Carbonation curing helps plain concrete but may shorten blended-cement life

Accelerated carbonation curing modestly extended the modeled service life of ordinary Portland cement mortar but shortened the predicted time to cover cracking in fly-ash and blast-furnace-slag blends under the specific conditions tested. The study combined chloride-transport measurements, electrochemical modeling and accelerated corrosion experiments to follow deterioration from chloride arrival at reinforcement to visible cracking of the concrete cover. Its central result is that the effect of carbonation curing depended strongly on the binder: it reduced the chloride diffusion coefficient in ordinary Portland cement, whereas it increased that coefficient in the two supplementary-cementitious-material systems. That contrast propagated through all three modeled stages of deterioration, although the experiments used mortar rather than structural concrete and the service-life values remain model predictions requiring field validation.

Accelerated carbonation curing exposes newly cast cementitious material to concentrated carbon dioxide, promoting carbonate formation and densification of the near-surface pore network. The process can consume some of the carbon dioxide released during cement manufacture while also increasing early-age strength. The researchers built on their earlier work with the same mixtures by extending the assessment beyond chloride diffusion and depassivation. Their semi-empirical hybrid framework divides deterioration into a diffusion-controlled stage, Pd, ending when chloride at the reinforcement reaches an adopted threshold; a corrosion-controlled stage, Pco, ending when calculated corrosion current becomes sustained and measurable; and a mechanical-controlled stage, Pcr, ending when surface cracking reaches 0.2 millimeters. These are engineering transitions rather than isolated physical stages, because chloride ingress, corrosion and cracking continue to overlap.

For the diffusion stage, the team used Fick’s second law with chloride coefficients measured in earlier rapid chloride penetration tests on identical mixtures. The model assumed a 50-millimeter concrete cover, a surface chloride concentration of 1.8 percent by binder mass and a critical threshold of 0.4 percent by binder mass. The threshold is an engineering reference rather than a direct measurement of passive-film breakdown. At 28 days, the non-steady-state migration coefficients were 21.9 × 10^-12 square meters per second for ordinary Portland cement and 11.2 × 10^-12 after carbonation curing. For fly-ash mortar, the values were 7.0 × 10^-12 without curing and 7.9 × 10^-12 with curing; for the slag blend, they were 3.1 × 10^-12 and 5.3 × 10^-12, respectively. Thus, carbonation lowered the coefficient for ordinary cement but raised it for both blended systems, indicating reduced chloride transport resistance in the fly-ash and slag mixtures under the investigated curing conditions.

The electrochemical model coupled ionic conduction through the pore solution, anodic dissolution of iron, cathodic oxygen reduction and diffusion-limited oxygen supply. Chloride ingress changed the free-chloride concentration at the steel surface and therefore the anodic reaction kinetics. A two-dimensional finite element representation included the ribs of a 10-millimeter deformed bar so that geometric discontinuities could act as localized corrosion sites, approximating the non-uniform anodic and cathodic regions seen in reinforced structures. The model calculated the corrosion potential and local current densities while enforcing charge conservation. It did not prescribe corrosion current directly. Pco was defined as the time when the maximum local current density reached 0.5 microamperes per square centimeter, a criterion associated with the boundary between low-to-moderate and moderate-to-high corrosion activity. The predicted corrosion-current histories for the blended mixtures plateaued only slightly above this threshold, so Pco was particularly sensitive to the selected criterion.

To establish the final cracking criterion, the researchers tested reinforced mortar specimens that had been water-cured for one year. A constant 5-volt impressed current accelerated corrosion in 3 percent sodium chloride solution, while linear polarization resistance, electrochemical impedance and crack width were monitored. Cumulative steel loss was estimated from the applied charge using Faraday’s law. The specimens showed an initial period in which corrosion mass accumulated without visible cracking, followed by rapid crack growth; alternating-current resistance fell sharply and corrosion current rose as cracking developed. Near the 0.2-millimeter criterion, the measurements were scattered because corrosion products formed unevenly and some specimens developed several cracks instead of one dominant path. The researchers therefore used the mean mass loss associated with the target width, approximately 0.9 grams, as a common comparative threshold rather than a universal material constant. Six representative specimens were selected across three test batches because comparable single-crack behavior was difficult to obtain consistently.

The experimental threshold was also the main bridge to the 50-millimeter numerical cover. The test specimens had 20 millimeters of cover, so the model multiplied the threshold by 2.5, producing a target natural-exposure mass loss of 2.25 grams. This linear scaling assumed that fracture energy, reinforcement expansion and critical mass loss were directly proportional to cover depth. It simplifies nonlinear fracture and omits stochastic variation, and the accelerated test also produced corrosion products that may differ from those formed under natural chloride exposure. The researchers applied the same threshold to all six mixtures to support comparison, not absolute prediction. They also used literature-derived electrochemical constants rather than system-specific measurements and assumed complete Faradaic efficiency during accelerated testing, leaving uncertainty from possible side reactions unquantified. The model stopped at crack initiation, so it did not simulate how an open crack subsequently changes transport, resistivity or corrosion rate.

Across the modeled sequence, carbonation curing produced the same directional effect at Pd, Pco and Pcr within each binder system: it improved ordinary Portland cement and shortened the fly-ash and slag blends. For ordinary cement, the delay between reaching the chloride threshold and developing sustained corrosion was very short, indicating nearly immediate active corrosion after depassivation in the model. Blended systems remained passive-like for much longer and therefore retained substantially more life after chloride arrival. Their predicted Pco values were about 100 years for the uncarbonated fly-ash mortar and 104 years for the uncarbonated slag blend in the baseline analysis. Pcr occurred far later than Pd and Pco, showing that the interval of active corrosion before cover cracking can occupy a large share of total durability. Carbonation curing’s modest benefit for ordinary cement was therefore accompanied by a much larger difference between plain and blended binders.

Sensitivity analysis showed that the absolute stage times were highly dependent on the thresholds chosen to define them, while the comparative conclusions were more stable. Lowering the adopted chloride threshold from 0.4 to 0.3 percent shortened Pd by 19–30 percent, whereas raising it to 0.6 percent extended Pd by 41–85 percent. Changing the corrosion-current criterion from 0.4 to 0.6 microamperes per square centimeter shortened Pco by 17–27 percent or extended it by 76–78 percent for ordinary-cement systems. Several blended systems did not reach the higher criterion within the 151-year simulation window, illustrating the limited precision of Pco as an absolute measure. By contrast, a 20 percent change in the mass-loss threshold altered Pcr by approximately 12–19 percent, and varying the crack-width criterion from 0.1 to 0.3 millimeters changed Pcr by roughly 17–26 percent in the lower direction or 14–21 percent in the higher direction. For the perturbations that could be evaluated, the direction of carbonation-curing effects within each binder system was retained, although some rankings involving unreached or insufficiently long histories remained unresolved.

The framework’s practical value lies in extending durability assessment beyond a diffusion-only endpoint without requiring a computationally intensive fracture-mechanics model. Its finite element analysis handles electrochemical behavior and local rib geometry, while the accelerated test supplies a manageable cracking threshold. Still, the study does not demonstrate that carbonation curing will extend the life of an existing reinforced-concrete structure. The mortar lacked coarse aggregates and aggregate-paste interfacial zones that can change chloride pathways and crack confinement; exposure was represented as constant chloride without wetting and drying; the carbonated and uncarbonated layers were represented by one effective transport coefficient; and no natural-exposure validation was performed. The study therefore supports a narrow, comparative conclusion: under one day of sealed pre-curing followed by one day in 10 percent carbon dioxide at 55 percent relative humidity and 20 degrees Celsius, carbonation curing improved ordinary-cement durability in the modeled assessment but reduced chloride resistance and modeled service life in the tested fly-ash and slag blends. Recalibration with concrete-specific transport data and cracking thresholds, plus validation under natural exposure, is required before the method can guide absolute life prediction.

Subject of Research: Modeling accelerated carbonation curing effects on chloride ingress, steel corrosion and concrete cover cracking in mortar

Article Title: From chloride transport to cover cracking: A semi-empirical service life assessment of accelerated carbonation-cured concrete

Article References: Liyew, G., Alemu, A. S., Lee, H. K., Kim, J., Park, K.-T., Kim, G., & Kim, H.-K. (2026). From chloride transport to cover cracking: A semi-empirical service life assessment of accelerated carbonation-cured concrete. Case Studies in Construction Materials, 25, Article e06632. https://doi.org/10.1016/j.cscm.2026.e06632

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06632

Keywords: accelerated carbonation curing, reinforced concrete, chloride ingress, steel corrosion, cover cracking, ordinary Portland cement, fly ash, blast furnace slag, service life prediction, finite element modeling, chloride, transport