Highway Pavement Materials and Design 215Porous macadam
Known as porous asphalt, unlike conventional bituminous materials which
provide an impermeable layer and protect the underlying layers from the ingress
of rainwater, porous macadam is an open graded material containing a high
proportion of voids whose primary function is to allow the rapid drainage of
water. The impervious nature of the lower layers together with the camber of
the road allows the rainwater to flow laterally through the porous asphalt,
thereby escaping quickly from the structure.
This type of surfacing greatly improves wheel grip on the road while also
reducing water spray and substantially reducing the general noise levels ema-
nating from a highway. Due to its high voids content, this material is not as
durable as the more impervious macadams and should not be used in areas of
particularly high traffic loading. The bitumen in the mix can be stiffened by the
addition of hydrated lime in order to reduce the likelihood of the binder being
stripped away from the aggregate. A relatively high bitumen content should be
employed. Table 7.12 illustrates the properties of 20 mm nominal size porous
asphalt.
Heavy duty
macadam
BS sieve size 28 40
37.5 mm 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 7–11 7–11
Bitumen content 3.4–4.6 2.9–4.1
(% by mass of total)
Grade of binder (pen) 50 50Table 7.11Heavy duty
macadam (DBM)
compositions
(Specification for
Highway Works, 1998)BS sieve size 20 mm porous asphalt
28 mm 100
20 mm 100–95
14 mm 75–55
6.3 mm 30–20
3.35 mm 13–7
0.075 mm 5.5–3.5
Bitumen content 3.4–4.5
(% by mass of total)
Grade of binder (pen) 100–200Table 7.12Pervious
macadam/porous
asphalt (PA)
composition
(Specification for
Highway Works, 1998)