Highway Pavement Materials and Design 217
requires the application of precoated chippings to its surface while still plastic,
in order to embed them firmly into the surface of the mix and give a roughened
finish.
It results in a long-life, low-maintenance surfacing in highly trafficked pre-
dominantly urban locations. The labour intensive nature of its application
makes it costly relative to other bituminous wearing courses.
7.6.6 Aggregates
The maximum nominal aggregate size is determined from both the required
thickness of the material when put in place and the surface texture called for.
The following are typical nominal aggregate sizes used at different levels within
a bituminous pavement:
Wearing course
14 mm dense wearing course macadam
10 mm or 6 mm pervious macadam
Base course
40, 28 or 20 mm dense macadam
Location Roadbase Basecourse Wearing course
Designation 60/20 50/14 30/14 30/10
BS sieve size grading
28 mm 100 — — —
20 mm 90–00 100 100 —
14 mm 30–65 90–100 85–100 100
10 mm — 65–100 60–90 85–100
6.3 mm — — — 60–90
2.36 mm 30–44 35–55 60–72 60–72
0.600 mm 10–44 15–55 45–72 45–72
0.212 mm 3–25 5–30 15–50 15–50
0.075 mm 2–8 2–9 8–12 8–12
Bitumen content
(% by mass of total mixture)
Crushed rock/steel slag 5.7 6.5 7.8 7.8
Gravel 5.5 6.3 7.5 7.5
Blast furnace slag of bulk
density
1440 kg/m^3 5.7 6.6 7.9 7.9
1280 kg/m^3 6.0 6.8 8.1 8.1
1200 kg/m^3 6.1 6.9 8.2 8.2
1120 kg/m^3 6.3 7.1 8.3 8.3
Layer depth (mm) 45–80 25–50 40 35
Table 7.13Examples of hot rolled asphalt roadbase, basecourse and wearing course bituminous
mixes (BS 594) (BSI, 1992)
