The Constructor

Properties and Uses of Different Types of Concrete Admixtures

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What are Concrete Admixtures?

Ingredients other than cement, water, and aggregates that impart a specific quality to either plastic (fresh) mix or the hardened concrete (ASTM C496) is called concrete admixture.

Why use Concrete Admixtures?

  1. Reduce the cost of concrete construction
  2. Achieve specific concrete properties more effectively
  3. Ensure quality of concrete during mixing, transporting, placing, and curing in adverse weather condition
  4. Overcome emergencies during operations

Classification of Concrete Admixtures:

  1. Air entrainers
  2. Water reducers
  3. High-range water reducers-superplasticizers
  4. Retarders
  5. Accelerators
  6. Fine minerals
  7. Specialty admixtures

Air entraining concrete admixture:

Air entraining concrete admixtures produce tiny air bubbles in the hardened concrete to provide space for the water to expand upon freezing.

How do they work?

They are anionic (water-hating) agents that form tough, elastic, air-filled bubbles. These bubbles reduce stresses caused by movement or freezing of water. They provide more volume for expansion and shorter flow path.

Benefits of air-entraining admixture:

Difficulties: Composition of Air Entrainers: Usually liquid meets ASTM C260 specifications.

Water Reducing concrete admixture:

Water reducers can result in 3 things:
  1. Increased slump at constant w/c
  2. Increased strength, by lowering the water content
  3. Reduced cost of the cement
How do they do this? Water reducers increase the mobility of the cement particles in the plastic mix, allowing the same workability to be achieved at lower water contents.


Superplasticizers are "high-range" water reducers. Superplasticizers are used when placing:
  1. Thin sections or around tightly spaced reinforcing steel
  2. concrete underwater
  3. Concrete by pumping
  4. consolidating the concrete is difficult
Note: When superplasticizers are used, the fresh concrete stays workable for only a short period of time (30 min to 60 min), which is why they are usually added at the site

Retarding concrete admixture:

Used to delay the initial set of concrete. Why do we use them?
  1. To offset the effect of hot weather
  2. Allow for unusual placement or long haul distances
  3. Provide time for special finishes
Possible adverse effects of retarders 1. Reduce early age strength 2. Reduction of time between initial and final set Possible advantages of retarders
  1. Air entrainment
  2. Increased workability
  3. Reduction of time between initial and final set
Note: The use of retarders must be evaluated experimentally before incorporation in mix design

Accelerating concrete admixture:

Used to reduce the time required to develop final strength characteristics in concrete Possible reasons for using accelerators:
  1. Reduce the amount of time before finishing operations begin
  2. Reduce curing time
  3. Increase rate of strength gain
  4. Plug leaks under hydraulic pressure efficiently
  5. Offset effect of cold weather
Calcium Chloride is the most widely used accelerator. Initial and final set times reduced
CaCl2 by weight Initial Set Time in Hrs.
0% 6
1% 3
2% 2
The PCA (Portland Cement Association) recommends against using calcium chloride when:
  1. Concrete is prestressed
  2. Concrete contains embedded aluminum such as conduits
  3. Concrete is subjected to alkali-aggregate reaction
  4. Concrete is in contact with water or soils containing sulfates
  5. Concrete is placed during hot weather
  6. Mass application of concrete
Alternatives to CaCl2
  1. High early strength cement (type III)
  2. Increase cement content
  3. cure at higher temperature (if feasible)
  4. Triethanolamine, sodium thiocyanate, calcium formate, calcium nitrite or calcium nitrate

Fine Minerals as concrete admixtures:

Fine mineral admixtures added in large amounts (20% to 100% of cement weight) to improve the characteristics of plastic and hardened concrete. Classification based on chemical and physical properties

1. Cementitious

2. Pozzolanic

Classification of fly ash and natural pozzolans

Fly ash is the most commonly used pozzolan in civil engineering structures

Special concrete admixtures:

Other admixtures are available to improve concrete quality in a number of ways, such as workability agents, bonding agents, etc. Table: Effect of Mineral Admixtures on Fresh Concrete
Quality Measure Effect
Water Requirements Fly ash reduces water requirements Silica fume increases water requirements
Air Content Fly ash and silica fume reduce air content It can be avoided by increasing air-entrainer.
Workability Fly ash, ground slag, and inert materials generally increase workability. Silica fume reduces workability; compensates by using super-plasticizers.
Hydration Fly ash reduces the heat of hydration. Silica fume may not affect, but superplasticizers used with silica fume can increase heat.
Set Time Fly ash, natural pozzolans, and blast furnace slag increase set time; Can compensate by using the accelerator.
Table: Effect of Mineral Admixtures on Hardened Concrete
Quality Measure Effect
Strength Fly ash increases the ultimate strength but reduces rate of strength gain. Silica fume has less effect on rate of strength gain than pozzolans.
Drying shrinkage and creep Low concentrations usually have single effect. High concentrations of ground slag or fly ash may increase shrinkage. Silica fume may reduce shrinkage.
Permeability and absorption Generally reduced permeability and absorption. Silica fume is especially effective.
Alkali-aggregate reactivity Generally reduced reactivity, extend of improvement depends on type of admixture.
Sulfate resistance Improved due to reduced permeability.
Read More: Concrete Admixtures – Types, Selection, Properties and Applications Different Types of Concrete Curing Compounds, their Properties and Uses Factors Affecting Concrete Admixtures Performance Anti-Washout Admixture for Underwater Concreting -Types, Advantages and Uses
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