Microbial Self-Healing Concrete: Denitrification as an Enhanced and Environment-Friendly Approach

ID No : 43 ABSTRACT Concrete tends to crack due to its relatively low tensile strength which jeopardizes the good condition of steel reinforcement bars. Concrete cracks start at micro level and facilitate ingression of corrosive substances such as SO4 , Cl, O2. Corrosion of the rebar may result in failure of the structure. Self-healing concrete can provide crack repair immediately after crack initiation. One of the self-healing mechanisms is the use of bacteria to induce CaCO3 precipitation in the crack environment. So far two different bio-chemical pathways, aerobic oxidation of lactate and ureolysis were used for microbial self-healing concrete. Despite the reported successful results, there are critical drawbacks of these two metabolic pathways to be improved such as oxygen limited performances, toxic byproducts, odor and negative effects on mechanical properties of concrete. To overcome these issues, we proposed an alternative pathway based on microbial NO3 reduction (denitrification) which also leads to CaCO3 precipitation.Concrete tends to crack due to its relatively low tensile strength which jeopardizes the good condition of steel reinforcement bars. Concrete cracks start at micro level and facilitate ingression of corrosive substances such as SO4 , Cl, O2. Corrosion of the rebar may result in failure of the structure. Self-healing concrete can provide crack repair immediately after crack initiation. One of the self-healing mechanisms is the use of bacteria to induce CaCO3 precipitation in the crack environment. So far two different bio-chemical pathways, aerobic oxidation of lactate and ureolysis were used for microbial self-healing concrete. Despite the reported successful results, there are critical drawbacks of these two metabolic pathways to be improved such as oxygen limited performances, toxic byproducts, odor and negative effects on mechanical properties of concrete. To overcome these issues, we proposed an alternative pathway based on microbial NO3 reduction (denitrification) which also leads to CaCO3 precipitation. We tested two axenic NO3 reducing cultures, Pseudomonas aeruginosa and Diaphorobacter nitroreducens for microbial self-healing through denitrification. Bacterial agents were encapsulated within expanded clay particles (0.5-2mm) and tested for multiple crack closure (crack range 100-500 μm) in mortar specimens under wet and wet/humid conditions. Under wet conditions, specimens with bacteria could completely close the cracks up to 250 μm in 2 weeks and 350 μm in 4 weeks. After 4 weeks, for 235 ± 35 μm original crack width, mortar prisms containing P. aeruginosa and D. nitroreducens absorbed 47% and 51% less water than reference specimen, respectively. Overall, the denitrification pathway was found to be as effective as existing methods while it is more environment-friendly.