Preliminary research for long lasting self-healing effect of bacteria-based concrete with lightweight aggregates

A typical phenomenon in concrete structures is crack formation. Shrinkage cracks may appear in early stage of concrete. In late age cracks may appear due to process of serviceability and external weather conditions. Larger cracks interfere structural integrity, micro-cracks may result in durability problems as increase of water absorption, which results in concrete degradation and reinforcement corrosion. Manual maintenance of the concrete structure is costly and labour consuming; therefore, functionality of self-healing phenomenon of concrete structures has been inspected. In this study self-healing phenomenon of bacteria-based concrete has been investigated. Calcium lactate and yeast medium with spore-forming alkaliphilic bacteria Bacillus Lentus have been chosen. Bacillus Lentus spores with medium were integrated (by using vacuum technique) into expanded clay granules (LECA) with a size of 2-4 mm. Direct addition of bacteria (without immobilization) in concrete limits the survival of bacteria from 1 to 2 months. In this study, to increase viability of bacteria at high pH media in concrete and thus the time-related functionality of self-healing agents, bacteria were immobilized in LECA. The mechanism of crack healing in bacteria-based concrete presumably occurs through metabolic conversion of calcium lactate to calcium carbonate. The influence of bacteria on the mechanical and physical properties of samples was estimated after 28 days and after 6 years. From the results obtained, it can be concluded that concrete samples with Bacillus Lentus showed an increase of 22% in compressive strength compared to the reference sample. The main task of this scientific research is to identify, if self-healing phenomenon happens for the samples, which are 28 days old and for samples which are 6 years old. Results from the optical investigation showed that self-healing phenomenon (after 14 days of healing) has occurred for all samples and cracks in concrete were partly closed with calcium carbonate.