Concrete EPD comparative assertion: declared unit, fly ash and critical review
Two ready-mix concrete mixes are compared in a manufacturer's EPD. Mix A is C30/37 with 320 kg cement/m³; Mix B is C50/60 with 420 kg cement/m³ and 80 kg fly ash/m³. The declared unit is 1 m³ of concrete. The draft EPD states that Mix B has 18% lower GWP than Mix A because the fly ash replaces cement, but the calculation compares 1 m³ of each mix, not an equivalent structural function. Fly ash is modelled as a waste with zero burden, and the end-of-life is excluded. The manufacturer wants to publish the EPD with the comparative statement on its website. The EPD programme requires an independent critical review, and the product category rules require the declared unit to be 1 m³ of concrete of a specified strength class.
1. Is the 18% comparative claim defensible? Identify the functional-unit and allocation problems.
Read the worked answer
No. The declared unit 1 m³ is a unit of volume, not a unit of equivalent function. Mix B has 420 kg cement/m³ and a higher strength class; a structure designed in C50/60 may use less concrete than one in C30/37, so comparing 1 m³ can overstate or understate the difference depending on the application. The correct functional unit is the delivered structural function, for example a slab or beam with a defined load and service life, or a comparison within the same strength class.
The fly ash zero-burden assumption is a second issue: it is defensible only if fly ash is a true waste with no demand-changing effect and the PCR allows it; otherwise allocation or system expansion is needed. Excluding end-of-life also matters because carbonation and demolition differ. The 18% claim is therefore not defensible as stated.
2. What critical review process does ISO 14044 require for a comparative assertion disclosed to the public, and what would you check?
Read the worked answer
ISO 14044 requires critical review for comparative assertions disclosed to the public. The review should be by a panel of interested parties, not only an internal check, and should cover goal and scope, LCI, LCIA, interpretation and the disclosure. The panel should include the concrete industry, a user or specifier, an environmental NGO and an LCA expert.
Check that the PCR is followed, the declared unit is correct, allocation of fly ash and slag is documented, the LCIA method and GWP100 are stated, and the comparative claim is limited to what the study supports. Under ISO 14025 the EPD also needs programme verification. The reviewer should see the sensitivity analysis, not just the final table, and the EPD should state the functional unit and strength class explicitly.
3. What alternative comparison would you propose, and what sensitivity analyses would you run?
Read the worked answer
Propose an equal-strength or equal-function comparison. Mix A at C30/37 and Mix B at C50/60 should be compared only after normalising to the same structural requirement, for example the concrete volume needed for a beam with a given load, or compare Mix B with a C50/60 reference mix without fly ash. A second option is to compare within the same strength class, such as a C30/37 mix with fly ash versus the C30/37 control.
Run sensitivity on fly ash allocation (zero burden versus economic versus mass), transport distance, cement factor, and end-of-life carbonation. If the claim survives only on a per-m³ basis, it should be withdrawn or reworded as a declaration, not a comparative assertion.