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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 influence the concrete’s freshly mixed and hardened properties, mixture proportions, and economy. Fine aggregates
(Fig. 5-1) generallyconsist 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 11⁄2 in.). Some
natural aggregate deposits, called pit-run gravel, consist
of gravel and sand thatcan 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, boulders, 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. Somevariation in the type, quality, cleanliness, 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 naturally occurring solid substance with anorderly internal
structure and a chemical composition that ranges within
narrow limits. Rocks, which are classified as igneous, sedimentary, or metamorphic, depending on origin, are generally 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. Conventional stone crushing equipment can be used, and new
equipment has been developed to reduce noise and dust.
Aggregates must conform to certainstandards for
optimum engineering use: they must be clean, hard,
strong, durable particles free of absorbed chemicals, coatings 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 undesirable. Aggregates containing any appreciable amounts
of shale or other shaly rocks, soft andporous materials,

Fig. 5-1. Closeup of fine aggregate (sand). (69792)

Video

Fig. 5-2. Coarse aggregate. Rounded gravel (left) and
crushed stone (right). (69791)
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Design and Control of Concrete Mixtures

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should be avoided; certain types of chert should be especially avoided since they have low resistance to weathering and can cause surface defects such as popouts.Identification of the constituents of an aggregate
cannot alone provide a basis for predicting the behavior of
aggregates in service. Visual inspection will often disclose
weaknesses in coarse aggregates. Service records are
invaluable in evaluating aggregates. In the absence of a
performance record, the aggregates should be tested
before they are used in concrete. The most commonly usedaggregates—sand, gravel, crushed stone, and air-cooled
blast-furnace slag—produce freshly mixed normal-weight
concrete with a density (unit weight) of 2200 to 2400
kg/m3 (140 to 150 lb/ft3). Aggregates of expanded shale,
clay, slate, and slag (Fig. 5-3) are used to produce structural lightweight concrete with a freshly mixed density
ranging from about 1350 to 1850 kg/m3 (90 to 120 lb/ft3)....