Tensile behavior of bio-cemented, fiber-reinforced calcareous sand from coastal zone

Abstract

Calcareous sand erosion under natural environment changes or human activities is a major concern to many coastal zones, where weak and loose calcareous sands are usually encountered. To improve the tensile behavior and structural integrity of such sands, we combine the fiber reinforcement method and the bio-cementation technique based on microbially induced carbonate precipitation (MICP) for calcareous sand treatment. Various fiber-sand mixtures are prepared by mixing calcareous sand with different amount of fibers, including 0.00%, 0.05%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30% and 0.40%, before the bio-cementation process. To investigate the tensile strength as well as the post-peak response, we conduct the direct tensile test on these bio-cemented, fiber-reinforced sand samples. Experimental results highlight the interplay of MICP and fiber inclusions in strengthening microscale inter-particle binding and improving macroscopic tensile behaviors. The tensile strength of the sample increases with the increasing calcium carbonate content. The inclusion of fibers increases the sand ductility, but excessive amount of fibers may cause negative effects on the sand tensile strength due to the possible presence of more micro-voids and the resulting structural heterogeneity. Given varying fiber contents, we identify two types of stress-strain responses under the direct tensile load. With higher fiber content, the overall stress strain response follows four phases, with phase I representing the linear breakage of cemented sand body, phase II responding to the sudden stress drop due to the brittle failure of cemented sand, phase III related to the elastoplastic deformation of fibers, and phase IV for the pull-out or snapping of fibers. In comparison, with lower fiber content, phase III does not exist because of the much higher stress distributed to each individual fiber. Scanned Electron Microscopy observations further reveal that the increasing calcite precipitations strengthen the fiber reinforcement effect and the increasing fibers provide more surfaces that facilitate the calcite precipitations. This study provides new insights into the tensile behavior of bio-cemented, fiber-reinforced sands and contributes to possible future implementations for coastal erosion control.