pozzolanic nature of diatomaceous earth. Since nutrients and microbes (depending
on the amount and the type used) are known to reduce the concrete strength [172],
the use of diatomaceous earth can compensate the potential concrete strength loss
emanating from incorporation of nutrients. This remarkable material has small pores,
and the microbes probably adsorb on the surface instead of getting inside the pores
[209], which makes the protective effect of diatomaceous earth debatable. Thus, it is
assumed that it protects only a portion of the cells/spores used, and this may be the
reason why the healing is slow; a major crack healing was observed after
40 days [209].
The effect of microencapsulated cells/spores on self-healing of concrete has also
been studied. Encapsulation of cells by impermeable microcapsules greatly
enhanced the concrete self-healing efficiency [172]. In this study, it was observed
that the microencapsulated B. sphaericus healed a larger crack area than the control
without bacteria. The encapsulated spore specimen healed 49–80 mm
2 of crack area,
whereas the non-bacterial specimen healed only 13–58 mm
2 of crack area autogenously. The average healed crack area in the bacterial-based system was about 50%
higher than that of the non-bacterial specimen. The maximum crack width healed by
the bacterial-based system was about 970 μm, which is about four times the value of
the non-bacterial specimens. The water permeability in bacteria-containing samples
was about ten times lower than that of the bacteria-free references.
Studies made on using microencapsulated spores revealed that liquid water is
important for efficient healing. For instance, concrete samples maintained at 95%
relative humidity did not show any self-healing [221]. Liquid water is essential for
bacterial activities, and it is needed for germination of spores, hydrolysis of urea by
urease, reaction of carbonate and calcium, and redissolution of nutrients to make
them available for the microbes. Due to these important roles of water in microbialbased self-healing process, studies on self-healing have been done by incubating
cracked specimens at full submersion or wet-dry cycles. However, from application
point of view, such submersion may not be realistic. On the other hand, using
carriers which have the capacity to take up water or moisture from the surroundings
and can keep the water for a long time could be very interesting, and this has brought
the use of hydrogels as water reservoir for microbial-based self-healing system into
focus [11, 217]. Indeed, the use of hydrogels resulted in attractive results. When
Pluronic-based hydrogel-encapsulated microbe-based self-healing system was used,
significant crack closure happened after 7 days. A 0.5 mm crack width was almost
completely healed within 7 days [11], and the healing ratio was about 20–60%
higher than for the specimens without bacteria. Hydrogels can take up moisture from
the surroundings, and this could be desirable for concrete self-healing in the absence
of liquid water (but this has yet to be proven). In principle, hydrogels can be used as
carriers for protection of spores during concrete mixing process and serve as water
reservoir for spore germination and bacterial activity when cracking occurs. The
hydrogel should be easy/safe to make and should not affect the property of the
concrete. Unfortunately, unless it is prepared separately, the use of Pluronic-based
hydrogel requires UV curing which is not good for viability of spores and genetic
stability. Such methods should be avoided. The other disadvantage of this hydrogel
is that it resulted in a drastic reduction in comprehensive strength, a staggering 50%
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
319
on the amount and the type used) are known to reduce the concrete strength [172],
the use of diatomaceous earth can compensate the potential concrete strength loss
emanating from incorporation of nutrients. This remarkable material has small pores,
and the microbes probably adsorb on the surface instead of getting inside the pores
[209], which makes the protective effect of diatomaceous earth debatable. Thus, it is
assumed that it protects only a portion of the cells/spores used, and this may be the
reason why the healing is slow; a major crack healing was observed after
40 days [209].
The effect of microencapsulated cells/spores on self-healing of concrete has also
been studied. Encapsulation of cells by impermeable microcapsules greatly
enhanced the concrete self-healing efficiency [172]. In this study, it was observed
that the microencapsulated B. sphaericus healed a larger crack area than the control
without bacteria. The encapsulated spore specimen healed 49–80 mm
2 of crack area,
whereas the non-bacterial specimen healed only 13–58 mm
2 of crack area autogenously. The average healed crack area in the bacterial-based system was about 50%
higher than that of the non-bacterial specimen. The maximum crack width healed by
the bacterial-based system was about 970 μm, which is about four times the value of
the non-bacterial specimens. The water permeability in bacteria-containing samples
was about ten times lower than that of the bacteria-free references.
Studies made on using microencapsulated spores revealed that liquid water is
important for efficient healing. For instance, concrete samples maintained at 95%
relative humidity did not show any self-healing [221]. Liquid water is essential for
bacterial activities, and it is needed for germination of spores, hydrolysis of urea by
urease, reaction of carbonate and calcium, and redissolution of nutrients to make
them available for the microbes. Due to these important roles of water in microbialbased self-healing process, studies on self-healing have been done by incubating
cracked specimens at full submersion or wet-dry cycles. However, from application
point of view, such submersion may not be realistic. On the other hand, using
carriers which have the capacity to take up water or moisture from the surroundings
and can keep the water for a long time could be very interesting, and this has brought
the use of hydrogels as water reservoir for microbial-based self-healing system into
focus [11, 217]. Indeed, the use of hydrogels resulted in attractive results. When
Pluronic-based hydrogel-encapsulated microbe-based self-healing system was used,
significant crack closure happened after 7 days. A 0.5 mm crack width was almost
completely healed within 7 days [11], and the healing ratio was about 20–60%
higher than for the specimens without bacteria. Hydrogels can take up moisture from
the surroundings, and this could be desirable for concrete self-healing in the absence
of liquid water (but this has yet to be proven). In principle, hydrogels can be used as
carriers for protection of spores during concrete mixing process and serve as water
reservoir for spore germination and bacterial activity when cracking occurs. The
hydrogel should be easy/safe to make and should not affect the property of the
concrete. Unfortunately, unless it is prepared separately, the use of Pluronic-based
hydrogel requires UV curing which is not good for viability of spores and genetic
stability. Such methods should be avoided. The other disadvantage of this hydrogel
is that it resulted in a drastic reduction in comprehensive strength, a staggering 50%
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
319