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% CO 2 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.