Reinforced concrete was the defining structural material of 20th-century brutalist and industrial architecture. While perceived as virtually permanent, unmaintained concrete structures suffer from progressive chemical degradation once atmospheric protection and roof integrity fail.
1. The Chemistry of Carbonation
Fresh concrete maintains a highly alkaline pore solution (pH 12.5–13.5) due to calcium hydroxide generated during cement hydration. This high pH forms a passive oxide layer around embedded steel rebar, preventing rust formation. Over time, atmospheric carbon dioxide penetrates concrete pores, reacting with calcium hydroxide to form calcium carbonate (CaCO₃). This reaction drops the pore solution pH below 9, dissolving the protective passivation layer.
2. Oxidation Expansion & Spalling
Once moisture and oxygen reach the depassivated steel rebar, oxidation begins. Iron oxide (rust) occupies up to six times the volume of original steel. This volumetric expansion generates immense internal tensile stresses exceeding concrete's tensile strength, leading to cracking, delamination, and severe concrete spalling.
3. Implications for Archival Field Safety
Understanding carbonation depth helps field researchers identify impending ceiling collapses in abandoned industrial plants and brutalist halls before visual indicators become obvious.