Executive Summary
In a Triple Bottom Line (TBL) comparison of various pavement rehabilitation design solutions for local roads (White & Young, AustStab, 2024), stabilised road pavements were shown to incur a much lower financial, social and environmental cost in construction than conventional unbound granular pavements. For seven pavement design types, twelve scenarios were modelled for various combinations of design traffic and subgrade strength. In most scenarios, lightly-bound stabilised pavement designs with spray seals incurred embodied emissions of 17 kg.e.CO2/m2; typically 85-90% lower than their structurally equivalent designs utilising natural gravels and spray seals. The mass of materials transacted (disposal + consumption) was typically 97-99% lower. Construction costs were 70-80% lower.
Renolith nanotechnology prevents cracking problems in cementitiously bound materials, and greatly enhances the material engineering properties. A small quantity of Renolith 2.0 admixture (5% w/w binder) can improve the pavement life (traffic capacity) by an order of magnitude and greatly reduce the risk of potholes and other water-induced premature failures. For a similar triple bottom line construction cost (financial, social & environmental), the life of a lightly-bound stabilised pavement might be enhanced from 20 years to over 100 years.
The TBL benefits of cementitious stabilisation are compelling. The TBL benefits of Renolith nanotechnology-enhanced cementitious stabilisation are extraordinary, particularly in challenging scenarios (heavy traffic, poor subgrade).
Rehabilitation of Local Roads – Triple Bottom Line
Local roads in Australia often require partial or full-depth rehabilitation due to their construction with lower quality materials compared to highways. In a paper presented at the 3rd Australian Pavement Recycling and Stabilisation Conference in August 2024, White & Young compared the triple bottom line of various pavement rehabilitation design solutions for local roads.
The study used the Triple Bottom Line (TBL) concept to evaluate the financial, social, and environmental costs of different pavement rehabilitation designs, where:
- Social. The social cost of each pavement design option was calculated as the mass of new quarried material consumed in the construction of the rehabilitated pavement, added to the mass of existing pavement excavated and sent to landfill.
- Environmental. The environmental cost of each pavement design option was calculated as the quantity of each material, based on the thickness and density, and the embodied carbon associated with each material’s production, supply and construction, summed for all materials in the pavements.
- Financial. The financial costs were calculated in a similar manner to the environmental costs, except typical cost rates (in Australia) were used in place of the embodied carbon rates. The cost rates were set based on experience and included the material supply, production and construction costs, as appropriate.
The research compared seven structural equivalent pavement rehabilitation options, including new granular and two stabilised pavement options, each with sprayed seal and asphalt surface options, as well as a full-depth asphalt pavement. The research calculated 84 structural equivalent pavement thicknesses based on three traffic levels and four subgrade strengths using the CIRCLY software. Stabilisation techniques included lightly bound stabilisation with a cementitious binder and foamed bitumen stabilisation, both of which improve the properties of existing materials. Pavement thicknesses were determined using Australian road pavement design practices, with specific modulus values and failure criteria for different materials.
The study concluded that stabilisation of existing pavement structures consistently provides the lowest cost solution for local road rehabilitation, while new granular pavement reconstruction is the most expensive. Similarly, stabilisation methods are more cost-effective and environmentally friendly compared to traditional new granular reconstruction.
The research recommends existing pavement stabilisation as the preferred method for local road rehabilitation unless other factors make it unviable.
Note that the triple bottom line model adopted does not consider resilience. See https://renolith.com.au/resilient-roads/ for insights on the resilience of various pavement options.
Renolith Enhancement
The data presented in the paper was extrapolated to include an additional 3 pavement variants for the three traffic levels and four subgrade strengths:
- Renolith-enhanced bound base and No wearing course (light-traffic only)
- Renolith-enhanced bound base and Spray seal wearing course
- Renolith-enhanced bound base and Asphalt wearing course
The extrapolated model (Renolith Triple Bottom Line Estimator) is published as an interactive Microsoft Excel spreadsheet tool at https://renolith.com.au/resources/ The tool allows the user to quickly estimate and compare the triple bottom line construction cost ($, carbon footprint, materials usage) of the various pavement rehabilitation options. Selected scenarios are shown below.
Scenario 1 – Easy mode
Scenario 1 considers a benign situation with light traffic and good subgrade. In this scenario there are several sensible options for the base layer:
- Foamed bitumen
- Lightly-bound
- Renolith-enhanced bound
One of the unique things that can be done with Renolith is to create an impermeable, very heavily bound base layer with Unconfined Compressive Strength (UCS) in the 3-10 MPa range that won’t suffer deleterious shrinkage cracking. If the bound material provides sufficient erosion and skid resistance, a separate wearing course might be avoidable in some cases. This is a useful option when the TBL cost of a spray seal is hard to justify.
Scenario 2 – Hard mode
Weak subgrades and heavy traffic present a challenge to pavement designers. Unbound granular bases need to be very thick to disperse the loads (rows 9&10), resulting in very high TBL costs. Thickness can be reduced by foamed bitumen or cementitious stabilisation of the base (rows 5-8), with a corresponding reduction in TBL costs. Full depth asphalt is the thinnest option, but is expensive. A Renolith-enhanced bound base and spray seal wearing course results in the lowest TBL construction cost in every category. Further, due to the enhanced resilience and traffic capacity, the whole-of-life TBL costs are substantially lower.
Conclusion
For the rehabilitation of local roads, the triple bottom line benefits of stabilisation are compelling. The triple bottom line benefits of Renolith nanotechnology-enhanced cementitious stabilisation are extraordinary.