Cement and Concrete

SOURCE; Portland Concrete Association website

Concrete Basics

Concrete is a mixture of two components: aggregates and paste. The paste, comprised of cement and water, binds the aggregates (usually sand and gravel or crushed stone) into a rocklike mass as the paste hardens because of the chemical reaction of the cement and water. Supplementary cemen tious materials and chemical admixtures may also be included in the paste.

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What are the unit weights (densities) of cement and concrete?

A: Cement: Cement (finely ground gray or white powder used to bind concrete mixtures) weighs between 830 kg/m3 and 1650 kg/m3 (52 lbs/ft3 and 103 lbs/ft3) depending on its handling. The weight of cement that has been pneumatically loaded into a cement silo may be as low as 830 kg/m3 (52 lbs/ft3), while cement that has been stored for a period of time exposed to vibration may be as heavy as 1650 kg/m3 (103 lbs/ft3). It is standard practice to consider a 94 lb bag of cement to be one cubic foot when freshly packed.

|[pic] |Both 500-mL beakers contain 500 grams of dry powdered cement. |

| |On the left, cement was simply poured into the beaker. On the |

| |right, cement was slightly vibrated—imitating consolidation |

| |during transport or packing while stored in a silo. The 20% |

| |difference in bulk volume demonstrates the need to measure |

| |cement by mass instead of volume |

| |for batching concrete. |

Concrete: Concrete is a mixture of cement, coarse and fine aggregates, water, and sometimes supplementary cementing materials and/or chemical admixtures. A normal weight concrete weighs approximately 2300 kg/m3 (145 lbs/ft3). The unit weight (density)...