Comparative study on engineering properties of cement-based backfill material for sustainable urban area

Backfill materials used in urban areas should have suitable mechanical properties (e.g., strength and flowability) for safety and serviceability of constructed geo-infrastructures. The objective of this study was to estimate the relative permittivity and compressional-wave velocity to correlate them with the unconfined compressive strength of a controlled low-strength material (CLSM) for backfill. CLSMs with fines content (FC) of 30 %, 50 %, and 90 % were prepared in sensing molds instrumented by a time domain reflectometry probe and piezo disk element. The test results show that the relative permittivity decreases with respect to the curing time, whereas the compressional-wave velocity and unconfined compressive strength increase. In addition, the relative permittivity increases as the FC increases owing to the water content, while the compressional-wave velocity and unconfined compressive strength decrease owing to the FC effect on the force chain. In this context, the FC might disturb the load-carrying matrix, and it results in remaining water and delays the curing time within 672 h. From the correlations, the relationship between the unconfined compressive strength and compressional-wave velocity with respect to the relative permittivity can be reliably constructed at an R² of 0.83. For this reason, it is considered that the water content from the relative permittivity can improve the relationship between strength and stiffness. Consequently, the time domain reflectometry probe can be effectively used for reliable assessment of CLSM strength during curing.