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Circular economy – a challenge for geotechnical engineering

1. Background

The demand for raw materials continues to grow unabated, and efforts to decouple economic performance from resource consumption have so far been largely unsuccessful. According to estimates, global raw material consumption will at least double over the next 30 years (1).

The construction sector plays a very important role in terms of efficient resource use. In Germany alone, approximately 550 million Mg of natural mineral materials are extracted and used for the production of new building materials every year (2). The associated land consumption is around 4 ha/d (= 14 million m²/a). The extraction and processing of these raw materials accounts for 37% of CO₂ and SO₂ emissions, 18% of NOx emissions, 30% of energy consumption in the western federal states and 75% of household energy consumption (11).

Even though mineral material flows predominate in the construction sector, this sector is also responsible for almost 20% of plastic consumption in Europe, with around 10 million Mg/a (3). The total amount of plastics consumed in Germany is approximately 12 million Mg/a, of which around 2.6 million Mg/a are used in the construction sector (4). The use of plastics in the construction sector is diverse:

• Polyvinyl chloride – including plastic pipes, window frames, floor coverings, wallpaper, roofing, cable sheathing,
• Polyethylene or polypropylene – including pipes, sleeves, cable insulation,
• Polystyrene or polyurethane – including insulating foams for façade insulation,
• Polyamide or polyester – e.g. geosynthetics.

The main reason for using plastics in construction is their technical performance. With increasing demands on their potential uses, processing, resistance, durability, resilience, malleability, elasticity, design, etc., plastics have found widespread use in the construction industry. Both homogeneous materials and composites made of several components are used.

On the other hand, there is growing public scepticism about plastic materials, as they may contain substances that are classified as harmful to health or the environment (see, for example, the discussion about the flame retardant HBCD in composite thermal insulation systems). On the other hand, the lack of or inadequate recovery processes for plastics contributes to the long-term loss of construction products and the raw materials they contain, or to substances entering the environment in an uncontrolled manner.

2. Establishing a circular economy

In the strategy outlined in the EU's Circular Economy Package, the European Commission formulates conceptual ideas for the sustainable transformation of the European economy into a comprehensive circular economy (5). This goes well beyond the concept of a circular economy as defined in the Waste Framework Directive at the end of a product's useful life.

The model of a comprehensive circular economy formulated by the European Commission requires a life-cycle-oriented approach. This means that issues relating to resource recovery must be a focus right from the product design stage and the selection of materials and ingredients, as well as in the development of innovative production processes. This also requires a rethink of the definition of forms of use and business models (e.g. product sharing and leasing).

Against this backdrop, the EU has also formulated the following objective in the recast of the Waste Framework Directive, which came into force in 2018: to transform waste management into sustainable materials management and a model of comprehensive circular economy in order to, among other things, reduce resource consumption, decrease the Union's dependence on resource imports and thus create new opportunities for the economy and long-term competitiveness (6).

At national level, the Circular Economy Act and, for example, the Commercial Waste Ordinance have formulated corresponding guidelines that specify these objectives.

In this context, particular attention is also paid to the construction sector, where the high-quality recycling of construction products has been identified as one of the major challenges. While the focus in this context is initially on the mineral fractions that are relevant in terms of mass, the plastics contained therein are also considered to be of great importance due to the above-mentioned circumstances (see also "Plastics Strategy – A European Strategy for Plastics in a Circular Economy") (14).

3. Situation regarding plastic waste in the construction sector

The increasing use of resources (see Chapter 1) means that more and more raw materials are being used in short- or long-life products, thus creating anthropogenic stocks at various levels. Figure 1 shows the quantities used in building construction and civil engineering in the example year 2010, the quantities that ended up as waste from the system and the quantities that remained in storage.

Fig. 1: Material flows in building construction and civil engineering in 2010 (7)

Estimates (7) show that a total of around 28.2 billion Mg of material is now bound in buildings, pipe-based infrastructure, building services and capital and consumer goods in Germany (see Fig. 2). Per capita (E), this corresponds to 341 Mg/E of material, including 318 Mg/E of mineral materials, 4.3/E Mg of metals, primarily steel, as well as approximately 100 kilograms/E of copper, 4.3 Mg/E of wood and 3 Mg/E of plastics.

Fig. 2: Stockpiles in Germany in 2010 (7)

Around 55% of the stock is tied up in residential and non-residential buildings. The remaining 45% is found in civil engineering, which includes infrastructure for transport, drinking water and wastewater, energy, and information and communication networks. The group comprising building services and goods accounts for significantly less than 1% (7).

In the plastics sector, too, there is an annual increase in stockpiles in Germany of around 2 million Mg of new plastics. This is associated with growing challenges, particularly in terms of recycling the materials produced to the highest possible standard. These measures are made more difficult by the considerable lack of information about the materials used. Despite various attempts to describe the building stock, there are still gaps in our knowledge, particularly with regard to the materials used, the quantities involved and the type of installation of the plastics.

The approximately 0.5 million Mg of plastic waste from the construction sector (as of 2016) consists of both large components (e.g. plastic windows) and small components (e.g. plastic dowels). The plastics are often contaminated with high proportions of minerals, which makes material recycling considerably more difficult. In Germany in 2016, only 27.7% of plastic waste from the construction sector was recycled, 69.9% was used for energy recovery and 2.4% was disposed of in landfills (4).

The increasing use of multifunctional building and construction materials is also making the dismantling and recovery of (high-quality) secondary raw materials from plastic increasingly difficult for demolition and recycling companies and is often uneconomical for them. The structural complexity of these composite materials poses a major challenge for material cycles, as no circular economy technologies exist for these products yet.

The landfill ban in force in Germany and the limited availability of waste incineration plant capacity for these material flows in the medium and long term further significantly restrict disposal options.

Due to their long service life, the dismantling of geosynthetic structures is currently only taking place in individual cases or for temporary structures. In the medium term, however, it will also be necessary to renew or modify structures manufactured in this way. Due to the economic and structural advantages of geosynthetic structures, increased use of temporary structures, in particular support structures with geogrid reinforcement, is also to be expected in the future. In both cases, the plastic installed in the ground must be removed. To date, there are no comprehensive and mature concepts for dismantling such structures and further handling the materials that are then removed. The use of the removed geosynthetic material for further recycling is not currently planned and is also not possible for certain product groups.

4. Obstacles and challenges for a resource-efficient circular economy in the construction industry

Construction and demolition measures are characterised by the fact that they often involve a large number of different actors at various levels of action (planners, construction companies, building material producers, demolition companies) with different scopes and depths of responsibility (e.g. general contractors or subcontractors).

The insufficient consideration of recycling in the planning of buildings to date is evident from the facts presented in Table 1 for the individual levels of action.

Table 1: Barriers to recycling at the various levels of action in construction and demolition measures (13)

Although the players at the various levels of action are endeavouring to develop solutions, these are individual activities, e.g. the European gypsum industry with its "Gypsum to Gypsum – From production to recycling" project launched in 2013 (9) or the take-back system offered for aluminium by system providers and extrusion plants for construction profiles (10). However, such individual approaches have little effect on the resource productivity of a building as a whole.

5. Approaches for a resource-efficient circular economy in construction

Buildings designed to be dismantled are much easier to repair in the event of damage or renovation and, when demolished, enable more effective use of secondary raw materials and, as a result, significantly higher resource efficiency by reducing the amount of waste to be disposed of in waste incineration plants or landfills.

This requires planning tools that enable appropriate assessment. These tools should be used to evaluate designs objectively, i.e. on the basis of clearly defined criteria, in terms of material selection and demolition costs, and to compare common, non-detachable (e.g. bonded) connections with detachable and thus forward-looking alternatives. These tools must be designed in such a way that they can be integrated into existing methods and planning aids for sustainable construction. Environmental product declarations (EPDs) can be a first step in this direction.

In order to achieve further application at the planning level, the criteria developed should be incorporated into engineering-oriented, practical overviews in which qualitative assessments of dismantling friendliness, recyclability and the material flows obtained can be read from current detailed plans.

Comparable component catalogues (in which, however, comprehensive consideration of the aspect of demolition planning has been lacking to date) have proven their worth in building construction. In practice, appropriate tools are suitable for transferring the results to a (digital) building passport (keyword BIM) on a project-specific basis, so that this can then be used at a later date as a planning basis for demolition and maintenance measures.

In the certification system of the German Sustainable Building Council (DGNB) and the Sustainable Building for Federal Buildings (BNB) assessment system of the Federal Ministry of the Interior, Building and Community (BMI), the dismantling capabilities of the structures and the separability of the materials have a direct influence on the classification of the technical quality of a building. The 2015 version of the DGNB's sustainable building system introduces indicators for assessing dismantlability, for which standard components evaluated in the associated calculation tool are stored.

In the 2018 system version, the Tec 1.6 profile "Demolition and recycling friendliness" also takes into account, among other things, the waste hierarchy according to the Circular Economy Act and the separability of components. However, the two indicators are not combined. As standardised evaluation criteria are still lacking, the evaluation remains dependent on the subjective assessment of the auditors. Although the dismantling costs and the amount of disposal costs are taken into account in the life cycle costs, these are only considered as an economic criterion and not as a technical criterion (15). Comparable ideas for civil engineering are currently lacking, but are also conceivable for this sector.

Another approach in this context is the introduction of a mandatory demolition concept (including a cost estimate), which should be submitted during the planning or construction phase (13). At this point, all essential information, such as construction plans, design drawings and parts lists, is available. This means that all relevant data on the materials actually used can be recorded. Furthermore, the creation of a demolition concept in parallel with the planning process creates incentives for the development of recycling-friendly designs.

In order to increase the intensity of use and resource efficiency of the materials used, in addition to the technical modifications mentioned above, the services provided by the building material manufacturer can also be a solution instead of purchasing building products. In this case, the manufacturer assumes responsibility for its products throughout their entire life cycle. A prerequisite for a functional, resource-efficient business model is the interaction of technical cycles (product design, production, installation, use, maintenance, dismantling, recycling options) within a supportive legal and commercial framework.

6. Conclusion

The stronger social and legal focus on resource use and consumption, as well as increasing (legal) requirements, e.g. in the form of recycling quotas and the prevention of uncontrolled release into the environment, also require early consideration of resource-efficient planning, construction, operation and dismantling, as well as recycling of the materials used, for structures made of geosynthetics. The challenges here are to develop resource-saving production processes, to assume product responsibility throughout the life cycle and to maintain data on the materials and compounds.

Fig. 3: Future life cycle of geosynthetics

This requires a life cycle-oriented assessment of the relevant structures and geosynthetic products in order to take into account, in particular, the qualitative change processes during the service life and the influences from the dismantling processes, which have a significant impact on the quality of the secondary raw materials. An assessment system based on the DGNB or BNB systems could also be developed for civil engineering. In addition to technical solutions, however, it is also necessary to create commercial and legal framework conditions. This means considering future business models and clarifying issues such as ownership and warranties.

Literature

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