Change Adaptation Socioecol. Syst. 2017; 3: 119–132

Research Article

Georg Schiller, Karin Gruhler, Regine Ortlepp* Quantification of anthropogenic using spatially differentiated continuous MFA https://doi.org/10.1515/cass-2017-0011 across the country. In Germany, such disparities mean that received April 13, 2017; accepted January 16, 2018 there will be a shortfall in RA of 6.3 Gt by the year 2020, Abstract: Coefficient-based, bottom-up material flow while the technically available but unusable RA (due to a analysis is a suitable tool to quantify inflows, outflows regional mismatch of potential supply and demand) will and stock dynamics of materials used by societies, and total 3.2 Gt. Comprehensive recycling strategies have to thus can deliver strategic knowledge needed to develop combine high-quality recycling with other lower-grade circular economy policies. Anthropogenic stocks and flows applications for secondary raw materials. Particularly in are mostly of bulk nonmetallic mineral materials related the case of building materials, essential constraints are to the construction, operation and demolition of buildings not only technical but also local conditions of construction and infrastructures. Consequently, it is important to be and demolition. These interrelations should be identified able to quantify circulating construction materials to and integrated into a comprehensive system to manage help estimate the mass of secondary materials which can the of materials in support of circular be recovered such as recycled aggregates (RA) for fresh economy policies. in new buildings. Yet as such bulk materials are high volume but of low unit value, they are generally Keywords: continuous (C-MFA), produced and consumed within a region. Loops are thus regionalized MFA, ewMFA, building material, bounded not only by qualitative and technical restrictions construction and demolition waste (C&D waste), recycled but also spatially to within regions. This paper presents aggregates (RA), circular economy a regionalized continuous MFA (C-MFA) approach taking account of these restrictions of local consumption, quality standards and technical limitations, illustrated using the example of Germany. Outflows and inflows of stocks are 1 Introduction quantified at county level and generalized by regional The circular economy is an economic model that promotes type, considering demand and supply for recycled “effective flows of materials, energy, labor and information materials. Qualitative and technical potentials of recycling so that natural and social capital can be rebuilt” [1]. This loops are operationalized by defining coefficients to model is gaining popularity in Europe and around the reflect technologies and engineering world as a potential way for societies to increase prosperity standards. Results show that 48% of outflows of concrete while reducing dependence on primary materials and and bricks are suitable for high-quality recycling, while energy [2]. In Germany, the role of the circular economy 52% of outflows do not fulfill the quality requirement is strongly anchored within the general revision of the and must be recovered or disposed of elsewhere. The German Resource Efficiency Program [3]. Similarly, the achievable inflow to RA is limited by the building activity EU Commission has drawn up a new Circular Economy as well as the requirements of the construction industry, Strategy, developed in the context of leading initiatives of e.g. the RA fraction of fresh concrete must not exceed 32%. the Europe 2020 growth strategy [4]. In addition, there exist spatial disparities in construction Circular economy policies must be based on detailed knowledge of the socio-economic metabolism [5]. This can be obtained through material flow analysis (MFA), *Corresponding author: Regine Ortlepp, Leibniz Institute of which is a “systematic assessment of the flows and stocks Ecological Urban and Regional Development (IOER), Dresden, Germany, E-mail: [email protected] of materials within a system defined in space and time” Georg Schiller, Karin Gruhler, Leibniz Institute of Ecological Urban [6], p.3. With regard to basic conceptual principles, we and Regional Development (IOER) can distinguish between deductive approaches (top-down

Open Access. © 2017 Georg Schiller et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution- NonCommercial-NoDerivs 4.0 License. 120 G. Schiller, et al. models) and inductive approaches (bottom-up models). flows, especially as building waste usually consists of a Economy-wide MFA (EW-MFA) follows a top-down logic heterogeneous mixture of different, partly contaminated and is generally reliant on official statistics [7], [8], [9], materials which hampers a high quality recovery of the [10]. Using this data it is possible to calculate material material.” Indeed, the current authors have been unable flows into and out of the stock. However, up to now these to find such a concept in the international literature. methods have either neglected the question of waste This article attempts to remedy this deficit by materials or only taken account of it in rough fashion [11]. presenting a new method to quantify material flows along Such limitations can be remedied by designing expanded a closed material cycle in the building sector in order to models that make use of coefficients. Methods of data analyze regionally-specific outflow/inflow constellations. extrapolation using material coefficients (bottom-up models) allow us to analyze anthropogenic stock and flows more precisely. These bottom-up methods gather 2 Concepts and methods information on stock variables to estimate in-use stock and to infer the behavior of flows following the principle: 2.1 High-quality recycling loops quantity of final good in-use × specific material content = material stock of a specific material [12]. This method offers The approach presented below has been developed a high degree of flexibility, whether for the description of to estimate the technical potential of the high-quality flows or the analysis of material stocks [13]. recycling of mineral C&D waste within the building The vast majority of anthropogenic stocks and flows are stock. The term “high-quality” means that C&D waste is building materials related to the construction, operation processed in such a way as to enable usage in the same and demolition of buildings and infrastructures [11], [14]. products from which it originated. Recycling streams A number of MFA studies have already investigated the “from buildings to buildings” are analyzed by considering built environment by analyzing stocks and flows and their concrete, specifically the aggregates used to produce dynamics [13], [15], [16]. These studies consider resource concrete, as a mass construction material. Hence, the consumption and waste management at both ends of the focus with regard to inflows into the building stock is material flow chain [17] as well as streams of C&D waste concrete. Germany’s building code permits the use of a [18]. In particular, various methods have been proposed to certain proportion of recycled aggregates (RA) from waste implement urban , defined as the quantification, concrete and brick to produce fresh concrete. The focus management and exploitation of materials embedded in with regard to outflows is therefore on recycling-relevant the anthropogenic stock [19], [20], [21]. material for the production of concrete, namely flows of In Germany, more than 90% of building material waste concrete and brick. inflows, outflows and stocks are made up of non-metallic mineral materials [11]. These bulk materials are usually consumed near to the place of extraction and processing 2.2 The continuous MFA approach due to their low economic value in terms of weight as well as the high relative costs of transportation [22]. This Inflows and outflows of the building stock are calculated is true for raw materials as well as recycled aggregates using the “classical method” of a coefficient-based, [23]. Consequently, bulk building materials are generally bottom-up MFA (cf. Section 1), considering specific traded within regional markets. material composition indicators (MCIs) for building types In recent years, scholars of have and deriving the stock dynamic from official statistics on investigated the inflows and outflows of non-metallic newly constructed and demolished buildings. building materials. Two lines of research can be discerned: As described by [6], “inflows”, “stock dynamic” outflow-oriented studies focusing on waste management and “outflows” are the main components of the MFA. and waste treatment (e.g. [24], [25], [26], [27], [28], [29]) and Here, three additional modules, namely “capture”, inflow-oriented studies focusing on the use of RC material “processing” and “admixture” (Fig. 1), constitute the in building materials (e.g. [30], [31], [32], [33], [34]). methodological expansion of the C-MFA according to Most of these have considered closed material loops [35]. These new modules encompass the various steps yet without providing a method to address the missing involved in extracting the relevant waste fractions and links between inflows and outflows quantitatively. It their processing into recycled products (see subsection is the view of Schultmann and Sunke [29] that “rarely 2.6) as well as legal and technical regulations specifying any concept exists for the circularity of these material the permitted uses of such products as building materials Quantification of anthropogenic metabolism using spatially differentiated continuous MFA 121

(cf. Section 2.7) (e.g. [30], [31], [32], [33], [34], [36], [37], –– the settlement structure (urban and rural), as this can [38], [39]). A review of the relevant literature revealed only determine the composition of the building stock; a small number of publications addressing the issues of –– demographic dynamics as one main factor behind the material capture and processing (e.g. [24], [28], [40]). development of building stock.

To meet these requirements, the current study applies the 2.3 Spatial differentiation MFA at the level of counties. The adopted data (from 2005) encompasses 438 German counties distributed over a Transportation distances and costs are major factors total area of 357,376 km². This gives an average diameter of determining the regional spread of markets for bulk building about 32 km, assuming that counties are roughly circular. materials, including recycled aggregates. According to [41], Although this is only an approximation, it does indicate road haulage costs “can equal the production (mining and that the county level reflects the typical geographical processing) costs of aggregates at truck haul extent of markets for mass building materials. The pre- distances of 30 to 50 miles” (approx 48 to 80 km). Wilbrun existing typology of spatial planning regions provided and Thomas [42] calculate a trucking distance of 56 km to by the German Federal Office for Building and Regional be the break-even point at which the cost of transporting Planning (BBR) classifies counties into nine clusters of the lower layers of road base material exceeds the purchase regional types ([47], Fig. 2) by considering settlement price of the product. Miliutenko [43] claims maximum structural features (e.g. urban/rural), general spatial distances of between 30 and 50 km. Although features as well as population trends. In this way the BBR transportation by rail or is much cheaper [41], road typology can be used to model markets for mass building haulage is often the only viable alternative for transporting materials. The concept can be said to follow a generalized construction [44]. Therefore, it can be assumed approach of spatial differentiation operationalized by that average transport distances of construction materials regional types. One basic assumption is to exclude the are less than 50 km. This is also true for C&D waste and possibility of material flows between the counties, in recycled aggregates [45]. Consequently, markets for mass line with the regionalized nature of markets for bulk building materials tend to be geographically localized. mineral building materials. The concept is reasonable if Such spatial confinement of markets is reflected in the we remember that assumptions on population dynamics model by defining spatial system boundaries. Small-scale and building stock dynamics are classified by regional studies often adopt real administrative units such as type Thus, all counties allocated to one regional type are electoral divisions or municipal boundaries to meet these marked by the same type of dynamic. requirements, e.g. [46]. When larger areas are considered, for example national territories, spatial typologies used for spatial planning purposes can provide a structure for 2.4 Stock dynamic spatial disaggregation. The following requirements should be considered when devising a market typology for building A bottom-up approach, whereby the stock acts as a materials: driver for inflows and outflows, is adopted to model the –– the typical spatial extents of markets for bulk building changing residential building sector. It is assumed that materials; the stock of service units (e.g. dwellings) is the driver

Fig. 1: Methodological framework of the continuous MFA approach to building materials 122 G. Schiller, et al. for the inflows and outflows of residential buildings non-domestic stock classified by building types1 as well and thus in turn for material flows. The stock can be as on population trends during the period 1991 to 2006 estimated by an assigned “development pattern” [48] [54]2. or “stock expansion rate” [49], [50]. Alternatively, the stock dynamic can also be defined as a function of the population and the lifestyles of citizens, e.g. [51]. The 2.5 Bottom-up calculation of material flows approach adopted here closely reflects that of [51], supplemented by a component to take into account the Based on the stock dynamic (cf. section 2.4), the authors intensity of stock use, specified in terms of the vacancy apply a bottom-up approach using material composition rate. The stock dynamics are analyzed for the year indicators (MCI) as described in [13] and [55] to calculate 2020, the final year of a 15-year period of investigation. material flows. The bottom-up calculations provide Assumptions on the development of the housing stock estimates of concrete inflow and outflow as well as the are based on existing studies by Banse and Effenberger outflow of bricks (cf. Section 2.1) [52] that estimate future additions to and removals from the stock using official population forecasts [53]. These estimates are supplemented by ex post studies 2.6 Capture and Processing of the demand for dwelling types, of construction and demolition levels as well as changes in the stock of vacant The outflows calculated using the MCIs correspond to housing [52]. Changes in the stock of non-residential the building materials of demolished buildings. Further buildings are determined from population-based rates of new construction and demolition. In particular, 1 Statistical data distinguishes between institutional buildings, of- calculations are based on data on new and demolished fice and administrative buildings, agricultural buildings, factory and workshop buildings, trade and storage buildings, hotels and restau- rants. 2 Calculations from 2007

Fig. 2: Regional types according to the BBR [47], p.12 Quantification of anthropogenic metabolism using spatially differentiated continuous MFA 123 steps of waste management are required to create the 2.7 Admixture secondary materials from these outflows, namely material capture and processing. In order to produce high-quality Various studies has been undertaken around the world secondary materials, the material fractions contained to compare potential admixtures of recycled aggregate in the waste have to be identified and separated as concrete (RAC) and their properties with natural aggregate carefully as possible at the time of demolition [45], [56], concrete (NAC), for example [25], [26], [66] (Germany), [57]. However, complete separation is impossible. The [30] (UK), [34] (United Arab Emirates), [63] (Spain), [67] inevitable losses in material capture have thus to be (Brazil), [68] (USA), [69] (India), [70] (China), [71] (Israel), modeled within the C-MFA approach (Fig. 1). Based on [72] (Egypt), [73] (Portugal), [74] (Serbia), [75] (Finland) estimates of waste management experts, we assume the and [76] (Belgium). capture loss to be 20% of outflows. Three types of recycled aggregates are distinguished Recycled aggregates are produced through the in the international literature: processing of construction waste, specifically the 1. recycled aggregates (RA), containing concrete rubble process steps of crushing, grading and sorting [56]. The and masonry rubble, which may include different inevitable material loss during processing also has to block materials (bricks, sandy , autoclaved be modeled within the C-MFA approach (Fig. 1). The aerated or lightweight concrete blocks), as physical properties of the resulting aggregates as well as well as plaster; the aggregate (particle) size distribution (in the form of 2. recycled concrete aggregates (RCA), containing standardized grading curves) are regulated by European concrete rubble only; standards such as EN 12620:2013 [58], see [59]. Studies on 3. recycled masonry aggregates (RMA), containing the sieving of aggregates (e.g. [60-63]) demonstrate that masonry rubble only. the grading curves of the RA can vary strongly depending on the origin of the waste building materials (Fig. 3). RA is the basic term, designating aggregate that contains varying proportions of RCA and RMA (RA = RCA + RMA). EU standards and related national specifications are oriented on these categories3 (see [75], [77], [78], [58]). In the example given below (cf. Section 3), the authors understand RCA to be pure concrete rubble and RMA to be special -brick rubble. The DAfStb4 guideline on permissible recycled aggregates in concrete [65] defines two delivery types of RA for Germany, called D1 and D2 (Table 1). The properties of these delivery types comply with EN 12620:2013 [58] and DΙΝ 4226−100:2002−02 [79]. Using these delivery types, for structural use may be manufactured up to strength class C 30/37. Fig. 3: Diverse grading curves of recycled masonry rubble determi- Depending on the class of exposure, i.e. the field of ned by Müller A. [62] application, different proportions of RA are permitted (Table 2). Sorting is another important processing step in order to In practice, concrete in Germany is produced almost remove impurities from the RA, which otherwise could exclusively using pure RCA [62], [80]. For this reason, the impair the desired properties of the fresh concrete. These authors suggest considering a further delivery type of impurities are mainly found within the fine aggregates. 100% RCA in addition to the two delivery types shown in For example, investigations by Nicolai [64] show that Table 1. This delivery type is denoted in the following as 70-90% of gypsum is contained in the fraction of particle D*. size 0/16 mm separated out by aggregate sieving. For this reason, German guidelines [65] exclude the use of fine aggregates (particle size <= 2 mm) in the production of 3 Different abbreviations are used in EN standards for the same ma- concrete. Such regulations result in further losses, which terial groups. have to be modeled within the C-MFA approach (Fig. 1). 4 German Committee for (Deutscher Ausschuss für Stahlbeton – DAfStb); further information see supplementary ma- terial. 124 G. Schiller, et al.

Table 1: RA delivery types acc. to the DAfStb guideline on recycled aggregates in concrete [65] Constituent materials Content (percentage by mass)

Description1) D13) D23)

Concrete, concrete products, mortar, concrete masonry units and ≥ 902) ≥ 702) unbound aggregate, natural stone, water-bound aggregate Clay masonry units (i.e. bricks and tiles), calcium silicate masonry units, aerated non-floating concrete ≤ 103) ≤ 303) Bituminous materials ≤ 1 ≤ 1 Floating material in volume ≤ 1 ≤ 2 Other (cohesives, i.e. clay and soil), miscellaneous: metals (ferrous and non-ferrous), non-floating wood, ≤ 2 ≤ 2 plastic and rubber, gypsum plaster) and glass 1) acc. to EN 12620:2013 [58] 2) corresponds to RCA + NA 3) corresponds to RMA

Table 2: Permissible proportions of RA acc. to the DAfStb guideline on recycled aggregates in concrete [65] Exposure classes under EN 206:20135 Permissible proportions of RA (percentage by volume)

D1 D2

X0, XC1–XC4 ≤ 45% ≤ 35% XF1, XF3 ≤ 35% ≤ 25% XA1 ≤ 25% ≤ 25%

2.8 Comparing supply and demand from the demolition of dwellings and non-residential buildings. In the regional types 1, 2, 3, which refer to The supply and demand of RA was analyzed at county western Germany, the housing stock can be seen to grow level and summed at the level of regional types in order significantly in comparison to the moderate growth to evaluate deviations between supply and demand indicated in east German hinterland counties (RT6). that result either in a regional surplus or shortfall of Regional types 2 and 3 in eastern Germany are marked by RA. Again, it should be noted that no material flows are shrinkage and a significant domination of deconstruction. assumed between the counties in this model (Section 2.3). However, the situation is different in the case of the non- Consequently, an evaluation at the level of regional types residential buildings. Based on the modeling assumptions can be undertaken without contradicting the principle of (Section 2.4), an increase in the non-residential building spatial differentiation between regional markets for bulk stock is calculated for all regional types. 5 mineral building materials.

3 Method applied to Germany’s 3.2 Outflows and inflows building stock in order to illus- The outflows and inflows are calculated using the MCIs listed in Table 4. The focus of the case study here is the trate the potential for resource potential for high-grade recycling, specifically RA in fresh conservation concrete (cf. Section 2.1). According to current regulations (Section 2.7, Table 1), materials designated for recycling are concrete and clay bricks within C&D waste outflows. 3.1 Regional stock dynamic The MCIs given in Table 4 refer to these two materials.

The regional stock dynamic, differentiated by regional type (RT), was calculated using the methods and data 5 Exposure conditions are chemical and physical conditions to described in Sections 2.3 and 2.4. Table 3 shows the which the concrete structure is exposedand are classified in exposu- resulting inflows due to new construction and outflows re classes (EN 206:2013 [77]); for more detailed information see sup- plementary material. Quantification of anthropogenic metabolism using spatially differentiated continuous MFA 125

Table 3: Estimates of dynamic stock changes in Germany’s building sector in 2020 Type of building activity Building activities broken down by regional and building type

RT1 RT2 RT3 RT4 RT5 RT6

Residential buildings [1000 dwellings per year] New construction 51 25 111 3 9 5 Demolished buildings 23 10 31 11 35 4 Non-residential buildings [1000 buildings per year] New construction 8.8 5.7 16.5 0.8 3.4 0.7 Demolished buildings 4.0 2.6 7.4 0.4 1.5 0.3

Table 4: Material Composition Indicators (MCIs) of concrete and clay brick classified by building type

Material Residential buildings [metric tonnes per dwelling]

Size / age class to 1918 1919 - 1948 1949 - 1968 1969 - 1990 after 1990

Concrete SFH 73 83 142 197 169 MFH 48 45 90 130 123 Clay bricks SFH 125 116 4 9 19 MFH 72 68 8 6 14

Non-residential buildings [metric tonnes per building]

Institutional Office and Agricultural Factory and Trade and storage Hotels and buildings administration buildings workshop restaurants Concrete 954 777 41 474 358 214 Clay bricks 954 489 42 196 203 377

The inflows and outflows are calculated by multiplying the regionalized changes in stock (cf. Section 3.1) with the MCI values to give a spatially differentiated picture (Fig. 4). While there exist regions with a clearly dominant material inflow (RT 1, 2, 3 and 6), we can also point to regions where the outflows dominate (RT 4 and 5).

3.3 Material capture and processing

Outflows (minus the losses) determine the potential supply of recycled aggregates. Based on expert estimates, the loss during material capture is assumed to be 20%. Due to the sieving of the fine aggregates (cf. Section 2.7), we can assume an addition loss of 40% of the remaining mass. This reduces the potential supply of recycled aggregates from outflows of recycling-relevant building materials by 52% (Table 5).

3.4 Admixture

Fig. 4: Outflows and inflows of recycling-relevant material in the Estimates of the demand for RA for concrete production German building stock in 2020, classified by material and regional type are based on the assumption that the fraction of concrete 126 G. Schiller, et al. aggregates in the concrete is 72% by volume6. The technically available but unusable RA totals 3.185 million proportions of concrete rubble to clay brick rubble (RCA: metric tonnes, while there is a deficit of 6.294 million RMA) were determined to be (cf. Section 2.7): metric tonnes elsewhere of unmet demand for RA. –– delivery type D*: RCA:RMA = 100:0, The imbalance between supply and demand for RA –– delivery type D1: RCA:RMA = 90:10, can be reduced by optimizing the admixture quantity. This –– delivery type D2: RCA:RMA = 70:30. approach is shown in Fig. 5b. Here, the delivery type is Table 6 shows the substitution potentials of RA in selected in each region type to allow the use of maximum inflows assuming that a maximum of 45% by volume (in quantities of available RC aggregates for any given demand. the case of delivery types D* and D1) and a maximum of In the regional types RT 1, 2, 3 and 6, this is the case when 35% by volume (in the case of delivery type D2) of the total the selected delivery type allows the most exhaustive use aggregate volume in the concrete is replaced by RA. of existing RA (delivery type D2 in RT1, 2 and 3 and delivery type D1 in RT6). Although the mass of RCA admixture is thereby reduced, the additional use of RMA will result in 3.5 Resource saving and utilization potenti- greater total RA. As a result, surpluses and shortages are als by comparing supply and demand reduced. In the shrinking regions (RT4, RT5), on the other hand, the type of delivery which offers the greatest quantity Fig. 5a shows the varying regional surpluses and shortages of admixture is preferred (delivery type D*). of RCA and RMA (calculated in Section 3.3) with regard to This strategy of optimizing delivery types can the demand for delivery type D* (calculated in Section 3.4). significantly increase the savings potential in growing Demand exceeds supply in regional types with growing regions by an average of 21%. building stocks, whereas the condition is reversed in In Fig. 6, outflows are evaluated with regard regional types marked by shrinkage. In the former (RT1, to different utilization potentials. They are further RT2, RT3, RT6) there is a shortage of RCA as a function of differentiated by material type, by qualitative aspects, the potential demand; in the latter (RT4, RT5) there is a and by the constraints arising from the supply-demand surplus. The results are somewhat different in regard to relations. The outflows can be divided into three groups: RMA: Here we note a surplus in all regions. Nationally, the –– RA used in new concrete products for buildings (RCA and RMA), 6 Average value based on mixture calculation; derivation see sup- –– surplus RA (RCA and RMA not used in new concrete plementary material. products for buildings), and

Table 5: Material losses during capture and processing of outflows (concrete and clay bricks)

Captured amounts and losses RT1 RT2 RT3 RT4 RT5 RT6 Total

million metric tonnes per year

Outflows (concrete and clay bricks) 4.9 2.7 8.6 1.6 5.3 0.6 23.7 100%

Losses during capture 1.0 0.5 1.7 0.3 1.1 0.1 4.7 20% Losses during processing 1.6 0.9 2.8 0.5 1.7 0.2 7.6 32% Resulting RA 2.3 1.3 4.1 0.8 2.5 0.3 11.4 48%

Table 6: Admixture RT1 RT2 RT3 RT4 RT5 RT6 Total

million metric tonnes per year

Inflows Concrete 9.6 5.6 22.1 0.7 2.7 1.1 42.1 Aggregates 7.2 4.1 15.9 0.5 1.9 0.8 30.3

Permitted admixtures

Potentials for resource D* 100% RCA 3.1 1.8 6.9 0.2 0.8 0.3 13.2 substitution under current D1 90% RCA 2.7 1.5 6.1 0.2 0.7 0.3 11.5 technical regulations 10% RMA 0.3 0.2 0.7 0.0 0.1 0.0 1.3 D2 70% RCA 1.6 0.9 3.5 0.1 0.4 0.2 6.6 30% RMA 0.7 0.4 1.5 0.0 0.2 0.1 2.8 Quantification of anthropogenic metabolism using spatially differentiated continuous MFA 127

–– losses resulting from capture and processing. permitted admixture level, with no demand for RMA. This fraction, therefore, remains as surplus aggregate The “used RA” group comprises the material masses even if there is high demand for concrete (e.g. RT 1, 2, 3, which can be used as high-quality recycling material in a 6). The surplus can be reduced by choosing an admixture circular economy. strategy that permits the use of RMC. Secondly, the The “RA surplus” group refers to those material masses demand for RA is limited by the general level of demand which meet the quality requirements for high-quality in the construction sector. This constitutes a basic upper recycling but cannot be used in building construction limit when managing material flows as part of a resource because of a lack of demand. There can be two reasons management strategy (e.g. RT 4, 5, 6). for this. The first is related to the technical demands of The group of material “losses” encompasses material complying with existing standards, reflected in the model that is basically unsuitable for recycling because of by means of the delivery types. In delivery type D*, for inferior quality. example, there is a demand for RCA up to the maximally

Fig. 5: Regional mismatch between supply and demand of RCA and RMA in the cases of admixture strategies D* and Dopt

Fig. 6: Regionally differentiated profile of potentials for the use of anthropogenic resources, i.e. concrete and clay bricks from buildings, differentiated according to admixture strategies D* and Dopt 128 G. Schiller, et al.

4 Discussion within the transportation distances that determine typical catchment areas of markets for bulk building materials. The main aims of the paper were to introduce a This is important if materials are traded in regional markets methodological refinement of material flow analysis in and the dynamics of these markets differ significantly. The order to achieve a closed loop MFA of bulk materials in the first of these conditions is certainly true for bulk building construction industry along continuous material flows, as materials. The second condition holds true not only for well as to discuss the benefits a spatial differentiation of Germany but also other highly industrialized nations as the approach to model potential loops of bulk material. well as strongly growing economies featuring a mix of The starting point of the presented approach is an thriving urban areas and stagnating rural areas, e.g. [81]. MFA using bottom-up logic. Such a bottom-up perspective In Germany, spatial disparities in construction mean that is a flexible way of examining the technical potentials there will be a shortfall of 6.3 Gt RA by the year 2020, of closing material loops while obeying the boundary while the technically available but unusable RS (due to conditions of supply and demand imposed by the market, a regional mismatch of potential supply and demand) cf. [11]. The flexibility offered by bottom-up approaches will total 3.2 Gt. Without regionally-based analysis these is found in the qualitative dimension, namely the disparities will remain hidden. The supply and demand differentiation of MFA according to materials and material of RA can be more optimally balanced by considering groups, as well as the spatial dimension, namely the recycling methods on a regional basis. Standardized spatial differentiation of MFA. regulations that require maximum recycling ratios with The qualitative differentiation of materials serves to respect to inflows without taking outflow quantities and bridge the gap between outflows and inflows, thereby qualities into account can result in poor utilization of realizing a continuous flow model by considering existing recycling potentials. process-related and qualitative framework conditions that In this context, it is important to discuss the exchange determine the extents of potential loops. In this paper of materials not only within one sector but also between such differentiation is achieved by means of the modules different sectors. For example, an intensified use of “Capture” and “Processing”, which allow us to directly secondary raw materials in building construction can lead quantify material flows and consider their implication to a lack of secondary raw materials in road construction. for waste management strategies, for example regarding This can have a negative effect on resource conservation. alternative forms of material gathering or the influence of It is possible to address and support such issues using innovative recycling processes. The module “Admixture” the presented methodology of CL-MFA by extending the takes account of restrictions in the use of RA due to technical sectoral approach and defining inter-sectoral interfaces to specifications and regulatory codes. One basic question to close inter-sectoral loops (Fig. 7). be answered is how to select the most appropriate input variables for the model calculations. These are basically oriented around currently accepted maximum values under available technologies (reflected in general standards) as well as around maximum values that research shows to be technically feasible. Both of these can be modeled using this approach. In fact, there is an enormous range of analytical possibilities that can be applied over the entire material loop in order to answer diverse research questions. Regarding the limits of recycling, we note again that only 48% of outflows of concrete and bricks are suitable for high-quality recycling. This means that more than half of these outflows do not meet quality requirements and must be disposed of elsewhere. The theoretically achievable inflow to RA is limited by the building activity as well as by the requirements of the construction industry. According to this, in Germany the RA share in fresh concrete may not exceed 32%. The flexibility offered by the spatial differentiation Fig. 7: Expanded framework: from a sectoral towards an integrated of MFA allows us to consider local building stocks lying inter-sectoral approach Quantification of anthropogenic metabolism using spatially differentiated continuous MFA 129

Alongside the advantages of bottom-up models, there in Germany by the year 2020, the technically available are several drawbacks. Bottom-up approaches always but unusable RA (due to regional mismatch of supply and neglect some part of the goods under consideration (e.g. demand) will total 3.2 Gt. Recycling rates can be increased buildings) [11]. In the case at hand, it is clear that not by means of the regional adaptation of recycling methods. all construction and demolition activities are reported Nevertheless, there will still remain some materials in official statistics, which provide the main data source unsuited to high grade recycling. These material masses for bottom-up estimates. Hence, for certain questions can be quantified in order to support waste management a hybrid model may be more appropriate, e.g. one that policies of recovering and disposal. Although materials combines flexible and detailed bottom-up C-MFA with should be circulated as far as possible within functional a comprehensive but slightly differentiated economy- sectors, if recycling is to be taken seriously, then clearly it is wide MFA model (ewMFA). This could take the form of necessary to consider alternatives, such as implementing differentiated recycling rates, which are analyzed and exchange relations between different sectors in order to defined using a C-MFA approach, and implemented into close inter-sectoral loops. ewMFA approaches, such as already described by Haas et Furthermore, parameters can be introduced into the al. [82]. An interesting field of application would be the regionalized C-MFA approach to create hybrid models development of scenarios in order to reveal the potentials for a more refined analysis of social metabolism, thereby and limits of recycling in a comprehensive context of providing additional knowledge to assist in drawing social material streams. up circular economy policies, e.g. by considering In summary, the discussion on resource preservation the potentials and limits of recycling. This opens up can be enriched by revealing the socio-technical various lines of discussion, such as the exploration and conservation potential of RA substitution for NA. This development of secondary resources in the anthropogenic enables us to connect closed loop policies with planning stock to secure the supply of materials needed by society issues such as how best to safeguard the supply of raw as well as ensuring resource-oriented waste management material. By analyzing outflows in terms of the potentials that simultaneously aims to remove harmful substances. and limitations of material recycling, the authors can directly link the MFA approach with the tasks of waste management. References

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