Page 17 - Masonry and Concrete

Understanding Concrete
16 CHAPTER TWO

2.2.2 Aggregates
The aggregates most commonly used in concrete are sand, gravel,
crushed stone, crushed slag, and pumice. Cement and water are mixed
with aggregates to produce concrete. Concrete contains both fine and
coarse aggregates. When cement is mixed only with fine aggregate, it is
called cement mortar, which is used typically for patching and small
repairs, or for coating a concrete surface to provide a smooth, even fin-
ish. Masonry mortar is different from a simple cement mortar because
it contains other ingredients as well (see Chapter 3).
Cement paste coats the aggregates, binding them together and cur-
ing to form concrete. Aggregates add strength to concrete and reduce
its potential for shrinkage. Aggregates actually make up 60 to 80% of
the volume of hardened concrete, so their properties and characteris-
tics are very important.
The coarse aggregates most commonly used in residential concrete
are gravel and crushed stone. Aggregates must be sound, volume stable,
nonreactive, abrasion resistant, suitably shaped, rough textured, well
graded, and clean. Each characteristic of the aggregate has an effect on
the resulting concrete (Figure 2-5). Unsound aggregates produce
unsound concrete which is weak, has poor appearance, low durability,
and may experience cracking, popouts, and spalling. Chemical reactiv-
ity, especially with the alkalis in cement, causes internal expansion,
cracking, and disintegration of the concrete. Low abrasion resistance
results in low strength and excessive wear in floors and pavements.
Particle shape affects workability, and surface texture affects bond of
the cement paste to the aggregate. Aggregate that is too absorptive pro-
duces concrete that has low durability and may suffer from scaling,
popouts, and excessive shrinkage. Dirty or contaminated aggregate
bonds poorly with the cement paste, can increase mixing water require-
ments, delay setting and hardening of the concrete, cause stains or
popouts, lower strength and durability, and increase shrinkage.
Gravel generally has smoother, more rounded shapes than crushed
stone and thus produces concrete with better workability. The worka-
bility of concrete made with crushed stone, however, can be improved
by air entrainment. Regardless of the type of aggregate or its particle
shape, coarse aggregate should include a well-graded range of sizes
from small (1/4 in.) to large (3/4, 1, or 1-1/2 in.). Smaller particles fill
in the spaces between the larger ones, making the mix both stronger

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