Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 335

layer material is remarkably similar to the limestone substrate. Moreover, biode-
position follows the same natural process that formed many limestones. Finally,
they concluded that bacterial mortars or cement could be used to affix small pieces
broken from statues and to fill small cavities on limestone surfaces. Nemati and
Voordouw ( 2003 ) noticed a decrease in the permeability of sandstone cores after
injecting CaCO 3 forming reactants. Dick et al. ( 2006 ) studied such deposition on
degraded limestone by Bacillus species. They concluded that such deposition
reduces the water absorption rate of limestone. They studied the capillary water
absorption and absorption under vacuum. A decrease in the ability to absorb water
will result in a deceleration of the weathering process. The positive results of
carbonate deposition on the sandstone and limestone attracted many researchers to
apply this technique on mortar and concrete to reduce permeability. Microbial
carbonate deposition (biodeposition) has also been reported to decrease the per-
meation properties of mortar and concrete. De Muynck et al. ( 2008a , b ) investi-
gated the effects of biodeposition on the permeability of concrete and mortar. To
determine the increase in resistance toward water penetration, they carried out a
sorptivity test. They coated the mortar specimens with Bacillus sphaericus, oven
dried then dipped into 10 ± 1 mm of water. They found that the presence of only
bacteria resulted in a significant decrease of the water uptake compared to
untreated specimens (a reduction of 45, 43, and 24 % with increasing w/c). When
a calcium source was added to the medium an additional significant decrease of the
water absorption coefficient was noticed. They also studied the durability of the
treated surface by measuring the resistance to carbonation and chloride ingress, as
chloride ions are one of the corrosion-causing agents. They obtained a 19 %
decrease of the chloride migration coefficient, by biotreatment, compared to
untreated cubes. The addition of bacterial biomass resulted in a significant smaller
carbonation rate compared to untreated cubes. The deposition of a layer of calcium
carbonate induced by Bacillus sphaericus on the surface of the mortar specimens
resulted in a decrease of water absorption and gas permeability (De Muynck et al.
2008a , b ). The biodeposition treatment on the surface of concrete materials should
be regarded as a coating system. This could be attributed to the fact that the
carbonate deposition was mainly a surface phenomenon due to the limited pene-
tration of the bacteria in the porous matrix. Ramakrishnan et al. (1998) investi-
gated the effect of this technique on the durability of concrete. The presence of
bacteria was observed to increase the resistance of concrete toward alkali, sulfate,
and freeze thaw attack and drying shrinkage; the effect being more pronounced
with increasing concentrations of bacterial cells. The authors attributed this to the
presence of a calcite layer on the surface, as confirmed by XRD analysis, lowering
the permeability of the specimens. Recently, Achal et al. ( 2011a , b , c ) reported that
the mortar cubes treated with Bacillus sp. a layer of biodeposition crystals on the
surface of the mortar resulted in a decrease of the permeation properties. As a
consequence, the ingress of harmful substances may be limited. From these
reports, it is clear that the presence of a layer of carbonate crystals on the surface
has the potential to improve the resistance of concrete materials toward degra-
dation processes. Biodeposition treatment was similar as conventional treatments.