Admixtures are classified by the following chemical and functional physical characteristics (Mamlouk, 2006)7:Air entrainers (ASTM C 260): produce tiny air bubbles in the hardened concrete to provide space for water to expand upon freezing.
Water reducers: increase the mobility of the cement particles in the plastic mix, allowing workability to be achieved at lower water contents.
Retarders: used to delay the initial set of concrete.
Hydration controller admixtures: stops and reactivates the hydration process of concrete allowing for extended use of ready-mixed concrete.
Accelerators (ASTM D 98): used to develop early strength of concrete at a faster rate than that developed in normal concrete.
Supplementary cementitious admixtures: byproducts of other industries that are used to improve some properties of concrete and to reduce the problem of discarding them. Fly Ash (ASTM C 618), Ground Granulated Blast Furnace Slag (ASTM 989, AASHTO M 302), Silica Fume (ASTM C 1240, AASHTO M 307), and Natural Pozzolans (ASTM C595) are common admixtures.
Specialty admixtures: several admixtures are available to improve concrete quality in different ways. These admixtures include but are not limited to workability agents, corrosion inhibitors, pumping aids, and bonding agents.
According to ASTM C33 coarse aggregate consists of gravel, crushed gravel, crushed stone, air-cooled blast furnace slag, crushed hydraulic-cement concrete, or a combination. The use of crushed hydraulic-cement concrete may require some additional precautions. Although it regularly produces satisfactory results; additional mixing water and the affects of freeze-thaw resistance and air void properties may need to be further tested.3
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