1. CON 122
Concrete Admixtures
Session 1
Introduction

2. What is an Admixture?
“A material other than
water, aggregates, hydraulic cement, and
fiber reinforcement, used as an ingredient
of a cementitious mixture to modify its
freshly mixed, setting, or hardened
properties and that is added to the batch
before or during its mixing.”
-ACI 116

3. General History of Admixtures
 Milk and Lard – Romans
 Eggs – Middle Ages in Europe
 Rice paste, lacquer, tung oil, molasses and
bananas – China
 Cactus juice and rubber latex – Peru
 Bark extracts - Mayans

4. Chemical Admixture Purpose –
ACI 212.3R
“Chemical admixtures are used to
enhance the properties of concrete
and mortar in the plastic and
hardened state.”

5. Concrete Chemical Admixtures
 Added to concrete mixture before or
during mixing
 Primarily water soluble
 Ingredients other than
 Cement
 Water
 Aggregates
 Fibers

6. Benefits of Admixtures in
Fresh Concrete
 decrease water content
 increase workability
 retard or accelerate time of setting
 entrain air / reduce unit weight
 reduce segregation
 reduce rate of slump loss
 improve
pumpability, placeability, finishability

7. Benefits of Admixtures in
Hardened Concrete
 modify rate and/or amount of bleeding
 increase strength
 increase durability
 improve aesthetics

8. Concrete Prerequisites
 Workable
 Finishable
 Strong
 Durable
 Watertight
 Wear Resistant

9. Concrete Chemical Admixtures
Key Points
 Admixtures are materials added to
concrete mixtures to modify concrete
properties such as air content, water
requirement, and setting time.
 Admixtures should complement, not
substitute, for good concrete
proportioning and practice.

10. Concrete Chemical Admixtures
Key Points
 Admixtures may have unintended side
effects. Therefore, run trial batches with
job materials and under job conditions.
 Generally, for every 1% entrained
air, concrete loses about 5% of its
compressive strength

11. Concrete Chemical Admixtures
Key Points
 Air-entraining admixtures are specified by
ASTM C 260.
 Water-reducing and set-modifying
admixtures are specified by
ASTM C494 / AASHTO M 194.

12. Main Classes of Chemical
Admixtures
Water Reducers Other
 Normal, mid-range,  Viscosity modifiers &
high-range Rheology control
Set Control  High Performance
 Accelerators, retarders
admixtures
 Coloring admixtures
Durability Enhancing
 Other
 Air-entrainers
 Corrosion inhibitors
 Shrinkage reducers
 ASR inhibitors

13. Admixture Cautions
 Change of type, source, or amount of
cement
 Change of aggregate type or grading
 Change of mixture proportions
 Change of job placement conditions
 Reinforcement of concrete
 Concrete appearance

14. Properties Influenced by Chemical
Admixtures
 Plastic Concrete
 Mix
 Transport
 Placing
 Finishing
 Frost Resistance
 Strength
 Early (1-7days)
 Late (28 days)

15. Effects of Chemical Admixtures

16. Effects of Chemical Admixtures
 Air entrainers: Air-entraining admixtures
work by stabilizing small air bubbles in the
paste.
 Several different chemical forms of air
entrainers are available, all with differing
stability and differing effects on bubble size.
 In general, the greater the slump, the easier
it is for air to be entrained.
 Retarders: Set-retarding admixtures may
increase air content.

17. Chemical Admixture Testing and
Sampling
 Plant production samples
 Admixtures tested to comply with specifications
 Quality control procedures
 Uniformity

18. Water-Reducing Admixtures
 PURPOSE: Water
reducers are used to
reduce the quantity of
mixing water required
to produce concrete
of a given
consistency.

19. Accelerating Admixtures
 PURPOSE: Accelerating admixtures are added
to concrete to shorten time of set and accelerate
early-age strength development.
 Main Types…
 Chloride
 Non-Chloride
 Sub-Freezing

20. Accelerating Admixtures
 Reduce time of set
 Increase early-age strength / fast-track
 Reduce heating/protection time in cold
weather
 Permit earlier stripping/reuse of forms
 Facilitate earlier use of structure

21. Retarding Admixtures
 PURPOSE: Retarding admixtures are added to
concrete to offset acceleration and unwanted
effects of elevated temperatures to keep
concrete workable during placement and
consolidation.
 Benefits include…
 Long hauls
 (extended-set
control types)

22. Air-Entraining Admixtures
 PURPOSE: Air
entrainers are
primarily used to
stabilize tiny bubbles
in concrete during
mixing to provide
protection against
cyclic freezing and
thawing.
Also used to improve workability

23. Air-Voids in Hardened Concrete

24. Corrosion-Inhibiting Admixtures
 PURPOSE: Corrosion
Inhibitors are added
to concrete to
mitigate chloride-
induced corrosion of
embedded reinforcing
steel.
 Main Types…
 Calcium Nitrite
(inorganic)
 Amine & Esters
(organic)

25. Corrosion-Inhibiting Admixtures
 MECHANISMS
 Function by forming a protective oxide layer
at the steel surface, as a result of a chemical
reaction with ferrous ions.
 Example: Calcium Nitrite
 Function by forming a protective layer at the
steel surface AND by reducing chloride ion
ingress.
 Example: Amine/Ester-Based

26. Shrinkage-Reducing
Admixtures (SRAs)
 PURPOSE: Shrinkage reducers are used to
reduce the drying shrinkage of concrete.
 Benefits include…
 Reduced drying shrinkage
cracking
 Reduced curling in slabs
 Improved aesthetics
& water-tightness

27. Cost Benefits
 Job Specified  Construction Methods
Concrete  High Performance
 Cementitious Materials Concrete
 Concrete Proportions  High Volume
 Aggregates Placement
 Job placement/curing  Finishing and Forming
 Adjustment of Yield

28. Chemical Admixture Considerations
 Admixture has not been used before
 Special cementitious materials specified
 Dosage of multiple admixtures
 Specific mixing and placement
requirements

29. Method of Preparation and
Batching
 Chemical Admixtures Affect
 Properties
 Performance
 Uniformity
 Chemical Admixture Introduction
 Concrete Plant
 Truck-Mounted Tanks for Job-site

30. Batching and Discharging Systems
 Pumps
 Meters
 Timers
 Calibration Tubes
 Valves
 Required Tolerances

31. Chemical Admixture
Recommendations
 Ready-to-use
 Dosage Rate
 Standard Solutions (% Solids)
 Degree of Agitation or Recirculation
 Storage and Dispensing Systems
 Temperature Storage
 Solid
 Premeasured Containers
 Batched by mass

32. Dispensing Chemical Admixtures
 Timing of Batching Sequence
 Addition of dosage and mixing
 Addition of multi-admixtures
 Compatibility
 Affect of water demand
 Flocculation
 Accurate calibration and visual observation
 Plant operator training

33. Concrete Admixtures
Please return to Blackboard and watch the
following video:
 Video 1: Chemical Admixture Testing and
Sampling

34. Admixtures
Please return to Blackboard and watch the
following video:
 Video 1: Admixture Dispensers