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Significant greenhouse gas (GHG) emissions will result from constructing, maintaining and rehabilitating the world road network using current practices.
In this article, we will demonstrate the potential to significantly reduce road construction industry GHG emissions by pivoting to in-situ stabilisation / cold-recycling using low-carbon binders and Renolith 2.0 nanopolymer admixture.
Road construction has a large carbon footprint
Roads play a key role in movements of goods and people but require large amounts of materials emitting greenhouse gases to be produced. We estimate road material stock to be of 254 Gt. If we were to build these roads anew, raw material production would emit 8.4 GtCO2-eq.
Material Stock and Embodied Greenhouse Gas Emissions of Global and Urban Road Pavement, Environmental Science & Technology 2022 56 (24), 18050-18059, DOI: 10.1021/acs.est.2c05255
The road network is growing
…It is expected that the world will need to add nearly 25 million paved road lane‐kilometres (km) and 335 000 rail track kilometres (track‐km), or a 60% increase over 2010 combined road and rail network length by 2050. In addition, it is expected that between 45 000 square kilometres (km²) and 77 000 km² of new parking spaces will be added to accommodate passenger vehicle stock growth.
Dulac, J., 2013. Global land transport infrastructure requirements. Paris: International Energy Agency, 20, p.2014.
Roads require maintenance and refurbishment
Asian countries tend to adopt a policy promoting short design life (about 10 years) to save on construction costs and because of the uncertainty connected with predicting long-term traffic volumes. However, initial cost savings under this strategy are often offset by mid-term and overall life-cycle costs required for maintenance and rehabilitation, which may result in increased GHG emissions. Though vehicle overloading is a major issue in Asia, it has rarely been taken into account at the design stage. This has commonly resulted in premature end of pavement life. Overloaded vehicles adversely and significantly affect GHG emissions, not only because they decrease road serviceability life, but also because of resulting increases in maintenance costs, vehicle operating costs, and road safety.
Transport – Greenhouse gas emissions mitigation in road construction and rehabilitation: A toolkit for developing countries (English). Washington, D.C.: World Bank Group. p.2012. (Hereafter: GHG emissions mitigation toolkit)
A life cycle assessment (LCA) is a methodology for detailing resource flows and associated environmental impacts. LCA is often used to assess the cradle-to-grave climate impact of a product or service, such as transport infrastructure. Various LCA software tools exist to estimate the GHG emissions from road projects. LCA requires project specific data to yield meaningful results, but in general, to minimise total GHG emissions from road construction, we seek to:
For our model, we will use a simple method to estimate the GHG emissions for an unbound granular pavement vs a bound pavement incorporating Renolith admixture.
A simplified approach to estimate GHG emissions during construction are estimated using the model and data from the GHG emissions mitigation toolkit. Only materials and transport emissions are considered.
For expressways and national roads, GHG emissions from the fabrication and extraction of construction materials are the main contributor, at about 90 percent of total emissions; they are less important for provincial and rural roads, at about 80 percent.
Transport of materials represents about 30 percent of the GHG emissions of a road project. Of that amount, about 50 percent is related to local (less than 25 km) transport.
Extraction and material transport are therefore the main activities that must be considered to significantly improve the GHG impact of a road construction project.
To build one kilometre of two-lane highway requires about 14,000 tonnes (or 400 truckloads) of construction aggregates.
A typical two-lane bitumen road with an aggregate base can require up to 25 000 tonnes of material per kilometre.
SBEnrc, “Reducing the environmental impact of road construction,” Sustainable Built Environment National Research Centre (SBEnrc), Perth WA, 2013
For our model, we will assume a requirement for 7000 tonnes of aggregate per lane km (excluding asphalt layer) and emissions intensity of 8 kg eq CO₂e / tonne. Transport of aggregate materials is assumed to be 30 percent of total GHG emissions.
In the Brenner Autobahn rehabilitation project, cement binder was applied at 25kg/m² (88 tonnes per lane km) – approximately 4% wt of pavement at 30cm depth. The project used cement CEM 32.5, which is similar to GP cement per AS 3972, with estimated emissions intensity of 670 kg CO₂e / tonne. The transport component was assumed to be via heavy truck (diesel) with emissions intensity of 1.58 kg CO₂e / veh.km.
Cement has high embodied energy and carbon footprint. Accordingly, there is much interest substituting lower-carbon binders for cement. Austroads AGPR4L-09, Guide to Pavement Technology Part 4L: Stabilising Binders, Ed1.1 Table 5.2 lists the typical range of commercial cementitious binder pozzolan blends available for pavement construction in Australia. These blends contain recycled supplementary cementitious materials (SCMs) (slag and/or fly ash), with a correspondingly lower emissions intensity. Renolith is compatible with all blends listed in Austroads AGPR4L-09. Therefore, a third design approach is considered, which assumes the same parameters as the Brenner case study but substitutes a lower emissions binder for cement. The binder blend is assumed to have emissions intensity of 335 kg eq CO₂ eq / tonne, which is half that of ordinary portland cement (OPC). Note that Austroads AGPR4L-09 lists dozens of blends with recycled SCM content ranging from 10% to 80%. In general, the higher the recycled SCM content, the lower the emissions of the blend. However, the performance of blends with very high SCM content may diverge from low to mid-range blends.
The chart below shows the estimated emissions from the the materials and transport component of construction for the three designs. A significant reduction in GHG emissions is achieved using a mid-range cement/SCM blend binder in combination with Renolith. Potentially (subject to testing), emissions could be reduced even further via use of a very low emissions cementitious binder, such as a high-blend cement with reduced clinker content.
Whole of Life Considerations
Considerations (italicised) in the table below are derived from the GHG emissions mitigation toolkit The associated advantages of a Renolith pavement are listed.
Cold recycling with cementitious binder and Renolith produces a stiff bound pavement layer at a much lower cost than conventional unbound granular construction methods. The improved economics of construction can reduce the pressure on the project to design for short life.
The fatigue resistance of cementitiously-bound layers is very sensitive to the thickness of the course and the stiffness of the layer. As an approximation, a 10% reduction in either thickness or stiffness of a cementitiously-bound layer could lead to about a 90% reduction in fatigue life.
AustStab TN05, “AustStab Technical Note No.5, Cement Stabilisation Practice,” AustStab, Sydney, 2012.
Consider the inverse of this heuristic: an 11% increase in thickness, at less than 11% additional construction cost, can yield a massive 900% increase in fatigue life.
The whole-of-life emissions advantages of Renolith pavements are clearly substantial, but difficult to quantify. The Brenner Autobahn case study suggests that a long pavement life can be achieved despite difficult environmental conditions (lateral water afflux, frost) and high traffic loading (>2,000,000 heavy goods vehicles per annum). By contrast, the 2022 floods in Australia highlighted the susceptibility of conventional pavement designs to water damage, causing $3.8B in damage. As the pace and intensity of extreme weather events increases, water damaged roads will need repairs and reconstruction, with associated GHG emissions from this activity. This perpetual problem could be substantially mitigated with Renolith, since the bound composite is essentially impermeable.
There is potential to significantly reduce road construction industry GHG emissions by pivoting to in-situ stabilisation / cold-recycling using low-carbon binders and Renolith 2.0 nanopolymer admixture as the primary pavement construction method. This potential arises from attractive economics, a low carbon footprint at construction plus high pavement design life and durability.
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