What is Concrete and How can we Control quality of Concrete,
Why is concrete
an excellent building material
·
What is concrete
Concrete
is derived from the Latin word “concretus” meaning grown together or compounded.
Concrete
is basically a mixture of two
components: aggregates and paste. The
paste, comprised of portland cement and water, binds the aggregates (usually
sand and gravel or crushed stone) into a rock like mass as the paste hardens because
of the chemical reaction of the cement and water Supplementary cementitious
materials and chemical admixtures may also be included in the paste.
Aggregates
are generally divided into two groups: fine and coarse. Fine aggregates consist
of natural or manufactured sand with particle sizes ranging up to 9.5 mm (3⁄8 in.); coarse
aggregates are particles retained on the 1.18 mm (No. 16) sieve and ranging up
to 150 mm (6 in.) in size. The maximum size of coarse aggregate is typically 19
mm or 25 mm (3⁄4 in. or 1 in.).
An intermediate-sized aggregate, around 9.5 mm (3⁄8 in.), is sometimes added to
improve the overall aggregate gradation.
The
paste is composed of cementitious materials, water, and entrapped air or
purposely entrained air. The paste constitutes about 25% to 40% of the total volume
of concrete. Fig. 1-2 shows that the absolute volume of cement is usually
between 7% and 15% and the water between 14% and 21%. Air content in
air-entrained concrete ranges from about 4% to 8% of the volume. Since
aggregates make up about 60% to 75% of the total volume of concrete, their
selection is important. Aggregates should consist of particles with adequate strength
and resistance to exposure conditions and should not contain materials that
will cause deterioration of the concrete. A continuous gradation of aggregate
particle sizes is desirable for efficient use of the paste. Throughout this
text, it will be assumed that suitable aggregates are being used, except where
otherwise noted.
·
Concrete Quality
The
quality of the concrete depends upon the quality of the paste and aggregate,
and the bond between the two. In properly made concrete, each and every
particle of aggregate is completely coated with paste and all of the spaces
between aggregate particles are completely filled with paste, For any
particular set of materials and conditions of curing, the quality of hardened concrete
is strongly influenced by the amount of water used in relation to the amount of
cement Unnecessarily high water contents 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 increased water tightness
• 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 properly. Smaller amounts of mixing water result in stiffer
mixtures; but with vibration, stiffer mixtures can be easily placed. Thus,
consolidation by vibration permits improvement 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.
increase workability,
4.
intentionally entrain air,
5.
adjust other fresh or hardened
concrete properties.
·
Why is Concrete
an excellent building material
After
completion of proper proportioning, batching, mixing, placing, consolidating,
finishing, and curing, concrete hardens into a strong, noncombustible, durable,
abrasion-resistant, and watertight building material that requires little or no
maintenance.
Furthermore, concrete is an excellent building
material because it can be formed into
awide variety of shapes, colors, and textures for use in an unlimited number of
applications
For
More Information please download the attached file