6505-7 - Admixtures Definition in ASTM C125 An admixture is...

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1 Admixtures • Definition in ASTM C125: An admixture is a material other than water, aggregate, hydraulic cements and fiber reinforcement, used as an ingredient of concrete or mortar and added to the batch immediately before or during mixing. • About 71% of the concrete placed in the U.S. contains one or more admixtures. • Three categories of admixtures: (1) Surface-active chemicals - (A) Air-entraining (B) Water-reducing (2) Set-controlling chemicals - (A) Accelerating (B) Retarding (3) Mineral admixtures - (A) Blast-furnace slag (B) Fly ash (C) Silica fume (D) Natural Pozzolans Surface-Active Chemicals (or Admixtures) • Consist of long-chain organic molecules, with one end being hydrophobic and the other end being hydrophilic. • When the hydrophilic end contains non-polar groups, the chemicals are air-entraining admixtures. • When the hydrophilic end contains one or more polar groups, the chemicals are water-reducing admixtures. Air-Entraining Admixtures • Consist of salts of wood resins, proteinaceous materials and petroleum acids, and some synthetic detergents. • These surface-active chemicals make the cement surface to be hydrophobic so that air can displace water and remain attached to the cement particles as bubbles.
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2 Air-Entraining Admixtures • Effects: – Minute air bubbles, 10 to 100 m in diameter, are entrained in the cement paste. These air bubbles are not interconnected and are well distributed. • Properties of Air-Entrained Concrete: – Improved freeze-thaw resistance. – Improved resistance to deicers and salts (damage due to pressure caused by formation of salt crystals). – Improved sulfate resistance. – Improved resistance to alkali-silica reactivity. – Each percent of entrained air reduces the compressive strength of concrete by about 2 to 6%. – Improved workability of fresh concrete. Resistance to freezing and thawing increases as the entrained air content increases.
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3 Expansion due to alkali reactivity decreases as entrained air content increases. Air-entrained concrete after 20 years of exposure to calcium chloride solution Non-air-entrained concrete after 20 years of exposure to calcium chloride solution
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4 The water demand for the same workability of fresh concrete decreases as entrained air content increases. The required proportion of sand in concrete decreases as the entrained air content increases.
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5 ASTM C260 Air-Entraining Admixtures Note: The control samples are made using a reference admixture (neutralized Vinsol resin). Water-Reducing Admixtures
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This note was uploaded on 05/26/2011 for the course CGN 6505 taught by Professor Mang during the Spring '11 term at University of Florida.

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6505-7 - Admixtures Definition in ASTM C125 An admixture is...

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