AGGREGATES - Materials for Civil and Construction Engineers - page 167

FIGURE 5.6 Los Angles abrasion machine.

The Los Angeles abrasion test (ASTM C131, C535) evaluates the aggre-
gates' toughness and abrasion resistance. In this test, aggregates blended to
a fixed size distribution are placed in a large steel drum with standard sized
steel balls that act as an abrasive charge (see Figure 5.6). The drum is rotated,
typically for 500 revolutions. The material is recovered from the machine
and passed through a sieve that retains all of the original material. The per-
centage weight loss is the LA abrasion number. This is an empirical test; that
is, the test results do not have a scientific basis and are meaningful only
when local experience defines the acceptance criteria.

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5.5.4 Absorption

Although aggregates are inert, they can capture water and asphalt binder in
surface voids. The amount of water the aggregates absorb is important in the
design of portland cement concrete, since moisture captured in the aggregate
voids is not available to improve the workability of the plastic concrete and
to react with the cement. There is no specific level of aggregate absorption
that is desirable for aggregates used in portland cement concrete, but aggre-
gate absorption must be evaluated to determine the appropriate amount of
water to mix into the concrete.
Absorption is also important for asphalt concrete, since absorbed as-
phalt is not available to act as a binder. Thus, highly absorptive aggregates
require greater amounts of asphalt binder, making the mix less economical.