For general work gravel or broken stone always is preferred to other
coarse aggregate. Other materials at times are easier to obtain and, when
used with discretion, will provide a satisfactory concrete.
Broken terra cotta, brick, and old concrete, if hard and strong, may
be used for unimportant work where no great strength is required, but
special care should be taken that the particles do not show on the
finished surface.
The maxim that a chain is only as strong as its weakest link applies
to concrete. If the coarse aggregate is weaker than the cement mortar,
as in the case of some sandstones, it should be used with caution.
The aggregate may have properties that render it unsuitable for use
under certain conditions; for instance, cinders should not be used
if water-tightness or strength is expected, but they are useful for
fireproofing. Material that disintegrates or flakes when heated is
undesirable in places exposed to high temperature; thus marble and some
limestones should not be used in fireplaces. Some aggregates when exposed
at the surface of concrete are apt to cause discolorations, and when this
would be objectionable aggregates of this type should be avoided. Flat
or elongated slab-like fragments should be avoided, as particles of this
shape do not bond well; slate and shale are examples.