Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 339

matrix due to the microbial biomass. To achieve greatest improvement in the
compressive strength, cell concentrations/microbial doses need to be optimized.
They concluded that biodeposition enhanced the compressive strength of cement
mortar cubes. Biodeposition has unique features and performance that have
encouraged innovative high-performance composite applications in construction
materials. The application of biodeposition in concrete is a potential field for
further research efforts. Compressive strength of concrete with B. pasteurii as
microbes had marginal (5-10 %) increase in strength (in case of BU and BP
bacterial suspended in urea-CaCl 2 , bacteria suspended in phosphate buffer when
conc. with BW (Bacteria suspended in water) had marginal decrease in strength
(10 %) when compared to controlled concrete samples. Park et al. ( 2010 ) used
four different species of calcite-forming bacteria (Sporosarcina soli, Bacillus
massiliensis, Arthrobacter crystallopoietes, and Lysinibacillus fusiformis) for
compressive strength improvement in mortar. The 28-day strength test for con-
solidated mortar showed that the cube treated with Arthrobacter crystallopoietes
had the greatest improvement in compressive strength, and the observed change in
compressive strength from 7 to 28 days was 22 %. Ghosh et al. ( 2005 ) demon-
strated the positive effect of biodeposition on the compressive strength of mortar
specimens. They found that the strength of mortar cubes increased at all levels of
anaerobic microbe addition. For these samples, the presence of a fibrous material
inside the pores could be noticed. As a result, a modification of the pore size
distribution was observed. The positive effect of biodeposition improved with
increasing curing times. The greatest improvement in compressive strength was
reported to occur at cell concentrations of 10 5 cells/ml for all ages (3, 7, 14, and
28 days). For a concentration of 10 5 cells/ml, an increase of the compressive
strength of 17 and 25 % was observed after 7 and 28 days, respectively. However,
no increase of the compressive strength was observed with additions of
Escherichia coli (non-urease-producing microbe) to the mortar mixture. This led
the authors to suggest that the choice of the bacteria plays an important role in the
improvement of the compressive strength. Jonkers and Schlangen (2007) inves-
tigated that the addition of a high number of bacterial spores (10 8 /cm 3 ) by spore-
forming bacteria (Bacillus pseudofirmus and Bacillus cohnii) resulted in 10 %
increase of strength. Achal et al. ( 2009 ) showed that nutrients to grow bacteria also
play an important role when used with construction materials. To economize the
overall process, they replaced standard nutrient with some industrial byproducts
such as lactose mother liquor (LML) and corn steep liquor (CSL). They used
Sporosarcina pasteurii in mortar cubes with LML medium and found 17 %
improvement in compressive strength at 28 days (26.3 MPa) compared to control
(23.2 MPa). There was no significant difference when LML medium was replaced
by standard nutrient medium. Further, medium containing CSL resulted in sig-
nificantly higher compressive strength even when compared with commercially
available medium. All the reports suggest significant increase in the compressive
strength at the age of 28 days compared to early ages (3 and 7 days). The overall
trend of an increase in compressive strength up to 28 days might be attributed to
the behavior of microbial cells within the cement mortar matrix. During the initial