Functional Unit Choice for Comparative Pavement LCA Involving Use-Stage with Pavement Roughness Uncertainty Quantification (UQ)

An analysis of the use-stage for pavement life-cycle assessment (LCA) is presented within a framework of an LCA tool developed for Illinois Tollway. Methodological choices that can significantly affect LCA results were evaluated in this study. The share of the use-stage in a comprehensive pavement LCA framework was evaluated with all life-cycle stages including materials, construction, use, maintenance, and end of life. The scope of the use-stage includes albedo, carbonation, and rolling resistance (including pavement roughness and texture components). Uncertainty of the pavement roughness and its effect on the results are investigated using Monte Carlo sampling technique. A discussion on the choice of functional unit and its effect in comparative LCA is presented and an appropriate functional unit is suggested. Four projects were selected as case studies to demonstrate the capabilities of the tool and proposed framework. A multi-point environmental performance evaluation was performed using four environmental indicators for comprehensive interpretation of results. The effect of each stage of LCA was evaluated with emphasis on the results of the use-stage. Additional fuel consumption and emissions, resulting from roughness and texture, constituted the largest share of use-stage impacts while the effect of carbonation was limited. As traffic reduced and the share of the materials and construction stage increased, the share of the use-stage could be decreased to 50% levels. than energy and GHG as outlined by characterization methods such as TRACI (Bare, 2011), CML (Guinée et al., 2002). This is provided by a multi-point environmental assessment opportunity, rather than relying on only energy and GHG. Common challenges in performing pavement LCA were reviewed and discussed in details by Santero et al. (2011a, b) through a comprehensive review of existing pavement LCA literature and modeling tools as of 2010. Data and modeling gaps were identified in pavement LCAs and in particular for the use-stage, feedstock energy of bitumen, impact of traffic delay, maintenance phase, end-of-life stage, and inconsistencies for various methodological choices were summarized. Inyim et al. (2016) and AzariJafari et al. (2016) reviewed the current literature and pointed out similar gaps in more recent studies. One of the challenges in performing comparative LCA is the choice of functional unit (FU). The functional unit is a unit of measurement of system components to which inputs and outputs of LCA are normalized. According to ISO 14044:2006, functional unit is “quantified performance of a product system for use a reference unit”. ISO standards recommends to choose functional consistent with the goal and scope of the study where primary function and performance characteristics of the product are specified. The purpose of the functional unit is to quantify the services delivered by the product system. Therefore, the functional unit shall declare the relevant functions of the product with some performance characteristics. The flaws in some of the commonly used FUs are discussed in the following sections and a proper FU is proposed. While comprehensive methodological choices are needed to implement the pavement LCA, there are many uncertainty sources associated with pavement LCA. Some of these uncertainties include, but are not limited to: input variability, human errors, source credibility, and parameter and model uncertainty. Studies have focused on the uncertainty analysis of pavement LCA (Noshadravan et al., 2013; Gregory et al., 2016). Uncertainty quantification (UQ) methods are mathematical methods that help identify, propagate, quantify, and interpret the uncertainties in the system. Each stage of the pavement LCA may need different handling of uncertainty depending on the type of the source. Quantifying uncertainties would ultimately help understand the variations in the pavement LCA analysis and to what extent these variations affect the reliability of the outcome. Moreover, this would let decision-makers consider ranges of outcomes rather than deterministic values, henceforth, helping in informative decision-making with better understood consequences. The main goal of this paper is to present the essential components needed to perform a complete LCA (cradle-to-grave analysis) to compare pavement systems with varying designs, and traffic characteristics. The main components targeted in this study include selection of an appropriate functional unit and uncertainty quantification. Uncertainty quantification of the use stage is performed by studying the variability in the pavement roughness. Both of these components are presented with illustrative examples through case studies. 2 METHODOLOGICAL CHOICES AND LIFE-CYCLE INVENTORY ANALYSIS 2.1 Goal of the Study The goal of the study is to develop an LCA tool and framework for the Illinois Tollway in order to perform a comparative LCA between projects constructed in the past and planned for future. The intended application is to evaluate the progress toward the agency’s sustainability goal from environmental performance perspective by reporting and comparing environmental performance of past and present projects, and pavement type and design selection using environmental performance in addition to cost and performance.