Admixtures belong to some ingredients in concrete or mortar except for hydraulic cement (OPC), water& aggregate and mineral additives like Pozzolana or slag and fiber reinforcement which are included with the concrete mix promptly before or throughout its to alter one or more definite properties of concrete in plastic or hardened condition to attain the required workability when water cement ratio is low as well as improve the setting time of concrete for long distance conveyance of concrete.
Concrete admixtures are used to modify the properties of concrete to achieve desired workability in case of low water cement ratio, and to enhance setting time of concrete for long distance transportation of concrete.
Therefore, a site civil engineer should be well versed with the properties of admixtures to choose admixtures in an efficient manner and use them properly in concrete works.
The admixtures mainly alter the following properties of concrete
Rate of hydration
There exist two elementary types of admixtures which range from chemical & mineral. Admixtures like flyash, silicate fume, slag falls in the category of mineral admixtures. They are included with the concrete to improve the workability, resistance strength to thermal cracking and alkali–aggregate reaction as well as allow reduction in cement content.
The functionality of an admixture is based on various factors including type and quantity of cement, water content, mixing time, slump and temperatures of the concrete and air.
Admixtures are used for the following purposes:
To decrease the cost of concrete construction.
To modify the performance of hardened concrete.
To attain specific properties in concrete more efficiently.
To retain the quality of concrete throughout different stages of mixing, transporting, placing, compacting and curing in unfavorable climatic conditions.
To speed up or delay the initial set of concrete.
To make the flowability & pumpability of concrete.
To minimize the segregation in concrete.
To improve the strength of concrete with reduction in the water content.
To enhance the longevity of concrete.
To raise the impermeability of concrete.
To manage alkali-aggregate reaction.
To lessen the corrosion of reinforcement in concrete.
To raise the resistance capacity against chemical attack.
To lessen the heat of hydration.
To make the bond superior among concrete & steel.
To make the bond better among old and new concrete surfaces.
To develop cellular concrete, colored concrete.
To form non-skid wearing surface.
To reduce the unit weight of concrete.
To form concrete of fungicidal, germicidal and insecticidal properties.