Biochar Storage in the Built Environment

This Module details the requirements for storage of carbon as biochar in the built environment, including reversal risk and durability. Within this Module, a built material is defined as a product used in the construction of buildings or infrastructure projects, including concrete, cement, and asphalt. Durability refers to the stability of CO₂e stored as biochar in the built material and the length of time for which CO₂ is removed from Earth's atmosphere. Within this Module, concrete durability is used to refer to the structural durability of concrete. This Module is intended for use in conjunction with other Isometric Protocols and Modules, and assumes the following:

• The full quantification of the net tonnes of CO₂e removed for Certificate issuance follows the Isometric Biochar Production and Storage Protocol.
• All environmental and social safeguards have been followed according to the ESS Section in the Biochar Production and Storage Protocol and this Module.

This Module applies to biochar that is physically incorporated into conventional built materials (e.g., concrete and asphalt) during their manufacture. It does not apply to CO₂ stored in built materials as carbonate minerals, which is addressed by the CO₂ Storage via Carbonation in the Built Environment Module. Where a built material both incorporates biochar and stores CO₂ through carbonation, only the carbon stored as biochar is Certified under this Module.

The durability horizon for biochar in built materials is 200 years, and Certificate issuance under this Module is limited to the inertinite fraction of the biochar, quantified by random reflectance against the inertinite benchmark¹ and refined to the recalcitrant fraction by thermogravimetric analysis². The horizon is shorter than the 1,000-year default applied elsewhere by the Isometric Standard. It is not a finding that the inertinite degrades after 200 years. It limits the durability claim to the period over which the fate of the material can be documented rather than extrapolated. The durability discount in this Module (Appendix A: Reversal risk) depends on how many times the material is demolished, milled, or recycled, and on the end-of-life pathway at each event. Service lives and reprocessing intervals are documented over approximately one service life plus first reuse; across 1,000 years the number of reprocessing cycles and the pathway at each cycle are unconstrained. This applies with particular force to asphalt, which is milled and reprocessed at intervals of decades. A 200-year horizon corresponds approximately to the design life of a built asset, indicatively 50 years for common buildings and 100 years for monumental structures and bridges under EN 1990:2023, plus reuse.

A further consideration applies to cementitious materials only. Biochar in concrete and other cement-based materials is exposed to the pore solution during encapsulation, typically at pH 12.5 to 13.5³, which alters its structure⁴. While the material remains intact this is not itself a carbon loss pathway, but altered and aged carbon is more susceptible to oxidation⁵, so the exposure may leave the biochar more vulnerable to degradation once the material is broken up and the carbon is exposed to the open environment. Direct evidence on the degradation of biochar recovered from cured cementitious material is not yet available. This consideration does not apply to asphalt, in which the binder is a hydrophobic bituminous matrix rather than an alkaline aqueous environment.

The stability of carbon within the built environment, and thus its durability, depends on interactions with the surrounding environment and the intended use case. Built materials used in the construction of buildings or infrastructure Projects may be exposed to variable environmental conditions during their use life. Risk factors that may affect the expected durability of built materials include, but are not limited to, interactions with acidic fluids such as acid rain and low-pH groundwater, building fire, abrasion, comminution during recycling, and high-temperature closed-loop recycling of concrete. For the purpose of this Module, built materials are treated as an open system storage mechanism. Reversal risk over the durability horizon is assessed on a Project-specific basis and applied as an uncertainty discount, as set out in Appendix A: Reversal risk.

Biochar storage in the built environment is a novel CDR approach, so this Module incorporates requirements that may be more stringent than some current relevant regulations or other Protocols related to biochar for CDR. This Module will be reviewed when there is an update to scientific published literature that would affect net CO₂ removal quantification, durability claims, or the monitoring guidelines outlined in this Module. Future versions of this Module may be altered, particularly regarding requirements for demonstrating durability of biochar, as the stability of carbon in biochar is better demonstrated and documented; quantification of biochar degradation is further improved and refined; and the overall body of knowledge and data regarding all processes, from feedstock supply to conversion and to permanent storage, is significantly increased.

Projects seeking Certificates for biochar stored in the built environment are subject to the requirements set out in Applicability and Biochar Characterization, and to the reversal risk assessment set out in Appendix A: Reversal risk.

Projects seeking Certificates under this Module must meet the following criteria:

• R-A4BY-1Built materials produced with incorporated biochar must meet the same performance requirements as a conventional product for the intended use case. Any required admixtures or processing steps must be standard industry practice and must not result in disproportionately greater resource use or emissions compared to conventional materials. This comparison is made for the product system alone, excluding the carbon stored in the biochar; it is an applicability condition, not a test of the net removal. Emissions from admixtures or processing steps required as a consequence of using biochar are Project emissions: they fall outside the exclusion of downstream emissions set out in System Boundaries and must be included in $CO_2e_{Emissions,RP}$, so they are deducted from the removal.
• R-XS7K-1Built materials produced with incorporated biochar must not require additional products or activities related to installation and maintenance as compared to conventional products.

Only biochar incorporated into asphalt that is laid in a Sub-Surface Layer is eligible for certification under this Module. Biochar in a surface course is not eligible.

The surface course is excluded because its service life prevents a conservative, verifiable estimate of the biochar retained. It is the only bound layer under direct tire contact, so part of its biochar is liberated continuously during service as road pavement wear particles. Reported abrasion rates span more than an order of magnitude (0.04 to 0.5 mm per year¹³) depending on traffic, tire type, studded-tire use, and climate, and the abraded mass cannot be measured for an individual pavement or verified after the fact. The surface course is also the layer milled at each resurfacing, roughly every 15 to 25 years¹⁴, and its reclaimed material may be returned to a new surface course, so the biochar can remain in the most exposed position across successive cycles. Wear particles, which peak at 63 to 106 μm¹⁵, are carried to roadside soil, road runoff, stormwater and wastewater systems, and street-sweeping waste. Some of these routes end in sewage-sludge or waste incineration, which destroys the carbon, and the share following each route cannot be determined for a Project. The exclusion rests on the inability to quantify and verify the retained fraction, not on a finding that dispersed inertinite degrades in every receiving environment.

Isometric will reconsider this exclusion on evidence that allows in-service loss from a surface course to be bounded conservatively and verified for a Project, including measured retention of biochar carbon in cores taken from in-service surface courses across a range of traffic levels and climates, a field method for quantifying biochar in asphalt cores and wear particles that can be audited by a VVB, and data on the fate of biochar-bearing wear particles across receiving pathways.

Biochar is a durable carbon-rich solid material produced from the pyrolysis of waste biomass. The carbon content in biochar can be further separated into stable (with inertinite as a proxy) and degradable (labile) fractions. These distinct carbon fractions have different carbon durabilities, with the stable fraction of biochar being stored durably for a time horizon of 1,000 years and above¹ while the labile fraction can degrade over shorter time horizons in the range of a few years to decades, when used in conventional soil amendment applications¹⁶ (see Isometric Biochar Production and Storage Protocol). Different production processes and feedstocks directly impact biochar's quality and stability, as they may result not only in variations in the proportions of labile and fixed (stable) fraction but also changes in biochar structure.

Biochar in the built environment is subject to two categories of process. The first is physical liberation and dispersal: loss of fines, particle size reduction during reuse or recycling, and abrasion of the host material. This acts on the biochar regardless of which carbon fraction it belongs to. The second is chemical and biological degradation, including natural mineralization, photodegradation, and leaching driven by water flow or freeze thaw cycling¹⁷. This acts principally on the labile fraction, and only once that fraction is exposed.

This Module certifies the stable fraction only. The physical processes that liberate biochar from the host matrix are therefore the dominant determinant of durability, and they are the basis for the inertinite retention factor set out in Appendix A: Reversal risk. The stable fraction is durable over geological timescales once formed¹; the question this Module answers is not whether that carbon degrades in place, but whether it remains held within the built material.

In recent years, the incorporation of biochar in construction materials has been receiving increasing interest, owing to its unique properties such as porosity, high specific surface area, water retention capacity and organic carbon content. Biochar's unique structure and porosity makes it a versatile and innovative material that can be utilized in multiple applications including concrete, asphalt, bricks, tiles, insulation materials⁸, composites, panels, plasters and geopolymers. These uses not only contribute to permanent carbon removal but may also enhance specific material properties¹⁸.

Previous studies have focused on integrating biochar in construction materials as a substitute, filler, additive or modifier, mainly in concrete and asphalt production^9,18. Applications that have gained traction include biochar used in:

• Concrete production process as an aggregate replacement¹⁸
• Cement production as a filler material, reducing the amount of carbon intensive clinker and concrete
• Asphalt applications used as an asphalt modifier and/or binder¹⁹

These applications are discussed in further detail in the Built Environment section.

New research on biochar stored in built materials has shown that these novel materials may capture additional CO₂ from the air, particularly in the case of biochar-cement composites²⁰. Replacement of the aggregates in concrete production with biochar improves pore size distribution¹⁸ and can enhance the CO₂ carbonation process and increases concrete's ability to sequester carbon⁷.

At the time of writing, there is not sufficient evidence to include this in the quantification framework for net CDR achieved by biochar storage in the built environment. This will be revisited in future editions of the Module as new evidence is published and/or presented to Isometric by Project Proponents as proof of quantification.

Concrete, made from cement, sand, aggregate, water, and chemical admixtures, is the most widely used construction material in the world, with approximately 4.2 billion tonnes of cement produced globally in 2020 (Global Cement and Concrete Association, Cement and Concrete Around the World). Compared to other structural materials, concrete has a relatively low cost and embodied carbon, while also providing strength, durability and adaptability. However, because of the large volume of concrete produced and used globally, the concrete supply chain has a large carbon impact, accounting for approximately 5 to 8% of total global CO₂ emissions²¹. The vast majority of emissions during concrete production result from manufacturing Portland Cement (PC) via calcination. During this process, crushed limestone (CaCO₃) is heated in a kiln to high temperatures (~1400°C), causing degradation to CaO (lime) and CO2:

$$CaCO_3 \to CaO + CO_2$$

(Equation 1)

Though a portion of emissions are associated with processing limestone and heating the kilns, approximately half of the process emissions are the result of direct emissions from calcination²².

Using less material in the built environment and using lower embodied carbon alternatives to PC both play a key role in efforts to reduce the climate impact of the construction industry. Several recent studies (summarized well in Legan et al.²⁰) have shown that addition of biochar in building materials, notably cement, can improve several key performance indicators while reducing the environmental impact of concrete production. For example, researchers have observed enhanced mechanical properties of biochar-amended concrete, including compressive, flexural and tensile strength; thermal stability and conductivity; bulk density; ductility; flowability; water absorption and penetration potential; and albedo²³. However, some studies have also noted decreased flexural strength²³ and decreased compressive strength^6,9 at biochar dosages up to 40% by mass of cement.

Limits to the amount of biochar addition in construction materials are dictated from material specification and performance standards, especially in terms of the fresh and hardened mechanical properties of concrete and other cementitious composites. The impact on the mechanical performance and other properties is dependent on the biochar type (e.g., feedstock used, temperature of production, post-processing), percentage of biochar incorporated, and concrete mix design. Previously reported limit values of biochar in built materials ranged from 0.5 to 40%²⁰, although these thresholds can vary depending on the material and application. Biochar is typically added at rates of up to 20% and 15% in concrete and asphalt production, respectively^24,25,26.

Asphalt is a viscoelastic material that is commonly used in pavement construction, roofing and other building applications²⁷. It is a composite material typically including a mixture of aggregate materials, such as crushed stone, sand and gravel, bound together with bitumen, a highly viscous material derived from petroleum. Asphalt production requires significant energy consumption and is consequently an emissions-intensive process²⁸. Conventional methods of asphalt production are referred to as 'hot-mix asphalt', in which aggregates are heated to remove moisture and are combined with bitumen at high temperatures (typically 170 to 190°C)²⁹.

Several alternatives to hot-mix asphalt have been proposed to reduce the environmental impact of pavement construction. These include warm-mix asphalt, which produces a similar product at lower temperatures (130 to 150°C)²⁸; recycled asphalt pavements; and bio-asphalt, which encompasses both the partial replacement of fossil fuel-derived bitumen with plant-derived bio-oil³⁰ and addition of biochar to the mixture as an asphalt modifier²⁶. Cold-mix asphalt, including mixtures bound with bitumen emulsion or foamed bitumen and cold recycling of reclaimed asphalt, is produced at ambient or near-ambient temperatures, with lower energy use and volatile release than hot-mix production. Recent studies have shown that addition of modest amounts of biochar can maintain the rheological behaviors and enhance the oxidation resistance of asphalt³¹, improve deformation resistance²⁶, and inhibit volatile organic compounds (VOCs)³². However, surface asphalt is prone to significant abrasion and loss through tyre wear, and therefore the durability and environmental fate of the biochar is uncertain.

Performance requirements for built assets and individual components are influenced by a complex set of factors, including the intended design life, the exposure environment and the mechanical properties needed to meet the requirements of the installation and application.

The design life of a built asset is the assumed period for which it can be used for its intended purpose. Examples of common asset types and design lives (as defined in EN 1990:2023 and CD 226) are summarized in Table 1. Assets with slow rates of change, such as transport or public health infrastructure, will typically be designed with a longer service life and the minimization of maintenance in mind. Corporate office buildings, which are more likely to change in alignment with trends, may be designed with replaceability, adaptability or deconstructability in mind. It is important to note that design life of an asset is not necessarily equivalent to functional service life, which may be longer or shorter than the design life based on a variety of factors such as asset ownership, changes in regional regulatory environments, societal preferences and asset performance. A study of buildings in North America found that the majority of demolished buildings were less than 50 years old³³ and the Royal Institute of British Architects highlight that, in the UK, the functional service lives of buildings within the commercial sector are 25 to 30 years³⁴.

Table 1: Overview of common asset types and their design lives as defined in EN 1990:2023 and CD 226.

Within a given asset, different components will have different expected design lives. For some element groups, such as the structural skeleton of buildings or the piers of a bridge, which serve the key functional purpose of providing the structure of the asset, the goal is a long service life. Elements with more transient functional requirements, such as internal space partitions, or those prone to disturbance, such as paving over buried services or utilities, will typically have shorter design lives.

Concrete may be used in one or more types of asset throughout its functional life, depending on regional policies and trends surrounding end-state concrete use. This topic is covered in further detail in the Section 3.2.3.2.1 End-State Use Cases (Concrete) . Similarly, asphalt may be recycled into new pavement or other civil engineering projects throughout its functional life (see Section 3.2.3.2.2 End-State Use Cases, Asphalt).

Over the 200-year durability horizon, a built material may pass through one or more construction, demolition, and reuse transitions, and each transition carries a Project-specific reversal risk described in Appendix A: Reversal risk. Concrete is typically recycled at most once as a bound material before reaching a terminal downcycled state, so the number of transitions within the horizon is small. The design life of the asset determines how many transitions occur, and the uncertainty in the location and condition of the biochar increases with each one.

Figure 1 summarizes the typical life cycle of concrete, from procurement of raw material, through to processing, use and end-of-life scenarios. In the context of this Module, end-state use case refers to the fate of a given amount of biochar-amended concrete co-located within a single asset after the functional service life of that asset. How concrete is used after the end of an asset's life is influenced by a variety of complex factors, notably the geographic location and the economic and regulatory landscape for particular waste routes. Here, we discuss five possible end-state use cases and implications for CO₂ stored within concrete.

1. In-situ Reuse

In-situ reuse refers to extending the use of concrete in its original form. Some level of repair may be required. Because the concrete remains in its original form, it is not subjected to additional reversal risk and stored biochar is considered stable.

2. Ex-situ Reuse

Ex-situ reuse refers to reuse of concrete in its original form, but in a different asset. This is uncommon practice for reinforced structural concrete elements due to the difficulty of extracting these components, lack of demand and the relatively low cost of new concrete. There are greater opportunities for ex-situ reuse with precast concrete elements, as these may be designed to facilitate deconstruction. There are currently limited regulatory frameworks worldwide that incentivize extensive re-use, though some examples do exist (see Norwegian Standard NS 3682). Frameworks currently under development will likely limit concrete reuse based on exposure class, potentially resulting in more extensive reuse of lower performance elements. Because the concrete remains in its original form, it is not subjected to additional reversal risk and stored biochar is considered stable.

3. Closed-loop Recycling

Closed-loop recycling refers to waste processing that occurs in a closed loop, allowing the waste to retain its original value and return to the original production process. Two routes must be distinguished, because they have opposite consequences for stored carbon.

In mechanical closed-loop recycling, concrete is crushed and the recovered material is used as aggregate in new concrete. The biochar is liberated from its original matrix and re-encapsulated, but it is not exposed to conditions that destroy the stable fraction.

In thermal closed-loop recycling, the recovered cement paste is reprocessed to clinker at the calcination temperatures described in Concrete. This destroys the biochar carbon, including the stable fraction. Technologies for thermal closed-loop concrete recycling remain nascent and have not been deployed at scale. At the time of drafting there are a limited number of pilot projects operating at small scale, including in the United Kingdom, France, and Canada, and policy in several jurisdictions is directed toward expanding this route.

Both routes are included in the reversal assessment set out in Appendix A: Reversal risk. Earlier versions of this Module excluded closed-loop recycling from the reversal calculation on the grounds that it was not operating at scale. That exclusion no longer holds over the durability horizon of this Module.

4. Downcycling

Downcycling refers to waste processing where the resultant product has lower value than the original product. This is the most common end-state use case for concrete. Approximately 90% of concrete and demolition waste in the UK is recovered (DEFRA, UK Statistics on Waste)³⁵. The process typically involves crushing of concrete and subsequent use in applications such as road subbase, coastal defenses, engineering fill, and external landscaping. Downcycled concrete may also be used as aggregate in new concrete products where contamination from other waste products is low. But only a low precent of aggregates are reused, and generally the secondary aggregate replacement rates are typically capped in new structural concrete mixes.

Downcycling increases uncertainty about the location of the biochar and exposes additional surface area at the surfaces created by crushing. It does not expose the biochar to temperatures that destroy the stable fraction, and material placed in an unbound downcycled application is not subsequently routed to high-temperature reprocessing, so downcycling is treated as a terminal state for the purposes of the reversal assessment. The biochar is not re-encapsulated, however, and the alkaline pre-conditioning described above applies to it, so downcycled material is not equivalent to biochar that remains held within an intact matrix.

5. Landfill

Concrete is landfilled when it cannot be effectively used in another application through reuse, closed-loop recycling or downcycling. Landfilled concrete can be assumed to remain within a single storage location for long timescales, decreasing geographical uncertainty. However, additional reversal risks may be introduced (e.g. acidic groundwater) depending on regional environmental characteristics and regulations.

A report published by the National Asphalt Pavement Association in 2024³⁶ found that 99 percent of reclaimed asphalt pavement (RAP) is recycled, with 93 percent reused in new asphalt pavements and the remainder used in other civil engineering projects, such as unbound aggregate bases. A small percentage, reported at 0.2 percent in 2022, is landfilled where it cannot be used in construction, for example if it contains hazardous materials.

A pavement is built in layers. The surface course is the uppermost bound layer and is in direct contact with vehicle tires. Beneath it sit the binder course, the base, and, where it is bituminous-bound, the subbase. These lower layers are together defined in this Module as the Sub-Surface Layer.

Material is removed from the surface course over time by abrasion caused by friction between tire treads and the pavement, generating tire wear particles (TWP) and road pavement wear particles (RPWP). The road abrasion rate depends on driving conditions and traffic volume and has been reported at 0.04-0.5 mm per year¹³. A recent study¹⁵ found that, for asphalt pavement, wear particle size distributions peaked at 63-106 μm, with 84% of the wear particles falling in the size range of 38-212 μm. Particles at that size are readily transported from the road surface by wind and water, so biochar co-abraded from the surface course disperses to the roadside environment and its subsequent location cannot be tracked.

Biochar placed in a Sub-Surface Layer is shielded from direct tire contact. It is liberated principally when the layer itself is milled or reclaimed, at rehabilitation or reconstruction, rather than progressively during service. This Module certifies biochar in Sub-Surface Layers only, as set out in Applicability.

Reversal in asphalt is assessed through the liberation and dispersal of biochar during milling and recycling. This is a different approach from the one used for concrete, and both are set out in Appendix A: Reversal risk

Biochar incorporated into a built material is not contained for the whole of its service life. Constituents may leach from the matrix into runoff, soil, and groundwater; biochar-bearing fines are released when the material is milled, crushed, or demolished; and workers are exposed to dust and fume at each point where the biochar and the biochar-amended material are handled, blended, and placed.

This Module does not set built-environment-specific numeric contaminant thresholds. A single numeric limit cannot distinguish a contained sub-surface placement from a crushed and downcycled one, and would not identify the exposure pathway that actually applies to a given Project. Instead, The Project Proponent must declare contaminant concentrations against the World Biochar Certificate material thresholds (Required measurements) and must separately assess and mitigate the exposure arising from The Project's biochar, placement, and end-of-life pathway.

As outlined in the Biochar Production and Storage Protocol, there are some potential risks to environmental and human health associated with biochar composition, for example:

• Heavy metals, which may impact soil health and may be transported through groundwater flow
• Contamination by Persistent Organic Pollutants

Activities that were already occurring and would continue to occur without The Project may be omitted from the system boundary of the GHG accounting, if evidence that the activity was already occurring and would have continued to occur in the absence of the biochar storage activity can be provided.

Emissions associated with upstream and downstream activities associated with the biochar storage process may be omitted from the system boundary if the biochar-amended product meets the same performance requirements as a conventional product for the intended use case and does not require additional products or activities related to product manufacturing, installation, maintenance and end-of-life, as compared to conventional products. For asphalt, the conventional product is a mixture of the same production type (hot-, warm-, or cold-mix) for the same pavement layer.

Transport of biochar from the production facility to the point of incorporation, and any intermediate storage and handling of biochar before incorporation, are within the Project system boundary and must be accounted for in accordance with the Biochar Production and Storage Protocol. The exclusion described above applies only to activities associated with the manufacture, installation, maintenance, and end-of-life of the biochar-amended product. Transport of the finished product to the end-use site may be excluded only on the basis set out below.

This is based on the assumption that emissions associated with biochar-amended product manufacturing, installation, maintenance and end-of-life would have occurred anyway had a different product been produced and used. The exception to this rule is that additional information is required for exclusion of transport emissions, including transport of biochar-amended built materials to the end-use site (defined as the final location where the biochar-amended product is installed or used). This is to negate the risk of induced transport associated with lower availability of novel biochar-amended products.

Emissions associated with all activities and equipment related to biochar storage in the built environment must be fully accounted for within the Project system boundary. Project Proponents relying on this exclusion must confirm that it applies and provide supporting evidence in the PDD, as set out in Section 8.1.

Table 2: Requirements for physical characterization of biochar

This section details the calculation of net CO₂ storage in biochar-amended built materials. The monitoring requirements are set out in Section 8.0 Monitoring Requirements.

Accurate quantification of net CO₂ stored requires a robust quantification of associated Project emissions within the project boundary. In the context of this Module, a Project refers to the point of CO₂ capture and the point of CO₂ storage. This Module defines the point of CO₂ storage as the point of incorporation of biochar into a built material. This Module applies only to biochar that is durable for 200 years.

This horizon is shorter than the Isometric 1,000-year default and is specific to biochar in built materials. Over 1,000 years the fate of the biochar through demolition, recycling, and end-of-life reprocessing is neither guaranteed nor observable for this pathway, and, for cementitious materials, encapsulation in the high-pH cement pore solution may leave the biochar more vulnerable to loss once the host material is later broken up. The 200-year horizon reflects approximately the service life of the asset plus its first reuse, a period over which fate is documented rather than extrapolated. It is not a finding that the certified carbon degrades after 200 years, nor a claim that built materials are less durable in general.

Projects are required to provide detailed descriptions of Project boundaries for all activities associated with certificate issuance. Project boundaries must include all areas where removal mechanisms and processes may occur, removed carbon storage locations and forms, and potential reversal and loss pathways.

$$CO_2e_{stored,RP} = C_{biochar} \cdot m_{biochar} \cdot F_{durable,material} \cdot \frac{44.01}{12.01}$$

(Equation 2)

Where:

• $CO_2​e_{Removal,RP​}$ is the total CO₂ removed from the atmosphere and stored as organic carbon in the biochar for the Reporting Period, $RP$, in tonnes of CO₂e.
• $C_{biochar}$ is the carbon content of the biochar.
• $m_{biochar}$ is the dry mass of biochar applied.
• $F_{durable,material}$ is the fraction of durable biochar that remains in the built material for the full duration of the certification timeline (200 years), and can be certified under this Module.
• $\frac{44.01}{12.01}$ is the mass fraction of carbon dioxide and elemental carbon.

This Module only certifies the durably stored organic carbon fraction of biochar, which is used to calculate $CO_2​e_{Removal,RP​}$. Thus, $C_{biochar}$ is calculated using the following equation:

$$C_{\text{biochar}} = \text{Total Carbon Content} - C_{\text{inorg}}$$

(Equation 3)

Where:

• $\text{Total Carbon Content}$ is the total carbon content of the biochar as analyzed using the methods described in Table 2.
• $C_{inorg}$ is the inorganic carbon content of the biochar as analyzed using the methods described in Table 2, in %.

Please refer to Section 8.3.1 of the Biochar Production and Storage Protocol for full guidelines on number of samples required for the measurement of biochar carbon content, $C_{biochar}$.

Please refer to Section 8.4 of the Biochar Production and Storage Protocol for full guidelines on measurement of mass of biochar applied, $m_{biochar}$.

$F_{durable}$ is the fraction of the incorporated biochar certified as durably stored over the 200-year horizon. This Module certifies the inertinite (recalcitrant) fraction of the biochar only, quantified by random reflectance and thermogravimetric analysis ($F_{durable}$).

$$F_{durable,material} = (1 - U) \cdot F_{durable}$$

(Equation 4)

Where:

• $F_{durable,material}$ is the certified durable fraction.
• $F_{durable}$ is the durable fraction of the biochar over the 200-year horizon.
• $U$ is the project-specific durability discount. The fraction of the certified durable carbon that is physically liberated and dispersed to the open environment through wear, comminution, and recycling over the 200-year horizon. It is determined by the reversal risk questionnaire in Appendix A.

The durable fraction is quantified by the random-reflectance inertinite method, following Sanei et al. (2024)¹ for the $R_0$inertinite benchmark, refined by Sanei et al. (2025)² tto account for only the recalcitrant fraction, with the reactive fraction discounted via thermogravimetric analysis. The only difference from that Module is the horizon label (200 years for built materials) and the downstream reversal discount $U$ (Equation 4).

As outlined in Section 6.0 of this Module, Project Proponents must report a set of at least 500 measurements of R₀, calculated at the maceral-level, for at least three replicate samples of their biochar, for the percentage of their biochar which passes the 2% R₀ benchmark, as outlined in Sanei et al. (2024)¹ . The histogram of the R₀ values must be submitted at the point of project verification. This method was further updated in Sanei et al., (2025)² to refine the methodology to only account for the recalcitrant fraction of biochar (discounting the reactive fraction, determined by thermogravimetric analysis).

To ensure a conservative approach when issuing certificates against biochar durability, we account for uncertainty in both the fraction of biochar passing the 2% R₀ benchmark and the proportion of carbon that is non-reactive. Specifically, for each replicate sample we determine the fraction of R₀ measurements that meet or exceed the 2% benchmark. The certified inertinite fraction is the mean of these fractions across all samples analyzed, reduced by one standard deviation of the fractions (Equation 5). This is applied to the non-reactive carbon fraction, itself reduced by one standard deviation (Equation 6), to give the durable fraction of the biochar (Equation 7). This ensures the durability estimate reflects a lower-bound confidence level, mitigating the risk of overestimating long-term carbon storage.

As such, $F_{durable}$ is calculated through:

Calculating the sample standard deviation of $R_0$ quantifying the typical deviation of individual $R_0$ measurements from their mean, which is used to account for uncertainty and conservatively adjust the estimated durable fraction of biochar carbon, as:

$$s_{R_0} = \sqrt{\frac{1}{n_{R_0}-1} \sum_{i=1}^{n_{R_0}} (R_{0,i} - \bar{R_0})^2}$$

(Equation 5)

Where:

• $s_{R_0}$ is the standard deviation of the fractions passing the 2% benchmark.
• ${n_{R_0}}$ is the number of samples analyzed (i.e. ≥ 3).
• $R_{0,i}$ is the fraction of R₀ measurements ≥ 2% in the $i$-th sample.
• $\bar{R_0}$ is the mean of the fractions (≥ 2%) across all samples.

Calculating the sample standard deviation of $C_{non-reactive}$ quantifying the typical deviation of individual $C_{non-reactive}$ measurements from their mean, which is used to account for uncertainty and conservatively adjust the estimated durable fraction of biochar carbon, as:

$$s_{C_{non-reactive}} = \sqrt{\frac{1}{n_{C_{non-reactive}}-1} \sum_{i=1}^{n_{C_{non-reactive}}} \left(C_{\text{non-reactive},i} - \bar{C}_{\text{non-reactive}}\right)^2}$$

(Equation 6)

Where:

• $s_{C_{non-reactive}}$ is the standard deviation of $C_{non-reactive}$ measurements
• $n_{C_{non-reactive}}$ is the number of samples analyzed (i.e. ≥ 3).
• ${C_{non-reactive,i}}$ is the individual measurement for the $i$-th sample.
• $\bar{C}_{\text{non-reactive}}$ is the mean of all $C_{non-reactive}$ measurements

Then:

$$F_{durable} = min(0.95 , max( 0 , (\bar{R_0} - s_{R_0}) (\bar{C}_{\text{non-reactive}} − s_{C_{non-reactive}}) ) ) )$$

(Equation 7)

Where:

• $F_{durable}$ is the fraction of durable (inert) carbon in the biochar after 200 years, adjusted conservatively for uncertainty.
• $\bar{R_0}$ is the mean of the fractions (≥ 2%) across all samples.
• $s_{R_0}$ is the standard deviation of the $R_0$ passing the 2% benchmark.
• $\bar{C}_{\text{non-reactive}}$ is the mean of all non-reactive carbon measurements.
• $s_{C_{non-reactive}}$ is the standard deviation of $C_{non-reactive}$.

The maximum and minimum functions are applied to ensure that the fractions are bounded.

$U$ is the project-specific durability discount for physical liberation and dispersal of the certified durable carbon through wear, comminution, and recycling over the 200-year horizon. It does not re-discount the labile fraction (which is not certified), the one-standard-deviation conservatism already contained within $F_{durable}$, or acid and matrix dissolution: acid dissolves the host matrix and can expose biochar, but does not mineralize the inertinite fraction, which is the same basis on which the inertinite fraction is treated as permanent in the Biochar Storage in Low Oxygen Burial Environments Module.

$U$ is determined per Project, and per distinct placement or mix design where a Project spans several, by the reversal risk questionnaire in Appendix A. The questionnaire scores placement and incorporation, physical exposure, and end-of-life and recycling factors, applies gating exclusions, and maps the net score to a durability discount band.

For biochar production, the calculation of $CO_2e_{Counterfactual,RP}$ is determined by the requirements laid out in the Counterfactuals Section of the Biomass Feedstock Accounting Module.

$CO_2e_{Emissions,\ RP}$ is the total greenhouse gas emissions associated with a given Reporting Period, RP.

Equations and emissions calculation requirements for $CO_2e_{Emissions,\ RP}$ including emissions associated with reactor operations and reaction monitoring, are set out in the relevant Protocol and are not included in this Module. Specific considerations for CO₂ stored as biochar in built materials are set out here.

CO₂ removals must not be double counted, regardless of the production of product Environmental Product Declarations. The product must still comply with all relevant emission accounting regulations and requirements, which may mean emissions are double counted; however, removals must not be double counted. This is the most conservative approach to take. Certificate issuance claims must be transparently reported and must not form part of marketing for a separate product.

Allocation procedures may be undertaken to separate emissions associated with the concrete production and the emissions associated with the CO₂ storage process. Allocation procedures must follow the following emissions allocation procedure:

• Procedure 1: Allocate all emissions to CDR. Projects may opt to allocate all emissions to CDR.
• Procedure 2: Divide the process into sub-processes. Where possible, the process may be divided into sub-processes. For example it may be possible to isolate processes relating to processing and storing CO₂ only. Only sub-processes and relevant inputs that are physically separable may undergo division, for example electricity usage for equipment with separate electricity meters. Sub-processes that are physically separable and do not contribute to CDR may be excluded from the CDR system boundary.

Transport emissions must be considered if the average transport distance for biochar-amended built materials is more than the average transport distance of normal products produced by the production facility. All other downstream emissions may be excluded from the system boundary if these activities were already occurring and would continue to occur in the absence of The Project. This can be evidenced by providing documentation that the biochar-amended product meets the same performance requirements of a conventional product for the intended use case, as described under the Section 2.0 Applicability.

Projects using this Storage Module are typically deemed to have No Observable Risk of reversal, according to the Isometric Standard Risk Assessment Questionnaire (also found in the relevant Protocol), as all reversal risk is accounted for using the reversal risk modelling and Uncertainty Discount approach. This results in a 0% buffer pool for Projects using this Storage Module. This reversal risk will be reassessed at the renewal of the Crediting Period, or when new scientific research and knowledge are produced.

The requirements in this section apply to the process steps and equipment within the Project system boundary. Where the exclusion of downstream manufacturing emissions set out in the System Boundaries section is relied upon, the boundary terminates at the point at which biochar or a biochar-containing material transfers to the downstream manufacturer, and process data for the manufacturer's own operations is not required. Requirements relating to the mass of biochar incorporated, the mix into which it was incorporated, and the pavement layer in which it was laid apply irrespective of that exclusion.

Process specific details, including energy consumption, curing steps and processing temperatures, are required only where the exclusion of downstream manufacturing emissions set out in the System Boundaries section is not relied upon. Where the biochar-amended product meets the same performance requirements as a conventional product for the intended use case and requires no additional products or activities related to product manufacturing, installation, maintenance and end-of-life, those emissions are omitted from the system boundary and the underlying process data is not required.

Production batch records must identify the issuing facility, the batching system from which they are extracted and the date of extraction, must cover the full Reporting Period, and must reconcile to the Project Proponent's records of biochar dispatched to that facility. For asphalt, the documentation provided must identify the mix designation (e.g., EN 13108 or Superpave) for each batch, so that the pavement layer can be established as required under the Asphalt Layer Eligibility section.

Where biochar is added to an asphalt mixture as a separate component rather than through the binder, the Project Proponent must state the point at which it is added in the plant (for example, directly to a batch-plant mixer, or through an additive or filler feed on a drum plant).

Where production batch records are not available, mix-level evidence may be provided instead to establish placement: for asphalt, a job mix formula that identifies the biochar and its dosage and restricts it to a mix designated for a Sub-Surface Layer; for concrete, an element and mix specification that ties the biochar-amended mix to its placement. The mass of biochar incorporated must still be established by one of the forms listed above. Placement evidenced in this way is scored under the mix-level response to question A3 of the reversal risk questionnaire (Appendix A).

The process flow diagram must be prepared to ISO 10628 or an equivalent convention, and must show:

• Each processing step as a discrete unit operation.
• The material inflows and outflows of each step, including waste and loss streams.
• The energy inputs to each step and their source.
• The measurement point and instrument for each metered stream (e.g., flow meters, temperature sensors, weighbridges).

Project Proponents must also provide an equipment list stating, for each item of equipment within the boundary, its function, make and model or asset identifier, and rated throughput capacity.

Process steps should include acceptable processes including, but not limited to:

• Particle size reduction grinding or milling.
• Mixing, stirring or blending.
• Calcination.
• Drying and heating.

An applicability criterion for certification with this Module is that the biochar-amended built materials must meet the same performance requirements as conventional products for the intended use case.

Evidence and data, resulting from research studies of biochar addition in built materials such as asphalt, cement or other building elements, have shown the potential for additional benefits and improvements on the final product quality⁹,⁴¹,⁴². Biochar addition can impact a range of different properties (mechanical, chemical and physical properties) including but not limited to the following:

• Flexural strength (N/mm²)
• Sound absorption (Hz) (applicable for concrete and other building elements like insulation or plaster)
• Compressive strength (N/mm²)
• Tensile strength (N/mm²)
• Density (kg/m³)
• Electrical resistance (Ω)
• Sorption capacity
• Fire resistance (fire retardant or fire resistance)

Project Proponents should include any additional product characterization, analysis and reported properties of the final produced materials. Changes or deviations from the baseline product values should also be included and relevant product standards referenced in the PDD Appendix.

Departures from standard may be permissible on a case-by-case basis. Project Proponents seeking a departure from standard must submit documentation on the requested departure and proof of approval by the relevant regulatory body to Isometric and the VVB.

This Module certifies biochar in built materials over a 200-year durability horizon, rather than the Isometric 1,000-year default. The grounds are set out in full in Section 1.0 and summarized here. They are specific to biochar rather than general to built materials. Containment over 1,000 years is neither guaranteed nor observable for this pathway, because the fate of the material through demolition, recycling, and end-of-life reprocessing across that span is extrapolated rather than documented. For cementitious materials, a further consideration applies. Biochar is exposed to the pore solution of a cementitious matrix during encapsulation, typically at pH 12.5 to 13.5³, which alters its structure, including pore enlargement and matrix defectiveness⁴. While the material remains intact this is not itself a carbon-loss pathway, because there is no oxygen, no export route, and the condensed inertinite core is chemically resistant¹. Altered and aged carbon is, however, more susceptible to oxidation⁵, so the exposure may leave the biochar more vulnerable to degradation once the material is broken up and the carbon is exposed to the open environment. This consideration does not apply to asphalt, in which the binder is a hydrophobic bituminous matrix rather than an alkaline aqueous environment; for asphalt, the horizon rests solely on the extent to which the fate of the material can be documented. The 200-year horizon corresponds approximately to the design life of a built asset, indicatively 50 years for common buildings and 100 years for monumental structures and bridges under EN 1990:2023, plus first reuse.
A second consideration is the uncertainty in the location of the material at any given time. As with the CO₂ Storage via Carbonation in the Built Environment Module, a built material may pass through many use cases across its life, depending on the initial use case, local regulatory requirements, and economic incentives. In the traditional construction industry there has been no need to track built materials past the life of an asset, particularly where elements are designed to perform with minimal maintenance or intervention. Biochar-amended built materials are therefore likely to transition through several construction-demolition-reuse cycles with minimal ability to track these changes. Over the 200-year horizon this leads to uncertainty in both the storage location of the material and, consequently, whether it is exposed to conditions that lead to loss of the certified carbon. This treats built-environment storage as an open system and necessitates the conservative, project-specific approach to reversal set out below.

This Module certifies inertinite (recalcitrant) fraction of the biochar only (Equation 7). That fraction is durable over geological timescales once formed¹ and is refined to the thermally non-reactive residual by thermogravimetric analysis². It is chemically resistant while it remains encapsulated in the host matrix, and while it remains in contained chunks after the matrix is broken up. The reversal concern is therefore not chemical mineralization of encapsulated carbon, but physical liberation of the certified inertinite to mobile fines that are dispersed into the open environment. Once dispersed, the location and fate of the fines cannot be verified, and they are exposed to pathways that can return the carbon to the atmosphere, including oxidation of fine, high-surface-area and aged material⁵ and thermal destruction where sewage sludge, road runoff solids, or sweepings are incinerated. Consistent with the conservative approach this Module takes to a novel storage pathway, carbon is certified only where its retention can be quantified and verified, and certified carbon liberated to mobile fines is therefore treated as not retained. This treatment is adopted because the retention of dispersed fines cannot be demonstrated; it is not a finding that dispersed inertinite degrades in every receiving environment. Isometric will revisit it if evidence on the fate of dispersed biochar fines becomes available. Liberation happens through wear, comminution, and recycling. The pathways that act on liberated, dispersed material include:

• Physical loss: abrasion or erosion of the material surface, delamination or spalling, and comminution during demolition, crushing, and recycling ⁴³.
• Abiotic degradation: chemical reactions with environmental factors, including natural mineralization, photodegradation, and leaching by water flows and freeze-thaw cycles¹⁷.
• Microbial degradation: through the action of aerobic or anaerobic microorganisms and lichen colonization^44,45.

Two related mechanisms are handled outside this dispersal term. Acid and matrix dissolution can expose biochar as the host matrix is lost, but does not mineralize the inertinite fraction, so it is treated as an exposure driver in the questionnaire (Section B), not a carbon-loss term. Thermal exposure during hot asphalt recycling (approximately 150 to 190°C) is highly unlikey to reverse the certified fraction, because that fraction is by construction the thermally non-reactive residual determined by thermogravimetric analysis (Equation 7), which is stable well above that range: road-grade biochar loses only about 1% of its mass to 150°C and about 5% to 400°C⁴⁶, and oxidation onset rises with pyrolysis temperature, well above the hot-mix range⁴⁷. High-temperature reprocessing of concrete to clinker (approximately 1,400°C, the calcination temperature described in the Concrete background) destroys even the inertinite, and is handled as a gating exclusion (below).

Liberation of biochar from concrete occurs when the material is lost or broken up, either during use or at end of life. During use, the relevant drivers are surface loss by physical or chemical weathering, including exposure to acid rain and contact with acidic groundwater or soils (most relevant for foundation uses). The end-of-life pathways and their reversal treatment are set out in the Section 3.2.3.2.1 End-State Use Cases (Concrete) background and summarized here:

• In-situ and ex-situ reuse: the material remains in its original form, so no additional carbon is liberated.
• Downcycling (the dominant pathway in most regions): crushing liberates a fraction of the biochar to fines, but the majority remains bound in coarse, re-encapsulated aggregate. Downcycling is treated as a terminal state, though the alkaline pre-conditioning⁴ applies to the exposed material.
• Mechanical closed-loop recycling: the biochar is liberated and re-encapsulated in new concrete, and is not exposed to conditions that destroy the stable fraction.
• G-B0T4-0Thermal closed-loop recycling to clinker: reprocessing at calcination temperatures destroys the certified inertinite. This pathway is a gating exclusion (G3): a Project with a credible, unconstrained likelihood of it is ineligible unless a binding, evidenced restriction against it is in place.
• Landfill: the material remains in a single location, reducing geographic uncertainty, though acidic leachate exposure may apply depending on region.

Only biochar in a Sub-Surface Layer is eligible for certification under this Module; biochar in the surface course, and in bituminous surface treatments, is an ineligible practice, as set out in the Section 2.0. Surface-course material abrades directly under tire contact, generating tire and road pavement wear particles at a size (peaking at 63 to 106 μm)¹⁵ that is readily transported from the road by wind and water. The abraded fraction cannot be quantified or verified for an individual pavement, and its subsequent location and fate cannot be determined (see Asphalt Layer Eligibility). For eligible Sub-Surface placements, the reversal concern is the biochar comminuted to mobile fines during milling and recycling over the horizon, which is quantified through the questionnaire and the Uncertainty Discount ($U$) derivation below. Exposure to chlorides (road salt) and sulfates (surface water) may increase the reactivity of dispersed material.

Reversal is assessed for each Project using a questionnaire. The net score maps to a banded durability discount, ($U$). Each band's value of $U$ is derived from a comminution and dispersal model of how the certified inertinite is physically liberated over the horizon, set out under Derivation of the Durability Discount below.

Reversal risk is assessed on a project-by-project basis. The Project Proponent must complete the questionnaire for each Project, for each distinct placement or mix design where a Project spans several, and for each new storage site added to the Project after validation (see Calculation of Uncertainty Discount, $U$). Because built-environment storage is largely unobservable over the horizon, the output is a durability uncertainty discount on the certified durable fraction ($U$ applied in Equation 4), instead of a buffer-pool contribution. This assessment is conservative by default: any variable not characterized with verifiable evidence is scored at its worst option, and gating answers override the total and can result in ineligibility of the Project.

Note that Projects may choose to provide documentations specific to the region of operations to calculate a Project-specific reversal risk. This documentation may include, but is not limited to:

• Geologic maps of bedrock lithology (from USGS or national equivalent) and groundwater pH.
• Climatic data, including average yearly temperature, precipitation volume and precipitation pH.
• Estimated transport radius of produced materials.
  • If this is unknown for a given Project, Project Proponents may use either the maximum possible transport distance or a conservative average distance.
• Types of assets that materials may be used in and a breakdown by percentage.
• Proof of material suitability for the range of applications (e.g. certificates of compliance with concrete quality and performance standards).
• Risk of external acid attack, based on:
  • Regional groundwater and precipitation pH.
  • Likely exposure risk of the concrete containing stored carbon.
    • This can be determined through analysis of possible asset types and well-justified assumptions about the location of the concrete within the asset.
• Chain-of-custody evidence for placement, or, where unavailable, a probability analysis of the concrete or asphalt end-state based on regional trends.

Table A1: Gating Criteria.

Table A2: Section A (placement and incorporation).

Table A3: Section B (physical exposure over the horizon).

Table A4: Section C (end-of-life and recycling).

For asphalt, C1 and C2 are answered for the biochar-bearing layer. C1 is the expected time until that layer is first milled, removed, or reprocessed. C2 is the number of such events within the horizon, and must be consistent with $N$ as determined under Derivation of the Durability Discount U. Where a layer-specific interval is not evidenced, C1 and C2 are answered on the resurfacing interval.

For C3, engineered landfill means disposal at a landfill operated under a permit issued under the applicable waste regulation for the class of waste concerned (for example, Council Directive 1999/31/EC on the landfill of waste, or equivalent national regulation), with the basal barrier, leachate controls, and capping required for that landfill class, and with records of the waste accepted. Disposal at unpermitted sites or open dumps, or use as unregulated fill, is not engineered landfill and is scored as Unknown. To score C3 as contained on the basis of landfill, the Project Proponent must evidence, from national or regional waste statistics, that landfill at such facilities is the dominant end-of-life pathway for the material in the region, and must identify the regulatory regime under which those facilities operate.

Table A5: Section D (containment safeguards and mitigants). Subtract from the Section A to C total.

Completion of the questionnaire for each new storage site added to the Project, and its re-completion at each Crediting Period renewal or on material change, are required of all Projects (Calculation of Uncertainty Discount, $U$) and do not score under Section D. Where D3 is claimed, the register entry and the asset owner's agreement must be provided in the PDD, works notified under the agreement must be reported at the next Verification, and notified works trigger re-completion of the questionnaire.

Scoring. Sum Sections A to C, then subtract Section D to give the net score. Map the net score to the risk rating and durability discount $U$ in the banded discount schedule below (Derivation of the Durability Discount U). A Project that does not complete the questionnaire, or that leaves variables uncharacterized, receives the worst-case discount.

Responses to Section C are forecasts over the 200-year horizon, based on the evidence available at the time of assessment, and are not commitments by the Project Proponent. A response other than the worst option must be supported by documented evidence, such as the design working life of the asset under the applicable design standard, the maintenance, rehabilitation, and reconstruction policy of the asset owner or road authority, and national or regional data on end-of-life and recycling practice, and the highest-risk response consistent with that evidence must be selected. The questionnaire is completed for each new storage site added to the Project and re-completed at each Crediting Period renewal or on material change, and a revised discount applies to Certificates issued after the re-assessment.

$U$ is the project-specific durability discount as specified in Equation 4, Section 7. The term also represents the fraction of the certified inertinite that may be physically liberated to mobile fines and dispersed into oxic surface environments over the 200-year horizon. The calculation for $U$does not re-discount the labile fraction, which is not certified under this Module; the one-standard-deviation conservatism is already accounted for in $F_{durable}$; or acid and matrix dissolution, which exposes but does not mineralize inertinite¹. Thermal destruction (clinker reprocessing) and surface abrasion are handled as gating exclusions, so they are also excluded from the graded discounts here.

$U$ is derived from the comminuted-inertinite retention model as shown by the equation below:

$$U = 1 - (1 - f_{disp})^{N}$$

(Equation 8)

Where:

• $f_{disp}$ is the fraction of biochar comminuted to mobile fines (sub-0.075 mm) per comminution or recycling round.
• $N$ is the number of comminution rounds over the 200-year horizon for the placement band.

$N$ follows from the service life of the element or, for asphalt, of the biochar-bearing layer. The basis is the design standards and documented maintenance and replacement-interval data. Design working lives under EN 1990:2023 are 50 years for common buildings, 100 years for monumental structures and bridges, and 25 years for agricultural buildings; foundations are frequently left in situ.

For asphalt, $N$ is the number of events within the 200-year horizon at which the biochar-bearing layer is milled, removed, or reprocessed, because the biochar is comminuted only when its own layer is disturbed. A resurfacing event counts for a layer where the milling or removal depth reaches any part of that layer. Rehabilitation, reconstruction, full-depth reclamation, and in-place recycling count where they reach that layer.

By default, every resurfacing event is treated as disturbing the biochar-bearing layer. $N$ is then set by the resurfacing interval, roughly 15 to 25 years¹⁴, rather than the 40-year pavement design-life label in Table 1. A longer interval may be used only where the Project Proponent evidences, for the application type and region:

• the depth of routine resurfacing relative to the depth of the biochar-bearing layer; and
• the interval at which that layer is milled, removed, or reconstructed.

This must be set out as required under Storage Site. The basis may be the applicable pavement design standard or the published maintenance policy of the road authority or asset owner, together with the layer thicknesses in the pavement design. Where the evidence supports a range of intervals, the shortest interval is used.

Table A6: Comminution rounds N over the 200-year horizon, by placement band.

The Reclaimed Asphalt Pavement (RAP) multiple-recycling literature shows the binder degrades cycle-on-cycle⁴⁸, so later asphalt cycles are downcycling rather than re-laying, but this does not reduce $N$, because the biochar is comminuted at each event that mills or reclaims the biochar-bearing layer, whether the material is re-laid or downcycled.

Recycling of biochar-bearing reclaimed asphalt into a new bound mixture, whether by hot, warm, or cold recycling, is not treated as a reversal of the biochar it contains. The durability discount applies only to the fraction comminuted to mobile fines at each milling or processing round ($f_{disp}$); the remainder is treated as retained, whether it is re-bound in a new pavement or used in another construction application.

$f_{disp}$, no study measures biochar carbon retention through actual asphalt or concrete recycling and this represents a gap in the current literature. As such, $f_{disp}$ is inferred from the fines (sub-0.075 mm) generated per processing round in the aggregate-recycling literature:

• Asphalt (RAP milling and crushing): roughly 5 to 10% passing the 200 (0.075 mm) sieve per round⁴⁹, so $f_{disp}$ approximately 0.05 to 0.10.
• Concrete, coarse crush: the sub-0.075 mm fraction is usually minimal, and the biochar stays bound in coarse re-encapsulated aggregate⁵⁰, so $f_{disp}$ 0.02 to 0.05. This is the physical basis for treating concrete inertinite retention as near-complete for thick elements.
• Concrete, fine crush (thin elements, repeated crushing): fine recycled aggregate carries roughly 4.86 times the sub-0.075 mm crush dust of natural sand⁵¹, and repeated crushing produces progressively finer distributions⁵², so $f_{disp}$ approximately 0.05 to 0.10.

The high end of each range is taken because biochar is more friable than mineral aggregate and the dispersed fraction is untraceable.

These values are a conservative proxy for the liberation of biochar to mobile fines, pending direct measurement. Isometric will recalibrate $f_{disp}$ as direct measurements of biochar carbon retention through asphalt and concrete milling, crushing, and recycling become available.

Applying Equation 8 across the sourced parameter ranges of $f_{disp}$ gives: concrete structural ($f_{disp}$ 0.03 to 0.05, N 1 to 2) roughly 0 to 10%; concrete above-ground (0.04 to 0.05, N 3.5 to 4) roughly 13 to 19%; asphalt base (0.05 to 0.075, N 7) roughly 30 to 42%; concrete thin (0.06 to 0.08, N 9 to 10) roughly 43 to 57%; and asphalt binder (0.075 to 0.10, N 10 to 13) roughly 63 to 65%. These model outputs cluster into four risk bands shown in Table A7. The Module adopts one durability discount $U$ per band; the Table below maps each net score and risk rating to that discount and to the representative placement and parameter basis behind it.

Table A7: Durability discount U by risk band.

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