably
acts as a catalyst in the hydration processes. It is important that
this admixture is thoroughly and uniformly distributed in the mix,
and is best achieved by dissolving it in the mixing water. However,
the use of calcium chloride reduces the resistance of cement to
sulphate attack. It also increases shrinkage and creep. At the same
time, the beneficial effect is an increase in the resistance of
concrete to erosion and abrasion. There is also a possibility of
corrosion of reinforcing steel by integral calcium chloride.
**AcceleratorsThis admixture accelerates the setting/hardening
time of fresh concrete. It increases the early strength of concrete
but has little effect on the 28-days strength. In practice, the
setting time is reduced as described in BS5075: Part 1:1982. The
most common accelerator is Calcium Chloride (CaCl2), which
accelerates the primary early strength development of concrete.
This admixture is sometimes used when concrete is to be placed at
low temperatures (2 to 4oC) or when urgent repair work is required
because it increases the rate of heat development during the first
few hours after mixing. Calcium Chloride probably acts as a
catalyst in the hydration processes. It is important that this
admixture is thoroughly and uniformly distributed in the mix, and
is best achieved by dissolving it in the mixing water. However, the
use of calcium chloride reduces the resistance of cement to
sulphate attack. It also increases shrinkage and creep. At the same
time, the beneficial effect is an increase in the resistance of
concrete to erosion and abrasion. There is also a possibility of
corrosion of reinforcing steel by integral calcium chloride.
**RetardersThese admixtures delay the setting time of concrete.
They are described in BS 5075:Part 1:1982. Retarders are useful
when concreting in hot weather, when the normal setting time is
shortened by the higher temperature, and in preventing the
formation of cold joints between successive lifts. Generally, with
a retarder, a delay in hardening also occurs a property which is
useful to obtain an architectural surface finish of exposed
aggregate. The setting time of concrete is increased by delaying
the adding of the retarding admixture to the mix. Compared with a
normal concrete mix, the use of retarding admixtures reduces the
early strength but later the rate of strength development is
higher, so that the longer-term strength is not much different. In
addition, the retarders tend to increase the plastic shrinkage
because the plastic stage is extended, but the drying shrinkage is
unaffected.
**RetardersThese admixtures delay the setting time of concrete.
They are described in BS 5075:Part 1:1982. Retarders are useful
when concreting in hot weather, when the normal setting time is
shortened by the higher temperature, and in preventing the
formation of cold joints between successive lifts. Generally, with
a retarder, a delay in hardening also occurs a property which is
useful to obtain an architectural surface finish of exposed
aggregate. The setting time of concrete is increased by delaying
the adding of the retarding admixture to the mix. Compared with a
normal concrete mix, the use of retarding admixtures reduces the
early strength but later the rate of strength development is
higher, so that the longer-term strength is not much different. In
addition, the retarders tend to increase the plastic shrinkage
because the plastic stage is extended, but the drying shrinkage is
unaffected.
**Plasticizers (Water reducers)These admixtures are used for
three purposes:To achieve higher strength by decreasing the
water/cement ratio at the same workability as an admixture free
mix.To achieve the same workability by decreasing the cement
content so as to reduce the heat of hydration in mass concreteTo
increase the workability so as to ease placing in inaccessible
locations.The principal active components of water-reducing
admixtures are surface-active agents. The dispersing ability of
water-reducing admixtures results in greater surface area of cement
exposed to hydration, and for this reason there is an increase in
strength at early ages compared with admixture-free mix of the same
water/cement ratio. Long term strength may also be improved because
of a uniform distribution of the dispersed cement throughout the
concrete. Generally, they are effective with all types of cement.
There are no detrimental effect on other long-term properties of
concrete, and when the admixture is used correctly, the durability
can be improved.**Plasticizers (Water reducers)These admixtures are
used for three purposes:To achieve higher strength by decreasing
the water/cement ratio at the same workability as an admixture free
mix.To achieve the same workability by decreasing the cement
content so as to reduce the heat of hydration in mass concreteTo
increase the workability so as to ease placing in inaccessible
locations.The principal active components of water-reducing
admixtures are surface-active agents. The dispersing ability of
water-reducing admixtures results in greater surface area of cement
exposed to hydration, and for this reason there is an increase in
strength at early ages compared with admixture-free mix of the same
water/cement ratio. Long term strength may also be improved because
of a uniform distribution of the dispersed cement throughout the
concrete. Generally, they are effective with all types of cement.
There are no detrimental effect on other long-term properties of
concrete, and when the admixture is used correctly, the durability
can be improved.**Plasticizers (Water reducers)These admixtures are
used for three purposes:To achieve higher strength by decreasing
the water/cement ratio at the same workability as an admixture free
mix.To achieve the same workability by decreasing the cement
content so as to reduce the heat of hydration in mass concreteTo
increase the workability so as to ease placing in inaccessible
locations.The principal active components of water-reducing
admixtures are surface-active agents. The dispersing ability of
water-reducing admixtures results in greater surface area of cement
exposed to hydration, and for this reason there is an increase in
strength at early ages compared with admixture-free mix of the same
water/cement ratio. Long term strength may also be improved because
of a uniform distribution of the dispersed cement throughout the
concrete. Generally, they are effective with all types of cement.
There are no detrimental effect on other long-term properties of
concrete, and when the admixture is used correctly, the durability
can be improved.**Plasticizers (Water reducers)These admixtures are
used for three purposes:To achieve higher strength by decreasing
the water/cement ratio at the same workability as an admixture free
mix.To achieve the same workability by decreasing the cement
content so as to reduce the heat of hydration in mass concreteTo
increase the workability so as to ease placing in inaccessible
locations.The principal active components of water-reducing
admixtures are surface-active agents. The dispersing ability of
water-reducing admixtures results in greater surface area of cement
exposed to hydration, and for this reason there is an increase in
strength at early ages compared with admixture-free mix of the same
water/cement ratio. Long term strength may also be improved because
of a uniform distribution of the dispersed cement throughout the
concrete. Generally, they are effective with all types of cement.
There are no detrimental effect on other long-term properties of
concrete, and when the admixture is used correctly, the durability
can be improved.**Plasticizers (Water reducers)These admixtures are
used for three purposes:To achieve higher strength by decreasing
the water/cement ratio at the same workability as an admixture free
mix.To achieve the same workability by decreasing the cement
content so as to reduce the heat of hydration in mass concreteTo
increase the workability so as to ease placing in inaccessible
locations.The principal active components of water-reducing
admixtures are surface-active agents. The dispersing ability of
water-reducing admixtures results in greater surface area of cement
exposed to hydration, and for this reason there is an increase in
strength at early ages compared with admixture-free mix of the same
water/cement ratio. Long term strength may also be improved because
of a uniform distribution of the dispersed cement throughout the
concrete. Generally, they are effective with all types of cement.
There are no detrimental effect on other long-term properties of
concrete, and when the admixture is used correctly, the durability
can be improved.**
