AGGREGATES - Materials for Civil and Construction Engineers - page 161

present: calcareous (limestone, chalk, etc.), siliceous (chert, sandstone, etc.),
and argillaceous (shale, etc.).
Metamorphic rocks form from igneous or sedimentary rocks that are drawn
back into the earth's crust and exposed to heat and pressure, re-forming the
grain structure. Metamorphic rocks generally have a crystalline structure, with
grain sizes ranging from fine to coarse.
All three classes of rock are used successfully in civil engineering ap-
plications. The suitability of aggregates from a given source must be evalu-
ated by a combination of tests to check physical, chemical, and mechanical
properties, and must be supplemented by mineralogical examination. The
best possible prediction of aggregate suitability for a given application is
that based on historical performance in a similar design.

5.3 Evaluation of Aggregate Sources

Civil engineers select aggregates for their ability to meet specific project re-
quirements, rather than their geologic history. The physical and chemical
properties of the rocks determine the acceptability of an aggregate source for
a construction project. These characteristics vary within a quarry or gravel
pit, making it necessary to continually sample and test the materials as the
aggregates are being produced.
Due to the quantity of aggregates required for a typical civil engineering
application, the cost and availability of the aggregates are important when
selecting an aggregate source. Frequently, one of the primary challenges fac-
ing the materials engineer on a project is how to use the locally available ma-
terial in the most cost-effective manner.
Potential aggregate sources are usually evaluated for quality of the
larger pieces, the nature and amount of fine material, and the gradation of
the aggregate. The extent and quality of rock in the quarry is usually in-
vestigated by drilling cores and performing trial blasts (or shots) to evalu-
ate how the rock breaks and by crushing some materials in the laboratory
to evaluate grading, particle shape, soundness, durability, and amount of
fine material. Cores are examined petrographically for general quality,
suitability for various uses, and amount of deleterious materials. Potential
sand and gravel pits are evaluated by collecting samples and performing
sieve analysis tests. The amount of large gravel and cobble sizes deter-
mines the need for crushing, while the amount of fine material determines
the need for washing. Petrographic examinations evaluate the nature of
aggregate particles and the amount of deleterious material (Meininger and
Nichols 1990).
Price and availability are universal criteria that apply to all uses of ag-
gregates. However, the required aggregate characteristics depend on how
they will be used in the structure; they may be used as base material, in as-
phalt concrete, or in portland cement concrete.