Construction World July 2020

PRODUCTS & SERVICE

ADMIXTURES NOW COMMON AS PART OF CONCRETE MIX

Admixtures have in the past three or four decades become increasingly popular as part of the concrete mix for a variety of reasons. Here Bryan Perrie, MD of The Concrete Institute, outlines the rationale for, and type of, admixtures most commonly found in the mix.

A dmixtures are chemicals added to the concrete mix to modify some of the properties of the mix but should never be regarded as a substitute for good mix design, sound workmanship, or use of the correct materials. The most common reasons for using admixtures in concrete are: • To increase workability without changing water content. • To reduce water content without changing workability. • To achieve a combination of the above. • To adjust setting time. • To reduce segregation and/or bleeding. • To improve pumpability. • To accelerate or retard the rate of strength development at early stages. • To improve potential durability and reduce permeability. • To reduce the total cost of the materials used in the concrete. • To compensate for poor aggregate properties. The various types of admixtures include: Plasticisers When added to a concrete mix, plasticisers (water-reducing agents) are absorbed on the surface of the binder particles, causing them WR UHSHO HDFK RWKHU DQG GH ͥRFFXODWH 7KLV UHVXOWV LQ LPSURYHG workability and provides a more even distribution of the binder particles through the mix. The main types of plasticisers are lignosulfonic acids and their salts, hydroxylated carboxylic acids and WKHLU VDOWV DQG PRGLͤFDWLRQV RI ERWK Plasticisers usually increase the slump of concrete with a given water content and can reduce the water requirement of a concrete mix for a given workability by about 10%. The addition of a plasticiser makes it possible to achieve a given strength with a lower cement content, and they may also improve pumpability. Superplasticisers These admixtures are chemically different from normal plasticisers DQG WKHLU DFWLRQ PRUH PDUNHG :KHQ XVHG WR SURGXFH ͥRZLQJ concrete, a rapid loss of workability can be expected and therefore

they should be added just prior to placing. Superplasticisers are usually chemical compounds such as sulphonated melamine IRUPDOGHK\GH VXOSKRQDWHG QDSKWKDOHQH IRUPDOGHK\GH PRGLͤHG lignosulphonates, and polycarboxylate-based materials. Air-entrainers An air-entraining agent introduces air in the form of minute bubbles distributed uniformly throughout the cement paste. The main types include salts of wood resins, animal or vegetable fats and oils and sulphonated hydrocarbons. Uses include where improved resistance RI KDUGHQHG FRQFUHWH WR GDPDJH IURP IUHH]LQJ DQG WKDZLQJ LV required; to add workability, especially in harsh or lean mixes; and to UHGXFH EOHHGLQJ DQG VHJUHJDWLRQ HVSHFLDOO\ ZKHQ D PL[ ODFNV ͤQHV Accelerators These admixtures speed up the chemical reaction of the cement and water and so accelerate the rate of setting and/or early gain in strength of concrete. Among the main types are chloride based, non- chloride based and shotcrete accelerators. Shotcrete accelerators react almost instantaneously, causing stiffening, rapid setting and hardening of the shotcrete. Accelerators’ uses include where rapid setting and high early strengths are required (e.g. in shaft sinking); where rapid turnover of moulds or formwork is needed; and where concreting takes place under very cold conditions. Retarders These admixtures slow the chemical reaction of the cement and water leading to longer setting times and slower initial strength gain. The most common retarders are hydroxylated carboxylic acids, lignins, sugar and some phosphates. They are mainly used when placing concrete in hot weather, particularly when the concrete is pumped; to prevent cold joints due to duration of placing; and for concrete that has to be transported for a long time. ƒ

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CONSTRUCTION WORLD JULY 2020