Civil Engineering Reference
In-Depth Information
Fig. 1-3. Cross section of hardened concrete made with (left) rounded siliceous gravel and (right) crushed limestone. Cement-
and-water paste completely coats each aggregate particle and fills all spaces between particles. (1051, 1052)
For any particular set of materials and conditions of
curing, the quality of hardened concrete is strongly influ-
enced by the amount of water used in relation to the
amount of cement (Fig. 1-4). Unnecessarily high water con-
tents dilute the cement paste (the glue of concrete).
Following are some advantages of reducing water content:
• Increased compressive and flexural strength
• Lower permeability, thus lower absorption and in-
creased watertightness
• Increased resistance to weathering
• Better bond between concrete and reinforcement
• Reduced drying shrinkage and cracking
• Less volume change from wetting and drying
The less water used, the better the quality of the
concrete—provided the mixture can be consolidated prop-
erly. Smaller amounts of mixing water result in stiffer
mixtures; but with vibration, stiffer mixtures can be easily
placed. Thus, consolidation by vibration permits improve-
ment in the quality of concrete.
The freshly mixed (plastic) and hardened properties of
concrete may be changed by adding chemical admixtures
to the concrete, usually in liquid form, during batching.
Chemical admixtures are commonly used to (1) adjust
setting time or hardening, (2) reduce water demand, (3) in-
crease workability, (4) intentionally entrain air, and (5) ad-
just other fresh or hardened concrete properties.
After completion of proper proportioning, batching,
mixing, placing, consolidating, finishing, and curing, con-
crete hardens into a strong, noncombustible, durable, abra-
sion-resistant, and watertight building material that re-
quires little or no maintenance. Furthermore, concrete is an
excellent building material because it can be formed into a
wide variety of shapes, colors, and textures for use in an
unlimited number of applications.
FRESHLY MIXED CONCRETE
Freshly mixed concrete should be plastic or semifluid and
generally capable of being molded by hand. A very wet
concrete mixture can be molded in the sense that it can be
cast in a mold, but this is not within the definition of “plas-
tic”—that which is pliable and capable of being molded or
shaped like a lump of modeling clay.
In a plastic concrete mixture all grains of sand and
pieces of gravel or stone are encased and held in suspen-
sion. The ingredients are not apt to segregate during trans-
port; and when the concrete hardens, it becomes a
homogeneous mixture of all the components. During plac-
ing, concrete of plastic consistency does not crumble but
flows sluggishly without segregation.
In construction practice, thin concrete members and
heavily reinforced concrete members require workable, but
never soupy, mixes for ease of placement. A plastic mixture
is required for strength and for maintaining homogeneity
during handling and placement. While a plastic mixture is
suitable for most concrete work, plasticizing admixtures
may be used to make concrete more flowable in thin or
heavily reinforced concrete members.
Fig. 1-4. Ten cement-paste cylinders with water-cement
ratios from 0.25 to 0.70. The band indicates that each
cylinder contains the same amount of cement. Increased
water dilutes the effect of the cement paste, increasing
volume, reducing density, and lowering strength. (1071)
