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CONCRETE ADMIXTURESCONCRETE ADMIXTURES
Lecture 6Lecture 6
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TEST 1
Monday 23rd August 2010 (Week 7)
M47 212 & 213
8.30 9.30 p.m. Two questions up to concrete
Q1. a)
b)
c)
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Admixtures
A material added to plastic concrete or
mortar, other than water, aggregates,
cement, and fibre TO change one ormore properties of fresh or hardened
concrete
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When to use admixture?
When the desired modification of
properties of fresh or hardened
concrete cannot be achieved bychanges in the composition of the mix
proportion
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Functions of Admixtures
Pg. 61
To accelerate or retard initial setting
To enhance workability
To increase durability of concrete
Etc.
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Example
High-strength concrete
Concrete grade 80
Cement = 450kg/m3
Water/Cement ratio =0.35
Coarse Aggregate = 1120kg/m3
Fine Aggregate = 980
kg/m3
WHAT WILL BE THEPROBLEM?
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Form of Admixtures
Liquid
F
lake Powder
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Chemical Admixture
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CHEMICAL ADMIXTURES
Chemical added in very
small amount or
quantity to the mixture
to develop special
properties in fresh or
hardened concrete
1) Accelerating
admixture
2) Retarding admixture
3) Water-reducing/high-range water-reducing
admixture
4) Air-entraining
admixture
5) others
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Accelerating admixture
To accelerate the setting and early strength development
of concrete, particularly in cold weather applications
E.g. calcium chloride (CaCl2)
Early removal of formwork
Compressive strength at 3 days at least 25% higher than
concrete without admixture
The amount of CaCl2 should be limited to 2% by weight of
cement
Read page 63-64
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Cont.
Large dosage may cause severe
corrosion and loss of strength at later
ages
CaCl2 is not recommended in hot weather
concreting and for pre-stressed concrete
and may be ineffective at temperatures
below freezing because concrete canfreeze before it reaches the initial set.
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Problems Associated with Accelerator
Increase the heat of hydration
Found to aggravate corrosion ofreinforcing and pre-stressing steel
Increase creep and shrinkage
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Retarding Admixture
Added to concrete to delay its setting and hardening,
especially in hot weather applications (temperature
greater than 32r
C) Hot weather increases the rate of hardening which makes
it difficult to place and finish the plastic concrete
Recommended in large structures and difficult situations
such as large piers and foundations, to delay the initial
set as well as to keep concrete workable throughout the
entire placing period (eliminate cold joint)
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Cont.
Sugar in any form is a
retarder (coke?)
The use of retardermay reduce the
strength at early age
Read pg. 64-65
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Water-Reducing Admixture
Used to reduce the quantity of mixing water
required, which increase the compressive
strength, and to produce concrete of desired
consistency or high slump. Can reduce water required by 12% or more
Its effect is due to increased dispersion of
cement particles, causing a reduction in the
viscosity of the concrete.
Read pg. 70
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Pumping Concrete
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High-Range Water Reducer
Also called Superplasticizer
Added in small dosage with mixing
water (typically 0.2% by weight ofcement) to increase slump
Flowing concrete slump greater than
180 mm
Dosage rate may vary from brand to
brand for different types of cement.
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Without SP
With SP
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Cont.
By using superplasticizer, it is possible
to have w/c ratio in the range of 0.3 to
0.45 which would be impossibleotherwise.
Addition of superplasticizer increases
cost by 5% BUT savings in labour can
be as high as 33%
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Effect of Superplasticizer
Increase in workability
Increase in compressive strength Eliminates segregation
Allow good dispersion of cement
particles in water
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Superplasticizer how it works?
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Concrete with Superplasticizer
Has smooth surface
Much less likely tochip and spall
Has fewer plasticshrinkage cracks
Higher compressivestrength
because of rapidgain in strength
Early removal offormwork
Less shrinkage
Low permeability
Less time to place
and finish the
concrete
An accelerating
curing period
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Air-Entraining Admixture
Added to entrain air in concrete
Air entrainment is a process by which a
large number of microscopic air bubblesare dispersed throughout the concrete
Diameter of air bubbles in the range of
20 to 200 m
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Air Bubbles
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Advantages
It causes the disruption of the continuity
of capillary pores result in reduction in
permeability and reduces internal
stresses caused by expansion of the porewater on freezing.
Foaming agents are added in small
amount (typically about 0.05% by weightof cement) during mixing and can be used
to entrain 3 to 10% air.
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Cont.
The use of air-entraining admixture can:
Improve workability
Reduce bleeding Reduce segregation
However:
Lower compressive strength
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Chemical Admixture
DescriptionDescription ApplicationsApplications
WaterWater--reducingreducing
AdmixtureAdmixture
To get dense concrete, to improveTo get dense concrete, to improve
workabilityworkability
Retarding admixtureRetarding admixture To delay setting and hardening,To delay setting and hardening,hot weather concreting, largehot weather concreting, large
structuresstructures
Accelerating admixtureAccelerating admixture To accelerate setting and earlyTo accelerate setting and early
strength development, coldstrength development, cold
weather concretingweather concreting
AirAir--entraining admixtureentraining admixture To improve durability andTo improve durability and
workabilityworkability
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POZZOLANIC / MINERAL
ADMIXTURES
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Mineral Admixtures
Are natural pozzolanic materials orindustrial by-products that are used inconcrete to replace part of cement
It is known as supplementarycementing material
Are generally added in relatively large
quantities in comparison with chemicaladmixtures
Contains reactive silica (SiO2)
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Pozzolans
Are siliceous or siliceous and aluminousmaterials which in themselves possess littleor no cementitious property
BUT in finely divided form and in thepresence of moisture, these materials canreact with calcium hydroxide of concrete atordinary temperatures to form compoundsof cementitious properties.
This reaction is called POZZOLANICREACTION
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Reactions
Hydration process:
Cement + H2O C-S-H gel + Ca(OH)2
Pozzolanic reaction:
SiO2
+ Ca(OH)2
C-S-H gel
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Effect of Pozolanic Admixture
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Example of Pozzolans
Ground Granulated Blastfurnace Slag
PulverisedF
uel Ash Silica Fume
Rice Husk Ash
Palm Oil Fuel Ash
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Method of Replacement
Replacing part of the cement
content (by weight of cement)
Can be added durning mixingprocess or grind together with
cement (pozzolan cement)
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Effect of pozzolanic admixtures
Lower early strength
Lower heat of hydration
Higher ultimate strength
Less permeable
Reduce cost Increase workability (some)
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Slag
Is a non-metallic product, consisting essentially
of silicates and aluminates of calcium bases
Not only pozzolanic but also cementitious
It is develop in a molten condition simultaneously
with iron in a blastfurnace
Is the glassy granular material formed when
molten blastfurnace slag is rapidly chilled as byimmersion in water
Amount replacement : up to 80%
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Slag
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Fly Ash
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Microstructure ofFly Ash
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Rice Husk Ash (RHA)
Is an agricultural fly ash which is
obtained from burning and
grinding of rice husk
Contains silica
Amount replacement : 10 to 30%
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Factors Affecting the Suitability and
Performance of Admixtures in Concrete
Type of cement Mix proportions and method of mixing
Temperature and time when the admixture is
added into the mix
Chemical composition of the admixture
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Monitoring and Precaution
Selection of material
Trial mix
Short-term and long-term effect Eliminate overdosing on site
Manufacturers advice
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Recommended