Lesson 04: End-of-Life (Modules C1–C4), Circularity, and Beyond the Lifecycle (Module D)

Learning objectives

By the end of this lesson, you will be able to:

• Quantify emissions across all End-of-Life modules (C1–C4) per BS EN 15978 and RICS standards.
• Define the End-of-Waste boundary that separates Module C3 waste processing from Module D circularity credits.
• Apply the Net Scrap Formula under BS EN 15804+A2:2019 to prevent double counting of recycled content between Modules A1–A3 and Module D.
• Balance biogenic carbon accounting from cradle to grave under the static $-1/+1$ international standard rule.
• Formulate circular economy design strategies (such as Design for Deconstruction) that convert landfill burdens into secondary material credits.

Key concepts

1. Stage C: The End-of-Life Phase (Modules C1–C4)

When a building reaches the conclusion of its functional life, the deconstruction, sorting, and disposal activities are categorized within Stage C [S01, S04]:

• Module C1 (Deconstruction & Demolition): On-site plant fuel (diesel excavators with hydraulic shears, concrete pulverizers, mobile cranes) consumed during tearing down the building structure and separating finishes.
• Module C2 (Transport to Waste Processing / Disposal): Freight logistics transporting demolished rubble, scrap steel, and mixed waste from the site to local transfer stations, recycling depots, or landfills.
• Module C3 (Waste Processing for Reuse, Recovery & Recycling): Mechanical processing required to prepare secondary materials for a subsequent product system (e.g., crushing concrete into aggregate, shredding timber for biomass, baling scrap steel).
• Module C4 (Final Disposal): Permanent deposition of unrecovered waste into landfills or incineration without energy recovery.

Empirical Benchmark Values for Stage C

In typical building life cycle assessments, Modules C1–C4 contribute approximately $15 \text{ to } 40\text{ kgCO}_2\text{e}/\text{m}^2 \text{ GIA}$ (typically 3% to 7% of total cradle-to-grave emissions) [S04, S05]:

• C1 (Demolition): $5 \text{ to } 12\text{ kgCO}_2\text{e}/\text{m}^2$ (based on diesel intensity of $0.035 \text{ to } 0.050\text{ liters/t}$ of demolished mass).
• C2 (Transport): $3 \text{ to } 8\text{ kgCO}_2\text{e}/\text{m}^2$ (assuming default distance of 50 km to transfer station).
• C3/C4 (Processing & Landfill): $10 \text{ to } 25\text{ kgCO}_2\text{e}/\text{m}^2$.

2. Module D: Benefits and Loads Beyond the System Boundary

Module D quantifies the net environmental benefits or loads resulting from materials, products, or energy that exit the building system boundary and displace virgin production in another subsequent product system [S01, S02, S03]:

• Recycled scrap: Displacing virgin iron ore, virgin bauxite, or virgin glass sand.
• Reused elements: Displacing newly fabricated structural beams or bricks.
• Energy recovery: Incinerating waste timber with heat and power recovery, displacing fossil electricity or gas heating on the municipal grid.

The “End-of-Waste” State

Under EN 15804+A2 [S02], activities up to the point where a discarded material reaches the End-of-Waste state belong inside the building’s own inventory (Module C3). Once the material has been cleaned, crushed, or sorted so that it can directly substitute for a primary raw material in an industrial process, all further transformations and the resulting displacement credits belong in Module D.

Demolition (C1) -> Haulage (C2) -> Sorting & Crushing (C3) | [END OF WASTE] | -> Substituted Virgin Production (Module D)

3. The Net Scrap Method: Preventing Double Counting

A major pitfall in carbon accounting is double counting recycled content [S04, S05]. If a structural steel beam already benefits from high recycled content during its upfront manufacturing stage (Module A1–A3), the project cannot claim credit for sending all of that steel to a recycler at end of life without adjusting for the scrap consumed at the outset!

Under BS EN 15804+A2 [S02] and RICS 2023 [S04], Module D must be calculated using the Net Scrap formula:

Where:

• $M_{\text{recovered at EoL}} = M_{\text{installed}} \times R_{\text{recovery}}$ (where $R_{\text{recovery}}$ is the end-of-life recovery rate, e.g., 95% for structural steel).
• $M_{\text{recycled scrap input}} = M_{\text{purchased}} \times R_{\text{recycled input}}$ (the scrap percentage in the original product).
• $\text{EF}_{\text{virgin primary}}$ = Carbon factor for manufacturing 100% virgin material (e.g., BF-BOF primary steel: $\approx 2.30\text{ kgCO}_2\text{e}/\text{kg}$) [S05, S07].
• $\text{EF}_{\text{scrap recycling}}$ = Carbon factor for remelting scrap (e.g., EAF secondary steel: $\approx 0.45\text{ kgCO}_2\text{e}/\text{kg}$) [S05, S07].

Interpreting the Net Scrap Result

• If a building is a Net Producer of Scrap ($M_{\text{recovered}} > M_{\text{scrap input}}$), Module D yields a negative carbon value (a net climate benefit/credit).
• If a building is a Net Consumer of Scrap ($M_{\text{scrap input}} > M_{\text{recovered}}$), Module D yields a positive carbon value (a net burden).
• If a building uses $100\%$ virgin steel and recycles $95\%$ of it at end of life, its A1–A3 carbon is very high, but its Module D credit is very large.
• If a building uses $95\%$ recycled steel and recycles $95\%$ at end of life, its $\text{Net Scrap} \approx 0$, so its Module D is near zero—but its A1–A3 carbon was already exceptionally low!

4. Biogenic Carbon Dynamics: The Static $-1/+1$ Rule

Timber and bio-based materials sequester atmospheric carbon dioxide during tree growth ($1\text{ kg}$ of dry wood stores approximately $1.83\text{ kgCO}_2$) [S02, S05].

Under BS EN 15804+A2 [S02] and ISO 21930 [S03], lifecycle standards enforce a closed-loop static mass balance:

1. In Module A1–A3, biogenic carbon uptake is recorded as a credit: $$\text{GWP-biogenic}_{A1-A3} = -1.83\text{ kgCO}_2/\text{kg dry wood}$$
2. In Modules C3/C4, when the timber building is demolished and the wood is either incinerated or rots in landfill, that stored carbon is released back into the atmosphere: $$\text{GWP-biogenic}_{C3-C4} = +1.83\text{ kgCO}_2/\text{kg dry wood}$$
3. Over the full cradle-to-grave cycle ($A1–C4$), the net biogenic carbon balance equals: $$-1.83 + 1.83 = 0.00\text{ kgCO}_2\text{e}$$

[!IMPORTANT] Permanence & Energy Recovery: If timber is incinerated with energy recovery at end of life, the $+1.83\text{ kgCO}_2$ is booked into Module C3/C4, while the avoided fossil fuel emissions from the generated district heat or power are credited in Module D [S02, S04].

Worked example

Problem Statement

A structural engineer is evaluating the end-of-life performance and Module D circularity credit for $150\text{ tonnes}$ of structural steelwork installed in a 5-story commercial frame.

Project Data:

• Installed structural steel mass ($M_{\text{installed}}$): $150\text{ tonnes}$ ($150{,}000\text{ kg}$).
• Wastage rate in A5 ($W$): $2\%$ ($M_{\text{purchased}} = 153\text{ tonnes}$).
• Recycled scrap content in original product ($R_{\text{recycled input}}$): $35\%$ ($0.35$).
• Demolition & sorting diesel consumption (C1): $0.040\text{ liters diesel/tonne}$ of steel.
  • Diesel emission factor: $3.18\text{ kgCO}_2\text{e}/\text{liter}$ [S04].
• Transport to scrap processing depot (C2): $40\text{ km}$ via HGV ($\text{EF}_{\text{trans}} = 0.088\text{ kgCO}_2\text{e}/(\text{t}\cdot\text{km})$) [S04].
• End-of-Life steel recovery rate ($R_{\text{recovery}}$): $96\%$ ($0.96$ is recovered for recycling; $4\%$ is lost to landfill) [S05].
• Scrap shredding and sorting processing (C3): $12.0\text{ kgCO}_2\text{e}/\text{tonne}$ [S04].
• Landfill disposal of unrecovered metal (C4): $8.0\text{ kgCO}_2\text{e}/\text{tonne}$ [S04].
• Material replacement carbon factors for Module D:
  • Virgin steel production ($\text{EF}_{\text{virgin}}$): $2{,}300\text{ kgCO}_2\text{e}/\text{tonne}$ [S05, S07].
  • Scrap remelting production ($\text{EF}_{\text{remelt}}$): $450\text{ kgCO}_2\text{e}/\text{tonne}$ [S05, S07].

Calculate:

1. Total End-of-Life emissions across Modules C1, C2, C3, and C4.
2. The Net Scrap tonnage exiting the system boundary.
3. The Module D circularity credit.
4. Explain how the results are reported alongside the building’s cradle-to-grave inventory.

Step-by-Step Solution

Step 1: Module C1 (Deconstruction / Demolition)

• Total diesel consumed: $$\text{Diesel} = 150\text{ tonnes} \times 0.040\text{ liters/tonne} = 6.0\text{ liters}$$
• Carbon emissions: $$\text{EC}_{C1} = 6.0\text{ liters} \times 3.18\text{ kgCO}_2\text{e}/\text{liter} = 19.08\text{ kgCO}_2\text{e} \approx 0.02\text{ tCO}_2\text{e}$$

Step 2: Module C2 (Transport to Scrap Depot)

• Carbon emissions: $$\text{EC}_{C2} = 150\text{ tonnes} \times 40\text{ km} \times 0.088\text{ kgCO}_2\text{e}/(\text{t}\cdot\text{km}) = 528.0\text{ kgCO}_2\text{e} \approx 0.53\text{ tCO}_2\text{e}$$

Step 3: Modules C3 and C4 (Processing & Disposal)

• Recovered Steel Mass for Recycling (C3):

  $$M_{\text{recovered}} = 150\text{ tonnes} \times 0.96 = 144.0\text{ tonnes}$$
  $$\text{EC}_{C3} = 144.0\text{ tonnes} \times 12.0\text{ kgCO}_2\text{e}/\text{tonne} = 1{,}728.0\text{ kgCO}_2\text{e} \approx 1.73\text{ tCO}_2\text{e}$$

• Unrecovered Steel Mass to Landfill (C4):

  $$M_{\text{landfill}} = 150\text{ tonnes} \times 0.04 = 6.0\text{ tonnes}$$
  $$\text{EC}_{C4} = 6.0\text{ tonnes} \times 8.0\text{ kgCO}_2\text{e}/\text{tonne} = 48.0\text{ kgCO}_2\text{e} \approx 0.05\text{ tCO}_2\text{e}$$

• Total Stage C Emissions (C1–C4):

  $$\text{EC}_{C1-C4} = 19.08 + 528.0 + 1{,}728.0 + 48.0 = 2{,}323.08\text{ kgCO}_2\text{e} \approx 2.32\text{ tCO}_2\text{e}$$

Step 4: Module D Net Scrap Calculation

• Scrap Input at Production (A1–A3):

  $$M_{\text{scrap input}} = M_{\text{purchased}} \times R_{\text{recycled input}} = 153.0\text{ tonnes} \times 0.35 = 53.55\text{ tonnes}$$

• Recovered Scrap Output at EoL:

  $$M_{\text{recovered}} = 144.0\text{ tonnes}$$

• Net Scrap Output:

  $$\text{Net Scrap} = M_{\text{recovered}} - M_{\text{scrap input}} = 144.0 - 53.55 = 90.45\text{ tonnes of net scrap}$$

• Unit Avoided Burden Factor:

  $$\Delta \text{EF} = \text{EF}_{\text{virgin}} - \text{EF}_{\text{remelt}} = 2{,}300 - 450 = 1{,}850\text{ kgCO}_2\text{e}/\text{tonne}$$

• Total Module D Credit:

  $$\text{Module D} = -\left( \text{Net Scrap} \times \Delta \text{EF} \right)$$
  $$\text{Module D} = -(90.45\text{ tonnes} \times 1{,}850\text{ kgCO}_2\text{e}/\text{tonne}) = -167{,}332.5\text{ kgCO}_2\text{e} \approx -167.33\text{ tCO}_2\text{e}$$

Step 5: Reporting Compliance

The project reports:

• Stage C End-of-Life Total (C1–C4): $+2.32\text{ tCO}_2\text{e}$ (included in the cradle-to-grave sum).
• Module D Circularity Potential: $-167.33\text{ tCO}_2\text{e}$ (reported in a separate column).
• Under EN 15978 [S01] and EN 15804+A2 [S02], the $-167.33\text{ tCO}_2\text{e}$ is never subtracted from the building’s A1–C4 total, ensuring transparent reporting of actual lifecycle emissions while demonstrating circular potential.

Visual explanation

The diagram below tracks materials passing through demolition and the End-of-Waste boundary into the circular secondary materials market.

Reading the Diagram

1. Operations inside Stage C represent activities funded and managed as part of the building’s decommissioning.
2. The End-of-Waste state is the regulatory and physical threshold where processed material becomes commercially tradable secondary raw material.
3. Operations inside Module D calculate the avoided carbon burden conferred upon future industries, strictly kept outside the building’s cradle-to-grave inventory.

Practice

Exercise 1: Net Scrap with High Recycled Input

Consider the same $150\text{ tonnes}$ of structural steelwork from the worked example, but this time fabricated via an Electric Arc Furnace with a very high recycled scrap content of $95\%$ ($R_{\text{recycled input}} = 0.95$).

• Installed mass: $150\text{ tonnes}$; Purchased mass ($2\%$ waste): $153\text{ tonnes}$.
• EoL Recovery rate: $96\%$ ($144\text{ tonnes}$ recovered).
• $\Delta \text{EF} = 1{,}850\text{ kgCO}_2\text{e}/\text{tonne}$.

Calculate:

1. The mass of scrap input consumed during upfront manufacturing (A1–A3).
2. The Net Scrap tonnage at end of life.
3. The Module D credit.
4. Explain why Module D is drastically smaller here than in the worked example, and why this is fair.

Exercise 2: Biogenic Carbon End-of-Life Pathways

A cross-laminated timber (CLT) structure contains $400\text{ tonnes}$ of dry timber.

• In Module A1–A3, biogenic uptake was booked as $-1.83\text{ kgCO}_2/\text{kg} = -732{,}000\text{ kgCO}_2$ ($-732\text{ tCO}_2$).
• At Year 60, two end-of-life disposal scenarios are evaluated:
  • Scenario A (Incineration with Energy Recovery): $100\%$ of the timber is burned in an energy recovery plant, releasing $+732\text{ tCO}_2$ into Module C3, but displacing $450\text{ tCO}_2$ of fossil gas grid heating in Module D.
  • Scenario B (Landfill): $100\%$ of the timber is placed in an anaerobic landfill, where $50\%$ permanently remains stored as lignin, while $50\%$ decomposes into landfill gas ($50\% \text{ CO}_2$ and $50\% \text{ CH}_4$). Methane has a GWP of 28.

Evaluate:

1. The net cradle-to-grave biogenic carbon balance ($A1–C4$) for Scenario A.
2. The net Whole Life Carbon (including Module D) for Scenario A.
3. Explain why Scenario B (landfill methane) can be disastrous for Whole Life Carbon despite “permanent storage” of part of the wood.

Answers and hints

Exercise 1 Answers

1. Scrap input in A1–A3: $$M_{\text{scrap input}} = 153\text{ tonnes} \times 0.95 = 145.35\text{ tonnes}$$
2. Net Scrap: $$\text{Net Scrap} = M_{\text{recovered}} - M_{\text{scrap input}} = 144.0 - 145.35 = -1.35\text{ tonnes}$$
3. Module D: $$\text{Module D} = -(-1.35\text{ tonnes} \times 1{,}850\text{ kgCO}_2\text{e}/\text{t}) = +2{,}497.5\text{ kgCO}_2\text{e} \approx +2.50\text{ tCO}_2\text{e}$$
4. Explanation: Module D is near zero (in fact, slightly positive, representing a negligible net burden of $+2.5\text{ tCO}_2\text{e}$) because the building consumed as much scrap at the start ($145.35\text{ t}$) as it returned at the end ($144.0\text{ t}$). This is equitable because the project already captured the massive carbon benefit upfront in Module A1–A3 through an exceptionally low initial carbon footprint ($0.45\text{ kgCO}_2\text{e}/\text{kg}$ instead of $2.30$)!

Exercise 2 Answers

1. Scenario A ($A1–C4$): $$\text{Net Biogenic } (A1–C4) = -732\text{ tCO}_2 \text{ (A1–A3)} + 732\text{ tCO}_2 \text{ (C3)} = 0.00\text{ tCO}_2$$
2. Scenario A including Module D: $$\text{Net WLC} = 0.00\text{ tCO}_2 + (-450\text{ tCO}_2 \text{ in Module D}) = -450\text{ tCO}_2$$
3. Scenario B (Landfill): Even though half the carbon stays stored, the methane ($\text{CH}_4$) released from the decomposing half has a global warming potential 28 times greater than $\text{CO}_2$! The release of fugitive methane can generate far higher warming than the initial biogenic sequestration, turning the timber into a substantial net climate driver unless landfill gas capture rates exceed 90% [S02, S04].

Next steps

In this lesson, you mastered the End-of-Life modules (C1–C4), the Net Scrap circularity formula, and the rules governing Module D and biogenic carbon.

Proceed to Lesson 05: Practical Calculation Methodology, Benchmarking, and Case Study for the synthesis lesson. We will connect bills of quantities, EPDs, uncertainty analysis, and climate benchmarks into a comprehensive comparative case study of a real building.

You can also simulate various circular recovery rates ($10\% \text{ to } 95\%$) in the Whole Life Embodied Carbon Simulator.

References

• [S01] British Standards Institution / CEN. (2011). BS EN 15978:2011: Sustainability of construction works — Assessment of environmental performance of buildings — Calculation method. BSI Standards Limited. URL: https://knowledge.bsigroup.com/products/sustainability-of-construction-works-assessment-of-environmental-performance-of-buildings-calculation-method
• [S02] British Standards Institution / CEN. (2019). BS EN 15804:2012+A2:2019: Sustainability of construction works — Environmental product declarations — Core rules for the product category of construction products. BSI Standards Limited. URL: https://standards.iteh.ai/catalog/standards/cen/5e3df5f5-ef85-48b9-823a-fcf7e19ec355/en-15804-2012-a2-2019
• [S03] International Organization for Standardization. (2017). ISO 21930:2017: Sustainability in buildings and civil engineering works — Core rules for environmental product declarations of construction products and services. ISO. URL: https://www.iso.org/standard/62545.html
• [S04] Royal Institution of Chartered Surveyors. (2023). Whole life carbon assessment for the built environment (2nd ed.). RICS Professional Standard. URL: https://www.rics.org/profession-standards/rics-standards-and-guidance/sector-standards/building-surveying-standards/whole-life-carbon-assessment
• [S05] Gibbons, O. P., Orr, J. J., & Arnold, W. H. (2022). How to calculate embodied carbon (2nd ed.). The Institution of Structural Engineers (IStructE). URL: https://www.istructe.org/resources/guidance/how-to-calculate-embodied-carbon/
• [S07] Jones, C., & Hammond, G. (2019). Inventory of Carbon and Energy (ICE) Version 3.0. Circular Ecology / University of Bath. URL: https://circularecology.com/embodied-carbon-footprint-database.html
• [S09] Pomponi, F., & Moncaster, A. (2016). Embodied carbon mitigation and reduction in the built environment – What does the evidence say? Journal of Environmental Management, 181, 687–700. DOI: 10.1016/j.jenvman.2016.08.036