Materialia 15 (2021) 101051 Contents lists available at ScienceDirect Materialia journal homepage: www.elsevier.com/locate/mtla Full Length Article Comprehensive microbiological studies on screening bacteria for self-healing concrete Chereddy Sonali Sri Durga a,∗ , Nerella Ruben a , Madduru Sri Rama Chand b , Mikkili Indira c , Chava Venkatesh a a Department of Civil Engineering, Vignan’s Foundation for Science, Technology & Research, Deemed to be University, Vadlamudi, Andhra Pradesh, India b Department of Civil Engineering, Sree Chaitanya College of Engineering, Karimnagar, Telangana, India c Department of Biotechnology, Vignan’s Foundation for Science, Technology & Research, Deemed to be University, Vadlamudi, Andhra Pradesh, India a r t i c l e i n f o Keywords: Bacterial solution Calcite crystals Gram staining Microbes Nutrient broth medium a b s t r a c t The present study majorly focused on the selection of suitable bacteria through microbiological studies for con- crete crack healing. However, Bacillus species of non-pathogenic type bacteria’s Viz., Bacillus subtilis, Bacillus cereus, Bacillus licheniformis, and Bacillus halodurans were used as self-healing agents. Also conducted various mi- crobiological studies such as gram staining process, biofilm, Exopolysaccharide, Endospores, Urease Enzyme, and Calcium Carbonate formations. Similarly, the growth pattern of bacteria was measured in different temperatures (25 °C, 30 °C, 35 °C, 37 °C, and 60 °C), environments (such as p H of 3,7,8,9,10,11,12,13, and 14), and concrete composition medium. The gram staining process has identified that the above four bacteria have gram-positive strains and a rod-like shape. The B.halodurans and B.licheniformis have shown more spore formation, urease en- zyme production, Exopolysaccharide production, biofilm formation, and calcium carbonate formation than other cultures. The B.licheniformis has shown the highest growth rate at various temperatures and environmental con- ditions. However, in the case of concrete composition medium B.halodurans has shown the highest growth rate. To determine the above four bacteria’s crack healing ability, 40 MPa strengthened concrete samples are prepared and, then 65% and 75% of stress level concentration were applied to form cracks. However, Ultra-sonic pulse velocity values and water absorption values are taken for the concrete specimens to assess the healing capability. From the obtained results, B.halodurans has shown an ultrasonic pulse velocity value of 4.6 km/s and with 1.71% of water absorption, which is optimum among other bacterial cultures. The present study strongly recommends using B.halodurans as self-healing agents in concrete repair works. 1. Introduction The disintegration of concrete due to cracking is one of the major cul- prits endangering the durability, safety, and integrity of concrete struc- tures [1]. To overcome this situation, self-healing thought can be in- corporated into cement concrete and asphalt concrete structures [2,3]. In the active mode of self-healing (autogenous self-healing) [4], the hu- man interface is not required as unreacted cement grains may react with moisture exposure and seal the cracks. On the other hand, in the pas- sive mode mentioned as autonomous self-healing, a human interface is required as the self-healing material is external material added to heal cracks [5]. This autonomous self-healing process can be broadly classified into various categories like intrinsic self-healing [6], micro- bial self-healing [7,8], capsule-based self-healing [9,10], and vascular self-healing [11]. The utilization of all these materials reduces cement production and the replacement of structures [12]. A new self-healing ∗ Corresponding author. E-mail address: chereddysony@gmail.com (C.S. Sri Durga). approach was developed as the fungi can also be used as a healing agent to fill the concrete’s cracks [13,14]. Autogenous self-healing can only heal narrow cracks whose widths are <0.3mm-polymeric material fills cracks in concrete by converting healing agents to foam in the presence of moisture. Although the chem- icals released from hollow fibers embedded in a concrete matrix can heal cracks, these materials are incompatible with concrete composi- tions due to variance in mechanical properties and thermal expansion coefficients, leading to enlargement and propagation of existing cracks [15]. Out of all these approaches, biomineralization has drawn many researchers’ attention as it is a sustainable, biodegradable one and pos- sesses eco-friendly nature [16]. Owing to the above-addressed limitation of autogenous self-healing and vascular self-healing, using of biological approach through calcium mineral-producing microbes becomes par- ticularly enviable. The thought of microbial self-healing has come from biological systems, as many plants and animals can recover from serious damage to a full extent. This thought has been implemented for many https://doi.org/10.1016/j.mtla.2021.101051 Received 25 December 2020; Accepted 16 February 2021 Available online 23 February 2021 2589-1529/© 2021 Acta Materialia Inc. Published by Elsevier B.V. All rights reserved.