| The Australian and New Zealand Standards AS 1478 and NZS 3113 Chemical Admixtures for Concrete define an admixture as a material, other than water, aggregate and cementitious materials, used as an ingredient of concrete, and added to the batch in controlled amounts immediately before or during its mixing to produce some desired modification to the properties of the concrete.
A wide variety of materials comply with this definition but all are not recognised as admixtures. Some are used to produce special types of concrete, e.g. pigments to produce coloured concrete, fibres in fibre concrete and polymers in polymer concrete. True admixtures are used to impart certain desired characteristics to the fresh or hardened concrete. Increased workability, acceleration or retardation of the rate of hydration of cements, and added resistance to freezing and thawing are examples of these effects.
The Correct Use of Admixtures
Admixtures fulfill an important role in the production of quality concretes. When used in a controlled environment, a high quality outcome is possible. Users of admixtures need to be familiar with the instructions for each product being used. The manufacturer's guidelines on admixture storage, dispensing and mixing procedures need to be followed closely. Failure to do so can lead to undesirable outcomes; two such examples being prolonged retardation of setting times, or excessive air entrainment.
Generally, admixtures are classified either by their characteristic effect on the concrete (e.g. set-retarding) or by the type of material or chemical which is the principal constituent (e.g. polysaccharides). Table 1 outlines the more common types of admixture used in concrete and indicates their relationship to one another.
||Water- reducing/ set- controlling
||Permeability reducing Durability enhance- ment
||Set- accelerating (Type Ac)
||Normal (Type WR)
||Water- reducing/ set- accelerating (Type WRAc)
||Water- reducing/ set- accelerating (Type WRAc)
||Set- retarding (Type Re)
||High-range water- reducing - super- plasticisers (Type HWR)
||Water- reducing/ set- accelerating (Type WRRe)
||High-range water- set- retarding - super- plasticisers (Type HWRRe)
Use of Admixtures
Table 2 summarises their fields of application and general effects on concrete. The wide range of materials used as admixtures and the continual development of new or modified products makes it impractical to list other than they main types met in practice. Many admixtures used in ready-mixed concrete are combinations of basic types.
Table 2. Admixtures for concrete
||To enhance freeze/ thaw resistance. To increase workability. To reduce bleedwater of harsh mixes.
||Produces a large number of small air bubbles in the concrete.
||Efficiency is reduced by increases in temperature, high cement content and by the presence of fly ash.
||For cold-weather concreting. To permit early finishing. To permit early formwork removal. To plug leaks against hydraulic pressure. To expedite completion of structure or repair.
||Shortens setting time. Increases early strength of concrete. May reduce long-term strength.
||Overdosing may lead to the flash set of concrete and to reduced ultimate strength. Those containing chloride ions have tendency to promote the corrosion of metals embedded in, or in contact with, the concrete.
||For hot weather concreting. To facilitate the use of delayed finishes. For mass concrete and slow pours. To eliminate cold joints. For long travel times to site.
||Delays setting of concrete. Reduces early strength of concrete (up to 7 days).
||Overdosing may lead to excessive retardation, delays in the development of concrete strength (in severe cases up to several days) and reduction in the ultimate strength.
|Water-reducing (WR, WRRe and WRAc)*
||To increase workability. To increase strength at same workability. A combination of the two above applications. To improve properties of concrete incorporationg poorly-graded aggregates.
||Disperses cement particles and increases the fluidity of the concrete. Reduces the water demand of the mix. May affect setting time (retard or accelerate) depending on the formulation of the admixture.
||The chloride content should be ascertained. Overdoes of lignosulphonates may cause excessive air entrainment with subsequent effect on strength and porosity.
|High range water reducing superplasticisers (HWR and HWRRe)
||To facilitate placing and compacting (e.g. in heavily reinforced members). To increase strength. For the provision of high-quality formed surfaces. To facilitate pumping.
||Increases the fluidity of the concrete with little effect on setting times.
||Compatibility with other admixtures in the mix should be checked. Retempering of the concrete more than once to restore slump may cause reductions in the ultimate strength.
||To facilitate pumping over greater distances at lower pressure. To improve lubrication and reduce segregation.
||Increase the visocosity of the cement paste.
||Will not convert unpumpable concrete into pumpable concrete - the mix must be designed specifically for pumping.
||To offset volume change (in concrete, mortar and grout). For grouting of anchor bolds, masonry prestressing ducts and prepacked-aggregate concrete. For bedding of machines and columns. For underpinning. To produce self-stressed concrete.
||Provides expansion of concrete, mortar or grout (in the plastic or hardened state).
||Excessive dosage of the admixture, or the presence of unsuitable combinations of admixtures, or of admixture and cement could generate excessive forces that disrupt the concrete, mortar or grout.
||To reduce transmission of moisture.
||Fills the pores with reactive, inert or water-repellent materials.
||Will not convert poor-quality concrete into water-tight concrete. Any reduction in premeability is mainly due to improved workability and, in turn, better workmanship.
|TWRE, WRC, NZ References.