Introduction to Microprocesso programming and interfacing.pptx
Admixtures
1.
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