SAPEM Chapter 2 - Sanral

South African Pavement Engineering Manual
Chapter 2: Pavement Composition and Behaviour
friction between tyre and road, damages the surface and unpaved roads could not cater for this. This resulted in use
of the following in the 20 th century:
Empirical design systems: experience- and observation-based designs
Mechanistic design systems: linking performance to critical pavement properties and failure mechanisms
Mechanistic-empirical design systems: linking critical pavement properties to experience based limits
In Scotland and Ohio, expensive solutions were found through experimentation. At the same time, experiments were
undertaken to investigate tar or natural asphalt, found in a lake in Trinidad, and split (aggregate). Skid resistance
and a lack of bond to existing layers, proved problematic. Typical problems of the time, which included impassibility
of many unsurfaced roads in wet conditions, can be seen in Figure 5, taken from Floor (1985).
Figure 5.
Impassable Unsurfaced Roads Due to Wet Conditions, Early 20 th Century
In the period between the First and Second World Wars, the growing importance of roads drove necessary
improvements in pavement design. After the Second World War, the growth in traffic, loads and tyre pressures, and
the higher speeds, necessitated the development of pavement technology beyond empiricism or designs based on
experience only. Functional performance had to be defined, being the basis of the service that is provided to the road
users in relation to the cost. This is indicative of fitness for use. Performance also needed to be better understood
and more predictable. This required knowledge of structural behaviour and pavement distress in relation to time.
This motivated the AASHO road test.
2.2.1 AASHO Road Test
The AASHO Road Test took place in Ottawa, Illinois about 100 km south-west of Chicago between 1956 and 1958. It
was an enormous effort to systematically quantify the complex interaction between road deterioration, traffic and
composition of the pavement structure on a closed loop test track with trucks. The test track is illustrated in Figure 6.
AASHO stands for American Association of State Highway Officials and later became AASHTO (Highway and
Transportation).
The aims of the AASHO road test are still very relevant:
Developing satisfactory pavement design procedures to meet the growing demands of traffic.
Aid legislators in setting user taxation and control of vehicle size and weight.
Section 2: History
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