the wrong ruler

for concrete's lifetime

Problem statement

Replacing Portland clinker with slag, fly ash, calcined clay or limestone is "one of the most feasible and rapidly implementable solutions" for cutting concrete's CO₂, but every such blend carbonates faster than plain Portland cement, and the durability rules that decide whether a concrete may be used in a given exposure class treat carbonation depth (the advance of the pH-neutralised front toward the reinforcement) as the measure of corrosion risk. After six years and 120+ members, RILEM Technical Committee 281-CCC concluded that this ruler is bent in three places. There are "at least 16 different standards or recommendations" for carbonation testing "with significant differences in sample curing, pre-conditioning, carbonation exposure conditions, and methods used for determination of carbonation depth"; an interlaboratory test across 22 laboratories found "the variation between laboratories was even larger than the potential effect of raising the CO₂ concentration from 1 to 4%," and "accelerated and natural carbonation rates cannot be converted into each other without applying correction factors" that do not yet exist — "it remains unknown how best to translate carbonation rates obtained via accelerated tests to carbonation rates under natural conditions for different types of concrete." Worse, the premise is shaky: reviewing documented practical cases, the committee found that "a reduced pH alone is not sufficient to lead to significant steel corrosion in concrete," so "from practical experience there does not seem to be a direct correlation between reduction of pH associated with concrete carbonation, and corrosion of steel reinforcement." Meanwhile, at the other end of the life cycle, the ReCreate reuse project found that applying the standard carbonation rates in EN 16757:2022 to a 1967 Swedish precast building "led to the conclusion that most of the precast elements would not be reusable," whereas with contextual carbonation rates "all elements were deemed suitable for reuse." The unsolved problem is a qualification regime for concrete durability that measures a proxy, measures it inconsistently, and cannot yet be replaced because the real predictors of corrosion in carbonated low-clinker concrete are not established.

Why this matters

Cement is among the largest industrial sources of CO₂ and clinker substitution is its main near-term lever (a general-knowledge framing, not from the source); if low-clinker concretes are excluded from, or over-designed for, common exposure classes because of an accelerated test that over-predicts their carbonation, decarbonisation is slowed by the very rules meant to protect service life. The committee notes that "this mindset has long dictated the research priorities surrounding the developments towards new, low-emission binders" — the industry has been optimising against the wrong target — and that the finding "has profound consequences for the established approach of assessing the durability performance based on carbonation testing and modelling." Cover-depth requirements, and therefore concrete volume and embodied carbon, follow directly; the committee even suggests "the concrete cover prescribed in standards for design of concrete for different exposure classes could be revised considering the loading in service condition," having found carbonation depths 9–16% lower under 30% compressive load and up to 70% higher under 60% tensile load. Getting the ruler right also matters for CO₂ accounting (how much CO₂ concrete re-absorbs) and for the reuse economy that depends on estimating remaining service life of existing elements.

What’s been tried and why it hasn’t worked

The TC's five working groups produced a database of natural and accelerated carbonation data spanning years of exposure, a predictive equation in which the water-to-reactive-CaO ratio is "a decisive factor," a recommendation for testing carbonation under load, and a critical review of standards. The 22-lab round robin showed that 91-day accelerated tests rank cement types consistently and are "a good estimate of the potential resistance to carbonation," but "large variations within and between laboratories did not allow to draw clear conclusions regarding the effect of sample pre-conditioning and carbonation exposure conditions." For low-calcium alkali-activated materials, "even at the relatively modest concentration of 1% CO₂, accelerated testing may lead to inaccurate predictions of the carbonation resistance under natural exposure conditions." Natural exposure requires "at least one year" to yield a stable rate, so standards default to accelerated tests. And the corrosion review found the influencing factors "leading to considerable corrosion damage" are "the moisture state, the microstructure of the carbonated concrete, various species that may be present — even in minor amounts — in the concrete pore solution, and the cover depth" — none of which a carbonation-depth test measures. Attempts to fix the standards have run into this: harmonising 16 protocols is a multi-year committee process, and no agreed alternative metric exists to harmonise toward. On the reuse side, ReCreate found EN 16757's carbonation rates "may be overly conservative and hinder the reuse of concrete elements," yet the standard is what a permitting engineer must apply.

What would unlock progress

Progress needs (1) correction factors, or better a physics-based mapping, between accelerated and natural carbonation for each binder family, built from natural-exposure programmes that record the microclimate; (2) a qualification test that measures what actually drives corrosion in carbonated concrete — moisture state and pore-solution chemistry of the cover — rather than pH front position, particularly "in novel cementitious systems with a limited service track record"; and (3) standards revision (EN 12390-10/-12, EN 16757, CEN/TR 17310, exposure-class rules) that admits contextual measurement for existing elements. Adjacent precedents: fatigue and creep qualification in metals, where accelerated tests are tied to service conditions by validated equivalence models; and corrosion engineering in pipelines, which qualifies by moisture and electrochemical state rather than by a single chemical proxy.

Entry points for student teams

A materials team could set up a comparative programme on one or two low-clinker mixes — accelerated carbonation at 1% and 3–4% CO₂ under two of the standard protocols, plus indoor natural exposure — and quantify how protocol choice changes the durability verdict, adding a data point to the RILEM database. A team with electrochemical skills could instrument carbonated specimens of different binders at controlled moisture states to test the "carbonation alone does not corrode" finding and prototype a moisture-based screening test. A structural-reuse team could re-run the ReCreate service-life assessment on a local existing precast building, comparing the EN 16757 rate with measured carbonation depths, and draft the assessment guidance a permitting authority would accept. Relevant skills: cement chemistry, corrosion electrochemistry, statistics, standards analysis.