Optimised design of concrete recycling networks: The case of North Rhine-Westphalia

Highlights

• Construction and demolition waste is either landfilled or used for roadworks.

• Recycled concrete aggregates can be reused for the construction of new buildings.

• Optimisation model minimises the total costs and the mining of natural aggregates.

• Concrete with recycled aggregates is viable without natural aggregates supply.

• Minimum quota of recycled concrete allows to strengthen closed loop material cycle.

Abstract

In Germany, construction and demolition waste is either landfilled or used for roadworks after being treated in recycling plants. However, recycled concrete aggregates from building demolition can be reused for the construction of new buildings, which allows to close the material loop of concrete. The aim of this study is to develop a decision support system (DSS) in order to investigate the economic impacts and consequences of recycling concrete from building demolition in a closed loop material cycle. To do so, we propose a bi-objective mixed-integer linear optimisation model that provides information on the number and location of installed sorting screens and defines the material flows within a comprehensive regional recycling network covering all processes from building demolition to construction of new buildings. While the economic objective function minimises the total costs of the recycling network, the circularity objective function minimises the amount of primary natural aggregates required for concrete manufacturing. We apply our optimisation model to the German federal state of North Rhine-Westphalia. Results show that under current conditions the manufacturing of concrete with recycled concrete aggregates is economically viable predominantly in areas without local supplies of natural aggregates. In addition, analysis of the trade-offs between the two objectives as well as recommendations on the design of political regulations and economic instruments to foster such recycling networks are presented.

Introduction

With 800 million tonnes and a high recycling potential, construction and demolition waste (CDW) is a priority waste stream within the European Union (EU) (European Commission, 2019). The Waste Framework Directive 2008/98/EC sets a target of 70% material recovery of CDW which should have been achieved by 2020 (Directive 2008/98/EC). Even though most of the EU states have accomplished this recycling goal (Eurostat, 2020), the quality of recycled materials and their markets vary (Gálvez-Martos et al., 2018).

The most widespread recovery practice so far is the usage of recycled concrete from building demolition in roadworks (Di Maria et al., 2018, Coelho and de Brito, 2013a). According to current legal regulations, this is defined as recycling, but does not allow to close a loop of the material cycle as the concrete cannot be used for construction of buildings again (Fig. 1 (a)). In contrast, in a closed material loop, concrete is kept within the system after extraction from demolished buildings and is used for the construction of new buildings (Schultmann and Rentz, 2002) (Fig. 1(b)). Due to the substitution of virgin material this type of recycling activity allows to reduce the amount of virgin material used for high grade applications like building construction. In a closed material loop, the consequences of mining virgin material like harmful emissions and solid wastes are reduced (Silva et al., 2017).

In Germany, less than 21% of recycled CDW from building demolition are applied for building construction, while 75% end up in roadworks and earthworks (Bundesverband Baustoffe — Steine und Erden e.V., 2019). The amount of recycling aggregates for roadworks is steadily increasing in Europe, while material demand for road construction is expected to remain stable or even to be reduced (Umweltbundesamt, 2015). Consequently, there is already an excess supply of low-quality recycled CDW in Belgium and the Netherlands (Hu et al., 2013). To avoid this in the German market, it is important to intensify activities aimed at closing the material cycle of concrete used for building construction in the existing recycling network. However, this needs extra investment for the installation of sorting screens to produce concrete aggregates suitable for the construction of new buildings.

The aim of this study is to develop a decision support system (DSS) in order to investigate the economic impacts and consequences of recycling concrete from building demolition in a closed loop material cycle. To do so, we propose a bi-objective mixed-integer linear optimisation model that provides information on the number and location of installed sorting screens and defines the material flows for a comprehensive regional recycling network that covers all processes from building demolition to construction of new buildings. The model identifies the existing recycling plants best suitable for the installation of sorting screens to produce recycled aggregates that can be used for concrete manufacturing for building construction. The objectives of the model are to minimise the total costs of the recycling network across the lifespan of the sorting screens (economic objective) and to decrease the amount of virgin material used for concrete manufacturing (circularity objective).

The remainder of this paper is structured as follows. Section 2 presents our planning problem. Section 3 provides an overview of the existing literature and outlines the research gap. In Section 4, we present the optimisation model. Furthermore, we describe the framework and assumptions of the case study. Section 5 presents the results of our case study for North Rhine-Westphalia (NRW) and provides recommendations for political instruments to intensify CDW recycling in the region. Finally, in Section 6, we draw conclusions and describe the outlook for further research.