Highway Pavement Materials and Design 213
precoated with a thin layer of binder in order to promote their swift adhesion
to the binder film during the laying process. Rolling should be carried out using
pneumatic-tyred rollers. The process should result in a single layer of chippings
covering the entire surface, firmly held within the binder film.
7.6.3 Recipe specifications
Some of the most important bituminous materials used within highway pave-
ments in the UK are:
Coated macadam (dense bitumen macadam, high density macadam, pervi-
ous macadam)
Asphalt (mastic asphalt, hot rolled asphalt).
The main uses for these materials within a highway pavement are shown in Table
7.9.
Bituminous material Location in pavement
Dense bitumen macadam Roadbase, basecourse, wearing course
High density macadam Roadbase, basecourse
Pervious macadam Wearing course
Mastic asphalt Wearing course
Hot rolled asphalt Roadbase, basecourse, wearing course
Table 7.9Location in
pavement of different
bituminous materials
Hot rolled asphalt, dense bitumen macadam and porous macadam are the
most prominent recipe-based bituminous materials used in major highways. The
recipe method uses a cookbook-type procedure for the selection of the type and
relative proportions of the materials within the mixture. This selection is based
on both experience over many years and empirical judgement rather than strict
theoretical engineering principles.
It involves the specification of the type of aggregate together with its grading,
the grade of the bitumen and the relative proportions of the bitumen and aggre-
gate. The method of mixing, placement and compaction will also be stipulated.
This mixture is specified on the basis that it has been adjudged by experts within
the industry to have performed to an acceptable level over a time span of years.
It is the method that is concentrated on within this text. It does however have
some limitations, most notably its inability to allow for the inclusion of more
innovative road materials or to provide a workable specification where unusual
traffic or climatic conditions may prevail. Furthermore, it may, in certain situa-
tions, prove impossible to ensure that the mixture has been produced exactly as
the specification requires. The method, by its very nature, cannot take full
account of the engineering properties of the mixture. The engineering design
approach to bituminous surfacings was put forward and details of it be found
elsewhere (O’Flaherty, 2002).
