Building Materials

Admixture and their Properties

 Definition
A material other than water, aggregate and hydraulic cement and additives like pozzolana or slag and fibre reinforcement used as an ingredient of concrete or mortar and added to the batch immediately before or during its mixing to modify one or more of the properties of concrete in the plastic or hardened state.

 IS Classification
i. Accelerating admixtures / Accelerator.
ii. Retarding admixtures / Retarder.
iii. Water reducing admixtures / Workability aid.
iv. Air Entraining admixtures.
v. Super plasticizing admixtures.
vi. Retarding super plasticizing admixtures.

 Special category admixtures
i. Grouting admixtures.
ii. Bonding admixtures.
iii. Damp proofing and permeability reducing admixtures.
iv. Air detraining admixtures.
v. Corrosion inhibiting admixtures.

 Function of admixtures
i. To accelerate the initial set of concrete i.e. to speed up early strength development.
ii. To retard the initial set i.e. to keep concrete workable for longer period.
iii. To improve workability.
iv. To improve penetration and pumpability of concrete.
v. To reduce segregation in grout.
vi. To increase the strength of concrete by reducing the water content.
vii. To increase the durability of concrete.
viii. To increase the resistance against chemical attack.
ix. To reduce heat of hydration.
x. To increase the bond between old and new concrete surfaces.
xi. To decrease the weight of concrete per cubic meter.
xii. To reduce the corrosion of reinforcement.

 Accelerator
These admixtures are added to concrete to –
a) Increase the rate of hydration of cement.
b) Shorten the time of set.
c) Increase the rate of hardening or strength development.

Thus these admixture are used when there is –
a) Earlier requirement of structure in service.
b) Earlier removal of formwork.
c) Earlier finish of surface.
d) Underwater concreting.
e) Coldwater concreting.

 Examples of accelerators

i. CaCl2 (Calcium Chloride)
 Dose
1.5 to 2% by weight of cement.
 Functions
Reduce setting time by 1/3 rd.
Increase 7 day compressive strength.
Increase flexural strength.
 Disadvantage
Exposed to alkali-aggregate reaction.
Sulphate attack.
Corrosion of reinforcement.

ii. Calcium formate.
iii. NaCl.
iv. Na2SO4.
v. NaOH.
vi. KOH.
vii. Fluoro silicates.

 Retarding Admixtures
These admixture are added to concrete to –
a) Delay the setting of cement paste.
b) Slowdown the initial rate of hydration of cement.

Thus these admixtures are used when –
a) There is requirement keep the concrete workable for longer period for transportation and placing.
b) To reduce the damaging effect during hot weather, low humidity and high wind velocity concreting conditions.
c) Ready mix concrete.

 Examples of Retarding admixtures
i. Sugar
 Dose
0.05 to 1% by weight of cement.
 Function
Delay initial setting time by 4 hrs.
ii. Starch.
iii. Dextrin.
iv. Zinc hydroxide.
v. Lead hydroxide.
vi. Calcium borate.

 Water reducing admixture
These admixture are added to concrete to –
a) Increase the workability of concrete without increasing water content.
b) Maintain wokability with reduced amount of water.
Thus these admixtures are used when –
a) There is faster requirement of concrete placement.
b) Heavily reinforced concrete sections.
c) Reduction of cement content.
d) Higher flowable concrete.

 Benefits of water reduction in hardened state of concrete.
i. Increased strength.
ii. Density.
iii. Durability.
iv. Volume stability.
v. Abrasion resistance.
vi. Reduced permeability & cracking.

 Water reducing admixtures are also termed as plasticizing admixtures. The mechanism involved are (i) Dispersion (ii) Retarding effect.

 Dispersion
When plasticizers are added thy get absorbed on cement particles. The absorption of charged polymer creates repulsive force, which results in deflocculation and dispersion. Thus increases the fluidity of concrete.

 Retarding effect.
On addition of plasticizers the following mechanism takes place –
a) Reduction in surface tension of water.
b) Induce electrostatic repulsion between particles of cement.
c) Lubricating film between cement particles.
d) Surface hydration of the cement particles, leaving more water to fluidify the mix.

 Examples of water reducing admixtures
i. Derivatives of lignosulphonic acids and their salts (e.g. Calcium Lignosulphonate).
(Reduce water requirement by 8 to 15 %. Dose 0.1 to 0.2 % by weight of cement. )
ii. Hydroxylated carboxylic acids and their salts.

 Air Entraining Admixtures
These admixtures are added which causes –
Air to be incorporated in the form of minute bubbles in concrete usually to increase workability and resistance to freezing and thawing and disruptive action of de-icing salts.

These admixtures are used when –
a) High workability is desired.
b) Freezing and thawing condition.
c) Cellular concrete.
d) Light weight aggregate concrete.

 Mechanism
These admixture when added acts as surfactants which are absorbed on to the cement particles. These surfactants forms bubbles on agitation which stabilize as microscopic spheres which increases mobility of concrete. These air entrainment bubbles reduces capillary forces in concrete. During freezing these bubbles produce extra space for expansion of ice and again regain their original size during thawing.
Thus segregation and bleeding is also reduced due to formation of homogenous mix. However increase of air entrainment reduces compressive strength.

 Properties effecting due to addition air-entrainment admixtures –
i. Segregation, bleeding get reduced.
ii. Improves permeability.
iii. Reduces aggregate alkali action and chemical attack.
iv. Reduction of sand and water content.
v. Lowers unit weight.
 Examples of Air Entraining Admixtures
i. Natural wood resins.
ii. Vegetable fats and oils.
iii. Olive oils.
iv. Stearic and oleic acids.

 Superplasticizing Admixtures
These admixtures are used to impart –
a) Very high workability.
b) Large decrease in water content for a given workability.

These admixtures are principally surface reactive agents. They confer negative charge on individual cement particles such that they are kept in a dispersed or suspended state due to interparticle repulsion. Thus they confer high mobility to the particles.

 Uses of Superplasticizers
i. Produce flowing concrete (Slump ≥ 200mm) without causing segregation and bleeding.
ii. Produce concrete with very low water cement ratio maintaining same workability.
iii. Produce high performance concrete.
iv. Produce concrete mix with reduced cement content.

 Examples of Superplasticizing Admixtures
i. Sulphonated Melamine Formaldehyde (SMF)
(3% by weight of cement)
ii. Sulphonated Naphthalene Formaldehyde (SNF)
(e.g. Poly B-nepthalene, imparts longer period of workability retention)
iii. Modified Lignosulphonates (MLS)
(Greatest workability retention)
iv. Carboxylated Acrylic ester Co-polymers (CAEC)
(Produce concrete with lower slump loss)

 Retarding Superplasticizing Admixtures
These admixtures imparts prolonged workability and retards setting.
e.g. SNF and MLS.

 Requirements of Admixtures.
Admixture in general sence should be avoided. If used its suitability or conformance to specified requirements should be compared with identical concrete without the admixtures under the following requirements.
i. Percentage reduction in water content.
ii. Slump.
iii. Initial and final setting time deviation.
iv. Deviation of compressive strength.
v. Deviation of flexural strength.

 Tests to be performed on fresh concrete
i. Tests of workability (Slump and C.F. test)
ii. Loss of workability on standing (45 mm and at 2hrs.)
iii. Test for air content.
iv. Test for time of setting.
v. Test for bleeding (Absorbtion of aggregates)
vi. Test for water content.
(Water absorption of aggregate, W/C ratio, Mass of cement/ Unit volume)
 Tests to be performed on hardened concrete
i. Compressive Strength.
ii. Flexural Strength.
iii. Test for length change.

 Information required for selection of a admixture
a) Physical state. (Liquid or Solid and Colour)
b) Generic type. (Lignosulphonate, Organic hydroxyl carboxylic)
c) Dry material content.
d) Ash content.
e) Relative density of liquid admixture.
f) Chloride ion content.
g) Recommended storage conditions, maximum storage time etc.
h) Recommended dosage.
i) Effect of under dose and overdose.
j) Expiry date.

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