Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 332

shown. The top views were taken from realistic cracks after the specimens were
subjected to the water permeability test. Cross-sections were obtained by sawing
samples with repaired standardized cracks after ultrasonic measurements were
taken. Figure 15.1 a shows a non-treated crack. At both crack faces, crystal
deposition can be observed, showing that the untreated specimens had undergone a
certain extent of autogenous healing during the water permeability test. For the
specimens with treated cracks, no crystals were observed after performance of the
water permeability test. This can be explained as follows: for the untreated sam-
ples, the water flow was so fast that the upper compartment, of the test setup,
became completely empty between two successive readings. This brought the
concrete surface into contact with the atmosphere and led to carbonation of
Ca(OH)2 into CaCO3 crystals. In Fig. 15.1 b, a crack treated with BS + CaCl2 is
shown. No CaCO3 crystals were detected by means of the microscope used,
probably, because bacteria were not protected against the high pH in concrete.
However, when the bacteria are immobilized in sol-gel, complete filling of the
cracks occurs as shown in Fig. 15.1 c. Figure 15.1 d, e shows a cross-section of a
standard crack filled with grout and epoxy, respectively. Cement grout only covers
the surface of the samples and does not fill the cracks because of the big grain size
of the grout. Epoxy treatment, by contrast, resulted in complete filling of cracks of
both 10 and 20 mm deep. Treatment with only sol-gel or with sol-
gel + BS + CaCl 2 resulted in cracking of the gel matrix as shown in Fig. 15.1 g,
h. When the gel hardens, it shrinks and this gives rise to cracking. Samples treated
with BS in sol-gel + CaCl2 or Ca(NO 3 ) 2 or Ca (CH 3 COO) 2 were placed in a
urea-calcium solution immediately after filling of the cracks with silica gel and
bacteria. During immersion, bacteria started to precipitate CaCO3 resulting in a
compete filling of the cracks (see Fig. 15.1 c). However, complete filling was only
feasible for 10-mm deep cracks (see Fig. 15.1 f). As shown in Fig. 15.1 i, these
treatments were not able to fill 20-mm deep cracks. This was also observed from
the ultrasonic measurements. The 10-mm deep cracks treated with BS in sol-
gel + CaCl2 or Ca(NO3)2 or Ca(CH3COO)2 performed almost as good as cracks
treated with epoxy, which was no longer the case for 20-mm deep cracks, which
were only completely filled when epoxy was used.
Microbe cement as a biogrouting could consolidate loose particles to improve
mechanical properties. Microbe cement has drawn much attention because of the
ever increasing awareness of environmental protection. This paper confirms the
feasibility of binding loose sand particles using microbe cement and details the
cementation mechanism of microbe cement. In addition, the microstructure and
properties of representative bio-sandstones have been analyzed by X-ray computed
tomography (XRCT), scanning electron microscopy (SEM), and mercury intrusion
porosimetry (MIP). The experimental results indicate that the compressive strength
of bio-sandstone could be up to 6.1 MPa and the bottom region microstructure in
bio-sandstone is denser and less fragile than the top region due to more calcite
precipitated in the former one (Rong et al. 2012 ). Kim et al. ( 2013 ) placed the