Material metabolism and lifecycle GHG emissions of urban road system (URS)

The intensive activities of urban infrastructure construction lead to a considerable amount of materials flowing into cities. With these processes of material metabolism, a huge level of energy consumption occurs throughout the lifecycle. Identifying the metabolism processes enables decision makers to better understand the causes of resource and environmental issues, therefore to propose corresponding measures for optimizing urban management. Focusing on the road systems in megacities, we probed into the construction standards and conducted in-site surveying. Based on the stock model and inventory building, studies on the metabolic processes, lifecycle energy consumption, and GHG emissions were carried out. The results demonstrated that: 1) The current road system in Beijing features large stocks and small flows. In particular, the in-use stock is 164.6 Mt, mainly involving 13 types of general construction materials. Of the total stock, 79.2% is stored in roads, and the rest is stored in ancillary facilities. 2) Microcirculation roads account for relatively smaller proportions in road network rationing. Regulating the grade proportions and the spatial layouts are feasible ways to reduce the input of infrastructure resources. 3) The two main stages with massive GHG emissions are production and maintenance, in which clinker production is the largest contributing process. The key to solid waste control and emission reduction in the long term is to further improve the recycling rates and technique upgrading for new pavement materials, not simply prolonging the service years.