the amount of CaCO 3 precipitated, the higher the amount of CaCO 3 formed, the
thicker the layer becomes to effectively decrease the water absorption by the
concrete [193]. The rate of precipitation and organic matter presence are known to
affect the deposit density and cohesion [124]. As loose layers are not effective [124],
the layer not only be thicker but also should be dense and coherent to be protective
enough.
Another interesting application of the biodeposition is improving the concrete
aesthetically, which is so far given little attention. It is not uncommon to see
unevenness on finished concrete surfaces. This phenomenon has effect on the
appearance of the structure. The use of calcite precipitating microbes in addition to
filling small cracks and pores on the surface can improve the smoothness of the
surface by depositing calcite layers. In fact, a very encouraging smoothing effect has
been reported by Richardson et al. [194]. Their study showed a reduction in
differential surface topography using Alicona scanner. These authors also indicated
that if the white-colored calcite layer is not aesthetically pleasing, it is possible to get
the desired color by using suitable pigment additives.
3.3 Microbial-Based Self-Healing Concrete
A passive way of fixing concrete cracks involves constant inspection. When only
the cracks are noticed, any of the available and appropriate method(s) will be
implemented to repair and maintain the concrete structure. This is an expensive
process, and countries often allocate a considerable annual budget for repair and
maintenance of existing concrete structures [2, 195]. Despite that concrete production cost is in the range of $65 and $80 per m
3 , the direct cost of concrete cracks
repair and maintenance is estimated to be around $147 per m
3 of concrete [196]. The
indirect costs are even much higher at least for structures such as bridge repair.
According to the FHWA [195] report, bridge maintenance indirect cost includes a
traffic tie-up or detour that can result in wear and tear on automobiles, increased
gasoline use, delays in product transport, missed appointments, and other inconveniences that incur economic loss. The report stated that the indirect cost of steel
bridge maintenance can be tenfold higher than the direct cost; thus one can assume a
similar trend for maintenance of concrete bridges. This (the sum of the direct and
indirect costs) clearly indicates the enormous amount of resources consumed annually and globally to repair concrete structures. Therefore, preventive approaches to
confine and heal crack at early stage are vital, and the use of self-healing durable
concretes can substantially reduce the maintenance and repair cost of concrete
structures.
Studies on the use of microbial-based preparations to manually repair existing
cracks revealed many interesting results as discussed in Sect. 3.1. This approach is
notable not only by its potential for long-lasting effect but also for being environmentally benign. Furthermore, it prevails the conventional repair techniques due to
its bonding capacity and compatibility with concrete content [165]. These desired
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thicker the layer becomes to effectively decrease the water absorption by the
concrete [193]. The rate of precipitation and organic matter presence are known to
affect the deposit density and cohesion [124]. As loose layers are not effective [124],
the layer not only be thicker but also should be dense and coherent to be protective
enough.
Another interesting application of the biodeposition is improving the concrete
aesthetically, which is so far given little attention. It is not uncommon to see
unevenness on finished concrete surfaces. This phenomenon has effect on the
appearance of the structure. The use of calcite precipitating microbes in addition to
filling small cracks and pores on the surface can improve the smoothness of the
surface by depositing calcite layers. In fact, a very encouraging smoothing effect has
been reported by Richardson et al. [194]. Their study showed a reduction in
differential surface topography using Alicona scanner. These authors also indicated
that if the white-colored calcite layer is not aesthetically pleasing, it is possible to get
the desired color by using suitable pigment additives.
3.3 Microbial-Based Self-Healing Concrete
A passive way of fixing concrete cracks involves constant inspection. When only
the cracks are noticed, any of the available and appropriate method(s) will be
implemented to repair and maintain the concrete structure. This is an expensive
process, and countries often allocate a considerable annual budget for repair and
maintenance of existing concrete structures [2, 195]. Despite that concrete production cost is in the range of $65 and $80 per m
3 , the direct cost of concrete cracks
repair and maintenance is estimated to be around $147 per m
3 of concrete [196]. The
indirect costs are even much higher at least for structures such as bridge repair.
According to the FHWA [195] report, bridge maintenance indirect cost includes a
traffic tie-up or detour that can result in wear and tear on automobiles, increased
gasoline use, delays in product transport, missed appointments, and other inconveniences that incur economic loss. The report stated that the indirect cost of steel
bridge maintenance can be tenfold higher than the direct cost; thus one can assume a
similar trend for maintenance of concrete bridges. This (the sum of the direct and
indirect costs) clearly indicates the enormous amount of resources consumed annually and globally to repair concrete structures. Therefore, preventive approaches to
confine and heal crack at early stage are vital, and the use of self-healing durable
concretes can substantially reduce the maintenance and repair cost of concrete
structures.
Studies on the use of microbial-based preparations to manually repair existing
cracks revealed many interesting results as discussed in Sect. 3.1. This approach is
notable not only by its potential for long-lasting effect but also for being environmentally benign. Furthermore, it prevails the conventional repair techniques due to
its bonding capacity and compatibility with concrete content [165]. These desired
314
G. Mamo and B. Mattiasson