to deterioration and structural failure of the concrete.
Alkalis, acids, and sulfates in the water tend to react with
the chemicals in the cement. The result is inadequate
cementing and weakened concrete. Water must be free
of these chemicals to be used in concrete mixing.
SEAWATER.— The salts in seawater are normally
thought of as being corrosive; however, seawater is used
sometimes in concrete mixing with satisfactory results.
A loss of 10 to 20 percent in compressive strength can
be expected when the same amount of seawater as fresh
water is used. That can be compensated somewhat by
reducing the water-cement ratio.
Aggregates
The aggregates normally used for concrete are
natural deposits of sand and gravel, where available. In
some localities, the deposits are hard to obtain and large
rocks must be crushed to form the aggregate. Crushed
aggregate usually costs more to produce and will require
more cement paste because of its shape. More care must
be used in handling crushed aggregate to prevent poor
mixtures and improper dispersion of the sizes through
the finished concrete. At times, artificial aggregates,
such as blast-furnace slag or specially burned clay, are
used.
TYPES OF AGGREGATE.— Aggregates are
divided into two types as follows:
. FINE AGGREGATE. “Fine aggregate” is
defined as material that will pass a No. 4 sieve and will,
for the most part, be retained on a No. 200 sieve. For
increased workability and for economy as reflected by
use of less cement, the fine aggregate should have a
rounded shape. The purpose of the fine aggregate is to
fill the voids in the coarse aggregate and to act as a
workability agent.
. COARSE AGGREGATE. Coarse aggregate is a
material that will pass the 3-inch screen and will be
retained on the No. 4 sieve. As with fine aggregate, for
increased workability and economy as reflected by the
use of less cement, the coarse aggregate should have a
rounded shape. Even though the definition seems to
limit the size of coarse aggregate, other considerations
must be accounted for.
When properly proportioned and mixed with
cement, these two groups yield an almost voidless stone
that is strong and durable. In strength and durability,
aggregate must be equal to or better than the hardened
cement to withstand the designed loads and the effects
of weathering.
It can be readily seen that the coarser the aggregate,
the more economical the mix. Larger pieces offer less
surface area of the particles than an equivalent volume
of small pieces. Use of the largest permissible maximum
size of coarse aggregate permits a reduction in cement
and water requirements.
One restriction usually assigned to coarse aggregate
is its maximum size. Larger pieces can interlock and
form arches or obstructions within a concrete form. That
allows the area below to become a void, or at best, to
become filled with finer particles of sand and cement
only. That results in either a weakened area or a
cement-sand concentration that does not leave the
proper proportion to coat the rest of the aggregate. The
maximum size of coarse aggregate must be no larger
than the sizes given in table 13-1. The capacity of the
mixing equipment may also limit the maximum
aggregate size.
GRADATION.— Gradation of aggregate refers to
the amount of each size of particle used in the mix. Too
large a proportion of coarse aggregate leaves voids that
require more cement paste to fill. That affects the
Table 13-1.—Maximum Recommended Size of Coarse Aggregate
13-21
