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

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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 <strong>2010</strong>/JULY <strong>2010</strong>
The Road to Innovation Ever increasing truck traffic, extreme climates and the continuous need for maintenance and growthbased construction pressure road budgets around the world every day. Our wide range of polymerbased solutions not only deliver improved durability and resistance to rutting and cracking, but also provide flexibility to overcome the most challenging sources of bitumen. For all your paving needs, Kraton Polymers is your single source for highly innovative solutions, service and support. Delivering solutions to your toughest challenges is what we do best. For more information, call +61-419 375055 or email us at vincent.conserva@kraton.com
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Asphalt Review - Volume 29 Number 2 (June / July 2010)

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