Environmental assessment of road constructions: Life cycle assessment of Swiss road pavements and an accompanying analysis of construction and maintenance costs

The importance of overall sustainable development in guaranteeing an appropriate quality of life for upcoming generations was underlined in recent years by the fact that several nations, societies and economic sectors agreed in various international protocols and contracts to reduce their environmental pollution and to enhance social living or working conditions. To integrate road infrastructure into the concept of sustainable development, infrastructure owners should take environmental and social aspects into account in addition to their economic based construction and maintenance concepts. This dissertation assesses representative Swiss asphalt, concrete and composite road pavements over chosen analysis periods with the goal of determining environmental potentials, i.e. reduction potentials regarding chosen environmental indicators, given by the processes of the life cycle phases of the pavements, and the influence of these potentials on costs occurring over the periods analyzed. This dissertation was directed by an advisory board containing 14 experts from all parts of the Swiss road infrastructure sector (governmental and cantonal departments, contractor associations, engineering consultants and experts, construction companies, road material producers, and research). The road pavements analyzed are pavements for national and cantonal roads. It was specified that processes of the use phase, i.e. use related processes such as vehicle operation, lighting, road cleaning, etc., would be excluded from the study. Hence, this study focuses on the environmental potentials and the economic influence of new construction and maintenance processes occurring over the life cycles of the evaluated pavements. Due to the fact that road infrastructure is maintained frequently to ensure an adequate level of service, average life times, i.e. the life cycle of different road constructions, are difficult to define. Therefore, the road pavements are analyzed over three chosen analysis periods (25, 50 and 75 years) applying three different maintenance intervention strategies for each pavement type (asphalt, concrete, and composite). The methodologies used to evaluate the road pavements chosen to be representative for the Swiss national and cantonal road network are the Life Cycle Assessment (LCA) according to ISO 14040 and Life Cycle Cost Analysis (LCCA) according to ISO 15686-5, which observe a product over its entire life cycle to determine its environmental (LCA) and economic (LCCA) impacts. The LCA results are expressed by several environmental indicators (Global Warming Potential (GWP) indicator, Cumulative Energy Demand (CED), Ecological Scarcity 2006 indicator, ReCiPe Midpoint (MP) and Endpoint (EP) indicators and the Cumulative Exergy Demand (CExD)). In order to determine the reliability of the results of the Life Cycle Assessments, Monte Carlo simulations were performed. This thesis is a cumulative dissertation and is divided into seven main chapters. Chapter 1 gives a brief introduction of the problem statement, the goal, the state of research, the research questions, the research approach, as well as the relevance and feasibility of the study. Chapter 2 includes descriptions of road infrastructure in Switzerland, the concept of sustainability, the state of the art for the field of sustainable (road) infrastructure, the quantitative research approach, as well as all applied methodologies and environmental indicators including a critical view on all methodologies and indicators. The four following main chapters of the thesis are formed by five research papers (Papers I to IV or rather Chapters 3 to 6). The first two papers were published in peer-reviewed scientific journals (Paper I: Journal of Materials in Civil Engineering - Life Cycle Assessment of the Production of Swiss Road Materials, Paper II: Journal of Management in Engineering - Hidden Ecological Potentials in the Production of Materials for Swiss Road Pavements). Paper III was submitted to a peer-reviewed scientific journal (Paper III: Structure and Infrastructure Engineering - Environmental Analysis of New Construction and Maintenance Processes of Road Pavements in Switzerland) and Paper IV is about to be submitted to a peerreviewed scientific journal (Paper IV: Environmental Science & Technology - Environmental and Economic Analysis of Representative Swiss Road Pavements). The contents of the papers correspond to the temporal occurrence of the pavements life cycle phases, i.e. starting with the material production, the life cycle phases are analyzed step by step until the overall analysis of the road pavements over the chosen analysis periods has been completed. Papers I-III focus on the environmental analysis, i.e. the LCAs, of the road pavements life cycle phases (material production, new construction, maintenance and different LCA-models) with regard to the Global Warming Potential indicator, the non-renewable Cumulative Energy Demand and the Ecological Scarcity indicator. In Paper IV, the step-bystep analysis of the road pavements is finalized by a combined LCA and LCCA study, which investigates the road constructions over the three analysis periods taking the three maintenance strategies into account (Figure S-1). Chapter 3 (Paper I) is the first part of the stepwise analysis of the life cycles of the different road pavements and the detailed analyses of the production of all materials applied within the road pavements chosen to be representative for the Swiss national and cantonal network. Accordingly, the environmental reduction potentials within the analyzed material production processes regarding the Global Warming Potential indicator, the non-renewable Cumulative Energy Demand and the Ecological Scarcity indicator are demonstrated for each material by applying the cradle-to-gate LCA approach, which analyses all production process of a road material, from raw material acquisition until the moment the finished product leaves the production plant. The results of Chapter 3 show that material production processes for one cubic meter of road material offer reduction potentials regarding the applied environmental indicators of up to 81 % for asphalt mixtures (in comparison to the Non-renewable CED results of standard and best case production of AC F 32), 38 % for concrete mixtures (in comparison to the Ecological Scarcity results of standard and best case production of bottom concrete), and 69 % for subbase mixtures (in comparison to Ecological Scarcity results of standard and best case production of hydraulically bound subbase mixtures). Chapter 4 (Paper II) combines the results and potentials of the material production processes analyzed in Chapter 1 according to the superstructures of the representative cantonal and national road pavements. The results of Chapter 4 show reduction potentials with regard to the utilized environmental indicators of up to 59 % for asphalt pavements (in comparison to Non-renewable CED results of standard and best case production of T4 S4 asphalt pavements), 54 % for concrete pavements (in comparison to Non-renewable CED results of standard and best case production of T4 S4 concrete pavements) and 38 % for composite pavements (in comparison to Nonrenewable CED results of standard and best case production of T6 S3 composite pavements). In Chapter 5 (Paper III) the environmental analysis of all processes needed to construct and maintain the representative road pavements is described with a focus on a detailed evaluation of material transport, pavement construction and deconstruction processes, as well as upgrading processes of reclaimed material to reusable recycled material. Results are expressed in terms of Climate Change, non-renewable CED and Ecological Scarcity for a full new construction and a full replacement of the analyzed pavement types. The results in Chapter 5 show the great influence of the material production processes on the LCA results for the complete new construction (on average 90 % of the total impact of all new construction processes of the three pavement types) and the total replacement (on average 75 % of the total impact of all processes needed to fully replace the three pavement types) of all pavement types. The influence of the material transport to the building site with an average transportation distance of 25 km was determined to be 9 % of the total impact of all new construction processes (on average for all three indicators) and 8 % of all processes needed for a full replacement of all pavement types (on average for all three indicators). The impact of construction and deconstruction processes can be described as insignificant with an average influence of 1 % of the impact of the complete new construction and the full replacement of the three pavement types. The "upgrading" process of reclaimed hydraulically bound materials (concrete and hydraulically bound foundation layers) has an average impact of 10 % of the impact of the full replacement of concrete or composite pavements, but also of asphalt pavements containing a hydraulically bound subbase (with regard to all three indicators). Bituminous bound materials are already upgraded to reusable asphalt granulate by the mill cutter applied for the deconstruction process. In Chapter 6 (Paper IV) the stepwise evaluation of the road pavements is finalized by a combined Life Cycle Assessment and Life Cycle Cost Analysis study investigating the representative pavements over the chosen analysis periods applying the different maintenance strategies. This overall analysis assesses the pavements regarding the whole variety of environmental indicators mentioned before. Results demonstrate the influence of all analyzed processes and maintenance strategies on the overall environmental and economic results of the assessed pavements. The results in Chapter 6 show reduction potentials given by the material production processes (comparison of standard and best case production) occurring over the different analysis periods of up to 42 % (Non-renewable CED) for asphalt, 39 % (Non-renewable CED) for concrete and 29 % (GWP) for composite pavements. Regarding the application of different maintenance strategies (variant 1, 2 and 3) reduction potentials of up to 21 % (Non-renewable CED – comparison of variant 1 and variant 3) for asphalt pavements, of up to 31 % for concrete pavements (GWP – comparison of variant 1 and variant 3) and of up to 30 % for composite pavements (GWP – comparison of variant 1 and variant 3) were determined. Over 75 years, overall reduction potentials (a combination of optimized production processes and longer service life) of up to 51 % (Non-renewable CED) for asphalt, 52 % (GWP) for concrete and 48 % (GWP) for composite pavements were identified. The influence of environmental potentials offered by material production processes is not reflected in the economic results, because typically applied positions of construction tenders in Switzerland do not specify recycling and production characteristics for the different materials. The reduction potentials for the economic results given by longer service lives, i.e. the optimized (variant 2) and the aspired maintenance strategies (variant 3), were determined to be up to 22 % for concrete pavements, up to 23 % for composite pavements and up to 15 % for asphalt pavements. The thesis is brought to a close in Chapter 7, which summarizes and discusses the results and the findings of the Chapters 3 to 6. The material production processes and the applied maintenance strategy turned out to be the most influential factors on the results of the overall LCAs (from-cradle-to-grave). Furthermore, it was demonstrated that none of the analyzed pavement types (asphalt, concrete, and composite) offers the best results regarding all analyzed environmental indicators. It was also stated that the positive influence of longer service lives on the environmental and economic results does not signify that road owners should wait as long as possible to perform maintenance interventions. The results of this dissertation should encourage road owners to utilize materials with reduced environmental impacts and technical advantages, which under given conditions (traffic load, budget, geographic and climatic influences, etc.) guarantee optimum service life for the different layers of the road pavements. Based on the discussions and conclusions, recommendations for future research and investigation were expressed, such as a focus on the environmental performance of road materials and their production processes, as well as the implementation of the Life Cycle Assessment methodology into pavement management systems.