AGGREGATES - Materials for Civil and Construction Engineers - page 163

concrete depends mostly on interparticle friction between the aggregates
and, to a limited extent, on the binder.

5.5 Aggregate Properties

Aggregates' properties are defined by the characteristics of both the individ-
ual particles and the characteristics of the combined material. These proper-
ties can be further described by their physical, chemical, and mechanical
characteristics, as shown in Table 5.1 (Meininger and Nichols, 1990). There
are several individual particle characteristics that are important in determin-
ing if an aggregate source is suitable for a particular application. Other char-
acteristics are measured for designing portland cement and asphalt concrete
mixes (Goetz and Wood 1960).

■ Particle Shape and Surface Texture

The shape of the individual aggregate particles, Figures 5.3 and 5.4, deter-
mines how the material will pack into a dense configuration and also deter-
mines the mobility of the stones within a mix. There are two considerations
in the shape of the material: angularity and flakiness. Crushing rocks pro-
duces angular particles with sharp corners. Due to weathering, the corners of
the aggregates break down, creating subangular particles. When the aggre-
gates tumble while being transported in water, the corners can become com-
pletely rounded. Generally, angular aggregates produce bulk materials with
higher stability than rounded aggregates. However, the angular aggregates
will be more difficult to work into place than rounded aggregates, since their
shapes make it difficult for them to slide across each other. Flakiness de-
scribes the relationship between the smallest and largest dimensions of the
aggregate.
The roughness of the aggregate surface plays an important role in the way
the aggregate compacts and bonds with the binder material. Aggregates with
a rough texture are more difficult to compact into a dense configuration than
smooth aggregates. Rough texture generally improves bonding and increases
interparticle friction. In general, natural gravel and sand have a smooth tex-
ture, whereas crushed aggregates have a rough texture.
For the purpose of preparing portland cement concrete, it is desirable to
use rounded and smooth aggregate particles to improve the workability of
fresh concrete during mixing. However, angular and rough particles are de-
sirable for asphalt concrete and base courses in order to increase the stability
of the materials in the field and to reduce rutting. Flaky and elongated aggre-
gates are undesirable for asphalt concrete, since they are difficult to compact
during construction and are easy to break.
Many specifications for aggregates used in asphalt concrete require a
minimum percentage of aggregates with crushed faces as a surrogate shape

5.5.1