Shrinkage Control & Self-Healing of Cement-Stabilized Unsaturated Soil through Sequential Hydration and Calcite-Biofilm-Biochar-Composite

Seth Owusu Tawiah

doi:10.7273/000007095

The durability and benefits of infrastructures constructed with cement-stabilized soils (CSSs) are often limited by the unavoidable cracks that develop during their service life. To ensure the functionality of such infrastructure at the risk of cracking is sustained, close monitoring and periodic mitigation are required. This work developed an innovative approach that couples a synergistic calcite-biofilm-biochar-composite (CBBC) with carbon sequestration benefits and sequential hydration methodology for self-healing in cement-stabilized saturated and unsaturated soil. This study developed a nature-inspired self-healing biomineralization approach that allows two different bacteria to co-exist in an environment with minimized competition, while promoting the most effective microbial activities. The two bacteria included one that produces a brittle precipitate (i.e., calcite), while the other a soft polymer (i.e., biofilm). The calcite-biofilm in a synergistic condition yields a microbial-induced ‘soft-hard’ precipitate’s (MIS-HP) composite material that could allow deformation while maintaining self-healing benefits. This study selected bacteria 1 from Bacillus genus specifically pseudofirmus and bacteria 2 from Leuconostoc genus specifically Mesenteroides for calcite and biofilm precipitation respectively. Well-graded sand with silt (SW-SM) stabilized with Type I Portland cement was used as the host material, while biochar was used as a cargo for the healing agents and nutrients. Calcium dioxide (CaO2) was used for oxygen enhancement. Samples were healed under saturated and unsaturated conditions over a 90-day period. Sequential hydration (SH) is a methodology proposed to help mitigate the development of shrinkage cracking of CSSs while enhancing the mechanical properties (λ(s), 𝑐′, ∅′) required for ultimate limit designs. The performance tests showed that during construction of road pavement for example, controlling the initial compaction moisture content to about 80% of optimum, followed by SH and compaction after 3 days before placement of asphalt, could minimize shrinkage and mitigate cracking potential by over 6000 % in the early stages, and thus could extend the service life of pavement. The restrained shrinkage results revealed that the development of tensile stresses and the travel direction of the stress intensity governed the cracks development and propagation over wetting-drying cycles. Furthermore, the concentration of tensile stresses varied spatially and did not stay at a specific location over wetting-drying cycles. Besides, the locations with the highest porosity intensity and the largest pore size diameter shifted spatially with wetting-drying cycles. Consequently, shrinkage cracks that result from shrinkage stresses were revealed to be controlled by the total porosity and not by the intensity of porosity. This is because the pattern of tensile stresses development was influenced by the initial compaction moisture content. In the self-healing study, all types of treatments of the CSSs considered, including the pure biochar, showed self-healing behavior by either calcite precipitation or calcite-biofilm-composite with varying ages and ductility as confirmed by the microscopic images, SEM, TGA and XRD data. The healing products showed different precipitates-biochar-soil (CSS) interactions as was revealed by SEM images and EDXS results. The results imply that a targeted healing platform could be designed to address cracks (shrinkage and/or fatigue-induced) associated with specific problems in a wide variety of geotechnical structures. In terms of the oxygen enhancement (OE) optimization for self-healing, 17 g/L of CaO2 showed a higher oxygen release rate than the 25 g/L, consequently oxygen (O2) was released to bacteria quickly by the 17 g/L than the 25 g/L. Hence, the O2 was consumed more readily which affected long-term self-healing benefits, as was revealed by the growth curves and the CSS performance test results. During the self-healing, it seemed the bacteria migrated from within to the outer surface of the healing samples under low O2 conditions, while they stayed longer before the migration under higher O2 conditions. Therefore, while true crack healing occurred under higher O2 conditions, the early migration led to superficially healed cracks with a corresponding low strength and strain recoveries. The implication is that a CSS matrix containing bacteria-impregnated biochar with oxygen enhancement would enhance self-healing, benefit immensely from SH, and could lead to a novel design for cement-stabilized structures if implemented in practice.

Shrinkage Control & Self-Healing of Cement-Stabilized Unsaturated Soil through Sequential Hydration and Calcite-Biofilm-Biochar-Composite

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