7.6.5 Asphalts
Two asphalts are discussed within this section: hot rolled asphalt (HRA) and
mastic asphalt.Hot rolled asphalt
Hot rolled asphalt is similar to a coated macadam. It is a dense material with
low air voids content, consisting of a mixture of aggregate, fines, binder and a
filler material, but in this case the grading is far less continuous (gap-graded)
with a higher proportion of both fines and binder present in the mix. The mate-
rial is practically impervious to water, with the fines, filler and bitumen forming
a mortar in which coarse aggregate is scattered in order to increase its overall
bulk.
Hot rolled asphalt wearing courses typically have from zero to 55% coarse
aggregate content, with basecourses having either 50% or 60% and roadbases
normally at 60%.
There are two recipe mixes for gap-graded rolled asphalt wearing course: Type
F,characterised by the use of sand fines, and Type C, characterised by the use
of crushed rock or slag fines. F denotes a finer grading of the fine aggregate with
C denoting a coarser grading of the fine aggregate.
Table 7.13 details a range of mixes for hot rolled asphalt to be used at road-
base, basecourse and wearing course levels within the pavement. Each mix has
a designation composed of two numbers, with the first relating to the percent-
age coarse aggregate content in the mix and the second to the nominal coarse
aggregate size. (The wearing courses in Table 7.13 are both Type F.)
As hot rolled asphalt wearing course is a smooth-textured material, precoated
chippings should be spread over and rolled into its surface while plastic in order
to increase skid resistance.Mastic asphalt
Mastic asphalt is a very durable heavy-duty, weather-proof wearing course
material. It consists of a mixture of asphaltic cement (low-penetration grade
bitumen), fine aggregate and filler in proportions which result in a low-void
impermeable mass. It contains a low percentage of coarse aggregate, all of which
must pass the 14 mm sieve and be retained on the 10 mm sieve. The mix consists
of a high percentage of fine aggregate, with no less than 45% and no more than
55% passing the 0.075 mm sieve and at least 97% passing the 2.36 mm sieve in
addition to high proportions of both filler material and binder. The grade of
the binder is very high (10 to 25 pen) with less than 1% voids in the mix.
The mix is applied manually using wooden floats, at a temperature of between
175°C and 225°C approximately. It requires considerable working, with the fin-
ished layer measuring between 40 mm and 50 mm. Its low skidding resistance216 Highway Engineering