2. What Are They?
Ingredients other than:
◦ Cement
◦ Water
◦ Aggregates
Added before or during mixing.
3. Why Are They Used?
To modify properties of fresh & hardened concrete
To ensure the quality of concrete during mixing, transporting,
placing & curing
To overcome certain unexpected emergencies during concrete
operations (ie, set retarders)
to reduce the cost of concrete construction
4. How Applied?
Most admixtures are supplied in a ready-to-use form and
added at plant or jobsite.
Pigments and pumping aids are batched by hand in VERY
small amounts
5. Effectiveness
Factors effecting results of use:
◦ Type & amount of cement:
Chemistry effects due to portland components
◦ Water content & Slump
Flowability will effect how well admixtures are activated
due to internal agitation actitivity
◦ Mixing time
Effects of admixtures are dependent on time allowed to
react.
6. 5 Typical Functions of Admixtures:
Air entraining
Water-reducing
Retarders
Set Accelerators
Plasticizers (superplasticizers)
Some other “specialty” types exist:
◦ Color, corrosion inhibitors, etc.
7. Air Entrainment
The entrained air bubbles act as a physical buffer against the
cracking caused by the stresses due to water volume
augmentation in freezing temperatures.
Developed in 1930s
Today, recommended for all concretes exposed to freeze/thaw
cycles
Imposes microscopic air cells that relieve internal pressure of
freezing water
High resistance to cycles of wetting and drying
High degree of workability
High degree of durability
8. Water Reducers
Internal lubricant
Allows for reduction of water/cement ratio while maintaining
workability (increased strength)
Can reduce water requirement at least 5-10%
Obtain higher slump without adding water
lower CO2 emissions and energy usage per volume of
concrete produced
With this type of admixture, concrete properties are improved and
help place concrete under difficult conditions.
Mid-range water reducers result in at least 8%
◦ Mid-range water reducers provide more consistent setting times
than standard water reducers.
9. Water Reducer: Pozzolith
Performance:
◦ Improves workability at low w/c ratio
◦ Normal to retarded set times
◦ Helps eliminate “cold joints”
10. Set Retarders
Set retarding concrete admixtures are used to delay the
chemical reaction that takes place when the concrete
starts the setting process. These types of concrete
admixtures are commonly used to reduce the effect
of high temperatures that could produce a faster initial
setting of concrete.
Slows curing rate
Used to counteract hot weather conditions that cause increased
rate of hardening.
◦ This makes placing and finishing difficult
◦ Pozzolith 961
11. Set Accelerators
Accelerating concrete admixtures are used to increase the rate
of concrete strength development, or to reduce concrete
setting time. Calcium chloride could be named as the most
common accelerator component
Increase curing rate for achievement of high early strength
Speeds up start of finishing operations
Used for speeding curing rate in cold weather concreting
Pozzolith 122
13. Superplasticizers
The main purpose of using super plasticizers is to produce
flowing concrete with a high slump in the range of 7-9 inches
to be used in heavily reinforced structures and in placements
where adequate consolidation by vibration cannot be readily
achieved. The other major application is the production
of high-strength concreteat w/c's ranging from 0.3 to 0.4. It
has been found that for most types of cement, super plasticizer
improves the workability of concrete.
Also known as high-range water reducers (HRWR)
Reduce water requirement by 12-30%
Can make low slump concrete flow like high-slump mix
Makes mix highly fluid and can be placed with little or no
vibration or compaction
Effect lasts only 30-60 minutes and is followed by rapid loss
of workability
Usually added at jobsite
