This document discusses different types of admixtures used in concrete, including their uses and effects. It covers mineral admixtures like fly ash and silica fume that can reduce costs and improve properties. It also discusses chemical admixtures, specifically highlighting water reducers, superplasticizers, accelerators, and retarders. Water reducers and superplasticizers can lower the water-cement ratio while maintaining workability, leading to increased strength. Accelerators can reduce setting time while retarders have the opposite effect of delaying setting.

2. MILITARY COLLEGE OF ENGINEERING
RISALPUR
CE 308 – PRC I - LECTURE 4
ADMIXTURES

3. ADMIXTURES
Often, instead of using a special cement, it is possible to
change some of the properties of the more commonly used
cements by incorporating a suitable additive or an admixture

4. TYPES OF ADMIXTURES
 Mineral admixtures
Chemical admixtures
Mineral Admixtures are supplementary cementing
materials. Main use is to reduce cost as it is
replacing cement, improve workability and durability
and to reduce heat of hydration
Classified as pozzollanic materials (fly ash and
silica fume) and cementations materials (slag)
Chemical admixtures are essentially water
reducers (plasticizers and super plasticizers), set-
retarders and accelerators. ASTM C 494-92
classified them as Type A, B and C respectively

5. Precautions in use

6. MINERAL ADMIXTURES

7.  Use of fly ash
The most extensively uses and widely available
material
By product of coal after burning in power plants
Economical
Use of silica fumes
By product in the manufacture of silicon metal
and alloys
Very fine grains
Pozzolanic material

8. Pozzolanic material
 Glassy silica reacts with CH formed from cement
hydration
Composition of C-S-H produced by FA pozzolanic
reaction is similar to that formed in regular hydration
 In terms of reaction heat, addition of FA leads to
lowering the amount of early heat evolution and
reduced early strength but not long term strength
Addition of FA leads to an overall increase in solid
volume and eventually a decrease in porosity of the
paste, resulting in higher strength and durability
compared with plain paste

9. Pozzolanic material

10. Chemical admixtures have
formulated chemical composition
special chemical action
used to modify certain properties of concrete
Primarily used to
reduce the cost of concrete construction;
to modify the performance of hardened concrete;
to ensure the quality of concrete during mixing,
transportation, placing, compacting and curing;
to overcome certain emergencies during concreting
operations.
CHEMICAL ADMIXTURES

11. Properties commonly modified are
the rate of hydration
setting times,
workability,
dispersion
and air entrainment
Generally added in a relatively small quantity.
Ensure proper quantity of an admixture, as an excess
quantity may be detrimental to the properties of concrete.
Most admixtures are supplied in ready-to-use liquid form
and are added to the concrete.
CHEMICAL ADMIXTURES

12. Chemical Admixtures

13. Air-entraining admixtures help to incorporate a controlled
amount of air, in the form of millions of minute bubbles
distributed throughout the body of concrete, during mixing,
without significantly altering the setting or the rate of
hardening of concrete.
A proper amount of entrained air results in improved
properties of plastic concrete like workability, easier placing
and finishing, increased durability, better resistance to frost
action and reduction in bleeding and segregation.
The entrained air bubbles, ranging approximately from
0.05 to 0.25 mm diameter and spaced 0.003 mm apart
reduce the capillary connectivity in concrete.
AIR-ENTRAINING ADMIXTURES

14.  Increasing the amount of air in concrete
The volume of air required to produce optimum
frost action is 9% by volume of the mortar fraction
and 2 to 8 % by volume of concrete depending on
the size of coarse aggregates used
The natural air content is about 1 to 3 % by volume
of concrete with naturally entrapped air (bigger pore
size, irregularly distributed)
The air content increased by 3 to 4 % by entrained
air (smaller pore size, uniformly distributed)
Air entrainment

15. AIR-ENTRAINING ADMIXTURES

16. MECHANISM
At the air-water interface the polar groups are oriented
toward the water phase lowering the surface tension,
promoting bubble formation, and counteracting the
tendency for the dispersed bubbles to coalesce (combine).
At the solid-water interface where directive forces exist in
the cement surface, the polar groups become bound to the
solid with the nonpolar groups oriented toward the water,
making the cement surface hydrophobic so that air can
displace water and remain attached to the solid particles as
bubbles.

17. Air entrainment

18. Affects of air entrainment

20. Affects of air entrainment
 Fresh concrete
Enhanced workability
Air entraining agents help to produce more uniform
and well compacted concrete
Reduced amount of sand and water required at a
desired slump
Hardened Concrete
Reduction in compressive strength (1 % of
entrained air, 5 % reduction in strength)
Optimum durability (weathering resistance) at air
entrainment of 7 to 8 %)

23. Most fine ingredients of concrete tend to flocculate or
clump together.
Flocculated fines cause an increase in viscosity by
entrapping a part of the water and by physically resisting
the flow – in turn effect workability.
More water is required to achieve desired slump - Water
can be added up to a certain point, beyond which the
intended plastic and hardened physical properties of
concrete are compromised.
Water reducers are often used the solution…
The organic or combinations of organic and inorganic
substances to achieve these objectives are termed as
water reducers or plasticizing admixtures.
PLASTICIZING ADMIXTURES

24. TYPES OF PLASTICIZERS

25. Mechanism of Water
Reduction
Green is
admixture
MECHANISM

26. SUPERPLASTICIZERS

27. Superplasticizers - a relatively new category and improved
version of plasticizers, the use of which was developed in
Japan and Germany during 1960 and 1970 respectively.
They are chemically different from normal plasticizers.
Use of superplasticizers permit the reduction of water to
the extent up to 30 % without reducing workability.
The use of superplasticizer is practiced for production of
flowing, self leveling, self compacting and for the production
of high strength and high performance concrete.
The mechanism of action of superplasticizers is more or
less same as in case of ordinary plasticizer. Only thing is
that the superplasticizers are more powerful as dispersing
agents.
They are called High Range Water Reducers in American
literature.
SUPERPLASTICIZERS

28. Superplasticizer made it possible to have a w/c ratio as low
as 0.25 or even lower and yet make flowing concrete to
obtain strength of 120 MPa or more.
Superplasticizer made it possible to use fly ash, slag and
particularly silica fume to make high performance concrete.
Superplasticizers can produce:
At the same w/c ratio much more workable concrete;
For same workability, it permits the use of lower w/c ratio;
For increased strength with lower w/c ratio, it permits a
reduction of cement content.
The superplasticizers also produce a homogeneous,
cohesive concrete generally without any tendency for
segregation and bleeding.
SUPERPLASTICIZERS

29. BENEFITS

30. • Series A, had a 300 kg/m3 cement content and a 0.62
water-cement ratio; the fresh concrete showed 50-mm
slump, and the hardened concrete gave 25- and 37-MPa
compressive strength at 7 and 28 days, respectively.
• With Test Series B, the purpose was to increase the
consistency of the reference concrete mixture without
adding more cement and water. This was easily achieved
by incorporating a small dosage of the water-reducing
admixture. Such an approach is useful when concrete is to
be placed in heavily reinforced sections by pumping. (only
slump increased as an effect of plasticizer, by dispersing
the flocculated cement particles)
SUPERPLASTICIZERS

31. • With Test Series C, the object was to achieve higher
compressive strengths without increasing the cement
content or reducing the consistency of the reference
concrete mixture. Incorporating the same amount of the
water-reducing admixture as used with Series B made it
possible to reduce the water content by 10 percent (from
186 to 168 kg/m3) while maintaining the 50-mm slump.
• As a result of reduction in the water-cement ratio, the 7-
day compressive strength increased from 25 to 34 MPa
and the 28-day strength from 37 to 46 MPa. This approach
may be needed when job specifications limit the maximum
water cement ratio but require high early strength to
develop.
SUPERPLASTICIZERS

32. • Test Series D demonstrates how the addition of the water-
reducing admixture made it possible to affect a 10 percent
cement saving without compromising either the
consistency or the strength of the reference concrete.
Besides cost economy, such cement savings may be
important when reduction of the temperature rise in mass
concrete is the primary goal.
SUPERPLASTICIZERS

33.  Water reducing admixtures are ASTM C 494,
C 1017
Chemical admixtures used for lowering the water
required to attain a given slump. Lowering the water
content means the reduction in cement content for
same w/c ratio (at a given strength)
Economical concrete by cutting the amount of
cement used (expensive ingredient)
Increase in cracking index, i.e. lowering potential
early age hydration heat , shrinkage etc. (low w/c
ratio)
Facilitates difficult placements by increasing slump
without increasing w/c ratio
Water reducing admixtures

35. The benefits of reduced time of setting may include:
Early finishing of surface; and
Reduction of pressure on forms or of period of time
during which the forms are subjected to hydraulic
pressure.
With the availability of powerful accelerators, the under
water concreting, the basement waterproofing operations,
the repair work of the waterfront structures in the tidal
zones have become easy.
With proper proportion these admixtures partly
compensate for the retardation of strength development
due to low temperatures in cold weather concreting.
ACCELERATING ADMIXTURES

36. TYPES OF ACCELERATORS

37. Set-accelerating admixtures

38. High temperatures, low humidity, and wind cause rapid
evaporation of water from the mix during summer. This
drying of concrete leads to the cracking of the surface.
Retarders delay setting of cement either by forming a thin
coating on the cement particles and thus slowing down
their dissolution in and reaction with water or by increasing
the intra-molecular distance of reacting silicates and
aluminates from water molecules.
RETARDING ADMIXTURES

39. Set-retarding admixtures

40. IMPACT ON HEAT EVOLUTION

41. Impact on compressive strength

42. Thank You.