Effects of Bacillus pumilus precipitation on the flexural strength of jute fibre reinforced concrete

This study investigates the influence of Bacillus pumilus-induced calcium carbonate precipitation (MICP) on the flexural performance and durability of jute fibre-reinforced concrete (JFRC). A nominal 1:2:4 concrete mix with 1% jute fibre (treated and untreated) was prepared and dosed with three bacterial concentrations (B1.5, B12, and B24). Prismatic beams (150 × 150 × 500 mm) were cured and tested at 7, 14, 21, and 28 days under three-point bending, and a total of twelve beams per mix (n = 12), corresponding to three replicate specimens at each curing age, were evaluated. Fresh properties (slump and compaction factor), mass and dimensional loss, SEM microstructural observations, and statistical analysis (two-way ANOVA and Tukey HSD) were used to interpret results. Findings show that bacterial dosage strongly governs performance: low dosage (B1.5) produced minor early-age gains; moderate dosage (B12) yielded delayed but measurable improvements; and high dosage (B24) produced the greatest 28-day flexural enhancement, although with reduced workability. Durability tests indicated improved resistance to acid attack and lower mass and dimensional loss for bacterial mixes, with B1.5 and B24 dosages showing the most favourable performance depending on the metric assessed. SEM observations confirmed progressive CaCO₃ deposition with increasing bacterial concentration, enhancing fibre-matrix bonding and reducing microcrack connectivity. Overall, the study demonstrates a measurable synergy between jute fibres and microbially induced calcite precipitation, indicating that appropriately dosed B. pumilus can significantly enhance the flexural behaviour and durability of JFRC. These findings provide insight into the development of low-cost, bio-enhanced natural-fibre composites for sustainable construction applications.