Highway Engineering

214 Highway Engineering

7.6.4 Coated macadams

With these pavement materials, graded aggregate is coated with bituminous

binder, generally penetration grade bitumen. It is classified in terms of the

nominal size of the aggregate, its grading and the location within the pavement

for which it is intended. Densely graded materials have a high proportion of

fines producing dense and stable macadam. Open graded materials have less

fines. This results in less dense and less stable macadam.

Let us examine the three coated macadams used within the roadbase/base-

course/wearing courses of flexible pavements – dense bitumen macadam

(DBM), heavy duty macadam (HDM) and porous macadam.

Dense bitumen macadam

Dense bitumen macadam is well graded and is the most common material used

in the roadbase and basecourse of major roadways (trunk roads/motorways)

within the UK. Table 7.10 illustrates the properties of two types of dense

bitumen macadam – 28 mm and 40 mm nominal size.

Dense bitumen

macadam

BS sieve size 28 40

37.5 mm 100–100 95–100

28 mm 90–100 70–94

20 mm 71–95 —

14 mm 58–82 56–76

6.3 mm 44–60 44–60

3.35 mm 32–46 32–46

0.3 mm 7–21 7–21

0.075 mm 2–9 2–9

Bitumen content 3.4–4.6 2.9–4.1

(% by mass of total)

Grade of binder (pen) 100 100

Table 7.10Dense

bitumen nacadam

(DBM) compositions

(Specification for

Highway Works, 1998)

Heavy duty macadam

Heavy duty macadam is used in roadbases and basecourses for major highways

with high traffic loadings. It contains more of the finer material (filler) and uses

a harder bitumen grading than DBM. The result is a stiffer mixture that will

provide greater protection against cracking and deformation over the life of the

pavement. Table 7.11 illustrates the properties of two types of heavy duty

macadam – 28 mm and 40 mm nominal size.