Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 331

immobilized with a silica sol to protect B. sphaericus from the alkaline pH con-
ditions. Upon the addition of a salt, a bioceramic material (biocer) was formed,
which was able to bridge the crack. Subsequent addition of a urea and calcium
chloride solution resulted in the deposition of carbonate crystals inside the pores of
the biocer and concomitantly sealing of the crack. As a result, a decrease in the
water permeability, similar to that obtained with traditional epoxy injections, was
observed. Cracks filled with a mixture of S. pasteurii and sand showed a significant
increase in compressive strength and stiffness, compared to cracks without cells.
Microscopy confirmed the presence of calcite crystals and cells near the surface of
the cracks. Ramachandran et al. ( 2001 ) studied the effect of microbiological calcite
deposition of cracks of various depths on the compressive strength values in
Portland cement mortar cubes and found an increase of the strength in the presence
of B. pasteurii in the cubes prepared with the deepest cracks (25.4 mm), whose
microbial remediation increased the compressive strength by approximately 61 %
of that of the control concrete. Recently, Qian et al. ( 2010a ) reported that the
compressive strength of the treated specimens could be restored to 84 % and
demonstrated that this biorestoration method is effective in repairing surface
defects of cementitious materials. Sand particle surfaces were covered by CaCO 3
deposition which could cement loose sand and supply conjunction between indi-
vidual particles. SEM results indicated that CaCO 3 crystals were precipitated on
crack surface, whereas sand was consolidated and cemented by CaCO 3 crystals
resulting in compressive strength recovery.
Some form of enhanced crack repair might be obtained through biodeposition
treatment with B. sphaericus culture, which is incorporated in a gel matrix and a
calcium source is provided, and silica gel was used to protect the bacteria against
the high pH in concrete. Protection of the bacteria by means of this gel matrix
seemed to be effective as CaCO 3 crystals were precipitated inside the matrix which
was not the case if only bacteria were used, without immobilization in the silica
gel (see Fig. 15.1 ). Crack sealing by means of this biological treatment resulted in
a decrease in water permeability. However, it was seen that the decrease in water
flow was also obtained if autoclaved bacteria were used instead of active bacteria.
This corroborates that the greater part of the decrease in water permeability is
attributed to crack filling by the sol-gel matrix. TGA analysis on the crack repair
material showed only in the case of active bacteria the presence of CaCO 3 crystals.
Precipitation of these crystals inside the gel matrix may enhance the durability of
this repair material. Efficiency of this biological treatment was also evaluated by
means of ultrasonic transmission measurements and visual examination. Crack
treatment with B. sphaericus, immobilized in silica gel, resulted in an increase in
ultrasonic pulse velocity, indicating that crack bridging was obtained. Visual
examination of the cracks proved that this technique resulted in complete filling of
the cracks. The use of this biological repair technique is highly desirable because
the mineral precipitation induced as a result of microbial activities is pollution-free
and natural. However, further experiments have to be done to examine the dura-
bility of this crack repair technique (Van Tittelboom et al. 2010 ). In Fig. 15.1 ,
some top views of the specimens and some cross-sections of the treated cracks are