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Executive Summary
There is an urgent global need to improve the resilience and sustainability of road networks. Pavement recycling and stabilisation with low-carbon binders is inherently a highly sustainable road construction method. However, some of the plant (machinery) required is specialised and typically capital intensive. This can be a barrier to adoption. It is possible to substitute optimal plant with low-budget alternatives. This approach may not conform to best practice but is viable in most cases. The combination of budget plant and Renolith 2.0 nanotechnology unlocks the potential for a low-cost, rapidly scalable and highly sustainable pavement construction system. Such a system would be suitable for adoption in numerous markets, such as developing countries and remote communities.
Introduction
Stabilisation is the improvement of a soil or pavement material usually through the addition of a binder. In-situ stabilisation techniques have long been used by engineers in Australia and many places throughout the world for pavement (usually roads) construction and rehabilitation. These techniques have historically been chosen by managers of pavements because of their significant cost advantages. The economic and sustainability benefits of in-situ recycling and stabilisation are compelling. Nonetheless, more expensive and environmentally destructive pavement construction methods are still commonplace.
Pavement recycling / stabilisation with cementitious binders and Renolith 2.0 admixture enables a low-cost, low-carbon method of building resilient pavements. Various best practice methods are available, such as those published by AustStab, Austroads and state road authorities.
The machinery required for pavement recycling and stabilisation according to best practice is specialised and typically capital intensive. This can be a barrier to adoption, particularly in developing countries.
This article explores the need and opportunity for pavement construction using low-cost equipment.
The Need
The world’s road network is large and growing. A significant proportion of the world’s roads are unfit for purpose, in poor condition and degrade quickly. This is unsustainable.
Figure 1: The world’s road construction system is unsustainable
The Global Roads Inventory Project (GRIP) gathered, harmonized and integrated nearly 60 geospatial datasets on road infrastructure into a global roads dataset, covering 222 countries and over 21 million km of roads. Of these roads, only 35% are paved and 50% have all year accessibility. Most of the remainder would benefit from a simple, low-cost, low-capital road construction/rehabilitation approach.
Aim
This article outlines the potential to substitute optimal plant with low-budget alternatives. This approach, in conjunction with Renolith nanotechnology, could enable rapid adoption of more sustainable road construction practices in under-serviced markets such as developing countries.
Best Practice
Queensland Government Department of Transport and Main Roads Technical Specification, MRTS07B Insitu Stabilised Pavement using Cement or Cementitious Blends is an example of a modern, comprehensive, best-practice stabilisation specification. The specified plant is listed in the table below.
Table 1: MRTS07B – Minimum requirements and numbers of particular plant
The capital cost for this specified minimum plant would typically exceed USD $1million. Such an investment is only viable for large organisations and specialist stabilisation contractors.
Stabilisation - critical success factors
Pavement recycling and stabilisation is very effective when done well but can fail dramatically if done badly. Accordingly, to mitigate risks, best practice methods impose controls on:
- Soil/aggregate composition
- Binder type, quantity & quality
- Mixing methods
- Water content
- Construction sequence & timing
- Compaction methods and performance
- Process quality control
Often the most challenging aspect is optimising the binder quantity. Too little binder will yield inadequate material performance gain; too much may result in unacceptable shrinkage cracking. In Australia, stabilised base layers are often designed to be ‘lightly-bound’ (28d UCS 1-2MPa) to mitigate shrinkage cracking effects. Renolith 2.0 admixture enables heavily bound layers with low risk of shrinkage cracking. This assists the construction process significantly:
- Thinner pavements are viable, which can be mixed and compacted with lighter machinery.
- The upper limit on binder quantity is relaxed, allowing much greater flexibility in mix design and construction quality control.
Renolith 2.0 admixture also lubricates the mix, reducing the compaction effort required. Hence, lighter compaction plant can be used.
Budget plant
Stabilisers can be self-propelled or tractor towed, light or heavy, premium or budget brands. There is a huge difference in the capital costs.
Figure 2: Premium vs budget stabilisers
A simple tractor stabiliser attachment with power take-off (PTO) and hydraulic height adjustment via 3-point hitch can be acquired for less than USD $2000. Such a machine would not meet the MRTS07B specification and has a low production rate compared to a heavy stabiliser. Nonetheless, it is likely to be adequate for simple road construction. The working pace would need to be constrained to ensure thorough mixing. Often the mixing stages are the limiting factor on pavement production rate, so concurrent operation of two or more tractors with stabiliser attachments may be optimal.
Commercial-off-the-shelf (COTS) tractor attachments are also available for all the other plant required to construct a stabilised pavement. This considerably reduces the capital outlay required. Suitable tractor(s) (nominally 150HP+ with PTO) could be bought, leased or hired as needed for the project.
Figure 3: Cement spreader trailer
Various manufacturers produce tractor towed automated cementitious binder spreaders. The typical list price on Alibaba.com is USD $10,000-$15,000 (8m3 capacity).
Figure 4: Water cart
Many manufacturers produce water carts. The typical list price on Alibaba.com is USD $5,000-$8,000 (~5T capacity). Suitable carts would require modification with a spray bar or dribble bar system with accurate flow and dispersal control.
Figure 5: Vibrating steel roller tractor attachment
Vibrating smooth drum rollers are not specialised stabilisation plant and tend to be readily available. If not, a vibrating steel roller tractor attachment may be an option. Typical list price on Alibaba.com for a budget 8T vibrating self-propelled roller or equivalent tractor attachment is USD $10,000.
For a typical road pavement of 200mm, a padfoot roller is not essential. For unsealed roads, padfoot compaction is not desirable because the indentations are often not fully removed by trimming. A padfoot roller might only be required for a deep pavement of heavy clay materials.
Multi-tyre pneumatic rollers can be useful for improving the pavement finish. However, in the interests of minimising the required equipment and capital outlay, a pneumatic roller is considered optional.
A vibrating steel roller is the most versatile compaction option and should suffice for most scenarios.
Various studies (Savan et al, Mazumber et al) have shown that Intelligent Compaction (IC) systems can improve construction productivity and reduce total cost of ownership for road asset owners. State-of-the-art multi-sensor integrated systems such as the SPARC Intelligent Compaction Kit cost in the order of USD $50,000, which makes the business case challenging for IC acquisition in a low-capital scenario. A cursory online search did not find any budget (<USD $10,000) COTS IC kits. However, the individual components of an IC system (eg. GPS, LiDAR, accelerometer, data logger) are inexpensive, so it is conceivable that budget IC kits could be developed.
Figure 6: Tractor towed grader
Graders are not specialised stabilisation plant and are usually readily available. Alternatively, various tractor grader blades and attachments are available. The list price on Alibaba.com for the example attachment above is approximately USD $4000.
The table below summarises the required plant. The total capital outlay is a fraction of that required for a typical set of heavy stabilisation plant.
Table 2: Minimum requirements and numbers of particular plant – budget option
Conclusion
The minimum plant for heavy pavement recycling and stabilisation is specialised and capital intensive – typically $USD 1M+. It is possible to substitute optimal plant with budget alternatives. This approach may not conform to best practice but is viable in most cases. The combination of budget plant and Renolith 2.0 admixture unlocks the potential for a low-cost, rapidly scalable and highly sustainable pavement construction system. Such a system would be suitable for adoption in numerous markets, such as developing countries and remote communities.