Complementing Urea Hydrolysis and Nitrate Reduction Matabolisms to Enhance Microbial Self-Healing Perfromance in Cementitıous Composites

Abstract

Concrete is a commonly used material in construction, but it tends to crack due to many reasons. When the size and number of cracks exceed a certain threshold, they threat concrete durability and shorten its lifespan. Since recycling of the concrete components is difficult, it leads to the accumulation of concrete waste worldwide. Researchers suggesting the use of bacteria and exploitation of biomineralization to repair concrete cracks and to extend the service life of structures. This phenomenon is known as "bioconcrete." In this study, the use of biogranules, non-axenic microbial granules capable of urease hydrolysis and nitrate reduction, is proposed for the improvement of concrete crack healing. These granules were produced in a cylindrical sequencing batch reactor (SBR) at laboratory scale. The granules were then harvested, and their resuscitation performance was tested after drying. The study showed that biogranules can be stored in a dried form and reactivated when needed. Biogranules efficiently consumed urea and NO3-N. The study determined that biogranules had the capacity to consume 1 g/L of urea in 6 hours and 200 mg/L of NO3-N in 3 hours. Furthermore, the study revealed that the granule production process can be adapted to minimal nutrient conditions (Phase I) and alkaline pH conditions (Phase 2) as well as enables regular granule harvesting (Phase 3). The analyses and granule samples obtained from the reactor demonstrated its ability to adapt to these conditions effectively. After the resuscitation and confirmation of the activity of the dried biogranules, they were added to cementitious composites. The cementitious composites were cracked in a controlled manner to obtain crack widths ranging from 100±20 µm to 600±30 µm in the samples. These cracks were observed weekly under a light microscope. By adding these biogranules to the cementitious composites, biogranules contained bacteria capable of consuming nutrients that entered the cracks with water, such as urea and nitrate, and producing calcium carbonate, which is a natural mineral that can fill and repair cracks. This research explores the crack healing performance of biotic samples in diverse environmental conditions. Notably, crack healing thresholds reaching 90% recovery were highest in rainwater, tap water, and seawater, measuring at 156, 230, and 253 µm, respectively. Further analysis revealed varying closure percentages within specific crack width ranges, with rainwater, tap water, and seawater exhibiting distinct behaviors. Additionally, a 70 µm disparity between biotic and abiotic samples in marine water signifies the extent of microbial healing, while rainwater showed no significant advantage for both abiotic and biotic healing at higher percentages. In tap water, microbial healing was observed at 55 µm. In particular, in rainwater, microbial healing at the 80% healing threshold was approximately 70 µm. These findings shed light on the environmental factors influencing biomortar crack healing and constitute a study on the application of granular bacterial concrete.