CE601
LECTURE – 3
ADMIXTURES
ADMIXTURES
• Definition:
• Ingredients in concrete, other than cementitious
materials, aggregates and water that are added to
the concrete mixture before or during mixing.
• Uses:
• To modify the properties of fresh and hardened
concrete.
• To make the concrete more suitable for specific
work.
• To decrease the cost of concrete construction.
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ADMIXTURES
• Required to obtain the desired fresh and hardened properties of concrete.
• Impart physical and economic benefits to concrete.
• Initial cost is high, but there will be overall savings, example saving in cost of
labor for consolidation, reduction in cement content, decrease in the water
content thereby increasing the strength and durability.
• Though admixtures impart good properties to concrete, their use should not
override the need for good quality constituents, proper mix proportions,
poor workmanship in transporting, placing and consolidation.
• They may be inorganic or organic.
TYPES OF ADMIXTURES (ASTM C 494)
TYPE
FUNCTION
A
B
Water-reducing
Retarding
C
D
E
Accelerating
Water-reducing and retarding
Water-reducing and accelerating
F
G
High-range water-reducing or superplasticizing
High-range water-reducing and retarding or superplasticizing and
retarding
BS standards: BS EN 934-2; BS EN 480
USE OF ADMIXTURES
• May be in solid or liquid form. Latter is preferable to control the diluent, in most cases
water. Water in the admixture should be used to calculate the mix water.
• Generally added with the mix water in one go or in instalments.
• Dosage of an admixture is normally expressed by weight of cement. The dosage is
normally recommended by the manufacturer.
• Effectiveness may vary depending on the dosage and also on the constituents of the
mix, especially the cement.
• The performance may vary with temperature, for this reason their performance at
extreme temperatures should be ascertained prior to use.
ACCELERATING ADMIXTURES (ASTM C494 TYPE C)
• Also known as accelerators.
• Function is to accelerate the early strength development of concrete.
May also accelerate the setting time (Set accelerators).
• Generally used when concrete is to be placed at low temperature (2
to 4 C). In prefabricated concrete where a rapid removal of form
work is required.
• It allows early finishing of the concrete and for putting the structure
in use.
• Use of accelerators may lead to shrinkage cracking in hot weather
conditions. Proper precautions need to be taken for this.
ACCELERATING ADMIXTURES (CONTD.)
• Calcium chloride is generally used as an accelerator.
• Accelerates the hydration of C3S, possibly by altering the alkalinity of the
pore water.
• May accelerate reinforcement corrosion and decrease the sulfate resistance.
• May also increase the erosion and abrasion.
• May also increase the drying shrinkage.
• Other chloride-free accelerators include:
• Calcium nitrite
• Calcium nitrate
• Calcium formate
• Triehtanolamine
• Chloride-free accelerators are recommended.
STANDARD REQUIREMENTS FOR ACCELERATORS
• ASTM C494 (Type C)
• Final set should be at least 1 hour earlier than that of the control mix but not more
that 3.5 hours,
• Compressive strength at 3 days should be 125% that of the control mix.
• The strength beyond 28 days can be similar or marginally lower than that of the
control mix.
• Strength retrogression is not allowed.
• BS EN 934: Setting time, strength and air content requirements are prescribed.
RETARDING ADMIXTURES (ASTM C494 TYPE B)
• Generally referred as retarders.
• Delays the setting time. Useful under hot weather conditions.
• Some retarders may slow down the hardening of concrete.
• Useful in architectural concrete or exposed aggregate concrete or mozzaic
and terrazzo.
• Common retarders are:
• Sugar
• Carbohydrate derivatives
• Soluble zinc salts
• Soluble borates, etc.
RETARDERS (CONTD.)
• Sugar acts as retarder by slowing down the formation of C-S-H.
• A large quantity of sugar (02. to 1%) will prevent the setting of time.
• The early strength of concrete with sugar is severely reduced.
However, after 7 days the strength is significantly increased. It is
equal to or more than that of the control concrete.
• Retarders may increase the plastic shrinkage cracking. However, the
chances of drying shrinkage cracking are similar to normal concrete.
• Trial mixtures should be tested to evaluate the retardation effect and
the slump retention and the hardened concrete properties prior to the
selection of the retarding admixtures.
REQUIREMENTS FOR RETARDERS
• ASTM C494:
• Initial set should be at least 1 hour but not more than 3.5 hours.
• The compressive strength from the age of 3 days can be 10% less than
that of the control concrete.
• BS 5075: Broadly similar to that of ASTM C494.
WATER-REDUCING ADMIXTURES (ASTM TYPE A)
• Only water reducing “Type A”
• Water reducing and retarding “Type D”
• Water reducing and accelerating admixtures “Type E”
• Main function is to reduce the water content or increase the workability for the
same water content.
• Up to 5 to 10% reduction in the water content is possible.
• Water reducers improve the workability of harshly graded aggregate concrete.
• Can also be used in pumped concrete or concrete placed by tremie.
• Types: Lignosulfonic acids and hydroxylated carboxylic acids.
• Acts like a coagulant, dispersing the cement particles in water.
WATER REDUCING ADMIXTURES (CONTD.)
• Though water reducers affect the rate of hydration of cement, the nature of
the hydration products and their structure is unchanged. However,
segregation and bleeding may be compromised.
• The hardened properties may not be affected.
• The effectiveness with respect to strength varies with the composition of
cement, it is better with low alkalinity or low C3A cements.
• A slight delay in the mixing of the admixture, i.e. after mixing of water, is
beneficial.
• Some water-reducing admixtures may increase shrinkage of concrete.
• Two stage mixing is known to be beneficial.
WATER REDUCING ADMIXTURES (CONTD.)
• The effectiveness of a water reducing admixture is dependent on:
• Cement content
• Water content
• Type of aggregate
• Presence of air entertaining agents
• Pozzolans
• Temperature.
• Selection of the type and dosage of a water reducing admixture
should be based on the materials to be used. Manufacturer’s data is
not sufficient.
• Trial mixtures are recommended.
SUPERPLASTICIZERS
• Also called high-range water reducers. High workability with low water/cement ratio can be
obtained.
• Classified as “Type F” admixtures if they are only superplasticizers.
• Superplasticizers with retarding action are called “Type G” admixtures.
• Types of superplasticizers:
• Sulfonated melamine-formaldehyde condensates-melamine-based.
• Sulfonated naphthalene-formaldehyde condensates – naphthalene-based.
• Modified lignosulfonates
• Sulfonic-acid esters
• Carbohydrate esters.
SUPERPLASTICIZERS
• Most of the salts are in the form of sodium salts, but calcium salts are also
produced. However, calcium has low solubility, therefore other sodium salts
are preferred.
• Use of sodium increases the alkalinity of the pore solution, thereby
encouraging alkali-silica reaction. Therefore, a limit on the alkalinity is
imposed in areas with reactive aggregates.
• Modifications to the structure of the superplasticizer is normal to meet
certain specific requirements.
SUPERPLASTICIZERS
• The molecules in a superplasticizer coat the cement and provide negative
charge thereby they repel each other. This results in deflocculation and
dispersion of cement particles.
• They do not alter fundamentally the structure of hydrated cement, the main
effect being a better distribution of cement particles and consequently, their
better hydration.
• The increase in workability can be exploited in two ways: producing
concrete with high workability or concrete with a very high strength due to
low w/c.
• High workability of concrete can be used for producing flowable concrete
(Self consolidating concrete).
• Production of normal workability concrete but with low water cement ratio
resulting in high strength concrete.
DOSAGE OF SUPERPLASTICIZER
• For increasing the workability of the mix, the normal dosage of
superplasticizer is between 1 and 3 liters per cubic meter of concrete.
• When used to decrease the water content the of the mix, the dosage is
much higher, 5 to 20 liters per cubic meter.
• In calculating the total water content, the water in the superplasticizer
should be taken in to account.
• Compatability with cement and other constituents should be checked.
USE OF SUPERPLASTICIZERS
• In dry concrete with low/cement ratio to produce high performance
concrete.
• To produce self consolidating concrete.
• In silica fume concrete.
• Superplasticizer does not influence shrinkage, creep, modulus of elasticity
or resistance to freezing and thawing and durability of concrete.
• The use of superplasticizer with an air-entraining agent admixture requires
caution as sometimes the actual amount of entrained air is modified by the
superplasticizer.
AIR ENTRAINING AGENTS
Required to increase the air content in concrete. Air bubbles so formed accommodate the ice
formation at low temperatures.
About 2 to 5% AEA, by weight of cement, is normally added to concrete.
Addition of AEA should not alter the properties of concrete. 


WATER
• Water is used for both mixing and curing of
concrete.
• The substances in water should not:
• Interfere with the setting of concrete.
• Adversely affect concrete strength.
• Cause staining of the concrete surface.
• Lead to corrosion of the reinforcing steel.
• Have a harmful effect on concrete.
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IMPURITIES IN WATER
• Oils
• Acids
• Organic
Matter
• Chlorides
• Sulphates
• Alkalis
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• Algae
Sweet
Water
Raw
Water
Gulf
Water
Sodium
98
641
13,200
Calcium
8
248
516
Magnesium
3
88
1,690
Sulphate
19
407
3,240
Chloride
120
1,257
23,700
Carbonate
0
0
6
Bicarbonate
62
209
103
Total Dissolved Solids (TDS)
313
2,848
42,500
Ions
TYPICAL COMPOSITION OF WATER
IN SAUDI ARABIA
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Type of Impurity
Suspended matter (turbidity)
2,000
500 – 1,000
Algae
Carbonates
Bicarbonates
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Maximum
Concentration
(ppm)
1,000
400 – 1,000
Remarks
Salt, Clay, Organic Matter
Entrain Air
Decrease Setting Time
400 ppm for Bicarbonates of Ca, Mg
Sodium Sulphate
Magnesium Sulphate
10,000
Sodium Chloride
Calcium Chloride
Magnesium Chloride
20,000
Iron Salts
Phosphates, Arsenates, Borates
Salts of Zn, Cu, Pb, Mn, Sn
40,000
Inorganic Acids
10,000
Reduce Alkalinity of Concrete, may attack the
Concrete
Sodium Hydroxide
500
May cause Alkali-Silica (Aggregates) Reaction
Sodium Sulfide
100
Should Test Concrete
Sugar
500
Affects Setting Behavior
40,000
50,000
40,000
500
Increase early Strength but Reduces Later
Strength, May Contribute to Sulphate Attack
Decrease Setting Time, Increase early Strength
and Reduce Ultimate Strength, will Contribute to
Steel Corrosion
Retard Set
500
TOLERABLE LEVELS OF IMPURITIES IN MIXING WATER
FOR PLAIN CONCRETE