Roadbase
40 or 28 mm dense macadam.
The size of aggregate must not be greater than the required layer thickness. The
layer thickness should be approximately 2^1 / 2 times the nominal maximum aggre-
gate size, with a minimum layer thickness of 1^1 / 2 times the nominal maximum
aggregate size in order to minimise the likelihood of the larger stones being
crushed during rolling.7.6.7 Construction of bituminous road surfacings
The production of a successful bituminous road surfacing depends not just on
the design of the individual constituent layers but also on the correctness of the
construction procedure employed to put them in place. In essence, the con-
struction of a bituminous pavement consists of the flowing steps: Tr ansporting and placing the bituminous material
Compaction of the mixture If required, the spreading and rolling of coated chippings into the surface
of the material.Transporting and placing
The bituminous material is manufactured at a central batching plant where, after
the mixing of its constituents, the material is discharged into a truck or trailer
for transportation to its final destination. The transporters must have metallic
beds sprayed with an appropriate material to prevent the mixture sticking to it.
The vehicle should be designed to avoid heat loss which may result in a decrease
in temperature of the material, leading to difficulties in its subsequent place-
ment – if it is too cold it may prove impossible to compact properly.
It is very important that the receiving surface is clean and free of any foreign
materials. It must, therefore, be swept clean of all loose dirt. If the receiving
layer is unbound, it is usual to apply a prime coat, in most cases cutback
bitumen, before placing the new bituminous layer. A minimum ambient tem-
perature of at least 4°C is generally required, with BS 594 stating that a wearing
course should not be laid when the temperature of the course being covered is
less than 5°C, and work stops completely when the air temperature hits 0°C on
a falling thermometer. Work may, however, recommence if the air temperature
hits -1°C on a rising thermometer, provided the surface is ice-free and dry.
Steps must be taken to ensure that the surface being covered is regular. If it
is irregular, it will not be possible to attain a sufficiently regular finished surface.
A typical surface tolerance for a bituminous basecourse or wearing course
would be ±6 mm.218 Highway Engineering