concrete structures. Studies made on the effect of NO 2
À on corrosion have been
made using steel coupons. Results from the studies indicated effective corrosion
inhibition of steel coupons due to the NO 2
À accumulation during biological NO 3
À
reduction by denitrifiers [227]. In line with this, non-axenic culture of ACDC
reduced corrosion in corrosive electrolyte solution (0.05 M NaCl) by producing
about 57 mM NO 2
À in 1 week [226]. Moreover, corrosion experiments have been
carried out by immersing cracked concretes specimens containing an embedded steel
rod in 0.4 M chloride solution that mimics the chloride concentrations in seawater.
Moreover, a 0.3 mm crack was completely healed. The use of ACDC system that
generates corrosion inhibitor could postpone the time to corrosion initiation by a
factor of 2.4 [19].
The solubility of the electron acceptor is an important parameter in determining
the efficiency of the system. Since the solubility of O 2 in water (9.1 mg/L at 20
C) is
up to 105 times lower than that of NO 3
À , the nitrate reduction seems more effective,
and it is expected to be accompanied by an efficient calcite precipitation. The higher
solubility of nitrate may favor the use of NO 3
À as an electron acceptor instead of
O 2 during microbial oxidation of organic matter. Even enhanced CaCO 3 precipitation can be potentially achieved in nutrient-poor environments which makes the
denitrification-based self-healing system feasible for self-healing concrete [59, 227].
This self-healing system is particularly attractive for applications in anoxic zones or
healing deep cracks. However, it has been shown that the CaCO 3 precipitation
efficiency of the denitrification system is much lower than that of ureolysis process
[65]. Moreover, if the reduction process is incomplete, it may produce the nasty
by-product N 2 O, especially under aerobic condition [228]. Thus, it may be necessary
to study the microbes intended for this application if they produce these highly
potent ozone-depleting greenhouse gas during the healing process. So far, there is no
available information in this regard.
4 Factors That Determine the Efficiency of Microbes
in Enhancing Concrete Durability
The success of microbial preparations in maintenance and repair of concrete structures primarily relies on the efficiency of calcite precipitation. Usually, the more the
precipitate is, the more efficient it becomes. To effectively seal cracks or coat
the surface of a structure, it is important that the calcite precipitated is enough.
The efficiency of MICCP is affected by several factors such as the type of organism
used, the cell load, pH of the precipitation environment, temperature, concentration
of calcium and carbonate, etc. [229–232]. These factors are important in microbialbased concrete, manual repair, self-healing, and biodeposition applications, and are
discussed below.
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
323
À on corrosion have been
made using steel coupons. Results from the studies indicated effective corrosion
inhibition of steel coupons due to the NO 2
À accumulation during biological NO 3
À
reduction by denitrifiers [227]. In line with this, non-axenic culture of ACDC
reduced corrosion in corrosive electrolyte solution (0.05 M NaCl) by producing
about 57 mM NO 2
À in 1 week [226]. Moreover, corrosion experiments have been
carried out by immersing cracked concretes specimens containing an embedded steel
rod in 0.4 M chloride solution that mimics the chloride concentrations in seawater.
Moreover, a 0.3 mm crack was completely healed. The use of ACDC system that
generates corrosion inhibitor could postpone the time to corrosion initiation by a
factor of 2.4 [19].
The solubility of the electron acceptor is an important parameter in determining
the efficiency of the system. Since the solubility of O 2 in water (9.1 mg/L at 20
C) is
up to 105 times lower than that of NO 3
À , the nitrate reduction seems more effective,
and it is expected to be accompanied by an efficient calcite precipitation. The higher
solubility of nitrate may favor the use of NO 3
À as an electron acceptor instead of
O 2 during microbial oxidation of organic matter. Even enhanced CaCO 3 precipitation can be potentially achieved in nutrient-poor environments which makes the
denitrification-based self-healing system feasible for self-healing concrete [59, 227].
This self-healing system is particularly attractive for applications in anoxic zones or
healing deep cracks. However, it has been shown that the CaCO 3 precipitation
efficiency of the denitrification system is much lower than that of ureolysis process
[65]. Moreover, if the reduction process is incomplete, it may produce the nasty
by-product N 2 O, especially under aerobic condition [228]. Thus, it may be necessary
to study the microbes intended for this application if they produce these highly
potent ozone-depleting greenhouse gas during the healing process. So far, there is no
available information in this regard.
4 Factors That Determine the Efficiency of Microbes
in Enhancing Concrete Durability
The success of microbial preparations in maintenance and repair of concrete structures primarily relies on the efficiency of calcite precipitation. Usually, the more the
precipitate is, the more efficient it becomes. To effectively seal cracks or coat
the surface of a structure, it is important that the calcite precipitated is enough.
The efficiency of MICCP is affected by several factors such as the type of organism
used, the cell load, pH of the precipitation environment, temperature, concentration
of calcium and carbonate, etc. [229–232]. These factors are important in microbialbased concrete, manual repair, self-healing, and biodeposition applications, and are
discussed below.
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
323