The standard procedure used in mending concrete follows monitoring, detecting,
and repairing steps. Thus, the cracks must be discovered to perform the repair.
However, this is not an easy task. High tensile stresses due to external loads, volume
shrinkage that imposes deformations (e.g., due to temperature gradient or confined
shrinkage), or volume expansion (initiated by reinforcement corrosion, alkali silica
reaction, or sulfate attack) often result in micro-cracks in the concrete structure
[8, 9]. Some of these micro-cracks (with width of less than 300 μm) may autogenously heal [10, 11], but the rest requires human intervention. However, the microcracks could be too small for visual detection or be located deep inside the concrete
structure, making early detection and subsequent repair almost impossible. On the
other hand, if these cracks, even the smallest ones, are not repaired in time, they often
expand further and can reach to the reinforcement bar [12]. The cracks provide
passage for water and gases that may contain aggressive substances such as chlorine,
sulfate, carbon dioxide, and oxygen that attack both the concrete and its reinforcement [9, 12]. This attack potentially impairs the strength and durability of concrete
structures. Thus, if prolonging the service life of the concrete structures is of interest,
it is ideal that the micro-cracks which are precursors for concrete structural failures
[13] should be repaired in time. However, repair of these small cracks is difficult not
only due to invisibility but also by inaccessibility. Even if the micro-cracks are
discovered, proper application of the repair agents to these cracks is very difficult.
The repair agents are applied from outside and should penetrate to reach the internal
cracks. Although this approach is quite efficient for repairing large cracks, it is not
suitable for small and deep cracks, which often remain inaccessible to the applied
repair agent. Therefore, there has been interest for alternative repair methods that
potentially are suitable to reach all sorts of cracks in concrete structures. This
challenge has partly contributed to the emergence of the idea of developing concretes with self-healing properties [1, 14–17]. Currently, one of the rapidly
expanding research areas in the field of concrete materials is the development of
self-healing concrete, and the use of microbes as self-healing agents has become a
very attractive strategy [18, 19]. In fact, the potential of microbes in making smart
concrete that autonomously heals its crack is enormous. The conventional, concrete
repair is labor intensive, and its direct and indirect costs are very high. Thus, the use
of concrete that fixes its crack independent of human intervention is expected to have
remarkable economic, environmental, and technical advantages.
Microbial-based self-healing concrete potentially seals cracks without human
intervention they are formed anywhere in the structure and when they are yet
small. Thus, in addition to alleviating inaccessibility, it avoids the difficult and
costly monitoring and detection of cracks in traditional crack repair procedure. In
this approach, the repair agents are added during concrete mixing and casting,
and upon crack formation, the repair agents seal the crack autonomously [20].
Alkaliphiles have also been tried as surface coating agents for concrete structures.
The application of microbial-based preparation on the surface forms calcite which
coats and plugs openings (i.e., pores and cracks). This limits the water and aggressive substance ingress to the subsurface of the concrete structure [21].
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
297
and repairing steps. Thus, the cracks must be discovered to perform the repair.
However, this is not an easy task. High tensile stresses due to external loads, volume
shrinkage that imposes deformations (e.g., due to temperature gradient or confined
shrinkage), or volume expansion (initiated by reinforcement corrosion, alkali silica
reaction, or sulfate attack) often result in micro-cracks in the concrete structure
[8, 9]. Some of these micro-cracks (with width of less than 300 μm) may autogenously heal [10, 11], but the rest requires human intervention. However, the microcracks could be too small for visual detection or be located deep inside the concrete
structure, making early detection and subsequent repair almost impossible. On the
other hand, if these cracks, even the smallest ones, are not repaired in time, they often
expand further and can reach to the reinforcement bar [12]. The cracks provide
passage for water and gases that may contain aggressive substances such as chlorine,
sulfate, carbon dioxide, and oxygen that attack both the concrete and its reinforcement [9, 12]. This attack potentially impairs the strength and durability of concrete
structures. Thus, if prolonging the service life of the concrete structures is of interest,
it is ideal that the micro-cracks which are precursors for concrete structural failures
[13] should be repaired in time. However, repair of these small cracks is difficult not
only due to invisibility but also by inaccessibility. Even if the micro-cracks are
discovered, proper application of the repair agents to these cracks is very difficult.
The repair agents are applied from outside and should penetrate to reach the internal
cracks. Although this approach is quite efficient for repairing large cracks, it is not
suitable for small and deep cracks, which often remain inaccessible to the applied
repair agent. Therefore, there has been interest for alternative repair methods that
potentially are suitable to reach all sorts of cracks in concrete structures. This
challenge has partly contributed to the emergence of the idea of developing concretes with self-healing properties [1, 14–17]. Currently, one of the rapidly
expanding research areas in the field of concrete materials is the development of
self-healing concrete, and the use of microbes as self-healing agents has become a
very attractive strategy [18, 19]. In fact, the potential of microbes in making smart
concrete that autonomously heals its crack is enormous. The conventional, concrete
repair is labor intensive, and its direct and indirect costs are very high. Thus, the use
of concrete that fixes its crack independent of human intervention is expected to have
remarkable economic, environmental, and technical advantages.
Microbial-based self-healing concrete potentially seals cracks without human
intervention they are formed anywhere in the structure and when they are yet
small. Thus, in addition to alleviating inaccessibility, it avoids the difficult and
costly monitoring and detection of cracks in traditional crack repair procedure. In
this approach, the repair agents are added during concrete mixing and casting,
and upon crack formation, the repair agents seal the crack autonomously [20].
Alkaliphiles have also been tried as surface coating agents for concrete structures.
The application of microbial-based preparation on the surface forms calcite which
coats and plugs openings (i.e., pores and cracks). This limits the water and aggressive substance ingress to the subsurface of the concrete structure [21].
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
297