ingress. There are many alternative chemical-based products which upon application
to cracks can stop water ingress and the subsequent physical deterioration. However,
it has been shown that some of these conventional crack repair methods such as the
application of chemicals and polymers are sources of health- and environmental
risks, and more importantly, they are effective only for short term due to several
factors such as difference in the thermal expansion coefficient of the repairing
material and susceptibility to biological and chemical degradation [2–5]. Thus,
treatment methods that are environmentally benign, safe, long lasting, and sustainable are in high demand. The biological way of mending concrete cracks by biogenic
CaCO 3 precipitation is potentially cheap, exhibit compatible properties to concrete,
and it is environmentally benign and safe.
Biomineralization can be used in two ways to fix cracks in concrete. In one of
these methods, microbial cells together with nutrients, calcium source, and urea
(in most studies) are added into the concrete during mixing and casting. Later, when
cracks happen, the organisms are activated and form calcite that seals the cracks, a
method widely known as self-healing. This will be discussed below under Sect. 3.3.
In the second approach, which is a passive way, after localizing cracks during
inspection, the microbial agents are applied on the identified crack surface. This
method is sometime referred to as bio-based manual crack fixing or bioremediation
of concrete. The results among the first studies on using bacteria to repair concrete
crack was encouraging [122, 162, 163]. Artificially made cracks on mortar cubes
were filled with Sporosarcina pasteurii (formerly Bacillus pasteurii) cells, nutrients,
and sand. Then, the cube was immersed in urea-CaCl 2 solution for 28 days. Cubes
with their cracks filled by the biomineralization process exhibited a significant
increase in compressive strength compared to specimens without bacteria [163]. Similar results of higher compressive strength of microbial-treated concrete over
untreated samples have been widely reported [158, 164–166]. The interesting
observation is that the CaCO 3 precipitate serve as special cement that glue the
sand particles together, which indicates the potential of the microbially induced
mineralization in making better repair. The glue property may be partly attributed
due to the bacterial exopolysaccharides [124]. In addition to increasing the compressional strength, the precipitated calcite effectively mends cracks. The sealed
cracks exhibit reduction in water permeability (10
À10
–10
À11 m/s coefficient) and
crack bridging which was demonstrated by an increase in ultrasonic pulse velocity
[4, 9]. It has also been reported that the microbial-based crack fixing resulted in a
60% strength regain in mortar prisms. The presence of calcite in the repair material
has also been confirmed by thermogravimetry [4], scanning electron microscopy
(SEM) [167], and X-ray diffraction (XRD) [168] analyses. Over the years, several
research publications appeared which report on the use of different microorganisms,
different sources of calcium and nutrients, cell protection methods, etc. A summary
of the efficiency of some of these microbial-based concrete crack repair studies
reported by different researchers is given in Table 3, which is partly adapted and
modified from Achal and Mukherjee [64].
In general, the results from studies made so far clearly indicate the great potential
of microbial-induced calcite precipitation for repairing concrete cracks. However,
there are still some issues that need to be properly addressed. For instance, analysis
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
307
to cracks can stop water ingress and the subsequent physical deterioration. However,
it has been shown that some of these conventional crack repair methods such as the
application of chemicals and polymers are sources of health- and environmental
risks, and more importantly, they are effective only for short term due to several
factors such as difference in the thermal expansion coefficient of the repairing
material and susceptibility to biological and chemical degradation [2–5]. Thus,
treatment methods that are environmentally benign, safe, long lasting, and sustainable are in high demand. The biological way of mending concrete cracks by biogenic
CaCO 3 precipitation is potentially cheap, exhibit compatible properties to concrete,
and it is environmentally benign and safe.
Biomineralization can be used in two ways to fix cracks in concrete. In one of
these methods, microbial cells together with nutrients, calcium source, and urea
(in most studies) are added into the concrete during mixing and casting. Later, when
cracks happen, the organisms are activated and form calcite that seals the cracks, a
method widely known as self-healing. This will be discussed below under Sect. 3.3.
In the second approach, which is a passive way, after localizing cracks during
inspection, the microbial agents are applied on the identified crack surface. This
method is sometime referred to as bio-based manual crack fixing or bioremediation
of concrete. The results among the first studies on using bacteria to repair concrete
crack was encouraging [122, 162, 163]. Artificially made cracks on mortar cubes
were filled with Sporosarcina pasteurii (formerly Bacillus pasteurii) cells, nutrients,
and sand. Then, the cube was immersed in urea-CaCl 2 solution for 28 days. Cubes
with their cracks filled by the biomineralization process exhibited a significant
increase in compressive strength compared to specimens without bacteria [163]. Similar results of higher compressive strength of microbial-treated concrete over
untreated samples have been widely reported [158, 164–166]. The interesting
observation is that the CaCO 3 precipitate serve as special cement that glue the
sand particles together, which indicates the potential of the microbially induced
mineralization in making better repair. The glue property may be partly attributed
due to the bacterial exopolysaccharides [124]. In addition to increasing the compressional strength, the precipitated calcite effectively mends cracks. The sealed
cracks exhibit reduction in water permeability (10
À10
–10
À11 m/s coefficient) and
crack bridging which was demonstrated by an increase in ultrasonic pulse velocity
[4, 9]. It has also been reported that the microbial-based crack fixing resulted in a
60% strength regain in mortar prisms. The presence of calcite in the repair material
has also been confirmed by thermogravimetry [4], scanning electron microscopy
(SEM) [167], and X-ray diffraction (XRD) [168] analyses. Over the years, several
research publications appeared which report on the use of different microorganisms,
different sources of calcium and nutrients, cell protection methods, etc. A summary
of the efficiency of some of these microbial-based concrete crack repair studies
reported by different researchers is given in Table 3, which is partly adapted and
modified from Achal and Mukherjee [64].
In general, the results from studies made so far clearly indicate the great potential
of microbial-induced calcite precipitation for repairing concrete cracks. However,
there are still some issues that need to be properly addressed. For instance, analysis
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
307