What is the Concrete Admixtures Classified and the benefits

What is the Concrete Admixtures Classified and the benefits

Admixtures are added to cement, water, and aggregates either immediately before or during mixing. The primary reasons for using an admixture are: 
  • To reduce the cost of concrete construction 
  • To achieve certain properties in concrete more effectively than by other means 
  • To ensure the quality of concrete placed and cured in adverse weather conditions 
  • To overcome certain emergencies during concreting operations

Concrete admixtures may be classified by function as: 

  • Air-entraining 
  • Superplasticizing 
  • Water-reducing 
  • Finely divided mineral admixtures 
  • Set accelerating
  • Miscellaneous admixtures, including bonding, dampproofing, 
  • Set retarding coloring, corrosion inhibiting, and pumping admixtures
Although admixtures can enhance the quality of concrete, they are not a substitute for good concreting practice. Water-reducing admixtures reduce the quantity of mixing water used in the concrete by 5 to 10%, and are used to either lower the water-cement ratio or increase slump. Set retarding admixtures are used to slow the rate of concrete setting. In temperatures above 850F, concrete has an increased rate of hardening, which makes placing and finishing more difficult. 

An alternative to set retarding admixtures is to cool the mixing water or aggregates. Set accelerating admixtures increase concrete compressive strength at an early age. Other means of achieving this effect are to use Type III portland cement, add cement to lower the water-cement ratio, or cure the concrete at higher temperatures.

Calcium chloride is the most common ingredient in set accelerating admixtures. However, calcium chloride is not an antifreeze agent and does not take the place of cold weather precautions. Although non-chloride accelerators are available, they are not as effective as calcium chloride and are more expensive. 

The table below lists recommended maximum chloride ion content for corrosion protection. Calcium chloride is not recommended under the following conditions:
  • In prestressed concrete due to possible corrosion of embedded metals 
  • In concrete containing embedded aluminum (conduit, for example) due to possible corrosion 
  • In concrete subjected to alkali-aggregate reaction or exposed to soil or water containing sulfates 
  • In floor slabs intended to receive dry-shake metallic finishes 
  • In hot weather, generally 
  • In massive concrete placements
Superplasticizers are high-range water reducers added to concrete with low-to-normal slump and water- cement ratio to make high-slump, flowing concrete. Flowing concrete is defined as a highly fluid but workable concrete that can be placed with little or no vibration or compaction, but is still free of excessive bleeding or segregation. Superplasticizers are more effective, but more expensive, than regular water-reducing admixtures and are commonly used in the following applications: 
  • In thin section placements 
  • In areas of closely spaced and congested reinforcing steel
  • In underwater placements
  • In pumped concrete 
  • In areas where conventional consolidation methods are impractical

Concrete Admixtures by Classification

Type of Admixture
Desired Effect(s)
Common Ingredients
(ASTM C494, Type C)
Accelerate setting and early strength
calcium chloride (ASTM D98), Triethanolamine, sodium thiocyanate,
calcium formate, calcium nitrite, calcium nitrate
Decrease air content
Tributyl phosphate, dibutyl phthalate, octyl alcohol, waterinsoluble
esters of carbonic and boric acid, silicones
Air-entraining admixtures
(ASTM C260)
Improve durability in environments
of freeze-thaw, deicers, sulfate, and
alkali reactivity.
Improve workability.
Salts of wood resins (vinsol resins), some synthetic detergents,
salts of sulfonated lignin, salts of petroleum acids, salts of proteinaceous
material, fatty and resinous acids and their salts,
alkylbenzene sulfonates, salts of sulfonated hydrocarbons
Alkali-reactivity reducers
Reduce alkali-reactivity expansion
Pozzolans (fly ash, silica fume), blast-furnace slag, salts of lithium
and barium, air-entraining agents
Bonding admixtures
Increase bond strength
Rubber, polyvinyl chloride, polyvinyl acetate, acrylics, butadienestyrene
Coloring agents
Colored concrete
Modified carbon black, iron oxide, phthalocyanine, umber, chromium
oxide, titanium oxide, cobalt blue (ASTM C979
Corrosion inhibitors
Reduce steel corrosion activity in
chloride environments
Calcium nitrite, sodium nitrite, sodium benzoate, certain phosphates
or fluosilicates, fluoaluminates
Dampproofing admixtures
Retard moisture penetration into dry
Soaps of calcium or ammonium stearate or oleate, butyl
stearate, petroleum products
Fungicides, germicides,
and insecticides
Inhibit or control bacterial and fungal
Polyhalogenated phenols, dieldrin emulsions, copper compounds
Gas formers
Cause expansion before setting
Aluminum powder, resin soap and vegetable or animal glue,
saponin, hydrolized protein
Grouting agents
Adjust grout properties for specific
See air-entraining admixtures in Section 1.3.3, accelerators,
retarders, workability agents
Permeability reducers
Decrease permeability
Silica fume, fly ash (ASTM C618), ground slag (ASTM C989),
natural pozzolans, water reducers, latex
Silica fume, fly ash (ASTM C618), ground slag (ASTM C989),
natural pozzolans, water reducers, latex
Improve pumpability
Organic and synthetic polymers, organic flocculents, organic
emulsions of paraffin, coal tar, asphalt, acrylics, bentonite and
pyrogenic silicas, natural pozzolans (ASTM C618, Class N), fly
ash (ASTM C618, Classes F and C), hydrated lime (ASTM C14

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