What Is Carbonation in Concrete?
Carbonation is the slow reaction of atmospheric CO2 with the calcium hydroxide in concrete, lowering the concrete's pH from ~13 to ~9 starting from the surface and moving inward over decades. The pH drop destroys the passive layer protecting embedded rebar, leaving the steel vulnerable to corrosion. It is the quiet durability threat in older concrete structures — slower than chloride attack, but just as real.
Best price · Quotes back fast · PE stamps backed by Apex Grid Engineering
How it progresses
Carbonation advances roughly with the square root of time — fast in the first years, slowing as the front moves deeper. In dry interior environments it can reach rebar depth (typically 1.5-3 inches of cover) in 20-50 years; in wet or exterior exposures, other deterioration usually arrives first. Poor-quality concrete (high water-cement ratio, inadequate curing) carbonates far faster than dense, well-cured concrete.
Why it matters structurally
Once the carbonation front reaches the rebar and moisture is present, the steel corrodes, expands, and spalls the concrete cover — the classic rust-stained, delaminated concrete on aging parking structures and facades. Unlike chloride-induced corrosion (which is localized and aggressive), carbonation corrosion is broad and uniform. The repair is the same expensive proposition: remove unsound concrete, clean or replace steel, and patch with compatible materials.
Prevention and assessment
Prevention is adequate cover (the code minimums exist partly for this), low water-cement ratios, proper curing, and protective coatings in aggressive exposures. Assessment uses phenolphthalein indicator on fresh fracture surfaces (pink = uncarbonated, colorless = carbonated) plus cover-meter surveys and half-cell potential mapping. For structures past 30 years with thin cover, a durability assessment ($5,000-$15,000) is prudent maintenance.
The bottom line
Carbonation lowers concrete pH over decades, eventually depassivating rebar and causing corrosion. Prevent with adequate cover and good concrete; assess older structures with indicator testing and cover surveys.
Torch Engineering PLLC · (480) 490-0064 · info@torchengineering.com — PE stamping backed by Apex Grid Engineering.