maximum strength and durability will still not be
attained unless the sand and coarse aggregate you use
consist of well-graded, clean, hard, and durable
particles free of undesirable substances (figure 6-1).
WATERTIGHTNESS OF CONCRETE
The ideal concrete mix is one with just enough
water required for complete hydration of the cement.
However, this results in a mix too stiff to pour in
forms. A mix fluid enough to be poured in forms
always contains a certain amount of water over and
above that which will combine with the cement. This
water eventually evaporates, leaving voids, or pores,
in the concrete. Penetration of the concrete by water
is still impossible if these voids are not inter-
connected. They may be interconnected, however, as
a result of slight sinking of solid particles in the mix
during the hardening period. As these particles sink,
they leave water-tilled channels that become voids
when the water evaporates. The larger and more
numerous these voids are, the more the watertightness
of the concrete is impaired. The size and number of
the voids vary directly with the amount of water used
in excess of the amount required to hydrate the
To keep the concrete as watertight as
possible, you must not use more water than the
minimum amount required to attain the necessary
degree of workability.
GENERAL REQUIREMENTS FOR GOOD
The first requirement for good concrete is to use a
cement type suitable for the work at hand and have a
satisfactory supply of sand, coarse aggregate, and
water. Everything else being equal, the mix with the
best graded, strongest, best shaped, and cleanest
aggregate makes the strongest and most durable
Second, the amount of cement, sand, coarse
aggregate, and water required for each batch must be
carefully weighed or measured according to project
Third, even the best designed, best graded, and
highest quality mix does not make good concrete if it
is not workable enough to fill the form spaces
thoroughly. On the other hand, too much fluidity also
results in defects. Also, improper handling during the
overall concrete making process, from the initial
aggregate handling to the final placement of the mix,
causes segregation of aggregate particles by sizes,
resulting in nonuniform, poor-quality concrete.
Finally, the best designed, best graded, highest
quality, and best placed mix does not produce good
concrete if it is not proper] y cured, that is, properly
protected against loss of moisture during the earlier
stages of setting.
LEARNING OBJECTIVE: Upon completing
this section, you should be able to identify the
ingredients essential for good concrete.
The essential ingredients of concrete are cement,
aggregate, and water. A mixture of only cement and
water is called cement paste. In large quantities,
however, cement paste is prohibitively expensive for
most construction purposes.
Most cement used today is portland cement. This
is a carefully proportioned and specially processed
combination of lime, silica, iron oxide, and alumina.
It is usually manufactured from limestone mixed with
shale, clay, or marl.
Properly proportioned raw
materials are pulverized and fed into kilns where they
are heated to a temperature of 2,700°F and maintained
at that temperature for a specific time. The heat
produces chemical changes in the mixture and
transforms it into clinkera hard mass of fused clay
and limestone. The clinker is then ground to a
fineness that will pass through a sieve containing
40,000 openings per square inch.
Types of Cement
There are five types of Portland cement covered
under Standard Specifications for Portland Cement.
These specifications are governed by the American
Society for Testing and Material (ASTM) types.
Separate specifications, such as those required for
air-entraining portland cements, are found under a
separate ASTM. The type of construction, chemical
composition of the soil, economy, and requirements
for use of the finished concrete are factors that
influence the selection of the kind of cement to be
TYPE I. Type I cement is a general-purpose
cement for concrete that does not require any of the
special properties of the other types. In general, type
I cement is intended for concrete that is not subjected