Highway Engineering

porous asphalt (PA). (If PA is used, it is assumed to contribute only 20 mm to

the overall thickness of the pavement for design purposes.) The bitumen within

dense bitumen macadam roadbases and basecourses must be at least 100 pene-

tration grade, with hot-rolled asphalt containing 50 pen binder.

8.2.2 Road Note 29

Pavement thickness design methods have historically been empirically based,

with the performance of pavements being analysed and design charts being

compiled based on the information obtained from the on-site observations of

researchers. This approach led to the publication of Road Note 29 (Department

of the Environment, 1973). This document was based on scrutiny during the

1960s of the behaviour of sections of highway pavement along the A1 trunk

road in Cambridgeshire trafficked by up to 10 million standard axles. It formed

the basis for pavement design philosophy in the UK from then until the mid

1980s.

Road Note 29 (RN29) takes account of increasing axle loads and vehicle

numbers while also differentiating between the performance characteristics of

different roadbase materials. (The roadbase is assumed to satisfy the entire

strength requirements for the entire pavement, with the surfacing considered to

make no significant contribution to the strength of the pavement. The primary

function of the surface material is to provide surface texture and regularity.) The

RN29 procedure is best explained as a series of design steps.

Step 1

Determine the cumulative number of commercial vehicles expected to use the

highway from its first day of use to the end of its design life, taken as 20 years.

Step 2

Determine the cumulative number of commercial vehicles expected to use the

‘design lane’ over its design lifetime. (The design lane is the most heavily

trafficked lane in any given direction.)

Step 3

Determine the equivalent number of standard axles incident on the road over

its design life, based on the commercial vehicle usage. Based on a standard axle

230 Highway Engineering

Wearing course

Basecourse

Roadbase

Surface course

Binder course

Base

Will become

Figure 8.1Proposed renaming of pavement layers.