Early Warning of Aggregate Size Effect in Tunnel Lining Concrete Based on NMR-AE

To reveal the correlation mechanism between aggregate particle size and the early-age pore structure–damage evolution of tunnel lining concrete, and to clarify the differences in acoustic emission (AE) responses of concretes with different aggregate sizes under loading, concretes prepared with 0 mm~5 mm fine aggregate and 10 mm~15 mm coarse aggregate were taken as research objects. After 7 days of natural curing, nuclear magnetic resonance (NMR) was used to obtain the pore T₂ spectra and fractal dimensions, while uniaxial compression tests were simultaneously conducted with AE monitoring. Furthermore, an ARIMA model was employed to predict the increasing trend of the cumulative AE count before failure. The results show that the porosity of fine-aggregate concrete (26.98%) is significantly higher than that of coarse-aggregate concrete (17.58%), with the proportion of air voids or cracks reaching 90.14%. The overall T₂ pore fractal dimension of fine-aggregate concrete is smaller than that of coarse-aggregate concrete, indicating that the pore structure of coarse-aggregate concrete exhibits stronger multi-scale heterogeneity. The AE b-value of fine-aggregate concrete averages 1.15 and remains relatively stable, while that of coarse-aggregate concrete averages 1.32, showing a distinct peak and marked fluctuations in the middle-to-late loading stage, reflecting more intensive microcrack initiation and more active evolution. The ARIMA model, with parameters (2, 0, 0) and (2, 2, 0) respectively, predicts the cumulative AE count before failure for the two types of concrete; the predicted curves are in good agreement with the measured values, and the predicted acceleration intervals coincide with the b-value decline/fluctuation phases. In summary, the mechanism by which aggregate particle size influences the AE temporal evolution via regulation of pore fractal characteristics is clarified, and the ARIMA model can provide a quantitative basis for early damage warning of tunnel lining concrete.