Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 330

commonly isolated from soil, water, sewage, and incrustations on urinals. The
participation of S. pasteurii in sand consolidation has been demonstrated by
Kantzas et al. ( 1992 ). Gollapudi et al. ( 1995 ) further investigated the use of S.
pasteurii for the plugging of sand columns. Although the bacteria were mixed with
the sand slurry, consolidation mainly occurred near the surface.
The use of bacteria products as a long-term remediation tool has promised high
potential for crack healing of various structural formations such as granite and
concrete (Gollapudi et al. 1995 ). Stocks-Fischer et al. ( 1999 ) showed that bacteria
were directly involved in deposition by providing a nucleation site and by creating
an alkaline environment which favored carbonate deposition. Zhong and Islam
( 1995 ) used the consolidation of sand mixtures for the remediation of cracks in
granite. Cracks in granite were packed with a mixture of bacteria, nutrients, and a
filler material. Among the different materials that were mixed with B. pasteurii, the
silica fume (10 %) and sand (90 %) mixture lead to the highest compressive
strength and lowest permeability. Bacterially enhanced crack remediation has been
reported by Bang et al. ( 2001 ). They used Bacillus pasteurii to induce CaCO 3
deposition. Scanning electron micrography (SEM) and X-ray diffraction (XRD)
analysis has shown the direct involvement of bacteria in the process of deposition.
As a further extension to this research, Ramachandran et al. ( 2001 ) investigated the
microbiological remediation of cracks in concrete and proposed carbonate depo-
sition as an effective way to seal cracks. The appearance of cracks and fissures is
an inevitable phenomenon during the aging process of concrete structures upon
exposure to weather changes. If left untreated, cracks tend to expand further and
eventually lead to costly repair. Specimens with cracks filled with bacteria,
nutrients, and sand demonstrated a significant increase in compressive strength and
stiffness values when compared with those without cells. The presence of calcite
was, however, limited to the surface areas of the crack. B. pasteurii grows more
actively in the presence of oxygen. Still, the highly alkaline pH (12.5) of concrete
was a major hindering factor to the growth of the moderate alkaliphile B. pasteurii,
whose growth optimum is around a pH of 9. To retain high metabolic activities of
bacterial cells at such a high pH, immobilization technology (where microbial cells
are encapsulated in polymers) can be applied. Bang et al. ( 2001 ) found that
physicochemically versatile polyurethane is an effective enhancement tool in
microbiologically induced calcite deposition in concrete cracks. In order to protect
the cells from high pH, Day et al. ( 2003 ) investigated the effect of different filler
materials on the effectiveness of the crack remediation. Beams treated with bac-
teria and polyurethane showed a higher improvement in stiffness compared to filler
materials such as lime, silica, fly ash, and sand. The porous nature of the poly-
urethane minimizes transfer limitations to substrates and supports the growth of
bacteria more efficiently than other filling materials, enabling an accumulation of
calcite in deeper areas of the crack. No differences could be observed between the
overall performances of free or polyurethane-immobilized cells in the deposition
of carbonate. As an extension to their research on biodeposition on cementitious
materials, De Muynck et al. ( 2008a , b ) further investigated the use of bacterially
induced carbonate deposition for the repair of concrete cracks. Bacteria were