Admixtures are materials added to the concrete during mixing for the purpose of altering the properties of the concrete mix.
Chemical and air-entraining admixtures including super plasticizers, solid or liquid or emulsion may be added to cement concrete at the time of mixing so as to achieve the desired property in concrete, in the plastic or hardened state.
Approval of admixtures shall be subjected to extensive trials to demonstrate the suitability, adequacy of dosing arrangements and performance.
The manufacturer or supplier shall furnish a series of at least ten trial mixes which clearly indicate that the use of the admixture has consistently manifested the required properties of concrete including absorption, permeability and poring values.
2. Type of admixtures
Mineral admixtures: A material other than water, aggregates, cement and additives like pozzolana or slag used as an ingredient of concrete 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.
Fiber reinforcement admixtures: Fibers shall serve as secondary reinforcement for concrete, and to protect concrete from stresses which cause cracking initially after placement.
Accelerating admixture or accelerator: An admixture when added to concrete, mortar or grout, increases the rate of hydration of cement, shortens the time of set, or increases the rate of hardening or strength development.
Retarding admixture or retarder: An admixture which delays the setting of cement paste, and hence of mixtures, such as mortar or concrete containing cement.
Water reducing admixture or workability aid: An admixture which eir increases workability of freshly mixed mortar or concrete without increasing water content or maintains workability with a reduced amount of water.
Air-entraining admixtures: An admixture for concrete or mortar which causes air to be incorporated in the form of minute bubbles in the concrete or mortar during mixing, usually to increase workability.
Super plasticizing admixtures: An admixture for mortar or concrete which imparts very high workability or allows a large decrease in water content for a given workability.
Retarding superplasticizing admixture: Superplasticizing admixture that imparts prolonged workability retention and retards setting.
Admixtures shall comply with the requirements of IS: 9103. In addition to the standard requirements for the approval of materials, approval of admixtures shall be subject to extensive trials to demonstrate the suitability, adequacy of dosing arrangements and performance, when a proven history of performance can be provided to the satisfaction of the Engineer.
Concrete made with admixtures when compared with identical concrete made without the admixture shall conform to the requirements appropriate to the type of admixture given in table 1A and table 1B of IS: 9103.
The methods of use and the quantities of admixture used shall be in accordance with the manufacturer’s instruction and subject to the Engineer’s approval after evaluation in trial mixes. Admixtures should not impair durability of concrete nor combine with the constituent to form harmful compounds nor increase the risk of corrosion of reinforcement.
The relative density of liquid admixtures shall be checked for each drum containing admixtures and compared with the specified value before acceptance.
If two or more admixtures are used simultaneously in the same concrete mix, data should be obtained to assess their interaction and to ensure their compatibility.
A report of the effects of accidental overdose of the admixture and the remedial measures to be taken if an overdose occurs shall be taken care.
No admixtures containing chlorides shall be used. In particular, the use of acceleration admixtures containing calcium chloride shall not be used.
The use of the admixtures shall be strictly controlled to ensure correct dosages as prescribed by the manufacturer and justified by trial mixes to be used. A calibrated dispenser or flow meter shall be used for the addition of the admixture.
1.IS: 6925-Methods of test for determination of water soluble chlorides inconcrete admixtures.