Page 16 - Masonry and Concrete

Understanding Concrete
15
UNDERSTANDING CONCRETE

bubbles in the concrete mix. When manufactured in this way, the
cements are said to be air entrained, and are designated as Types IA,
IIA, and IIIA. Air-entrained cements require mechanical mixing.
Finely ground cement increases the workability of harsh mixes,
making them more cohesive and reducing tendencies toward segrega-
tion. Coarsely ground cement reduces stickiness. Cement packages
that are marked ASTM A150 meet industry standards for both physi-
cal and chemical requirements.
Portland cement comes in three colors—grey, white, and buff.
The white and buff are more expensive and typically used in com-
mercial rather than residential projects to achieve special color
effects. Liquid or powder pigments can be added to a concrete mix,
and liquid stains can be used to color the surface of cured concrete,
but both will add to the cost. For most applications, ordinary gray
concrete made with gray cement is suitable. Colored concrete
should be reserved for special areas like a front entrance, a patio, or
a pool deck.
In the United States, portland cement is packaged in bags con-
taining exactly one cubic foot of material and weighing exactly 94
lbs. This standardized packaging, which all American manufactur-
ers use, allows consistency in proportioning and mixing concrete
by either weight or volume measurement. Bags should be stored on
wooden pallets and covered to prevent wetting. Portland cement
must remain dry and free-flowing until it is ready for use. If the
bags get wet or absorb moisture from the soil or from a concrete
slab, the cement will begin to harden prematurely and will produce
weak, slow-curing concrete. Hard lumps which cannot be easily
pulverized by hand indicate excessive wetting, and the cement
should be discarded or used only for minor work such as setting
fence posts.
Packaged concrete mixes contain cement, sand, and gravel in
appropriate proportions and require only the addition of water to pro-
duce fresh concrete. These packaged mixes, marketed under a variety
of trade names, are very convenient for small items like setting a sin-
gle mailbox post or doing minor repairs. The most commonly available
1
sizes are 40-, 60-, and 80-lb. bags. The 40-lb. bag makes about /3 cu. ft.
1
of concrete. A 60-lb. bag makes about /2 cu. ft., and an 80-lb. bag about
2 /3 cu. ft. of concrete.

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