From 1900 to 2010, the amount of materials accumulated in buildings and infrastructure worldwide increased 23 times. We are consuming resources at an unprecedented rate. Now is not to start extracting dwindling raw materials from nature at higher and higher costs, but time to start reusing the raw materials in our cities’ buildings and infrastructure.
We have been trying to find material resources in cities that can be “mined” for reuse. In the case study, we modeled more than 13,000 buildings in central Melbourne, Australia. We estimated the amount of building materials and the specific energy, water, and greenhouse gas emissions associated with the construction of these buildings (if buildings are built today). We also simulated material changes over time and in the future.
The exploitation and transformation of resources have extensive environmental impact. These include resource depletion, loss of biodiversity, soil and water pollution, and greenhouse gas emissions, all of which are driving climate change.
In addition to these challenges, there is also a large amount of waste generated, especially in the construction sector due to construction, renovation and demolition activities. Every time building materials are discarded, all the embodied energy, water and emissions generated are also wasted.
In our two recent studies, we have proposed a model that can help us “mine” our cities and quantify the environmental benefits of such urban mining.
Model all buildings in the city
Let’s take the city of Melbourne as an example, which represents the central business district and surrounding areas. This is because the City of Melbourne provides open source code and high-resolution data, allowing us to model the materials in a single building.
By combining the land use and employment census database with the building footprint database, we can obtain the basic geometry and building-related data of each of the 14,385 buildings in the area in 2015. Among them, we modeled 13,075 models, or unique buildings (stadiums, train stations, etc.) must be discarded. We used 48 architectural prototypes to represent different buildings in the city based on the building type (such as office), construction year (such as 1987) and height (such as 25 meters).
The results show that there are 1.5 million tons of material stocks per square kilometer in Melbourne. Merely replacing worn-out materials to maintain the inventory of such buildings is estimated to require 26,000 tons of materials per year.
Today, the reconstruction of the city of Melbourne will use about 10 oil equivalents of energy and 17.7 million cubic meters of water. The building will emit 605,000 tons of greenhouse gases per square kilometer.
These resource requirements are simply huge. For example, the energy consumed per square kilometer is enough to drive 700,000 cars from Melbourne to Sydney. Imagine the development of all cities in the world, and then try to imagine all the resources needed.
In addition to generating these overall estimates, the real value of our model is that it provides detailed information about the use of materials in cities. It can answer questions such as when and where building materials and quantities are needed. And what are the related environmental impacts?
Visual logistics
For the 13,075 buildings modeled, we finally obtained more than 500 million data points. We need to provide effective data visualization to inform decision makers how to mine cities.
The most important results we found are displayed in a map, which shows the quantity of each material in each building. We can do this in any given year or period. This allows us to track the quantity and quantity of materials expected to be replaced.
These maps allow us to begin to think of cities as mines and places of material production (supply), not just consumption (demand).
We can imagine how a new construction project can investigate what materials are available at the beginning, and how best to reuse these materials and incorporate them into the design. This will save a lot of energy and water, while avoiding further ecosystem degradation caused by greenhouse gas emissions and raw material extraction (usually far from the city).
We borrowed the concept of age pyramid from population research to represent the material inventory in the city’s buildings. This allows us to understand which materials are kept in inventory and for how long.
It can also help us better predict when large amounts of materials need to be replaced, which can lead to construction waste and the need for new materials. These maps show where it will take place.
The long journey of circular economy
Although we have developed tools that can help urban mining, for the construction industry, we are far from reaching a true circular economy.
First, our model has a lot of uncertainty: it is only as good as the data we provide. Although the City of Melbourne has high-quality data on its buildings, it is really difficult for us to estimate how long the materials can be used, especially on the scale of the building. We need more research and data to better estimate the material composition of the building, the period of material replacement or building renovation, and the materials that have been released and are available for reuse.
Second, we need to better design buildings for demolition. In most cases, the replaced material cannot be reused directly due to wear or damage (perhaps during the removal process).
If we design and prefabricate modular buildings that can be easily disassembled, the potential for reuse and recycling can be greatly increased. This will help us move towards a zero waste city.
The third way forward is to reconsider the design and purpose of building materials and systems. The company does not need to buy carpet for the office, but can pay for the “carpet-covered floor area” service. This will transfer ownership of the carpet to the supplier.
In turn, this will incentivize people to produce durable, high-quality materials that can be reused and recycled rather than eliminated as planned. This business model has already begun to emerge.
With the growth of the global population, limited resources and climate change, we need to quickly move towards a more circular construction industry to help ease the pressure on the earth’s resources and ecosystems. A model such as the one we are developing is a long journey.
