Asphalt is a solid or semi-solid mixture of bitumens obtained from
native deposits or as a petroleum by-product that is commonly used
for paving, roads, and waterproofing.
Asphalt was originally used as a hard lining by the spreading asphalt
aggregate mix on the subgrade of the channel to form a hard surface
and seal the channel. This material is also known as ‘asphalt
concrete’. The layer would normally have been 6-12mm thick
to be regarded as a hard surface liner. The lining was either installed
as an exposed liner susceptible to wear, erosion, the elements
and the channel operations, or as a buried liner to protect the
asphalt against potential damage.
These asphaltic linings when properly constructed were comparable
to concrete linings in many respects. However, the expected service
life of an asphalt lining is shorter and ranges between 10 and
20 years (Kraatz, 1977). It was necessary to include subgrade sterilisation
as an integral part of the construction operation to prevent plants
from growing and penetrating the lining, although this may not
apply to channels constructed in desert areas. Advantages of this
approach compared to cement include:
The possibility of placement during freezing weather.
Better
adjustment to subgrade changes.
The possibility of using slightly
poorer quality aggregate.
Any overall advantage in the use of asphalt is contingent upon
a considerable price differential in favour of asphalt and the
suitability of local aggregates for asphalt constructions.
