Colloidal Silica-Stabilized Subgrade for Self-Sensing Vehicle Stress Affected by Unsaturation and Crack

Colloidal silica can seep through calcareous sand in the subgrade, forming colloidal-silica-cemented sand with self-sensing ability—that is, it is sensitive to stress changes caused by vehicle loading. Its self-sensing sensitivity is higher than that of traditional Portland-cement-based self-sensing materials. The self-sensing mechanism is attributed to the ionic conductive network formed by seawater. However, a change in tidal water level causes an unsaturated state, and foundation deformation leads to cracking of the roadbed. The effect of unsaturation and cracking on self-sensing remains unclear, and they have not been studied in the previous literature. The aim of this paper is to study the self-sensing ability of subgrades formed via colloidal-silica-cemented sand under unsaturated and cracked states, as well as to explore the underlying mechanisms. Specimens with different degrees of saturation and different levels of joint roughness in precracks were prepared; then, the self-sensing ability was tested using the four-electrode method for each specimen under cyclic stress loading. NMR (nuclear magnetic resonance) and an unsaturated triaxial apparatus were also used to investigate the underlying mechanisms. This paper discovers that (1) either unsaturation or crack alone can increase self-sensing, but their self-sensing sensitivities are on the same order; (2) under the coupled effect of unsaturation and cracking, the self-sensing sensitivity increases by one order of magnitude, which is higher than when only unsaturation or cracking exists; and (3) the joint roughness of precracks does not affect self-sensing in the saturated state, but it affects self-sensing dramatically in the unsaturated state. The NMR test demonstrated the conductive ionic water within nanopores, which forms the conductive network for self-sensing. Unsaturation causes suction-induced shrinkage based on the unsaturated triaxial apparatus, while unsaturation increases self-sensing sensitivity, indicating that shrinkage is accompanied by self-sensing improvement. This paper provides the effects of unsaturation and cracking on the self-sensing capabilities of colloidal-silica-cemented sand, and the findings can contribute to the knowledge of subgrades formed via colloidal-silica-cemented sand for stress-sensing under traffic loading.