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Validating Economic and Environmental Feasibility of DURABROADS Asphalt MixesSeptember 2017

Divya Deepankar, Research Engineer, BSRIA Sustainable Construction Group

In continuation with the previous article on DURABROADS – Life Cycle Costing Analysis of Road System, this is the final paper to present the results obtained for the life cycle assessment and life cycle costing analysis of the DURABROADS asphalt mixes. BSRIA has been primarily involved in conducting the life cycle assessment (LCA) and life cycle costing (LCC) analysis of the potential new road layer mixtures formulated by the DURABROADS project partners to compare their environmental impacts and life cycle costs with the traditional materials. BSRIA presented the final results at the conference on – ‘Enhancing Research and Innovation in Europe: the Road Sector Experience’ held in Madrid on 12th September, 2017.

In the analysis, three pavement mixtures - AC (Asphalt Concrete), BBTM (Béton Bitumineux Mince), and PA (Porous Asphalt) were considered. A bitumen mixture for DURABROADS - referred as Durabroads Bitumen here - was designed to be used with all three cases, following which the overall mixtures were also modified with different additives such as evotherm, carbon black and SBS (Styrene Butadiene Styrene); and steel slag, basalt as aggregates. This analysis was carried out for six cases where the base case was the asphalt concrete road named AC Base. The other five pavement alternatives namely AC Alt, BBTM Base, BBTM Alt, PA Base and PA Alt were compared against the AC Base case to select the one that has the lowest environmental impacts and maximizes net savings. The functional unit that the study reports on is: “The impact assessment of a road cross- section of 7.4 m*1000 m (width*length) for a life cycle of 50 years”.

A lifecycle of 50 years has been chosen for the study because the service life of a typical well-maintained road subjected to high-traffic load is between 9-15 years before its surface layer is relayed and its average lifetime is 24 years before any major roadwork repair is done. Hence, a study period of 50 years would provide the optimum number of iterative cycles of road maintenance, surface relay and total road recycling across the lifecycle to generate well-informed, reliable results.This study has focused on a method in which the road construction, road maintenance and road relaying processes have been broken down into small process units. These process units have been used to build up a dynamic LCA and LCC model for each scenario.

Figure 1: LCA of total road system in the AC, BBTM and PA Base vs Alt by ReCiPe EndPoint (H)

Life Cycle Assessment (LCA)

Within the scope of this study, the LCA has been done for the following:

  • Cradle-to-Gate Assessment of 1kg of Bitumen
  • Cradle-to-Gate Assessment of Initial Construction Process of Road Surface Layer
  • Cradle to Grave Assessment of Total Road System

In the analysis of 1kg of Bitumen, a weighted average in each method shows a significantly higher environmental impact of the DURABROADS Alternative bitumen mixture which is due to the higher percentage of Carbon Black. However, this does not imply that the overall road system using the DURABROADS alternative material will also have higher environmental impacts. From Figure 1, it can be seen that overall, AC Base has the highest environmental impact while the environmental damage caused by BBTM Alt is the lowest for the life cycle assessment of the total road system.

Figure 2: Net present value of costs incurred for the total road system over its lifecycle of 50 years

Life Cycle Costing Analysis (LCCA)

The LCC Model was calculated as the net present value over a 50 year period using a discount rate of 6% per year. The 50 year period has been selected to match the period used for the environmental impact assessment done in the LCA while the 6% discount rate was selected as it is a typical value used for a private sector construction project undertaken by a trusted investor. The total life cycle costs include the construction, maintenance and transportation costs. From Figure 2, it can be seen that BBTM Alt has the lowest life cycle costs while AC Base has the highest.

Final Results of LCA and LCC

Finally, the study concluded with graphical results of life cycle assessment and life cycle costing analysis to select the scenario that has the lowest environmental impacts and the lowest life cycle costs for the total road system. Based on the LCA and LCC analysis of the total road system across a life cycle of 50 years, the BBTM Alt has the lowest environmental impacts and lowest life cycle costs while AC Base causes maximum environmental damage with the highest life cycle costs. Overall each DURABROADS Alternative case had lower environmental impacts and lower life cycle costs across the 50 year life cycle when compared to its base case.

It is essential to understand that the LCA and LCC analysis is only a comparison between the base case against the alternatives. Hence, the results obtained do not represent absolute values but a comparative improvement or diminishment with respect to the overall base case scenario.

About DURABROADS and this article

This article has been produced in the context of the DURABROADS Project. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement No.605404. All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability. For the avoidance of all doubts, the European Commission has no liability in respect of this document, which is merely representing the authors view.

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