Understanding material stocks in a built environment

Buildings and construction are major sources of economic activity, employment and material throughput globally. The sector is also very wasteful, with estimates in the UK, for example, of between 7 and 15% of products not being used in the final construction, much of it landfilled. Over the past century, the overall use of construction materials has increased by a factor of 42; the same period has seen a 23-fold increase in the accumulation of materials (792 Gt) within stocks of buildings and infrastructure.

The built environment is a vast amalgamation of numerous structures and units, made of different resources, which together aid in human activities and facilitate social and economic exchange. The rapid growth in the population and the continuous desire for accommodating the needs of this population has caused an explosion of new construction and infrastructural projects, consuming surplus amounts of resources and simultaneously causing waste and emissions.

A suggestive routine for eliminating the quarrying or mining of virgin resources and also to minimize the wastes would be to circulate the materials back to the new construction projects at the end of the service life of the structure (Ness & Xing, 2017). Such materials, which are used again or reused are called secondary materials. These materials can also be recycled or upcycled depending on their properties and the requirements of the project in consideration. For the purpose of recirculating the materials, it is necessary to understand and determine which materials are available and in how much quantity (Volk et al., 2019). This is possible by the determination of material stocks. ‘Material stocks’ is nothing but a simple accounting of available resources in a structure or product, the quantity and quality of these resources, along with the tentative timeline of when these resources will be available for reuse (Deetman et al., 2020). Detailed analysis and understanding of material stocks aids in bringing the materials back to the economy – thereby reducing wastes and pollution and at the same time-saving costs and conserving resources.

The analysis of material stocks can be as elaborate a process or a simple calculation depending on the structure or product for which this analysis is performed. For example, let us consider the case of a city with buildings. A building is a composite structure made of steel, concrete, timber, glass, plaster, etc. Let us suppose that the building is a residential building and has a life span of 50 – 60 years. In general, material stock accounting typically uses four types of methods which are bottom-up material accounting, top-down material accounting, demand-driven modelling and satellite driven remote sensing methods (Tanikawa & Hashimoto, 2009). In this example, we use the top-down approach. Based on the general model, the inflows, stocks and outflows are estimated based on the annually constructed floor area, a total number of buildings, apartments or total floor area and demolition data. This data can be obtained from the regional administrative bodies and further details from building layouts.

The total building stock is calculated from the number of total housing units according to annual housing reports. Housing unit drawings and layouts are used to calculate the building components. For individual materials, the data is assimilated from the regional construction authorities. The general model applied for material calculation is given by:

Stock=Inventory of Material x Material Intensity Factor

Where Stock represents the total volume or weight of individual material, Inventory of Material is defined by the number of the total units while the Material Intensity Factor is defined by the material intensity by volume or weight per unit. Total stock represents the sum of individual material stocks. A framework for the assessment of material stocks in a structure is shown in figure 1.

Figure 1: Framework for material stock assessment (Arora et al., 2019)

Concrete Usage Index (CUI) is used for material intensity factor comparison among construction projects. CUI is an indicator of the amount of concrete required to construct a superstructure which includes structural and non-structural elements. CUI is defined as the volume of concrete in cubic metres to cast a square metre of constructed floor area.

To account for various building components, dwelling construction plan and layouts are studied to list each component in a specific dwelling type. With the annual demolition data retrieved from annual reports, total dwellings demolished are calculated. Based on demolitions data, the overall outflow of materials and building components is estimated for the selected timeline.

In India, the scope for incorporating the material stock analysis is extensive. The very fact of India being a developing country provides a broad horizon for the implementation of such innovative ideas. New cities can be planned while incorporating the accounting of material stocks. Also, existing cities can be analysed for material stocks. This can aid in the reuse of these materials and also account for developers for the disposal of construction and demolition wastes. India needs more policies in this forte as the current policies are essentially based on managing waste instead of efficiently using the resources.

Construction minerals account for the highest extraction rate of raw materials worldwide, and buildings present the largest material stock. Creating a circular economy building system requires an ability to couple closely the recovery and reuse of products from end-of-life buildings to stock replacement and maintenance. Thus, determining material stocks can aid in material recovery and reuse in a more efficient manner. In a circular economy, building and construction system demand will be created through a combination of factors, including efficient and proven techniques for selective deconstruction and segregation of products, cost-effective remanufacturing and reuse certification processes, that creates competitively priced products and breeds confidence, coupled with building designs that are better equipped to incorporate reused products and shifts in procurement policies and regulation to stimulate reused product. Individual innovations such as online marketplaces and exchanges for building wastes and products.

About the Author

Purva Mhatre (LinkedIn) is a Doctoral Researcher at the National Institute of Industrial Engineering, Mumbai working in the department of Sustainability Management. My research is based on the incorporation of the Circular Economy in Built-environment.

References

Arora, M., Raspall, F., Cheah, L., & Silva, A. (2019). Residential building material stocks and component-level circularity: The case of Singapore. Journal of Cleaner Production, 216, 239–248. https://doi.org/10.1016/j.jclepro.2019.01.199

Deetman, S., Marinova, S., van der Voet, E., van Vuuren, D. P., Edelenbosch, O., & Heijungs, R. (2020). Modelling global material stocks and flows for residential and service sector buildings towards 2050. Journal of Cleaner Production, 245, 118658. https://doi.org/10.1016/j.jclepro.2019.118658

Ness, D. A., & Xing, K. (2017). Toward a Resource-Efficient Built Environment: A Literature Review and Conceptual Model. Journal of Industrial Ecology, 21(3), 572–592. https://doi.org/10.1111/jiec.12586

Tanikawa, H., & Hashimoto, S. (2009). Urban stock over time: Spatial material stock analysis using 4d-GIS. Building Research and Information, 37(5–6), 483–502. https://doi.org/10.1080/09613210903169394

Volk, R., Müller, R., Reinhardt, J., & Schultmann, F. (2019). An Integrated Material Flows, Stakeholders and Policies Approach to Identify and Exploit Regional Resource Potentials. Ecological Economics, 161(August 2018), 292–320. https://doi.org/10.1016/j.ecolecon.2019.03.020