[179]. The treatment was solvent free and resulted in a significant protective and
consolidating effect. The drilling resistance achieved was more than doubled just
with two rounds of spray applications in a day. However, in these studies, hygroscopic salts such as calcium chloride, calcium acetate, and calcium formate have
been used, and this created a concern that this may later damage the stone. It is
believed that substitution of the calcium salt with less hygroscopic sources could
alleviate this concern. So far, the biodeposition application in this regard is limited to
the on-site bioconsolidation of calcareous cultural heritage buildings. However, the
findings of these application studies clearly indicated that the technique has
improved the surface strength, reduced the water permeability, and enhanced the
freeze-thaw resistance, which positively contribute to the stone protection [97, 124,
185, 187]. Moreover, these effects are long lasting as shown in treated limestones
that did not require new treatment for more than a decade [77]. A comparison of
different biodeposition methods have been extensively reviewed by De Muynck
et al. [18].
The encouraging results of biodeposition applications on stone surfaces led to use
of microorganisms on the treatment of concrete surfaces. This is indeed very
attractive as degradation of concrete structures often starts from the surface. Surface
treatment of concrete is believed to be an effective way in enhancing the durability
of concrete structures. Although the principle is the same as for the treatment of
stone surfaces, the nature of stone and concrete is somehow different; hence, the
biodeposition process should be done in a way that suits the concrete nature. One of
the differences is the high alkalinity of concrete. Thus, the use of microorganisms
that are tolerant to high pH is a requirement. S. pasteurii and B. sphaericus are alkalitolerant and are the most commonly used organisms in concrete biodeposition
studies. The other difference is the porosity. Compared to porous limestone, concrete
is more compact and hence less efficient in retaining microorganisms on its surface
layer. However, a number of studies have successfully applied the biodeposition
treatment on concrete surfaces [2, 3, 187–189].
The microbial agent and the precipitating precursor substances can be applied in
two steps [2, 3, 190] or in a single step [187]. In the two-step immersion process,
concrete specimens were first immersed in the bacterial culture, and then the
specimens were wiped with a towel to remove the excess surface liquid and
immersed in solution of urea and Ca
2+ for 72 h for deposition of CaCO 3 . In the
single-step process, the concrete specimens were immersed in medium containing
the bacterial culture, urea, and Ca
2+ . In this single-step immersion procedure, the
precipitation of CaCO 3 on the top surface of the specimens formed into a dense and
coherent layer which has a thickness of 150–290 μm. However, the CaCO 3 deposition was not only restricted to the concrete surface but it had also been observed in
the bulk solution. On the other hand, in the two-step immersion system, the deposition of CaCO 3 was performed by the bacteria that had been adsorbed on the
concrete surface.
In both cases, the presence of CaCO 3 layer on the concrete surface dramatically
reduced the capillary water absorption and gas permeability, which concomitantly
enhanced the resistance to carbonation, chloride penetration, and freezing and
312
G. Mamo and B. Mattiasson
consolidating effect. The drilling resistance achieved was more than doubled just
with two rounds of spray applications in a day. However, in these studies, hygroscopic salts such as calcium chloride, calcium acetate, and calcium formate have
been used, and this created a concern that this may later damage the stone. It is
believed that substitution of the calcium salt with less hygroscopic sources could
alleviate this concern. So far, the biodeposition application in this regard is limited to
the on-site bioconsolidation of calcareous cultural heritage buildings. However, the
findings of these application studies clearly indicated that the technique has
improved the surface strength, reduced the water permeability, and enhanced the
freeze-thaw resistance, which positively contribute to the stone protection [97, 124,
185, 187]. Moreover, these effects are long lasting as shown in treated limestones
that did not require new treatment for more than a decade [77]. A comparison of
different biodeposition methods have been extensively reviewed by De Muynck
et al. [18].
The encouraging results of biodeposition applications on stone surfaces led to use
of microorganisms on the treatment of concrete surfaces. This is indeed very
attractive as degradation of concrete structures often starts from the surface. Surface
treatment of concrete is believed to be an effective way in enhancing the durability
of concrete structures. Although the principle is the same as for the treatment of
stone surfaces, the nature of stone and concrete is somehow different; hence, the
biodeposition process should be done in a way that suits the concrete nature. One of
the differences is the high alkalinity of concrete. Thus, the use of microorganisms
that are tolerant to high pH is a requirement. S. pasteurii and B. sphaericus are alkalitolerant and are the most commonly used organisms in concrete biodeposition
studies. The other difference is the porosity. Compared to porous limestone, concrete
is more compact and hence less efficient in retaining microorganisms on its surface
layer. However, a number of studies have successfully applied the biodeposition
treatment on concrete surfaces [2, 3, 187–189].
The microbial agent and the precipitating precursor substances can be applied in
two steps [2, 3, 190] or in a single step [187]. In the two-step immersion process,
concrete specimens were first immersed in the bacterial culture, and then the
specimens were wiped with a towel to remove the excess surface liquid and
immersed in solution of urea and Ca
2+ for 72 h for deposition of CaCO 3 . In the
single-step process, the concrete specimens were immersed in medium containing
the bacterial culture, urea, and Ca
2+ . In this single-step immersion procedure, the
precipitation of CaCO 3 on the top surface of the specimens formed into a dense and
coherent layer which has a thickness of 150–290 μm. However, the CaCO 3 deposition was not only restricted to the concrete surface but it had also been observed in
the bulk solution. On the other hand, in the two-step immersion system, the deposition of CaCO 3 was performed by the bacteria that had been adsorbed on the
concrete surface.
In both cases, the presence of CaCO 3 layer on the concrete surface dramatically
reduced the capillary water absorption and gas permeability, which concomitantly
enhanced the resistance to carbonation, chloride penetration, and freezing and
312
G. Mamo and B. Mattiasson