crack. At present, the use of these chemical-based concrete crack repair agents is the
most important approach and applicable to many existing concrete structures.
However, it is known that these chemical-based repair agents suffer from various
limitations such as poor weather resistance, sensitivity to moisture, unsustainability,
poor bonding with concrete, susceptibility to degradation, delamination with age,
and/or have different thermal expansion coefficient than that of the concrete [2–
5]. Moreover, most of the repair agents contain chemicals that pose environmental
and health hazards. Thus, there has been an interest to develop concrete crack repair
agents that are efficient, long lasting, and safe. These desires have brought the use of
microorganisms as concrete crack repair agents into focus.
Studies made so far on development of microbial-based repair agents primarily
focus on calcite (CaCO 3 ) precipitation (crystallization). Concrete deterioration is
often associated with penetration of water, which often carries aggressive substances
such as chlorides and sulfates. In addition to cracks, concrete is intrinsically porous
in nature and often has many openings on its surface that may let in water to the
structure. When microbial-based preparations are applied on concrete, the microbes
deposit calcite that fills the cracks, surface pores, and cavities. This deposition of
calcite blocks the water ingress to the concrete structure. The water ingress prevention in turn protects the concrete from deterioration, and hence the application of
calcite precipitating microbes is expected to extend the durability of concrete
structures.
Calcite precipitating organisms are ubiquitous and use different mechanisms to
precipitate CaCO 3 such as through degradation of urea, dissimilatory sulfate reduction, metabolism of organic acids, ammonification, methane oxidation, etc. [6]. However, only some of these calcite precipitating mechanisms are suitable for concrete
crack repair applications. For example, organisms that precipitate calcite through
methane oxidation or ammonification cannot be considered for this application.
Even among microbes with suitable mechanisms of calcite precipitation, only few
have the potential to effectively treat concrete. Several factors determine the efficiency of microbially driven concrete crack repair. The nature of concrete is one of
the most important factors that dictate which organisms to be considered. Concrete is
very alkaline (pH of about 12.5) in nature; therefore microbes intended for concrete
crack repair application should be able to thrive in high pH environment and
precipitate copious amount of calcite under the harsh condition. Thus, alkaliphiles
that precipitate calcite are the most desirable organisms for this application.
The application of alkaliphiles in concrete goes beyond the repair of existing
cracks. It has also a great potential in making more durable concrete. Concrete repair
has been neither cheap nor easy. Annually, the world spends billions of dollars to
repair concrete infrastructures. For example, in Europe, it is estimated that half of the
annual construction budget allocated by EU countries is consumed by the repair
works [7]. Moreover, in addition to direct costs, there are punishingly high indirect
costs [1] related to disuse of the facility, road blockage, occurrence of traffic jams,
overall inconvenience, etc. The use of more durable concrete is expected to substantially cut down these repair costs and the associated inconveniences. This has
triggered the effort to find ways in making more durable concrete.
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G. Mamo and B. Mattiasson
most important approach and applicable to many existing concrete structures.
However, it is known that these chemical-based repair agents suffer from various
limitations such as poor weather resistance, sensitivity to moisture, unsustainability,
poor bonding with concrete, susceptibility to degradation, delamination with age,
and/or have different thermal expansion coefficient than that of the concrete [2–
5]. Moreover, most of the repair agents contain chemicals that pose environmental
and health hazards. Thus, there has been an interest to develop concrete crack repair
agents that are efficient, long lasting, and safe. These desires have brought the use of
microorganisms as concrete crack repair agents into focus.
Studies made so far on development of microbial-based repair agents primarily
focus on calcite (CaCO 3 ) precipitation (crystallization). Concrete deterioration is
often associated with penetration of water, which often carries aggressive substances
such as chlorides and sulfates. In addition to cracks, concrete is intrinsically porous
in nature and often has many openings on its surface that may let in water to the
structure. When microbial-based preparations are applied on concrete, the microbes
deposit calcite that fills the cracks, surface pores, and cavities. This deposition of
calcite blocks the water ingress to the concrete structure. The water ingress prevention in turn protects the concrete from deterioration, and hence the application of
calcite precipitating microbes is expected to extend the durability of concrete
structures.
Calcite precipitating organisms are ubiquitous and use different mechanisms to
precipitate CaCO 3 such as through degradation of urea, dissimilatory sulfate reduction, metabolism of organic acids, ammonification, methane oxidation, etc. [6]. However, only some of these calcite precipitating mechanisms are suitable for concrete
crack repair applications. For example, organisms that precipitate calcite through
methane oxidation or ammonification cannot be considered for this application.
Even among microbes with suitable mechanisms of calcite precipitation, only few
have the potential to effectively treat concrete. Several factors determine the efficiency of microbially driven concrete crack repair. The nature of concrete is one of
the most important factors that dictate which organisms to be considered. Concrete is
very alkaline (pH of about 12.5) in nature; therefore microbes intended for concrete
crack repair application should be able to thrive in high pH environment and
precipitate copious amount of calcite under the harsh condition. Thus, alkaliphiles
that precipitate calcite are the most desirable organisms for this application.
The application of alkaliphiles in concrete goes beyond the repair of existing
cracks. It has also a great potential in making more durable concrete. Concrete repair
has been neither cheap nor easy. Annually, the world spends billions of dollars to
repair concrete infrastructures. For example, in Europe, it is estimated that half of the
annual construction budget allocated by EU countries is consumed by the repair
works [7]. Moreover, in addition to direct costs, there are punishingly high indirect
costs [1] related to disuse of the facility, road blockage, occurrence of traffic jams,
overall inconvenience, etc. The use of more durable concrete is expected to substantially cut down these repair costs and the associated inconveniences. This has
triggered the effort to find ways in making more durable concrete.
296
G. Mamo and B. Mattiasson