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According to advances in various scientific fields, concrete technology has also evolved. However, nowadays, lightweight concrete has many advantages that is widely used in the construction industries, mechanical properties of lightweight concrete, is weaker in compared to normal concrete. An appropriate solution for improving the mechanical properties of lightweight concrete is adding fiber although fiber might disrupt the integrity of the cement matrix by creating pores in the concrete mix. So presence of fiber and lightweight aggregate that is porous, cause weakness in durability of concrete.Due to the negative side-effects of certain chemical techniques to reduce the porosity of concrete, in recent years, biological methods have been used as an environmental friendly strategy by filling the concrete pores and repair cracks with calcite sediments. In this regard, calcium carbonate precipitation induced by micro-organisms has found wide applications in construction technology for its effect on quality improvement of building materials.This research is an experimental study to evaluate the effect of calcium carbonate sediments induced by the activity of four microbial strains known as Sporosarcina Pasteurii, Bacillus Megateterium, Sporosarcina Ureaea and Bacillus Licheniformis, with 107 cell/ml concentration, on structural lightweight concrete, which included Leca aggregate and polypropylene fiber. To study the effect of these bacteria along with fiber, three mix designs with different fiber volume are considered, including:1-non-fiber specimens, 2- specimens containing 0.5% fiber, which is almost the most optimal case, 3-specimens containing 1% fiber, which creates the phenomenon of balling. Generally, 240 specimens were made for compressive strength, splitting tensile strength, flexural strength, water obserption, water impermeability and freeze-thaw test.By comparing the bacterial specimens can be stated, Bacillus licheniformis, Bacillus megateterium, Sporosarcina pasteurii, Sporosarcina ureae had the best performance in producing more calcium carbonate sediment respectively. The results indicate that the combination effect of presence of bacteria and fiber has been effective and has increased compressive strength, tensile strength and flexural strength. It also has reduced water absorption and permeability of concrete specimens and decrease damage of samples during the freeze-thaw cycles. In addition, the SEM and EDX analyses show formation of calcium carbonate crystals and existence of bacterial mass in concrete matrix. In fact, the microbial mineral precipitation has improved the mechanical properties and durability of concrete by filling the pores and creating denser concrete.
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