Geoscience Reference
In-Depth Information
Bituminous
mixtures
10
INTRODUCTION
Bituminous mixtures contain two main ingredients,
which are aggregate (coarse and fine) and the binder
(petroleum bitumen, natural bitumen, road tar). Various
grades of each may be used in various proportions to
produce mixes with different design properties. Fillers
(fine-grained material such as limestone dust, Portland
cement or PFA) may also be included in the mix to
change its physical characteristics.
Various bituminous mixtures have evolved to suit the
range of different circumstances in which they are used.
They can broadly be divided into two groups, the
macadams and asphalt. Macadams have a high content
of well graded (dense graded) aggregate and a low
content of bitumen binder, giving a dense stable structure
with the load being transmitted through the aggregate. In
The term 'bituminous mixtures' is used to denote all
materials in which an aggregate is bound with a
hydrocarbon binder. They are most extensively used as
surfacing material for road construction, where they are
commonly referred to as 'blacktop'. They were first
produced in the 1870s and became extensively available
by the 1930s.
Roads are built from a number of layers that together
are called a pavement. Bituminous mixtures are used in
the upper layers of flexible or flexible-composite
pavements, as opposed to rigid pavements where the top
layers comprise concrete. Typical flexible pavement
structure is illustrated in Figure 363 .
363
Layer
Properties
Wearing course
Upper surfacing layer providing the running surface for
traffic. Usually comprises hot-rolled asphalt (bituminous
mixture)
Basecourse
Lower surfacing layer that provides a well-shaped surface
for the wearing course to be applied to. Usually comprises
dense bitumen macadam or can be hot-rolled asphalt
(both bituminous mixtures)
Roadbase
Main structural layer of the pavement. May comprise
bituminous mixtures (either macadam or asphalt),
unbound aggregate mixtures, hardcore, or stabilised soil
Sub-base
Provides a surface of uniform strength for roadbase and
surfacing layers. Usually comprises unbound aggregate
Capping layer
Forms a substructure to sub-base. Only required where
(if required)
the subgrade is particularly weak
Subgrade
The in situformation (usually soil). Imported fill may be
added to make up levels or to replace unstable formation
material
363 Typical flexible pavement structure.
 
 
 
 
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