the subsequent calcite precipitation. Since the organisms used in these studies such
as S. pasteurii are aerobic and accumulates the calcite relatively faster at the surface
where there is no oxygen limitation. The other reason is directly related to the calcite
accumulated at the crack surface, which becomes a barrier for the ongoing diffusion
of calcium, urea, and even water. Hence, the calcite precipitation in the deeper part of
the crack will dwindle and with it diminishes the healing efficiency. To tackle this
problem, Bang et al. [162] investigated the use of polyurethane-immobilized
B. pasteurii to repair cracks. The pores in polyurethane foam can both increase the
surface area and minimize the diffusion limitation for calcium, urea, water, and
oxygen and thus promote the formation of calcite in the deeper part of the crack.
B. pasteurii cells were immobilized on polyurethane strips and manually placed in
artificial cracks (width ¼ 3.18 mm, depth ¼ 25.4 mm) in the cement mortar. This
preparation was immersed in the urea-CaCl 2 solution for 28 days. Although a
remarkable compressive strength increase was achieved in the specimens with the
immobilized cells compared to the cell-free controls, it was observed that the calcite
precipitation did not improve the tensile strength and the stiffness of the treated
specimen. This may be due to the lack of chemical bonding between the polyurethane matrix and the concrete crack surface. The polyurethane and the associated
precipitated calcite simply plugged the crack but did not make the desired kind of
sealing. Thus, it is a possibility that the concrete treated with microbial agents may
not gain its full strength. It will be beneficial to look for a means that can improve
calcite strength and reduce its brittleness such as through composite formation.
Another issue that needs attention is the filling material that can be used with the
microbial-based agent in repairing concrete cracks. Although the filling materials
play an important role, little has been done on its screening and selection. Even
among the limited work done, it seems that some of them led to ambiguity. For
example, S. sphaericus immobilized on silica gel was manually injected into concrete cracks, and the specimens were immersed into urea and calcium source
solution [4, 13]. The permeability of water was greatly reduced in the samples
treated with silica gel-immobilized cells as well as specimens treated with cell-free
silica gel. This indicates that the effect that resulted in reduced permeability was the
filling of the crack by silica gel. However, the authors believe that the calcite
precipitated inside the silica gel can enhance the durability of the repair material.
Researchers have also tried concrete crack fixing without any filling materials, by
immersing the cracked concrete specimens into a microbial medium which consisted
of bacterial cells, urea, and CaCl 2 for 28 days. In such a way, it was possible to fill
the entire crack by the precipitating calcite [173]. Although the authors have shown
the crack sealing potential of calcite without any other filler, they failed to mention
the size of the cracks in their specimen. However, considering that it happened
without other filling materials, one can assume that the cracks were narrow and
shallow.
Almost all the studies that have been reported are lab-scale investigations, often
carried out in ideal conditions where there is no limitation of calcium supply, urea, or
nutrients to support the activities of the microorganisms. It is obvious that there is a
lot of data trickling in from such studies and enriching the field with substantial
amount of information. However, it is very important to make field trial studies that
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
309
as S. pasteurii are aerobic and accumulates the calcite relatively faster at the surface
where there is no oxygen limitation. The other reason is directly related to the calcite
accumulated at the crack surface, which becomes a barrier for the ongoing diffusion
of calcium, urea, and even water. Hence, the calcite precipitation in the deeper part of
the crack will dwindle and with it diminishes the healing efficiency. To tackle this
problem, Bang et al. [162] investigated the use of polyurethane-immobilized
B. pasteurii to repair cracks. The pores in polyurethane foam can both increase the
surface area and minimize the diffusion limitation for calcium, urea, water, and
oxygen and thus promote the formation of calcite in the deeper part of the crack.
B. pasteurii cells were immobilized on polyurethane strips and manually placed in
artificial cracks (width ¼ 3.18 mm, depth ¼ 25.4 mm) in the cement mortar. This
preparation was immersed in the urea-CaCl 2 solution for 28 days. Although a
remarkable compressive strength increase was achieved in the specimens with the
immobilized cells compared to the cell-free controls, it was observed that the calcite
precipitation did not improve the tensile strength and the stiffness of the treated
specimen. This may be due to the lack of chemical bonding between the polyurethane matrix and the concrete crack surface. The polyurethane and the associated
precipitated calcite simply plugged the crack but did not make the desired kind of
sealing. Thus, it is a possibility that the concrete treated with microbial agents may
not gain its full strength. It will be beneficial to look for a means that can improve
calcite strength and reduce its brittleness such as through composite formation.
Another issue that needs attention is the filling material that can be used with the
microbial-based agent in repairing concrete cracks. Although the filling materials
play an important role, little has been done on its screening and selection. Even
among the limited work done, it seems that some of them led to ambiguity. For
example, S. sphaericus immobilized on silica gel was manually injected into concrete cracks, and the specimens were immersed into urea and calcium source
solution [4, 13]. The permeability of water was greatly reduced in the samples
treated with silica gel-immobilized cells as well as specimens treated with cell-free
silica gel. This indicates that the effect that resulted in reduced permeability was the
filling of the crack by silica gel. However, the authors believe that the calcite
precipitated inside the silica gel can enhance the durability of the repair material.
Researchers have also tried concrete crack fixing without any filling materials, by
immersing the cracked concrete specimens into a microbial medium which consisted
of bacterial cells, urea, and CaCl 2 for 28 days. In such a way, it was possible to fill
the entire crack by the precipitating calcite [173]. Although the authors have shown
the crack sealing potential of calcite without any other filler, they failed to mention
the size of the cracks in their specimen. However, considering that it happened
without other filling materials, one can assume that the cracks were narrow and
shallow.
Almost all the studies that have been reported are lab-scale investigations, often
carried out in ideal conditions where there is no limitation of calcium supply, urea, or
nutrients to support the activities of the microorganisms. It is obvious that there is a
lot of data trickling in from such studies and enriching the field with substantial
amount of information. However, it is very important to make field trial studies that
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
309