Environmental Engineering Reference
In-Depth Information
induced by Bacillus sp. was effective in reducing the water absorption rate of
limestone. Such researches provided way to choose other building materials. An
increase in the resistance of concrete toward alkali, sulfate, freeze thaw attack and
drying shrinkage was observed with calcite precipitating bacteria (Ramakrishnan
et al.
1998
). While studying the durable effect of Bacillus sphaericus on mortars,
De Muynck et al. (
2008
) found a significant decrease of the water uptake com-
pared to untreated specimens (a reduction of 45, 43, and 24 % with increasing w/c)
and 19 % decrease of the chloride migration coefficient. Later, they (De Muynck
et al.
2008
) concluded that the carbonate precipitation was mainly a surface
phenomenon due to the limited penetration of the bacteria in the porous matrix,
resulted in a decrease of water absorption and gas permeability from mortars.
Bacillus sp. CT-5, isolated from commercially available cement, was used to
prepare mortars and a sorptivity test was performed on it (Achal et al.
2011b
).
Over a period of 168 h, the mortars with bacterial cells absorbed nearly six times
less water than the control cubes. The presence of bacteria resulted in a significant
decrease of the water uptake compared to untreated mortars. The deposition of a
layer of calcium carbonate crystals on the surface resulted in a decrease of the
sorptivity.
Achal et al. (
2011c
) performed the water impermeability test on the concrete
cubes of dimension 150 mm (M20 grade), prepared with mutant S. pasteurii grown
in commercially available nutrient broth (NB) and economic corn steep liquor
(CSL) media with urea as substrate and calcium chloride as calcium source. The
results indicated that the permeability of the concrete cubes prepared with bacterial
cells was lower than that of the control irrespective of media used. The penetration
at the sides of concrete was higher than that at the top due to better compaction and
closing of pores at the top by calcite precipitated by bacterial cells. Further they
reported that the resistance of concrete to chloride penetration increased with
MICP. The permeability class type was recorded ''moderate'' for control concrete
specimens, while the class changed to ''low'' type of concrete with bacterial cells
as per ASTM C1202-05. For control samples, the average charge passed was
3,177 C, whereas for samples prepared with bacterial cells in NB and CSL media
it was 1,019 and 1,185 C, respectively.
14.4.4 Microbial Concrete and Corrosion
The corrosion of steel and reinforcing bar is a predominant factor causing wide-
spread premature deterioration of concrete constructions worldwide (Raupach and
Schiebl
2001
). Corrosion and permeability goes together, higher the permeability,
more would be corrosion and vice versa. The ingress of moisture, chloride ions, and
carbon dioxide initiates corrosion through the concrete to the steel surface. Chloride-
induced corrosion of reinforcing steel is one of the most pressing problems world-
wide that the construction industry is facing today. The corrosion products of iron
oxides/hydroxides expose the reinforcement to direct environmental attack that
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