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
Accelerators (ASTM C494, Type C)	Accelerate setting and early strength development	calcium chloride (ASTM D98), Triethanolamine, sodium thiocyanate, calcium formate, calcium nitrite, calcium nitrate
Airdetrainers	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 copolymers
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 concrete	Soaps of calcium or ammonium stearate or oleate, butyl stearate, petroleum products
Fungicides, germicides, and insecticides	Inhibit or control bacterial and fungal growth	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 applications	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