Effect of bacteria on strength, durability and permeability of structural lightweight concrete with natural aggregate reinforced by steel fiber Farnaz Salmasi Date of Submission: November 21, 2018 Department of Civil Engineering Degree: M.Sc. Language: Farsi Supervisor: Dr. Davood Mostofinejad, prof. Undoubtedly, nowadays concrete is one of the most used building materials in the world, the application of which is becoming more and more widespread. The reason for having a special position is that the concrete is responsive to the technical, economic and environmental needs of human societies. The implementation of reinforced concrete structures as the cheapest and most durable structures has led researchers to find ways to improve concrete properties. The significant weight of the dead load of building components is one of the most important problems in the design and implementation of concrete buildings. The use of structural lightweight concrete in concrete structures reduces traort costs by reducing dead weight and reducing the dimensions of beams and columns. Another advantage of weight loss is its strength against earthquake forces, because earthquake forces are mostly directly related to dead load. Lightweight concrete also results in economics due to the thermal insulation effect. On the other hand, the presence of light and porous aggregates in concrete structures increases water absorption and chloride ions penetration in contaminated environments. The microbiological method can be used to increase the durability of concrete and repair the finest cracks in it. The use of bacteria producing calcium carbonate sediments has been confirmed by the researchers in filling the pores and improving the properties of ordinary concretes. These bacteria help urea hydrolysis with the production of urease enzymes as catalysts. When this hydrolysis occurs in calcium-containing environments, solid calcium carbonate deposits are formed and will fill the pores of the concrete. In order to achieve the research objectives, the samples were made in three general groups. In the first group, no bacteria were used in the samples as control samples. It was made exclusively with two mixing designs, including samples containing and without steel fibers. For this purpose, 72 cylindrical cylinders of 100 and 200 mm in diameter and height, 24 cubic samples with a dimension of 100 mm and 12 prism samples with dimensions of 200 × 200 × 120 mm were made. Subsequently, according to the duration of treatment required for each experiment, the samples were placed in three treatment environment, such as water, urea-calcium lactate solution and nutrients solution. In the second group, samples of bacteria with a specific concentration cell of bacteria with their liquid culture medium were used as concrete mixing water. The number of samples made, the experiments, the type of environment and the treatment time were the same as those of the first group; with the difference, in addition to them, 36 specimens of spore-containing bacteria were also used to perform tests to determine the water absorption, electrical resistance and compressive strength was built with the same dimensions and ages. In the third group, bacteria were used to make surface treatment gel and the surface of the samples was cured. In order to study the improvement of concrete durability, water absorption percentage, electrical resistance, chloride ion permeability, compressive strength in open air and magnesium sulfate solution, determination of carbonation depth and impermeability of concrete against water were carried out on samples. The results of these experiments show that the simultaneous use of bacteria and steel fibers generally reduces water absorption, carbonation depth, transient electric charge and lower penetration depth and increases the compressive strength and electrical resistance. Regarding the use of spore bacteria, activity and its ability to produce calcium carbonate precipitation were lower than living bacteria. Among the treatment environments, the urea-calcium lactate environment has had a more favorable effect on bacteria and surface treatment. Key words: stuctural lightweight concrete, steel fibers, bacteria, surface treatment