Asphalt Review - Volume 29 Number 2 (June / July 2010)

ASPHALT REVIEW
occur over time. However, such rate of softening is mainly
determined by the stiffness of the support, which determines
the degree of shear in the layer. However, owing to the higher
cohesive strength, these layers are less sensitive to the support
stiffness than unbound granular materials (higher modular ratio
limit).
Table 3 summarises the limits applicable to the modular ratio
and maximum stiffness rules.
Condensed version - full version available from Conference
papers 2009 - aapa@asn,com.au
Designing pavement using the PN method
See full paper - Condensed version - full version available from
Conference papers 2009 - aapa@asn,com.au
The beauty of this method lies in its simplicity. The procedure
commences by defining the pavement structure in terms of layer
thickness and material class for each component layer.
ECONOMIC AND ENVIRONMENTAL
BENEFITS
The benefits of using BSMs in pavement structures were shown
by comparing different options for the rehabilitation of a specific
pavement, each with the same structural capacity. The three
options selected (illustrated in Figure 4) represent different
technologies.
Option 1 calls for the distressed asphalt layers to be removed
by milling, the upper portion of the underlying base repaired
before replacing the asphalt with fresh material.
Option 2 reuses the existing pavement layers, recycled with
cement as a new subbase layer.
Option 3 reuses the existing pavement layers recycled with
bitumen as a new BSM base layer, requiring only a relatively
thin asphalt wearing course. .
Economic comparison
An estimate of the type and extent of maintenance work
during a 20-year service life suggested that the asphalt
surfacing layer would need replacing every seven to eight
years for all options. Being susceptible to moisture ingress,
an extra 35mm thickness of asphalt would be needed for
Option 2 after some 14 years. Then, at the end of the service
life, rehabilitation measures were determined, based on the
failure condition of the various pavements:
Option 1 (failure condition: fatigue cracks through the full
thickness of asphalt over 10% of the length). Rehabilitate by
applying a 60mm thick asphalt overlay plus a new asphalt
surfacing;
Option 2 (failure condition: moisture activated distress
of the graded crushed stone base with potholes affecting
10% of the road length). To be recycled to create a BSM
base layer plus an asphalt surfacing (similar to the original
rehabilitation Option 3);
Option 3 (failure condition: permanent deformation of
20mm in 10% of the total length of wheel paths). This can
be addressed by milling off the asphalt surfacing, paving a
levelling layer (35mm nominal thickness) and replacing the
asphalt surfacing.
Construction costs tend to be country specific and, in
some cases, regional specific. A comparison of construction
costs in money terms is therefore not a satisfactory means of
comparing the three options. Determining the unit cost per
kilometre for the life cycle (rehabilitation to rehabilitation)
and reducing cost to a relative index does, however, provide a
unit free basis for comparison. Accordingly, each option was
priced using average unit contract rates in the South African
construction industry (expressed in US$) and divided by the
cost of Option 1 to obtain an index, as shown in Table 4.
This exercise indicates that recycling with a bitumen
stabilising agent is some 30% more cost efficient than the
other two options where the initial rehabilitation called for
a 20-year service.
Energy consumption comparison
The increased level of awareness of climate change is
making society more aware of energy consumption. The
construction industry is not exempt and several studies have
been undertaken to estimate the amount of energy being
consumed, particularly in the construction of roads where
large machinery is employed and the quantities of material
either consumed or moved is high.
A detailed exercise carried out in New Zealand in 2008
(Reference 8, Patrick) reported on the energy consumption
data for various construction activities. Using this data, a
similar exercise to that carried out above to determine costs
was undertaken to estimate the total energy consumed for
each option over a 20-year period. The results are shown in
Table 5.
This exercise indicates that combined construction activities
over a 20-year cycle are almost 30% more energy efficient for the
BSM option (Option 3) than Option 1 which, in turn, is some
30% more energy efficient than Option 2.
CONCLUSIONS
The publication of TG2 Second Edition has provided a refreshing
new insight into the behaviour of BSMs and introduced a new
separate class of material for use in pavement structures.
In recognising that the end product is similar regardless of
whether the bitumen is applied in an emulsified or foamed
state, this publication has effectively eliminated the conflict
between the bitumen emulsion and foamed bitumen lobbies.
In addition, TG2 Second Edition provides a relatively simple
set of guidelines for the competent design and construction of
pavements that include these materials.
Maximising the reuse of existing pavement materials by
recycling minimises the consumption of new materials,
thereby providing both economic and environmental
benefits. The addition of a bitumen stabilising agent
enhances the performance of the recycled material, providing
both flexibility and durability. Due to their durability
properties, BSMs offer a lower whole-of-life cost through
lower maintenance and other interventions required to
achieve an acceptable level of service over the design life of
the pavement, as well as the cost of rehabilitation when the
terminal condition is reached. The true value of BSMs is only
now starting to receive the attention they deserve. Meanwhile,
environmental considerations are receiving more attention
in the provision of pavements, a long overdue focus brought
about by global concerns over climate change. Increasing
emphasis on the environmental impact of road construction
and rehabilitation has led to sufficient data becoming
available for use in analysis and decision making. Decisions
based solely on initial construction costs will, in future, be
replaced by a more complex model that incorporates both
energy consumption and whole-of-life costs.
32 ROADS JUNE 2010/JULY 2010