Influence of Different Diffusion Depths on Chloride Migration Coefficient and the Calibration Methodology

In the rapid chloride migration (RCM) test, the chloride concentration at the chromogenic boundary often differs from the standard value of 0.07 mol/L, leading to an overestimated migration coefficient, especially when the penetration depth is shallow. This study investigates the effect of penetration depth on the measured migration coefficient and proposes a practical correction method. RCM tests were carried out on four concrete mixtures with fly ash and slag under various voltages, two curing ages, and multiple test durations. The results show that the migration coefficient decreases as the penetration depth increases. A simple empirical correction model is introduced, using a chromogenic error ε obtained by fitting the experimental data. After correction, most of the modified migration coefficients fall within ±20% of the true values. The proposed model provides a useful engineering tool for rapid estimation of chloride migration coefficients in field laboratories where direct chloride concentration measurement is not available.