As per IS:9103-1999, an admixture is defined as a material other than water, aggregates, hydraulic cement and additives like pozzolana or slag and fibre reinforcement used as an ingredient of concrete or mortar and added to the batch immediately before or during its mixing to modify one or more of the properties of concrete in the plastic or hardened state.
Admixtures are divided into five main categories as per IS 9103-1999. These are Accelerating, retarding, Water-reducing, Air entraining and Superplasticizing or High Range Water Reducing Admixtures (HRWRA). Superplasticisers are the most common admixtures used these days. Superplasticisers help in lowering the water content in the mix, thereby help reducing the water/cement ratio and contributing in enhancement of strength, durability of concrete. The super plasticising admixtures are particularly useful for high workability mixes or when the concrete is to be pumped. Slumps of 175 or more are easily attained by the use of super plasticisers.
The performance of concrete admixtures depend upon its compatibility with cement, mixing water, aggregate shape, silt present in finer component of concrete and to some extent cement chemistry.
Whenever admixtures are decided to be used for a project, it is necessary to evaluate the admixtures for specific use with the concrete making materials and mix proportions to be used for the work. Admixtures are also evaluated for conformity to the requirements of IS 9103-1999.
DIFFERENT TYPES OF ADMIXUTRES
ACCELERATORS
This admixture when added to concrete, mortar or grout increases the rate of hydration of hydraulic cement, shortens the time of set or increases the rate of hardening or strength development. They help in maintaining schedules during cold weather working, allow early removal of formwork e.g. in precast industry etc. These admixtures are particularly useful for shotcreting/guniting works for high early strengths and for reducing rebound loss. Triethanolamine, calcium nitrite, calcium formate, lithium oxalate and certain aluminates are some of the chemicals used for accelerating admixture. The use of calcium chloride in reinforced concrete is discouraged because it can promote corrosion of the reinforcement.
RETARDERS
Retarders slow down the rate of setting of cement. They assist in hot weather concreting and in casting and consolidating of large numbers of pours without the formation of cold joints. They do this by extending the vibration time. However, once the reaction has got through the setting stage the hydration continues at the normal rate and sometimes greater. Retarding admixtures are also useful for ready-mixed concrete industry as they can enhance the time of transportation of concrete. Materials comprising retarders are:
Unrefined ligno-sulphonates containing sugars.
Modified derivatives of the above.
Hydroxy-carboxylic acids and their salts.
Carbohydrates including sugars.
Heptonates which are related to the sugars and starches.
It is thought that retarding admixtures are absorbed on the C3A phase in cement forming a film around the cement grains and preventing or reducing the reaction with water. After a while this film breaks down and normal hydration proceeds. Retarders also interact with the C3S phase since retardation can be extended to a period of many days which cannot be accounted for by the C3A/ water reaction alone.
WATER REDUCERS
As per IS:9103-1999, water reducing admixtures are put in a separate category than superplasticizers. Water reducers reduce 5% to 6% water and superplasticizers are high range water reducers and reduce water more than 12%.
These admixtures can be utilised to perform the following functions:
Their addition to a concrete mix will result in increase in workability.
For a given workability the amount of water, i.e. w/c ratio can be reduced.
Cement reductions can be made without any detrimental effect on workability or strength.
These materials posses surface activity through which they are able to de-flocculate or disperse cement particles, resulting in increase in workability. The admixtures are usually based on either calcium or sodium salts of ligno-sulphonic acid or purified products.
AIR ENTRAINING ADMIXTURES
A normal air-entraining admixture (AEA) can be defined as any substance which can be utilised to intentionally entrain a controlled amount of air in the right form, into a concrete mix without significantly altering the setting or rate of hardening characteristics of the concrete. It is necessary to distinguish between deliberately entrained air and that trapped accidentally. Entrained air takes the form of discrete bubbles whose magnitude is of the order of 0.05 mm to 1.25 mm. The primary purpose of using an air-entraining admixture is for improved durability.
All AEA’s are surface-active agents which involve a physico-chemical process occurring at the surface of constituent materials in a system. Surfactants which will produce a stable foam with water and in concrete a stable dispersion of bubbles of the specified size and spacing. There are relatively few surfactants that will form stable foams in concrete but the following are the more commonly used:
Animal and vegetable fats and oils.
Natural wood resins and their sodium salts.
Alkali salts of sulphated and sulphonated organic compounds.
SUPER PLASTICIZERS
These belong to a new class of water reducers chemically different from the normal water reducers and capable of reducing water contents more than 12%.
These are broadly classified into four groups:
Sulphonated naphthalene-formaldehyde condensate.
Sulphonated melamine-formadehyde condensate.
Modified lignosulphonates.
Sulphonic-acid esters, carbohydrate esters, etc.
Superplasticized concrete permits placement in congested reinforcement. These are very useful for high workability mixes and for pumpable concretes. These also have a serious disadvantage, namely the large decrease of concrete flowability with time. Which is referred to as “slump loss”.
IS 9103-1999 covers two types of superplasticisers, ie. Normal type and retarding type.
Effect on properties of plastic concrete:
Admixtures are divided into five main categories as per IS 9103-1999. These are Accelerating, retarding, Water-reducing, Air entraining and Superplasticizing or High Range Water Reducing Admixtures (HRWRA). Superplasticisers are the most common admixtures used these days. Superplasticisers help in lowering the water content in the mix, thereby help reducing the water/cement ratio and contributing in enhancement of strength, durability of concrete. The super plasticising admixtures are particularly useful for high workability mixes or when the concrete is to be pumped. Slumps of 175 or more are easily attained by the use of super plasticisers.
The performance of concrete admixtures depend upon its compatibility with cement, mixing water, aggregate shape, silt present in finer component of concrete and to some extent cement chemistry.
Whenever admixtures are decided to be used for a project, it is necessary to evaluate the admixtures for specific use with the concrete making materials and mix proportions to be used for the work. Admixtures are also evaluated for conformity to the requirements of IS 9103-1999.
DIFFERENT TYPES OF ADMIXUTRES
ACCELERATORS
This admixture when added to concrete, mortar or grout increases the rate of hydration of hydraulic cement, shortens the time of set or increases the rate of hardening or strength development. They help in maintaining schedules during cold weather working, allow early removal of formwork e.g. in precast industry etc. These admixtures are particularly useful for shotcreting/guniting works for high early strengths and for reducing rebound loss. Triethanolamine, calcium nitrite, calcium formate, lithium oxalate and certain aluminates are some of the chemicals used for accelerating admixture. The use of calcium chloride in reinforced concrete is discouraged because it can promote corrosion of the reinforcement.
RETARDERS
Retarders slow down the rate of setting of cement. They assist in hot weather concreting and in casting and consolidating of large numbers of pours without the formation of cold joints. They do this by extending the vibration time. However, once the reaction has got through the setting stage the hydration continues at the normal rate and sometimes greater. Retarding admixtures are also useful for ready-mixed concrete industry as they can enhance the time of transportation of concrete. Materials comprising retarders are:
Unrefined ligno-sulphonates containing sugars.
Modified derivatives of the above.
Hydroxy-carboxylic acids and their salts.
Carbohydrates including sugars.
Heptonates which are related to the sugars and starches.
It is thought that retarding admixtures are absorbed on the C3A phase in cement forming a film around the cement grains and preventing or reducing the reaction with water. After a while this film breaks down and normal hydration proceeds. Retarders also interact with the C3S phase since retardation can be extended to a period of many days which cannot be accounted for by the C3A/ water reaction alone.
WATER REDUCERS
As per IS:9103-1999, water reducing admixtures are put in a separate category than superplasticizers. Water reducers reduce 5% to 6% water and superplasticizers are high range water reducers and reduce water more than 12%.
These admixtures can be utilised to perform the following functions:
Their addition to a concrete mix will result in increase in workability.
For a given workability the amount of water, i.e. w/c ratio can be reduced.
Cement reductions can be made without any detrimental effect on workability or strength.
These materials posses surface activity through which they are able to de-flocculate or disperse cement particles, resulting in increase in workability. The admixtures are usually based on either calcium or sodium salts of ligno-sulphonic acid or purified products.
AIR ENTRAINING ADMIXTURES
A normal air-entraining admixture (AEA) can be defined as any substance which can be utilised to intentionally entrain a controlled amount of air in the right form, into a concrete mix without significantly altering the setting or rate of hardening characteristics of the concrete. It is necessary to distinguish between deliberately entrained air and that trapped accidentally. Entrained air takes the form of discrete bubbles whose magnitude is of the order of 0.05 mm to 1.25 mm. The primary purpose of using an air-entraining admixture is for improved durability.
All AEA’s are surface-active agents which involve a physico-chemical process occurring at the surface of constituent materials in a system. Surfactants which will produce a stable foam with water and in concrete a stable dispersion of bubbles of the specified size and spacing. There are relatively few surfactants that will form stable foams in concrete but the following are the more commonly used:
Animal and vegetable fats and oils.
Natural wood resins and their sodium salts.
Alkali salts of sulphated and sulphonated organic compounds.
SUPER PLASTICIZERS
These belong to a new class of water reducers chemically different from the normal water reducers and capable of reducing water contents more than 12%.
These are broadly classified into four groups:
Sulphonated naphthalene-formaldehyde condensate.
Sulphonated melamine-formadehyde condensate.
Modified lignosulphonates.
Sulphonic-acid esters, carbohydrate esters, etc.
Superplasticized concrete permits placement in congested reinforcement. These are very useful for high workability mixes and for pumpable concretes. These also have a serious disadvantage, namely the large decrease of concrete flowability with time. Which is referred to as “slump loss”.
IS 9103-1999 covers two types of superplasticisers, ie. Normal type and retarding type.
Effect on properties of plastic concrete:
- Workability: The ability of the super plasticisers to increase the slump of concrete depends on the type, dosage and time of addition of the superplasticising, w/c ratio, nature and the amount of cement, aggregate and the temperature etc.
- Slump loss: Becuase of slump loss the superplasticizer has to be added at the point of discharge of concrete. If it has to be incorporated at the ready mix operations some methods have to be adopted to retain the slump. Slump loss depends on the type of superplasticisers and also on chemical and mineralogical composition of cement. However, beneficial effect can be obtained by using retarding type superplasticisers with respect to slump loss.
- Segregation and Bleeding: If superplasticiser is used as a water reducer no undue segregation or bleeding occurs. Bleeding and segregation may occur in flowing concrete if precautions are not taken. The mix must have sufficient fines.
- Air content: They entrain some air in concrete.
- Setting times: Generally superplasticizers retard the setting of concrete. The setting times may be retarded or accelerated when superplasticizers are used in combination with other admixtures.
- Strength: The increase in mechanical properties i.e. compressive strength and flexural strength and modulus of elasticity is generally commensurate with reductions in water-to cement ratio. The ability of superplasticizers to reduce water and achieve very high strengths is of special importance for the precast concrete industry where very high early strengths are needed.
- Shrinkage and creep: The shrinkage of superplasticized concrete is comparable to or is less than that of the reference concrete.
