MATERIALS ENGINEERING CONCEPTS - Materials for Civil and Construction Engineers - page 22

possible misuse of the structure, such as improperly hanging an object
from a truss roof,

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degree of accuracy of considering the proper response of materials dur-
ing design, such as assuming elastic response although the material
might not be perfectly elastic.

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1.3 Nonmechanical Properties

Nonmechanical properties refer to characteristics of the material, other than
load response, that affect selection, use, and performance. There are several
types of properties that are of interest to engineers, but those which are of
the greatest concern to civil engineers are density, thermal properties, and
surface characteristics.

1.3.1 Density and Unit Weight
In many structures, the dead weight of the materials in the structure signifi-
cantly contributes to the total design stress. If the weight of the materials can
be reduced, the size of the structural members can be also reduced. Thus, the
weight of the materials is an important design consideration. In addition, in
the design of asphalt and concrete mixes, the weight-volume relationship of
the aggregates and binders must be used to select the mix proportions.
There are three general terms used to describe the mass, weight, and vol-
ume relationship of materials. Density is the mass per unit volume of mate-
rial. Unit weight is the weight per unit volume of material. By manipulation
of units, it can be shown that

g = rg (1.12)

where

g = unit weight

r = density

g = acceleration of gravity

Specific gravity is the ratio of the mass of a substance relative to the mass
of an equal volume of water at a specified temperature. The density of water
is in SI units and in English units at 4°C (39.2°F). Ac-
cording to the definition, specific gravity is equivalent to the density of a
material divided by the density of water. Since the density of water in the
metric system has a numerical value of 1, the numerical value of density and
specific gravity are equal. This fact is often used in the literature where den-
sity and specific gravity terms are used interchangeably.
For solid materials, such as metals, the unit weight, density, and spe-
cific gravity have definite numerical values. For other materials such as

1 Mg/m 3 62.4 lb/ft 3