5.1. Methodologies Adopted in the Literature to Implement Buildings as Material banks
Based on the results presented in the previous section, several qualitative and quantitative methodologies were adopted in the literature for research involving DRC management through the evaluation of buildings as material banks, which vary according to the topic addressed.
Along these lines, 34% of the articles addressed the topic of Construction and Demolition Waste Management, with many of these proposing methodologies for quantifying the generation of construction and demolition waste in cities, as well as the amounts recycled, destined for landfills or discarded from irregularly, through case studies and statistical analyses. According to [
44], knowledge of these data can support public policies to encourage more sustainable initiatives, such as applying sanctions for polluting practices, as well as supporting the population's awareness of this environmental problem. However, limited data in many regions still makes it difficult to accurately estimate the waste generated.
For [
27,
45], studies on CDW management based on the circular economy still address recycling processes much more than reuse or reducing resource consumption. In the articles evaluated in this research, this behavior was also observed, in which many studies evaluate the use of recycled aggregate as the most sustainable alternative for CDW. Also according to [
27], to make the evaluation of alternatives more robust, LCA, which is applied in many cases, must predict credits due to reuse and other life cycles, as well as incorporate life cycle cost assessments.
Regarding material quantification studies in urban regions, most of the studies evaluate the intensity of materials such as concrete, wood, steel, plaster and glass, and their geographic distribution. Among the studies that go beyond these analyses, [
43] also evaluate environmental indicators of greenhouse gas emissions and water use, in addition to land use for each type and height of building, for the city of Melbourne, Australia. Approaches like this are relevant to relate the materials available in cities with their environmental impact, in addition to waste management, promoting better urban mining strategies [
43]. Furthermore, it is possible to establish a relationship between the spatiality of stocks and population density, checking obsolete buildings that can be immediate sources of resources [
46].
As the only global assessment initiative, [
47] proposed a method for quantifying the stock of materials present in rural and urbanized areas, of residential buildings, based on information from databases on population, energy consumption demand, built area per captures, and literature on material intensity. Although the research results have great uncertainties about the accuracy of the data, due to the specificities of each region and data limitations in some locations, it can be a starting point for a global analysis of the stock of available materials and future flows of materials and waste, which can be predicted based on different scenarios of climate and socioeconomic changes.
Many of these researches are based on a Material Flow Analysis (MFA), which considers the inputs, stocks and outputs of the system to estimate the expected amount of waste, and can use top-down or bottom-up approaches. However, these are still very specific studies and difficult to replicate, as they depend on the availability of data in the region evaluated [
48]. Given these difficulties, some studies combine the use of tools such as the Geographic Information System (GIS), BIM, statistical analyses, among others. [
34] also suggest the use of sensitivity analyzes to validate material flow analyses.
Among the studies that propose these methodologies, [
49] carried out a statistical analysis, using machine learning algorithms, to estimate a linear regression that could estimate the amount of demolition waste in the city of Hong Kong. Using waste data from 71 demolished buildings, the authors obtained a regression that described the amount of waste through six variables: year of construction, total number of floors, building height, total area, building perimeter and type of use of the building. These variables are the most used in studies on this topic.
Regarding the type of building, of the studies that applied methods that required this classification, 36.4% evaluated residential buildings, 45.5% residential and non-residential, and 4.5% only non-residential, data that are similar to the results obtained by [
50] in which 50% of the articles evaluated residential constructions. However, the authors highlight the importance of evaluating data for non-residential buildings, since they have a shorter useful life and, consequently, end up generating a greater amount of waste. As for systematization, BIM and statistical analysis are used as support technologies.
In order to summarize the material flow process proposed in the research,
Figure 8 presents the urban mining process scheme proposed by [
51].
Regarding research categorized as pre-deconstructive audit tools or methodologies, they include both studies on the evaluation of deconstruction or demolition alternatives, as well as the disposal of waste and components after this stage. According to [
52], defining the ideal rates of reused components in a new project is a multidisciplinary task, which must involve environmental, economic, technical and logistical criteria. For [
53], the main factors that affect the economic decision between demolition and deconstruction with recovery of components are the operational costs for deconstruction, environmental impact costs, recovery potential and market value of secondary materials, in addition to the rates of landfill. In this way, these studies present tools that can facilitate the process for decision makers.
Another topic covered was the use of technologies to support circular construction. Among the most relevant studies is that of [
54], who developed a unified virtual platform for managing the process of construction, demolition and marketing of recycled products. The initiative was applied in Italy, for public works, and is a possibility to be replicated for any other location or company, as long as interested parties are willing to get involved in the process of developing and providing data. An alternative to facilitate the process may be the use of block chain technologies, which decentralizes data management processes.
Business models for the circular economy were also a topic covered by only 4 articles. Among them, [
55] and [
56] applied the life cycle assessment and case study methodology to validate the proposals. However, [
56] assess that only environmental indicators are not enough to promote the circular product chain, therefore, in their study they proposed material flow analysis and input-output analysis, with the aim of monetizing the indicators and verifying the sectors and companies that have the most influence on the purchasing process of secondary products.
It can be seen from the evaluation of the methodologies used that the gaps observed by [
40], in the development of innovative circular construction models so that the industry adopts these principles in their activities, has not yet been resolved, as only 4 studies have addressed the topic of business models for circular economy. Therefore, more research on the topic is needed. To achieve this, it is also necessary to carry out a strategic analysis of the processes, and the SWOT Matrix is then proposed, which is discussed below.
5.2. Potential Applications of Buildings as Material Banks in Mitigating the Environmental Impacts of Construction and Demolition Waste
Through the analysis of the SWOT matrix, referring to strengths, it is observed that designing buildings as future material banks reduces the environmental impact by incorporating reused and recycled materials in new projects, mainly reducing the carbon embodied in the production of new materials. This is because reuse avoids or postpones new construction, and its embedded impacts [
57]. Therefore, project design and execution practices must be valued, even though the construction industry is focused on profit and is resistant to change [
58].
To enable this, design alternatives for disassembly and adaptability (designing layers of different useful life independently), and a more judicious selection of materials, which extends their useful life, are potential to be applied [
57,
59]. In this case, the greater the durability and life cycles of the component, the lower its environmental impact will be, by postponing the extraction of materials for new production. For this analysis, criteria for LCA of multiple cycles must also be established, as well as the environmental impact resulting from each material alternative must be considered [
60].
As for the project for disassembly and adaptability, it is a tool to be used in new buildings, in order to promote the future reuse of the building, whether through the removal of its components or through adaptive reuse. In this case, disassembly avoids the extraction of resources for the production of new products [
61], and adaptive reuse (ST14) promotes a new function for the existing park, optimizing land use and ensuring the updating of the building for new energy and energy requirements [
62].
According to [
63], these techniques must be combined with Eco design, standardization, modularization and prefabrication (ST7) methods, which rationalize the construction process, reducing waste generated at the stage. In addition to these, the design for manufacturing and assembly also makes construction more efficient, since in the case studies evaluated by [
64] the reduction was between 60% and 90% of the waste generated. 3D concrete printing was also mentioned as an option by the authors, an example being Dubai, which plans to build 25% of concrete components for construction with 3D printing by 2030, although the methodology still requires technological developments.
For [
65], the use of prefabricated construction systems with threaded connections are the most efficient for disassembly and subsequent reuse. Furthermore, steel and wood structures are preferable due to their reuse possibilities, compared to concrete, which can generally only be recycled [
66]. However, it is important to relate these studies to the energy consumption of buildings throughout their life cycle [
59], as with the greater energy efficiency of buildings, a large part of the impacts remain in the production and construction phase [
67]. Considering the possibilities of building rehabilitation, methodologies must combine indicators of energy consumption, water and materials and waste management [
68].
Furthermore, attention must be paid to the carbon incorporated in the processing and transportation of the material for reuse, as well as the source of data for LCA, as these are factors that directly impact the positive result of carbon savings [
55]. Regarding the risk of the existence of dangerous materials in the built stock, there are impacts from the point of view of the threat of contamination due to the use in older constructions of materials that are now prohibited, such as asbestos, in addition to the costs necessary for their safe removal [
69].
Two other benefits of better material stock management are the reduction in land use due to waste being sent to landfills, as this will be an increasingly scarce resource. Furthermore, when it comes to mineral resources, the reduction of extractive activities can provide the recovery of large areas that currently serve as quarries to supply materials for civil construction [
70].
Regarding the management of built stock, most of the studies focus on quantifying waste available in cities, which has as one of the benefits of enabling better urban planning, optimizing resource and waste management, mitigating the environmental impacts of urbanization and being able to strengthen businesses places for reusing components, as long as they are encouraged by public authorities [
71].
Even so, several authors point to the unavailability of accurate data on construction materials and components as one of the major challenges for research [
48,
71]. Other obstacles are the lack of standardized methodologies for quantifying and evaluating the environmental impact of CDW, the costs and environmental impacts of transporting materials, which have a high density [
72], and the short reuse gap, due to construction deadlines from commercial partners and storage difficulties.
To mitigate these negative impacts, information systematization tools to speed up the purchasing process, as well as mechanisms for displaying and marketing secondary materials can be applied [
63]. Regulated databases must be created, to establish a standard of information, and open, so that interested parties can supply them [
73]. Furthermore, these platforms can provide for the commercialization of materials, using Industry 4.0 technologies to support them, bringing the construction industry closer to the manufacturing industries, although studies on this need to be developed [
74].
[
75] performed a material flow forecast, based on data from the building stock and proposing future resource optimization scenarios through stakeholder perspectives and German resource management policies. For the authors, stakeholder involvement is fundamental for formulating policies and estimating future use of resources. Research like this allows better management of urban planning and building renovation, for greater energy efficiency and for promoting the circularity of materials in public or private works. This is because, once mapped, it is possible to predict when their replacement will take place, the volume of waste in a given location and the future demand for new materials [
76].
For [
77], this makes it possible to evaluate urban mining alternatives, being able to establish policies to encourage the circular economy more effectively. [
51] assesses that urban mining processes are essential to achieving sustainable development objectives in cities. However, to achieve the circular economy, it is necessary to assess the demand for reapplication of these components, strengthening recycling with high added value.
To operationalize this process, according to [
50] the use of GIS software allows companies to manage demolitions, logistics and commercialization of these materials. In a future scenario, manufacturers themselves could track, monitor and perform reverse logistics for these construction products. Thus, a more regional mapping of materials is possible, enabling the commercialization of resources with less transport impact over long distances. However, the existence of fragmented and uncertain databases on the existing stock of materials remains a barrier to this [
75].
When evaluating the stock of materials, distinguishing according to function (structural or non-structural) is relevant to better manage whether the resource will be available throughout the useful life of the building, or only at the end [
50]. Regarding structural systems, [
33] proposed a management system for reuse, in which the structural engineer can consult the elements available for reuse when designing a new building, based on the required resistance parameters. The idea is to reduce physical stock, promoting a just-in-time flow of materials. The authors highlight the need to make these structural elements traceable, in order to monitor their performance throughout the operation, and predict residual performance for new use. To operationalize this system, the authors propose using BIM, BigData, and RFID sensors technologies.
These tools, as well as artificial intelligence technology, are a key piece in implementing systematic circularity in construction [
78]. For the authors, simulation tools, learning algorithms, 3D printing, among others, can support the systematization of circular solutions at all stages of a building's life cycle. According to [
79], these technologies, which make up the fourth industrial revolution, called Industry 4.0, are essential for the transformation from a linear model to a circular economy in manufacturing industries.
According to the authors, smart cities align EC principles for government and society, around an urban context with low environmental impact [
79]. When dealing with these technologies, BIM is also presented as a tool to be combined with LCA, in order to evaluate the environmental impacts of the entire life cycle of the building through a digital tool [
80].
Material passports are another concept that integrates digital technologies with the management of construction resources, serving as a digital document that follows and tracks construction materials and components throughout their life cycle, having their information on performance [
81]. They serve as an inventory of the building's elements, their composition and possibilities for reuse and recycling, and can be a key piece for urban mining processes. Thus, it can be used in the initial phases of the project, to verify the most sustainable alternatives [
82].
For countries that already have a robust methodology for environmental audits of existing buildings, the methodology proposed by [
69] can be applied as a way to systematize and validate a limited set of environmental inventory data. These can be compared to the existing stock of buildings to predict waste materials in those buildings.
However, for countries that do not yet have these methodologies, the creation of a standardized digital database is an alternative for greater efficiency in analyzing this information. In this sense, [
83] apply a methodology in a case study of the construction of popular housing in Mexico, with which it is possible to estimate the amount of waste generated throughout the construction and its impact in terms of embodied energy, carbon emissions and disposal of waste.
Evaluating buildings individually, [
84] highlight the need for evaluation protocols that are more accessible to professionals to encourage selective deconstruction and reuse of components in the initial phases of the project. The authors developed a simplified assessment method, with five parameters covering environmental, economic and social aspects. However, it is worth highlighting that very generic protocols can mask results by generalizing the possibilities of reuse according to the type of material, disregarding how the construction techniques used can impact the instruments needed for disassembly and transport equipment. This has consequences both for the time spent on disassembly, as well as for carbon emissions and estimated reuse of components [
84].
Other difficulties highlighted in the literature are the additional costs for deconstruction (even though the greater demand for labor can have a positive social impact); the difficulty in reusing elements of the existing park due to technical, structural and performance problems, and consequently; the need to incorporate other materials to ensure safe reuse [
84].
To contain these barriers, it is necessary to create guidelines to standardize and optimize these processes, as well as regulations and certifications for reused materials. Furthermore, in the case of demolition, the high cost and low availability of landfills for CDW disposal must be considered, which can encourage reuse [
85]. In contrast to the additional costs, in a case study applied to a bridge built in London, [
86] evaluated that replacing the original materials with more circular solutions would reduce construction costs by up to 62%. The type of concrete was replaced, from traditional to another with a higher residue content, granite to limestone and asphalt with reduced binder content.
Regarding CDW management processes, [
83] warn that in less developed countries, such as Mexico, the laws are not strict, and there is no supervision over the disposal of this waste, which ends up in irregular landfills. [
87] assess that there is an underreporting of the generation of construction and demolition waste, which in addition to irregular disposal, can be reused in the informal market. In their study, the estimated waste generated annually reached almost 4 times the amount of waste formally notified in Cape Town. Likewise, in Bolivia, [
41] their study reached an amount of waste generation twice as high as that estimated by the government.
In order to produce revenues that encourage the correct disposal of waste, recycled aggregates (AR) plants can be a proposal. To this end, it is important to focus on sorting waste directly at the source, in order to ensure higher product quality [
41]. Another possibility to contain this threat is the creation of local business models, which promote a closed cycle of products, strengthening the local economy and reducing the environmental impacts of irregular disposal [
83]. It is noteworthy that of the studies evaluated, recycling of aggregates was the main strategy to promote the circularity of construction waste, despite not being at the top of the waste hierarchy.
For underdeveloped regions, [
72] assess that expected revenues may not be sufficient to cover the investments required for recycling facilities, requiring political incentives. Furthermore, the management of recycled waste must take transport distance into account, and it is recommended to shorten distances by creating small-scale recycling plants, closer to large urban centers [
88]. From a technical point of view, [
42] also analyze that there is a high rate of mass loss for the AR to meet the resistance requirements. For reuse in structural elements in Germany, the loss would reach 52%, and it is only possible to incorporate 35% to 45% of recycled masonry and concrete aggregates, respectively.
As for business models for the circular economy, [
55] warn of two threats to the material reuse process: restricted access to secondary materials and market restrictions on the use of secondary materials. [
89] found in The Netherlands that secondary materials would not yet have the capacity to meet the demand for new construction within the limits of the evaluated municipality. To contain these threats, opportunities include the formation of partner networks to supply secondary materials and the identification of customer segments that value products with a lower environmental impact. However, to make these partnerships effective, companies need to actively act in the sector, sometimes having to get involved in deconstruction processes [
55].
From an organizational point of view, an alternative to enable better waste management can be public-private partnerships (PPP), although the bureaucracy of these contracts can be a challenge. It is necessary for governments to be involved in the formulation of public policies that encourage connections between different stakeholders, in order to promote a multidisciplinary environment that assesses the environmental, economic and social impacts of the process, and can even finance circular business models [
90].
[
91] defend the need for integration between municipal management and micro and small businesses, through strategies that facilitate better waste management. The first is the creation of municipal CDW storage spaces, which the City Hall makes available for local companies to use, training them in the correct separation of waste. In addition, the municipal management must intensify construction site inspections to monitor CDW management, making companies aware of this, as well as the importance of keeping documentation relating to these processes up to date. Despite this proposal, in Brazil, most City Halls do not link small construction licenses to responsibility for correct waste management.
Public policies to encourage reused materials can also enhance urban mining [
55]. An existing example is a decree from Italy, which determines that at least 50% of the mass of construction components, excluding installations, must be subject to selective demolition at the end of their useful life and be recyclable or reusable, being at least 15% made up of non-structural materials [
65].
According to [
92], greater awareness of the correct disposal of construction waste comes from government initiatives such as tax incentives for recycling and greater supervision of irregular waste dumping, since professionals still do not perceive political control over this. [
72] assess that public pressure can generate more satisfactory results, on the other hand, [
64] found that charging fees from contractors on generated waste was the greatest incentive for the application of CE practices. According to [
70], an alternative may be the application of clauses for the incorporation of recycled content in public works.
In addition to this, [
93] assess that professionals are more willing to pay for more efficient CDW management processes when they are aware of the importance of these practices and the circular economy. Therefore, it is important to invest in actions to expand professionals' knowledge on the subject, as well as in strategies to improve transport, management and productivity of CDW management processes.
The low cost of primary materials is another threat that differs depending on each country's natural resource reserves. Countries like Brazil, for example, still have a substantial reserve of natural resources to subsidize construction activities, meaning the market is not concerned with initiatives to make the sector more sustainable [
80]. For [
94], Brazil, which has low-cost natural inputs for construction, is unable to make recycled or reused resources competitive, unlike Europe, which has greater recycling efficiency and more expensive raw materials.
To contain the problem, [
17] suggest targets to be imposed for value-added reduction, reuse and recycling (upcycling), together with tax incentives for companies that meet the targets. In the case of Brazil, [
80], they also warn of the market's concern with the accumulation of taxes on the product with each sale, which would make the different life cycles of a product unfeasible. Therefore, tax benefits for projects based on EC and using construction and demolition waste are essential and can be complemented with facilitated financing and reduced interest rates for this audience.
Innovations can also be applied at an organizational level, with the application of product-service systems, promoting a change in the construction production chain through the rental or sharing of reusable components, which would extend the useful life of these products and avoid the consumption of new materials. Or, alternatively, actions for urban industrial symbiosis are solutions that can be applied by the industry to better plan the reverse logistics of products, adding value in the process [
95]. However, they need understanding and acceptance from society to be successful.
[
11] see urban social mining as a new business model, which enables the creation of jobs, bringing economic and social benefits. Second, [
96] the application of these models must be considered in regional contexts, since imports or exports can cause transfer of impacts. For [
97] it is necessary to verify the available markets for secondary resources in the local context, as they depend on consumer acceptance for the circulation of products.
In view of these analyses, it appears that despite advances in research, little progress has been made towards forming and testing business models aimed at circular construction through buildings as material banks, still being in the theoretical field. However, indications from the literature can serve as guidelines for future work.