Sampling and measurements
The following sampling and measurement requirements only apply to projects measuring carbon storage using solid-sample CO2 storage measurement methods. Projects using gaseous inflow-outflow measurements are exempt from these sampling protocols, as their measurement approach directly tracks CO2 flux in real time.
Solid sample measurements
All solid sample measurements shall be done by an external laboratory, accredited and compliant with ISO/IEC 17025, or accredited by national accreditation bodies that certify compliance with ISO 17025 or equivalent, such as COFRAC (France), UKAS (UK) or ANAB (USA).
Homogenous materials (preferred approach)
Project Developers should prioritize high-accuracy measurements using one of the following:
Thermogravimetric analysis (TGA), e.g. ISO 11358-1:2014, ISO 19579:2006 and ISO 21687:2007
Dry combustion (Total Carbon Analysis, or TCA) methods, following e.g. ISO 10694:1995, ISO 13878:1998, and ISO 15178:2000
These measurements shall be done in 3 or more replicates.
Heterogeneous materials
For feedstocks that are highly heterogeneous in composition or particle size, where methods like TGA or dry combustion may result in significant sampling error, Project Developers may use alternative large-sample methods, such as:
Acid digestion with CO2 quantification
Furnace-based mass loss methods
These alternatives may only be used if the potential reduction in sampling error outweighs the increase in analytical (measurement) uncertainty.
These measurements shall be done in 6 or more replicates.
Results from one of these methods must be cross-validated against TGA or dry combustion results initially to validate this option for the project, and then at least once annually. Minor discrepancies (e.g. ±20%) are acceptable if justified by sampling uncertainty due to heterogeneity.
For all methods other than TGA, Project Developers must determine fraction of total measured carbon that is attributable to stable mineral carbonates (e.g., CaCO3 or MgCO3). This may be determined once, upfront, for the project's alkaline feedstock, and applied throughout the crediting period until a different feedstock is used. This ensures that only creditable carbon increases resulting from the project activity are counted, excluding carbon stored in less stable or non-mineral phases. Accepted methods for confirming carbonate speciation include, but are not limited to, X-ray diffraction (XRD) with Rietveld refinement and infrared spectroscopy (FTIR).
Centralized vs distributed sites
Carbonation projects may be highly distributed, deploying the same carbonation technology and process to many different sites. For example, carbonation may occur directly in concrete mixing trucks, with small amounts of carbon storage taking place at each carbonation site. Because of this, we distinguish between two types of projects: centralized vs distributed, with different sampling requirements.
A distributed project is defined as one where
Each carbonation site is expected to issue fewer than 100 RRCs annually, based on carbon storage via carbonation (i.e. excluding reduced cement avoidance), and
The following characteristics are consistent across sites in a group:
Carbonation reaction type, the metal(s) contributing to mineral formation, and expected carbonate compound
Expected carbonation timeline post-reactor, based on the kinetics of the specific technology (e.g. full carbonation occurs within 24 hours, 1 week, etc.)
Reactor operating temperature and pressure (within a target range of ±10%)
Distributed projects can use empirical data from a representative subset of sites to prove that carbonation rates are sufficiently similar across grouped sites (<10%) when alkaline feedstock is the same.
Once this is demonstrated, the project can shift to reduced sampling, where only one site continues to take samples and perform laboratory measurements. Those results may then be applied to other sites in the same group, under the conditions agreed upon by the Project Developer, the Rainbow Certification team and the VVB. The lowest-measured carbonation rate shall be used to issue credits.
Grouped sites must use the same alkaline feedstock. The criteria for determining whether feedstocks are considered the same, or should be treated as distinct, are outlined in Carbonation batch section.
This approach can be planned from the start and included in the project design, or implemented partway through the crediting period. In the latter case, the supporting data and updated Monitoring Plan must be reviewed and approved by the VVB.
Grouped sites that are not taking samples and measurements shall continue to provide all other data required for GHG quantification, such as alkaline feedstock type and amount, onsite energy use, and transport distances. These sites are only exempt from taking samples and laboratory measurements of carbonated materials.
To maintain data quality, Project Developers shall perform random cross checks annually, taking samples from non-measuring grouped sites to confirm that their carbonation rates remain within the expected range. The number of sites and samples depends on the project size, and shall be decided by the Rainbow Certification team and the Project Developer on a case by case basis.
Measurement frequency and representativeness
Project Developers must perform carbon storage measurements, in replicates of 3 or 6, on a representative sample, at the frequency outlined below. The approach to obtain a representative sample and measurement is:
At the start of verification, for the first carbonation batch, measurements shall be taken at high frequency until the project demonstrates stable operating conditions and establishes operational stability and consistent results.
It is recommended to conduct 10 separate measurements, each on a composite sample from 3 sub-samples each, in 3 or 6 replicates depending on the measurement type used, at the beginning of the crediting period. The variability of these measurements should not exceed an of 10%.
The lowest value of recent stable measurements shall be used for initial credit issuance once measurement consistency is established. This shall be updated regularly for ongoing verification.
This frequency may be adjusted based on the project’s technological maturity, existing data, and expected variability. For example, this step may be skipped if the project can prove stable operating conditions were established before the start of the crediting period.
For ongoing monitoring and verification, measurements shall be repeated:
For each carbonation batch (with batch validity limited to 1 year), or
At least once per quarter, or
Every 500 tonnes CO2 removed, whichever comes first.
Each ongoing verification measurement shall:
Be based on a . Best practice is to create this sample as a homogenized composite of at least 3 sub-samples taken from different points in the batch.
Be analyzed in replicates of
3 for TCA and dry combustion, and
6 for acid digestion with CO2 quantification and furnace-based mass loss methods.
Achieve an ≤ 10% between the replicate measurements; otherwise, measurements must be repeated.
The lowest of the replicate values shall be retained and used for crediting.
Taking representative samples of heterogeneous materials like concrete is challenging due to high baseline variability in carbonate and carbon content: factors not affected by the project activity or controlled by the Project Developer. To address this, it is recommended to separate the reactive fraction (e.g. cement paste) from aggregates before sampling. Otherwise:
Natural variability in aggregate carbonation may obscure the smaller, project-induced changes in carbonation in the cement fraction, and
The overall carbon content difference between project and baseline samples may not be statistically significant, even if a measurable effect exists within the cement fraction alone.
Ex-ante sampling plans
Project Developers shall submit a Sampling Plan, upon validation, that details:
procedure to ensure representative sampling, including:
tools or equipment for taking samples,
homogenization techniques,
sample storage conditions
the approach for random timing and location (i.e. within the pile, or the production line) of sub-samples,
the proposed frequency and number of measurements performed in the first reporting period, to establish a robust dataset (mean and distribution),
approach to ensure the control/uncarbonated samples are representative of project samples, and exposed to the same conditions,
the post-carbonation timing of sample collection (e.g. 24 hours, 1 week), justified in terms of the carbonation kinetics of the specific technology.
Ex-post sampling records
For each Reporting Period, Project Developers shall submit a Sampling Record documenting all sampling activities. This record must include the following information, for both project and control materials:
Date and time of carbonated material production
Date and time of sampling
Name or ID of the person(s) performing sampling
Amount of material in sub-sample and composite sample
Processing or preparation steps before analysis
Description of representative sampling method
Sample ID for traceability
Notes on anomalies, if any
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