Circular Economy in the Built Environment
The construction sector is one of the most resource intensive sectors. With over 3 billion tons of annual raw material input, a linear economy results in one-fifth of this ending up as waste. A circular economy in the built environment could not only reduce the stress on virgin resource mining but also have profound climate change impacts.
The expansion of the global economy and rising living standards have resulted in a radical increase in the consumption of raw materials, with a 60% increase since 1980. This is further projected to double by 2060.  According to World Economic Forum (2016), the engineering and construction sector dominates the overall resource use with over 3 billion tons of raw materials being used annually.  At the same time, a linear economy model of take, make, use and dispose in the construction sector results in one-fifth of the material extracted worldwide ending up as waste annually.
In India, the Construction and Demolition Waste Management Rules (2016) define construction and demolition (C&D) waste as ‘any waste comprising building materials, debris and rubble resulting from construction, remodeling, repair and demolition of any civil structure.’  It is estimated that new construction generates around 40 to 60 kg of C&D waste per sq. m of built area, whereas demolition waste generates around 300 to 500 kg per sqm of built area.  This aggravating demand for materials and resultant waste in the built environment have put a strain on virgin sources entailing significant loss of value all along the material chain. Therefore, a transition to the restorative model of the circular economy model could help ease the pressure on the existing natural resources and provide material supply security.
Circular Economy in the Built Environment is a regenerative design approach for achieving the UN Sustainable Development Goals by designing out waste from the product’s life cycle. Circular Economy can be applied at different scales in the built environment: materials, components, buildings, neighbourhoods, and cities.
Buildings as a ‘system of layers’
Buildings are generally conceived, designed, and constructed as static entities in time. However, they continuously change and adapt to user demands and environmental conditions over their lifespan, making it a ‘series of different buildings over time.’  CE considers time as a critical dimension of design, which is often neglected in a linear economy. This relation between space and time can be understood by the shearing layers concept. A building consists of six functional layers characterized by their lifespan: site (infinite); structure (30-300 years); skin (20-50 years); service (7-15 years); space plan (3-30 years), and stuff (0-3 years). Think of it as an onion. Some layers are more flexible and independent than others, such as space and stuff. The structure and skin can only be accessed once the outer layers are peeled. Understanding this dependency between the building layers is crucial to make conscious choices after one layer has reached its end of life.
Another way to distinguish the building layers is the hierarchy of material levels entailing the technical and physical dependencies between these layers. The framework characterizes the building as a hierarchy of subassemblies at the building, system, and component levels:
Building level represents the group of systems carrying out the primary function, e.g., load-bearing, enclosure, partitioning.
System level represents components carrying out the system function, e.g., finishing, insulation.
Component level represents the layered assembly of component functions, allocated through elements and materials at the lowest building assembly.
For instance, let’s consider the case of building facades. The average functional lifespan for facades is 30 years. However, the facade can consist of components with technical lifespans ranging between 10 to 100 years. It means the facade has to be independent on a functional level from other building components. At the same time, the arrangement of components and materials within the facade should also be an independent part of the system to enable transformations on the component and material level. The functional, technical and physical hierarchy defines the dependencies within the building layers. These are helpful to devise strategies to apply the circular economy principles – designing out waste, building resilience through diversity, relying on energy from renewable resources, thinking in systems, and converting waste to resource – to the built environment. 
To give an example, Delhi’s beloved Chanakya Cinema opened its doors back in 1971. As the requirements from that piece of land changed over time, it was decided to demolish the structure in 2005 and instead build a multiplex there instead. Materially speaking, it lived a short life of 36 years, about one-third of what it could have lived. Had circularity been a concept back then, perhaps space and function could have been adapted like a Rubik’s cube to accommodate the changing needs over time. Design for Disassembly (DfD), Design for Adaptability (DfA), Design for Remanufacturing (DfRem), Design for Reverse Logistics (DfRL) are few approaches to address circular economy in the built environment through design.
Peeling the building layers and defining their dependencies suggests that they are composed of three lifecycles: the cycle of the building, the cycle of its components, and the cycle of the materials used to manufacture the components. The life cycles, even though, become one during the use of the building, is not the same case before the construction and after demolition. The life that comes to an end is usually the service life of the material, component, or building. Hence, there lies a potential to extract the materials and components at the EO(s)L for their residual value. The usual end-of-life disposal strategy can then be replaced with a suitable R-strategy (reduce, reuse, refurbish, remanufacture, recycle) to prolong their life and reduce the quantity of C&D waste in the building industry. Identifying the buildings as a reservoir of materials and components can turn the ‘waste to resource’ for subsequent use cycles, thereby closing the material loops.
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Durmisevic, E., Transformable building structures, in Architectural Engineering and Technology 2006, Delft University of Technology: Netherlands.
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