Identification of Mechanical Properties of Bacterial Concrete with Partial Replacement of Fly Ash

Abstract

Concrete is the mostly used building material in the world. Concrete has a large load bearing capacity for the compressive loads. The application of concrete is rapidly increasing worldwide and therefore the development of sustainable concrete is urgently needed for environmental reasons and durability aspects. If a mechanism is developed that would contribute to a longer service life of concrete structures and make the material not only more durable but also more sustainable that would suffice the requirement of the durability of the concrete. One such mechanism that receives increasing attention in recent years is the ability for self repair (self healing), i.e., the autonomous healing of cracks in concrete.

Concrete is a material which takes compressive loads, but the material is weak in tension. That is why steel reinforcement bars are embedded in the material to take care of the tension. The steel bars take over the load when the concrete cracks in tension. The concrete covers and protects the steel bars from dampness etc. As the cost of the cement is going up day by day there is a need for finding an alternative to cement. For making it economical, a part of the cement by weight is replaced with a material called ‘fly ash’ which is cheaper in cost and abundantly available. On the other hand the cracks in concrete lead to leakage problems and there is a need to address these problems for future.

In the above context, the objective of the present investigation is to obtain the performance of the concrete by adding microbiologically induced special growth/filler and part of cement replaced by fly ash. One such thought leads to the development of very special concrete  known as bacterial concrete where bacteria is induced in the concrete  and part of the cement replaced by fly ash. A technique is adopted in the formation of concrete by utilizing

microbiologically induced calcite (CaCo₃) precipitation. Microbiologically induced calcite precipitation (MICP) is a technique that comes under a broader category of science called Bio-Mineralization.

It is a process by which living organisms form inorganic solids. ‘Bacillus Subtilis’, a common soil bacterium can induce the precipitation of calcite.  In this investigation it is to be checked by using the above technique, the possibility of improving the performance of the bacterial concrete using fly ash in longer duration. The compressive strength, tensile strength and compactness are checked on the samples of bacterial concrete by conducting the Laboratory tests like compressive test, split tensile test, pulse velocity test and diagonal tensile test in comparison with the normal concrete samples using fly ash as part replacement for cement in three different proportions.   

For the experimental investigation firstly cement mortar blocks are casted using fly ash as partial replacement of cement without bacteria and also with a common soil bacterium called ‘Bacillus Subtilis’ of different concentrations like 10⁴, 10⁵, 10⁶, 10⁷ and 10⁸ cells/ml. The cement mortar blocks are tested for 7 days and 28 days strength. Finally it is observed that the mortar blocks made with 10⁵ cells/ml. concentration of ‘Bacillus Subtilis’ attained good strength when compared with normal mortar blocks. Therefore, for further experimental investigations ‘Bacillus Subtilis’ culture samples with 10⁵ cells/ml. concentration are used for casting of samples of bacterial concrete using fly ash as partial replacement of cement.

Design mixes are prepared by adopting the IS code, IS: 10262-2009. To get the performance of the bacterial concrete when compared with the normal concrete 36 cubes, 36 cylinders and 36 prisms are casted with varying fly ash replacement for cement. And also the same numbers of samples are casted for normal concrete with varying fly ash replacements and tested for compressive strength, flexural strength, split tensile strength.

  From the experimental investigations it is observed that the compressive strength, flexural strength and split tensile strength are on par with the normal concrete strength parameters. The three strength parameters of bacterial concrete are found to be higher than that of the normal concrete.