Comparative Life Cycle Assessment of End-of-Life Scenarios of Carbon-Reinforced Concrete: A Case Study

Abstract

This study assesses the environmental performance in the end-of-life (EoL) of double walls made of carbon-reinforced concrete (CRC) and steel-reinforced concrete (SRC). The most feasible CRC EoL scenarios are evaluated using life cycle assessment and their environmental performances are then compared to those of SRC. The results showed that mechanical recycling is the best CRC EoL scenario, with a global warming potential (GWP) of 7.0 kg CO₂ eq., while the use of renewable energy can save over 50% of GWP. For SRC, the best scenario was obtained using a mobile recycling plant (GWP of 8.8 kg CO₂ eq.). In general, the further life of the reinforcements is hardly comparable. Steel can be recycled nearly without losses or downcycling, while a closed cycle of carbon fibers is not yet possible. Therefore, carbon fiber properties or EoL processes need to be improved for a closed loop with an optimized environmental performance.

1. Introduction

Building construction globally consumes 40% of materials and 40% of primary energy, causes 40% of waste, and generates 33% of the worldwide greenhouse gas emissions [1]. Furthermore, 8% of the anthropogenic CO₂ emissions are caused by global cement production [2].

Consequently, sustainability and resource efficiency in the construction industry are increasingly important, where the end-of-life (EoL) performance and circular economy come into play.

Technologies such as fiber-reinforced concrete, in the form of carbon- or glass-fiber-reinforced concrete, with high potential of material saving, play an important role in sustainable development [3]. Currently, steel-reinforced concrete is state-of-the-art in the construction industry. However, C3 e.V. assumes that by 2030, 20% of it can be substituted by carbon concrete [4]. Carbon-reinforced concrete (CRC) offers several advantages over steel-reinforced concrete (SRC), the most widely used building material worldwide [5]. For instance, steel reinforcement is susceptible to corrosion and requires a certain concrete cover serving as corrosion protection, which is not necessary for the load-bearing capacity of the material. In contrast, the corrosion-resistant carbon fiber reinforcement only requires a minimum concrete layer necessary for the bond (up to 50 mm concrete cover needed for steel-reinforced concrete; 10–15 mm concrete cover for CRC needed), enabling a component-dependent reduction of up to 80% in concrete consumption [6,7]. Thus, the environmental impacts related to the concrete cover can also be decreased.

To evaluate construction materials such as FRC, in terms of environmental sustainability, life cycle assessment (LCA) can be used, and is used, to quantify environmental impacts. The basis of LCA is the evaluation of different stages of a product’s life cycle, beginning with the extraction and processing of raw materials (cradle), associated with different production and use stages (gates), and ending with its end-of-life (EoL). The EoL describes the moment a product is no longer used and disposed, recycled, or utilized in another way (down- or upcycling) [8]. Subsequently, an important step is to be able to assess the sustainability of the product’s EoL already in the design process of the material. Especially for expensive and energy-intensive carbon fibers, a definition of sustainable concepts for a reutilization or recycling paths is essential [9]. Although a consideration of the entire life cycle is relevant, until now, only few LCA studies focus on the EoL or full life cycle of CRC (see Section 2.4).

In this study, we analyze how the recycling of CRC compares with the recycling of conventional SRC in terms of environmental impacts. The aim of this study is to identify different CRC EoL scenarios and to assess the environmental impacts of the most feasible ones currently available by means of an LCA. Furthermore, an LCA for the EoL of SRC is made and compared to the environmental impacts of the identified EoL scenarios of CRC.

1.1. Carbon-Reinforced Concrete

Carbon-reinforced concrete (CRC) is an innovative composite building material composed of concrete and reinforcement [7]. In the construction sector, carbon fibers are made of polyacrylonitrile (PAN), which is obtained from petroleum. The fibers are impregnated and, depending on the desired stability and flexibility of the reinforcing mesh, can be distinguished between two types of impregnation for application in the construction industry: epoxy resin (duromers; EP) or styrene–butadiene rubber (elastomers; SBR). For the reinforcement, up to fifty thousand filaments (individual fibers) are bundled into long fibers and spun into a roving (yarn) [3].

Since carbon reinforcement is not susceptible to corrosion and no thick concrete cover is needed in the construction element to avoid it, the consumption of aggregates, cement, and water, i.e., essential components of concrete, can be reduced [7]. Another significant advantage of CRC, resulting from the corrosion resistance and durability of the carbon reinforcement, is its above-average service life of an estimated 100 years, which is significantly longer than the estimated service life of SRC (50 years) [3,6,7,10].

In CRC, the carbon fibers primarily carry the loads and are significantly stiffer and stronger than the matrix [11]. Further material properties include very low creep; good vibration damping; low thermal expansion and conductivity; good electrical conductivity; low X-ray absorption; and high resistance to acid, alkaline, and organic solvents [12]. The negative properties of carbon fibers include low elongation and the resulting sharp-edged fractures; high price; and, in the event of a fire, the release of toxic gases, smoke, and respirable fibers at temperatures higher than 650 °C [11].

1.2. Life Cycle Assessment

LCA provides a robust, structured, and standardized method for evaluating the environmental impacts of systems and quantifying potential environmental emissions and impacts. In LCA, energy and material inputs and associated waste and emissions outputs are evaluated in terms of possible environmental impacts. LCA, according to the ISO 14040/44, consists of four phases: goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA), and interpretation [13,14]. In the goal and scope definition, the functional unit (which is the reference for the results of the assessment), system boundaries, data sources, and impact assessment methods are defined. This phase is followed by the LCI, in which all inputs and outputs in the form of material and energy flows within the system boundaries are collected. Furthermore, the inputs and outputs of the LCI are then classified in different impact categories and characterized according to the chosen impact assessment method in order to calculate the potential environmental impacts produced by the analyzed product (system). Finally, the results of the LCIA are interpreted (interpretation phase) based on the defined goals and recommendations which can be made. Furthermore, sensitivity analyses can be carried out to assess the impact of robustness on the results and assess further life cycle scenarios.

2. End-of-Life of CRC

End-of-life (EoL) describes the end of a product’s life cycle and starts when the product no longer accomplishes its function and is considered as waste. The EoL contemplates different routes, such as reuse without structural changes, remanufacturing to like-new condition, recycling treatment of waste products for use as raw material in the same or a different product cycle, as well as incineration with or without energy recovery or landfill [15]. For the EoL of CRC, a distinction is made between two types:

(1)

Separation of the demolition material into individual fractions that can be disposed of or recycled individually;

(2)

Non-separation of the demolition material into individual fractions, resulting in the reuse, recycling, or disposal of a heterogeneous material [3] (Figure 1).

Figure 1 shows all currently possible EoL scenarios for CRC—excluding landfilling, as this is not allowed in Germany for carbon fibers with an EP matrix, according to the Deponieverordnung (DepV) (09/1027). It represents both the complete separation of the materials and their continued use as a complete component. The carbon fiber fraction can be used for energy recovery, reinforcement in concrete and plastic, as well as a carbon source for industrial processes. These processes and applications are further discussed in Section 2.2. For the separated concrete fraction, the recycling and use of a filling material in road construction are possible scenarios which are discussed in more detail in Section 2.3. Furthermore, in Figure 1, a distinction between preferred, less preferred, and not preferred EoL scenarios is made—shown graphically with different frames. The terms “preferred” or “not preferred” refer to the properties of the materials at the end of the EoL scenario, based on whether or not their further use avoids downcycling. In general, scenarios are preferred in which the material properties are maintained.

Kimm [16] rules out the reuse of textile-reinforced concrete components. According to Kimm, reuse should be chosen over material recycling, but in practice a damage-free dismantling is not yet possible. Moreover, the current legal and political system of the construction sector does not promote the dissemination of reuse practices [17]. With the demolition and separation of the CRC components, it is expected that the highest-quality outputs for the processed materials (processed concrete and processed carbon fibers) will be achieved [3].

2.1. Separation of CRC

The following separation process is based on a German study [3] and was the result of experiments centered around separating CRC. CRC is selectively crushed with a carrier with a concrete pulverizer and sorting grab, resulting in pre-crushed coarse fragments of carbon concrete with carbon reinforcement. The fragments are loaded onto a dump truck with a hydraulic excavator and transported to a stationary preparation plant. The main crushing is performed by a jaw crusher, resulting in a heterogeneous mixture of building materials. The degree of disintegrating the carbon roving fragments from the concrete matrix is over 99% [3].

In the pre-separation stage, metallic-embedded parts are removed from the material flow by a stationary magnetic separator. Concrete fines (largest grain size of 2 mm), light plastic components (e.g., spacers), and carbon reinforcement with a mass fraction estimated at 10% are removed using a cross-flow classifier. After the pre-separation, the material stream contains a heterogeneous accumulation of concrete fragments of grain group 3/56 and exposed carbon roving fragments (average length 80 mm) [3].

A camera-based sorting unit is used for the main separation of fractions. According to Kortmann, 97.7% of the carbon roving fragments can be separated with this type of single-grain sorting. The result is the concrete fraction of grain group 3/56 (the utilization of which is not further discussed in this paper—mainly being used in road construction as gravel or partly in recycled concrete [18]) and the separated carbon fiber fraction including foreign mineral constituents of grain group 0/2 [3].

2.2. Separation of Impregnated Fibers

The main waste management strategies for carbon fibers are incineration with energy recovery and recycling [19,20,21]. Recycling can be in the form of mechanical, thermal, or chemical recycling [19,20].

Thermal recycling includes pyrolysis and fluidized bed pyrolysis, while chemical recycling includes solvolysis, acid digestion, and supercritical fluid solvolysis [22]. Among these, pyrolysis and mechanical recycling show better potential for industrial application when compared to chemical recycling or fluidized bed pyrolysis [19,22].

The sizing (impregnation) material is the most relevant component concerning fiber separation from concrete. Epoxy resin showed to provide the best separation since they dissolve well in a jaw crusher. FRC with elastomeric sizing can also be separated, but the reinforcement textile tends to be more damaged after the separation. Mineral or no sizing at all causes significant damage to the reinforcement and hinders the separation (Kimm et al., 2020).

2.2.1. Mechanical Recycling

An advantage of mechanical processing is that it addresses the growing amounts of carbon fiber waste [22]. The fiber composite material is shredded and milled through a multi-stage process [23]. Multi-shaft shredders and granulators are used as equipment, followed by a screening of the fibers to obtain a homogeneous particle size distribution [24,25]. The fibers cannot be completely separated from the matrix (both EP and SBR) [23,26]. However, recycling can also occur without separating the two components [27]. The shredded carbon fibers can be downcycled as filler in composites, concrete, asphalt, and coatings [24,26,27,28,29]. Moreover, inserting filler consisting of recycled carbon fibers can increase the mechanical (fatigue and fracture) and tribological (friction and wear) properties of new pure plastics [24,30]. In summary, fast processing and easy scalability are major advantages for this recycling method. Nevertheless, the length of carbon fibers is greatly reduced and contains resin residues, which affects their recyclability into new products [22].

2.2.2. Thermal Recycling: Pyrolysis

Compared to mechanical recycling, a fundamental advantage of pyrolysis is the complete separation of the polymer matrix, allowing the separate reuse of fibers and the matrix [24,27]. These are separated in an inert atmosphere (usually nitrogen) under atmospheric pressure at a temperature of at least 350 °C [31,32,33]. During this process, so-called pyrolysis gases (e.g., H₂, CH₄, CO, and CO₂) are generated from the polymer matrix [3]. Due to their high calorific value, they can be used as fuel to directly support pyrolysis and can offset some of the energy needed in the process [3,19,34,35]. Burning off the polymer matrix (EP or SBR) can cause soot adhesion to the carbon fiber surface, preventing the fibers from bonding with new resin [32,36]. Therefore, subsequent oxidation is necessary to remove the carbon black particles, negatively impacting the elastic modulus and tensile strength of the fibers [35,37,38,39,40]. The extent of this damage largely depends on the operating conditions, such as the pyrolysis and oxidation temperatures, the residence time, and the reaction atmosphere [22]. In this regard, the best mechanical properties (93% tensile strength and 96% elastic modulus) have been obtained at a pyrolysis and oxidation temperature of 500 °C, a pyrolysis time of one hour, and an oxidation time of two hours [41]. The oxidation process may additionally enrich the pyrolyzed fibers with oxidized groups, serving as a crosslink between recycled fibers and new resin [42]. However, the composites made with the recycled carbon fibers (rCF) tend to have poorer mechanical properties (reduced elastic modulus and tensile strength) compared to the composites made out of primary fibers [22]. rCF can be used to increase the strength of plastics in the injection-molded components as well as further processed into nonwovens [3]. Thus, the recycled fibers find application only in the field of non-structural composites, such as cladding in the automotive and aerospace industries or in lightweight sports equipment [3,38,42].

2.3. Use of Recycled Fibers and Concrete

After being separated from their sizing, the recycled carbon fibers can be used in different scenarios. For some scenarios, the sizing does not have to be removed. Separated carbon fibers can serve as reinforcement in fiber concrete (FC). In FC, unlike fiber-reinforced concrete (FRC), the fibers lay unoriented in the concrete matrix. They are added during the mixing process of the concrete. In comparison to steel FC, with carbon FC, 18% better mechanical properties can be achieved with about ¼ of the material input. The demand of FC is very high. Germany has a 50 kt/a need of steel FC. By substituting 10% of these steel fibers by rCF, 2/3 of Europe’s annual CF waste could be recycled (Kimm, 2019). Another possibility for rCF is the development of nonwovens as concrete or plastic reinforcement (Kimm et al., 2020). rCF nonwovens are ideal reinforcements for sophisticated composites [43]. As reinforcement for concrete, the nonwoven is unsuitable due to difficulties in soaking the nonwoven properly in concrete. The generated benefits do not justify the processing expense (Kimm et al., 2020). Further, a staple fiber hybrid yarn at Technische Universität Dresden was tested in composite test pieces and reached a tensile strength of over 80% of comparable pure carbon fiber composite. Because the rCF processed into a yarn can be oriented in the load direction, this method more effectively exploits the mechanical properties of the rCF. The yarn can be used in lightweight construction as, e.g., reinforcement in plastic composites for the automotive industry, where rCF’s conductive properties can be beneficial [44]. As another alternative for carbon fiber utilization, carbon fiber-reinforced plastics (CFRPs), as a substitute in calcium carbide production, have been investigated on an industrial scale [45]. Calcium carbide (CaC₂) is an important commodity chemical used for organic synthesis and nanotechnology [46]. In the production process, CFRP waste can replace up to 20% of the needed carbon source [45]. Further, the use of CFRP in the arc furnace steel production as a substitute for carbon sources has been investigated. The results are promising; nevertheless, the process has to be researched further on industrial scale [45,47].

If material recycling for carbon fibers is no option, thermal recycling can be considered [36]. The landfilling of carbon fibers with EP matrix is not allowed in Germany. The matrix must be removed. Considering the expensive removal process, landfilling is unfavorable in an economic and environmental way [3].

The recycling of concrete has been researched for a long time. The concrete fraction resulting from the FRC separation process is crushed to particle sizes of 3–56 mm and classified. For use in concrete production, it can be crushed and separated into 0–16 mm fractions. Subsequently, up to 45% volume fraction can be added in the production of new concrete parts for dry surroundings without further examination (Exposure-class XC1). Theoretically, concrete can be recycled like this many times [3]. Another common way of recycling is utilizing concrete aggregate as a filling material in road construction or in the construction of buildings, mines, and dams [3].

A promising approach for a closed life cycle in building industry is “urban mining”. Materials in buildings at its EoL, which are still usable, are reused as a whole building part. Helbig et al. (2019) investigated the reuse of 1 m² CRC façade panels. First, the panel is deconstructed as a whole. Subsequently, it is transported to a processing plant where it becomes dismantled, refurbished, and re-mantled. After the return to a construction site, the façade panel is assembled again. The strategy of reuse is regarded as a flexible and sustainable solution [48].

The literature shows a clear necessity of sustainable EoL scenarios for CRC. Regarding the separation of CRC, crushing by mills and camera-based sorting seems to ensure the best results by enabling good separation and the extraction of 97.7% of carbon fibers with an acceptable length (average 80 mm) for further processing. The EP matrix stays on the fibers. To separate the matrix from the fibers, a few different methods are available (pyrolysis, solvolysis, etc.). Pyrolysis is the only method already performed in an industrial scale. However, it should be considered that pyrolysis cannot be performed as many times as desired. Thus, alternative scenarios, e.g., use in steel or calcium carbide production, might represent a more suitable for carbon fibers after being pyrolyzed. Using concrete fraction in the production of new concrete is the most promising scenario because the value of the former product is achieved again in the recycled product.

2.4. LCAs of EoL of CRC

Only some LCA studies considering the EoL of CRC are available. Chen et al. (2010) [49] conducted an LCA of waste recycling in the case of cement. As the functional unit (FU), the binding equivalent value BE (kg/m³) = cem + k*SCM was used and the system boundaries were set from cradle to gate [49]. Colangelo et al. (2018) [50] researched the environmental damage of concrete production with LCAs. The functional unit was defined as 1 m³ of concrete, with a specific weight of 2400 kg/m³, and the system boundaries were included the production phase [50]. Colangelo et al. (2020) [51] analyzed the environmental impact of concrete with recycled aggregates and geopolymer mixtures. They used 1 m³ of reinforced concrete with a compressive strength of 30 Mpa as their functional unit and cradle-to-grave as their system boundaries [51]. Furthermore, Jehle et al. (2017) [52] investigated the key questions regarding the LCA of carbon-reinforced concrete, wherein several experiments were conducted [52]. Moreover, Kimm et al. (2018) [53] present the results of recent experiments on the recyclability of the textile components in textile-reinforced concrete [53]. Finally, Knoeri et al. (2013) [54] analyzed the life cycle impacts of 12 recycled concrete mixtures with two different cement types and compared them with corresponding conventional concretes for three structural applications. The functional unit was set to 1 m³ of concrete of a specific strength class with system boundaries, including the construction, use, and demolition stages [54].

For the above reasons, we will consider the EoL of CRC in the following case study and compare it with that of an equivalently functional steel-reinforced concrete component. The aim is to show EoL considerations already in the design phase, search for suitable solutions, and compare the long-term emissions of complete life cycles of the composite materials.

3. Case Study: LCA EoL of SRC and CRC

So far, the authors are not aware of any comparative LCAs of the EoL of CRC and SRC in the form of a double wall. In the following, the EoL of two double walls will be examined and compared using LCA. The aim is to draw attention to future challenges with regard to buildings made of CRC in terms of environmental emissions and impacts, and to suggest suitable solutions.

3.1. Goal and Scope

The aim of this case study is to present the environmental impact range of EoL (modules C1–C3 in DIN EN 15804) [55] scenarios of SRC and CRC, determined as a future outlook for 50 and 100 years from now [10]. The cut-off method is used as no environmental impacts prior to the demolition are considered [56]. Hotspots and optimization approaches are identified and a comparison with SRC is made. The following scenarios are based on ISO 14040/44 [13,14]. The impact categories used are those in CML2001 (August 2016) and modelled with GaBi ts [57]: abiotic depletion potential (ADP elements [kg Sb eq.], ADP fossil [MJ]), acidification potential (AP [kg SO₂ eq.]), eutrophication potential (EP [kg phosphate eq.]), freshwater aquatic ecotoxicity potential (FAETP inf. [kg DCB eq.]), global warming potential (GWP [kg CO₂ eq.]), human toxicity potential (HTP inf. [kg DCB eq.]), marine aquatic toxicity potential (MAETP inf. [kg DCB eq.]), ozone depletion potential (ODP, steady state [kg R11 eq.]), photochemical ozone creation potential (POCP [kg ethene eq.]), and terrestrial ecotoxicity potential (TETP inf. [kg DCB eq.]). All mentioned impact categories will be analyzed with a special focus on GWP.

For comparison reasons, the FU for this case study is defined as the mass of re-usable material of a double wall at its EoL with an equivalent load capacity and function (carbon-reinforced wall: 5 m × 2.5 m × 0.03 m vs. steel-reinforced wall: 5 m × 2.5 m × 0.06 m) (Figure 2) (according to Otto and Adam, 2019 [58]). The reason for the defined FU is a previous conducted cradle-to-gate LCA [59], which provided the starting point of this study and a precise mass definition given by Otto and Adam, 2019 [58].

The mass differences between a carbon-reinforced double wall and a steel-reinforced double wall are shown in

Table 1. Volume and weight of concrete and reinforcement (according to [58]).

Carbon-Reinforced Concrete	Amount	Unit	Steel-Reinforced Concrete	Amount	Unit
Total weight	1.43	t	Total weight	2.86	t
Total double wall	0.6	m3	Total double wall	1.1	m3
Concrete per double wall	1.42	t	Concrete per double wall	2.63	t
Carbon scrim per m3	0.0170	t	Steel scrim per m3	0.2100	t
Carbon scrim per double wall	0.0102	t	Steel scrim per double wall	0.2310	t
Carbon scrim per wall	0.0051	t	Steel scrim per wall	0.1155	t

. The total weight of the steel-reinforced double wall doubles the total weight of the comparative carbon-reinforced double wall. The concrete cover per double wall varies from 1.42 t concrete per CRC to 2.63 t concrete per SRC, representing the (not) needed corrosion protection. Further, the carbon scrim included as reinforcement per double wall has a weight of 10 kg, whereof the steel reinforcement brings 230 kg (Table 1). The double walls are additionally connected to each other in reality using a composite grid. This is shown in Figure 2 as a black dotted line between the two walls. Such a composite grid can, for example, be made of AR glass textile [60] or steel. This composite lattice (connecting pins) was not named in Otto and Adam’s study (2019), and is therefore not included in the current model. Moreover, possible insulation is also not considered in this case study.

3.2. Life Cycle Inventory

Conservative approaches are used in the selection of processes and in the assumptions, when not given as primary or clearly defined literature data. German datasets—the double wall is assumed to be demolished and processed in Germany—are used when possible; where this is not feasible, European (first) and global (second) datasets are used.

The life cycle inventory (LCI) for CRC is based on the processes outlined in Section 2, including the energy and fuel needed for demolition (from wall to small re-usable particles), transport routes of ~100 km, and the mechanical recycling or pyrolysis. The energy values and transports, including fuel, refer exclusively to the FU. The entire LCI is based on literature data or datasheets from German companies, which are converted to the target value, according to justified assumptions (Figure 3).

For SRC, we assume three different demolition scenarios: (A) a mobile plant at the demolition location, (B) a stationary plant with a conventional processing (dry preparation), and (C) a stationary plant with innovative processing (dry and wet preparation). For SRC, a distance of ~100 km for each transport is assumed. Figure 4 shows the modelled flow chart, starting from the selective demolition on site (Scenario A) or a stationary plant (Scenario B1 and B2). The steel reinforcement is unpacked, meaning it can be ready for recycling at any steel-producing site. The concrete can be demolished into different sizes of reusable material. Flows entering the scope of the assessment are fuel (diesel), electricity, and water.

4. Results

In the discussion, we place a strong emphasis on GWP; nevertheless, all CML2001 (August 2016) midpoint indicators are assessed.

It is important to note that the output of concrete demolition can vary greatly, from the result of large fragments, which can be used as crushed stone in road construction, as well as fine grains, which can be used again directly in fresh concrete. Depending on the grain size, the required energy and fuel consumption, and thus the resulting emissions, also vary. Therefore, we list below a range of possible emissions from concrete crushing, which range up to a fineness of 0/1 mm. Considering the GWP in kg CO₂e, aligning the concrete fraction for the CRC double wall results in a range of 2–14 kg CO₂e (Figure 5); depending on the processes, the final grain size and required energy can be assumed (and the mass can be demolished).

Mechanical recycling results in 4.9 kg CO₂e. In contrast, pyrolysis produces 18.7 kg CO₂e per double wall. Adding the concrete fraction and the two fiber EoL scenarios, the results in kg CO₂e per double wall from carbon reinforcement range from 7 kg CO₂e for concrete crushing and mechanical recycling to 22.4 kg CO₂e for concrete crushing and pyrolysis (Figure 6) (including different concrete demolition ranges).

Considering the individual EoL steps of the fibers (mechanical recycling and pyrolysis), the main drivers in mechanical recycling are the energy required (70%) and transport (30%) (Figure 7). For pyrolysis, 84% of the total GWP (18.7 kg CO₂e) can be attributed to the required electricity, whereas only 8% can be attributed to transport and another 8% to gas (Figure 7).

For steel recycling (

Table 2. LCA CRC-EoL CML2001.

EoL CRC	ADPe	ADPf	AP	EP	FAETP	GWP	HTP	MAETP	ODP	POCP	TETP
Concrete fraction	0.0	27.3	0.0	0.0	0.0	2.0	0.0	33.6	0.0	0.0	0.0
Mechanical recycling	0.0	164.0	0.0	0.0	0.1	4.9	0.3	521.1	0.0	0.0	0.0
Concrete fraction + mech. recycling	0.0	191.3	0.0	0.0	0.1	7.0	0.4	554.7	0.0	0.0	0.0
Pyrolysis	0.0	445.6	0.0	0.0	0.1	18.7	1.0	1849.0	0.0	0.0	0.1
Concrete fraction + pyrolysis	0.0	473.0	0.0	0.0	0.1	20.7	1.0	1882.6	0.0	0.0	0.1

), we assume that the crushed concrete can vary in the resulting grind sizes. We assume identical crushing steps to the ones for CRC (Figure 3). However, the results differ from those of the CRC due to a higher amount of concrete in SRC. The concrete demolition for the reinforced concrete is at a GWP of 6.9 kg CO₂e with a particle size of 8/22 mm (Scenario A). The treatment by a conventional stationary plant (scenario B1) leads to 9.5 kg CO₂e. The innovative stationary plant produced the finest grains (Scenario B2), which can be used directly in fresh concrete. Consequently, the largest amount of energy is required for crushing, which is also reflected in a GWP of 14 kg CO₂e, the highest GWP for concrete crushing scenarios.

For the steel and its EoL, we consider only the detachment from the surrounding concrete, as well as the transport of steel to the nearest steel mill, for melt down and recovery of 100% of steel. The melting itself is not included in the case study because after the steel is detached from the concrete, a reusable material already exists (goal: re-usable material out of double wall). Consequently, the GWP for the predefined steel-reinforced concrete double wall ranges from 5.6 kg CO₂e to 15.9 kg CO₂e, depending on the processes used—scenarios A, B1, or B2 (Figure 8).

By analyzing scenarios B1 and B2, the following hotspots can be identified in terms of total GWP. Scenario B1 requires a large fraction of fuel for the selective demolition, leading to a fraction of 32% of the total GWP (3.1 kg CO₂e of total 9.5 kg CO₂e). The transport accounts for 39% of the total GWP emissions (as the complete wall was transported to the processing plant) and the dry processing accounts for the remaining 28% (Figure 9). In scenario B2, the innovative stationary processing plant additionally uses wet processing, which accounts for the largest share of GWP (8 kg CO₂e of total 14 kg CO₂e) due to the required energy, fuel, and water. The wet processing corresponds to 29%. Furthermore, transport has a large share of the GWP with 27%, followed by dry processing and selective demolition (Figure 9).

In terms of ADP_fossil, carbon fraction and mechanical recycling lead to 191.3 MJ, while carbon fraction and pyrolysis amount to 473 MJ. For steel recycling, the impact is similar (412.9–475.5 MJ for stationary plants) or significantly higher in terms of the mobile plant (700.3–725.5 MJ). The HTP for carbon concrete recycling with pyrolysis is almost twice as high as the option with only mechanical recycling (1.0 vs. 0.4 kg DCB eq.), while the HTP for steel recycling ranges between 0.6 and 0.9 kg DCB eq. The MAETP_inf. for carbon concrete recycling again is significantly higher as the mechanical treatment (more the three times the impact: 1882.6 vs. 557.7 kg DCB eq.). The environmental impacts of recycling of SRC are higher than the mechanical treatment of carbon-reinforced concrete, but lower than the option which includes pyrolysis (769.6–1301.0 kg DCB eq.) (Table 2 and

Table 3. LCA SRC-EoL CML2001.

	EoL SRC	ADPe	ADPf	AP	EP	FAETP	GWP	HTP	MAETP	ODP	POCP	TETP
	Steel	0.0	25.2	0.0	0.0	0.0	1.9	0.0	30.98	0.0	0.0	0.0
A	Concrete fraction—mobile plant	0.0	700.3	0.0	0.0	0.3	6.9	0.9	860.2	0.0	0.0	0.1
	A: Steel + mobile plant	0.0	725.5	0.0	0.0	0.3	8.8	0.9	891.2	0.0	0.0	0.1
B1	Concrete fraction—stationary conventional	0.0	412.9	0.0	0.0	0.2	9.5	0.6	769.6	0.0	0.0	0.1
	B1: Steel + stat. plant con.	0.0	438.1	0.0	0.0	0.2	11.4	0.6	800.6	0.0	0.0	0.1
B2	Concrete fraction—stationary innovative	0.0	475.5	0.0	0.0	0.2	12.8	0.8	1270	0.0	0.0	0.1
	B2: Steel + stat. plant inno.	0.0	500.7	0.0	0.0	0.2	14.7	0.9	1301.0	0.0	0.0	0.1

).

5. Sensitivity Analysis

As shown in the evaluation of CRC using electricity, fuel, and gas, the main driver for all EoL processes is the required energy (see also Figure 3 and Figure 5 (dry processing and dry and wet processing)). Consequently, in our sensitivity analysis, only a variation in the energy supply is considered. This was previously set as a traditional German electricity mix, which is replaced by a renewable electricity mix.

In our scenarios, the concrete fraction is demolished and crushed individually with fuel, and consequently with diesel, which has no impact on energy use and the electricity mix; thus, the emissions remain unchanged. However, the key factor for CRC is mechanical recycling and, in particular, pyrolysis. We assume that both recycling options are purely based on renewable energies [61]. With the substitution of the energy source, emissions are reduced by more than 50% for both options (conventional in diagonally striped muster and renewable in grey, as shown in Figure 10). In pyrolysis, savings as high as 70% can be achieved (Figure 6).

6. Discussion and Limitations

The case study shows that the environmental impacts of CRC demolition and grinding are significantly influenced by the size of the grains. For GWP, a reduction of about 86% is possible comparing the best with the worst scenario (range: 2–14 kg CO₂e). Moreover, the use of renewable energy plays an important role in CRC as the use of renewable energy can reduce over 50% of environmental impacts. For SRC, wet processing (scenario B2) provides the largest emissions but offers the smallest particle size for further processing.

Although the CRC double walls consist of less material and might have a longer lifespan, the EoL emissions are partly higher than those for SRC. This is due to energy-intensive processes of pyrolysis. Thus, purely renewable energy can already help to reduce the environmental impacts. However, the further life of the reinforcements is hardly comparable. While steel can be recycled nearly without any losses or downcycling, a closed cycle of carbon fibers is, despite high emissions, realistically not possible today. Hence, fiber properties or EoL processes need to be greatly improved for an efficiently improved loop that provides more environmental benefits.

Some limitations of the study shall be addressed. For instance, the results are based on literature data and assumptions, as few primary data related to EoL of CRC currently exist. Furthermore, only the EoL is considered and no upstream and downstream processes are included. Downstream processes are not considered as steel and carbon fibers are not comparable if not used for the same purpose again. Moreover, the melting of steel is not accounted for, as a direct further use is assumed. Thus, the comparability can certainly be criticized depending on the use and should be considered accordingly in future studies.

7. Conclusions and Future Outlook

The aim of the study was to compare the environmental impacts of the end-of-life (EoL) of carbon-reinforced concrete (CRC) and steel-reinforced concrete (SRC). To do so, a life cycle assessment using literature data is performed—only focusing on the end-of-Life scenarios. In the background, we describe in detail all possible options for the EoL of CRC. The results show that the emissions of CRC EoL, despite lower input related to mass and an expected higher lifetime, do not necessarily decrease compared to SRC, but are partly higher. The best SRC scenario in terms of GWP is a mobile plant leading to 8.8 kg CO₂ eq., while the best CRC scenario is mechanical recycling (7.0 kg CO₂ eq. best case and 8.6 kg CO₂ eq. worst case), which does not include pyrolysis and does not provide material circularity. Moreover, the concrete grain size is identified as relevant for emissions; the smaller the particle sizes, the higher the environmental emissions. Carbon fibers cannot be completely looped and downcycling is likely. The use of CRC should therefore be carefully considered. While its use may have advantages in some applications, a general advantage may not be established. Even though it is based on secondary data, the study may provide relevant insight into the EoL for both academics and industry—already today focusing on challenges 100 years from now. A study with primary data and a comparison with glass-fiber-reinforced concrete, another alternative for fiber-reinforced concrete, should be performed in future research.