Environmental assessment of a disruptive innovation: comparative cradle-to-gate life cycle assessments of carbon-reinforced concrete building component

With growing global awareness and importance of sustainability, there is increasing interest in the development of methods that contribute to a better understanding of the different impacts a building has in its life cycle. In this study, the focus is only on the environmental dimensions of a life cycle sustainability assessment (LCSA) (Finkbeiner et al. 2010; Kloepffer 2008): the LCA of reinforced concrete. LCA identifies, analyzes, and evaluates all impacts that a product, service, or building has on the environment during its life cycle (Ding et al. 2016; Finkbeiner (editor) 2015), starting from resource extraction up to the end of life (EoL). The assessment (LCA) is standardized according to ISO 14040 and ISO 14044 (ISO 14040 2006; ISO 14044 2018), and in the building sector additionally by ISO 15686–5 (ISO 15686–5 2017) and DIN EN 15,804 (Deutsches Institut für Normung (DIN) e.V. 2012). The structure of LCA, as stated in the standards (ISO norms), in general and in this article in particular comprises four phases, named as goal and scope, life cycle inventory (LCI), life cycle impact assessment (LCIA), and interpretation (ISO 14040 2006; ISO 14044 2018). The LCA is considered the best methodology for evaluating the environmental impacts caused by a product during its life cycle in the construction sector.

LCA studies in the construction sector have been available since 1997 (Adalberth 1997). Various reviews (Backes and Traverso 2021a; Benli Yildiz et al. 2020; Buyle et al. 2013; Martínez-Rocamora et al. 2016; Ortiz et al. 2009; Sharma et al. 2011) show that the results of diverse construction related LCAs are not easy to compare. System boundaries vary significantly and many studies do not consider the entire life cycle. The following aspects are excluded for example: transportation, waste factors, maintenance, water consumption, use phase, and end of life (Benli Yildiz et al. 2020; Buyle et al. 2013). The functional units vary from a whole building, over m², over number of lives in a house, over kWh/m²/year, and over building components and material (Backes and Traverso 2021a; Buyle et al. 2013; Sharma et al. 2011). Furthermore, different lifetimes are assumed for the buildings or components, ranging from 1 year to 75 or even 100 years (Buyle et al. 2013; Sharma et al. 2011). The inclusion of regional factors such as location, climate, and technology as a result of studies conducted in different countries is often neglected (Benli Yildiz et al. 2020). The influence of different production sites (location, geographical differences) can have an impact on raw material sourcing, transport routes, technologies used and efficiency of these, energy used, and, by extension, on social and economic factors. Furthermore, there are also differences between the use and end-of-life phases, in the use phase in particular due to different climatic conditions, which influence early and late aging or, for example, corrosion; in the end-of-life, the state of the art in different countries and thus technologies and the use of energy compositions (purely renewable vs. nuclear power vs. coal power, for example) can have further influence. Accuracy also varies, i.e., some studies are crude and less detailed, considering only the most obvious products and processes (Buyle et al. 2013). Both midpoint and endpoint indicators are applied (e.g., CML, Eco-indicator99, and Carbon Footprint), sometimes a range of different methodologies are used, or results are examined to see if they meet policy objectives (Buyle et al. 2013). However, GaBi Database and Ecoinvent stand out for their integrity, ease of use, and dedicated resources, highlighted by Martínez-Rocamora et al. (2016). Less frequently, LCAs consider impact categories other than CED and climate change (Backes and Traverso 2021a). Many of these studies are simplified LCAs that only address the energy demand, especially the early studies. The results shed more light on building designs in general and less on the materials chosen (Backes and Traverso 2021a; Benli Yildiz et al. 2020; Buyle et al. 2013; Martínez-Rocamora et al. 2016; Ortiz et al. 2009; Sharma et al. 2011).

It is important to note at this point that we are not aiming for a literature review on LCAs in the construction sector, LCAs on steel-reinforced concrete, or LCA on carbon-reinforced concrete. The following studies are listed for the purpose of classification of the topic and are intended to illustrate the complexity of LCA in the construction sector.

Looking in more detail at the state of research of scientific LCAs on carbon-reinforced concrete, a literature search extended to basic materials such as carbon fiber–reinforced polymers (CFRP) and carbon fibers based on few studies on carbon concrete yields about 80 detailed and advanced studies. The studies consider a large variety of functional units, as for example 1 kg material (Maxineasa and Taranu 2018), 1 m unit length of a bridge system (Pang et al. 2015), 1 m² of sandwich wall (Scope et al. 2020), 1 m² of reinforced concrete (Williams Portal 2015), 1 m³ of concrete (Zingg et al. 2016), or an experimental facade of 60 m² (Laiblová et al. 2019). Chen et al. (2020) addressed the environmental impact of strengthening methods made of a carbon fiber-plastic composite, while Zhou (2013) conducted a comparative LCA of two alternative structural reinforcement techniques, both adopted a cradle-to-gate LCA approach (Chen et al. 2020; Zhou 2013). A cradle-to-grave approach was presented by Das (2011) (FU = 1 kg). Williams Portal et al. (2015) investigated textile-reinforced concrete (TRC) as a building material in a cradle-to-gate LCA (FU = 1 m²), with carbon fiber and steel reinforcement also being part of the study (Williams Portal et al. 2015). Laiblová et al. (2019) compared carbon fiber–reinforced TRC facades with ordinary steel-reinforced concrete (Laiblová et al. 2019). Maxineasa and Taranu (2018) investigate pultrusion molded carbon fiber strips used to reinforce concrete structures (Maxineasa and Taranu 2018). Ibrahim et al. (2020) compare a carbon fiber reinforcing bar to a steel reinforcing bar in a cradle-to-gate LCA (Ibrahim et al. 2020). Scope et al. (2020) designed a LCSA to compare exemplary building components and retrofit scenarios based on CRC with those made of SRC (Scope et al. 2020).

For SRC, a large number of studies have been published. The objects studied vary widely and include, for example, reinforced concrete structures in detail (Li et al. 2019), concretes (Abdulkareem et al. 2019; Ding et al. 2016; Knoeri et al. 2013; Xia et al. 2020), ceiling designs (Brambilla et al. 2019; Hájek et al. 2011), and strengthening techniques (Palacios-Munoz et al. 2018). The different studies have been carried out all over the world (e.g., Ding et al. 2016; Dong and Ng 2016; Evangelista et al. 2018; Maia de Souza et al. 2016) and consequently considered different construction methods, transport routes, and/or types of manufacturing. Different units were used as the FU for which the environmental impact was determined: as 1 m³ concrete (e.g., Ding et al. 2016), a building construction project (Dong and Ng 2016), m² area of the building per year (e.g., Evangelista et al. 2018), kg of reinforcing material or m of reinforcing span (e.g., Palacios-Munoz et al. 2018), concrete structure with specific structural form and mechanical properties (e.g., Xia et al. 2020), and the construction and maintenance of 1 m² of exterior wall (Maia de Souza et al. 2016). The main focus in all studies was on climate change (in kg CO₂e).

Since the aim of this study is not a literature review, we present some examples of studies mentioned above in more detail in order to be able to rank the results of this study — with its focus on CRC in comparison to SRC — in terms of completeness, consistency, and plausibility. In Table 1, climate change (CC) in kg CO₂e values were extracted from previously published studies, focusing CRC and SRC. The studies presented below have conducted an extensive and comprehensible LCA, the data basis is presented and explained, all results are reported in absolute numbers, and correspond to a comparable functional unit (or reference flows) and system boundaries. Unnamed studies, without claiming to be complete, have deviations or missing information with regard to data, functional unit, system boundaries, or results presented (e.g., in percentage). Table 1 differentiates in coating, carbon fiber, steel, concrete, and reinforced concrete, resulting in a range of kg CO₂e values for the reference flows or FUs of mass (1 kg, 1 t) or volume (1 m³) (Table 2), assessed for the system boundaries of cradle-to-gate. In addition, we integrated also a reference value for concrete from the Environmental Product Declaration (EPD) (A1–A3 = cradle-to-gate) (ibu-epd 2021).

None of the named studies considered and compared a functional unit at building component level (double wall), none made the direct comparison to steel-reinforced concrete, and additionally and especially, none used different concrete and fiber compositions, so that opportunities and risks can already be identified in the individual components, and the individual and reproducible inventory data. Furthermore, the current study represents a much greater level of detail, as the complete inventory as well as the process selection is presented in detail in the Supplementary Material.

Short digression: the double wall — our functional unit: The double wall in general offers an alternative to conventionally formed concrete walls, as it is an industrially prefabricated wall system. The double-skin wall element can be used as a statically loadable wall panel both in multi-storey construction and in basements and underground garages. This semi-prefabricated wall (double wall) consists of two wall shells, which are connected with lattice girders. The wall elements contain the statically required main and transverse reinforcement at the factory. Casted with in situ concrete, the overall cross-section looks like a monolithically manufactured wall. The result and the advantage of the double wall are perfect surfaces and a fast, economical construction process, as formwork is no longer required on site. In the example presented here, the basis for comparison of carbon- and steel-reinforced concrete are functionally identical components made of carbon and reinforced concrete in precast construction, which are equivalent in their load-bearing capacity. The same functionality results in components with different dimensions (Otto and Adam 2019).

Carbon-reinforced concrete is a composite construction material made of concrete and carbon fibers. Concrete forms the basis for both composite materials (CRC and SRC). Concrete consists of a mix of cement, aggregates, concrete additives (admixtures), and water (Stahr 2015). The composition of these starting materials and the respective material properties influence the workability of fresh concrete and subsequent properties of the hardened concrete. Therefore, the composition of concrete must be specifically planned so that it can meet requirements from the environment and use, such as frost resistance and durability (Stahr 2015). Cement, as part of concrete, consists primarily of limestone and clay, and in most cases also of quartz sand and iron ore (Wietek 2019). After grinding and burning processes at 1400 to 1500 °C, the so-called cement clinker is produced. This is mixed with further additives such as granulated blast furnace slag, fly ash, or limestone and ground (Grimm 2014) — resulting in cement, a hydraulic binder which, with the addition of water, forms a cement paste and solidifies (Stahr 2015). Concrete is processed as fresh concrete and acquires its dimensionally stable property only after the chemical reaction between cement, water, and the subsequent hardening processes. Twenty-four hours after the addition of water, concrete has largely solidified; nevertheless, the solidification process is only complete after 4 weeks to several months (Grimm 2014).

Carbon fibers are used, among other products, to produce the reinforcement of CRC. They consist of carbon-containing materials and are produced artificially. Due to their low density and good mechanical properties as well as processability, they are appreciated reinforcing fibers in fiber composite construction (Kortmann 2020). The starting material for carbon fibers produced for the construction sector is mainly polyacrylonitrile (PAN), made of petroleum. PAN-based fibers are produced from the starting material PAN using solvents. These are drawn and pre-stretched in a wet-spinning process, which straightens the molecular structure for the desired fiber properties. This is followed by further post-treatment, which consists of washing, drying, and sizing. Finally, the PAN-based fibers are wound onto spools (AVK 2014; Kortmann 2020). The conversion process of PAN-based fibers to carbon fibers includes stabilization and oxidation (1), carbonization (2) and possible graphitization (3), and surface treatment (4) of the fiber (Kortmann 2020), being energy intensive process steps. Values compiled from the literature vary between an electricity demand of 32.4 to 200 MJ/kg and a thermal energy requirement of 97.69 to 200 MJ/kg for the production of carbon fibers (Hohmann 2019). This is followed by the impregnation application, which allows the surface properties of the fiber to be specifically adjusted. The fiber is pulled through a sizing bath and absorbs a predetermined amount of impregnation and water. In the construction industry, a distinction is usually made between two impregnations, depending on the desired stability and flexibility of reinforcing scrim: epoxy resin (duromers; EP) or styrene-butadiene rubber (elastomers; SBR) (Kortmann 2020).

CRC refers to a new type of composite material. The material is categorized as a textile concrete because, unlike conventional fiber concretes, the fibers are not added loosely to the fresh concrete mix but are present as so-called continuous fibers in a fixed lattice or bar form (scrim). As a result, the reinforcement can be arranged in the tensile zone of a component in line with the force flow. Carbon-reinforced concrete thus combines an already known advantage of reinforced concrete with a high load-bearing capacity of carbon fibers. To produce the reinforcement, up to fifty thousand filaments (individual fibers) are bundled into long fibers and following into a roving. In textile machines, the rovings are processed into carbon fiber rods or a flat, grid-like scrim. Depending on the application of the carbon scrim, flexible matrices (for curved components, for example) or stiff matrices (for the production of large-format reinforcing scrims) are used (Kortmann 2020).

SRC is a composite material consisting of concrete and a reinforcement of steel (Weber 2013). Among all, steel-reinforced concrete is the most widely used building material in the construction sector (Stoiber et al. 2021). The steel industry in Germany can be considered as one of the countries’ most important sector. With an annual production of 42 million tons, it is even one of the largest crude steel producers worldwide. About 35% of the produced steel is used for the construction industry (14.7 million tons). In addition, about one-eighth of this amount is further used for reinforcing steel in concrete (1.8 million tons) (Seifert and Lieboldt 2020). Taking a look at the production of steel, two different processes are mainly used (Helmus and Randel 2014): basic oxygen steelmaking (BOS) and electric arc furnace (EAF). The production process of BOS, also known as the primary route, currently accounts for over 60% of the total crude steel production. Even though this production method is seen as efficient, it is highly carbon-intensive. Consequently, EAF has gained in importance in recent years owing to ecological benefits of this process (Helmus and Randel 2014). To be in line with EU targets (carbon neutrality by 2050), further carbon mitigation techniques have been developed (or are currently in development) for the steelmaking industry (Backes et al. 2021). An example is the hydrogen-based direct reduction processes as promising carbon–neutral steelmaking (Suer et al. 2022).

The popularity of SRC can be explained due to its beneficial characteristics (Stahr 2015). SRC is characterized by its economic efficiency, durability, and ready availability of raw materials (Poursaee 2016). Looking at the steel supplier network, it is characterized by short transport routes carried out by medium-sized concrete steel suppliers (Kortmann 2020). The advantages, however, are countered by disadvantageous properties. To provide general resistance and protection against corrosion, a minimum concrete cover must be maintained (Stahr 2015). A minimum concrete cover can be described as a barrier of concrete which suits as a physical and chemical corrosion resistance (Poursaee 2016). In general, a larger diameter of installed steel requires a thicker concrete cover (Stahr 2015).

When using CRC, concrete savings of about 50% can be achieved with the same static properties (Adam 2018) (Table 2: concrete cover), due to the corrosion resistance. The minimum concrete cover of 5–10 mm must be observed for all exposure classes when using a stainless reinforcement to ensure durability (Kortmann 2020). Consequently, no minimum concrete cover is required to protect the reinforcement from corrosion. Table 2 shows main differences between CRC and SRC.

Due to the low density of carbon, its performance in terms of weight is even greater in comparison, being almost 24 times greater than that of reinforcing steel (Otto and Adam 2019) (Table 2: performance). Since different concretes are used depending on the project and the CRC innovation, it is difficult to compare or differentiate the concrete used for steel and carbon, as for both similar concrete can be used. Differences and the rationale for diverse concrete mixes are primarily due to different strength classes of concrete and possibly different ways of manufacturing the component. As example, the extrusion process allows a much simpler integration of the reinforcement compared to 3D concrete printing. High demands are placed on the rheological properties of fresh concrete for the extrusion process, as the concrete must be transportable in the extruder. Another difference between CRC and SRC lies in the delivery network of the reinforcements. While steel reinforcement is characterized by fast procurement and short delivery routes, carbon components have to be supplied over much longer distances (Kortmann 2020). Despite the differences mentioned above, both reinforced concretes (CRC and SRC) can be used in equivalent performance — which is shown in this LCA with the example of a reinforced double wall.

The aim of the LCA scenarios in this paper is to present the environmental impact range of cradle-to-gate scenarios of CRC — determined in the design stage, hotspots, and optimization approaches will be identified, a comparison with SRC is made.

The following scenarios are based on ISO 14040/44 (ISO 14040 2006; ISO 14044 2018). The impact categories used are those in CML2001 (August 2016) (CML - Department of Industrial Ecology 2016): abiotic depletion (ADP elements [kg Sb eq.], ADP fossil [MJ]), acidification potential (AP [kg SO₂ eq.]), eutrophication potential (EP [kg Phosphate eq.]), freshwater aquatic ecotoxicity (FAETP inf. [kg DCB eq.]), global warming potential (GWP [kg CO₂ eq.]), human toxicity potential (HTP inf. [kg DCB eq.]), marine aquatic toxicity (MAETP inf. [kg DCB eq.]), ozone depletion potential (ODP, steady state [kg R11 eq.]), photochemical ozone creation potential (POCP [kg Ethene eq.]), and terrestric ecotoxicity potential (TETP inf. [kg DCB eq.]).

For comparison reasons, the FU and reference flows for the current study were differentiated in.

The functional unit of a double wall for the composite material (5 m × 2.5 m × 0.03 m) (Fig. 1) (according to Otto and Adam 2019),

The reference flow of 1 m³ for concrete (C) only (in accordance with product category rules), and.

The reference flow of 1 kg for fibers (F) only (in accordance with product category rules).

The reason for the FU and different reference flows were:

Simplified comparison possibilities to previous studies (in m³, kg, or t) (Table 2) and thus a plausibility check of the current results,

Construction component comparison to steel-reinforced concrete and its function, and.

In the possibility to show the challenge of the functional unit selection in LCA.

The baseline scenario represents a combination of Portland cement and PAN-based fiber with EP impregnation, produced in Germany (C1/F1). In total, nine concrete mixes (C = concrete (C1–C9)) and five carbon-scrim reinforcements (F = fiber (F1–F5)) (Fig. 1, Table 3) are represented, as well as 45 (9 × 5) different scenarios, supplemented by varying electricity mix, concrete mix, and steel processing (“2.18”). The reason for the different scenarios is that there is not a single concrete mix or a single fiber and impregnation combination for a carbon concrete component. Through the scenarios, we try to represent a whole range of possible realities and at the same time also show what material compositions with the lowest and highest possible emissions are. The detailed changes in concrete mixes and fiber compositions are shown in more detail below.

Mass differences between a carbon-reinforced double wall and a steel-reinforced double wall are shown in the following Table 3 — concrete in tons (t) needed for a carbon reinforcement is defined as 1.42 t compared to 2.63 t of concrete needed for a steel reinforcement (Table 3).

The double walls, whereof each wall is reinforced by a carbon fiber or steel matrix (Fig. 1), are additionally connected to each other in reality by a composite grid. This is shown in Fig. 1 as a black dotted line between the two walls. Such a composite grid can, for example, be made of AR glass textile (von der Heid et al. 2019) or of steel. This composite lattice (connecting pins) was not named in Otto and Adam (2019), which is why it is not included in the current model. Moreover, possible insulation was also not considered in this LCA.

The system boundaries are designated as cradle-to-gate (= EPD A1-A3 in DIN EN 15,804 (Deutsches Institut für Normung (DIN) e.V. 2012)). The use and EoL phases are not part of this study: The use phase differs depending on the function, which is why it is difficult to clearly assess it. The EoL phase is briefly outlined below, but requires research and assessment work due to the innovation of CRC. The equipment and machinery themselves used for production are not included within the study. The energy, however, required for these machines — such as the concrete mixer or the scrim production machine — is included in the balance sheet, based on primary and literature data (see Supp. Material). The scenarios assume production in Germany and Japan. Further scenarios consider the use of renewable electricity mixes, based on current German data from 2020 (Umweltbundesamt 2021). In the fiber production, about 5% cutoffs (by, e.g., cutting fibers) are assumed (by ita, RWTH Aachen University, Hohmann (2019)), which are fed to landfill and then converted in and fed into the grid as energy (through combustion; credit given). The transports between raw material (-extraction) and further processing are assumed with a general average distance of 100 km each and are transported by truck, operated with diesel. For a scenario with oversea production (Japan), a ship transport is assumed (see Life Cycle Inventory in Supp. Material).

Conservative approaches were used in the selection of processes and also in the assumption of data, when not given as primary or clearly defined literature data. Country-specific datasets were used as far as possible; where this was not feasible, first European and then global datasets were used. All data generated or analyzed during this study are included in this published article and its supplementary material (Supp. Material) — showing various concrete mixes and fibers including impregnation.

For a better understanding of the following life cycle impact assessment, the summary of the abbreviations is shown as a legend (Table 6).

Following, we set the objective of the study and compare CRC with SRC. Furthermore, we consider hotspots in detail as part of a sensitivity analysis.

Apart from the environmental aspect of sustainability, the choice of CRC or SRC for the double wall obviously needs to be broadened to focus on the other two dimensions of sustainability: LCC (life cycle costing) (cost/economic) and S-LCA (social life cycle assessment) (social). In future, this environmental assessment should be placed in the context of a LCSA. The reason for naming the LCSA at this point is that strong changes are expected, especially in the social sector (S-LCA), with the introduction of CRC at an industrial scale. CRC is already being 3D-printed (e.g., Mechtcherine et al. 2020 and Neef et al. 2020) or extruded (e.g., Kalthoff et al. 2022) at laboratory scales — which can significantly change the world of work in the construction industry. Furthermore, the fibers are largely sourced from Asia and the USA, which implies different social factors than pure production steps in Germany or the EU. In terms of cost, CRC will exceed SRC, making it questionable what impact economics will have in the long run in this three-dimensional construct, especially in contrast to the environmental dimension (LCA).

Recycling of this new type of composite material in particular may still present us with future challenges. Summarizing current findings on the end-of-life phase of textile-reinforced concrete, it is clear that downcycling due to damage to the textiles is unavoidable. A second reuse of the reinforcement textile in its original purpose can be excluded with current methods (Backes et al. 2022a, b; Kimm 2019; Kimm et al. 2020). In the future, the construction method itself and the component, i.e., our FU of a double wall, for the carbon-reinforced concrete will also have to be questioned. A double wall is not the optimal component in terms of emissions for the CRC, as shown in our results. At this point, cooperation with civil engineers is once again needed to design innovative constructions that make the CRC useful also in terms of emissions and not only in terms of resource savings and thus can seriously replace steel-reinforced concrete in some places.