4. Pavement Materials
4.1 Introduction
A pavement is a structure capable of bearing the applied
loads of traffic and distributing them over the natural soil in such a way
that excessive deformations do not occur.
Pavements are normally classified as either
A typical flexible pavement consists of a bituminous
surfacing, granular base and granular sub-base. A typical rigid pavement consists
of a portland cement concrete slab over agranular sub-base.
The soil foundation is termed the subgrade, and the
level of the surface of the subgrade is usually referred to as the formation
level.
4.2 Pavement Materials
Pavement materials for flexible pavements (the major
type of pavement used in Australia) are:
- Granular materials , such as crushed rock, soil
aggregate mixtures, and chemically modified natural materials (typically
naturally occurring soils with a very small percentage of cement or lime
added).
- Cemented materials, which are usually cement
or lime stabilised materials having a sufficient proportion of stabiliser
to create a semi-brittle material.
- Bituminous materials, which are usually mixtures
of aggregate particles bonded with a bituminous binder.
4.3 Gravels and Loams
The most commonly used materials for road pavement
construction are naturally occurring soil aggregate mixtures known as gravels
or loams. In Queensland a soil aggregate mixture having largest particles
larger than 5mm is known as a gravel, while a mixture with largest particles
less than 5mm is called a loam.
These materials are obtained from pits or quarries
created in ridges of harder materials or in former creek beds. The source
materials may be decomposed igneous rocks, sedimentary rocks, metamorphic
rocks, or fine grained sediments. Harder materials may have to be crushed
to produce a suitable product and sometimes a soil binder needs to be added
to a crushed rock to achieve the necessary cohesion within the material. Processing
of a soil aggregate mixture may involve crushing, the removal of excessive
oversize material, the removel of excessive fine material, or the modification
of the material with a stabilising agent (typically cement or lime).
4.4 Factors Influencing Selection of Pavement
Material
The material selected for a particular layer in a pavement
will be influenced by the following factors:
- the type of pavement (different requirements
for unsurfaced and surfaced roads),
- the position in the pavement (different materials
are used for areas just under the surface and those areas lower down in the
pavement structure),
- the climatic conditions (specification regarding
material plasticity may be more demanding in wetter areas),
- traffic (generally better quality materials
are required for more heavily trafficked roads), and
- availability.
4.5 Location and Investigation of Natural Gravel
Deposits
The location and investigation of natural gravel deposits
involves the following steps:
- Search to identify potential deposits - involves
one or more of:
- an examination of existing deposits,
- a study of available soil and geological maps
of the area,
- an examination of available aerial photographs
of the area, noting in particular changes in landform and vegetation,
- utilising existing local knowledge within
road personnel, local farmers, etc.
- Visual examination of potential deposits.
- Preliminary sampling of potential deposits -
involving the collecting of samples from a range of conditions across the
deposit, and the performance of basic classification tests (grading, liquid
limit, plastic limit, shrinkage limit, etc.) on each sample, with the aim
of determining the extent of usable material within the deposit.
- More extensive sampling and testing, utilising
strength tests such as the California Bearing Ratio (CBR) test.
- Collection of additional information needed
to establish the economic viability of the deposit, such as amount of clearing
and stripping required, length of haul, condition of access roads, land ownership,
etc.
Page last modified 25 June 2002.