Transport Infrastructure Ireland (TII) posed an important question: What does it mean to optimise a road? To reduce a road’s carbon footprint and lessen the carbon and economic costs across the asset’s lifecycle – not just through road use, but through the design of the road itself?

TII asked Arup’s circular economy and road-pavement design specialists to explore opportunities to innovate road design using circular-economy principles.  

The materials used in pavement design – concrete, asphalt, gravel and soil – make up the largest proportion of carbon emissions related to the construction and management of roads. We worked with TII to develop a design procedure that transitions from historical empirical design methods to a materials performance approach. This takes advantage of more detailed pavement modelling and an increase in digital data management capacity to harness the properties of the materials within the design process.  

Intervening early in the pavement materials design process enables a significant reduction in carbon impact, construction waste and long-term expense for a road. TII and Arup ran a case study on 15km of Ireland’s National Road dual carriageway, helping to reduce bituminous materials by 8% and reducing embodied carbon emissions by more than 450,000 kg CO2e.  

Employing circular economy principles

When TII set out to determine a way to optimise the vast network of roads across Ireland, they first questioned the potential to innovate road design fundamentally, as the old method of road design relies on a limited number of design factors, including significantly lower historic traffic levels. 

The design method developed by TII and Arup looks in detail at choosing the correct materials for road design by taking a performance-based approach.

Updating the pavement design methodology presents two opportunities: firstly, it makes modern analytical methods available to designers, and secondly, it takes into account the greater level of detail available about material performance characteristics. Using this updated methodology will reduce material consumption and carbon impact of designing new roads and repairing existing ones. 

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Designing sustainable transport infrastructure
Aerial view of a highway

Building on the ‘layers-based’ approach to infrastructure design, which focuses on the inter-connectedness of an asset’s elements, the new design method developed by TII and Arup looks in detail at choosing the correct materials for road design by taking a performance-based approach. This enables the assessment and use of new and innovative materials, allows designers to specify and use only the necessary amount of each relevant material for specific road conditions and contexts, and creates the opportunity to design for reuse.  

The result is a significant reduction in construction waste and cost, and, ultimately, carbon footprint of roads. 

Materials data is inputted to TII’s web-based software, allowing for iterative design testing and helping designers to consider multiple impact factors. These take into account material performance characteristics, as well as other critical factors such as traffic load patterns, pavement structure and layer thickness, environmental and ground conditions and performance risk. Having a digital record of the materials used in all road designs will help TII to improve monitoring, optimise costs and predict maintenance requirements.