carbonate interacts with the calcium and results in precipitation of CaCO 3 . Since
direct addition of spores and nutrient to concrete significantly reduce the viability of
spores as well as the concrete strength, Wiktor and Jonkers [10] used porous
expanded clay for immobilization of the microbial agents. The clay aggregate, in
addition to immobilizing the microbial agents and the nutrient-calcium sources
such as Ca-lactate, serves as a structural constituent of concrete. However, a slight
decrease in the concrete strength has been observed. The concrete specimens with
expanded clay immobilized spores and nutrients were immersed in water for
100 days. The oxygen consumption study revealed that the immobilization enhanced
the spore viability. The immobilized spores exhibited remarkably efficient healing
after 40 days. The study reported that the maximum crack width healed was
0.46 mm, which was about two times more than the reference specimens. The
interesting observation the authors stated is that there was no difference in the
healing efficiency in the first 20 days between the samples with microbial agents
and the reference samples without the bioagent. The possible reasons could be that it
took relatively longer time for the spores to germinate and become active cells, or
most of the spores lost viability, and the remaining few cells had to propagate to
reach critical mass that produces extra CaCO 3 to seal the cracks.
In general, this approach of using aerobic oxidation of organic acids to precipitate
calcite is interesting as it does not have any undesirable by-product and it is more
sustainable. When crack happens, the immobilized microbes released by breakage of
their protective capsules or porous materials. The released microbes will be activated
by the incoming water and produce CaCO 3 that heals the cracks. However, the
system is not as efficient as the ureolytic process and requires relatively high
concentrations of calcium source [225] which could possibly lead to undesirable
buildup of high level of salts in concrete. Furthermore, the efficiency of this
approach can be limited in healing deep cracks or concretes with low oxygen
environments such as underground structures. On the other hand, this system is
the first one tried in field application [175]. The irrigation canals in Ecuadorian
highland had a cracking problem, and the use of microbial-based self-healing
concrete has been proposed to solve the daunting problem. In 2014, self-healing
concrete mix prepared from locally available ingredients and healing agents was
used to make new concrete lining of canals, and the result is very encouraging.
3.3.3 Denitrification-Based Self-Healing System
The third system that has been used in developing self-healing concrete is the
dissimilatory nitrate reduction process. This process is primarily accomplished by
facultative heterotrophic microbes which enzymatically reduce nitrate to N 2 (Fig. 5).
The heterotrophs require organic carbon source to fuel its metabolic activity and get
O 2 for respiration either from air (atmospheric or water) or by reducing nitrate
(upon depletion of O 2 ). Thus, denitrification occurs during anoxic condition at
which nitrate is used as electron acceptor. The identification process, in the presence
of organic carbon, generates CO 2 and OH
À (Fig. 5), which leads, respectively, to
formation of CO 3
2- and alkalinity, conditions necessary for calcite precipitation.
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
321
direct addition of spores and nutrient to concrete significantly reduce the viability of
spores as well as the concrete strength, Wiktor and Jonkers [10] used porous
expanded clay for immobilization of the microbial agents. The clay aggregate, in
addition to immobilizing the microbial agents and the nutrient-calcium sources
such as Ca-lactate, serves as a structural constituent of concrete. However, a slight
decrease in the concrete strength has been observed. The concrete specimens with
expanded clay immobilized spores and nutrients were immersed in water for
100 days. The oxygen consumption study revealed that the immobilization enhanced
the spore viability. The immobilized spores exhibited remarkably efficient healing
after 40 days. The study reported that the maximum crack width healed was
0.46 mm, which was about two times more than the reference specimens. The
interesting observation the authors stated is that there was no difference in the
healing efficiency in the first 20 days between the samples with microbial agents
and the reference samples without the bioagent. The possible reasons could be that it
took relatively longer time for the spores to germinate and become active cells, or
most of the spores lost viability, and the remaining few cells had to propagate to
reach critical mass that produces extra CaCO 3 to seal the cracks.
In general, this approach of using aerobic oxidation of organic acids to precipitate
calcite is interesting as it does not have any undesirable by-product and it is more
sustainable. When crack happens, the immobilized microbes released by breakage of
their protective capsules or porous materials. The released microbes will be activated
by the incoming water and produce CaCO 3 that heals the cracks. However, the
system is not as efficient as the ureolytic process and requires relatively high
concentrations of calcium source [225] which could possibly lead to undesirable
buildup of high level of salts in concrete. Furthermore, the efficiency of this
approach can be limited in healing deep cracks or concretes with low oxygen
environments such as underground structures. On the other hand, this system is
the first one tried in field application [175]. The irrigation canals in Ecuadorian
highland had a cracking problem, and the use of microbial-based self-healing
concrete has been proposed to solve the daunting problem. In 2014, self-healing
concrete mix prepared from locally available ingredients and healing agents was
used to make new concrete lining of canals, and the result is very encouraging.
3.3.3 Denitrification-Based Self-Healing System
The third system that has been used in developing self-healing concrete is the
dissimilatory nitrate reduction process. This process is primarily accomplished by
facultative heterotrophic microbes which enzymatically reduce nitrate to N 2 (Fig. 5).
The heterotrophs require organic carbon source to fuel its metabolic activity and get
O 2 for respiration either from air (atmospheric or water) or by reducing nitrate
(upon depletion of O 2 ). Thus, denitrification occurs during anoxic condition at
which nitrate is used as electron acceptor. The identification process, in the presence
of organic carbon, generates CO 2 and OH
À (Fig. 5), which leads, respectively, to
formation of CO 3
2- and alkalinity, conditions necessary for calcite precipitation.
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
321