Bio Immune Steel Reinforced Cementitious Composites in Corrosive Environment

Abstract

The production of self-healing cementitious composites is the most anticipated approach for repair of unavoidable cracks through biological intrusions. The bio-inspired self-healing can systematically heal the cracks and limit the penetration of aggressive ions towards embedded steel. However, there are various bacterial species which could form biofilms on solid substrates and accelerate corrosion of metals especially upon exposure of saline environment. Thus, the microbial tendency to form biofilm and effect of biofilms on corrosion of reinforcing steel along with their calcification ability in marine environments holds several questions which need to be explored further. Therefore, the influence of 0% to 5% NaCl contents on the CaCO3 production by eleven distinct bacterial strains, being used in bio-inspired self-healing concrete, was investigated. Additionally, microbial in-vitro biofilm forming potential in simulated marine environment was also researched. The results indicated that on average 1.5% of NaCl content positively affects the bacterial CaCO3 production capability, moreover, altered the type of produced CaCO3 polymorph, microstructure, and crystallinity. The Bacillus safensis, Corynebacterium efficiens, Planococcus plakortidis and Glutamicibacter mysorens were the optimum strains in developing strong and sticky biofilms and hold potential to precipitate CaCO3 under the stress conditions of chlorides. Whereas the biofilms majorly consisted of extracellular polymeric substances which governed the adhesive characteristics of biofilms. The bacteria in cementitious matrices experience harsh environment due to the high shear stresses offered during mixing, heat exposure during hydration and stresses by the developed hydration products upon hardening. Hence the survival rate of directly added microbes’ declines, therefore, x the immobilization of bacteria through protective carrier compound is beneficial for their longterm survival. In this study, bacteria were immobilized through nano-micro sized bagasse biochar that was competent in housing bacteria inside its pores and imparting superior mechanical properties of cementitious composites. Whereas the biochar was produced through pyrolysis of sugarcane-bagasse at 500oC in the nitrogen environment. Then the four optimum bacterial strains were immobilized through the pyrolytic biochar and incorporated into the mortar for assessment of their calcite precipitating potential, transport attributes, strength development and microstructural characteristics. The bacterial immobilized samples resulted in enhanced compressive strength, flexural strength with boosted strain energy storing potential than control sample, respectively. About 92% gain in compressive strength was observed after 28 days of cracking at 85% of the ultimate compressive strength. Besides, the porosity of modified samples was noted lesser than the control samples. Additionally, a significant increase in relative healing degree was seen during ultrasonic pulse velocity measurements of uncracked and cracked samples. While the healing precipitate was recognized as CaCO3 through scanning electron microscopy and Fourier transform infrared spectroscopy. The percentage porosity analysis was done through binary image processing. The sorptivity trends of the bacterialimmobilized samples were optimum indicating the denser microstructure with minimum porosity alongside the lesser water uptake capability. The corrosion attributes of bacterial and non-bacterial formulations were tested under exposure of 3.5% NaCl as simulated marine environment. The electrochemical testing of uncracked and cracked samples was assessed. The effect of biofilm on embedded steel along with the effect of xi cracking was elucidated. Maximum corrosion inhibition tendency was seen by immobilized bacterial samples due to the filling action of biochar and microbial calcite precipitation in open pores. The cracking leads to higher corrosion rates than the uncracked samples. Still the bacterial immobilized sample was superior to the control sample as the generated cracks were completely healed by the bacterial precipitate and there was lesser contact of chlorides with the steel. The biofilm containing samples behaved superiorly in cracked conditions compared to the immobilized samples. Because the bacteria in the biofilm started precipitated calcite with the access of O2 and further densified the film and protected the steel from corrosion. Thus, the proposed combination of biotic and abiotic materials would be a good solution for recycling waste and enhancing mechanical and durability prospects of reinforced cementitious composites.