Bilateral effects of accelerated carbonation on concrete microstructure: Insights from one-year ultrasonic measurements

The long-term microstructural evolution of concrete under accelerated carbonation was investigated using ultrasonic wave velocity (V) and acoustic nonlinearity parameter (β) to assess multiscale material changes. Concrete specimens made with ordinary Portland cement (OPC) were exposed to 10 % CO2 for one year. During this period, V remained nearly constant until 100 days and then increased by ∼10 %, indicating stiffness enhancement. In comparison, β decreased by ∼50 % within 100 days due to densification but later rose to ∼200 %, reflecting the onset of microcracking. This trend in β was supported by SEM-BSE and MIP analyses, which revealed pore refinement alongside the formation of nanoscale voids (10–100 nm). The influence of slag incorporation (50 % replacement) and curing conditions on carbonation kinetics was also examined. The results show that carbonation-induced densification could be offset by shrinkage, highlighting the bilateral nature of carbonation and the strong potential of β for long-term field monitoring.