Evaluation of the Effect of Bacillus Pumilus Precipitate on the Strength and Durability of Concrete

Abstract

Microbiologically induced calcite precipitate (MICP) also called bio-mineralization is a process by which living organism’s forms inorganic solids. The basic principles of MICP involves the formation of urease by the Bacillus species and the hydrolyze urea produce ammonia and carbon dioxide, and the ammonia released to the surroundings subsequently increases pH leading to accumulation of insoluble calcite (CaCO₃). Bacillus pumilus is a common soil bacterium which can induce the precipitation of calcite. The effect of different concentrations of B. pumilus on the compressive strength and durability of concrete was studied. It was found that the concrete treated with bacteria (i.e. concrete mixed with bacteria and concrete cured in bacteria suspension) performed better than the control concrete. Though the treated concrete were stronger than the control in terms of compressive strength, the strength increase was more pronounced at early ages (7 & 14 days) than at later ages (21 & 28 days); the best performance at 28days was an increase of about 6% above the compressive strength of the control for concrete mixed with bacteria, at a suspension density of 1.5x10⁸ cells/ml. The durability performance improved with increase in the concentration of B. pumilus, though there was no significant difference between the performance of concrete mixed with bacteria and concrete cured in bacteria suspension. At 28 days, the weight loss was 16.5%, 11.8%, 17.8% and 14.2% for concrete mixed with bacteria at concentration of 0, 1.5E8, 12E8 and 24E8 cells/ml respectively. Similarly, at 28 days, the weight loss was 16.5%, 15.5%, 15.7% and 15% for concrete cured in cementation reagent containing various B. pumilus suspension at density of 0, 1.5E8, 12E8 and 24E8 cells/ml respectively. Furthermore, the general trend for loss of weight under acidic condition was increase in loss of weight with duration of immersion but the rate of loss decreased with age. The depositions of calcite inside the micro cracks of concrete by B. pumilus were analyzed under scanning electron microscope (SEM). The unique imaging and microanalysis capacities of SEM established the presence of calcite precipitation inside cracks, bacterial impressions and new calcite layer on the concrete surface of concrete. This calcite layer improves the compressive strength and impermeability of the concrete, thereby increasing its resistance to acidic environment especially at 1.5 x 10⁸cells/ml B. pumilus suspension.

Keywords: Bacillus Pumilus, Concrete, Compressive strength and Durability