Civil Engineering Reference
In-Depth Information
CHAPTER 5
Aggregates for Concrete
The importance of using the right type and quality of
aggregates cannot be overemphasized. The fine and
coarse aggregates generally occupy 60% to 75% of the
concrete volume (70% to 85% by mass) and strongly influ-
ence the concrete's freshly mixed and hardened proper-
ties, mixture proportions, and economy. Fine aggregates
(Fig. 5-1) generally consist of natural sand or crushed
stone with most particles smaller than 5 mm (0.2 in.).
Coarse aggregates (Fig. 5-2) consist of one or a com-
bination of gravels or crushed stone with particles
predominantly larger than 5 mm (0.2 in.) and generally
between 9.5 mm and 37.5 mm (
3
⁄
8
in. and 1
1
⁄
2
in.). Some
natural aggregate deposits, called pit-run gravel, consist
of gravel and sand that can be readily used in concrete
after minimal processing. Natural gravel and sand are
usually dug or dredged from a pit, river, lake, or seabed.
Crushed stone is produced by crushing quarry rock, boul-
ders, cobbles, or large-size gravel. Crushed air-cooled
blast-furnace slag is also used as fine or coarse aggregate.
The aggregates are usually washed and graded at the
pit or plant. Some variation in the type, quality, cleanli-
ness, grading, moisture content, and other properties is
expected. Close to half of the coarse aggregates used in
portland cement concrete in North America are gravels;
most of the remainder are crushed stones.
Naturally occurring concrete aggregates are a mixture
of rocks and minerals (see Table 5-1). A mineral is a natu-
rally occurring solid substance with an orderly internal
structure and a chemical composition that ranges within
narrow limits. Rocks, which are classified as igneous, sedi-
mentary, or metamorphic, depending on origin, are gener-
ally composed of several minerals. For example, granite
contains quartz, feldspar, mica, and a few other minerals;
most limestones consist of calcite, dolomite, and minor
amounts of quartz, feldspar, and clay. Weathering and
erosion of rocks produce particles of stone, gravel, sand,
silt, and clay.
Recycled concrete, or crushed waste concrete, is a
feasible source of aggregates and an economic reality,
especially where good aggregates are scarce. Conven-
tional stone crushing equipment can be used, and new
equipment has been developed to reduce noise and dust.
Aggregates must conform to certain standards for
optimum engineering use: they must be clean, hard,
strong, durable particles free of absorbed chemicals, coat-
ings of clay, and other fine materials in amounts that could
affect hydration and bond of the cement paste. Aggregate
particles that are friable or capable of being split are unde-
sirable. Aggregates containing any appreciable amounts
of shale or other shaly rocks, soft and porous materials,
Fig. 5-1. Closeup of fine aggregate (sand). (69792)
Fig. 5-2. Coarse aggregate. Rounded gravel (left) and
crushed stone (right). (69791)
