Role of ITZ and aggregates in concrete carbonation: Multiscale-coupled modeling and experimental validation

Abstract

Concrete carbonation poses a critical threat to the durability of reinforced concrete structures. Conventional homogeneous models fail to quantify the localized acceleration effect at the interfacial transition zone (ITZ), while neglecting the tortuosity effects induced by aggregates on CO<inf>2</inf> transport pathways. This study developed a multiscale mesoscopic carbonation model that innovatively decouples the reaction kinetics of calcium hydroxide (CH) and calcium silicate hydrate (CSH), integrating cement hydration parameters to establish a three-phase mesostructure incorporating cylindrical aggregates. Model validation against accelerated carbonation tests and phenolphthalein titration demonstrated high accuracy, with simulation errors below 10 % in long-term carbonation stages. Results reveal that the high porosity and interconnected transport pathways of the ITZ accelerate localized carbonation, with diffusivity 5–10 times that of the mortar matrix, and significantly affect microstructural carbonation patterns near aggregates. However, coarse aggregates predominantly govern macroscopic carbonation progression through diffusion path extension and overshadow the localized acceleration of ITZ. This research establishes a theoretical framework for precision durability design by highlighting the key role of the aggregate interface, which is particularly vulnerable to early-stage degradation.