3. Roads and Road Materials
3.1 Roads
Road: A route trafficable by motor vehicles,
and which may also cater for pedestrians, bicyclists,
animals, and animal-drawn vehicles.
Includes the whole width of land between boundaries
of abutting properties.
Purpose: To enable
- safe
- convenient
- effective, and
- efficient
transfer of goods and people.
Roads are for traffic, and traffic
is for people.
Therefore, the distribution of population
determines the need for roads.
For example, in Australia the population distribution
is determined largely by climatic factors, and to a lesser extent by topography.
Consequently most of Australia's population lives within a few hundred kilometres
of the coast, and this is where the majority of roads are located.
3.1.1 The Australian Road Network
Australia has three (3) levels of government:
- Federal Government (also known as the National
Government or Commonwealth Government)
- State Government (individual governments for
the 6 states and 2 territories), and
- Local Government (of which there are about 1000).
Each level of government is responsible for some
of the nation's road network. The Federal Government is responsible for direct
funding of the National Highway System, which are the major routes covering
the whole country. The State Governments are responsible for the major road
network within their individual states. This is usually known as the "declared"
road system (the roads are declared as important under a state Act of Parliament).
Responsibility for the remainder of the public road system falls to Local Governments
(Cities, Towns, Municipalities or Shires).
Each State has a State Road Authority, although
the name differs between States e.g.Queensland has a "Department of Main
Roads", New South Wales has a "Road and Traffic Authority", and Victoria
has "VICROADS".
In the state of Queensland their are six types of
declared roads:
- Urban Arterial
- Urban Sub-arterial
- State Highway
- Developmental Road
- Main Road
- Secondary Road
The Queensland road network consists of about 165,000
km. of road, of which 39,800 km. is declared road. The remaining 125,200 km.
is under the control of the local authorities (about 130).
LINKS TO SITES ON AUSTRALIAN ROADS.
Austroads has a
Road Facts 2005
site which has some useful information on the Australian road network.An example of a State Road Authorities site is the
Department of Main Roads, Queensland
.
3.2 Road Materials
Wheeled vehicles require a firm, smooth surface for
operation. Usually natural soil is not strong enough to support repeated
wheel loads without surface deformation. Therefore a pavement structure is
used to carry applied wheel loads, and to distribute these loads to the underlying
natural soil.
The main materials used for road pavements are:
- bituminous materials
- unbound granular materials (gravels or loams)
- cemented materials, and
- cement concrete
Road pavements are classified into two broad categories:
- Flexible pavements - which flex slightly under
the action of traffic, and which distribute load to the underlying soil in
the manner of classical soil mechanics theory, and
- Rigid pavements - which distribute load to
the underlying soil by a slab action.
3.3 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.
3.4 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).
3.5 Factors Influencing the Selection of Pavement
Materials
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.
3.6 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.
3.7 Testing of Granular Pavement Materials
The properties of importance for a granular pavement
material are:
- STABILITY - determined by strength testing
(typically CBR test), or inferred from classification tests.
- RESISTANCE TO WEAR - indicated by classification
and index tests.
- PERMEABILITY - inferred from classification
and index tests
- WORKABILITY - inferred from classification
tests
Testing of gravel samples therefore usually includes
the following tests:
- Particle size distribution (sieving, and hydrometer
analysis)
- Consistency limits (or Atterburg limits) - LL,
PL, SL, and hence calculation of PI)
- Linear shrinkage
- Maximum dry density and optimum moisture content
- California Bearing Ratio (CBR) (Dry and/or soaked)
Other tests may include:
- 10% Fines (indicates resistance to crushing,
and may be determined for dry or soaked condition)
- Wet / dry strength variation (comparison of dry
and wet 10% fines values)
- Washington degradation (to assess the extent
of alteration or weathering)
3.8 Bituminous Surfacing Materials
Bituminous surfacing takes two common forms:
- SPRAYED SURFACINGS- where bituminous binder
in a fluid state is sprayed over the road surface, and aggregate particles
are placed in the fluid material.
- PLANT MIX SURFACINGS - where a graded range
of aggregate particles are mixed together with a bituminous binder in a mixing
plant, and the resulting mixture is spread as a mat on the road surface (and
normally compacted to achieve certain density requirements).
Both surfacing types involve the use of:
- Aggregate particles to resist the wear of traffic,
and
- Bituminous binder to act as an adhesive to hold
the aggregate particles in place and to bond to the underlying pavement.
3.9 Road Surfacing Aggregate
Road surfacing aggregates are typically derived from:
- Unconsolidated sediments (eg river gravels)
- Crushed quarried rock (typically igneous rocks
such as granite and basalt)
- Crushed artificial rock (eg blast furnace slag)
An aggregate is characterised by its particle size
distribution.
3.10 Desirable Properties of a Road Surfacing Aggregate
Road surfacing aggregates need to be:
- Mechanically strong
- Resistant to wear
- Well shaped physically (the most desirable shape
is a cubical shape), and
- Possessing good surface texture, and having a
good resistance to polishing.
3.11 Testing of Road Surfacing Aggregates
The following tests are used for determining the
properties of road surfacing aggregates:
- STRENGTH AND WEAR RESISTANCE
- Los Angeles Abrasion test
- Aggregate Crushing Value
- RESISTANCE TO DECOMPOSITION
- Soundness (Sodium sulphate solution test)
- Wet / Dry Strength Variation
- TESTS FOR PARTICLE SHAPE AND SURFACE TEXTURE
- Flakiness Index
- Angularity Number
- RESISTANCE TO POLISHING
- Polished Aggregate Friction Values (PAFV)
3.12 Bituminous Materials
The bituminous binder predominantly used in road
surfacing work is of petroleum origin and in Australia is known simply as
bitumen. It is composed mainly of hydrocarbons and their derivatives.
Natural bitumen is probably the oldest petroleum
product to be used by man. The ancient Egyptians used it for embalming mummies
and in jewellery. Through the ages it has been used in Middle Eastern countries
for water-proofing and constructional jobs. The bitumen was obtained from
natural seepages out of the ground in various parts of the Middle East. Although
naturally occurring bitumens are still available they account for only a very
small percentage of the bitumen used today.
3.13 Production of Bitumen
Bitumens are produced from suitable crude petroleum
oils, essentially by a process of distillation.
Australian crude oils are unsuitable for bitumen
production as they are too light and too waxy in nature. Therefore bitumen
production in Australia is almost exclusively from Middle East crudes.
3.14 Testing of Bitumen
The characteristics of bitumen which make it a good
material for use in road surfacing are:
- good adhesiveness
- good water resistance
- flexibility
- durability
The classification of bitumen is based on its viscosity
at 60 degrees C. For example Class 170 bitumen has a viscosity in the range
140 to 200 Pa.s. (170 being the mid-point of the range) at 60 degrees C.
The classes of bitumen generally used for road making
purposes in Australia are 170 and 320.
The range of tests used for determining the characteristics
of bitumen is as follows:
- Viscosity at 60 degrees C (Pascal seconds)
- Viscosity at 135 degrees C.
- Penetration at 15 degrees C (mm)
- Density at 15 degrees (kg/l)
- Flashpoint (degrees C)
- Purity (matter soluble in toluene)
- Durability
3.15 Flux and Cutter
Flux and cutter are both petroleum which are added
to bitumen to change the viscosity.
In fluxing the aim is to achieve a relatively long
term change to the viscosity. A typical flux in use is diesel fuel oil. Fluxing
the bitumen means that the binder remains soft for a period of weeks or months.
Fluxed bitumen is used to manufacture pothole patching mixes which have to
be stored for a period of time before use. Flux may also be added to the bitumen
when sealing in colder climates during the cooler period of the year. In
this case flux helps to keep the bitumen fluid during very cold spells, and
helps prevent loss of aggregate from new seals due to bitumen brittleness
at low temperatures.
Cutting is the addition of a fairly volatile oil
which produces a temporary reduction in the viscosity of the binder. Cutter
is intended to be mainly lost by evaporation within a period of minutes
or hours. The use of cutter in sealing work is mainly to give enough time
to incorporate the aggregate particles firmly in the sprayed bitumen before
it becomes too hard, and subsequently to allow the development of a good
bitumen-aggregate bond. The cutter commonly used is power kerosene.
Fluxing and cutting may both be carried out at the
job site before sealing work is performed. However cuback bitumen can also
be purchased in bulk from bitumen refineries.
Fluxes and cutters have fairly low flashpoints (eg
power kerosene 38 degrees C) and therefore strict safety procedures must be
adopted when using these materials.
3.16 Cutback Bitumen
Cutback bitumen is classified into three categories:
- Slow Curing (using fuel oil cutter)
- Medium Curing (using kerosene cutter), and
- Rapid Curing (using white spirit cutter).
Curing refers to the time required for the altered
bitumen to return to its original properties. Medium curing cutbacks are
generally used for roadmaking purposes. Typical uses are:
| Grade of Cutback Bitumen |
Percent of Kerosene in Cutter |
Use
|
|
AMC00, AMC0, AMC1
|
56, 44, 34
|
Precoating, Priming |
|
AMC2, AMC3, AMC4
|
27, 21, 16
|
Primersealing, Light sealing, Cold Mix |
|
AMC5, AMC6, AMC7
|
11, 7, 3
|
Sealing |
3.17 Bitumen Emulsions
Bitumen emulsion is a mixture of bitumen and water,
with the bitumen suspended in water as very fine droplets. In order to achieve
this condition an emulsifying agent is used in the mixture. The stability
of an emulsion (ie its ability to retain the bitumen in droplet form) is controlled
by the amount and type of emulsifying agent used.
Emulsions are very fluid and have the advantage
that they allow the bitumen to be applied to the road surface without the
heating of the bitumen. However because of the very fluid nature of the material
it can only be applied in a relatively thin layer (thicker applications attempt
to run off the road surface). They are used for tack coats before the spreading
of plant mix, for maintenance patching, and to a limited extent for spray
sealing work. When emulsion is applied to a surface the emulsion "breaks",
with the bitumen droplets coalescing and the water evaporating off to leave
behind a thin bitumen layer.
Two types of emulsion are in common use and are
described by the type of emulsifying agent used in their manufacture. The
two type are 'anionic' and 'cationic'.
3.18 Adhesion Agents
An adhesion agent may be used in bituminous surfacing
work to assist with the adhesion between bitumen and aggregate particles.
They may be applied to the aggregate or by incorporating in the bitumen.
The performance of particular adhesion agents varies with different types
of aggregateand it is necessary for them to be tested for effectiveness with
the particular aggregate to be used. They are generally used in the proportion
of about 0.5 to 1% of the volume of the bitumen.
3.19 Precoating Materials
Precoating agents are used to improve the bond between
the bitumen binder and the aggregate particles. The precoating material
is applied to the aggregate particles before the sealing work takes place.
Materials used for precoating include bitumen based materials (such as AMC00
grade cutback bitumen) and oil based materials. The application rate for
precoating material varies according to the nature, size and surface area
of the aggregate, but is normally about 4 to 12 litres per cubic metre of
aggregate.
3.20 Polymer Modified Binders
Polymer modified binders (PMBs) have been in use
in Australia since the 1970s. Early materials used natural or synthetic rubber
at about 4 to 6 % by mass of binder.
PMBs can now be manufactured to produce specific
binder properties. The binder properties desired often include rutting resistance,
fatigue resistance, and crack control for asphalt binders; and crack control
and aggregate retention for sprayed seal binders.
Page last modified 5 July 2006.