Self-Healing Concrete Using Innovative Bio-Inspired Self-Healing Processes

Abstract

Autonomous healing of concrete composites via microbially induced calcite precipitation (MICP) emanates as an imperative remedial mode for plugging the fissures and pores which are induced by the stresses. However, this contemporary recurrent healing system is susceptible to microbial depletion in the highly alkaline cementitious environment. Therefore, researchers are probing for high alkali resistant calcifying microbes. In the present study, alkaliphilic microbes were isolated from different soil sources and screened for probable CaCO3 precipitation at pH 10. Non-ureolytic pathway (oxidation of organic carbon) was adopted for calcite precipitation to eliminate the production of toxic ammonia. For this purpose, calcium lactate Ca(C3H5O3)2 and calcium acetate Ca(CH3COO)2 were used as CaCO3 precipitation precursors. The quantification protocol for precipitated CaCO3 was established to select 10 superlative morphologically distinct microbial species for implementation in the alkaline cementitious systems besides 16SrRNA analysis. Sequenced microbes were identified as species of Bacillus, Arthrobacter, Planococcus, Chryseomicrobium, and Corynebacterium. Further, microstructure of precipitated CaCO3 was inspected through scanning electron microscopy (SEM), Xray diffraction (XRD) and thermal gravimetric (TG) analysis. It is pertinent to mention that Bacillus, Arthobacter and Planocuccus species have been reported as CaCO3 producers but Chryseomicrobium and Glutamicibacter were reported first time in the current research as CaCO3 precipitators After effective in-vitro performance of isolated strains, five high endurance alkaliphilic species were evaluated for their prolonged survival in cementitious environment via direct induction. Extensive experimental program was designed to examine mechanical, self-healing, microstructural modifications, durability and energy performance of self-healing concrete (SHC). Output of experimental program confirmed the survival of these isolated species in the harsh concrete environment. All investigated strains were capable of precipitating copious amount of calcite in formulated cracks with maximum of average crack healing rate of 0.8 mm. Densification of microstructure was evident from the microstructural evaluation and pore refinement. SHC offered a significant resistance against Cl- penetration and sulphate attack. Considering the fact of bacterial crushing inside the hard concrete matrix, most compatible bacterial strain was selected among five strains based on direct intrusion results for further enhancement the viability of bacteria via immobilization. For immobilization of selected bacterial strain, one mineral and one fibrous carrier media namely alumina and cellulose were opted. Concerning the dependency xv of SHC efficiency against the immobilizers size, carrier media were used in three variable sizes for optimization of grain size. Further, for provision of healing agent homogeneously inside the concrete matrix, a well dispersion scheme was devised having equally dispersed nano, micro and milli-sized particles. Results endorsed the effect of immobilizers size on SHC efficacy as well-graded dispersion of immobilizers media was effective remedy for all small as well as large crack widths. The immobilization of bacteria filled the wider cracks as compared to direct induction of microbes. Likewise, nano-sized carrier and uniform dispersion was better among all. 95% recovery rate was observed. Mineral carrier media portrayed good results against strength enhancement and regain in strength. Whereas, polymer media gave more crack healed widths. Taking into account the importance and diversity of civil structures, several curing methods with their specified durations have been suggested. So, in the last part of the study, the effect of curing condition on SHC efficacy was monitored against CO2, water curing, air curing and steamed autoclave curing using B.pumilus. As, modern accelerated curing techniques can open doors for industrialization of SHC. Among all mixes, carbonated bacterial concrete gave most propitious results of self-healing by filling average crack width of 0.8 mm with 97 % compressive strength regain. Overall, water cured bacterial concrete stood out as most durable having highest resistance towards water absorption, acid attack and chloride ion mitigation. Whereas air-cured normal concrete exhibited least resistance towards the same durability testing schemes. Mixes of autoclaved curing were observed to be most energy efficient having least value of coefficient for thermal conductivity. Conclusively, this study contributing the addition of novel calcifying alkaliphilic strains which were capable of producing copious amount of calcite using active pathway inside the cementitious matrix. Further, a strategy was devised for homogenized dispersion of healing agent inside the matrix. Additionally, isolated strains were viable against autoclave and CO2 curing conditions. Moreover, SHC formulated using isolated strains both directly intruded and immobilized have potential to impart sustainability in concrete structures by extending structural life, subsiding repairs cost, conserving natural resources and improving energy performance