COMPOSITES - Materials for Civil and Construction Engineers - page 456

that are subjected to tension. When the concrete structure is loaded, the con-
crete carries compressive stresses and steel carries tensile stresses. In such
cases steel allows concrete to be used as structural members carrying ten-
sion. Steel rebars are also used in areas subjected to compression, such as
columns, to share the load support. In such cases, steel reduces the required
cross-sectional area of the compressive member, since the compressive
strength of steel is larger than that of concrete. Steel reinforcing is also used
in prestressed concrete, where the reinforcement is prestressed under ten-
sion so that the concrete remains under compression even when it is exter-
nally loaded. In such cases, a smaller cross-sectional area of the concrete
member is required. Steel rebars can also be used to control cracking in con-
crete due to temperature change. For example, concrete pavement is some-
times reinforced by placing longitudinal and transverse steel bars at the
midheight of the concrete slab. In this case, when the concrete shrinks due
to reduction in ambient temperature, many cracks will develop; these cracks
are uniformly distributed within the pavement section, but each crack will
be tight. Typically, tight cracks are not harmful to concrete pavement, since
they transfer the load from one side of the crack to the other by interlocking.
In all applications of steel reinforced concrete, the bond between the rebars
and the concrete is important in order to allow the composite to work as one
unit. Therefore, bars have a deformed surface to prevent slipping between
steel and concrete.

■ 11.2.3 Asphalt Concrete

Asphalt concrete used in pavements is another composite material. It con-
sists of two materials with distinct properties, as presented in Chapter 9. As-
phalt concrete consists of approximately 95% aggregate and 5% asphalt
binder, by weight. When the traffic loads are applied on the asphalt concrete
composite, most of the compressive stresses are supported by the aggregate-
to-aggregate contact. The asphalt acts as a binder that prevents slipping of
aggregate particles relative to each other. When tensile stresses are ap-
plied due to bending of the asphalt concrete layer or due to thermal con-
traction, the aggregate particles are supported by the asphalt binder. One
important property of asphalt is that it gets soft at high temperatures and
brittle at low temperatures, whereas aggregate does not change its proper-
ties with temperature fluctuation. It is important, therefore, to properly
select the asphalt grade that will perform properly within the temperature
range of the region in which it is being used. Also, since aggregate repre-
sents a major portion of the mixture, it is important to use aggregate with
proper gradation and other properties. The asphalt binder content must be
carefully designed in order to ensure that aggregate particles are fully
coated, without excessive lubrication. When the asphalt concrete mixture
is appropriately designed and compacted, it should last for a long time
without failure.