Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 344

specimens treated with bacteria. This could be attributed to the inhibition of the
bacteria by the high pH and the lack of oxygen inside the mortar mixture. The
overall increase of strength, therefore, resulted from the presence of an adequate
amount of organic substances in the matrix due to the microbial biomass. How-
ever, an increase of the biomass, as dead cells in particular, resulted in a decreased
strength. According to the authors, this could be attributed to the disintegration of
the organic matter with time, making the matrix more porous (Ramachandran et al.
2001 ).
Ramakrishnan et al. ( 2001 ) investigated 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, 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 speci-
mens. The best results were obtained with the phosphate buffer. Ghosh et al.
( 2005 ) demonstrated the positive effect of the addition of Shewanella on the
compressive strength of mortar specimens. Contrary to the aforementioned
research, these authors did not intend mineral deposition, as these specimens were
cured in air and not in a nutrient containing medium. An increase of 25 % of the
28 days compressive strength was obtained for a cell concentration of about
10 5 cells mL -1 and a water to cement ratio of 0.4. 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 the addition of
Shewanella improved with increasing curing times. For a concentration of
105 cells mL -1 , an increase of the compressive strength of 17 and 25 % was
observed after 7 and 28 days, respectively. However, no increase of the com-
pressive strength was observed with additions of Escherichia coli to the mortar
mixture. This led the authors to suggest that the choice of the microorganism plays
an important role in the improvement of the compressive strength. More specifi-
cally, the production of EPS by the bacteria seemed to be of importance.
Figure 15.3 shows direct stereomicroscopic observation of cracks from control
and bacteria-based specimens before and after 100 days of immersion in tap water.
Width of completely healed cracks was significantly larger in bacteria-based
specimens (0.46 mm) compared to control specimens (0.18 mm).
Two-component biochemical self-healing agent, consisting of a mixture of
bacterial spores and calcium lactate, can be successfully applied to promote and
enhance the self-healing capacity of concrete as the maximum healable crack
width more than doubled. Moreover, oxygen measurements provided evidence that
concrete incorporating bacterial spores embedded in expanded clay particles and
derived active bacteria remain viable and functional several months after concrete
casting. The microbial enhanced crack-healing ability is presumably due to
combined direct and indirect calcium carbonate formation: (i) direct CaCO 3 pre-
cipitation through metabolic conversion of calcium lactate and (ii) indirect for-
mation due to reaction of metabolically produced CO 2 molecules with Ca(OH)2
minerals present in the concrete matrix leading to additional CaCO 3 precipitation.