Aggregates for Concrete - Design and Control of Concrete Mixtures - page 80

Table 5-1. Rock and Mineral Constituents in
Aggregates

should be avoided; certain types of chert should be espe-
cially avoided since they have low resistance to weather-
ing 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 used
aggregates—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/m 3 (140 to 150 lb/ft 3 ). Aggregates of expanded shale,
clay, slate, and slag (Fig. 5-3) are used to produce struc-
tural lightweight concrete with a freshly mixed density
ranging from about 1350 to 1850 kg/m 3 (90 to 120 lb/ft 3 ).
Other lightweight materials such as pumice, scoria,
perlite, vermiculite, and diatomite are used to produce
insulating lightweight concretes ranging in density from
about 250 to 1450 kg/m 3 (15 to 90 lb/ft 3 ). Heavyweight
materials such as barite, limonite, magnetite, ilmenite,
hematite, iron, and steel punchings or shot are used to
produce heavyweight concrete and radiation-shielding
concrete (ASTM C 637 and C 638). Only normal-weight
aggregates are discussed in this chapter. See Chapter 18
for special types of aggregates and concretes.
Normal-weight aggregates should meet the require-
ments of ASTM C 33 or AASHTO M 6/M 80. These speci-
fications limit the permissible amounts of deleterious
substances and provide requirements for aggregate char-
acteristics. Compliance is determined by using one or
more of the several standard tests cited in the following
sections and tables. However, the fact that aggregates
satisfy ASTM C 33 or AASHTO M 6/M 80 requirements
does not necessarily assure defect-free concrete.
For adequate consolidation of concrete, the desirable
amount of air, water, cement, and fine aggregate (that is,
the mortar fraction) should be about 50% to 65% by
absolute volume (45% to 60% by mass). Rounded aggre-
gate, such as gravel, requires slightly lower values, while
crushed aggregate requires slightly higher values. Fine
aggregate content is usually 35% to 45% by mass or
volume of the total aggregate content.

Minerals Igneous rocks Metamorphic rocks

Silica Granite Marble

Quartz Syenite Metaquartzite

Opal Diorite Slate

Chalcedony Gabbro Phyllite

Tridymite Peridotite Schist

Cristobalite Pegmatite Amphibolite

Silicates Volcanic glass Hornfels

Feldspars Obsidian Gneiss

Ferromagnesian Pumice Serpentinite

Hornblende Tuff

Augite Scoria

Clay Perlite

Illites Pitchstone

Kaolins Felsite

Chlorites Basalt

Montmorillonites
Mica

Sedimentary rocks

Zeolite Conglomerate

Carbonate Sandstone

Calcite Quartzite

Dolomite Graywacke

Sulfate Subgraywacke

Gypsum Arkose

Anhydrite Claystone, siltstone,

Iron sulfide argillite, and shale

Pyrite Carbonates

Marcasite Limestone

Pyrrhotite Dolomite

Iron oxide Marl

Magnetite Chalk

Hematite Chert

Goethite
lmenite
Limonite

For brief descriptions, see “Standard Descriptive Nomenclature of
Constituents of Natural Mineral Aggregates” (ASTM C 294).

CHARACTERISTICS OF AGGREGATES

The important characteristics of aggregates for concrete are
listed in Table 5-2 and most are discussed in the following
section:

Fig. 5-3. Lightweight aggregate. Expanded clay (left) and
expanded shale (right). (69793)

Grading

Grading is the particle-size distribution of an aggregate as
determined by a sieve analysis (ASTM C 136 or AASHTO