The life cycle analysis could look at a future steady state after full application, but a more sophisticated analysis would consider expected rates of hardscape replacement and market penetration of new practices over the analysis period. The analysis could include use stage interactions with other systems, vehicles in particular. Initially, a greatly simplifying assumption would be that all hardscape would need to meet the same functional requirements with regard to vehicle interaction. Effects of hardscape on vehicles in cities include vehicle degradation and increased fuel use due to roughness, and increased fuel use due to energy consumption from deformation of the pavement itself . Very few cities manage their pavements based on roughness, and many do not want to manage roughness because of difficulties in handling roughness caused by manholes and other accommodations for under-pavement utilities. Deformation related energy consumption is primarily an issue for asphalt pavement carrying heavy, slow-moving trucks on hot days. Processes associated with those hardscape material flows, both inside and outside the system boundary would be modeled, as well as transportation of the materials, following LCA principles. The flows are the scaling factors for the processes at the urban area scale. Alternative scenarios for changes in the amount and types of materials, structures, changes in the amount of hardscape in different functional types , and changes in the amount of recycling within the urban boundary to reduce material transportation,vertical planters for vegetables and changes in the processes both within and outside the urban boundary to supply urban hardscape could be compared with current practices.

First-level analysis would look at the life cycle material flows, such as materials by type, massdistances of transportation and where applicable, land area needed within the urban area for stockpiling and processing. Second-level analysis would include calculation of important midpoint LCA indicators providing quantification of impacts of changes in flows. Hydrologic modeling of the most intense storm events would be performed to consider the hydrological impacts of changes in urban hardscape to make it more permeable, including partially and fully permeable pavement where they can both be used to also meet other functions. The effects of the changes in material flows and impacts from changing those flows caused by use of this type of hardscape would also be analyzed.In general, the quantification framework developed in this study could help identify where on the curve shown in Figure 5 we stand as of today. Implementation of better practice can cut down imports and flows resulting in lower costs and other environmental impacts.Hardscape covers large portions of urban surface areas, and has potentially large influence on air emissions, truck traffic and its associated problems, and the potential for flooding. Modeling the inflows of hardscape materials and the outflows of demolished hardscape and other rockbased products from buildings and other civil infrastructure is expected to provide a means to find solutions for reducing these flows and their impacts. Modeling of urban hydrology with respect to the effects of hardscape on surface and groundwater flows from precipitation is expected to provide a means to find solutions that will reduce the risk of flooding and improve groundwater recharge. The sustainability of urban areas can potentially be improved through changes in the built infrastructure of urban hardscapes. A conceptual framework does not exist, but is needed, that considers the entire urban area in order to evaluate high-level effects of changes in the design, construction and renovation of urban hardscape.

The original idea for combining UM and LCA to investigate urban hardscapes came from a plenary presentation at the Pavement Life Cycle Assessment Symposium in Davis, CA by Stephanie Pincetl from UCLA . Using Pincetl and her colleagues’ work as a starting point, a system’s approach that considers rock products and hydrological flows was selected for the modeling. An urban area can be considered as a system, with the city or developed urban area limits defining the system boundary as shown in Figure 6. From the standpoint of material and resource flows, this system can hypothetically be optimized if it maximizes reuse of existing resources within the system boundary; i.e., energy and material flows are generated, consumed, and restored within the system. Hardscape materials are primarily sand, gravel and crushed stone, collectively called aggregate, by mass, with small amounts of binding agents such as asphalt binder and Portland Cement, and even smaller amounts of other materials such as recycled tire rubber, other polymers, and chemical additives. A substantial part of most pavement structures is comprised of base and subbase layers consisting of compacted aggregates. For pavement surface materials, aggregates comprise 80 to 85 percent by volume of typical asphalt concrete and 62 to 68 percent by volume of concrete . On a mass basis, aggregate is about 95 percent of the mass of asphalt concrete and 85 percent of the mass of hydraulic cement concrete because the aggregate is much denser than the binding agents. The facts that aggregate makes up the large portion of hardscape materials, compacted aggregate is very dense , and aggregate must typically be hauled to cities from quarries has strong effects on the fuel use for its transportation and on the damage to the roads over which it is hauled.

The environmental impacts of imports of new materials and exports of waste materials into and out of the urban area can be assessed using LCA-type calculations. If material use is reduced through recycling and reuse within urban boundaries, these environmental benefits or impacts of such changes can be quantified by comparison to current conditions. Potential unintended negative consequences of materials reuse include increased land use and environmental problems from storage and processing of reused materials in the urban area, which can also be quantified by UM-LCA. Innovative approaches to hardscape systems that move towards a balance in demand and supply from reuse of material resources will require less material from outside the system boundaries and may require less transportation. Furthermore, the system may potentially produce less waste and pollution in the urban area and will export less waste and/or pollution out of the system. Opportunities to move closer to such a system can be analyzed based on the flows of import and export of resources, waste and emissions within and outside the system for environmental impacts in a life cycle perspective. Changes in hardscape extent and technology that improve permeability so as to capture, detain and infiltrate storm water can be analyzed with respect to flooding, storm water quality and groundwater recharge. Some of the means for increasing the permeability of hardscape include the use of permeable pavement, bio-swales, and catchments,vertical farming technology all of which increase the infiltration of storm water through the surface. All of these approaches also slow the rate of runoff, and where infiltration occurs, reduce total runoff. There are various types of permeable pavement available, all of which include a sub-surface porous layer made with gravel to retain or detain the water.The needs of different urban areas for improvements in flood control, water quality and recharge will differ greatly depending on current and predicted climate change effects on precipitation patterns, their topography, size, drainage systems and land use patterns. Together, the potential changes in hardscape systems to be more permeable and re-usable within an urban system can be analyzed together with the proposed framework.The UM model accounts for the energy and material flows within urban areas and between the urban area and its surroundings, helping researchers to study the interactions of nature and human systems, affecting the environmental impact of a city and effects on human and natural ecosystems. Kennedy et al. defined urban metabolism as “the sum total of the technical and socioeconomic processes that occur in cities resulting in growth, production of energy and elimination of waste” .

Although others have studied the relationship of cities to their surroundings, Wolman in 1965 presented metabolism concept to address air and water quality in the U.S. and determined that increased urban population and industry growth created problems of water and air pollution. Furthermore, Wolman postulates that it is not the depletion of the water resource, but rather its mismanagement, that creates water problems. In subsequent years numerous cities worldwide including Brussels, Cape Town, Hamburg, Hong Kong, Lisbon, York, London, Singapore, Stockholm, Sydney, Tokyo, Toronto, and Vienna have been examined using the UM approach . Kennedy et al. studied data from urban metabolism studies since 1965 for eight metropolitan regions around the world and analyzed four fundamental cycles of energy, materials, water and nutrients. The authors found that water inputs per capita for six studies since the 1990s were higher than the four studies from the early 1970s. Due to the expansion of the cities, water table levels were found to be lower because of increased water demand for some cities such as Beijing and Mexico City while several other cities in the world water tables were found to be rising and water quality being highly affected mainly due to the discharge of wastewater into the ground. An increase in commercial and industrial waste was seen, however, the cities that had implemented recycling strategies at a large scale were producing less waste. The data also air pollution emissions over the same period. The study concluded that the urban metabolism results were different from city to city. Li et al. developed a general framework of an urban ecological infrastructure . Blue land , green land , grey land , exits and arteries build up the UEI system. All the sub-systems are tightly integrated and distributed amongst each other. Researchers have investigated transportation and street networks within the ‘grey land’ category have not been looked at in detail and almost no literature exists to guide roadway and other hardscape design in urban areas in light of responding to climate change effects on precipitation . Different methods such as the emergy or material flow analysis can be used in the UM approach. MFA is the most common and internationally accepted method that can be used to study the flows of resources into a city , followed by processing and consumption of resources, and flows of resources out of the city and emissions production . MFA approaches are typically focused entirely on the flows of materials or substances. However, researchers have also applied the UM approach under the umbrella of the three pillars of sustainability  and developed tools such as the integrated UM analysis tool . Others have suggested studying systems in a life cycle perspective and understanding urban sustainability by coupling UM and LCA, as is done in this white paper . Goldstein et al. applied the UM-LCA framework to five global cities and assessed the environmental impacts directly and indirectly caused by the mass and energy flows through the cities . The author concluded that the results of the study are first gross estimates due to inadequate data. Chester et al. presented evidence that better understanding of sustainable urban systems could be achieved by integrating UM and LCA frameworks, which would help in understanding physical flows and urban infrastructure system problems . A predecessor to this application of the UM-LCA framework to urban hardscape, Chester et al.used LCA to estimate the environmental impacts of building and operating parking spaces. Fraser and Chester used UM-LCA type analyses to estimate the materials and emissions investment in streets and roads in the Los Angeles area over time and the transition from heavy impacts from initial construction to increasing impacts from vehicle use and increasing demands for financial resources for maintenance and rehabilitation . This latter work indicates the importance of looking at urban impacts over a long enough analysis period, per LCA methods, to capture changes in impacts as the hardscape infrastructure both grows, or potentially shrinks if hardscape is converted to buildings or back to green space, and ages. Reyna and Chester evaluated the age and rate of demolition of building stock in Los Angeles . Recycling of concrete from building demotion recycling of all types of existing hardscape are the sources of raw aggregate based materials for new hardscape, the use of which is one of the cases proposed for exploration later in this document. LCA data and analysis approaches for use in an UM-LCA framework have greatly improved over the last ten years.